Understanding Synchronization Protection Basics in Transmission network

Let us considera ringnetwork. Normalsynchronizationworks arounda ring. In this case,Nodes B-F are line timed.Node A is timed to an external reference.When a sync source is failed,new time source should be selected in a reasonable amount of timeIfsynchronizationis not restored , BER will be increasing through time.

SPS Timing Loops (SPS =SynchronizationProtection Switching):Duringa ringfailure, simple reference switching would result in timing loops as shown below.

Operations Normal Flow :

Let us read about the operation. Following diagram shows the normal flow of operation.Synchronizationmessaging in normal operation.

S1 = Stratum 1 TraceableDU = Dont UseHO = Holdover

Operations Fiber Cut:In the ring, if fiber cuts between B & C,Node C goes into short term holdover as shown below.

Then,Node F switches to timing from Node A as shown below.

Finally ring is reconfigured and all nodes are again synchronized to BITS as shown below.

Please let me know if any clarification required :)

Basic SDH Network Topology & Advantages of SDHLet us read about the Basic SDHnetwork topology.Detailedtopology discussion will be done later.

Basic SDH Network Topology

SDH networks are usually deployed inprotected rings. This has the advantage of giving protection to the data, by providing analternate routefor it to travel over in the event of equipment ornetworkfailure.

Eachsideof the ring (known as A and B, or sometimes, East and West), consists of anindividualtransmit and receive fibre. Thesefibreswill take diverse physical paths to the distant end equipment to minimise the risk of both routes failing at the same time.

The SDH equipments have the ability to detect the problem and will automatically switch to thealternate route.

SDH multiplexers transmit on both sides of the ring simultaneously, But to speed up switching times, they only receive on one side at any time. This means that only the receiving end needs to switch, thus reducing the impact of a fault on the customers' data.Features and Advantages of SDHIn previous post we have seen the limitations ofPDH. Now let us see the advantages of SDH.

SDH permits the mixing of the existing European and North American PDH bit rates.

All SDH equipment is based on the use of a single masterreferenceclock source & hence SDH is synchronous.

Compatible with the majority of existing PDH bit rates

SDH provides for extraction/insertion, of a lower order bit rate from a higher order aggregate stream, without the need to de-multiplex in stages.

SDH allows for integrated management using a centralisednetworkcontrol.

SDH provides for a standard optical interface thus allowing the inter-working of different manufacturers equipment.

Increase innetworkreliability due to reduction of necessary equipment/jumpering.Scrambling SDH signal, why scrambler is used in SDH?

In SDH/SONET system, receivers recover clock based on incoming signal. Insufficient number of 0-1 transitions causes degradation of clockperformance. In order to avoid this problem and to guarantee sufficient transitions, SONET/SDH employ ascrambler.

All data except first row of section overhead is scrambled .Scrambleris 7 bit self-synchronizing X7+ X6+ 1 .Scrambleris initialized with ones

This type of shortscrambleris sufficient for voice data. But this is not sufficient for data which may contain long stretches of zeros. So, while sending data an additionalpayloadscrambleris used.

This modern standards use 43 bit X43+ 1. It run continuously on ATM payload bytes (suspended for 5 bytes of cell tax) . Run continuously on HDLC payloads

Scrambler:

Types of switching in SDH Rings.SPAN SWITCHING & RING SWITCHINGSpan switching :This type of switching uses only protection fibers on the span where fault is detected.

Ring switching :In this type of switching, traffic is switched away from filed span to adjacent node via the protection fibers on the long path.

REVERTIVE & NON-REVERTIVE SWITCHINGWe can implement two modes of protection switching in SDH networks, revertive or non-revertive.

In revertive switching, once the fault condition has cleared,the networkenters a "wait to restore state. One the configured WTR time is elapsed, traffic will be switched back to main path. This will be useful, wjen main path is much shorter than protection path.

In non-revertive mode,evenafterclearanceof fault condition, traffic will notswitch backto main path automatically.How Protection Switching is implemented in SDH? For protection switching, mainly K1, K2 bytes and B2 bytes in Multiplex Section Overhead of SDH frame are used. Normally K bytes carried in protection fiber are used to carry APS protocol. B2 bytes contain bit interleaved parity check of the previously transmitted MSOH plus the VC-n payload.

K1/K2 Byte strycture:

K1/K2 byte structure is as shown in above diagram. Upto maximum 16 nodes can be supported in a SDH ring with protection. This is because, only 4 bytes are used for source and destination ID. In 4F-rings, APS protocol is only active on the protection fibers. APS protocol is optimised for AU level of operation. Each node in the ring should be configured with a ring map. This ring map contains information about the channels that node handles. Also, Each node in the ring is given a unique Id number within the range 0 to 15.

At any point of time, each node will be knowing the current status of the ring ( normal or protected). When the protection switches are not active, each node sends K-bytes in each direction indicating "no bridge request". At the time of failure in the ring between two adjacent nodes, two paths may exist for communication. Short path is the one, which directly connects both the nodes. Longer path connects these two nodes via all other nodes on the ring. When a node receives a non-idle K-byte message containing a destination ID of another node on the ring, that node will change to pass through mode.

Let us read about types of ring protection in next postBLSR,Bi-directional Line Switched ringThere are two types of BLSR deployed in various networks.i. 2-fiber BLSRii. 4-fiber BLSR

2-fiber BLSR:This system is also known astwo fiber multiplex-section shared protection ring.Here, servicetraffic flowsbi-directionally. Both the fibers carries service and protectionchannels.

When the protectionchannelsare not required, they can be used to carry extra traffic, but at the time of protection switching, this extra traffic is dropped. Only ring switching is supported by this architecture. At the time of ring switching, thosechannelscarrying service traffic are switched to thechannelsthat carry the protection traffic in the opposite direction.

4-Fiber BLSR:This system is also known as four-fiber multiplex-section shared protection ring. This is the most robust ring architecture. This is most expensive to implementbecause of the extra optical hardware required.

In this system, bi-directional pairs of fibers are used toconnecteach span in the ring. One bi-directional pair carries theworkingchannels, while the other pair carries protectionchannels. 4F-BLSR supports both span switching and ring switching. ( but both not at the same time). Multiple span switches can coexist on the ring. This is because, only the protectionchannelsalong one span are used for each span switch.

What triggers a protection?Protection switching is triggered in following cases.

