A defect is understood as any serious or persistent event that holds up the transmission service. SDH defect processing reports and locates failures in either the complete end-to-end circuit (HP-RDI, LP-RDI) or on a specific multiplex section between adjacent SDH nodes (MS-RDI)

Alarm indication signal

An alarm indication signal (AIS) is activated under standardized criteria, and sent downstream in a path in the client layer to the next NE to inform about the event. The AIS will arrive finally at the NE at which that path terminates, where the client layer interfaces with the SDH network .

As an answer to a received AIS, a remote defect indication is sent backwards. An RDI is indicated in a specific byte, while an AIS is a sequence of “1s” in the payload space. The permanent sequence of “1s” tells the receiver that a defect affects the service, and no information can be provided.

Depending on which service is affected, the AIS signal adopts several forms:-

• MS-AIS: All bits except for the RSOH are set to the binary value “1.”
• AU-AIS: All bits of the administrative unit are set to “1” but the RSOH and MSOH maintain their codification.
• TU-AIS: All bits in the tributary unit are set to “1,” but the unaffected tributar- ies and the RSOH and MSOH maintain their codification.
• PDH-AIS: All the bits in the tributary are “1.”

Enhanced remote defect indication 

Enhanced remote defect indication (E-RDI) provides the SDH network with addi- tional information about the defect cause by means of differentiating:

• Server defects: like AIS and LOP;
• Connectivity defects: like TIM and UNEQ;
• Payload defects: like PLM.

Enhanced RDI information is codified in G1 (bits 5-7) or in k4 (bits 5-7), depending on the path.

Many times we heard that we should implement Unidirectional or Bidirectional APS in the network.Just some of the advantages of these are:-

Unidirectional and bidirectional protection switching

Possible advantages of unidirectional protection switching include:

• Unidirectional protection switching is a simple scheme to implement and does not require a protocol.
• Unidirectional protection switching can be faster than bidirectional protection switching because it does not require a protocol.
• Under multiple failure conditions there is a greater chance of restoring traffic by protection switching if unidirectional protection switching is used, than if bidirectional protection switching is used.

Possible advantages of bidirectional protection switching when uniform routing is used include:

• With bidirectional protection switching operation, the same equipment is used for both directions of transmission after a failure. The number of breaks due to single failures will be less than if the path is delivered using the different equipment.
• With bidirectional protection switching, if there is a fault in one path of the network, transmission of both paths between the affected nodes is switched to the alternative direction around the network. No traffic is then transmitted over the faulty section of the network and so it can be repaired without    further  protection switching.
• Bidirectional protection switching is easier to manage because both directions of transmission use the same equipments along the full length of the trail.
• Bidirectional protection switching maintains equal delays for both directions of transmission. This may be important where there is a significant  imbalance in the length of the trails e.g. transoceanic links where one trail is via a satellite link and the other via a cable link.
•  Bidirectional protection switching also has the ability to carry extra traffic on the protection path.

@above is extracted form ITU-G.*841

1.OverView

Availability is a probabilistic measure of the length of time a system or network is functioning.

• Generally calculated as a percentage, e.g. 99.999% (referred to as 5 nines up time) is carrier grade availability.
• A network has a high availability when downtime / repair times are minimal.
• For example, high availability networks are down for minutes, where low availability networks are down for hours.
• Unavailability is the percentage of time a system is not functioning or downtime and is generally expressed in minutes.
• Unavailability = (1 – Availability)*365*24*60
• Unavailability(U)=MTTR/MTBF
• The unavailability of a 99.999% available system is 5.3 minutes per year.
• Availability is generally measured as either failure rates  or mean time before failure (MTBF).
• Availability calculations always assume a bi-directional system.

2.Circuit vs. Nodal Availability

Circuit and nodal availability measure different quantities.  To help explain this clearly un-availability (Unavailability=1-Availablity) will be used in this section.

• Circuit un-availability is a measure of the average down time of a traffic demand / service.
  • A circuit is un-available only if traffic affecting components that help transport the demand / service have failed.
  • Circuit unavailability is calculated by considering the unavailabilities of components which are traffic affecting and by taking into consideration those components that are hardware protected.
  • For example, the failure of both 10G line cards on an NE can cause a traffic outage.
• Nodal un-availability is a measure of the average down time of a node.
  • Each time there is a failure in a node, regardless if it is traffic affecting or not, an engineer is required to visit the node to fix the failure.
  • Therefore nodal un-availability is based on calculated failure rates, it is still a direct measure of an operational expenditure.
  • Nodal unavailability is calculated by adding all components of a network element regardless of hardware protection, i.e. in series.
  • For example, failure of a protected switch card is non-traffic affecting but still requires a site visit to be replaced.

