The format of the label for SDH and/or SONET TDM-LSR link is:
https://docs.google.com/file/d/0BwE3NGerAe3tbXZ0NHhfUHpQbkk/edit?usp=sharing
0                    1                    2                    3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               S            |   U    |   K    |   L      |   M      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For SDH, this is an extension of the numbering scheme defined in
G.707 section 7.3, i.e. the (K, L, M) numbering. For SONET, the same
signaling scheme is used in order to provide easy interworking between
SDH and SONET signaling. For the S field a STS-3 group, which
corresponds with the SDH AUG-1 level is introduce. The U field indicates
the position of the STS-3c-SPE or STS-1-SPE within the STS-3 group

1. S is the index of a particular AUG-1/STS-3 group. S=1->N
indicates a specific AUG-1/STS-3 group inside an STM-N/STS-3xN
multiplex. For example, S=1 indicates the first AUG-1/STS-3 group,
and S=N indicates the last AUG-1/STS-3 group of this multiplex.
S is not significant for STM-0/STS-1.

2. U indicates a specific VC/STS-SPE inside a given AUG-1/STS-3
group or STM-0/STS-1. U=1 indicates a single VC-4/STS-3c-SPE in a
AUG-1/STS-3 group or the single VC-3/STS-1-SPE in a STM-0/STS-1, while U=2->4
indicates a specific VC-3/STS-1-SPE inside the given AUG-1/STS-3
group.

3. K is only significant for VC-4/STS-3c and must be ignored for
higher order VC-3/STS-1-SPE. For SDH it indicates a specific branch of a VC-4.
K=1 indicates that the VC-4 is not further subdivided and
contains a C-4. K=2->4 indicates a specific TUG-3 inside the VC-4.
For a SONET STS-3c-SPE it is fixed to K=1 as SONET doesn’t support
substructured STS-3c-SPE.

4. L indicates a specific branch of a TUG-3, VC-3 or STS-1 SPE.
It is not significant for an unstructured VC-4/STS-3c-SPE. L=1
indicates that the TUG-3/VC-3/STS-1 SPE is not further
subdivided and contains a VC-3/C-3 in SDH or the equivalent in
SONET. L=2->8 indicates a specific TUG-2/VT Group inside the
corresponding higher order signal.

5. M indicates a specific branch of a TUG-2/VT Group. It is not
significant for an unstructured VC-4, STS-3c-SPE, TUG-3, VC-3 or STS-1 SPE.
M=1 indicates that the TUG-2/VT Group is not further subdivided
and contains a VC-2/VT-6 SPE. M=2->3 indicates a specific VT-3
inside the corresponding VT Group, these values MUST NOT be used
for SDH since there is no equivalent of VT-3 with SDH. M=4->6
indicates a specific VC-12/VT-2 SPE inside the corresponding
TUG-2/VT Group. M=7->10 indicates a specific VC-11/VT-1.5 SPE
inside the corresponding TUG-2/VT Group. Note that M=0 denotes
an unstructured VC-4, VC-3 or STS-1 SPE (easy for debugging).

The M encoding is summarized in the following table:

M    SDH                          SONET
———————————————————-
0    unstructured VC-4/VC-3  unstructured STS-1 SPE
1    VC-2                    VT-6
2    –                       1st VT-3
3    –                       2nd VT-3
4    1st VC-12               1st VT-2
5    2nd VC-12               2nd VT-2
6    3rd VC-12               3rd VT-2
7    1st VC-11               1st VT-1.5
8    2nd VC-11               2nd VT-1.5
9    3rd VC-11               3rd VT-1.5
10   4th VC-11               4th VT-1.5

In case of contiguous concatenation, the label that is used is the
lowest label of the contiguously concatenated signal as explained
before. The higher part of the label indicates where the signal
starts and the lowest part is not significant. For instance, when
requesting an VC-4-16c the label is S>0, U=0, K=0, L=0, M=0.

Examples of labels:

Example 1: S>0, U=1, K=1, L=0, M=0
Denotes the unstructured VC-4/STS-3c-SPE of the Sth AUG-1/STS-3 group.

Example 2: S>0, U=1, K>1, L=1, M=0
Denotes the unstructured VC-3 of the Kth-1 TUG-3 of the Sth AUG-1.

Example 3: S>0, U>1, K=0, L=1, M=0
Denotes the Uth unstructured VC-3/STS-1 SPE of the Sth AUG-1/STS-3 group.

Example 4: S>0, U>1, K=0, L>1, M=1
Denotes the VC-2/VT-6 in the Lth-1 TUG2/VT Group in the Uth VC-3/STS-1 SPE of the Sth AUG-1/STS-3 group.

Example 5: S>0, U>1, K=0, L>1, M=9
Denotes the 3rd VC-11/VT-1.5 in the Lth-1 TUG2/VT Group in the Uth VC-3/STS-1 SPE of the Sth AUG-1/STS-3 group.

Example 6: S>0, U=1, K>1, L>1, M=5
Denotes the 2nd VC-12 in the Lth-1 TUG2 in the Kth TUG3 in the VC-4 of the Sth AUG-1.

This Topic is legacy now but just to keep reader aware about the basic concepts, compiling  all together SONET/SDH here.

• Bellcore  defines GR.253 (SONET) and  ITU-T defines G.691 (SDH)
• SONET means Synchronous Optical NETwork
• First North American Fiber-Optic Telecommunications Standard to overcome the limitations of the traditional asynchronous network.
• Formulated by the Exchange Carriers Standards Association for the American National Standards Institute
• Incorporated into the Synchronous Digital Hierarchy (SDH) recommendations of the International Telecommunications Union (ITU).
• SDH is mostly used outside North America and principally in Europe
• Synchronous systems (e. g. SONET) :
• — Average frequency of all clocks in the system are same or nearly—Clocking provided by a highly stable reference supply

  —Allows many STS- 1s to be stacked together without of bit- stuffing

  —STS- 1s as well as VTs easily accessed from higher rates signals

  —Pointers accommodate differences in reference source frequencies, and phase wander

  • STS-1: Synchronous Transport Signal – level 1
  • STM-1: Synchronous Transport Module – level 1
• The telecommunications industry adopted the Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely related standards that specify interface parameters, rates, framing formats, multiplexing methods and management for synchronous TDM  over fiber.
• SONET/SDH takes “n” bit streams, multiplexes them and optically modulates the signal, sending it out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) multiplied by “n.” Traffic arriving at the SONET/SDH multiplexer from four places at 2.5 Gigabits per second will go out as a single stream at 4 times 2.5 Gigabits per second, or 10 Gigabits per second.  

 SONET/SDH Digital Hierarchy

 SONET/SDH Tributaries

Optical Fiber Communication ..Once More by Sanjay Yadav

Optical Transport Network (OTN)

COMPARISON OF SDH AND OTN

1       Abbreviations

2       What is OTN/OTH

2.1        References

3       Optical transport network interface structure

3.1        Basic signal structure

3.1.1         OCh substructure

3.1.2         Full functionality OTM-n.m (n ≥ 1) structure

3.1.3         Reduced functionality OTM-nr.m and OTM-0.m structure

3.2        Information structure for the OTN interfaces

4       Multiplexing/mapping principles and bit rates

4.1        Mapping

4.2        Wavelength division multiplex

4.3        Bit rates and capacity

4.4        ODUk Time-Division Multiplex

5       OTUk, ODUk, OPUk Frame Structure

5.1        OPUk Overhead and Processing

5.1.1         Payload Structure Identifier (PSI)

5.1.2         Payload Type (PT)

5.2        ODUk Overhead and Processing

5.2.1         Path Monitoring (PM)

5.2.2         Tandem Connection Monitoring (TCM)

5.2.3         General Communication Channels (GCC1, GCC2)

5.2.4         Automatic Protection Switching and Protection Communication Channel (APS/PCC)

5.2.5         Fault Type and Fault Location reporting communication channel (FTFL)

5.3        OTUk Overhead and Processing

5.3.1         Scrambling

5.3.2         Frame Alignment Overhead

5.3.3         Section Monitoring (SM)

5.3.4         General Communication Channel 0 (GCC0)

6       OTN Maintenance Signals

6.1        OTUk maintenance signals

6.1.1         OTUk alarm indication signal (OTUk-AIS)

6.2        ODUk maintenance signals

6.2.1         ODUk Open Connection Indication (ODUk-OCI)

6.2.2         ODUk Locked (ODUk-LCK)

6.2.3         ODUk Alarm Indication Signal (ODUk-AIS)

6.3        Client maintenance signal

6.3.1         Generic AIS for constant bit rate signals

6.3.2         Client source ODUk-AIS

6.3.3         Client source ODUk-OCI

7       Defect detection

7.1        Default PLM (Payload Mismatch)

7.2        Default MSIM (Multiplex Structure Identifier Mismatch supervision)

7.3        Default LOFLOM (Loss of Frame and Multi-frame)

8       Synchronization

8.1        Introduction

8.2        Network requirements

9       Why use OTN

9.1        Forward Error Correction (FEC)

9.2        Tandem Connection Monitoring

9.3        Transparent Transport of Client Signals

9.4        Switching Scalability

1       Abbreviations

This document uses the following abbreviations:

0xYY                 YY is a value in hexadecimal presentation

3R                    Re-amplification, Reshaping and Retiming

ACT                  Activation (in the TCM ACT byte)

AI                     Adapted Information

AIS                    Alarm Indication Signal

APS                   Automatic Protection Switching

BDI                   Backward Defect Indication

BEI                    Backward Error Indication

BIAE                  Backward Incoming Alignment Error

BIP                    Bit Interleaved Parity

CBR                  Constant Bit Rate

CI                     Characteristic Information

CM                    Connection Monitoring

CRC                  Cyclic Redundancy Check

DAPI                 Destination Access Point Identifier

EXP                   Experimental

ExTI                  Expected Trace Identifier

FAS                   Frame Alignment Signal

FDI                   Forward Defect Indication

FEC                   Forward Error Correction

GCC                  General Communication Channel

IaDI                  Intra-Domain Interface

IAE                   Incoming Alignment Error

IrDI                   Inter-Domain Interface

JOH                  Justification Overhead

LSB                   Least Significant Bit

MFAS                 Multi-Frame Alignment Signal

MFI                   Multi-frame Indicator

MS                    Maintenance Signal

MSB                   Most Significant Bit

MSI                   Multiplex Structure Identifier

NNI                   Network Node Interface

OCh                  Optical channel with full functionality

OCI                   Open Connection Indication

ODU                  Optical Channel Data Unit

ODUk                Optical Channel Data Unit-k

ODTUjk             Optical channel Data Tributary Unit j into k

ODTUG              Optical channel Data Tributary Unit Group

ODUk-Xv            X virtually concatenated ODUk’s

OH                    Overhead

OMS                  Optical Multiplex Section

OMS-OH                        Optical Multiplex Section Overhead

OMU                  Optical Multiplex Unit

ONNI                 Optical Network Node Interface

OOS                  OTM Overhead Signal

OPS                   Optical Physical Section

OPU                  Optical Channel Payload Unit

OPUk                Optical Channel Payload Unit-k

OPUk-Xv            X virtually concatenated OPUk’s

OSC                  Optical Supervisory Channel

OTH                  Optical Transport Hierarchy

OTM                  Optical Transport Module

OTN                  Optical Transport Network

OTS                  Optical Transmission Section

OTS                  -OH Optical Transmission Section Overhead

OUT                  Optical Channel Transport Unit

OTUk                Optical Channel Transport Unit-k

PCC                  Protection Communication Channel

PM                    Path Monitoring

PMI                   Payload Missing Indication

PMOH                Path Monitoring OverHead

Ppm                  parts per million

PRBS                 Pseudo Random Binary Sequence

PSI                    Payload Structure Identifier

PT                    Payload Type

RES                   Reserved for future international standardization

RS                     Reed-Solomon

SAPI                  Source Access Point Identifier

Sk                     Sink

SM                    Section Monitoring

SMOH                Section Monitoring OverHead

So                     Source

TC                    Tandem Connection

TCM                  Tandem Connection Monitoring

TS                     Tributary Slot

TxTI                 Transmitted Trace Identifier

UNI                   User-to-Network Interface

VCG                  Virtual Concatenation Group

VCOH                Virtual Concatenation Overhead

vcPT                 virtual concatenated Payload Type

2       What is OTN/OTH

The Optical Transport Hierarchy (OTH) is a new transport technology for the OTN developed by the ITU. It is based on the network architecture defined in ITU G.872 “Architecture for the Optical Transport Network (OTN)”.

G.872 defines an architecture that is composed of the Optical Channel (OCh), Optical Multiplex Section (OMS) and Optical Transmission Section (OTS). It then describes the functionality that is needed to make OTN work. However, it may be interesting to note the decision made during G.872 development:

“During the development of ITU-T Rec. G.709, (implementation of the Optical Channel Layer according to ITU-T Rec. G.872 requirements), it was realized that the only techniques presently available that could meet the requirements for associated OCh trace, as well as providing an accurate assessment of the quality of a digital client signal, were digital techniques….”

“For this reason ITU-T Rec. G.709 chose to implement the Optical Channel by means of a digital framed signal with digital overhead that supports the management requirements for the OCh. Furthermore this allows the use of Forward Error Correction for enhanced system performance. This results in the introduction of two digital layer networks, the ODU and OTU. The intention is that all client signals would be mapped into the Optical Channel via the ODU and OTU layer networks.”

Currently there are no physical implementations of the OCh, OMS and OTS layers. As they are defined and implemented, they will be included in this document.

2.1      References

ITU-T Rec. G.709 (2009)            “Interfaces for the Optical Transport Network (OTN)”

ITU-T Rec. G.798 (2010)            “Characteristics of optical transport network hierarchy equipment functional blocks”

ITU-T Rec. G.872 (2001)            “Architecture of optical transport networks”

ITU-T Rec. G.873.1 (2006)         “Optical Transport Network: Linear protection”

ITU-T Rec. G.874 (2010)            “Management aspects of the optical transport network element”

ITU-T Rec. G.874.1 (2002)         “Optical transport network: Protocol-neutral management information model for the network element view”

ITU-T Rec. G.959.1 (2009)         “Optical transport network physical layer interfaces”

ITU-T Rec. G.8251 (2011)          “The control of jitter and wander within the optical transport network (OTN)”

3       Optical transport network interface structure

3.1      Basic signal structure

Figure 1 Structure of the OTN interfaces

3.1.1     OCh substructure

The optical channel layer is further structured in layer networks in order to support the network management and supervision functionalities:

• The optical channel with full (OCh) or reduced functionality (OChr), which provides transparent network connections between 3R regeneration points in the OTN.

• The optical channel transport unit (OTUk/OTUkV) which provides supervision and conditions the signal for transport between 3R regeneration points in the OTN.

• The optical channel data unit (ODUk) which provides:
• tandem connection monitoring (ODUkT)
• end-to-end path supervision (ODUkP)
• adaptation of client signals via the optical channel payload unit (OPUk)
• adaptation of OTN ODUk signals via the optical channel payload unit (OPUk)

3.1.2     Full functionality OTM-n.m (n ≥ 1) structure

The OTM-n.m (n ≥ 1) consists of the following layers:

• optical transmission section (OTSn)
• optical multiplex section (OMSn)
• optical channel (OCh)
• optical channel transport unit (OTUk/OTUkV)
• one or more optical channel data unit (ODUk)

3.1.3     Reduced functionality OTM-nr.m and OTM-0.m structure

The OTM-nr.m and OTM-0.m consist of the following layers:

• optical physical section (OPSn)
• reduced functionality optical channel (OChr)
• optical channel transport unit (OTUk/OTUkV)
• one or more optical channel data unit (ODUk)

3.2      Information structure for the OTN interfaces

Figure 2 Principal information containment relationships

The following layers are defined in OTN:

• OPUk: Optical channel payload unit k (k = 0, 1, 2, 3, 4)
• ODUk: Optical channel data unit k (k = 0, 1, 2, 3, 4)
• OTUk: Optical channel transport unit k (k = 1, 2, 3, 4)
• OCh: Optical channel, a single wavelength
• OMSn: Optical multiplex section of order n (Capacities for n = 0 and n = 16 are defined)
• OTSn: Optical transmission section of order n (Capacities for n = 0 and n = 16 are defined)

• OTM-n.m: Optical transport module of rate m with n optical channels. Possible values for m are:

1: 2.5 Gb/s

2: 10 Gb/s

3: 40 Gb/s

4: 100 Gb/s

Figure 3 shows how they are being used in a network.

Figure 3 OTN Network Layers

However for all intents and purposes there are only 4 layers

Figure 4 OTN Hierarchy

The OPUk, ODUk, and OTUk are in the electrical domain. The OCh is in the optical domain. There are more layers in the optical domain than just the OCh, but they are not being used now.