1. Signal Fail , detected as Loss of Signal (LOS) at receiver input. This may be due to faulty hardware in the upstreamnetworkequipment or due to broken fiber.2.Signal degrade, this is monitored bymonitoringB2 bytes.

In next post let us read about :How is protection switching implemented in SDH ( K1 & K2 Bytes)Ring networks - G.841 - Interview notes for UPSR Further toLinear protection, let us read about ring protection. ITU-T recommendation covers several types of ringnetworkarchitectures. Ring protection switching can be implemented at path level or at line level. Rings can be uni-directional or bi-directional. & they may utilise 2-fiber or 4-fibers.

UPSR : Uni-directional Path Switched Ring :In a uni-directional ring, servicetraffic flowsinone direction. ( clockwise in belowdiagram). Protectiontraffic flowsin opposite direction (counter clockwise)

In this example, traffic from C to B travels in clockwise direction via A. Traffic from B to C travels directly in clockwise direction. Thisconfigurationis also known asmultiplex section dedicated protection ring.This is because, one fiber carries service traffic, while the other is dedicated to protect the main path.All traffic is added in both directions. Decision as to which to use at drop point (no signaling).Normally non-revertive, so effectively twodiversitypaths

Main advantage of this configurations are :single ended switching, simple to implement and does not require any protocol. Single ended switching is always faster while compared to dual ended switching. Chances of restoring traffic under multiple fail conditions is high. Also, implementation of this architecture is least expensive.

However this arcitecture is Inefficient for core networks. There isno spatial reuse. Node needs to continuouslymonitorevery tributary to be dropped.

In next post let us read aboutBLSR - Bi-directional line switched ringTraffic Protection on SDH Optical Networks, Interview notes for SDH protection

Service survivability has become more important than ever. This is because telecommunication is used increasingly for vital transactions such as electronic fundtransfer, order processing, inventory control & many other business activities ( e.g : e-mail, internet access). Users are willing to pay more to get guaranteed service.

In SDH transmission system, Automatic Protection Switching ( APS) algorithms andperformance/alarm monitoringare built in. This system allows the construction of linear point-to-point networks and synchronous ring topology networks which are self- healing in the event of failure. Also, to minimize the disruption of traffic, the protection switching must be completed within the specified time limit ( sub 50ms) recommended by ITU-T G.783 (linear networks) and ITU-T G.841 (ring networks).

Upon detection of a failure (dLOS, dLOF, high BER),the networkmust reroute traffic (protection switching) fromworkingchanneltoprotection channel.TheNetworkElement that detects the failure (tail-end NE) initiates the protection switching. The head-end NE must change forwarding or to send duplicate traffic. Protection switching may be revertive (automatically revert toworkingchannel)

Key ITU-T recommendations :ITU-T recommendations define methods of protecting service traffic in SDH networks. Two important recommendations are :

1.Recommendation G.783 covers linear point to point networks.2.Recommendation G.841 covers various configurations of multiplex section rings.

Linear ( point to point) protection :In a linear network, protection is achieved through an extra protection fibre. It can protectthe networkfrom fiber or NE card failure. Different variants of linear protection are 1+1, 1:1 and 1:N.

How it works?

Head-end and tail-end NEs have bridges (muxes). Head-end and tail-end NEs maintain bidirectional signalingchannel. Signaling is contained in K1 and K2 bytes ofprotectionchannel. K1 tail-end status and requests. K2 head-end status .

Linear 1+1 protection :This is simplest form of protection. Can be at OC-n level (different physical fibers) or at STM/VC level (calledSubNetworkConnectionProtection) or end-to-end path (calledtrail protection)Head-end bridge always sends data on bothchannels. Tail-end chooseschannelto use based on BER, dLOS, etc. No need for signaling. For non-revertive cases, there is no distinction between.workingand protectionchannels. BW utilization is 50%.

Linear 1:1 protection :In this case, Head-end bridge usually sends data onworkingchannel. When tail-end detects failure it signals (using K1) to head-end. Head-end then starts sending data over protectionchannel. When not in use, protectionchannelcan be used for (discounted)extra traffic(pre-emptible unprotected traffic).

Linear 1:N protection:This is verymuch similar to 1:1 protection with a small difference. Here, in order to save BW we allocate 1 protectionchannelfor every Nworkingchannels. Here, N limited to 14.

Let us read aboutring networks in next post.Tributary Unit (TU) Frames, Interview notes on Tributary Unit frames in SDH Different sizes of Tributary Unit frames are used inSDH& we have seen basicSDH multiplexing structurein earlier post.

Different TU-Sizes provided in SDH are TU-11, TU-12. TU-2 and TU-3 .

1. TU-11: A TU-11 frame consists of 27 bytes, structured as 3 columns of 9 bytes. These 27 bytes provide atransportcapacity of 1.728 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 1.544 Mbps DS1 signal. 84 TU-11's can be multiplexed into aSTM-1 frame.Structureas asshown below.

2.TU-12 :A TU-12 frame consists of 36 bytes, structured as 4 columns by 9 bytes.These 36 bytes provide atransportcapacity of 2.304 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 2.048 Mbps E1 signal. 63 TU-12's can be multiplexed into aSTM-1 frame.Structureas asshown below.

3.TU-2 :A TU-2 frame consists of 108 bytes, structured as 12 columns by 9 bytes.These 108 bytes provide atransportcapacity of 6.912 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of DS2 signal. 21 TU-2's can be multiplexed into aSTM-1 frame.Structureas asshown below.

4.TU-3 :A TU-3 frame consists of 774 bytes, structured as 86 columns by 9 bytes.These 36 bytes provide atransportcapacity of 49.54 Mbpsat a frame rate of 8000 Hz. It will accommodate the mapping of 34 Mbps E3 signal and North American DS3 signal. 3 TU-3's can be multiplexed into aSTM-1 frame.Structureas asshown below.

Maintenance signals in SDH & abbreviationsLet us read about maintenance signals in SDH.

