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

FWM CPM and SPM.pptx by Sanjay Yadav

dB make it more simpler by Sanjay Yadav

This is a tricky question because 12 a.m. and 12 p.m. are ambiguous and should not be used.

To illustrate this, consider that “a.m.” and “p.m.” are abbreviations for “ante meridiem” and “post meridiem,” which mean “before noon” and “after noon,” respectively. Since noon is neither before noon nor after noon, a designation of either a.m. or p.m. is incorrect. Also, midnight is both twelve hours before noon and twelve hours after noon.

It is fair to say, however, that the shortest measurable duration after noon should be designated as p.m. For example, it would be applicable for a digital clock changing from 11:59:59 a.m. to 12:00:00 to indicate p.m. as soon as it the 12:00 appears, and not delay the display of the p.m. by a minute, or even a second. The same is true for midnight, but there is an added issue of which day midnight refers to (see below).

Hours of operation for a business or other references to a block of time should also follow this designation rule.
For example, a business might be open on Saturdays from 8 a.m. to noon or weekends from 3:30 p.m. until midnight.

Dawn occurs at the time that the geometric center of the Sun is 18° below the horizon in the morning. Respectively, dusk occurs when the geometric center of the Sun is 18° below the horizon in the evening. Twilight refers to the periods between the dawn and sunrise and sunset and dusk, where sunrise and sunset are defined as the exact times when the upper edge of the disc of the Sun is at the horizon. The hazy light during this period is an effect caused by the scattering of the sunlight in the upper atmosphere and reflecting towards Earth. It is very subjective as far as the time of day that twilight occurs, because it depends on the location and elevation of the observer, the time of year and local weather conditions.

In addition, twilight is divided into three durations based on the angle of the Sun below the horizon. Astronomical twilight is the period when the Sun is between 18° and 12° below the horizon. Nautical twilight is the period when the Sun is between 12° and 6° below the horizon, and civil twilight is the period when the Sun is between 6° below the horizon until sunrise. The same designations are used for periods of evening twilight.

DWDM_good for Beginners by Sanjay Yadav

optical switching comprehensive guide by Sanjay Yadav

Optical modulation for High Baud Rate networks…. 40G/100G  Speed Networks……

>> What is Higher-Order Modulation Method?

>> Why Do We Need Higher-Order Modulation Methods?

>> A Brief Tutorial on Higher-Order Modulation Methods

• Significant improvement in minimum OSNR by reducing the signal bandwidth
• Compatibility with the 50 GHz ITU-T channel spacing or at least with a spacing of 100 GHz
• Coexistence with 10 Gbps systems
• Transmission in networks that use ROADMs
• Scalable for 100 Gbps

>> Current Higher-Order Optical Modulation Methods Status

Four Wave Mixing (FWM) in WDM System..

>> Nonlinear Effects in High Power, High Bit Rate Fiber Optic Communication Systems

>> Basic Principles of Four-Wave Mixing

In the above equation, the term (28) represents the effect of SPM and CPM. The terms (29), (31), and (32) can be neglected because of lack of phase matching. Under suitable circumstances, it is possible to approximately satisfy the phase-matching condition for the remaining terms, which are all of the form ω_i + ω_j – ω_k, I,j k (ω_i, ω_j not necessarily distinct).

ω_ijk = ω_i + ω_j – ω_k,       i k, j k

This equation assumes a link of length L without any loss and chromatic dispersion. Here P_i, P_j, and P_k denote the powers of the mixing waves and P_ijk the power of the resulting new wave,  is the nonlinear refractive index (3.0x 10^-8 μm²/W), and d_ijk is the so-called degeneracy factor.

How to Test a Fiber Optic System with an OTDR (Optical Time Domain Reflectomer)

>> The Optical Time Domain Reflectometer (OTDR)

OTDR is connected to one end of any fiber optic system up to 250km in length. Within a few seconds, we are able to measure the overall loss, or the loss of any part of a system, the overall length of the fiber and the distance between any points of interest. OTDR is a amazing test instrument for fiber optic systems.

1. A Use for Rayleigh Scatter

As light travels along the fiber, a small proportion of it is lost by Rayleigh scattering. As the light is scattered in all directions, some of it just happens to return back along the fiber towards the light source. This returned light is calledbackscatter as shown below.

The backscatter power is a fixed proportion of the incoming power and as the losses take their toll on the incoming power, the returned power also diminishes as shown in the following figure.