4       Multiplexing/mapping principles and bit rates

Figure 5 shows the relationship between various information structure elements and illustrates the multiplexing structure and mappings (including wavelength and time division multiplexing) for the OTM-n.

Figure 5 OTM multiplexing and mapping structure

The OTS, OMS, OCh and COMMS overhead is inserted into the OOS.

4.1      Mapping

The OPUk encapsulates the Client signal (e.g. SDH) and does any rate justification that is needed. It is analogous to the path layer in SDH in that it is mapped at the source, de-mapped at the sink, and not modified by the network.

The OPUk is mapped into an ODUk. The ODUk performs similar functions as the path overhead in SDH.

The ODUk is mapped into an OTUk[V]. The OTUk[V] contains the FEC and performs similar functions as the section overhead in SDH.

After the FEC are added, the signal is then sent to a serializer/ deserializer to be converted to the optical domain. The OTUk[V] is mapped into an OCh[r] and the OCh[r] is then modulated onto an OCC[r].

4.2      Wavelength division multiplex

Up to n (n ≥ 1) OCC[r] are multiplexed into an OCG-n[r].m using wavelength division multiplexing. The OCC[r] tributary slots of the OCG-n[r].m can be of different size.

The OCG-n[r].m is transported via the OTM-n[r].m. For the case of the full functionality OTM-n.m interfaces the OSC is multiplexed into the OTM-n.m using wavelength division multiplexing.

4.3      Bit rates and capacity

The data rates were constructed so that they could transfer SDH and Ethernet signals efficiently. The bit rates are shown in the following tables:

Table 1 OTU types and capacity

Table 2 ODU types and capacity

Table 3 OPU types and capacity

4.4      ODUk Time-Division Multiplex

Figure 6 and Figure 7 show the relationship between various information structure elements and illustrate the multiplexing structure and mappings (including wavelength and time division multiplexing) for the OTM-n. In the multi-domain OTN any combination of the ODUk multiplexing layers may be present at a given OTN interface.

Figure 6 shows that a (non-OTN) client signal is mapped into a lower order OPU, identified as “OPU (L)”. The OPU (L) signal is mapped into the associated lower order ODU, identified as “ODU (L)”. The ODU (L) signal is either mapped into the associated OTU[V] signal, or into an ODTU. The ODTU signal is multiplexed into an ODTU Group (ODTUG). The ODTUG signal is mapped into a higher order OPU, identified as “OPU (H)”. The OPU (H) signal is mapped into the associated higher order ODU, identified as “ODU (H)”. The ODU (H) signal is mapped into the associated OTU[V].

The OPU (L) and OPU (H) are the same information structures, but with different client signals. The concepts of lower order and high order ODU are specific to the role that ODU plays within a single domain.

Figure 6 OTM multiplexing and mapping structure

Figure 7 shows that an OTU[V] signal is mapped either into an optical channel signal, identified as OCh and OChr, or into an OTLk.n. The OCh/OChr signal is mapped into an optical channel carrier, identified as OCC and OCCr. The OCC/OCCr signal is multiplexed into an OCC group, identified as OCG-n.m and OCG-nr.m. The OCG-n.m signal is mapped into an OMSn. The OMSn signal is mapped into an OTSn. The OTSn signal is presented at the OTM-n.m interface.

The OCGnr.m signal is mapped into an OPSn. The OPSn signal is presented at the OTM-nr.m interface.

A single OCCr signal is mapped into an OPS0. The OPS0 signal is presented at the OTM-0.m interface. The OTLk.n signal is mapped into an optical transport lane carrier, identified as OTLC. The OTLC signal is multiplexed into an OTLC group, identified as OTLCG. The OTLCG signal is mapped into an OPSMnk. The OPSMnk signal is presented at the OTM-0.mvn interface.

Figure 7 OTM multiplexing and mapping structure

5       OTUk, ODUk, OPUk Frame Structure

Figure 8 shows the overall frame format for an OTUk signal. The various fields will be explained in the following sub-sections.

Figure 8 OTN frame format

5.1      OPUk Overhead and Processing

The OPUk (k = 1,2,3) frame structure is organized in an octet-based block frame structure with 4 rows and 3810 columns.

Figure 9 OPUk frame structure

The two main areas of the OPUk frame are:

• OPUk overhead area
• OPUk payload area

Columns 15 to 16 of the OPUk are dedicated to OPUk overhead area.

Columns 17 to 3824 of the OPUk are dedicated to OPUk payload area.

NOTE – OPUk column numbers are derived from the OPUk columns in the ODUk frame

OPUk OH information is added to the OPUk information payload to create an OPUk. It includes information to support the adaptation of client signals. The OPUk OH is terminated where the OPUk is assembled and disassembled.

Figure 10 OPUk frame

5.1.1     Payload Structure Identifier (PSI)

The 256-byte PSI signal is aligned with the ODUk multi-frame (i.e. PSI[0] is present at ODUk multi-frame position 0000 0000, PSI[1] at position 0000 0001, PSI[2] at position 0000 0010, etc.).

PSI[0] contains a one-byte payload type. PSI[1] to PSI[255] are mapping and concatenation specific.

5.1.2     Payload Type (PT)

A one-byte payload type signal is defined in the PSI[0] byte of the payload structure identifier to indicate the composition of the OPUk signal.

5.2      ODUk Overhead and Processing

The ODUk (k = 1,2,3) frame structure is organized in an octet-based block frame structure with 4 rows and 3824 columns.

Figure 11 ODUk frame structure

The three main areas of the ODUk frame are:

• OTUk area
• ODUk overhead area;
• OPUk area.

Columns 1 to 14 of rows 2-4 are dedicated to ODUk overhead area.

Columns 1 to 14 of row 1 are reserved for frame alignment and OTUk specific overhead.

Columns 15 to 3824 of the ODUk are dedicated to OPUk area.

ODUk OH information is added to the ODUk information payload to create an ODUk. It includes information for maintenance and operational functions to support optical channels. The ODUk OH consists of portions dedicated to the end-to-end ODUk path and to 6 levels of tandem connection monitoring. The ODUk path OH is terminated where the ODUk is assembled and disassembled. The TC OH is added and terminated at the source and sink of the corresponding tandem connections, respectively.

Figure 12 ODUk overhead

5.2.1     Path Monitoring (PM)

Figure 13 ODUk path monitoring overhead

5.2.1.1     Trail Trace Identifier (TTI)

The TTI is a 64-Byte signal that occupies one byte of the frame and is aligned with the OTUk multi-frame. It is transmitted 4 times per multi-frame.

5.2.1.2     BIP-8

This byte provides a bit interleaved parity-8 (BIP-8) code.

The ODUk BIP-8 is computed over the bits in the OPUk (columns 15 to 3824) area of ODUk frame i, and inserted in the ODUk PM BIP-8 overhead location in the ODUk frame i+2.

5.2.1.3     Backward Defect Indication (BDI)

This is defined to convey the “Signal Fail” status detected at the path terminating sink function, to the upstream node.

5.2.1.4     Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE)

This signal is used to convey in the upstream direction the count of interleaved-bit blocks that have been detected in error by the corresponding ODUk path monitoring sink using the BIP-8 code. This count has nine legal values, namely 0-8 errors. The remaining seven possible values represented by these 4 bits can only result from some unrelated condition and are interpreted as 0 errors.

5.2.1.5     Path Monitoring Status (STAT)

They indicate the presence of a maintenance signal.

5.2.2     Tandem Connection Monitoring (TCM)

There are 6 TCM’s. They can be nested or overlapping.

Figure 14 ODUk tandem connection monitoring #i overhead

5.2.2.1     Trail Trace Identifier (TTI)

The TTI is a 64-Byte signal that occupies one byte of the frame and is aligned with the OTUk multi-frame. It is transmitted four times per multi-frame.

5.2.2.2     BIP-8

This byte provides a bit interleaved parity-8 (BIP-8) code.

Each ODUk BIP-8 is computed over the bits in the OPUk (columns 15 to 3824) area of ODUk frame i, and inserted in the ODUk TCM BIP-8 overhead location (associated with the tandem connection monitoring level) in ODUk frame i+2.

The BIP-8 is only overwritten at the start of a Tandem Connection. Any existing TCM is not overwritten.

5.2.2.3     Backward Defect Indication (BDI)

This is defined to convey the “Signal Fail” status detected at the path terminating sink function, to the upstream node.

5.2.2.4     Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE)

This signal is used to convey in the upstream direction the count of interleaved-bit blocks that have been detected as being in error by the corresponding ODUk tandem connection monitoring sink using the BIP-8 code. It is also used to convey in the upstream direction an incoming alignment error (IAE) condition that is detected in the corresponding ODUk tandem connection monitoring sink in the IAE overhead.

During an IAE condition the code “1011” is inserted into the BEI/BIAE field and the error count is ignored. Otherwise the error count (0-8) is inserted into the BEI/BIAE field. The remaining 6 possible values represented by these 4 bits can only result from some unrelated condition and are interpreted as 0 errors and BIAE not active.

5.2.2.5     TCM Monitoring Status (STAT)

For each tandem connection monitoring field three bits are defined as status bits (STAT). They indicate the presence of a maintenance signal (if there is an incoming alignment error at the source TCM, or if there is no source TCM active).

5.2.2.6     Tandem Connection Monitoring ACTivation/deactivation (TCM-ACT)

Its definition is for further study.

5.2.3     General Communication Channels (GCC1, GCC2)

Two fields of two bytes are allocated in the ODUk overhead to support two general communications channels between any two network elements with access to the ODUk frame structure (i.e., at 3R regeneration points). These are clear channels. The bytes for GCC1 are located in row 4, columns 1 and 2, and the bytes for GCC2 are located in row 4, columns 3 and 4 of the ODUk overhead.

5.2.4     Automatic Protection Switching and Protection Communication Channel (APS/PCC)

Up to 8 levels of nested APS/PCC signals may be present in this field. The APS/PCC bytes in a given frame are assigned to a dedicated level depending on the value of MFAS as follows:

Table 4 Multi-frame to allow separate APS/PCC for each monitoring level

For linear protection schemes, the bit assignments for these bytes and the bit-oriented protocol are given in ITU-T Recommendation G.873.1. Bit assignment and byte oriented protocol for ring protection schemes are for further study.

5.2.5     Fault Type and Fault Location reporting communication channel (FTFL)

One byte is allocated in the ODUk overhead to transport a 256-byte fault type and fault location (FTFL) message. The byte is located in row 2, column 14 of the ODUk overhead.

5.3      OTUk Overhead and Processing

The OTUk (k = 1,2,3) frame structure is based on the ODUk frame structure and extends it with a forward error correction (FEC). 256 columns are added to the ODUk frame for the FEC and the overhead bytes in row 1, columns 8 to 14 of the ODUk overhead are used for OTUk specific overhead, resulting in an octet-based block frame structure with 4 rows and 4080 columns.

The OTUk forward error correction (FEC) contains the Reed-Solomon RS(255,239) FEC codes. If no FEC is used, fixed stuff bytes (all-0s pattern) are inserted.

OTUk OH information is part of the OTUk signal structure. It includes information for operational functions to support the transport via one or more optical channel connections. The OTUk OH is terminated where the OTUk signal is assembled and disassembled.

Figure 15 OTUk Overhead

Figure 16 OTUk overhead

5.3.1     Scrambling

The OTUk signal needs sufficient bit timing content to allow a clock to be recovered. A suitable bit pattern, which prevents a long sequence of “1”s or “0”s, is provided by using a scrambler.

The operation of the scrambler is functionally identical to that of a frame synchronous scrambler of sequence length 65535 operating at the OTUk rate.

The generating polynomial is 1 + x + x³+ x¹² + x¹⁶.

The scrambler is reset to “FFFF” (HEX) on the most significant bit of the byte following the last framing byte in the OTUk frame, i.e. the MSB of the MFAS byte. This bit and all subsequent bits to be scrambled are added modulo 2 to the output from the x16 position of the scrambler. The scrambler runs continuously throughout the complete OTUk frame. The framing bytes (FAS) of the OTUk overhead are not scrambled.

Scrambling is performed after FEC check bytes computation and insertion into the OTUk signal.

5.3.2     Frame Alignment Overhead

5.3.2.1     Frame Alignment Signal (FAS)

A 6 byte OTUk-FAS signal is defined in row 1, columns 1 to 6 of the OTUk overhead.

OA1 is “1111 0110”. OA2 is “0010 1000”.

Figure 17 Frame alignment signal overhead structure

5.3.2.2     Multi-Frame Alignment Signal (MFAS)

Some of the OTUk and ODUk overhead signals span multiple OTUk/ODUk frames. A single multi-frame alignment signal (MFAS) byte is defined in row 1, column 7 of the OTUk/ODUk overhead The value of the MFAS byte will be incremented each OTUk/ODUk frame and provides as such a 256 frame multi-frame.

Individual OTUk/ODUk overhead signals use this central multi-frame to lock their 2-frame, 4-frame, 8-frame, 16-frame, 32-frame, etc. multi-frames to the principal frame.

5.3.3     Section Monitoring (SM)

Figure 18 OTUk section monitoring overhead

5.3.3.1     Trail Trace Identifier (TTI)

The TTI is a 64-Byte signal that occupies one byte of the frame and is aligned with the OTUk multi-frame. It is transmitted four times per multi-frame.

5.3.3.2     BIP-8

This byte provides a bit interleaved parity-8 (BIP-8) code.

The OTUk BIP-8 is computed over the bits in the OPUk (columns 15 to 3824) area of OTUk frame i, and inserted in the OTUk BIP-8 overhead location in OTUk frame i+2.

Note: The OPUk includes the Justification Bytes, thus an OTN signal can not be retimed without de-mapping back to the client signal.

5.3.3.3     Backward Defect Indication (BDI)

This is defined to convey the “Signal Fail” Status detected at the Section Terminating Sink Function, to the upstream node.

5.3.3.4     Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE)

This signal is used to convey in the upstream direction the count of interleaved-bit blocks that have been detected in error by the corresponding OTUk section monitoring sink using the BIP-8 code. It is also used to convey in the upstream direction an incoming alignment error (IAE) condition that is detected in the corresponding OTUk section monitoring sink in the IAE overhead.

During a IAE condition the code “1011” is inserted into the BEI/BIAE field and the error count is ignored. Otherwise the error count (0-8) is inserted into the BEI/BIAE field. The remaining six possible values represented by these four bits can only result from some unrelated condition and are interpreted as 0 errors and BIAE not active.

5.3.3.5     Incoming Alignment Error (IAE)

A single-bit incoming alignment error (IAE) signal is defined to allow the ingress point to inform its peer egress point that an alignment error in the incoming signal has been detected.

IAE is set to “1” to indicate a frame alignment error; otherwise it is set to “0”.

The egress point may use this information to suppress the counting of bit errors, which may occur as a result of a frame phase change of the OTUk at the ingress of the section.

5.3.4     General Communication Channel 0 (GCC0)

Two bytes are allocated in the OTUk overhead to support a general communications channel between OTUk termination points. This is a clear channel. These bytes are located in row 1, columns 11 and 12 of the OTUk overhead.

6       OTN Maintenance Signals

6.1      OTUk maintenance signals

6.1.1     OTUk alarm indication signal (OTUk-AIS)

The OTUk-AIS is a generic-AIS signal. Since the OTUk capacity (130 560 bits) is not an integer multiple of the PN-11 sequence length (2047 bits), the PN-11 sequence may cross an OTUk frame boundary.

The PN-11 sequence is defined by the generating polynomial 1 + x⁹ + x¹¹.

6.2      ODUk maintenance signals

Three ODUk maintenance signals are defined: ODUk-OCI, ODUk-LCK and ODUk-AIS.

6.2.1     ODUk Open Connection Indication (ODUk-OCI)

ODUk-OCI is specified as a repeating “0110 0110” pattern in the entire ODUk signal, excluding the frame alignment overhead (FA OH) and OTUk overhead (OTUk OH).

The presence of ODUk-OCI is detected by monitoring the ODUk STAT bits in the PM and TCMi overhead fields.

The insertion of this is under management control. There is no defect that inserts ODUk-OCI.

6.2.2     ODUk Locked (ODUk-LCK)

ODUk-LCK is specified as a repeating “0101 0101” pattern in the entire ODUk signal, excluding the Frame Alignment overhead (FA OH) and OTUk overhead (OTUk OH).

The presence of ODUk-LCK is detected by monitoring the ODUk STAT bits in the PM and TCMi overhead fields.

The insertion of this is under management control. There is no defect that inserts ODUk-LCK.

6.2.3     ODUk Alarm Indication Signal (ODUk-AIS)

ODUk-AIS is specified as all “1”s in the entire ODUk signal, excluding the frame alignment overhead (FA OH), OTUk overhead (OTUk OH) and ODUk FTFL.

The presence of ODUk-AIS is detected by monitoring the ODUk STAT bits in the PM and TCMi overhead fields.

