Alarm stress testing

Application note

OmniBER OTNcommunications

performance analyzer

G.709

2

Introduction

Standardization has played amajor part in the successfulevolution of the modern telecomnetwork. The introduction ofSONET and SDH standards in the1980s opened the door toincreased competition betweennetwork equipmentmanufacturers, leading to thereduced cost of networkequipment. Standardization alsoprovides the path towardsseamless integration ofequipment from differentvendors in the network. However,realizing this goal requiresvendors to interpret andimplement the standards in thesame way.

Ensuring that networkequipment designs faithfullyfollow the specifications laiddown in OTN, SONET and SDHstandards requires highly

specialized test equipment. Thetest equipment must be capableof fully stressing the designers’implementation. Failure to verifydesigns at this stage can lead toexpensive interoperability issueslater on.

This product note will show howthe OmniBER OTN’s powerfuland flexible test capability can beused to ensure designcompliance to the lateststandards.

The OmniBER OTN alarm stresstest capability covers SONET,SDH and the more recent ITU-TG.709 Interface for the OpticalTransport Network (OTN).

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Network management

Current optical transportnetworks allocate a portion ofthe bandwidth to managementfunctions. This allows real-timemonitoring of networkperformance, includingautomatic recovery andrestoration of some faults thatcan occur during normal day today operation.

The network management systemgains awareness of adverseconditions in the networkthrough alarm (defect) and error(anomaly) events. These aredetected by network equipmentand flagged to the managementsystem under defined conditions.

Alarms are only raised when asignificant event, such as abroken optical fiber, occurswhich may require someoperator intervention.Verifying that alarms aredetected and cleared as definedin the appropriate standards isvital. Compliance to standardsmust be made at the design stageto ensure and to reduce the riskof interoperability issues withother vendors’ equipment.

Alarm stress testing withOmniBER OTN

The OmniBER OTN has powerfulalarm stress testing capabilitythat allows frame by framecontrol of alarm generation.This paper will take you throughan example of out-of-frame(OOF) and loss-of-frame (LOF)stress testing, for ITU-T G.709frame synchronization.

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Figure 1Framing bytes

Frame synchronization

Let’s look at the frame alignmentprocess. The framing bytes (OA1/OA2) of an ITU-T G.709 signal arefound in columns 1 to 6 of thefirst row of the optical channelframe (see Figure 1). These bytesare unscrambled.

During the out-of-frame state, aframing device will constantlysearch for a 4-byte subset of thesix OA1/OA2 framing bytes.(It’s worth noting that G.709does not specify which 4 bytesubset of the six framing bytesshould be used).

In-frame takes place if thissubset is found and confirmed inthe same location, one framelater. (Two consecutive frameswith good framing bytes arerequired for in-frameconditions).

During in-frame, the signal iscontinually monitored for correctframe alignment using the OA1OA2 OA2 pattern found incolumns 3, 4 and 5 of the opticalchannel frame. The out-of-framestate occurs if this subset is notfound in the correct location forfive consecutive frames (SeeFigure 2). An out-of-framecondition that persists for 3 msbecomes loss-of-frame (LOF).

Figure 2Example of OTN OOF

stress test

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p, n and m stress testing

The p, n and m alarm stress-testing feature of the OmniBEROTN is used to verify entry andexit criteria for a wide range ofalarms.

Testing OOF

Figure 3 graphically shows howthe OmniBER OTN can be usedto stress test the framingalgorithm of a network element.

With the OmniBER OTN transmitinterface connected to the inputof the device under test (DUT),the alarm stress test feature isselected (see Figure 4). OOF isnow turned on, as this is thestarting point for the test.

The stress test feature allows theuser to transmit either a singlepulse of the alarm on/offcondition (number of framesdefined by the user).

Alternatively a single pulse (‘p’)followed by a repeating sequenceof frames with the alarmswitching between the on (‘n’)and off (‘m’) states.

A powerful feature of theOmniBER OTN stress test is theability to dynamically andseamlessly change the number offrames in the ‘n’ and ‘m’stresssequence while the test isrunning.

Figure 4 shows the OmniBEROTN settings required togenerate the stress sequence inFigure 3.