LOS Drop of incomming opticalpower levelcauses BER of 10-3 or worseOOF A1, A2 incorrect for more than 625 usLOF If OOF persists of 3msB1 Error Mismatch of the recovered andcomputedBIP-8MS-AIS K2 (bits6,7,8) =111 for 3 or more framesB2 Error Mismatch of the recovered andcomputedBIP-24MS-RDI If MS-AIS or excessive errors are detected, K2(bits6,7,8)=110MS-REI M1: Binary coded count of incorrect interleavedbit blocksAU-AIS All "1" in the entire AU including AU pointerAU-LOP 8 to 10 NDF enable or 8 to 10 invalid pointersHP-UNEQ C2="0" for 5 or more framesHP-TIM J1: Trace identifier mismatchHP-SLM C2: Signal label mismatchHP-LOM H4values(2 to 10 times) unequal to multiframesequence

B3 Error Mismatch of the recovered andcomputedBIP-8HP-RDI G1 (bit 5)=1, if an invalid signal is received in VC-4/VC-3HP-REI G1 (bits1,2,3,4) = binary coded B3 errors

TU-AIS All "1" in the entire TU incl. TU pointerTU-LOP 8 to 10 NDF enable or 8 to 10 invalid pointersLP-UNEQVC-3: C2 = all "0" for >=frames; VC-12: V5 (bits5,6,7) = 000 for >=5 framesLP-TIM VC-3: J1 mismatch; VC-12: J2 mismatchLP-SLM VC-3: C2 mismatch; VC-12: V5 (bits5,6,7) mismatchBIP-2 ErrMismatch of the recovered andcomputedBIP-2 (V5)LP-RDI V5 (bit 8) = 1, if TU-2 path AIS or signal failure receivedLP-REI V5 (bit 3) = 1, if >=1 errors were detected by BIP-2LP-RFI V5 (bit 4) = 1, if a failure isdeclared

Abbreviations :

AU Administration unitHP High pathLOF Loss of frameLOM Loss of miltiframeLOP Loss of pointerLOS Loss of signalLP Low pathOOF Out of frameREI Remote error indication (FEBE)RDI Remote defect indication (FERF)RFI Remote failure indicationSLM Signal label mismatchTIM Trace identifierTU Tributary unitUNEQ UnequippedVC Virtual ContainerC container

Detailed study of multiplexing process in SDH - Interview notes - Part III Let us continue fromprevious post, where we studied about multiplexing of VC-12 into VC-4. In this post, you will read about VC-4 Path overhead and Mapping of VC-4 into STM-1 frame.VC-4 Path Overhead: The VC-4 Path Overhead forms the start of the VC-4 payload area and consists of one whole column of nine bytes as shown below. The POH contains control and status messages (similar to the V5byte) at thehigher order.

J1 -Higher OrderPath trace. Thisbyteis used to provide a fixed length user configurable string, which can be used to verify the connectivity of 140 Mbit/s connections.

B3 - Bit Interleaved Parity Check(BIP-8). Thisbyteprovides an errormonitoringfunction for the VC-4 payload.

G1 -Higher OrderPath Status.Thisbyteis used to transmit back to the distant end, the results of the BIP-8 check in the B3byte

K3 -Automatic protection Switching (APS).K3 provides for automatic protection switching control with VC-4 payloads. Similar to the K4bitsin the 2 Mbit/s overheads

Mapping of a VC-4 into an STM-1 frame. An AU pointer is added to the VC-4 to form an AU-4 or Administrative Unit -4.The AU pointers are in a fixedpositionwithin the STM-1 frame and are used to show the location of the firstbyteof the VC-4 POH.

The AU-4 is then mapped directly into an AUG or Administrative Unit Group, which then has the Section Overheads or SOH, added to it. These section overheads provide STM-1 framing, sectionperformancemonitoringand other maintenance functions pertaining to the section path.

The VC-4 payload, plus AU pointers and Section Overheads, together form the complete STM-1 transport frame.

If you have any question , please write to meDetailed study of multiplexing process in SDH - Interview notes - Part II Further toprevious post, let us read about mapping VC-12 into TU-12, TU-12 to TUG-2, TUG-2 to TUG-3 & TUG-3 to VC4.

Mapping of a VC-12 into a TU-12 signal.

In order to detect the start of the 2 Mbit/s signal and thereby the start of the customers data, The V5 byte must be seen be the distant end. This is achieved by adding four overhead bytes to the multiframe, which together form a calculated byte count to the start of V5. This is called apointer valueand is known as theTU Pointer.

There are four pointer bytes called V1, V2, V3 and V4, which are used tocalculatethe location of V5.

Multiplexing of TU-12 into a TUG-2:

Each VC-12 consists of36bytes of information and these 36 bytes fill up exactly 4 columns of the STM-1 frame.3 separate TU-12's are directly mapped together to form a TUG-2.The 3 TU-12's will fit exactly into 12 columns of the STM-1 frame .

Mapping of a TUG-2 into a TUG-3 signal:

The mapping of TUG-2's into TUG-3's uses fixed column interleaving and is shown below.

Mapping of a TUG-3 into a VC-4 signal:

The three TUG-3's are column interleaved to form the VC-4 payload. At this point two columns of fixed stuffing are added and the 'VC-4 Path Overhead' is added to the start.

These are the steps followed in multiplexing VC-12 into VC-4.In next post, we will read about VC-4 Path over head

If you have anyquestions, you can add them in comments section. I will provide you theanswer.Detailed study of multiplexing process in SDH - Interview notes

Let us study the overview of the process followed by a 2 Mbit/s PDH input signal until it becomes part of an STM-1 frame. You can comparethe detailsof each individual stage toSDH multiplexing structure provided in previous post.

Mapping of a 2 Mbit/s PDH signal into a C-12: The 2 Mbit/sPDHinput signal is mapped into aContainer 12 (C-12). The input frame consists of 32 bytes of information and this fits directly into the C-12 as shown

Mapping of a C-12 into a VC-12:At the time of mapping a C-12 into a VC-12, we need to add four bytes of overheadcontrol information. But we can add only one byte per frame of customers' data So, this process takes place over4 consecutive frames& described below: -

Frame number Onehas two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overheadcontrol informationadded to the start.This byte ofover headis called the V5 byte and is known as the VC-12 Path OverHead (POH).

Two more importantfeaturesof the V5 byte are:

BIP-2is Bit Interleaved Parity Check-2. This looks at the data in theC-12. It counts all of the binary one's that it sees in theoddbitpositions(i.e. bits 1,3,5,7 etc) and then it counts all of the binary one's that it sees in theevenbitpositions(i.e. bits 2,4,6,8 etc). This BIP-2 is then recalculated at the distant end. If the count is different, then some bit corruption has occurred.