ODUk-AIS is generated if the OTUk input signal fails or it detects ODUk-OCI or ODUk-LCK on the input signal.

6.3      Client maintenance signal

6.3.1     Generic AIS for constant bit rate signals

The generic-AIS signal is a signal with a 2047-bit polynomial number 11 (PN-11) repeating sequence.

The PN-11 sequence is defined by the generating polynomial 1 + x⁹ + x¹¹.

During a signal fail condition of the incoming CBR2G5, CBR10G or CBR40G client signal (e.g. in the case of a loss of input signal), this failed incoming signal is replaced by the generic-AIS signal, and is then mapped into the OPUk.

During signal fail condition of the incoming ODUk/OPUk signal (e.g. in the case of an ODUk-AIS, ODUk-LCK, ODUk-OCI condition) the generic-AIS pattern is generated as a replacement signal for the lost CBR2G5, CBR10G or CBR40G signal.Maintenance Signal Insertion

6.3.2     Client source ODUk-AIS

During a signal fail condition of the incoming ODUj client signal (e.g. OTUj-LOF), this failed incoming signal will be replaced by the ODUj-AIS signal. This ODUj-AIS is then mapped into the respective timeslot in the ODUk.

6.3.3     Client source ODUk-OCI

For the case the ODUj is received from the output of a fabric (ODUj connection function), the incoming signal may contain (case of open matrix connection) the ODUj-OCI signal This ODUj-OCI signal is then mapped into the respective timeslot in the ODUk.

Not all equipment will have a real connection function (switch fabric) implemented; instead the presence/absence of tributary interface port units represents the presence/absence of a matrix connection.

If such unit is intentionally absent or not installed, the associated timeslot in the ODUk shall carry an ODUj-OCI signal.

If such unit is installed but temporarily removed as part of a repair action, the associated timeslot in the ODUk shall carry an ODUj-AIS signal.

7       Defect detection

There are no defects detected in the multiplexer. There are defects detected in the de-multiplexer.

7.1      Default PLM (Payload Mismatch)

Default PLM is declared if the accepted payload type (AcPT) is not equal to the expected payload type(s) as defined by the specific adaptation function. Default PLM is cleared if the accepted payload type is equal to the expected payload type(s).

A new payload type PT (AcPT) is accepted if a new consistent value is received in the PSI[0] byte in 3 consecutive multi-frames.

7.2      Default MSIM (Multiplex Structure Identifier Mismatch supervision)

Default MSIM is declared if the accepted MSI (AcMSI) is not equal to the expected multiplex structure identifier (ExMSI). dMSIM is cleared if the AcMSI is equal to the ExMSI. ExMSI is configured via the management interface. A new multiplex structure identifier MSI (AcMSI) is accepted if a new consistent value is received in the MSI bytes of the PSI overhead (PSI[2…5] for ODU2, PSI[2…17] for ODU3) in 3 consecutive multi-frames.

7.3      Default LOFLOM (Loss of Frame and Multi-frame)

If the frame alignment process is in the out-of-frame (OOF) state for 3 ms, default LOFLOM is declared. To prevent from the case of intermittent OOFs, the integrating timer is reset to 0 until an in-frame (IF) condition persists continuously for 3 ms. Default LOFLOM is cleared when the IF state persists continuously for 3 ms.

The ODUj frame and multi-frame alignment is found by searching for the framing pattern (OA1, OA2 FAS bytes) and checking the multi-frame sequence (MFAS byte) contained in the ODUj frame.

In the out-of-frame state the framing pattern searched for is the full set of the OA1 and OA2 bytes. The in-frame (IF) is entered if this set is found and confirmed one frame period later and an error-free multi-frame sequence is found in the MFAS bytes of the two frames.

In the in-frame state (IF) the frame alignment signal is continuously checked with the presumed frame start position and the expected multi-frame sequence. The framing pattern checked for is the OA1OA2 pattern (bytes 3 and 4 of the first row of the ODUj[/i] frame). The out of frame state (OOF) is entered if this subset is not found at the correct position in 5 consecutive frames or the received MFAS does not match with the expected multi-frame number in 5 consecutive frames.

The frame and multi-frame start are maintained during the OOF state.

There is one of these defects for each tributary.

8       Synchronization

8.1      Introduction

OTN is transparent to the payload it transports within the ODUk. The OTN layer does not need to transport network synchronization since network synchronization can be transported within the payload, mainly by SDH client tributaries.

Two types of mapping have been specified for the transport of CBR payload, e.g. SDH.

The first one is the asynchronous mapping, which is the most widely used, where the payload floats within the OTN frame. In this case, there is no frequency relationship between the payload and the OTN frame frequencies, thus simple free running oscillators can be used to generate the OTN frame.

The second is the synchronous mapping where the timing used to generate the OTN frame is extracted from a CBR client tributary, e.g. SDH. In case of LOS of the input client, the OTN frequency that does not transport payload is generated by a free running oscillator, without need for a holdover mode.

This specification allows for very simple implementation of timing in OTN equipments compared to SDH.

An OTN NE does not require synchronization interfaces, complex clocks with holdover mode nor SSM processing. Another difference with SDH is that there is no geographical option for the timing aspects of OTN.

OTN transports client signals into a G.709 frame, OTUk that is transported by an OCh on one lambda of the Optical Transport Module (OTM). Each lambda carries its G.709 frame with its own frequency; there is no common clock for the different OTUk of the OTM.

A trail through OTN is generated in an OTN NE that maps the client into an ODUk and terminated in another OTN NE that de-maps the client signal from the ODUk. Between the 2 OTN trail terminations, there might be 3R regenerators, which are equipments that perform complete regeneration of the pulse shape, clock recovery and retiming within required jitter limits.

The number of 3R regenerators that can be cascaded in tandem depends on the specification of this regenerator and on the jitter and wander generation and tolerance applicable to the OTUk interfaces; it is stated to be at least 50.

ODUk multiplexing has been standardized; its implication on timing has been taken into account in the relevant recommendations.

8.2      Network requirements

In an OTN, jitter and wander accumulate on transmission path according to the generation and transfer characteristics of interconnected equipments, 3R regenerators, client mappers, de-mappers and multiplexers, de-multiplexers. In order to avoid the effects of excessive jitter and wander, the ITU-T Recommendation G.8251 recommendation specifies the maximum magnitude of jitter and wander, and the minimum jitter and wanders tolerance, at OTN network interfaces.

The OTN generates and accumulates jitter and wander on its client signals due to the buffers of the mapping into ODUk and due to the ODUk multiplexing. The limits for such accumulation are given in the ITU-T Recommendation G.825 for SDH signal clients.

Jitter and wander is also accumulated on the OTN signals itself due to the ODUk multiplexing and 3R jitter generation. The network limits for this are given in the ITU-T Recommendation G.8251.

The ITU-T Recommendation G.8251 specifies the jitter and wander tolerance. As OTN clocks do not generate wander, no wander limit has been defined for OTN.

The ITU-T Recommendation G.8251 specifies the different type of clocks that are required to perform the following functions: the accuracy of these clocks depends on the definition of the G.709 frame and on the accuracy specified for the clients.

• Asynchronous mapping of a client into an ODUk and ODUk multiplexing: this ODCa clock is a free- running clock with a frequency accuracy of ± 20 ppm.

• Synchronous mapping of a client into an ODUk: this ODCb clock is locked on the client frequency.

• 3R regeneration: this ODCr clock is locked on an OCh input frequency which must be within ± 20 ppm.

• De-mapping a client signal from an ODUk and ODUk de-multiplexing: this ODCp clock is locked on an OCh input frequency which must be within ± 20 ppm.

The ITU-T Recommendation G.8251 specifies the jitter generation of these clocks and, when applicable, noise tolerance, jitter transfer and transient response.

All these clock functions are used for clock recovery and clock filtering of a particular signal. They never serve as an equipment synchronization source. Therefore there is no holdover mode specified for these clocks since there is no need for an accurate clock when the input signal disappears.

The ITU-T Recommendation G.8251 provides a provisional adaptation of the SDH synchronization reference chain to include OTN islands. This is an amendment of the reference chain being defined in the ITU-T Recommendation G.803. Considering that SDH may be transported by OTN islands, the SEC will no longer be present but replaced by OTN NEs. This leads to the definition of a reference chain where all SECs located between 2 SSUs are replaced by an OTN island. The local part of the reference chain, after the last SSU can still support 20 SECs in tandem. Each of these islands may be composed of OTN NEs performing mapping/de-mapping or multiplexing/de-multiplexing operations. This adaptation of the reference chain raises a buffer size constraint for the OTN NEs in order to keep the overall network wander performance within specified limits. Predominantly the mapping and the de-mapping functions of the OTN contribute to wander accumulation due to the buffers being involved in these functions. The size limit of these buffers is specified in the ITU-T Recommendation G.798. This allows inserting up to 10 mapping/ multiplexing nodes per OTN island. A total of 100 mapping/de-mapping functions can be performed on this synchronization reference chain.

The ITU-T Recommendation G.8251 presents a Hypothetic Reference Model for 3R regenerator jitter accumulation: according to this model, at any OTUk interface the jitter will remain within network limits in a chain of one mapping clock and up to 50 cascaded 3R regenerators plus a de-mapping clock. It reports the results of extensive simulations showing that it is possible to have 50 OTN regenerators without exceeding the network limits of OTUk interfaces, assuming the regenerators comply with the model defined in this Recommendation.

9       Why use OTN

OTN offers the following advantages relative to SDH:

• Stronger Forward Error Correction (FEC)
• More levels of Tandem Connection Monitoring (TCM)
• Transparent transport of client signals
• Switching scalability

OTN has the following disadvantages:

• Requires new hardware and management system

9.1      Forward Error Correction (FEC)

Forward error correction is a major feature of the OTN.

Already SDH has a FEC defined. It uses undefined SOH bytes to transport the FEC check information and is therefore called an in-band FEC. It allows only a limited number of FEC check information, which limits the performance of the FEC.

For the OTN a Reed-Solomon 16 byte-interleaved FEC scheme is defined, which uses 4 x 256 bytes of check information per ODU frame.

Figure 19 Error correction in OTN

According to ITU-T Recommendation G.709, an Reed-Solomon (255, 239) code with a symbol size of 8 is used for FEC. 239 input bytes are encoded in 255 output bytes. This code enables the detection of 2t = (n – k) = 16 errors in a codeword and the correction of t = (n – k)/2 = 8 of them.

FEC has been proven to be effective in OSNR limited systems as well as in dispersion limited systems. As for non-linear effects, reducing the output power leads to OSNR limitations, against which FEC is useful. FEC is less effective against PMD, however.

G.709 defines a stronger Forward Error Correction for OTN that can result in up to 6,2 dB improvement in Signal to Noise Ratio (SNR). Another way of looking at this is that to transmit a signal at a certain Bit Error Rate (BER) with 6,2 dB less power than without such a FEC.

The coding gain provided by the FEC can be used to:

• Increase the maximum span length and/or the number of spans, resulting in an extended reach. (Note that this assumes that other impairments like chromatic and polarization mode dispersion are not becoming limiting factors.)

• Increase the number of DWDM channels in a DWDM system which is limited by the output power of the amplifiers by decreasing the power per channel and increasing the number of channels. (Note that changes in non-linear effects due to the reduced per channel power have to be taken into account.)

• Relax the component parameters (e.g. launched power, eye mask, extinction ratio, noise figures, and filter isolation) for a given link and lower the component costs.

• but the most importantly the FEC is an enabler for transparent optical networks:

Transparent optical network elements like OADMs introduce significant optical impairments (e.g. attenuation). The number of transparent optical network elements that can be crossed by an optical path before 3R regeneration is needed is therefore strongly limited. With FEC an optical path can cross more transparent optical network elements.

This allows evolving from today’s point-to-point links to transparent, meshed optical networks with sufficient functionality.

9.2      Tandem Connection Monitoring

SDH monitoring is divided into section and path monitoring. A problem arises when you have “Carrier’s Carrier” situation where it is required to monitor a segment of the path that passes another carrier network.

Figure 20 Tandem Connection Monitoring

Here Operator A needs to have Operator B carries his signal. However he also needs a way of monitoring the signal as it passes through Operator B’s network. This is what a “Tandem connection” is. It is a layer between Line Monitoring and Path Monitoring. SDH was modified to allow a single Tandem connection. G.709 allows 6.

TCM1 is used by the User to monitor the Quality of Service (QoS) that they see. TCM2 is used by the first operator to monitor their end-to-end QoS. TCM3 is used by the various domains for Intra domain monitoring. Then TCM4 is used for protection monitoring by Operator B.

There is no standard on which TCM is used by whom. The operators have to have an agreement, so that they don’t conflict.

TCM’s also support monitoring of ODUk (G.709 w/o FEC) connections for one or more of the following network applications (refer to ITU-T G.805 and ITU-T G.872):

• optical UNI to UNI tandem connection monitoring; monitoring the ODUk connection through the public transport network (from public network ingress network termination to egress network termination);
• optical NNI to NNI tandem connection monitoring; monitoring the ODUk connection through the network of a network operator (from operator network ingress network termination to egress network termination);
• sub-layer monitoring for linear 1+1, 1:1 and 1:n optical channel sub-network connection protection switching, to determine the signal fail and signal degrade conditions;
• sub-layer monitoring for optical channel shared protection ring (SPRING) protection switching, to determine the signal fail and signal degrade conditions;
• Monitoring an optical channel tandem connection for the purpose of detecting a signal fail or signal degrade condition in a switched optical channel connection, to initiate automatic restoration of the connection during fault and error conditions in the network;
• Monitoring an optical channel tandem connection for, e.g., fault localization or verification of delivered quality of service.

A TCM field is assigned to a monitored connection. The number of monitored connections along an ODUk trail may vary between 0 and 6. Monitored connections can be nested, overlapping and/or cascaded.

Figure 21 ODUk monitored connections

Monitored connections A1-A2/B1-B2/C1-C2 and A1-A2/B3-B4 are nested, while B1-B2/B3-B4 are cascaded.

Overlapping monitored connections are also supported.

Figure 22 Overlapping ODUk monitored connections

9.3      Transparent Transport of Client Signals

G.709 defines the OPUk which can contain the entire SDH signal. This means that one can transport 4 STM-16 signals in one OTU2 and not modify any of the SDH overhead.

Thus the transport of such client signals in the OTN is bit-transparent (i.e. the integrity of the whole client signal is maintained).

It is also timing transparent. The asynchronous mapping mode transfers the input timing (asynchronous mapping client) to the far end (asynchronous de-mapping client).

It is also delay transparent. For example if 4 STM-16 signals are mapped into ODU1’s and then multiplexed into an ODU2, their timing relationship is preserved until they are de-mapped back to ODU1’s.

9.4      Switching Scalability

When SDH was developed, its main purpose was to provide the transport technology for voice services. Two switching levels were therefore defined. A low order switching at VC-12 level supports directly the E1 voice signals and a high order switching level at VC-4 level is used for traffic engineering. Switching levels at higher bit rates were not foreseen.

Over time the line rate increased while the switching rate was fixed. The gap between line rate and switching bit rate widened. Furthermore new services at higher bit rates (IP, Ethernet services) had to be supported.

Contiguous and virtual concatenation were introduce in order to solve part of the services problem as they allow to support services above the standard SDH switching bit rates.

The gap between line or service bit rate and switching bit rate however still exists as even with concatenation switching is performed at the VC-4 level.

For a 4 x 10G to 40G SDH multiplexer this means processing of 256 VC-4. This will result not only in efforts in the equipment hardware, but also in management and operations efforts.

For efficient equipment and network design and operations, switching at higher bit rates has to be introduced.

One could now argue that photonic switching of wavelengths is the solution. But with photonic switching the switching bit rate is bound to the bit rate of the wavelength and as such would be the service. An independent selection for service bit rates and DWDM technology is not possible.

A operator offering 2,5 Gbit/s IP interconnection would need a N x 2G5 DWDM system. When adding 10G services he has to upgrade some of its wavelengths to 10G. This would lead to inefficient network designs.

OTN provides the solution to the problem by placing no restrictions on switching bit rates. As the line rate grows new switching bit rates are added.

An operator can offer services at various bit rates (2G5, 10G …) independent of the bit rate per wavelength using the multiplexing and inverse multiplexing features of the OTN.

 OTN ALARM FLOW

WavelengthDivisionMultiplexingSystems by Sanjay Yadav

2013 Infonetics CRS 100G Market Share 01-08-13 by Sanjay Yadav

OTN MPLS Control Plane Service Provider Survey 05-01-13 by Sanjay Yadav

Understanding Synchronization Protection Basics in Transmission Network by Sanjay Yadav

Understanding SONET by K. Surya Prakash by Sanjay Yadav

Analog vs Digital Fiber Optic Transmission Comparison by Sanjay Yadav

All Coding Techniques Explained by Sanjay Yadav

Way Towards OTN -Optical Transport Network. by Sanjay Yadav

Why Coherent Optical Technology by Sanjay Yadav

Differences between SDH and SONET Technology by Optix Expert-Helvoort by Sanjay Yadav

>> What is PMD versus Differential Group Delay (DGD)?