3.Terms & Definitions

Failure rate

•  Failure rate is usually measured as Failures in Time (FIT), where one FIT equals a single failure in one billion (10⁹) hours of operation.
•  FITs are calculated according to industry standard (Telcordia SR 332).

MTBF- (Mean time between failure)

•  Average time between failures for a given component.
•  Measured either in hours or years.
• MTBF is inversely proportional to FITs.

MTTR-(Mean time to repair)

•  Average time to repair a given failure.
•  Measured in hours.
•  Availability is always quoted in terms of number of nines
•  For example, carrier grade is 5 9’s, which is 99.999%
• Availability is better understood in terms of unavailability in minutes per year
• Therefore for an availability of 99.999%, the unavailability or downtime is 5.3 minutes per year

Just prior to transmission, the entire SONET signal, with the exception of the framing bytes and the section trace byte, is scrambled.  Scrambling randomizes the bit stream is order to provide sufficient 0–>1 and 1–>0 transitions for the receiver to derive a clock with which to receive the digital information.

Actually every add/drop multiplexer sample incoming bits according to a particular clock frequency. Now this clock frequency is recovered by using transitions between 1s and 0s in the incoming OC-N signal. Suppose, incoming bit stream contains long strings of all 1s or all 0s. Then clock recovery would be difficult. So to enable clock recovery at the receiver such long strings of all 1s or 0s are avoided. This is achieved by a process called Scrambling.

Scrambler is designed as shown in the figure given below:-

It is a frame synchronous scrambler of sequence length 127. The generating polynomial is 1+x⁶+x⁷. The scrambler shall be reset to ‘1111111’ on the most significant byte following Z0 byte in the Nth STS-1. That bit and all subsequent bits to be scrambled shall be added, modulo 2, to the output from the x⁷ position of the scrambler, as shown in Figure above.Example:

The first 127 bits are:

111111100000010000011000010100 011110010001011001110101001111 010000011100010010011011010110 110111101100011010010111011100 0101010

The same operation is used for descrambling. For example, the input data is 000000000001111111111.

        00000000001111111111  <-- input data
        11111110000001000001  <-- scramble sequence
        --------------------  <-- exclusive OR (scramble operation)
        11111110001110111110  <-- scrambled data
        11111110000001000001  <-- scramble sequence
        --------------------  <-- exclusive OR (descramble operation)
        00000000001111111111  <-- original data

The framing bytes A1 and A2, Section Trace byte J0 and Section Growth byte Z0 are not scrambled to avoid possibility that bytes in the frame might duplicate A1/A2 and cause an error in framing. The receiver searches for A1/A2 bits pattern in multiple consecutive frames, allowing the receiver to gain bit and byte synchronization. Once bit synchronization is gained, everything is done, from there on, on byte boundaries – SONET/SDH is byte synchronous, not bit synchronous.

An identical operation called descrambling is done at the receiver to retrieve the bits.

Scrambling is performed by XORing the data signal with a pseudo-random bit sequence generated by the scrambler polynomial indicated above.  The scrambler is frame synchronous, which means that it starts every frame in the same state.

Descrambling is performed by the receiver by XORing the received signal with the same pseudo random bit sequence.  Note that since the scrambler is frame synchronous, the receiver must have found the frame alignment before the signal can be descrambled.  That is why the frame bytes(A1A2) are not scrambled.

References:http://www.electrosofts.com/sonet/scrambling.html

Frequency justification and pointers:+/-ve Stuffing mechanism in SONET/SDH

When the input data has a rate lower than the output data rate of a multiplexer, the positive stuffing will occur. The input is stored in a buffer at a rate which is controlled by the WRITE clock. Since the output (READ) clock rate is higher than the WRITE clock rate, the buffer content will be depleted or emptied. To avoid this condition, the buffer fill is constantly monitored and compared to a threshold. If the the content fill is below a threshold, the READ clock is inhibited and stuffed bit is inserted to the output stream. Meanwhile, the input data stream is still filling the buffer. The stuffed bit location information must be transmitted to the receiver so that the receiver can remove the stuffed bit.