FEBEis Far End Bit errors. This bit is set correspondingly to the result of theBIP-2check. If errors are received at the distant end then there needs to be a mechanism for informing the sender of the problem.

Frame number Twohas two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. It then has one byte of overheadcontrol informationadded to the start. This control byte inframe 2is theLower Order Path TraceorJ2byte. J2 is used to check continuity of a 2 Mbit/s path.

Frame number Threehas two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overheadcontrol informationadded to the start. This control byteN2,in frame3 is called theNetworkOperatororTandem Controlbyte.N2is used to transmit performance-monitoring information where the circuit spans differing vendors networks.

Frame number Fourhas one byte of fixed stuffing added to the end. It also has one byte of variable stuffing added to the start. One byte of overheadcontrol informationadded to the start. Control bytein frame4 is calledK4 and it is used for 2 Mbit/s Automatic Protection Switching or APS. APS is used to automatically switch a single 2 Mbit/s circuit to its alternate path if a fault condition occurs.

In the next post we will learn about :Mapping of a VC-12 into a TU-12 signal.SDH Concatenation, Interview notes on Contiguous concatenation

There are two types of concatenation in SDH. They areContiguous concatenation andVirtual concatenation.In this article, let us learn aboutcontiguous concatenation.

Contiguous Concatenation :

The SDH frame can be thought of as transport lorry. The data to be transported is placed in the VC-4 'Container'. This is thenhitchedto the SOH 'Cab unit' that 'drives' the data to its destination.The maximum carrying capacity of the vehicle is determined by the size of the 'container'.Thereforealthough the SDH signal is 155 Mbit/s in size, the largestsingle circuitthat can be transmitted at anyone timeby the customer is limited to the size of the VC-4 i.e. 140 Mbit/s.

When using higherrates ofSDH (STM-4, STM-16 etc), multiple 'containers' and 'cabs' are added one after another, to form a bigger vehicle. The customer is still limited to asingle circuitsize of 140 Mbit/s however, because eachindividual'container' is still the same size (140 Mbit/s). They can however transmit multiple 140 Mbit/s circuits simultaneously.

Standard STM-4structureis given below

The limitation of 140 Mbit/s perindividualcircuit is not a efficient way of managing bandwidth. In order to overcome this limitation, a method ofcombining 'containers'togetherhas been developed which is called'Concatenation'.

STM-4 concatenatedstructure(VC-4-4C) is as shown below

Concatenated paths are commonly defined asVC-4-xCcircuits (where x is size of the concatenation), as shown below:STM-4 concatenation (written as VC-4-4c), provides asingle circuitwith a bit rate ofapproximately600M (actually 599.04 Mbit/s).STM-16 concatenation (written as VC-4-16c), provides asingle circuitwith a bit rate ofapproximately2.2G (actually 2.2396160 Gbit/s).

STM-1 Frame Structure & Section Over head

STM-1 Frame StructureSTM-1 frame contains2430 bytesof information. Each byte contains8 databits(i.e. a 64kbit/schannel). Duration of STM-1 transport frame is125ms. Thenumberof frames per second is 1 second/125ms= 8000 Frames per second.

So, rate of STM-1 frame is calculated as follows: -

8bitsx 2430 bytes x 8000 per second=155,520,000bits/sor155 Mbit/s.

STM-1 frame chopped up into 9 segments, stacked on top of each other as shown in thediagrambelow. Thebitsstart at the top left with bytenumberone and are read from left to right and top to bottom. They are arranged as 270 columns across and 9 rows down.

STM-1 Section Overheads The STM-1 Section Overhead (SOH) consists of nine columns by nine rows as shown below. It forms the start of the STM-1 frame.The SOH contains control and status messages at the optical fibre level.

First three rows are RSOH ( Regenerator Section Overhead), Fourth row is AU-4 pointer. Fifth to Ninth row are MSOH ( Multiplexer section Overhead).

A1 & A2 - STM-1 Frame Alignment.These 6 bytes are used for STM-1 frame alignment. They are the first bytes transmitted. Frame alignment takes place overthreeSTM-1 frames.

J0 - STM-1 Section Path Trace. This byte is used to provide a fixed length user configurable string, which can be used to verifynetwork topologyconnections.

B1 - Byte Interleaved Parity Check 8 (BIP-8).This byte provides an errormonitoringfunction for the entire STM-1 frameafterencoding.

B2 - Byte Interleaved Parity Check 24 (BIP-24).These 3 bytes provide an errormonitoringfunction for the STM-1 framebeforeencoding.A comparison between the BIP-8 and BIP-24 checksrevealif there were any encoding errors.

D1 to D12 - Data CommunicationsChannel(DCC).These bytes provide a datachannelfor the use ofnetwork management systems.

K1 - Automatic protection Switching (APS). This byte is used to perform automatic protection switching of the optical fibre.

X- Reserved. These bytes are reserved for national use.

AllUnmarkedbytes are reserved for future international standardisation.SDH Principles and Interview questions on SDH Multiplexing structure

Overview The SDH standard defines a number of 'Containers' each corresponding to an existing PDH input rate. Information from the incoming PDH signal is placed into the relevant container.Each container then has somecontrol informationknown as the'Path Overhead' (POH)and stuffingbitsadded to it. The path overhead bytes allow the system operator to achieve end to endmonitoringof areas such as error indication, alarm indication andperformancemonitoringdata. The container and the path overhead together form a'Virtual Container' (VC).

Due to clock phase differences, the start of the customers' PDH data may not coincide with the start of the SDH frame. Identification of the start of the PDH data is achieved by adding a 'Pointer'. The VC and its relevant pointer together form a'Tributary Unit' (TU).

Tributary units are then multiplexed together instages(Tributary UserGroup 2(TUG-2)-Tributary User Group 3(TUG-3)- Virtual Container 4(VC-4)), to form anAdministrative Unit 4 (AU-4).Additionalstuffing, pointers and overheads are added during this procedure.This AU-4 in effect contains 63 x 2 Mbit/s channels and all thecontrol informationthat is required.

Finally,Section Overheads (SOH)are added to the AU-4.These SOH's contain the control bytes for the STM-1 section comprising of framing, sectionperformancemonitoring, maintenance and operationalcontrol information.An AU-4 plus its SOH's together form anSTM-1transport frame.

