Next Generation SONET/SDH devices - Keysightliterature.cdn.keysight.com/litweb/pdf/5988-8265EN.pdfNext Generation SONET/SDH devices include cost reduction and improved ease of operation.

Next GenerationSONET/SDH devices– the driver for multi-port,multi-channel test

Application note

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Introduction

This application note describesnext generation SONET/SDHtelecomm network equipmentand some of the new challengesfaced in testing the behavior andcharacteristics of suchequipment.

Next generation equipment ismuch more complex than legacyequipment due to theamalgamation of numerousfunctions into a single platform.This convergence of functionsprovides clear benefits, includinglower purchase and operatingcosts. Service provisioning is alsoeasier and faster, particularly inthe metro area of the network.The metro network is locatedbetween the edge or accessnetwork (the customer interface)and the core network, whichprovides long distancetransmission.

However, being in the metro areaalso means further complexity isnecessary to access the largenumber and variety of interfacetypes present at the edge of thenetwork.

Next generation devices arecapable of supporting multipleSONET/SDH rings, this in turnleads to the biggest challenges intesting and it soon becomesapparent that a new approach totesting is required.

This document deals only withissues raised by SONET/SDHtesting and doesn’t deal with GbEor any of the other interfaces.

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Next generationSONET/SDH

Value delivered in thenext generation networkelement

The values that NEMs are able todeliver to their customers withNext Generation SONET/SDHdevices include cost reductionand improved ease of operation.This is the driver for thedevelopment of these devices.

Costs are reduced by combiningthe functions of severaltraditional network elementssuch as grooming, switching,wavelength division multiplexing(WDM) and add/dropmultiplexing (ADM) into a singlemulti-function platform. Thisreduces capital costs simplythrough the fact that fewerelements are required in thenetwork. Operating costs are alsoreduced through lower floorspace requirements and reducedpower consumption.

Ease of operation is deliveredfrom the combination of telecom,data and multiple networkfunctions in a single platform.This allows the networkprovisioning system to cater forboth data and voice in a singlenetwork control environment,and permits “point and click”provisioning. Ease of operation isimportant because it providesoperators with significantimprovements in the ease andspeed of service provisioning,

enabling them to bring newservices to market faster. There isalso potential to improve marketshare by providing a greatervariety of services, or moreinnovative services to end-users.

Next Generation SONET/SDH isparticularly important in theMetro area, where multipleservices from customer sites,including Ethernet, IP and voiceneed to be combined fortransmission into the corenetwork. To be effective in thisarea a typical next generationdevice must be able toaccommodate interfaces for all ofthe varied technologies it mayencounter in the Metro area.These devices are often referredto as Optical Edge Devices(OEDs) or multi-serviceprovisioning platforms (MSPPs).

Some devices are focused onaddressing the needs of themetro edge. These containnumerous interfaces catering forEthernet, IP, voice, etc to acceptthe mix of services found atvarious customer sites. Theirinternal architecture will includeadaptation layers to transformthis mixture of traffic into theSONET/SDH frames, which arethen multiplexed on to metrorings or towards the networkcore itself.

Other devices are more focusedon the metro core, the link fromthe metro network into the corenetwork. These may contain alower variety and number oftributary interfaces but will havehigher rate SONET /SDH line sideinterfaces possibly includingDWDM capability.

By definition a next generationplatform is an amalgamation ofseveral network elements and asa consequence will feature manymore ports than legacyequipment. Some next generationdevices, such as the CiscoONS15600, Ciena Core Directorand Nortel Optera Metro arealready capable ofaccommodating several hundredports.

On the tributary side of thedevice, these ports would coverboth voice and data technologieswith a suitable variety of linerates. Where necessary theintroduction of new, arbitraryconcatenations such as STS-6c,STS-9c, STS-24c and their SDHequivalents, AU-4-2c, AU-4-3cand AU-4-8c are made accessible.

On the line or aggregate side ofthe device there is also a largernumber of ports. With traditionaldevices, a single SONET/SDHring would normally besupported. However, with nextgeneration devices multiple ringsmay be supported. Already somedevices accommodating 10 ringsor even more are available.

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Figure 1 shows how multiplefunctions are merged into asingle device in NG SONET/SDH.The orange colored boxes providethe interfaces and infrastructurefor receiving and transmitting arange of technologies includingPDH, ATM, Ethernet and IP. Thedata and voice signals areconnected through adaptationcomponents to a SONET/SDHstratum. The stratum containsoptical interfaces to handleSONET/SDH from STS-1 upwardsand controls the switchingfunctions. Note that SONET/SDHinterfaces may serve both thetributary and the line side of thedevice. Finally, the blue coloredboxes indicate the line side of thedevice, in this case a DWDMsystem.