When the input data has a rate higher than the output data rate of a multiplexer, the negative stuffing will occur. If negative stuffing occur, the extra data can be transmitted through an other channel. The receiver must need to kown how to retrieve the data.

Positive Stuffing

If the frame rate of the STS SPE is too slow with respect to the frame rate then the alignment of the envelope should periodically slip back or the pointer should be incremented by one periodically. This operation is indicated by inverting the I bits of the 10 bit pointer. The byte right after the H3 byte is the stuff byte and should be ignored. The following frames should contain the new pointer. For example, the 10 bit of the H1 and H2 pointer bytes has the value of ‘0010010011’ for STS-1 frame N.

        Frame #     IDIDIDIDID
        ----------------------
          N         0010010011
          N+1       1000111001  <-- the I bits are inverted, positive stuffing
                                    is required.
          N+2       0010010100  <-- the pointer is increased by 1

Negative Stuffing

If the frame rate of the STS SPE is too fast with respect to the frame rate then the alignment of the envelope should periodically advance or the pointer should be decremented by one periodically. This operation is indicated by inverting the D bits of the 10 bit pointer. The H3 byte is containing actual data. The following frames should contain the new pointer. For example, the 10 bit of the H1 and H2 pointer bytes has the value of ‘0010010011’ for STS-1 frame N.

        Frame #     IDIDIDIDID
        ----------------------
          N         0010010011
          N+1       0111000110  <-- the D bits are inverted, negative stuffing
                                    is required.
          N+2       0010010010  <-- the pointer is decreased by 1

SDH & SONET conversion deliverables: Number of Equivalent Voice Channels, SONET Virtual Tributary, SDH Virtual   Container, SONET Carrier, SDH Carrier & Administrative Unit.

For Education Purpose:Please download and use in slideshow mode for better view.

SDH/SONET alarms & performance monitoring from MapYourTech

The hex code F628 is transmitted in every frame of every STS-1.

This allows a receiver to locate the alignment of the 125usec frame within the received serial bit stream.  Initially, the receiver scans the serial stream for the code F628.  Once it is detected, the receiver watches to verify that the pattern repeats after exactly 810 STS-1 bytes, after which it can declare the frame found. Once the frame alignment is found, the remaining signal can be descrambled and the various overhead bytes extracted and processed.

Just prior to transmission, the entire SONET signal, with the exception of the framing bytes and the section trace byte, is scrambled.  Scrambling randomizes the bit stream is order to provide sufficient 0-1 and 1-0 transitions for the receiver to derive a clock with which to receive the digital information.

Scrambling is performed by XORing the data signal with a pseudo-random bit sequence generated by the scrambler polynomial indicated above.  The scrambler is frame synchronous, which means that it starts every frame in the same state.

Descrambling is performed by the receiver by XORing the received signal with the same pseudo random bit sequence.  Note that since the scrambler is frame synchronous, the receiver must have found the frame alignment before the signal can be descrambled.  That is why the frame bytes are not scrambled.

One more interesting answer from Standardisation Expert ITU-T Huub van Helvoort:-

The initial “F” is to provide four consecutive bits for the clock recovery circuit to lock the clock. The number of “0” and “1”in F628 is equal (=8) to take care that there is a DC balance in the signal (after line coding).

It has nothing to do with the scrambling, the pattern may even occur in the scrambled signal, however it is very unlikely that that occurs exactly every 125 usec so there will not be a false
lock to this pattern.

Explanation to Huub’s dc balancing w.r.t NEXT GENERATION TRANSPORT NETWORKS Data, Management, and Control Planes by Manohar Naidu Ellanti:

A factor that was not initially anticipated when the Al and A2 bit patterns were chosen was the potential affect on the laser for higher rate signals. For an ST S-N signal, the frame begins with N adjacent Al bytes followed by N adjacent A2 bytes. Note that for Al, there are more ls than Os, which means that the laser is on for a higher percentage of the time when the Al byte than for A2, in which there are more Os than ls. The A2 byte has fewer ls than Os. As a result, if the laser is directly modulated, for large values of N, the lack of balance between Os and 1s causes the transmitting laser to become hotter during the string of A 1 bytes and cooler during the string of A2 bytes. The thermal drift affects the laser performance such that the signal level changes, making it difficult for the receiver threshold detector to track. Most high-speed systems have addressed this problem by using a laser that is continuously on and modulate the signal with a shutter after the laser.