Graphical SDH Multiplexing Structure

Full SDH Multiplexing Structure :Diagram below shows full SDH Multiplexing structure.PDH signals enter on the right into the relevant container and progress across to the left through the various processes to form the STM frame.

2 Mbit/s Multiplexing Structure

Let us see the multiplexingstagesof 2 Mbit/s circuit. The relative bit rate and process is shown for eachstage.

If you like this post, please share the same with yourfriendsalsoSTM-1 Frame Structure & Section Over head

STM-1 Frame StructureSTM-1 frame contains2430 bytesofinformation. Each byte contains8 databits(i.e. a 64kbit/schannel). Duration of STM-1 transport frame is125ms. The number of frames per second is 1 second/125ms= 8000 Frames per second.

So, rate of STM-1 frame is calculated as follows: -

8bitsx 2430 bytes x 8000 per second=155,520,000bits/sor155 Mbit/s.

STM-1 frame chopped up into 9 segments, stacked on top of each other as shown in thediagrambelow. Thebitsstart at the top left with byte number one and are read from left to right and top to bottom. They are arranged as 270 columns across and 9 rows down.

STM-1 Section Overheads The STM-1 Section Overhead (SOH) consists of nine columns by nine rows as shown below. It forms the start of the STM-1 frame.The SOH contains control and status messages at the optical fibre level.

First three rows are RSOH ( Regenerator Section Overhead), Fourth row is AU-4pointer. Fifth to Ninth row are MSOH ( Multiplexer section Overhead).

A1 & A2 - STM-1 Frame Alignment.These 6 bytes are used for STM-1 frame alignment. They are the first bytes transmitted. Frame alignment takes place overthreeSTM-1 frames.

J0 - STM-1 Section Path Trace. This byte is used to provide a fixed length user configurable string, which can be used to verifynetworktopology connections.

B1 - Byte Interleaved Parity Check 8 (BIP-8).This byte provides an errormonitoringfunction for the entire STM-1 frameafterencoding.

B2 - Byte Interleaved Parity Check 24 (BIP-24).These 3 bytes provide an errormonitoringfunction for the STM-1 framebeforeencoding.A comparison between the BIP-8 and BIP-24 checks reveal if there were any encoding errors.

D1 to D12 - Data CommunicationsChannel(DCC).These bytes provide a datachannelfor the use ofnetwork management systems.

K1 - Automatic protection Switching (APS). This byte is used to perform automatic protection switching of the optical fibre.

X- Reserved. These bytes are reserved for national use.

AllUnmarkedbytes are reserved for future international standardisation.TDM : Positive Justification in PDH

Let us read the concept of positivejustification.

Thediagramabove illustrates the basic principle ofpositivejustification.There are 4 asynchronous inputs. All are brought to same frequency ( i.e.36 bps) by adding appropriate number of redundant bit to each tributary. Now all these 4 synchronous 36 bps inputs are multiplexed to get the output rate of 144 bps.

Revrse of this process takes place at the demultiplexer. From each tributary signals, redundant bits are removed to recover the original signal. These redundant bits are called stuffing or justification bits. The higher order stream will be having framestructureand framing bits so that interleaved tributary bits can be recovered.

If you like this post, please take 5 seconds to share this on web2 Mbps FRAME - FORMAT Let us study about standard 2 Mbps frame format G.704 / G.732.

Each 2 Mbps frame contains 256 bits ( 32 timeslots) at a repetition rate of 8 kb/s. The first timeslot i.e.TS 0 is reserved for framing, error-checking and alarm signals. Remaining 31channelscan be used for data traffic.Individualtimeslots /channelscan be used for 64 kbps PCM. Sometimes TS16 is reserved for signalling. For example - ISDN primary rate Dchannelsignalling (Q.931).

The start of 32 timeslot frame is signified by the frame alignment word 0011011 in TS0 of alternate frames. In the other frame, bit 2 is set to one and bit 3 contains the A-bit for sending alarm to the far end. If three frame alignment words in four are received in error, then the receiving terminal declares loss of frame alignment and initiates a resync process.

If you like this post, please take 5 seconds to share this onfacebook& google +Basic SDH Network Topology & Advantages of SDHLet us read about the Basic SDHnetworktopology.Detailedtopology discussion will be done later.Basic SDH Network Topology

SDH networks are usually deployed inprotected rings. This has the advantage of giving protection to the data, by providing analternate routefor it to travel over in the event of equipment ornetworkfailure.

Eachsideof the ring (known as A and B, or sometimes, East and West), consists of anindividualtransmit and receive fibre. Thesefibreswill take diverse physical paths to the distant end equipment to minimise the risk of both routes failing at the same time.

The SDH equipments have the ability to detect the problem and will automatically switch to thealternate route.

SDH multiplexers transmit on both sides of the ring simultaneously, But to speed up switching times, they only receive on one side at any time. This means that only the receiving end needs to switch, thus reducing the impact of a fault on the customers' data.Features and Advantages of SDHIn previous post we have seen the limitations ofPDH. Now let us see the advantages of SDH.

SDH permits the mixing of the existing European and North American PDH bit rates.

All SDH equipment is based on the use of a single masterreferenceclock source & hence SDH issynchronous.

Compatible with the majority of existing PDH bit rates

SDH provides for extraction/insertion, of a lower order bit rate from a higher order aggregate stream, without the need to de-multiplex in stages.

SDH allows for integrated management using a centralisednetworkcontrol.

SDH provides for a standard optical interface thus allowing the inter-working of different manufacturers equipment.

Increase innetworkreliability due to reduction of necessary equipment/jumpering.

Origin of SDH

As seen from the previous post aboutPDH,PDHis a workable but flawed system.At the beginningit wasthe bestavailable technology and was a giant leap forward in telecom transmission, As a result of growth in the field of silicon chips and integrated microprocessors, customer demand soon provided the need to introduce a new and better system.& it was expected to solve the existing limitations ofPDH.

As a next step, Bellcore introduced SYNTRAN (Synchronous Transmission) system. However this was only a development system. Soon it was replaced withSONET(SynchronousOptical Network).Initially SONET could only carry the ANSI (American NationalStandards Institute) bit rates i.e. 1.5, 6, 45 Mbit/s. Aim of the project was to provide easier international interconnection, Hence, SONET was modified to carry the European standard bit rates of 2, 8, 34 & 140 Mbit/s.