>> What is fiber PMD versus cable PMD?

>> What is a Link Design Value?

>> What is system PMD?

>> How much PMD can be tolerated in my system?

>> How do I guard against the transient nature of PMD?

>> Can I fix or compensate for PMD?

>> Are there standards for PMD?

>> There is so much ambiguity in PMD specifications. What should I be looking for?

>> What should I ask of my fiber cable supplier in terms of a PMD specification?

>> Is the sensitivity to PMD, fiber design – or manufacturer dependent?

>> How do I test an incoming cable or fiber for PMD?

>> Why is the minimum measurable PMD length dependent?

>> What are the limitations to measurement of low PMD?

>> What is a “low mode coupled” PMD measurement?

>> Is it possible to measure PMD with an OTDR?

>> I am not running high bit rates, like 10 or 40 Gbps, now, should I be concerned about PMD?

>> Is there any correlation between chromatic dispersion and PMD?

>> Should I worry about PMD in a multimode fiber?

source:http://www.fiberoptics4sale.com/wordpress/what-is-wavelength-selective-switchwss/

1. What Is a Wavelength Selective Switch (WSS)?

2. How Does a WSS Work?

1. The common input fiber enters the switch at point A where light is collimated by a microlens.
2. The following lens image the collimated beam on the diffraction grating at point C.
3. The wavelength dispersed beams fall then onto the MEMS device plane D
4. On MEMS device plane D, the beams are reflected with certain tilt angle depending on micromirrors’ setting.
5. All reflected beams are focused on point B again, where the angle to space conversion section will image the beam on the output fiber. Each output corresponds to a specific tilt angle of the micromirrors.

3. WSS Switching Engine Technologies

4. Switch Engine Technologies and Minimum Achievable Spot Sizes

2012-Infonetics-CRS-Optical-Leadership-SP-Survey….Know the Optical Product Manufacturer

OTN for Newbies

Learn the advanced features of SONET and SDH; specifically, the different ways of concatenating SONET and SDH signals, different techniques for mapping packet data onto SONET and SDH connections, transparency services for carrier’s carrier applications, and fault management and performance monitoring capabilities.

This section is procured from chapter 3 of  Optical Network Control: Architecture, Protocols, and Standards

• 3.1 INTRODUCTION
• 3.2 ALL ABOUT CONCATENATION
• 3.2.1 Standard Contiguous Concatenation in SONET and SDH
• 3.2.2 Arbitrary Concatenation
• 3.2.3 Virtual Concatenation
• 3.2.3.1 Higher-Order Virtual Concatenation (HOVC)
• 3.2.3.2 Lower-Order Virtual Concatenation (LOVC)
• 3.3 LINK CAPACITY ADJUSTMENT SCHEME
• 3.4 PAYLOAD MAPPINGS
• 3.4.1 IP over ATM over SONET
• 3.4.2 Packet over SONET/SDH
• 3.4.3 Generic Framing Procedure (GFP)
• 3.4.3.1 GFP Frame Structure
• 3.4.3.2 GFP Functions
• 3.4.4 Ethernet over SONET/SDH
• 3.5 SONET/SDH TRANSPARENCY SERVICES
• 3.5.1 Methods for Overhead Transparency
• 3.5.2 Transparency Service Packages
• 3.6 WHEN THINGS GO WRONG
• 3.6.1 Transport Problems and Their Detection
• 3.6.1.1 Continuity Supervision
• 3.6.1.2 Connectivity Supervision
• 3.6.1.3 Signal Quality Supervision
• 3.6.1.4 Alignment Monitoring
• 3.6.2 Problem Localization and Signal Maintenance
• 3.6.2.1 Alarm Indication Signals
• 3.6.2.2 Remote Defect Indication
• 3.6.3 Quality Monitoring
• 3.6.3.1 Blips and BIPs
• 3.6.4 Remote Error Monitoring
• 3.6.5 Performance Measures
• 3.7 SUMMARY

3.1 Introduction

In the previous chapter, we described TDM and how it has been utilized in SONET and SDH standards. We noted that when SONET and SDH were developed, they were optimized for carrying voice traffic. At that time no one anticipated the tremendous growth in data traffic that would arise due to the Internet phenomenon. Today, the volume of data traffic has surpassed voice traffic in most networks, and it is still growing at a steady pace. In order to handle data traffic efficiently, a number of new features have been added to SONET and SDH.

In this chapter, we review some of the advanced features of SONET and SDH. Specifically, we describe the different ways of concatenating SONET and SDH signals, and different techniques for mapping packet data onto SONET and SDH connections. We also address transparency services for carrier’s carrier applications, as well as fault management and performance monitoring capabilities. The subject matter covered in this chapter will be used as a reference when we discuss optical control plane issues in later chapters. A rigorous understanding of this material, however, is not a prerequisite for dealing with the control plane topics.

3.2 All about Concatenation

Three types of concatenation schemes are possible under SONET and SDH. These are:

• Standard contiguous concatenation
• Arbitrary contiguous concatenation
• Virtual concatenation

These concatenation schemes are described in detail next.

3.2.1 Standard Contiguous Concatenation in SONET and SDH

SONET and SDH networks support contiguous concatenation whereby a few standardized “concatenated” signals are defined, and each concatenated signal is transported as a single entity across the network [ANSI95a, ITU-T00a]. This was described briefly in the previous chapter.

The concatenated signals are obtained by “gluing” together the payloads of the constituent signals, and they come in fixed sizes. In SONET, these are called STS-Nc Synchronous Payload Envelopes (SPEs), where N = 3X and X is restricted to the values 1, 4, 16, 64, or 256. In SDH, these are called VC-4 (equivalent to STS-3c SPE), and VC-4-Xc where X is restricted to 1, 4, 16, 64, or 256.

The multiplexing procedures for SONET (SDH) introduce additional constraints on the location of component STS-1 SPEs (VC-4s) that comprise the STS-Nc SPE (VC-4-Xc). The rules for the placement of standard concatenated signals are [ANSI95a]:

1. Concatenation of three STS-1s within an STS-3c: The bytes from concatenated STS-1s shall be contiguous at the STS-3 level but shall not be contiguous when interleaved to higher-level signals. When STS-3c signals are multiplexed to a higher rate, each STS-3c shall be wholly contained within an STS-3 (i.e., occur only on tributary input boundaries 1–3, 4–6, 7–9, etc.). This rule does not apply to SDH.
2. Concatenation of STS-1s within an STS-Nc (N = 3X, where X = 1, 4, 16, 64, or 256). Such concatenation shall treat STS-Nc signals as a single entity. The bytes from concatenated STS-1s shall be contiguous at the STS-N level, but shall not be contiguous when multiplexed on to higher-level signals. This also applies to SDH, where the SDH term for an STS-Nc is an AU-4-Xc where X = N/3.
3. When the STS-Nc signals are multiplexed to a higher rate, these signals shall be wholly contained within STS-M boundaries, where M could be 3, 12, 48, 192, or 768, and its value must be the closest to, but greater than or equal to N (e.g., if N = 12, then the STS-12c must occur only on boundaries 1–12, 13–24, 25–36, etc.). In addition to being contained within STS-M boundaries, all STS-Nc signals must begin on STS-3 boundaries.

The primary purpose of these rules is to ease the development burden for hardware designers, but they can seriously affect the bandwidth efficiency of SONET/SDH links.

In Figure 3-1(a), an STM-16 (OC-48) signal is represented as a set of 16 time slots, each of which can contain a VC-4 (STS-3c SPE). Let us examine the placement of VC-4 and VC-4-4c (STS-3c and STS-12c SPE) signals into this structure, in line with the rules above. In particular a VC-4-4c (STS-12c SPE) must start on boundaries of 4. Figure 3-1(b) depicts how the STM-16 has been filled with two VC-4-4c (STS-12c) and seven VC-4 signals. In Figure 3-1(c), three of the VC-4s have been removed, that is, are no longer in use. Due to the placement restrictions, however, a VC 4-4c cannot be accommodated in this space. In Figure 3-1(d), the STM-16 has been “regroomed,” that is, VC-4 #5 and VC-4 #7 have been moved to new timeslots. Figure 3-1(e) shows how the third VC-4-4c is accommodated.

Figure 3-1. Timeslot Constraints and Regrooming with Contiguous (Standard) Concatenation

3.2.2 Arbitrary Concatenation

In the above example, a “regrooming” operation was performed to make room for a signal that could not be accommodated with the standard contiguous concatenation rules. The problem with regrooming is that it is service impacting, that is, service is lost while the regrooming operation is in progress. Because service impacts are extremely undesirable, regrooming is not frequently done, and the bandwidth is not utilized efficiently.

To get around these restrictions, some manufacturers of framers, that is, the hardware that processes the SDH multiplex section layer (SONET line layer), offer a capability known as “flexible” or arbitrary concatenation. With this capability, there are no restrictions on the size of an STS-Nc (VC-4-Xc) or the starting time slot used by the concatenated signal. Also, there are no constraints on adjacencies of the STS-1 (VC-4-Xc) time slots used to carry it, that is, the signals can use any combination of available time slots. Figure 3-2 depicts how the sequence of signals carried over the STM-16 of Figure 3-1 can be accommodated without any regrooming, when the arbitrary concatenation capability is available.

Figure 3-2. Timeslot Usage with Arbitrary Concatenation

3.2.3 Virtual Concatenation

As we saw earlier, arbitrary concatenation overcomes the bandwidth inefficiencies of standard contiguous concatenation by removing the restrictions on the number of components and their placement within a larger concatenated signal. Standard and arbitrary contiguous concatenation are services offered by the network, that is, the network equipment must support these capabilities. The ITU-T and the ANSI T1 committee have standardized an alternative, called virtual concatenation. With virtual concatenation, SONET and SDH PTEs can “glue” together the VCs or SPEs of separately transported fundamental signals. This is in contrast to requiring the network to carry signals as a single concatenated unit.

3.2.3.1 HIGHER-ORDER VIRTUAL CONCATENATION (HOVC)

HOVC is realized under SONET and SDH by the PTEs, which combine either multiple STS-1/STS-3c SPEs (SONET), or VC-3/VC-4 (SDH). Recall that the VC-3 and STS-1 SPE signals are nearly identical except that a VC-3 does not contain the fixed stuff bytes found in columns 30 and 59 of an STS-1 SPE. A SONET STS-3c SPE is equivalent to a SDH VC-4.

These component signals, VC-3s or VC-4s (STS-1 SPEs or STS-3c SPEs), are transported separately through the network to an end system and must be reassembled. Since these signals can take different paths through the network, they may experience different propagation delays. In addition to this fixed differential delay between the component signals, there can also be a variable delay component that arises due to the different types of equipment processing the signals and the dynamics of the fiber itself. Note that heating and cooling effects can affect the propagation speed of light in a fiber, leading to actual measurable differences in propagation delay.

The process of mapping a concatenated container signal, that is, the raw data to be transported, into a virtually concatenated signal is shown in Figure 3-3. Specifically, at the transmitting side, the payload gets packed in X VC-4s just as if these were going to be contiguously concatenated. Now the question is, How do we identify the component signals and line them up appropriately given that delays for the components could be different?

Figure 3-3. Mapping a Higher Rate Payload in a Virtually Concatenated Signal (from [ITU-T00a])

The method used to align the components is based on the multiframe techniques described in Chapter 2. A jumbo (very long) multiframe is created by overloading the multiframe byte H4 in the path overhead. Bits 5–8 of the H4 byte are incremented in each 125µs frame to produce a multiframe consisting of 16 frames. In this case, bits 5–8 of H4 are known as the multiframe indicator 1 (MFI1). This multiframe will form the first stage of a two-stage multiframe. In particular, bits 1–4 of the H4 byte are used in a way that depends on the position in the first stage of the multiframe. This is shown in Table 3-1.

Within the 16-frame first stage multiframe, a second stage multiframe indicator (MFI2) is defined utilizing bits 1–4 of H4 in frames 0 and 1, giving a total of 8 bits per frame. It is instructive to examine the following:

1. How long in terms of the number of 125µs frames is the complete HOVC multiframe structure? Answer: The base frame (MFI1) is 16 frames long, and the second stage is 2⁸ = 256 frames long. Since this is a two-stage process, the lengths multiply giving a multiframe that is 16 × 256 = 4096 frames long.
2. What is the longest differential delay, that is, delay between components that can be compensated? Answer: The differential delay must be within the duration of the overall multiframe structure, that is, 125µS × 4096 = 512mS, that is, a little over half a second.
3. Suppose that an STS-1-2v is set up for carrying Ethernet traffic between San Francisco and New York such that one STS-1 goes via a satellite link and the other via conventional terrestrial fiber. Will this work? Answer: Assuming that a geo-synchronous satellite is used, then the satellite’s altitude would be about 35775 km. Given that the speed of light is 2.99792 × 10⁸ m/sec, this leads to a round trip delay of about 239 ms. If the delay for the fiber route is 20 ms, then the differential delay is 209 ms, which is within the virtual concatenation range. Also, since the average circumference of the earth is only 40,000 km, this frame length should be adequate for the longest fiber routes.

Table 3-1. Use of Bits 1–4 in H4 Byte for First Stage Multiframe Indication (MFI1)

Now, the receiver must be able to distinguish the different components of a virtually concatenated signal. This is accomplished as follows. In frames 14 and 15 of the first stage multiframe, bits 1–4 of H4 are used to give a sequence indicator (SQ). This is used to indicate the components (and not the position in the multiframe). Due to this 8-bit sequence indicator, up to 256 components can be accommodated in HOVC. Note that it is the receiver’s job to compensate for the differential delay and to put the pieces back together in the proper order. The details of how this is done are dependent on the specific implementation.

3.2.3.2 LOWER-ORDER VIRTUAL CONCATENATION (LOVC)

The virtual concatenation of lower-order signals such as VT1.5s (VC-11), VT2 (VC-12), and so on are based on the same principles as described earlier. That is, a sequence number is needed to label the various components that make up the virtually concatenated signal, and a large multiframe structure is required for differential delay compensation. In the lower-order case, however, there are fewer overhead bits and bytes to spare so the implementation may seem a bit complex. Let us therefore start with the capabilities obtained.

LOVC Capabilities and Limitations

Table 3-2 lists the LOVC signals for SONET/SDH, the signals they can be contained in and the limits on the number of components that can be concatenated. The last two columns are really the most interesting since they show the range of capacities and the incremental steps of bandwidth.

LOVC Implementation

Let us first examine how the differential delay compensating multiframe is put together. This is done in three stages. Recall that the SONET VT overhead (lower-order SDH VC overhead) is defined in a 500 µs multiframe, as indicated in the path layer multiframe indicator H4. This makes available the four VT overhead bytes V5, J2, Z6, and Z7, from one SONET/SDH frame byte. Since a number of bits in these bytes are used for other purposes, an additional second stage of multiframe structure is used to define extended VT signal labels.

This works as follows (note that SDH calls the Z7 byte as K4 but uses it the same way): First of all, the V5 byte indicates if the extended signal label is being used. Bits 5 through 7 of V5 provide a VT signal label. The signal label value of 101 indicates that a VT mapping is given by the extended signal label in the Z7 byte. If this is the case, then a 1-bit frame alignment signal “0111 1111 110” is sent in bit 1 of Z7, called the extended signal label bit. The length of this second stage VT level multiframe (which is inside the 500 µs VT multiframe) is 32 frames. The extended signal label is contained in bits 12–19 of the multiframe. Multiframe position 20 contains “0.” The remaining 12 bits are reserved for future standardization.

Table 3-2. Standardized LOVC Combinations and Limits

Bit 2 of the Z7 byte is used to convey the third stage of the multistage multiframe in the form of a serial string of 32 bits (over 32 four-frame multi-frames and defined by the extended signal label). This is shown in Figure 3-4. This string is repeated every 16 ms (32 bits × 500 µs/bit) or every 128 frames.

Figure 3-4. Third Stage of LOVC Multiframe Defined by Bit 2 of the Z7 Byte over the 32 Frame Second Stage Multiframe

The third stage string consists of the following fields: The third stage virtual concatenation frame count is contained in bits 1 to 5. The LOVC sequence indicator is contained in bits 6 to 11. The remaining 21 bits are reserved for future standardization.

Let us now consider a concrete example. Suppose that there are three stages of multiframes with the last stage having 5 bits dedicated to frame counting. What is the longest differential delay that can be compensated and in what increments? The first stage was given by the H4 byte and is of length 4, resulting in 4 × 125 µs = 500 µs. The second stage was given by the extended signal label (bit 1 of Z7) and it is of length 32. Since this is inside the first stage, the lengths multiply, resulting in 32 × 500 µs = 16 ms. The third stage, which is within the 32-bit Z7 string, has a length of 2⁵ = 32 and is contained inside the second stage. Hence, the lengths multiply, resulting in 32 × 16 ms = 512 ms. This is the same compensation we showed with HOVC. Since the sequence indicator of the third stage is used to line up the components, the delay compensation is in 16 ms increments.

