1

T1/F

T1 T

est

ing

Overview

The demand for high-quality T1 circuits requiresconsistent maintenance and circuit analysis. To provideclean, error-free transmissions, the personnel who testthe performance of T1 circuits demand reliable instru-ments. The TTC® (Telecommunications TechniquesCorporation) FIREBERD Communications Analyzers areideal for T1 installation, acceptance testing, ongoingmaintenance, and fault isolation, all of which are essen-tial to providing quality T1 service.

This Application Note describes various T1and Fractional T1(FT1) fundamentals, and the impair-ments that can degrade transmission service. It thendescribes using the FIREBERD for in-service monitoringand out-of-service testing.

T1 Background

T1 circuits provide digital transmission of voice,data, and video signals at 1.544 Mb/s. They are used bycommon carriers, telephone companies, private net-works, and government agencies. T1 circuits are prima-rily used to transmit multiple voice and data signals thatare multiplexed and transmitted over a singlecommunication path. These circuits are found in bothpoint-to-point and network environments, as shown inFigure 1, on the next page.

Part A of Figure 1, on the next page, shows asimple point-to-point circuit connecting two pieces ofcustomer premise equipment (CPE) such as channelbanks or multiplexers. There are no intelligent devicessuch as digital cross-connect systems (DCSs) or higherrate multiplexers (e.g., M13) along the transmissionlink. This figure represents a typical small private net-work connecting two sites, or pieces of equipment.

Part B of Figure 1, on the next page, shows anetwork configuration that includes channel service units(CSUs), metallic local loops, span repeaters, digital sig-nal patch bays (DSXs), and transmission devices such asDCSs and M13 multiplexers. The CSU acts as a networkinterface between the customer premises equipment(CPE) and the service provider’s T1 network. Test accessis often provided through physical connections orloopback features. Span repeaters, placed at leastevery six thousand feet along the local loop,regenerate the signal in both directions. Office repeatersterminate the local loop and often feed the T1 signal toeither a DSX patch bay or an electronic DCS. The DSXpatch bay is used for manual signal routing and testaccess. The DCS enables multiple T1 circuits to berouted through the office electronically, and also pro-vides test access. Before being sent to the long-haulfacilities, the T1 circuits are typically multiplexed to aneven higher rate. To do this, M13 multiplexers combine28 T1 signals into one T3 signal (44.7 Mb/s). When theT3 signal reaches the far office, it is demultiplexed sothat each T1 signal may be transmitted across the desiredloop and into the far-end’s CPE. The T1 signal is thentransmitted across another local loop, or is transmittedto another central office (CO) by long-haul facilities(e.g., microwave, satellite, fiber optics).

T1/FT1 Testing with theFIREBERD 4000 and FIREBERD 6000®®

2T1

/FT1

Test

ing

28

MON

O R

O R

CSUCPE CSU CPE

Customer Premises

Local Loop

Central Office

A. Point-To-Point Circuit

MON

DCSO R

CSUM1328

Metallic Local Loop

MON

O RDCS

M13

CSU

CSU - Channel Service Unit - Span Repeater OR - Office Repeater CPE - Customer Premise Equipment

DSX-1 - Digital Signal Patch Bay DCS - Digital Cross-Connect System M13 - DS-1 TOS-3 Multiplexer

B. Network Configuration

Long-Haul Facilities

(Fiber Optic, Satellite, Radio)

DSX-1

DSX-1

DSX-1

Local Loop

Customer Premises

Central OfficeCustomer Premises

Central OfficeMetallic

Local LoopCustomer Premises

FT1 Background

Many users need the capabilities of a T1network, but few need a full complement of 24 DS0channels running into their facility. For these usersneeding bandwidth of less than 1.544 Mb/s, FT1allows them to select DS0 channels individually, rather

than in groups of 24. For example, 4 channels can beleased for video applications, 2 for voice and 2 fordata communications. As a result, users can meet allof their communication needs with FT1 service, in-stead of under-utilizing a full T1 circuit. Three mainchoices must be made when implementing FT1 service:bandwidth, Nx64 or Nx56 DS0s, and contiguous vs. non-contiguous channels.

Figure 1T1 circuitenvironments.

3

T1/F

T1 T

est

ing

Bandwidth

First, users must select the amount of bandwidththat’s needed. FT1 service typically offers DS0s ingroups of 2, 4, 6, 8, and 12 – at rates of 56 or 64 kb/sper channel. The most common fractional data ratesoffered are listed below:

Nx56 kb/s Nx64 kb/s 56 kb/s (x1) 64 kb/s (x1)112 kb/s (x2) 128 kb/s (x2)224 kb/s (x4) 256 kb/s (x4)336 kb/s (x6) 384 kb/s (x6)448 kb/s (x8) 512 kb/s (x8)672 kb/s (x12) 768 kb/s (x12)

Nx56 kb/s or Nx64 kb/s

Users must select between Nx56 or Nx64 kb/s,depending on the line coding supported by the serviceprovider. Nx56 kb/s is typically offered if the serviceprovider uses alternate mark inversion (AMI) linecoding, in which only 56 kb/s of the 64 kb/s DS0 isused for customer data. The remaining bandwidth isfilled by the forced insertion of ones to maintain onesdensity. This reduces throughput, but allows frac-tional service to be provided on a T1 span using AMI.