In 1989 theITU-T(International Telecommunications Union - Telecommunication's standardisation section), published recommendations which covered the standards for SDH. These were adopted inNorth Americaby ANSI (SONET is now thought of as a subset of SDH), making SDH a truly global standardPDH Basics | PDH Interview questions

Before 1970, worlds telephony systems were based on single line, voice frequency, and all connections were over twisted copper pair. During early 1970s digital transmission systems began to appear usingPulse Code Modulation (PCM). PCM enables analogue waveforms such as speech to be converted into a binary format suitable for transmission over long distances via digital systems.

PCM works by sampling the analogue signal at regular intervals, assigning a binary value to the sample and then transmitting this value as a binary stream. This process is still in use today and forms the basis of virtually all the transmission systems that we currently use.

Next step was multiplexing several PCM together over the same copper pair.

A standard was adopted in Europe where thirty-two, 64kbit/s channels were multiplexed together to produce astructurewith a transmission rate of 2.048 Mbit/s (usually referred to as 2 Mbit/s).

As demand fortelephony servicesgrew, Four X 2 Mbit/s signals were combined together to form an 8 Mbit/s signal (actually 8.448 Mbit/s). , This is because 2 Mbit/s signal was not sufficient to cope with the demands of the growing network, and so a further level of multiplexing was devised. Further, additional levels of multiplexingstructurewere added to include rates of 34 Mbit/s (34.368) and 140 Mbit/s (139.264).These transmission speeds are calledPlesiochronous Digital HierarchyorPDHrates.

PLESI means SimilarCHRONOUS means timing.

This means that PDH equipment operates with similar timing but not exactly the same timing.

Comparison of hierarchical PDH rates :A different hierarchicalstructurewas adopted in North America. Comparison between two systems are given below.

Disadvantages of PDH networksPDH signal is structured in such a way that, it is impossible to extract a single 2 Mbit/s signal from within a higher order (say 140 Mbit/s) stream. In order to crossconnect 2Mbit/s signal between one transmission system and another, it must be de-multiplexed back down to its primary rate first. This forms a multiplexer mountain.

So, we need to have a lot of equipments just toconnect 2Megs together.Due to this :

More usable space & power is taken up in racks in node sites by these equipment mountain & cause more maintenance-associated problems.

Equipment in different hierarchical levels synchronised from a different source and at a different rate, which may lead to clocking problems that can cause errors.

For asimple 2Meg signal, jumpering needs to be done at all levels, that make up theindividualtransmission system. Thisleadsto large amounts of physically bulky coax wiring.

Efficient use of bandwidth is achieved in PDH due to the fact that, it is having small overhead. But this limits the management ability of PDH. :

Automatic storage of route information is not available whichleadsto the requirement of accurate paper records to avoid problems.

It is not possible to remotely configure equipment and thealarm monitoringis only limited toreportingloss of inputs.

Protection of the transmission paths is generally using 1+1 protection and available at the higher PDH levels i.e.140 Mbit/s and above only.

Summary of PDH LimitationsInterconnection between different national systems were difficult (European/North American).

Clocking in different hierarchy levels are done individually, so slips possible.

'Multiplexer mountain' is costly and inflexible.

Limited management functionality

Path Protection available at higher rates only.

While comparing to today standards, more Prone to faults.All thesesystems worksfine in a stand-alone hierarchy. But it does make international inter-connection very difficult and costly.This was the major reason for the development of a new internationally agreed standard (SDH).Network Management Basics

Operational Tasks:

Following basic operational tasks are performed bynetwork management system:

Protection :Protection switching takes place within milliseconds ( sub 50 ms) & hence Circuit recovery in milliseconds ( failure should not be detected by voice customers)Restoration:By doing manual configuration, circuit recovery achieved in seconds orProvisioning:Allocation of capacity to preferred routes (according to certain time schedules)Consolidation:Moving traffic from unfilled bearers onto fewer bearers to reduce waste trunk capacityGrooming:Sorting of different traffic types from mixed payloads into separate destinations for each type of traffic.

OAM Functions and LayersLevel 1 - Regenerator Section: Loss of synchronization, signal quality degradationLevel 2 - Multiplex Section : Loss of frame synchronization, degraded errorperformanceLevel 3 Path : Assembly and disassembly, celldelineationcontrol.

Data Communication Channel (DCC)

DCC is a in-band channel to facilitate communication between all Network Elements (NE) in a network. This facilitates remote login, alarms reporting, software download, provisioningReference Clocks & alternative clock source in SDH network.

Reference Clocks:

Let us read aboutreferenceclocks. Precision of internal clock is classified into so called Stratum levels. Accuracy ofreferenceclock is defined as the ratio of bit slip happening (causing a bit error)

Stratum 1 => 1 x 10-11(synchronization toatomic clock)Stratum 2 => 1.6 x 10-9Stratum 3E => 1 x 10-6Stratum 3 => 4.6 x 10-6Stratum 4 => 32 x 10-6(typical forIP routers)

When we are distributing the clock inthe network, accuracy level might decrease at each hop in clock distribution. Originally providing Stratum 1 clocks for eachnetworkelement was far from being economical, even providing this service at multiple locations was too much demanding. So clockdistribution methodswere developed to minimize the number of high accuracy clocks needed inthe network.

GlobalPositioningSystem (GPS) includes Stratum 1atomic clockson the satellites. Cheap GPS receivers are available in themarketand they make it possible to have a Stratum 1 time source at almost any place. This reduces the need for time synchronizationnetwork(might even go away in the future).

Clock Distribution Methods :Various clockdistribution methodsare as described below.

When all equipment is at the same location, External clock input might be used. This is usually BITS = Building Integrated Timing Signal. It uses an empty T1 or E1 framing to embed clock signal. Might be provided as a dedicated bus reaching into each rack in a CO environment. BITS should be generated from a Stratum 1 clock. Typically it will be deployed with a hot spare alternative source for fail-over.

Networkelements not close to a BITS source should recover clock from the line. While distributing the clock, Clock distributionnetworkshould not have loops, so a tree distribution topology should be configured. Usually carriernetworkelement will have Stratum 3 accuracy when running free. By synchronization to thereferenceclock, this clock is running at the same rate as thereferenceclock (that is Stratum 1). Minimum requirement for anynetworkelement is 20 ppm (that is between Stratum 3 and Stratum 4).