Techniques to Combat Osnr in Dwdm Links

>> SPECIFICATION COMPARISION

>CHARACTERISTICS COMPARISON 

>>WDM BAND

*****************************************************************************************************************************References

EDFA-Erbium Doped Fiber Amplifier

EDFA block

RAMAN AMPLIFIER

SOA-Semiconductor Optical Amplifier

Introduction

Automatic Protection Switching (APS) is one of the most valuable features of SONET and SDH networks. Networks with APS quickly react to failures, minimizing lost traffic, which minimizes lost revenue to service providers.  The network is said to be “self healing.”  This application note covers how to use Sunrise Telecom SONET and SDH analyzers to measure the amount of time it takes for a network to complete an automatic protection switchover.  This is important since ANSI T1.105.1 and ITU-T G.841 require that a protection switchover occur within 50 ms.  To understand the APS measurement, a brief review is first given.  This is followed by an explanation of the basis behind the APS measurement.  The final section covers how to operate your Sunrise Telecom SONET and SDH equipment to make an APS time measurement.

What Is APS?

Automatic Protection Switching keeps the network working even if a network element or link fails.  The Network Elements (NEs) in a SONET/SDH network constantly monitor the health of the network.  When a failure is detected by one or more network elements, the network proceeds through a coordinated predefined sequence of steps to transfer (or switchover) live traffic to the backup facility (also called “protection” facility).  This is done very quickly to minimize lost traffic.  Traffic remains on the protection facility until the primary facility (working facility) fault is cleared, at which time the traffic may revert to the working facility.

In a SONET or SDH network, the transmission is protected on optical sections from the Headend (the point at which

the Line/Multiplexer Section Overhead is inserted) to the Tailend (the point where the Line/Multiplexer Section Overhead is terminated).

The K1 and K2 Line/Multiplexer Section Overhead bytes carry an Automatic Protection Switching protocol used to coordinate protection switching between the headend and the tailend.

The protocol for the APS channel is summarized in Figure 1.  The 16 bits within the APS channel contain information on  the APS configuration, detection of network failure, APS commands, and revert commands.  When a network failure is detected, the Line/Multiplexer Section Terminating Equipment communicates and coordinates the protection switchover by changing certain bits within the K1 & K2 bytes.

During the protection switchover, the network elements signal an APS by sending AIS throughout the network.  AIS is  also present at the ADM drop points.  The AIS condition may come and go as the network elements progress through their algorithm to switch traffic to the protection circuit.

AIS signals an APS event. But what causes the network to initiate an automatic protection switchover? The three most common are:

• • Detection of AIS (AIS is used to both initiate and signal an APS event)
• • Detection of excessive B2 errors
• • Initiation through a network management terminal

According to GR-253 and G.841, a network element is required to detect AIS and initiate an APS within 10 ms.  B2 errors should be detected according to a defined algorithm, and more than 10 ms is allowed.  This means that the entire time for both failure detection and traffic restoration may be 60 ms or more (10 ms or more detect time plus 50 ms switch time).

Protection Architectures

There are two types of protection for networks with APS:

• Linear Protection, based on ANSI T1.105.1 and ITU-T G.783 for point-to-point (end-to-end) connections.
• Ring Protection, based on ANSI T1.105.1 and ITU-T G.841 for ring structures (ring structures can also be found with two

types of protection mechanisms – Unidirectional and Bidirectional rings).

Refer to Figures 2-4 for APS architectures and unidirectional vs. bidirectional rings.

Protection Switching Schemes

The two most common schemes are 1+1 protection switching and 1:n protection switching.  In both structures, the K1 byte contains both the switching preemption priorities (in bits 1 to 4), and the channel number of the channel requesting action (in bits 5 to 8).  The K2 byte contains the channel number of the channel that is bridged onto protection (in bits 1 to 4), and the mode type (in bit 5); bits 6 to 8 contain various conditions such as AIS-L, RDI-L, indication of unidirectional or bidirectional switching.

1+1

In 1+1 protection switching, there is a protection facility (backup line) for each working facility.  At the headend, the optical signal is bridged permanently (split into two signals) and sent over both the working and the protection facilities simultaneously, producing a working signal and a protection signal that are identical.  At the tailend, both signals are monitored independently for failures.  The receiving equipment selects either the working or the protection signal.  This selection is based on the switch initiation criteria which are either a signal fail (hard failure such as the loss of frame (LOF) within an optical signal), or a signal degrade (soft failure caused by a bit error ratio exceeding some predefined value).

Refer to Figure 5.