3.3 Link Capacity Adjustment Scheme

Virtual concatenation allows the flexibility of creating SONET/SDH pipes of different sizes. The Link Capacity Adjustment Scheme or LCAS [ITU-T01a] is a relatively new addition to the SONET/SDH standard. It is designed to increase or decrease the capacity of a Virtually Concatenated Group (VCG) in a hitless fashion. This capability is particularly useful in environments where dynamic adjustment of capacity is important. The LCAS mechanism can also automatically decrease the capacity if a member in a VCG experiences a failure in the network, and increase the capacity when the fault is repaired. Although autonomous addition after a failure is repaired is hitless, removal of a member due to path layer failures is not hitless. Note that a “member” here refers to a VC (SDH) or an SPE (SONET). In the descriptions below, we use the term member to denote a VC.

Note that virtual concatenation can be used without LCAS, but LCAS requires virtual concatenation. LCAS is resident in the H4 byte of the path overhead, the same byte as virtual concatenation. The H4 bytes from a 16-frame sequence make up a message for both virtual concatenation and LCAS. Virtual concatenation uses 4 of the 16 bytes for its MFI and sequence numbers. LCAS uses 7 others for its purposes, leaving 5 reserved for future development. While virtual concatenation is a simple labeling of individual STS-1s within a channel, LCAS is a two-way handshake protocol. Status messages are continuously exchanged and consequent actions taken.

From the perspective of dynamic provisioning enabled by LCAS, each VCG can be characterized by two parameters:

• X_MAX, which indicates the maximum size of the VCG and it is usually dictated by hardware and/or standardization limits
• X_PROV, which indicates the number of provisioned members in the VCG

With each completed ADD command, X_PROV increases by 1, and with each completed REMOVE command X_PROV decreases by 1. The relationship 0 ≤ X_PROV ≤ X_MAX always holds. The operation of LCAS is unidirectional. This means that in order to bidirectionally add or remove members to or from a VCG, the LCAS procedure has to be repeated twice, once in each direction. These actions are independent of each other, and they are not required to be synchronized.

The protocols behind LCAS are relatively simple. For each member in the VCG (total of X_MAX), there is a state machine at the transmitter and a state machine at the receiver. The state machine at the transmitter can be in one of the following five states:

1. IDLE: This member is not provisioned to participate in the VCG.
2. NORM: This member is provisioned to participate in the VCG and has a good path to the receiver.
3. DNU: This member is provisioned to participate in the VCG and has a failed path to the receiver.
4. ADD: This member is in the process of being added to the VCG.
5. REMOVE: This member is in the process of being deleted from the VCG.

The state machine at the receiver can be in one of the following three states:

1. IDLE: This member is not provisioned to participate in the VCG.
2. OK: The incoming signal for this member experiences no failure condition. Or, the receiver has received and acknowledged a request for addition of this member.
3. FAIL: The incoming signal for this member experiences some failure condition, or an incoming request for removal of a member has been received and acknowledged.

The transmitter and the receiver communicate using control packets to ensure smooth transition from one state to another. The control packets consist of X_MAX control words, one for each member of the VCG. The following control words are sent from source to the receiver in order to carry out dynamic provisioning functions. Each word is associated with a specific member (i.e., VC) in the VCG.

• F_ADD: Add this member to the group.
• F_DNU: Delete this member from the group.
• F_IDLE: Indicate that this VC is currently not a member of the group.
• F_EOS: Indicate that this member has the highest sequence number in the group (EOS denotes End of Sequence).
• F_NORM: Indicate that this member is normal part of the group and does not have the highest sequence number.

The following control words are sent from the receiver to the transmitter. Each word is associated with a specific VC in the VCG.

• R_FAIL and R_OK: These messages capture the status of all the VCG members at the receiver. The status of all the members is returned to the transmitter in the control packets of each member. The transmitter can, for example, read the information from member No. 1 and, if that is unavailable, the same information from member No. 2, and so on. As long as no return bandwidth is available, the transmitter uses the last received valid status.
• R_{RS_ACK}: This is a bit used to acknowledge the detection of renumbering of the sequence or a change in the number of VCG members. This acknowledgment is used to synchronize the transmitter and the receiver.

The following is a typical sequence for adding a member to the group. Multiple members can be added simultaneously for fast resizing.

1. The network management system orders the source to add a new member (e.g., a VC) to the existing VCG.
2. The source node starts sending F_ADD control commands in the selected member. The destination notices the F_ADD command and returns an R_OK in the link status for the new member.
3. The source sees the R_OK, assigns the member a sequence number that is one higher than the number currently in use.
4. At a frame boundary, the source includes the VC in the byte interleaving and sets the control command to F_EOS, indicating that this VC is in use and it is the last in the sequence.
5. The VC that previously was “EOS ” now becomes “NORM” (normal) as it is no longer the one with the highest sequence number.

The following is a typical sequence for deleting the VC with the highest sequence number (EOS) from a VCG:

1. The network management system orders the source to delete a member from the existing VCG.
2. The source node starts sending F_IDLE control commands in the selected VC. It also sets the member with the next highest sequence number as the EOS and sends F_EOS in the corresponding control word.
3. The destination notices the F_IDLE command and immediately drops the channel from the reassembly process. It also responds with R_FAIL and inverts the R_{RS_ACK} bit.

In this example, the deleted member has the highest sequence number. If this is not the case, then the other members with sequence numbers between the newly deleted member and the highest sequence number are renumbered.

LCAS and virtual concatenation add tremendous amount of flexibility to SONET and SDH. Although SONET and SDH were originally designed to transport voice traffic, advent of these new mechanisms has made it perfectly suitable for carrying more dynamic and bursty data traffic. In the next section, we discuss mechanisms for mapping packet payloads into SONET and SDH SPEs.

3.4 Payload Mappings

So far, the multiplexing structure of SONET and SDH has been described in detail. To get useful work out of these different sized containers, a payload mapping is needed, that is, a systematic method for inserting and removing the payload from a SONET/SDH container. Although it is preferable to use standardized mappings for interoperability, a variety of proprietary mappings may exist for various purposes.

In this regard, one of the most important payloads carried over SONET/SDH is IP. Much of the bandwidth explosion that set the wheels in motion for this book came from the growth in IP services. Hence, our focus is mainly on IP in the rest of this chapter. Figure 3-5 shows different ways of mapping IP packets into SONET/SDH frames. In the following, we discuss some of these mechanisms.

Figure 3-5. Different Alternatives for Carrying IP Packets over SONET

3.4.1 IP over ATM over SONET

The “Classical IP over ATM” solution supports robust transmission of IP packets over SONET/SDH using ATM encapsulation. Under this solution, each IP packet is encapsulated into an ATM Adaptation Layer Type 5 (AAL5) frame using multiprotocol LLC/SNAP encapsulation [Perez+95]. The resulting AAL5 Protocol Data Unit (PDU) is segmented into 48-byte payloads for ATM cells. ATM cells are then mapped into a SONET/SDH frame.

One of the problems with IP-over-ATM transport is that the protocol stack may introduce a bandwidth overhead as high as 18 percent to 25 percent. This is in addition to the approximately 4 percent overhead needed for SONET. On the positive side, ATM permits sophisticated traffic engineering, flexible routing, and better partitioning of the SONET/SDH bandwidth. Despite the arguments on the pros and cons of the method, IP-over-ATM encapsulation continues to be one of the main mechanisms for transporting IP over SONET/SDH transport networks.

3.4.2 Packet over SONET/SDH

ATM encapsulation of IP packets for transport over SONET/SDH can be quite inefficient from the perspective of bandwidth utilization. Packet over SONET/SDH (or POS) addresses this problem by eliminating the ATM encapsulation, and using the Point-to-Point Protocol (PPP) defined by the IETF [Simpson94]. PPP provides a general mechanism for dealing with point-to-point links and includes a method for mapping user data, a Link Control Protocol (LCP), and assorted Network Control Protocols (NCPs). Under POS, PPP encapsulated IP packets are framed using high-Level Data Link Control (HDLC) protocol and mapped into the SONET SPE or SDH VC [Malis+99]. The main function of HDLC is to provide framing, that is, delineation of the PPP encapsulated IP packets across the synchronous transport link. Standardized mappings for IP into SONET using PPP/HDLC have been defined in IETF RFC 2615 [Malis+99] and ITU-T Recommendation G.707 [ITU-T00a].

Elimination of the ATM layer under POS results in more efficient bandwidth utilization. However, it also eliminates the flexibility of link bandwidth management offered by ATM. POS is most popular in backbone links between core IP routers running at 2.5 Gbps and 10 Gbps speeds. IP over ATM is still popular in lower-speed access networks, where bandwidth management is essential.

During the initial deployment of POS, it was noticed that the insertion of packets containing certain bit patterns could lead to the generation of the Loss of Frame (LOF) condition. The problem was attributed to the relatively short period of the SONET section (SDH regenerator section) scrambler, which is only 127 bits and synchronized to the beginning of the frame. In order to alleviate the problem, an additional scrambling operation is performed on the HDLC frames before they are placed into the SONET/SDH SPEs. This procedure is depicted in Figure 3-6.

Figure 3-6. Packet Flow for Transmission and Reception of IP over PPP over SONET/SDH

3.4.3 Generic Framing Procedure (GFP)

GFP [ITU-T01b] was initially proposed as a solution for transporting data directly over dark fibers and WDM links. But due to the huge installed base of SONET/SDH networks, GFP soon found applications in SONET/SDH networks. The basic appeal of GFP is that it provides a flexible encapsulation framework for both block-coded [Gorsche+02] and packet oriented [Bonenfant+02] data streams. It has the potential of replacing a plethora of proprietary framing procedures for carrying data over existing SONET/SDH and emerging WDM/OTN transport.

GFP supports all the basic functions of a framing procedure including frame delineation, frame/client multiplexing, and client data mapping [ITU-T01b]. GFP uses a frame delineation mechanism similar to ATM, but generalizes it for both fixed and variable size packets. As a result, under GFP, it is not necessary to search for special control characters in the client data stream as required in 8B/10B encoding,¹ or for frame delineators as with HDLC framing. GFP allows flexible multiplexing whereby data emanating from multiple clients or multiple client sessions can be sent over the same link in a point-to-point or ring configuration. GFP supports transport of both packet-oriented (e.g., Ethernet, IP, etc.) and character-oriented (e.g., Fiber Channel) data. Since GFP supports the encapsulation and transport of variable-length user PDUs, it does not need complex segmentation/reassembly functions or frame padding to fill unused payload space. These careful design choices have substantially reduced the complexity of GFP hardware, making it particularly suitable for high-speed transmissions.

In the following section, we briefly discuss the GFP frame structure and basic GFP functions.

3.4.3.1 GFP FRAME STRUCTURE

A GFP frame consists of a core header and a payload area, as shown in Figure 3-7. The GFP core header is intended to support GFP-specific data link management functions. The core header also allows GFP frame delineation independent of the content of the payload. The GFP core header is 4 bytes long and consists of two fields:

Figure 3-7. Generic Framing Procedure Frame Structure

Payload Length Indicator (PLI) Field

A 2-byte field indicating the size of the GFP payload area in bytes.

Core Header Error Correction (cHEC) Field

A 2-octet field containing a cyclic redundancy check (CRC) sequence that protects the integrity of the core header.

The payload area is of variable length (0–65,535 octets) and carries client data such as client PDUs, client management information, and so on. Structurally, the payload area consists of a payload header and a payload information field, and an optional payload Frame Check Sequence (FCS) field. The FCS information is used to detect the corruption of the payload.

Payload Header

The variable length payload header consists of a payload type field and a type Header Error Correction (tHEC) field that protects the integrity of the payload type field. Optionally, the payload header may include an extension header. The payload type field consists of the following subfields:

• Payload Type Identifier (PTI): This subfield identifies the type of frame. Two values are currently defined: user data frames and client management frames.
• Payload FCS Indicator (PFI): This subfield indicates the presence or absence of the payload FCS field.
• Extension Header Identifier (EXI): This subfield identifies the type of extension header in the GFP frame. Extension headers facilitate the adoption of GFP for different client-specific protocols and networks. Three kinds of extension headers are currently defined: a null extension header, a linear extension header for point-to-point networks, and a ring extension header for ring networks.
• User Payload Identifier (UPI): This subfield identifies the type of payload in the GFP frame. The UPI is set according to the transported client signal type. Currently defined UPI values include Ethernet, PPP (including IP and MPLS), Fiber Channel [Benner01], FICON [Benner01], ESCON [Benner01], and Gigabit Ethernet. Mappings for 10/100 Mb/s Ethernet and digital video broadcast, among others, are under consideration.

Payload Information Field

This field contains the client data. There are two modes of client signal payload adaptation defined for GFP: frame-mapped GFP (GFP-F) applicable to most packet data types, and transparent-mapped GFP (GFP-T) applicable to 8B/10B coded signals. Frame-mapped GFP payloads consist of variable length packets. In this mode, client frame is mapped in its entirety into one GFP frame. Examples of such client signals include Gigabit Ethernet and IP/PPP. With transparent-mapped GFP, a number of client data characters, mapped into efficient block codes, are carried within a GFP frame.

3.4.3.2 GFP FUNCTIONS

The GFP frame structure was designed to support the basic functions provided by GFP, namely, frame delineation, client/frame multiplexing, header/payload scrambling, and client payload mapping. In the following, we discuss each of these functions.

Frame Delineation

The GFP transmitter and receiver operate asynchro nously. The transmitter inserts GFP frames on the physical link according to the bit/byte alignment requirements of the specific physical interface (e.g., SONET/SDH, OTN, or dark fiber). The GFP receiver is responsible for identifying the correct GFP frame boundary at the time of link initialization, and after link failures or loss of frame events. The receiver “hunts” for the start of the GFP frame using the last received four octets of data. The receiver first computes the cHEC value based on these four octets. If the computed cHEC matches the value in the (presumed) cHEC field of the received data, the receiver tentatively assumes that it has identified the frame boundary. Otherwise, it shifts forward by 1 bit and checks again. After a candidate GFP frame has been identified, the receiver waits for the next candidate GFP frame based on the PLI field value. If a certain number of consecutive GFP frames are detected, the receiver transitions into a regular operational state. In this state, the receiver examines the PLI field, validates the incoming cHEC field, and extracts the framed PDU.

Client/Frame Multiplexing

GFP supports both frame and client multiplexing. Frames from multiple GFP processes, such as idle frames, client data frames, and client management frames, can be multiplexed on the same link. Client data frames get priority over management frames. Idle frames are inserted when neither data nor management frames are available for transmission.

GFP supports client-multiplexing capabilities via the GFP linear and ring extension headers. For example, linear extension headers (see Figure 3-7) contain an 8-bit channel ID (CID) field that can be used to multiplex data from up to 256 client sessions on a point-to-point link. An 8-bit spare field is available for future use. Various proposals for ring extension headers are currently being considered for sharing GFP payload across multiple clients in a ring environment.

Header/Payload Scrambling

Under GFP, both the core header and the payload area are scrambled. Core header scrambling ensures that an adequate number of 0-1 transitions occur during idle data conditions (thus allowing the receiver to stay synchronized with the transmitter). Scrambling of the GFP payload area ensures correct operation even when the payload information is coincidentally the same as the scrambling word (or its inverse) from frame-synchronous scramblers such as those used in the SONET line layer (SDH RS layer).

Client Payload Mapping

As mentioned earlier, GFP supports two types of client payload mapping: frame-mapped and transparent-mapped. Frame mapping of native client payloads into GFP is intended to facilitate packet-level handling of incoming PDUs. Examples of such client signals include IEEE 802.3 Ethernet MAC frames, PPP/IP packets, or any HDLC framed PDU. Here, the transmitter encapsulates an entire frame of the client data into a GFP frame. Frame multiplexing is supported with frame-mapped GFP. Frame-mapped GFP uses the basic frame structure of a GFP client frame, including the required payload header.

Transparent mapping is intended to facilitate the transport of 8B/10B block-coded client data streams with low transmission latency. Transparent mapping is particularly applicable to Fiber Channel, ESCON, FICON, and Gigabit Ethernet. Instead of buffering an entire client frame and then encapsulating it into a GFP frame, the individual characters of the client data stream are extracted, and a fixed number of them are mapped into periodic fixed-length GFP frames. The mapping occurs regardless of whether the client character is a data or control character, which thus preserves the client 8B/10B control codes. Frame multiplexing is not precluded with transparent GFP. The transparent GFP client frame uses the same structure as the frame-mapped GFP, including the required payload header.