Alternative Clock Sources:If the trail to thereferenceclock source is lost,the networkelement still continues normal operation. However, alarm might be generated. After some time the clock might drift away so much, that bit errors would occur. Some time is left for switching over to an alternative clock source. Thenthe networkelement gets into a holdover state. Requirement is to have less than 255 errors in 24 hours.

A hierarchy of potential clock sources should be configured at eachnetworkelement to achieve a high-availability operation. Typically a maximum 3 alternative timereferencesources might be configured. This is meaningful only if there are different paths to the alternative timereferencesources. If only one natural path exists to a single timereferencesource, then the path must be protected by automatic protection switching. This requires some extra signaling to do it properly, called SPS = Synchronization Protection Switching.Synchronization requirements & modes of timing in SDH transmission networks

For synchronization of atransmission network, Frequency variation of bits transmitted should be inside the limits determined by the next hops ability to transmit these bits further. Stuffing allows for some limited tolerance. In order to guarantee a low level of BER Frequencies should be synchronized all over the network. Usually Synchronization is done by recovering the embedded clock signal from the input signal . Synchronization source should have a very precise clock (referenceclock).Referenceclock might be reached only by multiple hops, but number of hops should be minimized.

Synchronization modes for transmission networks:In atransmission network, Each network element has to be configured for time synchronization. Timereferencedistribution should minimize delay.Various timing alternatives available are:ExternalLineLoopThrough.Let us seethe details.

External timing:In this mode, all signals transmitted from a node are synchronized to an external source received by that node; i.e. BITS timing source.

Line Timing :In this mode,All transmitted signals from a node are synchronized to one received signal.

Loop timing:In this mode, the transmit signal in a optical link, east or west, is synchronized to the received signal from the same optical link.

Through timing :In this mode, the transmit signal in one direction of transmission around the ring is synchronized to the received signal from that same direction of transmission.

Detailed operation of BLSR & Squelching.

Operation Traffic flow :

Bi-directional traffic between two nodes is transported over a subset of the "ring sections" or "spans". In thisconfiguration, Minimum capacity equalsline rate.Capacity is in general expressed asnumberof AU4, or bandwidth.The bandwidth is provided by an integernumberof AU4 payload.

Maximum bandwidth capacity :

Here, each span has, in each direction, a capacity of up to half thenumberof AU4 in the STM-N (i.e. 8 AU4 for an STM-16 section). All traffic from a node goes to adjacent nodes.

Max. capacity = 0.5 (line rate) xnumberof nodes.

Note:This Max is achieved only of the working traffic is transported only between two adjacent nodes.

Extra Traffic:

We can utilize shared protection bandwidth for Extra traffic. This extra traffic is not protected & it could be lost when a failure of working traffic occurs

Operations Fiber Cut :Let us consider a scenario, where fiber cuts between A&B. We have a working traffic from A-C and C-A. This failure interrupts A-C and C-A traffic . Now Node A and Node B detect failure

Now node A and node B will switch the traffic to protection path. No dedicated protection bandwidth - only used when protection required. Only nodes next to the failure know about the protection switch. No traffic lost.

Operations Node Failure:

Let us consider that we havelive trafficfrom D-F and F-D. If node B fails, Failure interrupts D-F and F-D traffic. NodeA and C detect failure

Now Both node A & C switch the traffic to protectionchannels.Only nodes next to the failure know about the protection switch.In this scenario, only Traffic to/from failed node lost.

Squelching Problem :

When a node fails, traffic terminating on those nodescut offby failures could be misconnected to other nodes on the ring in case of using a local fail-over decision.

Consider a scenario, where we have active traffic from Node F-B , B-F and E-B, B-E. If Node B fails,

Squelching misconnection occur :Node F now talking to Node E instead of Node B

This can be avoided by path AIS Insertion.STM Path AIS is inserted instead of the looped STM-1#7.No mis-connections

Squelching Summary :Squelching is in general used when extra traffic is used, it is used when normal traffic is switched to the protection entity and replaces the extra traffic. Squelching prevents that in case protection switch is active the normal traffic is output instead of the original extra traffic by outputting AU-AIS. You can also read clause 7.2.3.2 of ITU-T G.841

Squelching is required to assure that misconnections are not made. It isrequired forbidirectionalline switched rings only, since it is the only ring to provide a reuse capability of STM-1s around the ring. This is only required when nodes arecut offfrom the ring. Also this is only required for traffic terminating on thecut offnodes.

A ring map that includes all STM and VC Paths on the ring isavailableat every node on the ring.Squelching is also required for extra traffic since the extra traffic may be droppedwhen a protection switch is requiredS

PDH Basics | PDH Interview questions

Before 1970, worlds telephony systems were based on single line, voice frequency, and all connections were over twisted copper pair. During early 1970s digital transmission systems began to appear usingPulse Code Modulation (PCM). PCM enables analogue waveforms such as speech to be converted into a binary format suitable for transmission over long distances via digital systems.

PCM works by sampling the analogue signal at regular intervals, assigning a binary value to the sample and then transmitting this value as a binary stream. This process is still in use today and forms the basis of virtually all the transmission systems that we currently use.

Next step was multiplexing several PCM together over the same copper pair.

A standard was adopted in Europe where thirty-two, 64kbit/s channels were multiplexed together to produce astructurewith a transmission rate of 2.048 Mbit/s (usually referred to as 2 Mbit/s).

As demand fortelephony servicesgrew, Four X 2 Mbit/s signals were combined together to form an 8 Mbit/s signal (actually 8.448 Mbit/s). , This is because 2 Mbit/s signal was not sufficient to cope with the demands of the growing network, and so a further level of multiplexing was devised. Further, additional levels of multiplexingstructurewere added to include rates of 34 Mbit/s (34.368) and 140 Mbit/s (139.264).These transmission speeds are calledPlesiochronous Digital HierarchyorPDHrates.

PLESI means SimilarCHRONOUS means timing.

This means that PDH equipment operates with similar timing but not exactly the same timing.

Comparison of hierarchical PDH rates :A different hierarchicalstructurewas adopted in North America. Comparison between two systems are given below.

Disadvantages of PDH networksPDH signal is structured in such a way that, it is impossible to extract a single 2 Mbit/s signal from within a higher order (say 140 Mbit/s) stream. In order to crossconnect 2Mbit/s signal between one transmission system and another, it must be de-multiplexed back down to its primary rate first. This forms a multiplexer mountain.