Different NEMs devices willcontain some or all of thesesections with differing degrees ofpriority for each. For example, adevice that is intended for use atthe edge of the metro networkwill feature many of the orangeand green colored boxes ofFigure 1. A device that isintended for use nearer to thecore area of the metro networkwould have more blue and greencolored boxes.

Although there are advantages tothe network operator using nextgeneration devices, the internalcomplexity of these devices andthe high number of portsintroduce some significant designand test challenges for NEMs.Operators will also encounternew challenges in acceptancetesting prior to deployment. Therest of this application note willdescribe some of thesechallenges.

Figure 1:Generic architecture

of next generationSONET/SDH device

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Behavior and test challengesof legacy devices

A traditional SONET/SDH devicewill perform one of severalfunctions. For example,multiplexing/demultiplexing,cross connecting/switching, orgrooming. Let’s consider an add/drop multiplexer. If we regard thetributaries as the input side, andthe line or aggregate side as theoutput, then there are generallyfewer outputs than inputs.Normally only one ring issupported on the line side.Therefore a single portinstrument is adequate fortesting on the line (aggregate)side of the device. On the inputside only SONET or SDHtechnology is involved with thepossible addition of PDHinterfaces. There is unlikely to beany other technology such as GbEpresent. This type of device isshown in Figure 2.

Behavior and test challenges ofnext generation devices

In contrast to the traditionaldevice with a limited number oftributaries and supporting asingle ring, the next generationdevice has many tributaries ofvarying rates and technologiesand supports several rings. Thisis shown in Figure 3.

Figure 2:Legacy SONET/SDHadd/drop multiplexer

Figure 3:Next generationdevice supportingmultiple-rings

Normal operation

• Mixed payloads or tributaries

• Aggregate rings at higherrate (e.g. STS-192)

• Tributary traffic multiplexedinto aggergate payloads

• Tributary traffic alsodemultiplexed fromaggregate payloads

Next generation

• Support multiple rings

• Muxing of ADM withaggregation of multi-servicedata switch

• Integral DXC used to switchvirtual tributaries (i.e. assign VTto rings)

• IP/Ethernet MAC switching

• Single NE covers DS-1 to OC-192 and also handles datacominterface

• Protection switching eitherperformed at a card level or viaintegral DXC

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Multiple errors(errors on tributaries)

Let’s see what happens when weinject an error onto one of thetributaries.

• The tributary interfaceregisters B3 errors and as aresult B1 and B2 errors willalso appear on the tributary toreflect this state.

• The B3 errors are transportedin the payload through thedevice and on to one or severalof the rings on the line side tothe far end. The B1s and B2sare terminated in the device.

Figure 4:The effect of tributary errors

• The transported B3 errors aredetected by remote networkelements which then respondby generating REI-P (or REI-RSin SDH)

• Now consider a worst-casescenario. The B3 error in thetributary is carried to all of therings supported by our device.There is potential for floodingthe network with errors andalarms as each networkelement within each ringresponds by issuing REI-Puntil the signal is restored.

• Finally, protection switching isused to recover the tributarysignal if the error rate exceedsthe chosen error threshold, forexample 10–3. Alternatively,error suppression algorithmsare used to contain the errorrate below the threshold.

In the worst case, multiple pathson multiple rings are errored atthe same time. It is no longergood enough to test one channelor even one port at a time. Theonly way that you can measurethe true behavior of networkelements in this type ofenvironment is to usesimultaneous multi-channel,multi-port testing. There is noother way to achieve a completepicture of the network elementscharacteristics. The onlyalternative is to test less and takechances. For example, use asingle port tester and assumethat what you see on one ring isrepresentative of the others.

The support of multiple rings bynext generation equipment is afundamental change, and it isthis aspect more than almost anyother that presents significantand complex challenges fortesting the device. Two typicalchallenges found in nextgeneration devices are describedas follows.

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Multiple alarms(failure of tributary)

Now let’s see what happens ifthere’s a cut in the tributaryinterface in a multi-serviceplatform (MSPP).

• First, the tributary interfaceregisters a LOS.

• This LOS then causes an AIS-P(or AIS-RS in SDH) to begenerated in all impactedvirtual containers on the lineside of the device. Wheremultiple rings (multipleaggregate lines) are supported,this will occur across allaffected rings at the same time.

• As the AIS-Ps are transportedaround the rings, the NetworkElements associated withthese rings then respond withan RDI-P (or RDI-RS in SDH).

• Finally, protection switching isused to recover the tributarysignal.