3.4.4 Ethernet over SONET/SDH

As shown in Figure 3-5, there are different ways of carrying Ethernet frames over SONET/SDH, OTN, and optical fiber. Ethernet MAC frames can be encapsulated in GFP frames and carried over SONET/SDH. Also shown in the figure are the different physical layer encoding schemes, including Gigabit Ethernet physical layer, and 10Gigabit Ethernet physical (PHY) layer optimized for LAN and WAN. Gigabit Ethernet physical layer is 8B/10B coded data stream, and it can be encapsulated into GFP frames and carried over SONET/SDH. 10-Gigabit Ethernet WAN PHY is SONET/SDH encoded, and hence it can be directly mapped into STS-192/STM-16 frames.

3.5 SONET/SDH Transparency Services

SONET and SDH have the following notions of transparency built-in, as described in Chapter 2:

1. Path transparency, as provided by the SONET line and SDH multiplex section layers. This was the original intent of SONET and SDH, that is, transport of path layer signals transparently between PTEs.
2. SONET line and SDH multiplex section transparency, as provided by the SONET section and SDH regenerator section layers, respectively.
3. SONET section and SDH regenerator section transparency, as provided by the physical layer.

Of these, only (1) was considered a “user service” within SONET and SDH. There are reasons now to consider (2) and (3) as services, in addition to newer transparency services.

Figure 3-8 shows a typical scenario where transparency services may be desired. Here, two SONET networks (labeled “Domain 1”) are separated by an intervening optical transport network of some type (labeled “Domain 2”). For instance, Domain 1 could consist of two metro networks under a single administration, separated by a core network (Domain 2) under a different administration. The two disjoint parts of Domain 1 are interconnected by provisioning a “link” between network elements NE1 and NE2, as shown. The characteristics of this link depend on the type of transparency desired. In general, transparency allows NE1 and NE2 to use the functionality provided by SONET overhead bytes in various layers. For instance, section transparency allows the signal from NE1 to NE2 to pass through Domain 2 without any overhead information being modified in transit. An all-optical network or a network with transparent regenerators can provide section layer transparency. This service is equivalent to having a dedicated wavelength (lambda) between NE1 and NE2. Thus, the service is often referred to as a lambda service, even if the signal is electrically regenerated within the network. Section transparency allows NE1 and NE2 to terminate the section layer and use the section (and higher layer) overhead bytes for their own purposes.

Figure 3-8. Networking Scenario Used to Define SONET/SDH Transparency Services

If the OC-N to be transported between NE1 and NE2 is the same size (in terms of capacity) as those used within the optical network, then the section transparency service is a reasonable approach. If the optical network, however, deals with signals much larger than these OC-N signals, then there is the potential for inefficient resource utilization. For example, suppose the optical network is composed of DWDM links and switches that can effectively deal with OC-192 signals. A “lambda” in this network could indeed accommodate an OC-12 signal, but only 1/16th of the capacity of that lambda will be used. In such a case, the OC-12 signal has to be multiplexed in some way into an OC-192 signal. But SONET (SDH) multiplexing takes place at the line (multiplex section) layer. Hence, there is no standard way to convey the OC-12 overhead when multiplexing the constituent path signals into an OC-192 signal. This means that section and line overhead bytes presented by NE1 will be modified within Domain 2. How then to transfer the overhead bytes transparently across Domain 2? Before we examine the methods for accomplishing this, it is instructive to look at the functionality provided by overhead bytes and what it means to support transparency.

Tables 3-3 and 3-4 list the overhead bytes available at different layers, the functionality provided and when the bytes are updated (refer to Figures 2-4 and 2-5).

Table 3-3. SONET Section (SDH Regenerator Section) Overhead Bytes and Functionality

Table 3-4. SONET Line (SDH Multiplex Section) Overhead Bytes and Functionality

With standard SONET/SDH path layer multiplexing, the H1–H3 (pointer) bytes must be modified when the clocks are different for the streams to be multiplexed. The B2 byte must be updated when any of the line layer bytes are changed. Also related to timing is the S1 byte, which reports on the synchronization status of the line. This byte has to be regenerated if multiplexing is performed. Thus, it is not possible to preserve all the overhead bytes when the signal from NE1 is multiplexed with other signals within Domain 2. The additional procedures that must be performed to achieve transparency are discussed next.

3.5.1 Methods for Overhead Transparency

We can group the transport overhead bytes into five categories as follows:

1. Framing bytes A1 and A2, which are always terminated and regenerated
2. Pointer bytes H1, H2 and H3, which must be adjusted for multiplexing, and the S1 byte
3. General overhead bytes: J0, E1, F1, D1-D3, K1, K2, D4-D12, M0/M1, E2
4. BIP-8 error monitoring bytes B1 and B2
5. An assortment of currently unused growth bytes

With regard to the network shown in Figure 3-8, the following are different strategies for transparently transporting the general overhead bytes:

• Information forwarding: The overhead bytes originating from NE1 are placed into the OC-N signal and remain unmodified in Domain 2.
• Information tunneling: Tunneling generally refers to the encapsulation of information to be transported at the ingress of a network in some manner and restoring it at the egress. With respect to Figure 3-8, the overhead bytes originating from NE1 are placed in unused overhead byte locations of the signal transported within Domain 2. These overhead bytes are restored before the signal is delivered to NE2.

As an example of forwarding and tunneling, consider Figure 3-9, which depicts four STS-12 signals being multiplexed into an STS-48 signal within Domain 2. Suppose that the J0 byte of each of these four signals has to be transported transparently. Referring to Table 3-1, it can be noted that the J0 space in signals 2–4 of the STS-48 are reserved, that is, no specific purpose for these bytes is defined within Domain 2. Thus, referring to the structure of the multiplexed overhead information shown in Figure 2-5, the J0 bytes from the second, third, and fourth STS-12 signals can be forwarded unmodified through the intermediate network. This is not true for the J0 byte of the first STS-12, however, since the intermediate network uses the J0 byte in the first STS-1 to cover the entire STS-48 signal (Table 3-1). Hence, the J0 byte of the first STS-12 has to be tunneled by placing it in some unused overhead byte in the STS-48 signal at the ingress and recovering it at the egress.

Figure 3-9. Transparency Example to lllustrate Forwarding and Tunneling

Now, consider the error monitoring bytes, B1 and B2. Their usage is described in detail in section 3.6. Briefly, taking SONET as an example, B1 and B2 bytes contain the parity codes for the section and line portion of the frame, respectively. A node receiving these bytes in a frame uses them to detect errors in the appropriate portions of the frame. According to the SONET specification, B1 and B2 are terminated and regenerated by each STE or LTE, respectively. With regard to the network of Figure 3-8, the following options may be considered for their transport across Domain 2:

• Error regeneration: B1 and B2 are simply regenerated at every network hop.
• Error forwarding: As before, the B1 and B2 bytes are regenerated at each hop. But instead of simply sending these regenerated bytes in the transmitted frame (as in the previous case), the bytes are XOR’d (i.e., bit wise summed) with the corresponding bytes received. With this process, the B1 or B2 bytes will accumulate all the errors (at the appropriate layer) for the transparently transported signal. The only drawback of this method is that the error counts within Domain 2 would appear artificially high, and to sort out the true error counts, correlation of the errors reported along the transparent signal’s path would be required.
• Error tunneling: In this case, the incoming parity bytes (B1 and/or B2) are carried in unused overhead locations within the transport signal in Domain 2. In addition, at each network hop where the bytes are required to be regenerated, the tunneled parity bytes are regenerated and then XOR’d (bit wise binary summation) with the error result that was obtained (by comparing the difference between the received and calculated BIP-8s). In this way, the tunneled parity bytes are kept up to date with respect to errors, and the standard SONET/SDH B1 and B2 bytes are used within Domain 2 without any special error correlation/compensation being performed.

3.5.2 Transparency Service Packages

We have so far looked at the mechanisms for providing transparent transport. From the perspective of a network operator, a more important issue is the determination of the types of transparency services that may be offered. A transparency service package defines which overhead functionality will be transparently carried across the network offering the service. As an example, let us consider the network shown in Figure 3-9 again. The following is a list of individual services that could be offered by Domain 2. These may be grouped in various combinations to create different transparency service packages:

1. J0 transparency: Allows signal identification across Domain 2.
2. Section DCC (D1–D3) transparency: Allows STE to STE data communication across Domain 2.
3. B2 and M0/M1 transparency: Allows line layer error monitoring and indication across Domain 2.
4. K1 and K2 byte transparency: Allow line layer APS across Domain 2. This service will most likely be used with (3) so that signal degrade conditions can be accurately detected and acted upon.
5. Line DCC (D4-D12) transparency: Allows LTE to LTE data communication across Domain 2.
6. E2 transparency: Allows LTE to LTE order wire communication across Domain 2.
7. Miscellaneous section overhead transparency, that is, E1 and F1.

Whether overhead/error forwarding or tunneling is used is an internal decision made by the domain offering the transparency service, based on equipment capabilities and overhead usage. Note that to make use of equipment capable of transparent services, a service provider must know the overhead usage, termination, and forwarding capabilities of equipment used in the network. For example, the latest release of G.707 [ITU-T00a] allows the use of some of the unused overhead bytes for physical layer forward error correction (FEC). Hence, a link utilizing such a “feature” would have additional restrictions on which bytes could be used for forwarding or tunneling.

3.6 When Things Go Wrong

One of the most important aspects built into optical transport systems is their “self-diagnosis” capability. That is, the ability to detect a problem (i.e., observe a symptom), localize the problem (i.e., find where it originated), and discover the root cause of the problem. In fact, SONET and SDH include many mechanisms to almost immediately classify the root cause of problem. This is done by monitoring the signal integrity between peers at a given layer, and also when transferring a signal from a client (higher) layer into a server (lower) layer (Figure 2-17).

In the following, we first consider the various causes of transport problems. Next, we examine how problems are localized and how signal quality is monitored. Finally, we review the methods and terminology for characterizing problems and their duration.

3.6.1 Transport Problems and Their Detection

Signal monitoring functionality includes the following: continuity supervision, connectivity supervision, and signal quality supervision. These are described next.

3.6.1.1 CONTINUITY SUPERVISION

A fundamental issue in telecommunication is ascertaining whether a signal being transmitted is successfully received. Lack of continuity at the optical or electrical layers in SONET/SDH is indicated by the Loss of Signal (LOS) condition. This may arise from either the failure of a transmitter (e.g., laser, line card, etc.) or break in the line (e.g., fiber cut, WDM failure, etc.). The exact criteria for when the LOS condition is declared and when it is cleared are described in reference [ITU-T00b]. For optical SDH signals, a typical criterion is the detection of no transitions on the incoming signal (before unscrambling) for time T, where 2.3 µs ≤ T ≤ 100 µs. An LOS defect is cleared if there are signal transitions within 125 µs. When dealing with other layers, the loss of continuity is discovered using a maintenance signal known as the Alarm Indication Signal (AIS). AIS indicates that there is a failure further upstream in the lower layer signal. This is described further in section 3.6.2.1.

3.6.1.2 CONNECTIVITY SUPERVISION

Connectivity supervision deals with the determination of whether a SONET/SDH connection at a certain layer has been established between the intended pair of peers. This is particularly of interest if there has been an outage and some type of protection or restoration action has been taken. A trail trace identifier is used for connection supervision. Specifically,

• The J0 byte is used in the SONET section (SDH regenerator section) layer. The section trace string is 16 bytes long (carried in successive J0 bytes) as per recommendation G.707 [ITU-T00a].
• The J1 byte is used in the SONET/SDH higher-order path layer (e.g., SONET STS-1 and above). The higher-order path trace string could be 16 or 64 bytes long as per recommendation G.707 [ITU-T00a].
• The J2 byte is used in the SONET/SDH lower-order path layer (e.g., SONET VT signals). The lower-order path trace string is 16 bytes long as per recommendation G.707 [ITU-T00a].

For details of trail trace identifiers used for tandem connection monitoring (TCM), see recommendations G.707 [ITU-T00a] and G.806 [ITU-T00c]. The usage of this string is typically controlled from the management system. Specifically, a trace string is configured in the equipment at the originating end. An “expected string” is configured at the receiving end. The transmitter keeps sending the trace string in the appropriate overhead byte. If the receiver does not receive the expected string, it raises an alarm, and further troubleshooting is initiated.

3.6.1.3 SIGNAL QUALITY SUPERVISION

Signal quality supervision determines whether a received signal contains too many errors and whether the trend in errors is getting worse. In SONET and SDH, parity bits called Bit Interleaved Parity (BIP) are added to the signal in various layers. This allows the receiving end, known as the near-end, to obtain error statistics as described in section 3.6.3. To give a complete view of the quality of the signal in both directions of a bidirectional line, the number of detected errors at the far-end (transmitting end) may be sent back to the near-end via a Remote Error Indicator (REI) signal.

The following bits and bytes are used for near-end signal quality monitoring under SONET and SDH:

• SONET section (SDH regenerator section) layer: The B1 byte is used to implement a BIP-8 error detecting code that covers the previous frame.
• SONET line (SDH multiplex section) layer: In the case of SDH STM-N signals, a BIP N × 24 composed of the 3 STM-1 B2 bytes is used. In the case of SONET STS-N, a BIP N × 8 composed of the N B2 bytes is used. These cover the entire contents of the frame excluding the regenerator section overhead.
• SONET path (SDH HOVC) layer: The B3 byte is used to implement a BIP-8 code covering all the bits in the previous VC-3, VC-4, and VC-4-Xc.
• SONET VT path (SDH LOVC) layer. Bits 1 and 2 of the V5 byte are used to implement a BIP-2 code covering all the bits in the previous VC-1/2.

SONET/SDH provides the following mechanisms for carrying the REI information. For precise usage, see either T1.105 [ANSI-95a] or G.707 [ITU-T00a].

• Multiplex section layer REI: For STM-N (N = 0, 1, 4, 16), 1 byte (M1) is allocated for use as Multiplex Section REI. For STM-N (N = 64 and 256), 2 bytes (M0, M1) are allocated for use as a multiplex section REI. Note that this is in line with the most recent version of G.707 [ITU-T00a].
• Path layer REI: For STS (VC-3/4) path status, the first 4 bits of the G1 path overhead are used to return the count of errors detected via the path BIP-8, B3. Bit 3 of V5 is the VT Path (VC-1/2) REI that is sent back to the originating VT PTE, if one or more errors were detected by the BIP-2.

3.6.1.4 ALIGNMENT MONITORING

When receiving a time division multiplexed (TDM) signal, whether it is electrical or optical, a critically important stage of processing is to find the start of the TDM frame and to maintain frame alignment. In addition, when signals are multiplexed together under SONET/SDH, the pointer mechanism needs to be monitored.

Frame Alignment and Loss of Frame (LOF)

The start of an STM-N (OC-3N) frame is found by searching for the A1 and A2 bytes contained in the STM-N (OC-3N) signal. Recall that the A1 and A2 bytes form a particular pattern and that the rest of the frame is scrambled. This framing pattern is continuously monitored against the assumed start of the frame. Generally, the receiver has 625 µs to detect an out-of-frame (OOF) condition. If the OOF state exits for 3 ms or more then a loss of frame (LOF) state will be declared. To exit the LOF state, the start of the frame must be found and remain valid for 3 ms.

Loss of Multiframe

SDH LOVCs and SONET VTs use the multi-frame structure described earlier. The 500 µs multiframe start phase is recovered by performing multiframe alignment on bits 7 and 8 of byte H4. Out-of-multiframe (OOM) is assumed once when an error is detected in the H4 bit 7 and 8 sequence. Multiframe alignment is considered recovered when an error-free H4 sequence is found in four consecutive VC-n (VT) frames.

Pointer Processing and Loss of Pointer (LOP)

Pointer processing in SONET/SDH is used in both the HOVC (STS path) and LOVC (VT path) layers. This processing is important in aligning payload signals (SDH VC or SONET paths) into their containing signals (STM-N/OC-3N). Without correct pointer processing, essentially one per payload signal, the payload signal is essentially “lost.” Hence, pointer values are closely monitored as part of pointer processing [ITU-T00a, ITU-T00b]. A loss of pointer state is declared under severe error conditions.

3.6.2 Problem Localization and Signal Maintenance

Once a problem has been detected, its exact location has to be identified for the purposes of debugging and repair. SONET/SDH provides sophisticated mechanisms to this in the form of Alarm Indication Signals (AIS) and the Remote Defect Indication (RDI). These are described below.

3.6.2.1 ALARM INDICATION SIGNALS

Suppose that there is a major problem with the signal received by an intermediate point in a SONET network. In this case, a special Alarm Indication Signal is transmitted in lieu of the normal signal to maintain transmission continuity. An AIS indicates to the receiving equipment that there is a transmission interruption located at, or upstream, of the equipment originating the AIS. Note that if the AIS is followed upstream starting from the receiver, it will lead to the location of the error. In other words, the AIS signal is an important aid in fault localization. It is also used to deliver news of defects or faults across layers.