Normally, 1+1 protection switching is unidirectional, although if the line terminating equipment at both ends supports bidirectional switching, the unidirectional default can be overridden.  Switching can be either revertive (the flow reverts to the working facility as soon as the failure has been corrected) or nonrevertive (the protection facility is treated as the working  facility)

In 1+1 protection architecture, all communications from the headend to the tailend are carried out over the APS channel via the K1 and K2 bytes. In 1+1 bidirectional switching, the K2 byte signaling indicates to the headend that a facility has been switched so that it can start to receive on the now active facility.

1:n

In 1:n protection switching, there is one protection facility for several working facilities (the range is from 1 to 14) and all communications from the headend to the tailend are carried out over the APS channel via the K1 and K2 bytes.  All switching is revertive, that is, the traffic reverts back to the working facility as soon as the failure has been corrected. Refer to Figure 6.

Optical signals are normally sent only over the working facilities, with the protection facility being kept free until a working facility fails.  Let us look at a failure in a bidirectional architecture.  Suppose the tailend detects a failure on working facility 2.  The tailend sends a message in bits 5-8 of the K1 byte to the headend over the protection facility requesting switch action.  The headend can then act directly, or if there is more than one problem, the headend decides which is top priority.  On a decision to act on the problem on working facility 2, the headend carries out the following steps:

1. Bridges working facility 2 at the headend to the protection facility.
2. Returns a message on the K2 byte indicating the channel number of the traffic on the  protection channel to the tailend
3. Sends a Reverse Request to the tailend via the K1 byte to initiate bidirectional switch.

On receipt of this message, the tailend carries out the following steps:

1. 1. Switches to the protection facility to receive
   2. Bridges working facility 2 to the protection facility to transmit back

Now transmission is carried out over the new working facility.

Switch Action Comments

In unidirectional architecture, the tailend makes the decision on priorities. In bidirectional architecture, the headend makes the decision on priorities. If there is more than one failure at a time, a priority hierarchy determines which working facility will be backed up by the protection facility. The priorities that are indicated in bits 1-4 of the K1 byte are as follows:

1. Lockout
2. Forced switch (to protection channel regardless of its state) for span or ring; applies only to 1:n switching
3. Signal fails (high then low priority) for span or ring
4. Signal degrades (high then low priority) for span or ring; applies only to 1:n switching
5. Manual switch (to an unoccupied fault-free protection channel) for span or ring; applies only to 1+1 LTE
6. Wait-to-restore
7. Exerciser for span or ring (may not apply to some linear APS systems)
8. Reverse request (only for bidirectional)
9. Do not revert (only 1+1 LTE provisioned for nonrevertive switching transmits Do Not Revert)
10. No request

Depending on the protection architecture, K1 and K2 bytes can be decoded as shown in Tables 1-5.

Linear Protection (ITU-T G.783)

Ring Protection (ITU-T G.841)

collected from Sunrise Telecom APS application note..

GFP-T is very good for isocronic or delay sensitive-protocols, and also for Storage Area Networks (SAN) such as ECON or FICON. This is because it is not necessary to process client frames or to wait for arrival of the complete frame. This advantage is counteracted by lost efficiency, because the source MSPP node still generates traffic when no data is being received from the client. 

GFP enables MSPP nodes to offer both TDM and packet-oriented services, managing transmission priorities and discard eligibility. GFP replaces legacy mappings, most of them of proprietary nature. In principal GFP is just an encapsulation procedure but robust and standardised for the transport of packetised data on SDH and OTN as well.

GFP uses a HEC-based delineation technique similar to ATM, it therefore does not need bit or byte stuffing. The frame size can be easily set up to a constant length.

When using GFP-F mode there is an optional GPF extension Header (eHEC) to be used by each specific protocol such us source/destination address, port numbers, class of service, etc. Among the EXI types – ‘linear’ supports submultiplexing onto a single path, ‘Channel ID’ (CID) enables sub-multiplexing over one VC channel GFP-F mode.

CONCATENATION

Concatenation is the process of summing the bandwidth of X containers (C-i) into a larger container. This provides a bandwidth X times bigger than C-i. It is well indicated for the transport of big payloads requiring a container greater than VC-4, but it is also possible to concatenate low-capacity containers, such as VC-11,VC-12, or VC-2.

Figure An example of contiguous concatenation and virtual concatenation. Contiguous concatenation requires support by all the nodes. Virtual concatenation allocates bandwidth more efficiently, and can be supported by legacy installations.