The OTDR can continuously measure the returned power level and hence deduce the losses encountered on the fiber. Any additional losses such as connectors and fusion splices have the effect of suddenly reducing the transmitted power on the fiber and hence causing a corresponding change in backscatter power. The position and degree of the losses can be ascertained.

2. Measuring Distances

The OTDR uses a system rather like a radar set. It sends out a pulse of light and ‘listens’ for echoes from the fiber.

If it knows the speed of light and can measure the time taken for the light to travel along the fiber, it is an easy job to calculate the length of the fiber.

3. To Find the Speed of the Light

Assuming the refractive index of the core is 1.5, the infrared light travels at a speed of

This means that it will take

This is a useful figure to remember, 5 nanoseconds per meter (5 nsm^-1).

If the OTDR measures a time delay of 1.4us, then the distance travelled by the light is

The 280 meters is the total distance traveled by the light and is the ‘there and back’ distance. The length of the fiber is therefore only 140m. This adjustment is performed automatically by the OTDR – it just displays the final result of 140m.

4. Inside the OTDR

A. Timer

The timer produces a voltage pulse which is used to start the timing process in the display at the same moment as the laser is activated.

B. Pulsed Laser

The laser is switched on for a brief moment. The ‘on’ time being between 1ns and 10us. We will look at the significance of the choice of ‘on’ time or pulsewidth a little bit later. The wavelength of the laser can be switched to suit the system to be investigated.

C. Directive Coupler

The directive coupler allows the laser light to pass straight through into the fiber under test. The backscatter from the whole length of the fiber approaches the directive coupler from the opposite direction. In this case the mirror surface reflects the light into the avalanche photodiode (APD). The light has now been converted into an electrical signal.

D. Amplifying and Averaging

The electrical signal from the APD is very weak and requires amplification before it can be displayed. The averaging feature is quite interesting and we will look at it separately towards the end of this tutorial.

E. Display

The amplified signals are passed on to the display. The display is either a CRT like an oscilloscope, or a LCD as in laptop computers. They display the returned signals on a simple XY plot with the range across the bottom and the power level in dB up the side.

The following figure shows a typical display. The current parameter settings are shown over the grid. They can be changed to suit the measurements being undertaken. The range scale displayed shows a 50km length of fiber. In this case it is from 0 to 50km but it could be any other 50km slice, for example, from 20km to 70km. It can also be expanded to give a detailed view of a shorter length of fiber such as 0-5m, or 25-30m.

The range can be read from the horizontal scale but for more precision, a variable range marker is used. This is a movable line which can be switched on and positioned anywhere on the trace. Its range is shown on the screen together with the power level of the received signal at that point. To find the length of the fiber, the marker is simply positioned at the end of the fiber and the distance is read off the screen. It is usual to provide up to five markers so that several points can be measured simultaneously.

F. Data Handling

An internal memory or floppy disk can store the data for later analysis. The output is also available via RS232 link for downloading to a computer. In addition, many OTDRs have an onboard printer to provide hard copies of the information on the screen. This provides useful ‘before and after’ images for fault repair as well as a record of the initial installation.

5. A Simple Measurement

If we were to connect a length of fiber, say 300m, to the OTDR the result would look as shown in the following figure.

Whenever the light passes through a cleaved end of a piece of fiber, a Fresnel reflection occurs. This is seen at the far end of the fiber and also at the launch connector. Indeed, it is quite usual to obtain a Fresnel reflection from the end of the fiber without actually cleaving it. Just breaking the fiber is usually enough.

The Fresnel at the launch connector occurs at the front panel of the OTDR and, since the laser power is high at this point, the reflection is also high. The result of this is a relatively high pulse of energy passing through the receiver amplifier. The amplifier output voltage swings above and below the real level, in an effect called ringing. This is a normal amplifier response to a sudden change of input level. The receiver takes a few nanoseconds to recover from this sudden change of signal level.

6. Dead Zones

The Fresnel reflection and the subsequent amplifier recovery time results in a short period during which the amplifier cannot respond to any further input signals. This period of time is called a dead zone. It occurs to some extent whenever a sudden change of signal amplitude occurs. The one at the start of the fiber where the signal is being launched is called the launch dead zone and others are called event dead zones or just dead zones.

7. Overcoming the Launch Dead Zone

As the launch dead zone occupies a distance of up to 20 meters or so, this means that, given the job of checking a 300m fiber, we may only be able to check 280m of it. The customer would not be delighted.