1. OOF begins the sequence inthe on state.

2. A single two-frameopportunity for the DUT togain frame alignment.

3. Followed by a repeatingstress cycle of the alarm onfor four frames then off forone frame.

Figure 3OTN frame sync stress test

Figure 4

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Figure 6

The OmniBER OTN nevergenerates five bad frames in arow in this setup. Thereforethe DUT should never enterthe out-of-frame state.However, it will show frameerrors. The OmniBER OTN canalso be configured to test theOOF/LOF requirements asillustrated in Figure 5.

OTN, SONET and SDHalarm stress test

This paper gives one exampleof an alarm stress test on anITU-T G.709 compliant networkelement or sub-assembly. Otheralarms that can be tested in thismanner by the OmniBER OTNare as shown in the followingtable:

OTN SONET SDHalarms alarms alarms

LOS LOS LOS(10.7 Gb/s) (10.7 Gb/s) (10 Gb/s)

LOF LOF LOF

OOF SEF OOF

LOM AIS-L MS-AIS

OOM RDI-L MS-RDI

OTU-AIS AIS-P AU-AIS

OTU-IAE LOP-P AU-LOP

OTU-BDI UNEQ-P HP-UNEQ

ODU-AIS RDI-P HP-RDI

ODU-OCI

ODU-LCK

ODU-BDI

Figure 5OTN out of frame (OOF) stress test

LOS stress test

In the SONET and SDHstandards, there are two types ofanomalies that can be used todetect a loss of signal (LOS).

These are:

� A period of time with notransitions.

� Optionally, opticalpower below somethreshold level1

may also be used.

The OmniBER OTNhas the uniquecapability to stressnetwork equipmentto correctly detectLOS at 10 Gb/s. It does this bytransmitting runs of zeros (notransitions) for a user definedtime period.

The SONET and SDH standardsdiffer slightly when defining thedetection criterion for LOS. Thedefinitions from the standardsare shown in Figure 6. Putsimply, for optical signals, anLOS defect is defined as a periodof time with no transitions (lightpulses).

1 This method of LOS detection is implemented in the OmniBER OTN receiver

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LOS in SONET

In the SONET standard, a LOSdefect should be declared if thereare no transitions on anincoming signal for 100 ms orlonger. A LOS should NOT bedeclared if the period with notransitions lasts for 2.3 ms or less.If the LOS defect persists formore than 2.5 seconds, then aLOS alarm will be raised

What happens for periods of notransitions lasting greater than2.3 ms and less than 100 ms is notdefined in the SONET standardsand is left to the choice of theequipment designer.

LOS in SDH

For SDH, no timing is specified.Rather it states that “ The timingrequirements for detection of theLOS defect (is) in the province ofregional standards”. The examplein Figure 6 shows where a LOSwould be declared if notransitions were detected for aperiod T greater than 2.3 ms.

Testing LOS detection

Using the OmniBER OTN, it is asimple matter to verify that anetwork device complies with therelevant standard for LOSdetection. This is achieved bysending pulses of no transitionsinto the device under test (DUT).The test setup is shown inFigure 7. The OmniBER OTNoutput is connected to the inputof the DUT and the response ofthe DUT is monitored via thenetwork management system orcraft port.

All that is required is to set theOmniBER OTN to transmit acorrectly structured SONET, SDHor OTN signal. Then select Menu®�Test Functions ® Errors andAlarms. This provides access tothe LOS stress test shown inFigure 8. Enter any value from

0.1 to 2.3 ms and select the‘Transmit’ button on the display.The DUT should NOT declareLOS. Next, enter any value from100 to 110 ms, select the‘Transmit’ button on theinstrument display and the DUTSHOULD declare LOS.

Note: There is no specification inany of the standards that setsout a requirement for a numberof consecutive zeros that anetwork device should receivewithout errors.

Figure 7LOS test setup

Figure 8OmniBER OTNsetup

Agilent Technologies manufactures the

OmniBER OTN family under a quality

system approved to the international

standard ISO 9001 plus TickIT (BSI

Registration Certificate No FM 10987).

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Conclusions

This application note has shownhow the OmniBER OTN powerfulalarm stress test feature caneasily be used to verifyequipment conformance toSONET, SDH and ITU-T G.709(OTN) standards.

Precise sequences of alarm on/off conditions can be readilyconfigured by the user to testentry and exit criteria for majoralarms. This precise controloffers equipment designers theideal tool for verifying anddebugging new equipmentdesigns using an independenttest source.

The unique LOS (loss of signal)stress test also provides a simple,independent method forverifying compliance to therequirements for this criticalalarm.

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