There are two concatenation methods:

1. Contiguous concatenation: which creates big containers that cannot split into smaller pieces during transmission. For this, each NE must have a concatena- tion functionality.
2. Virtual concatenation: which transports the individual VCs and aggregates them at the end point of the transmission path. For this, concatenation functionality is only needed at the path termination equipment.

Contiguous Concatenation of VC-4

A VC-4-Xc provides a payload area of X containers of C-4 type. It uses the same HO-POH used in VC-4, and with identical functionality. This structure can be transported in an STM-n frame (where n = X). However, other combinations are also possible; for instance, VC-4-4c can be transported in STM-16 and STM-64 frames. Concatenation guarantees the integrity of a bit sequence, because the whole container is transported as a unit across the whole network .

Obviously, an AU-4-Xc pointer, just like any other AU pointer, indicates the position of J1, which is the first byte of the VC-4-Xc container. The pointer takes the same value as the AU-4 pointer, while the remaining bytes take fixed values equal to Y=1001SS11 to indicate concatenation. Pointer justification is carried out the same way for all the X concatenated AU-4s and X x 3 stuffing bytes .

However, contiguous concatenation, today, is more theory than practice, since other alternatives more bandwidth-efficient, such as virtual concatenation, are gaining more importance.

Virtual Concatenation

Connectionless and packet-oriented technologies, such as IP or Ethernet, do not match well the bandwidth granularity provided by contiguous concatenation. For example, to implement a transport requirement of 1 Gbps, it would be necessary to allocate a VC4-16c container, which has a 2.4-Gbps capacity. More than double the bandwidth that is needed.

Virtual concatenation (VCAT) is a solution that allows granular increments of bandwidth in single VC-n units. At the MSSP source node VCAT creates a continuous payload equivalent to X times the VC-n units . The set of X containers is known virtual container group (VCG) and each individual VC is a member of the VCG. All the VC members are sent to the MSSP destination node independently, using any available path if necessary. At the destination, all the VC-n are organized, according the indications provided by the H4 or the V5 byte, and finally delivered to the client

Differential delays between VCG member are likely because they are transported individually and may have used different paths with different latencies. Therefore, the destination MSSP must compensate for the different delays before reassembling the payload and delivering the service.

Virtual concatenation is required only at edge nodes, and is compatible with legacy SDH networks, despite the fact that they do not support concatenation. To get the full benefit from this, individual containers should be transported by different routes across the network, so if a link or a node fails the connection is only partially affected. This is also a way of providing a resilience service.

 Higher Order Virtual Concatenation

Higher Order Virtual Concatenation (HO-VCAT) uses X times VC-3 or VC4 containers (VC-3/4-Xv, X = 1 … 256), providing a payload capacity of X times 48384 or 149760 kbit/s.

The virtual concatenated container VC-3/4-Xv is mapped in independent VC-3 or VC-4 envelopes that are transported individually through the network. Delays could occur between the individual VCs, this obviously has to be compensated for when the original payload is reassembled .

A multiframe mechanism has been implemented in H4 to compensate for differential delays of up to 256ms:

• Every individual VC has a H4 multiframe indicator (MFI) that denotes the virtual container they belong to
• The VC also traces its position X in the virtual container using he SEQ number which is carried in H4. The SEQ is repeated every 16 frames

The H4 POH byte is used for the virtual concatenation-specific sequence and multiframe indication

 Lower Order Virtual Concatenation

Lower Order Virtual Concatenation LO-VCat. uses X times VC-11, VC12, or VC2 containers (VC-11/12/2-Xv, X = 1 … 64).

A VCG built with V11, VC12 or VC2 members provides a payload of X containers C11, C12 or C4; that is a capacity of X times 1600, 2176 or 6784 kbit/s. VCG members are transported individually through the network, therefore differential delays could occur between the individual components of a VCG, that will be compensated for at the destination node before reassembling the original continuous payload ).

A multiframe mechanism has been implemented in bit 2 of K4. It includes a sequence number (SQ) and the multiframe indicator (MFI), both enable the reordering of the VCG members. The MSSP destination node will wait until the last member arrives and then will compensate for delays up to 256ms. It is important to note that K4 is a multiframe itself, received every 500ms, then the whole multiframe sequence is repeated every 512 ms.