A SONET STE will originate an Alarm Indication Signal-Line (AIS-L) (MS AIS in SDH) upon detection of an LOS or LOF defect. There are two variants of the AIS-L signal. The simplest is a valid section overhead followed by “all ones” pattern in the rest of the frame bytes (before scrambling). To detect AIS-L, it is sufficient to look at bits 6, 7, and 8 of the K2 byte and check for the “111” pattern. A second function of the AIS-L is to provide a signal suitable for normal clock recovery at downstream STEs and LTEs. See [ANSI95a] for the details of the application, removal, and detection of AIS-L.

A SONET LTE will generate an Alarm Indication signal-Path (AIS-P) upon detection of an LOS, LOF, AIS-L, or LOP-P defect. AIS-P (AU AIS in SDH) is specified as “all ones” in the STS SPE as well as the H1, H2, and H3 bytes. STS pointer processors detect AIS-P as “111…” in bytes H1 and H2 in three consecutive frames.

A SONET STS PTE will generate an Alarm Indication signal-VT (AIS-V) for VTs of the affected STS path upon detection of an LOS, LOF, AIS-L, LOP-P, AIS-P, or LOP-V defect. The AIS-V signal is specified as “all ones” in the entire VT, including the V1-V4 bytes. VT pointer processors detect AIS-V as “111…” in bytes V1 and V2 in three consecutive VT superframes.

The SDH AIS signals for its various layers are nearly identical as those of SONET in definition and use as shown in Table 3-5.

3.6.2.2 REMOTE DEFECT INDICATION

Through the AIS mechanism, SONET allows the downstream entities to be informed about problems upstream in a timely fashion (in the order of milliseconds). The AIS signal is good for triggering downstream protection or restoration actions. For quick recovery from faults, it is also important to let the upstream node know that there is a reception problem downstream. The Remote Defect Indication (RDI) signal is used for this purpose. The precise definition of RDI, as per [ANSI95a], is

Table 3-5. SDH AIS Signals by Layer

A signal transmitted at the first opportunity in the outgoing direction when a terminal detects specific defects in the incoming signal.

At the line level, the RDI-L code is returned to the transmitting LTE when the receiving LTE has detected an incoming line defect. RDI-L is generated within 100 ms by an LTE upon detection of an LOS, LOF, or AIS-L defect. RDI-L is indicated by a 110 code in bits 6,7,8 of the K2 byte (after unscrambling).

At the STS path level, the RDI-P code is returned to the transmitting PTE when the receiving PTE has detected an incoming STS path defect. There are three classes of defects that trigger RDI-P:

1. Payload defects: These generally indicate problems detected in adapting the payload being extracted from the STS path layer.
2. Server defects: These indicate problems in one of the layers responsible for transporting the STS path.
3. Connectivity defects: This only includes the trace identifier mismatch (TIM) or unequipped conditions.

Table 3-6 shows current use of the G1 byte for RDI-P purposes (consult [ANSI95a] for details).

The remote defect indication for the VT path layer, RDI-V, is similar to RDI-P. It is used to return an indication to the transmitting VT PTE that the receiving VT PTE has detected an incoming VT Path defect. There are three classes of defects that trigger RDI-V:

Table 3-6. Remote Defect Indicator—Path (RDI-P) via the G1 Byte

1. Payload defects: These generally indicate problems detected in adapting the payload being extracted from the VT path layer.
2. Server defects: These generally indicate problems in the server layers to the VT path layer.
3. Connectivity defects: These generally indicate that there is a connectivity problem within the VT path layer.

For more information, see [ANSI95a] for details. RDI-V uses the Z7 bytes (bits 6 and 7).

One thing to note about RDI signals is that they are “peer to peer” indications, that is, they stay within the layer that they are generated. The AIS and RDI signals form the “fast” notification mechanisms for protection and restoration, that is, these are the primary triggers. Examples of their usage are given in the next chapter. The RDI signals in various SDH layers are nearly identical to those of SONET and they are summarized in Table 3-7.

Table 3-7. RDI Signals for Various SDH Layers

3.6.3 Quality Monitoring

3.6.3.1 BLIPS AND BIPS

The bit error rates are typically extremely low in optical networks. For example, in 1995, the assumed worst-case bit error rate (BER) for SONET regenerator section engineering was 10^-10, or one error per 10 billion bits. Today, that would be considered quite high. Hence, for error detection in a SONET frame, we can assume very few bit errors per frame.

As an example, the number of bits in an STS-192 frame is 1,244,160 (9 rows × 90 columns per STS-1 x 8 bits/byte × 192 STS-1). With a BER of 10^-10, it can be expected that there will be one bit error in every 8038 frames. The probability of two errors in the same frame is fairly low. Since the bit rate of an STS-192 signal is 10 Gbps (or 10¹⁰ bits per second), a BER of 10^-10 gives rise to one bit error every second on the average. This is why a BER of 10^-10 is considered quite high today.

Figure 3-10 shows the general technique used in SONET and SDH for monitoring bit errors “in-service” over various portions of the signal. This method is known as the Bit Interleaved Parity 8 Bits, or BIP-8 for short. Although the name sounds complex, the idea and calculation are rather simple. In Figure 3-10, X1-X5 represents a set of bytes that are being checked for transmission errors. For every bit position in these bytes, a separate running tally of the parity (i.e., the number of 1s that occur) is kept track of. The corresponding bit position of the BIP-8 byte is set to “1” if the parity is currently odd and a zero if the parity is even. The BIP-8 byte is sent, typically in the following frame, to the destination. The destination recomputes the BIP-8 code based on the contents of the received frame and compares it with the BIP-8 received. If there are no bit errors, then these two codes should match. Figure 3-10(b) depicts the case where one of the bytes, X2, encounters a single bit error during transmission, that is, bit 2 changes from 1 to 0. In this case, the received BIP-8 and the recomputed BIP-8 differ by a single bit and, in fact, the number of differing bits can be used as an estimate of the number of bit errors.

Figure 3-10. Example of BIP-8 Calculation and Error Detection

Note that the BIP-8 technique works well under the assumption of low bit error rates. The study of general mechanisms for error detection and correction using redundant information bits is known as algebraic coding theory (see [Lin+83]).

BIP-8 is used for error monitoring in different SONET/SDH layers. At the SONET section layer, the B1 byte contains the BIP-8 calculated over all the bits of the previous STS-N frame (after scrambling). The computed BIP-8 is placed in the B1 byte of the first STS-1 (before scrambling). This byte is defined only for the first STS-1 of an STS-N signal. SDH uses this byte for the same purpose. Hence, the BIP-8 in this case is calculated over the entire SONET frame and covers a different number of bytes for different signals, for example, STS-12 vs. STS-192.

At the SONET line layer, BIP-8 is calculated over all the bits of the line overhead and the STS-1 SPE (before scrambling). The computed BIP-8 is placed in the B2 byte of the next STS-1 frame (before scrambling). This byte is separately computed for all the STS-1 signals within an STS-N signal. These N BIP-8 bytes are capable of detecting fairly high bit error rates, up to 10^-3. To see this, consider an STS-1 line signal (i.e., an STS-1 frame without section layer overhead). The number of bytes in this signal is 804 (9 rows × 90 columns – 6 section bytes). Each bit in the line BIP-8 code is used to cover 804 bits (which are in the corresponding bit position of the 804 bytes in the line signal). Since a BER of 10^-3 means an average of one bit error every 1000 bits, there will be less than one bit error in 804 bits (on the average). This, the line BIP-8 code is sufficient for detecting these errors. Note, however, that BIP-8 (and any parity based error detection mechanism) may fail if there are multiple, simultaneous bit errors.

At the STS Path level, BIP-8 is calculated over all the bits of the previous STS SPE (before scrambling) and carried in the B3 path overhead byte. SDH uses this byte for the same purpose but excludes the fixed stuff bytes in the calculation. The path BIP-8, like the section BIP-8, covers a different number of bytes depending on the size of the STS path signal, that is, STS-3 vs. STS-12.

At the VT path level, 2 bits of the VT path level overhead byte V5 are used for carrying a BIP-2. The technique for this is illustrated in Figure 3-11. To save on overhead, the parity counts over all the odd and the even bit positions are combined and represented by the two bits of the BIP-2 code, respectively. Recall that the VT SPE is a multiframe spanning four SONET frames. The BIP-2 is calculated over all bytes in the previous VT SPE, including all overhead but the pointers (Figure 3-11).

Figure 3-11. BIP Calculation at the VT Path Level

Let us examine how effective the BIP-2 code is. The number of bits in the VT1.5 SPE is 832 ([(9 rows × 3 columns) – 1 pointer byte] × 8 bits/byte × 4 frames per SPE). Each bit of the BIP-2 code covers half the bits in the VT1.5 SPE, that is, 416 bits. Hence, BIP-2 can handle error rates of 1 in 500 bits (BER between 10^-2 and 10^-3). Now, a VT6 is four times the size of the VT1.5. In this case, each parity bit covers 1664 bits, handling a BER slightly worse than 10^-4.

3.6.4 Remote Error Monitoring

The error monitoring capabilities provided by SONET and SDH enables the receiver to know the error count and compute the BER on the received signal at various layers. Based on this information, it is useful to let the sender learn about the quality of the signal received at the other end. The following mechanisms are used for this purpose.

The STS-1 line REI (M0 byte) is used by the receiver to return the number of errored bits detected at the line layer to the sender. The receiver arrives at this number by considering the difference between the received and the recomputed BIP-8 (B2) codes. In the case of an STS-N signal, the M1 byte is used for conveying the REI information. Clearly, up to 8 × N errors could be detected with STS-N BIP-8 codes (as each STS-1 is covered by its own BIP-8). But only a count of at most 255 can be reported in the single M1 byte. Thus, in signals of OC-48 and higher rates, the number 255 is returned when 255 or more errors are detected.

At the path layer, the receiver uses the first four bits of the G1 path overhead to return the number of errors detected (using the path BIP-8) to the sender. At the VT path layer, the receiver uses bit 3 of the V5 byte to indicate the detection of one or more errors to the sender.

3.6.5 Performance Measures

When receiving word of a problem, one is inclined to ask some general questions such as: “How bad is it? “How long has it been this way?” and “Is it getting worse or better?” The following terminology is used in the transport world. An anomaly is a condition that gives the first hint of possible trouble. A defect is an affirmation that something has indeed gone wrong. A failure is a state where something has truly gone wrong. Whether an event notification or an alarm is sent to a management system under these conditions is a separate matter. Performance parameters in SONET and SDH are used to quantify these conditions.

A SONET or an SDH network element supports performance monitoring (PM) according to the layer of functionality it provides. A SONET network element accumulates PM data based on overhead bits at the Section, Line, STS Path, and VT Path layers. In addition, PM data are available at the SONET Physical layer using physical parameters. The following is a summary of different performance parameter defined in SONET. Similar performance parameters are also monitored and measured in SDH. For a detailed treatment on PM parameters on SONET refer to [Telcordia00].

Physical Layer Performance Parameters

The physical layer performance measurement enables proactive monitoring of the physical devices to facilitate early indication of a problem before a failure occurs. Several physical parameters are measured, including laser bias current, optical power output by the transmitter, and optical power at the receiver. Another important physical layer parameter is the Loss of Signal (LOS) second, which is the count of 1-second intervals containing one or more LOS defects^.

Section Layer Performance Parameters

The following section layer performance parameters are defined in SONET. Note that all section layer performance parameters are defined for the near-end. There are no far-end parameters at the Section layer.

• Code Violation (CV-S): The CV-S parameter is a count of BIP errors detected at the section layer. Up to eight section BIP errors can be detected per STS-N frame.
• Errored Second (ES-S): The ES-S parameter is a count of the number of 1 second intervals during which at least one section layer BIP error was detected, or an SEF (see below) or LOS defect was present.
• Errored Second Type A (ESA-S) and Type B (ESB-S): ESA-S is the count of 1-second intervals containing one CV-S, and no SEF or LOS defects. ESB-S is the count of 1-second intervals containing more than one but less than X CV-S errors, and no SEF or LOS defects. Here, X is a user-defined number.
• Severely Errored Second (SES-S): The SES-S parameter is a count of 1-second intervals during which K or more Section layer BIP errors were detected, or an SEF or LOS defect was present. K depends on the line rate and can be set by the user.
• Severely Errored Frame Second (SEFS-S): The SEFS-S parameter is a count of 1-second intervals during which an SEF defect was present. An SEF defect is detected when the incoming signal has a minimum of four consecutive errored frame patterns. An SEF defect is expected to be present when an LOS or LOF defect is present. But there may be situations when this is not the case, and the SEFS-S parameter is only incremented based on the presence of the SEF defect.

Line Layer Performance Parameters

At the SONET line layer, both near-end and far-end parameters are monitored and measured. Far-end line layer performance is conveyed back to the near-end LTE via the K2 byte (RDI-L) and the M0 or M1 byte (REI-L). Some of the important near-end performance parameters are defined below. The far-end parameters are defined in a similar fashion.

• Code Violation (CV-L): The CV-L parameter is a count of BIP errors detected at the line layer. Up to 8N BIP errors can be detected per STS-N frame.
• Errored Second (ES-L): The ES-L parameter is a count of 1-second intervals during which at least one line layer BIP error was detected or an AIS-L defect is present.
• Errored Second Type A (ESA-L) and Type B (ESB-L): ESA-L is the count of 1-second intervals containing one CV-L error and no AIS-L defects. ESB-L is the count of 1-second intervals containing X or more CV-L errors, or one or more AIS-L defects. Here, X is a user-defined number.
• Severely Errored Second (SES-L): The SES-L parameter is a count of 1-second intervals during which K or more line layer BIP errors were detected, or an AIS-L defect is present. K depends on the line rate and can be set by the user.
• Unavailable Second (UAS-L): Count of 1-second intervals during which the SONET line is unavailable. The line is considered unavailable after the occurrence of 10 SES-Ls.
• AIS Second (AISS-L): Count of 1-second intervals containing one or more AIS-L defects.

Path Layer Performance Parameters

Both STS path and VT path performance parameters are monitored at the path layer. Also, both near-end and far-end performance parameters are measured. Far-end STS path layer performance is conveyed back to the near-end STS PTE using bits 1 through 4 (REI-P) and 5 through 7 (RDI-P) of the G1 byte. Far-end VT path layer performance is conveyed back to the near-end VT PTE using bit 3 of the V5 byte (REI-V), and either bits 5 through 7 of the Z7 byte or bit 8 of the V5 byte (RDI-V). Some of the important near-end STS path performance parameters are defined below. The far-end parameters are defined in a similar fashion.

• Code Violation (CV-P): Count of BIP-8 errors that are detected at the STS-path layer.
• Errored Second (ES-P): Count of 1-second intervals containing one or more CV-P errors, one or more AIS-P, LOP-P, TIM-P, or UNEQ-P defects.
• Errored Second Type A (ESA-P) and Type B (ESB-P): ESA-P is the count of 1-second intervals containing one CV-P error and no AIS-P, LOP-P, TIM-P, or UNEQ-P defects. ESB-P is the count of 1-second intervals containing more than one but less than X CV-P errors and no AIS-P, LOP-P, TIM-P, or UNEQ-P defects. Here, X is a user-defined number.
• Severely Errored Second (SES-P): Count of 1-second intervals containing X or more CV-P errors, one or more AIS-P, LOP-P, TIM-P, or UNEQ-P defects. Here, X is a user-defined number.
• Unavailable Second (UAS-P): Count of 1-second intervals during which the SONET STS-path is unavailable. A path is considered unavailable after the occurrence of 10 SESs.
• Pointer Justification Counts: To monitor the adaptation of the path payloads into the SONET line, the pointer positive and negative adjustment events are counted. The number of 1-second intervals during which a pointer adjustment event occurs is also kept track of.
• 3.7 Summary
• SONET and SDH-based optical transport networks have been deployed extensively. It is therefore important to understand the fundamentals of these technologies before delving into the details of the control plane mechanisms. After all, the optical network control plane is a relatively recent development. Its primary application in the near term will be in SONET/SDH networks. In this context, it is vital to know about the low-level control mechanisms that already exist in SONET and SDH and how they help in building advanced control plane capabilities. The next chapter continues with a description of another key topic relevant to the control plane, that is, protection and restoration mechanisms in SONET and SDH networks. Following this, the subject of modern optical control plane is dealt with in earnest.

Virtual Concatenation: Knowing the Details .
They say the devil is in the details. That’s certainly the case when dealing with virtual concatenation. Clearly, designers at the chip, equipment, and carrier level have touted the wonders that virtual concatenation delivers. But, what often gets lost in these discussions are the real challenges that chip and equipment developers will face when implementing virtual concatenation in a real-world design.