To overcome this problem, we add our own patch cord at the beginning of the system. If we make this patch cord about 100m in length, we can guarantee that all launch dead zone problems have finished before the customers’ fiber is reached.

The patch cord is joined to the main system by a connector which will show up on the OTDR readout as a small Fresnel reflection and a power loss. The power loss is indicated by the sudden drop in the power level on the OTDR trace.

8. Length and Attenuation

The end of the fiber appears to be at 400m on the horizontal scale but we must deduct 100m to account for our patch cord. This gives an actual length of 300m for the fiber being tested.

Immediately after the patch cord Fresnel reflection the power level shown on the vertical scale is about –10.8dB and at the end of the 300m run, the power has fallen to about –11.3 dB. A reduction in power level of 0.5 dB in 300 meters indicates a fiber attenuation of:

Most OTDRs provide a loss measuring system using two markers. The two makers are switched on and positioned on a length of fiber which does not include any other events like connectors or whatever as shown in the following figure.

The OTDR then reads the difference in power level at the two positions and the distance between them, performs the above calculation for us and displays the loss per kilometer for the fiber. This provides a more accurate result than trying to read off the decibel and range values from the scales on the display and having to do our own calculations.

9. An OTDR Display of a Typical System

The OTDR can ‘see’ Fresnel reflections and losses. With this information, we can deduce the appearance of various events on an OTDR trace as seen below.

A. Connectors

A pair of connectors will give rise to a power loss and also a Fresnel reflection due to the polished end of the fiber.

B. Fusion Splice

Fusion splices do not cause any Fresnel reflections as the cleaved ends of the fiber are now fused into a single piece of fiber. They do, however, show a loss of power. A good quality fusion splice will actually be difficult to spot owing to the low losses. Any signs of a Fresnel reflection is a sure sign of a very poor fusion splice.

C. Mechanical Splice

Mechanical splices appear similar to a poor quality fusion splice. The fibers do have cleaved ends of course but the Fresnel reflection is avoided by the use of index marching gel within the splice. The losses to be expected are similar to the least acceptable fusion splices.

D. Bend Loss

This is simply a loss of power in the area of the bend. If the loss is very localized, the result is indistinguishable from a fusion or mechanic splice.

10. Ghost Echoes (False Reflection)

In the following figure, some of the launched energy is reflected back from the connectors at the end of the patch cord at a range of 100m. This light returns and strikes the polished face of the launch fiber on the OTDR front panel. Some of this energy is again reflected to be re-launched along the fiber and will cause another indication from the end of the patch cord, giving a false, or ghost, Fresnel reflection at a range of 200m and a false ‘end’ to the fiber at 500m.

As there is a polished end at both ends of the patch cord, it is theoretically possible for the light to bounce to and fro along this length of fiber giving rise to a whole series of ghost reflections. In the figure a second reflection is shown at a range of 300m.

It is very rare for any further reflections to be seen. The maximum amplitude of the Fresnel reflection is 4% of the incoming signal, and is usually much less. Looking at the calculations, even assuming the worst reflection, the returned energy is 4% or 0.04 of the launched energy. The re-launched energy, as a result of another reflection is 4% of the 4% or 0.04² = 0.0016 x input energy. This shows that we need a lot of input energy to cause a ghost reflection.

A second ghost would require another two reflections giving rise to a signal of only 0.00000256 of the launched energy. Subsequent reflections die out very quickly as we could imagine.

Ghost reflections can be recognized by their even spacing. If we have a reflection at 387m and another at 774 then we have either a strange coincidence or a ghost. Ghost reflections have a Fresnel reflection but do not show any loss. The loss signal is actually of too small an energy level to be seen on the display. If a reflection shows up after the end of the fiber, it has got to be a ghost.

11. Effects of Changing the Pulsewidth

The maximum range that can be measured is determined by the energy contained within the pulse of laser light. The light has to be able to travel the full length of the fiber, be reflected, and return to the OTDR and still be of larger amplitude than the background noise. Now, the energy contained in the pulse is proportional to the length of the pulse so to obtain the greatest range the longest possible pulsewidth should be used as illustrated in the following figure.

This cannot be the whole story, as OTDRs offer a wide range of pulsewidths.