So, we need to have a lot of equipments just toconnect 2Megs together.Due to this :

More usable space & power is taken up in racks in node sites by these equipment mountain & cause more maintenance-associated problems.

Equipment in different hierarchical levels synchronised from a different source and at a different rate, which may lead to clocking problems that can cause errors.

For asimple 2Meg signal, jumpering needs to be done at all levels, that make up theindividualtransmission system. Thisleadsto large amounts of physically bulky coax wiring.

Efficient use of bandwidth is achieved in PDH due to the fact that, it is having small overhead. But this limits the management ability of PDH. :

Automatic storage of route information is not available whichleadsto the requirement of accurate paper records to avoid problems.

It is not possible to remotely configure equipment and thealarm monitoringis only limited toreportingloss of inputs.

Protection of the transmission paths is generally using 1+1 protection and available at the higher PDH levels i.e.140 Mbit/s and above only.

Summary of PDH LimitationsInterconnection between different national systems were difficult (European/North American).

Clocking in different hierarchy levels are done individually, so slips possible.

'Multiplexer mountain' is costly and inflexible.

Limited management functionality

Path Protection available at higher rates only.

While comparing to today standards, more Prone to faults.All thesesystems worksfine in a stand-alone hierarchy. But it does make international inter-connection very difficult and costly.This was the major reason for the development of a new internationally agreed standard (SDH).What should the OSNR values be in DWDM networks?

By:Jean-Sbastien Tass| Category: Optical/Fiber Testing | Posted date: 2012-12-06 | Views:1563

I was surfing the Web when I ran into this question on a social media platform. Since this is a very important question that calls for a detailed answer, I figured I would break it down for you here. The short answer is that in general, you should target anOSNRvalue greater than 15 dB to 18 dB at the receiver, but this value will depend on many factors.The long answer is that many factors need to be taken into account, including: modulation format, data rate, location in the network, type of network and the target BER level.Here are a few initial guidelines: Dependency on the location. The required OSNR will be different for different locations in the light path. Closer to the transmitter, the OSNR requirement will be higher. Closer to the receiver, the OSNR requirement will be lower. This is because optical amplifiers and ROADMs add noise, which means that the OSNR value degrades after going through each optical amplifier or ROADM. To ensure that the OSNR value is high enough for proper detection at the receiver, the number of optical amplifiers and ROADMs needs to be considered when designing a network. Dependency on the type of network. For a metro network, an OSNR value of >40 dB at the transmitter might be perfectly acceptable, because there are not many amps between the transmitter and the receiver. For a submarine network, the OSNR requirements at the transmitter are much higher. Dependency on the data rate. As the data rate goes up for a specific modulation format, the OSNR requirement also increases. Dependency on the target BER. A lower target BER calls for a higher OSNR value.Those were the guidelines, and now for some specifics. The OSNR values that matter the most are at the receiver, because a low OSNR value means that the receiver won't detect or recover the signal. The exact requirements at the receiver will vary from one manufacturer to another (contact your system provider), but see the examples below for a few average OSNR figures to guarantee a BER lower than 10-8at the receiver: 10 Gbit/s NRZ: OSNR greater than approximately 11 dB 40 Gbit/s NRZ: OSNR greater than approximately 17 dB 40 Gbit/s DPSK: OSNR greater than approximately 14 dB 100 Gbit/s NRZ: OSNR greater than approximately 21 dB 100 Gbit/s DPSK: OSNR greater than approximately 18 dBAgain, the rule of thumb is really that the OSNR should be greater than 15 dB to 18 dB at the receiver, but this value will depend on many factors.If you'd like to get more information about this topic, we recently conducted awebinar, in collaboration withLight Reading, that is nowavailable online.OSNR in Next-Gen Networks: Polarization-Resolved Optical Spectrum Analysis Allows Fast and Accurate In-Band OSNR Measurement

By:Francis Audet| Category: Challenges/Solutions, Optical/Fiber Testing | Posted date: 2010-10-03 | Views:344

It has become widely accepted that, when applied to next-generation networks, traditional IEC-recommended optical signal-to-noise ratio (OSNR) measurement techniques fail to deliver the required accuracy. As these next-gen networks employ reconfigurable optical add-drop multiplexers (ROADM) and/or multiple-bit/symbol advanced modulation formats (as is the case with most 40 Gbit/s and 100 Gbit/s transmissions), use of traditional OSNR measurements leads to either over- or under-evaluation of the OSNR.When ROADMs were first introduced in the network topology, some commercial optical spectrum analyzers (OSAs) implemented the polarization-nulling method to measure OSNR within the filtered dense wavelength-division multiplexing (DWDM) channels. Unfortunately, the complexity of the networks has rendered polarization nulling unsatisfactory in many realistic cases. In this article, we will describe an approach that not only relies on the relative differences in the polarization properties of the data-carrying signal and noise but also leverages their respective spectral properties. Whats more, we will review both methods and discuss their limitations.Polarization NullingThe polarization-nulling approach is predicated on the normally realistic assumption that the light signal under test is typically highly polarized and that the superposed noise is not. The measurement setup includes a polarization controller, a polarization splitter and a dual-channel scanning monochromator (i.e. an OSA), as illustrated in Figure 1 (note that detection, signal processing and display electronics have been omitted for the sake of simplicity).

Figure 1. Schematic of an OSA used for the polarization-nulling techniqueThe polarization-nulling method involves the adjustment of the (internal) polarization controller in order to extinguish, to the highest degree practicable, the signal in one of the two detection channels of the OSA. Such adjustment corresponds to one of the two outputs of the polarization splitter (acting as a polarizer) to be aligned orthogonally with respect to the (polarized) signal. When this is achieved, the only light reaching the detector of that branch, at that particular wavelength, corresponds to half the noise power (since noise is assumed to be unpolarized, and hence split equally between the two branches).This provides a measured noise level, and since the OSA also measures the total power (noise + signal), the OSNR for the particular wavelength can be calculated. In order for polarization nulling to provide acceptable measurements, the DWDM channels under test and the OSA itself must meet the following criteria: Polarization-mode dispersion (PMD) on the link should be very modest, preferably near zero. PMD partially depolarizes the signal within the resolution bandwidth (RBW) of the OSA, so complete nulling becomes impossible. In practice, there is some tolerance to PMD, especially if the OSNR that will be measured is not extremely high (e.g.,