In this two-part series, we’ll examine the design issues that developers will encounter when implementing virtual concatenation in a system level design. In Part 1, we’ll examine the basic benefits of virtual concatenation, the difference between high- and low-order virtual concatenation pipes, and differential delay issues. In Part 2, we’ll take a detailed look at the link capacity adjustment scheme (LCAS).

Why VC is So hot
Much has already been said and written about the benefits of virtual concatenation over current payload mapping capabilities of Sonet and SDH. Table 1 summarizes the individual payload capacities of different commonly used Sonet or SDH paths. The table includes both high- and low-order paths with and without standard contiguous concatenation (denoted by the “c”).

While allowing a range of bandwidths to be provisioned, these current mappings do not have the granularity required to make efficient use of the existing network infrastructure. One other important point to note is that contiguous concatenation of VT1.5/VC-11s or VT2/VC-12s is not supported.

Table 1: Current Sonet and SDH Payload Capacities

Table 2 lists the payload capacities possible with virtual concatenation. As is shown, concatenation of VT1.5/VC-11s or VT2/VC-12s is supported and the concatenation of high-order paths is much more granular.

Table 2: Virtual Concatenation Payload Capacities

Making Use of Unused Overhead
In addition to allowing more flexible mapping, virtual concatenation also releases two of the restrictions upon which contiguous concatenation relies to reconstruct the signal being carried. These are phase alignment of the members of the concatenation and an inherent sequence order of the members. Consequently, in order to reconstruct the original signal from a virtually concatenated group (VCG), it is necessary to determine the phase alignment and sequence of the received members. The information required to support this is carried in previously unused Sonet/SDH path overhead, which is overhead that is generated by a payload mapper and effectively stays intact regardless of how the payload makes its way through the network to its destination. Note: In SDH, tandem connection monitoring involves the modification of some Path Overhead at an intermediate point.

To put this in context, Figure 1 illustrates the high- and low-order paths and their path overhead. For high-order paths, virtual concatenation uses the H4 byte while for low-order paths, virtual concatenation uses bit 2 of the Z7/K4 byte.

Figure 1: High- and low-order paths and overhead.

For both high- and low-order paths, the information required is structured in a multi frame format as shown in Figure 2. For high-order paths, the multi frame structure is defined by the virtual concatenation overhead carried in the H4 byte. For the low-order paths, on the other hand, the multi frame structure is phase aligned with the multi frame alignment signal (MFAS) of bit 1 of the Z7/K4 byte that carries the extended signal label.

Figure 2: Virtual concatenation multi frame formats.

High-Order Overhead
In high-order paths, the H4 multi frame structure is 16 frames long for a total of 2 ms. Within this structure, there are two multi frame indicators—MFI1 and MFI2. MFI1 is a 4-bit field which increments every frame while MFI2 is an 8-bit field which increments every multi frame.

The most significant and least significant nibbles of MFI2 are sent over the first two frames of a multi frame. Together with MFI1, they form a 12-bit field that rolls over every 512 ms (4096 x 125 μs). This allows for a maximum differential path delay of less than 256 ms to ensure that it is always possible to determine which members of a VCG arrive earliest (shortest network delay) and which members arrive latest (longest network delay).

If the differential delay were 256 ms or more, it would not be possible to know if a member with an {MFI2,MFI1}=0 is 256 ms behind or 256 ms ahead of a member with an {MFI2,MFI1}=2048.

The second piece of information conveyed in the H4 byte is the sequence indicator (SQ). This is an 8-bit field that, like the MFI2, is sent a nibble at a time over two frames in the multi frame. In this case if SQ, it is sent over the last two frames. Consequently, a high-order VCG can contain up to 256 members.

The number of members is obviously limited by the number of paths available in the transport signal. Thus, a 40-Gbit pipe would have to be a reality to have 256 STS 1 or VC 3 members. Referring back to the payload capacities of Table 1, for STS 1 256v/VC 3 256vs, the payload capacity is 256 x 48.384 Mbit/s = 12,386.304 Mbit/s. For STS 3c 256v/VC 4 256v, the payload capacity would be close to 50 Gbit/s.

Low-Order Overhead
Figure 1 above shows that low-order paths have an inherent multi frame structure of 4 Sonet/SDH frames (or 500 μs). As illustrated in Figure 2, the virtual concatenation multi frame structure, delineated by the MFAS pattern in the extended signal label bit (bit 1 of K4), is 32 of these 500 μs multi frames for a total VC multi frame (or should we say multi multi frame) duration of 16 ms.

Within the virtual concatenation multi-frame structure of bit 2 of the K4 byte, again, there is a multi frame indicator (MFI) and an SQ. In this case, the MFI is a 5-bit field sent over the first five 500 μs multi frames of the VC multi frame that rolls over every 512 ms (32 x 16ms). Again, this permits a maximum differential delay across all members of a low-order VCG of less than 256 ms.

The SQ for LO paths is a 6-bit field which is transmitted over virtual concatenation multi frames 6 through 11 allowing for up to 64 members on a low-order VCG. Again, using the values in Table 2, for VT1.5 64v/VC 11 64v, the payload capacity is 102.4 Mbit/s and the payload capacity of a VT2 64v/VC 12 64v is 139.264 Mbit/s.

Differential Delay alignment
When data is mapped into a VCG, it is essentially ‘demultiplexed’, on a byte by byte basis, across the members of the VCG in the sequence provisioned (reflected by the SQ bytes of each member). At the destination, these discrete paths must be ‘remultiplexed’ to form the original signal. Allowance for differential delay across the members of a VCG implies that all members must be delayed to that of the maximum member such that the ‘remultiplexing’ can be performed correctly.

As a concept, differential delay alignment is not particularly complex. Each member has its data written into a buffer upon reception along with some kind of indication as to where the MFI boundaries are. Data for a given MFI is then read out of each buffer, thus creating phase alignment of the members. The depth of each buffer (the difference between the read and write pointers) is a measure of the difference in delay between that member and the member that has the most network delay.

The main issue with differential delay is the amount of buffer space required. Designers can calculate the amount of buffer space required using the maximum number of members supported. For example, each VT1.5/VC 11 has a payload capacity of 1.6Mbit/s. The worst case is that a member would have to be delayed by just under 256 ms which represents 1.6 Mbit/s x 0.256 s = 400 kbit. Similarly, an STS 1/VC 3 requires a maximum payload capacity of 1.48 Mbit.

These numbers may not seem significant until one considers the number of paths in a given transport signal. Table 3 shows the memory requirements for some potential combinations of virtual concatenation path types and the transport signals that may carry them. Note: the calculations in Table 3 reflect maximum buffer sizes on all paths assuming only payload data is buffered. At least one member of each VCG, by definition, will have minimal buffering so the actual requirements will be slightly lower. If any Path Overhead is also buffered, then the requirements may rise.

Table 3: Virtual Concatenation Delay Buffer Requirements for Various Transport Signals

It is clear from Table 3 that, even for low bandwidth mapping/demapping devices (STS-3/STM-1) that support virtual concatenation, it is impractical to provide on-board buffers allowing for 256 ms of differential delay.

The obvious way to solve this problem is to equip mapping/demapping devices with interfaces to external memory that is large enough to hold the amounts of data listed above. Again this sounds straightforward but there is another consideration that complicates the solution. The data transfer rate between the mapper/demapper and the external buffer memory is twice that of the transport signal rate. This is because the data must be both written to and read from the buffers at the transport signal rate. For an OC 48/STM 16 this amounts to close to 5 Gbit/s. Even with 32-bit wide memory, this results in approximately 150 Mtransfers/s.

The memory options that support these rates are not plentiful. Essentially, these devices must support external SDRAM or SRAM. SDRAM may seem like a good solution due to the large capacities available and the apparent speed that DDR and QDR SDRAMs can support. These speeds can only be achieved, however, if access to the memory involves sustained bursts to sequential memory blocks where successive blocks sit in different pages within the SDRAM structure. This can’t easily be guaranteed, as the allocation of memory is entirely dependent on the type, number and delay supported of the members all VCGs being terminated by the device.

SRAMs, on the other hand, can easily keep up with the transfer rates required with no restriction on the order that data is either written or read but capacities of 500 Mbit can be prohibitive in cost and real estate. Consequently, component vendors must choose carefully how much differential delay and what type of external memory their mapper/demapper devices will support.

Virtual Concatenation: Knowing the Details continues
The hype behind virtual concatenation has been growing for more than a year now. And the link capacity adjustment scheme is one of the reasons why. LCAS enhances the capabilities provided by virtual concatenation, allowing operators to adjust virtually concatenated groups (VCGs) on the fly, thus improving network utilization even further.

But, like virtual concatenation, LCAS implementation can be quite challenging for today’s chip and equipment designers. In Part 1, we looked at virtual concatenation and the implementation issues designers will face using this technology. Now, in Part 2, we’ll focus our attention on describing how LCAS works and the design issues engineers will face when using this technology in a chip or system design.

Understanding LCAS
The link capacity adjustment scheme (LCAS) mainly attempts to address two of the tricky issues associated with virtual concatenation: ability to increase or decrease the capacity of a VCG and the ability to deal gracefully with member failures.

With LCAS, not all members of a VCG need to be active in order to pass data from the source (So), to the Sink (Sk). Once a VCG is defined, the So and Sk equipment are responsible for agreeing which members will carry traffic. There are also procedures that allow them to agree to remove or add members at any time. To achieve this, signaling between the source and sink is required and some of the reserved fields in the virtual concatenation overhead are used for this purpose.

Within LCAS, a control packet is defined that carries the following fields:

• Member status (MST)
• Re-sequence acknowledge (RS-Ack)
• Control (CTRL)
• Group ID (GID)
• CRC-3/CRC-8 (3 for LO, 8 for HO)

The position of these fields within the VC multi-frames for high- and low-order paths are shown in Figure 3. Note that, for high-order paths, the control packet begins with the MST field in MFI n and ends with the CRC-8 field in MFI n+1.

Figure 3: Signaling overhead associated with LCAS.

The MST field provides a means of communicating, from the Sk to the So, the state of all received VCG members. The state for each member is either OK or FAIL (1 bit). Since there are potentially more members than bits in the field in a given VC multi-frame, it takes 32 high-order virtual concatenation multi-frames and 8 low-order virtual-concatenation multi-frames to signal the status of all members. This signaling allows the Sk to indicate to the So that a given member has failed and may need be removed from the list of active members of the VCG.

The RS-Ack field is a bit that is toggled by the Sk to indicate to the So that changes in the sequence numbers for that VCG have been evaluated. It also signals to the So that the MST information in the previous multi-frame is valid. With this signaling, the So can be informed that the changes it has requested (either member addition or removal) have been accepted by the Sk.

The MST and RS-Ack fields are identical in all members of the VCG upon transmission from the Sk.

The Control Field
The control field allows the So to send information to the Sk describing the state of the link during the next control packet. Using this field, the So can signal that the particular path should be added (ADD) to the active members, be deleted (or remain deleted) from the active members (IDLE) or should not be used due to a failure detected at the Sk (DNU). It can also indicate that the particular path is an active member (NORM) or the active member with the highest SQ (EOS). Finally, for compatibility with non-LCAS VCAT the CTRL field can indicate that fixed bandwidth is used (FIXED).

The Group ID field provides a means for the receiver to determine that all members of a received VCG come from the same transmitter. The field contains a portion of a pseudo-random bit sequence (2¹⁵–1). This value is the same for all members of a VCG at any given MFI.

Finally, the CRC field provides a means to validate the control packet received before acting on it. In this way, the signaling link is tolerant of bit errors.

Basic LCAS Operation
When a VCG is initiated, all MSTs generated by the Sk are set to FAIL. It is then the responsibility of the So to add members to the VCG to establish data continuity. The So can set the initial SQ numbers of multiple members and set their CTRL fields to ADD. The Sk will then set all the corresponding MSTs to OK.

The first MST recognized by the So has its SQ renumbered to the lowest value and this re-sequence will be transmitted to the Sk. Multiple members can be recognized at the same time by the So and the re-sequence may involve more than one member.

The Sk acknowledges the re-sequence by toggling the RS-Ack in all members. After the RS-Ack is received by the So, it will set CTRL field for the corresponding members to NORM with highest SQ member being set to EOS. This process continues until all members have been added to the active group. At this point, the CTRL field for all but one added members will be NORM. The member with the highest SQ will have its CTRL field set to EOS.

Adding, Deleting Members
When members are to be added or deleted, the sequence is similar. The CTRL field for the member or members in question will be set by the So to either ADD or IDLE depending on the operation requested. The Sk will then respond with MST values of either OK or FAIL respectively. Again, the order that the updated MST values are seen and confirmed by the So will determine how the SQ values are updated.

In the event of a network failure resulting in a member failure, the Sk will set the corresponding MST (or MSTs) to FAIL. The So, upon seeing and confirming the status of this member (or members), will set the CTRL field for that member (or those members) to DNU.

If the last member has an MST of FAIL, then the next previous member that remains active will have its CTRL field changed from NORM to EOS. In the event that the failure is repaired, the MST (or MSTs) will be updated by the Sk to OK. At this point, the So can update the CTRL value (or values) to NORM to indicate that the member (or members) will again carry traffic at the next boundary.

In all cases, bandwidth changes take place on the high-order frame or low-order multi-frame following the reception of the CRC of the control packet where the CTRL fields change to or from NORM. Specifically, this is synchronized with the first payload byte after the J1 or J2 following the end of the control packet. This byte will be the first one either filled with data in the case of an added member or the first one left empty in the case of a deleted member.

LCAS Design Considerations
One of the most attractive features of LCAS is the fact that it provides a mechanism to map around VCG member failures by allowing them to be temporarily removed from a VCG without user intervention. Typically, however, paths will be protected in some fashion whether it is 1:N span protections or, more likely unidirectional path switched/subnetwork connection protection (UPSR/SNCP)-type protection within the network. If this is the case, then, on a network failure, it would easily be possible for an Sk to lose a member and signal that condition via the MST field and then regain that member after the So has already initiated temporary removal from the group. Without the ability to allow existing network APS schemes to settle before acting at the LCAS level, this kind of scenario can lead to a considerable amount of thrashing in the reestablishment of data continuity for the VCG after a network failure.

Similarly, while the flexibility of SQ assignment can allow for graceful inclusion or exclusion of VCG members, it can also create significant complication in managing the members. When an So chooses to add multiple members, it must arbitrarily set the SQ values for each member that it wishes to add to something greater than the maximum existing SQ value.

Once an MST=OK is received for any of those members, the So then sets that member’s SQ value to one greater than the highest active member. This means that the all the ‘new’ SQ values of the other members waiting to be added may need to be rearranged.

The RS-Ack is defined specifically so that the Sk can evaluate the new SQ information and acknowledge it before data is placed on the new member, but any table driven alignment scheme based on received SQ values must be tolerant of these changes. Also, software must be able to manage the changes in correlation between Sonet/SDH paths and their SQ values over time.

Additionally, when members are deleted from a VCG, their new SQ values can be any value greater than the highest active member. There is no restriction that these values be unique so many inactive members can share the same SQ. Again, context-switched state machines that run through the SQ values ensuring that all members are processed properly must handle this condition.

Other potential problems can arise from how unused members are handled. If unused paths are received with AIS or unequipped, then the path overhead will contain no virtual concatenation signaling of any kind. There is then no way to determine any kind of virtual concatenation multi-frame alignment of these members. So it must be possible to achieve alignment on the working members of a VCG regardless of the state of all other members.

Moving Processing to Software
As seen above, complications can arise in how different information is interpreted when using LCAS. Due to this complexity and the signaling durations involved (e.g. 64 or 128 ms required to update all MST), it is attractive to move some of the processing to software where more variables can, more easily, be considered. In fact some functions, such as waiting for APS to settle, can be better handled in software.

Care must be taken in establishing the hardware/software partition, however. For example, if a system needs to support hitless addition or deletion of members, the time between the reception of a control packet and when the data multiplexing configuration changes is just 55.6 μs for high-order paths and 250 μs for low-order paths. Software will typically not be able to reconfigure the data multiplexing quickly enough once it has determined what changes are about to occur. It is possible, depending on how many VCGs have changes going on at the same time, that software implementation will not even sort out the changes before they happen.

Wrap Up
Access equipment that supports both high- and low-order mapping allows the service provider to tailor the connectivity granularity and cost based on the requirements of the customers at each installation while only needing to worry about a limited product inventory. With virtual concatenation, the service provider can efficiently provide this appropriate level of connectivity without having to resort to statistical multiplexing techniques that complicate service level agreements (SLAs). With LCAS, bandwidth flexibility and fault tolerance are added.

Designing the systems and components to support virtual-concatenation-enabled Sonet and SDH infrastructures is not trivial, however. Designers have to draw on their experience with legacy equipment and the problems found in the network today to ensure a robust implementation of tomorrow’s network.

.

New Green Touch Telecom Consortium Reveals Energy-Saving Plan