Testing VCAT

When installing or maintaining VCAT it is important to carry out a number of tests to verify not only the performance of the whole Virtual Concatenation, but also every single member of the VCG. For reassembling the original client data, all the members of the VCG must arrive at the far end, keeping the delay between the first and the last member of a VCG below 256 ms. A missing member prevents the

reconstruction of the payload, and if the problem persists causes a fault that would call for the reconfiguration of the VCAT pipe. Additionally, Jitter and Wander on individual paths can cause anomalies (errors) in the transport service.

BER, latency, and event tests should verify the capacity of the network to perform the service. The VCAT granularity capacity has to be checked as well, by adding/removing. To verify the reassembly operation it is necessary to use a tester with the capability to insert differential delays in individual members of a VC.

Single-mode fibre selection for Optical Communication System

 This is collected from article written by Mr.Joe Botha

Looking for a single-mode (SM) fibre to light-up your multi-terabit per second system? Probably not, but let’s say you were – the smart money is on your well-intended fibre sales rep instinctively flogging you ITU-T G.652D  fibre. Commonly referred to as standardSM fibre and also known as Non-Dispersion-Shifted Fibre (NDSF) – the oldest and most widely deployed fibre. Not a great choice, right? You bet.  So for now, let’s resist the notion that you can do whatever-you-want using standardSM fibre. A variety of  SM optical  fibres with  carefully optimised characteristics are  available  commercially: ITU-T G.652, 653, 654, 655, 656 or 657 compliant.

Designs of SM fibre have evolvedover the decadesand present-day optionswould have us deploy G.652D, G.655 or G.656 compliantfibres. Note that G.657A is essentially a more expressive version of G.652D,with a superiorbending loss performance and should you start feeling a little benevolent towards deploying it on a longish-haul – I  can immediately confirm that this allows for a glimpse into the workingsof silliness. Dispersion Shifted Fibre (DSF) in accordance with G.653 has no chromatic dispersion at 1550 nm. However,they are limited to single-wavelength operation due to non-linear four-wave mixing. G.654 compliant fibres were developed specifically for underseaun-regenerated systems and since our focus is directed toward terrestrial applications – let’s leave it at that.

In the above context, the plan is to briefly weigh up G.652D,G.655 and G.656 compliantfibres against three parameters we calculate (before installation) and measure (after installation). I must just point-out that the fibre coefficients used are what one would expect from the not too shabby brands availabletoday.

Attenuation

Attenuation is the reduction or loss of opticalpower as  light travels through an opticalfibre and is measured in decibelsper kilometer (dB/km). G.652D offers respectable attenuation coefficients, when compared with G.655 and G.656. It should be remembered, however, that even a meagre 0.01 dB/km attenuation

improvement would reduce a 100 km loss budget by a full dB – but let’s not quibble. No attenuation coefficients for G.655 and G.656 at 1310? It was not, as you may immediately assume, an oversight. Both G.655 and G.656 are optimizedto support long-haul systems and thereforecould not care less about runningat 1310 nm. A cut-offwavelength is the minimum wavelength at which a particular fibre will support SM transmission. At ≤ 1260 nm, G.652 D has the lowest cut-off wavelength – with the cut-off wavelengths for G.655 and G.656 sittingat ≤ 1480 nmand ≤1450 respectively – which explainswhy we have no attenuation coefficient for them at 1310 nm.

PMD

Polarization-mode dispersion (PMD) is  an  unwanted effect caused by asymmetrical properties in an opticalfibre that spreads the optical pulse of a signal. Slight asymmetry in an

optical fibre causes the polarized modes of the light pulse to travel at marginally different speeds, distorting the signal and is reportedin ps / √km, or “ps per root km”. Oddly enough,G.652 and co all possess decent-looking PMD coefficients. Now then, popping a 40-Gbpslaser onto my fibre up againstan ultra-low 0.04 ps / √km, my calculator reveals that the PMD coefficient admissible fibre length is 3,900 km and even at 0.1 ps / √km, a distance of 625 km is achievable.

So far so good? But wait, there’s more. PMD is particularly troublesome for both high data-rate-per-channel and high wavelength channel count systems, largely because of its random nature.Fibre manufacturer’s PMD specifications are accuratefor the fibre itself,but do not incorporate PMD incurred as a result of installation, which in many cases can be many orders of magnitude larger. It is hardly surprising that questionable installation practices are  likely  to  cause imperfect fibre symmetry – the obvious implications are incomprehensible data streams and mental anguish.Moreover, PMD unlikechromatic dispersion (to be discussednext) is also affectedby environmental conditions, making it unpredictable and extremelydifficult to find ways to undo or offset its effect.