Figure 5:The effect of a tributary failure

If multiple rings are affected thenonce again there is potential forflooding the network with alarmsuntil the signal is restored. As inthe previous example, the onlyway that you can adequately testthe behavior of a device in thisscenario is to use simultaneousmulti-channel, multi-port testing.There is simply no other option.

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Error and alarm bursts

It is clearly useful to have atester that can re-create thenetwork conditions describedpreviously. These conditions maybe achieved with a suitable multi-port instrument capable ofrunning in through mode.

Once installed in the network,devices will regularly be exposedto bursts of errors and alarms asa result of the scenariosdescribed earlier. Performancemonitoring involves generatingerrors and alarms to a device andthen measuring the response tothem. This is a vital part of boththe operation and testing of anetwork element.

Performance monitoring iscovered in Telcordia standardGR-253, ITU-T standards G.821,G.826 and G.828 and in the ITU-M2100 series standards. Inessence, the standards define thenumber and frequency of errorsor alarms that may be toleratedon a network connection before itis deemed to have failed, orbefore an associated device isdeemed to have failed.

Manufacturers implementthresholds within the NE toensure stability during error andalarm flooding conditions. Animportant test of thesethresholds is to send bursts oferrors and alarms in a controlledmanner (e.g. a fixed number oferrors just below and then justabove the device threshold) andthen observe the behavior of theDUT. Does it respond correctly?

Is it stable? This can only be trulyassessed by applying realisticinputs across all ports andchannels simultaneously, andmeasuring the response of thedevice across all the ports andchannels.

Protection switching

The previous examples referredto the use of automaticprotection switching (APS) torestore the network to normal inthe event of error rates exceedingset thresholds. In next generationdevices with multiple rings APSbecomes much more complexthan before with a number ofnew protection schemes beingapparent in these new devices.APS will be the subject of aseparate application note.

Critical advantages ofmulti-port/multi-channelmeasurement

In conclusion, it is apparent thata new approach is required whentesting next generation networkelements (MSPPs). It is not somuch the specific test functionsthat are a challenge, it is thenumber of ports and the need forsimultaneous test that isimportant. Without simultaneoustest across a number of ports it iseasy to miss faults that occurintermittently and almostimpossible to gauge the truebehavior of a network elementwhen multiple errors and alarms.This is summarized in the Figures6a, 6b and 6c.

A fundamental requirement ofthe design of NG systems is theirability to remain stable duringoccurrences of error and alarmflooding. They must be able torespond correctly should thelevels of errors and alarmsexceed pre-set thresholds.Testing of this behavior isdescribed in the “Error andAlarm Bursts” section.Workarounds for running suchtests using splitters andamplifiers coupled to single porttesters are possible for a fewports at a time. However, thismethod simply isn’t adequate totest the high port counts nowbeing employed. In fact, it couldeven prove impossible to achievethe required error / alarmthresholds on enough ports forthe test to have any validity. Inthis situation the first chance amanufacturer would have todiscover a fault would be duringthe first customer installation.Not a risk that any manufactureris likely to be comfortable with.

Similarly, when considering theswitching and groomingcapabilities of a next generationdevice it is important to havemulti-channel measurements.This allows for the tracking of allpayload containers through thesedevices and allows the containersto be tracked on the line side ofthe device. Again this is notpossible on a single port, singlechannel device. You could testeach port in turn, but you can’tsee what’s happening on theother ports.

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Figure 6c:Thru-mode is essential fornext generation testing

Figure 6a:Conventional test equipmentwith next generation DUT

Figure 6b:Next generation multi-port testequipment makes the difference

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The key advantages with multi-port, multi-channel testequipment are:

• Visibility of the whole deviceunder test

• Ability to stress the wholedevice under test

• Speed and productivity oftesting numerous ports andchannels in parallel

and

• Security that any potentialfaults will be discoveredduring testing and not byoperators using the newequipment in their networks.

A new generation of networkequipment has created a new setof challenges that are now beingmet only by the new breed of testequipment being introduced.

The OmnBER XM network simulator –modular and scalable

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Related literature

For further information, pleaserefer to the followingpublications:

• “OmniBER XM networksimulator – redefining yourstrategy” brochurepub. no. 5988-6647EN

• J7241A/J7242A OC-192/STM-64 module data sheetpub no 5988-6665EN

• J7244A/J7245A OC-48/STM-16dual-port multi-rate moduledata sheetpub. no. 5988-6785EN

• J7246A/J7247A OC-12/STM-4dual-port multi-rate moduledata sheetpub. no. 5988-6786EN

• OmniBER XM configurationguidepub. no. 5988-6648EN

• OmniBER XM networksimulator J7263A 4-slotchassis technical overviewpub no. 5988-7996EN

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© Agilent Technologies, Inc. 2002Printed in USA, November 12, 20025988-8265EN

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