We have seen that light covers a distance of 1 meter every 5 nanoseconds so a pulsewidth of 100nm would extend for a distance of 20 meters along the fiber (see the following figure). When the light reaches an event, such as a connector, there is a reflection and a sudden fall in power level. The reflection occurs over the whole of the 20m of the outgoing pulse. Returning to the OTDR is therefore a 20m reflection. Each event on the fiber system will also cause a 20m pulse to be reflected back towards the OTDR.

Now imagine two such events separated by a distance of 10m or less as in the following figure. The two reflections will overlap and join up on the return path to the OTDR. The OTDR will simply receive a single burst of light and will be quite unable to detect that two different events have occurred. The losses will add, so two fusion splices for example, each with a loss of 0.2dB will be shown as a single splice with a loss of 0.4dB.

The minimum distance separating two events that can be displayed separately is called the range discrimination of the OTDR.

The shortest pulsewidth on an OTDR may well be in the order of 10ns so at a rate of 5nsm^-1 this will provide a pulse length in the fiber of 2m. The range discrimination is half this figure so that two events separated by a distance greater than 1m can be displayed as separate events. At the other end of the scale, a maximum pulsewidth of 10us would result in a range discrimination of 1 km.

Another effect of changing the pulsewidth is on dead zones. Increasing the energy in the pulse will cause a larger Fresnel reflection. This, in turn, means that the amplifier will take longer to recover and hence the event dead zones will become larger as shown in the next figure.

12. Which Pulsewidth to Choose?

Most OTDRs give a choice of at least five different pulse length from which to select.

Low pulse widths mean good separation of events but the pulse has a low energy content so the maximum range is very poor. A pulse width of 10ns may well provide a maximum range of only a kilometer with a range discrimination of 1 meter.

The wider the pulse, the longer the range but the worse the range discrimination. A 1us pulse width will have a range of 40 km but cannot separate events closer together than 100 m.

As a general guide, use the shortest pulse that will provide the required range.

13. Averaging

The instantaneous value of the backscatter returning from the fiber is very weak and contains a high noise level which tends to mask the return signal.

As the noise is random, its amplitude should average out to zero over a period of time. This is the idea behind the averaging circuit.

The incoming signals are stored and averaged before being displayed. The larger the number of signals averaged, the cleaner will be the final result but the slower will be the response to any changes that occur during the test. The mathematical process used to perform the effect is called least squares averaging or LSA.

The following figure shows the enormous benefit of employing averaging to reduce the noise effect.

Occasionally it is useful to switch the averaging off to see a real time signal from the fiber to see the effects of making adjustments to connectors etc. This is an easy way to optimize connectors, mechanical splices, bends etc. Simply fiddle with it and watch the OTDR screen.

14. OTDR Dynamic Range

When a close range reflection, such as the launch Fresnel occurs, the reflected energy must not be too high otherwise it could damage the OTDR receiving circuit. The power levels decrease as the light travels along the fiber and eventually the reflections are similar in level to that of the noise and can no longer be used.

The difference between the highest safe value of the input power and the minimum detectable power is called thedynamic range of the OTDR and, along with the pulse width and the fiber losses, determines the useful range of the equipment.

If an OTDR was quoted as having a dynamic range of 36 dB, it could measure an 18km run of fiber with a loss of 2 dB/km, or alternatively a 72 km length of fiber having a loss of 0.5 dB/km, or any other combination that multiplies out to 36 dB.

Why is BER difficult to simulate or calculate? 

For a given design at a BER (such as 10-12 and a line rate of OC-3, or 155 Mbps), the network would have one error in approximately 10 days. It would take 1000 days to record a steady state BER value. That is why BER calculations are quite difficult. On the other hand, Q-factor analysis is comparatively easy. Q is often measured in dB. The next question is how to dynamically calculate Q. This is done from OSNR.

In other words, Q is somewhat proportional to the OSNR. Generally, noise calculations are performed by optical spectrum analyzers (OSAs) or sampling oscilloscopes, and these

measurements are carried over a particular measuring range of Bm. Typically, Bm is approximately 0.1 nm or 12.5 GHz for a given OSA. From Equation 4-12, showing Q in dB in

terms of OSNR, it can be understood that if B0 < Bc, then OSNR (dB )> Q (dB). For practical designs OSNR(dB) > Q(dB), by at least 1–2 dB. Typically, while designing a high bitrate system, the margin at the receiver is approximately 2 dB, such that Q is about 2 dB smaller than OSNR (dB).