CD

CD (calledchromatic dispersion to emphasiseits wavelength-dependent nature) has zip-zero to do with the loss of light. It occurs because different wavelengths  of light travel at differentspeeds. Thus, when the allowable

CD is exceeded – light pulses representing a bit-stream will be renderedillegible. It is expressedin ps/ (nm·km). At 2.5- Gbps CD is not an issue – however, lower data rates are seldom desirable. But at 10-Gbps,it is a big issue and the issue gets even bigger at 40-Gbps.

What’s troubling is G.652D’shigh CD coefficient – which one glumly has to concede, is very poor next  to  the competition. G.655 and G.656, variants of non-zerodispersion-shifted fibre (NZ-DSF), comprehensively address G.652D’s shortcomings. It should be noted that nowadays some optical fibre manufacturers don’t bother with distinguishing between G.655 and G.656 – referring to their offerings as G.655/6 compliant.

On the face of it, one might suggest that the answer to our CD problemis to send light along an optical fibre at a wavelength where the CD is zero (i.e. G.653).The result? It turns out that this approach creates more problems than it is likely to solve – by unacceptably amplifying non-linear four-wave mixing and limiting the fibre to single-wavelength operation- in other words, no DWDM. That, in fact, is why CD should not be completely lampooned. Research revealed that the fibre-friendly CD value l i e s in the range of 6-11 ps/nm·km. Therefore, and particularly for high-capacity transport, the best-suited fibre is one in which dispersion is kept within a tight range, being neither too high nor too low.

NZ-DSFs are available in both positive(+D) and negative (-D) varieties. Using NZ-DSF -D, a reverse behavior of the velocity per wavelength is createdand therefore, the effect of +CD can be cancelled out. I almost forgot to mention,by the way, that short wavelengths travel faster than long ones with +CD and longer wavelengths travel faster than short ones with -CD. New sophisticated modulation techniques such as dual-polarized quadrature phase-shift keying (DP-QPSK) using coherent detection, yields high quality CD compensation. However, because of the addedsignal processing time (versussimple on-off keying) they require,this can potentially be a poor choice from a latency perspective.

WDM multiplies capacity

The use of Dense Wavelength Division Multiplexing (DWDM) technology and 40-Gbps(and higher) transmission rates can push the information-carrying capacityof a single fibre to well over a terabit per second.One example is EASSy’s (a 4-fibre submarine cable serving sub-Saharan Africa) 4.72-Tbps capacity. Now then, should my maffs prove to be correct, 118 x 40-Gbpslasers (popped onto only 4-fibres!) should give us an aggregatecapacity of 4.72-Tbps?

Coarse Wavelength Division Multiplexing (CWDM) is a WDM technology that uses 4, 8, 16 or 18 wavelengths for transmission. CWDM is an economically sensible option, often used for short-haul applications on G.652D,where signal amplification is not necessary. CWDMs large 20 nm channelspacing allows for the use of cheaper, less powerfullasers that do not require cooling.

One of the most important considerations in the fibre selectionprocess is the fact that optical signals may need to be amplified along a route. Thanks in no small part (get the picture?)to CWDM’s large channelspacing – typicallyspanning over several spectralbands (1270 nm to 1610 nm) – its signals cannot be amplifiedusing Erbium Doped-Fibre Amplifiers (EDFAs).You see, EDFAs run only in the C and L bands (1520 nm to 1625 nm). WhereasCWDM breaks the optical spectrum up into large chunks – by contrast,DWDM slices it up finely, cramming4, 8, 16, 40, 80, or 160 wavelengths (on 2-fibres)into only the C- and L-bands (1520nm to 1625nm) – perfectfor the use of EDFAs. Each wavelength can without any obvious effort support a 40-Gbps laser and on top of this, 100-Gbpslasers are chompingat the bit to go mainstream.

Making the right choice

On the whole, it is hard not to concludethat the only thing that genuinelyseparates fibre types for high-bit-rate systems is CD. The 3-things – the only ones that I can think of – that is good about G.652D – is that it is affordable, cool for CWDM and perfect for short-haul environments.Top  of the to-do lists of infrastructure providers pushing the boundaries of DWDM enabled ultra high-capacity transport over short, long or ultra long-haul networks – needlessto say, will be to source G.655/6compliant fibres. The cross-tabbelow indicates: Green for OK and oddly enough, Red for Not-OK