Nx64 kb/s service is offered if the providerhas incorporated bipolar eight zero substitution(B8ZS) line coding into their T1 network. B8ZSallows for utilization of the entire FT1 bandwidth byeliminating the forced insertion of ones.

Contiguous orNon-Contiguous DS0s

Finally, users must select either contiguous ornon-contiguous DS0s. Contiguous bandwidth is DS0sthat are routed together end-to-end to support applica-tions greater than 56 or 64 kb/s. Applications such asdata and video often require contiguous bandwidththroughout the T1 network, eliminating any delay

problems caused by different routing paths through adigital area cross-connect system (DACS). If the ser-vice user’s application is data oriented and requiresNx64 kb/s service, the service providers’ T1 networkmust support B8ZS line coding. If not, contiguousbandwidth can’t be used for the application.

Non-contiguous bandwidth refers to individualDS0s that are not adjacent, but are routed togetherthrough a DCS. Non-contiguous DS0 service fillsalternating timeslots of a T1 frame with a fixed eight-bit word, referred to as the idle code. The idle codemust contain at least two ones within each eight bitword. If Nx64 needs to be used for voice applica-tions, and the service provider doesn’t offer B8ZS linecoding, non-contiguous service must be used be-cause it maintains the T1 ones density requirement.However, it is not appropriate for fractional dataapplications because bandwidth for data must becontiguous (see Figure 2).

The T1 span between the customer premisesand the service provider office remains physicallyand electrically the same for FT1 service. FT1 serviceuses the same coding and framing techniques asstandard T1 service. The customer installs T1 multi-plexers to bring their individual DS0s up to T1(1.544 Mb/s) speed. If future channels are needed,the service provider can provision as needed.

F 5 6 7 13 14 15 16 17 18 19 208 9 10 11 12 21 22 23 24

Contiguous…

Non-Contiguous…

Active Channel: Inactive Channel:

F 2 4 6 13 14 15 16 17 18 19 208 9 10 11 12 21 22 23 24

*Inactive Channels (5-24) are filled with an idle code (00110000)

*Inactive Channels (2, 4, 6, 8-24) are filled with an idle code (00110000)

1 2 3

1 753

4

Figure 2Contiguous ornon-contiguousbandwidth.

4T1

/FT1

Test

ing

Causes of T1 Impairments

There are four main causes of T1 impairments:

1. Faulty Equipment. Any T1 equipment cancause errors when components fail or operateoutside of specification. Errors that may sug-gest faulty equipment include bipolar viola-tions (BPVs), bit errors, frame errors, jitter,slips, and excess zeros. For example, BPVscan occur due to faulty clock recovery circuitryin span repeaters. These errors occur as theequipment becomes older and begins to drift outof specification.

2. Improper Connections. Transmission errorsare created by improper connections or configu-rations. For example, intermittent errors canoccur when component or cable connections areloose, and timing errors can occur when im-proper or conflicting timing sources are con-nected together. Dribbling errors are oftencaused by loose or unconnected shield groundcables and by bridge taps. Further, upon instal-lation, the circuit may not work at all due tomislabeled pins on terminating cable blocks andto crossed wires: transmit-to-transmit instead oftransmit-to-receive. These errors are typicallydiscovered upon circuit installation and possiblyduring circuit acceptance when end-to-end testsare performed.

3. Environmental. Electrical storms, power lines,electrical noise, interference, and crosstalk be-tween transmission links can cause BPVs as wellas bit, frame, and CRC errors. Typically, theseconditions cause intermittent, bursty errors,which are among the most difficult to locate.Although the cause of these impairments isobvious, the one that’s difficult to pinpoint iscrosstalk. Crosstalk is sometimes caused byimproperly separated cable pairs. Transmit andreceive pairs should be between 25 and 100pairs apart.

4. Data Specific. Data characteristics, such asrepetitive patterns, can force equipment to createpattern-dependent jitter and code errors. Theseerrors may not exist when testing the transmis-sion path with standard pseudorandom patterns.For example, span repeaters regenerate the digi-tal signal and recover timing from the bipolarsignal. To facilitate timing recovery, there mustbe a sufficient number of transitions (e.g., no morethan 15 consecutive 0s), otherwise, the repeatercan’t properly recover timing from the signal.

Techniques andMeasurements

To analyze a T1 circuit’s performance and toisolate the causes of degraded service, the test instrumentmust perform these four basic types of measurements.

1. Installation. When installing a T1 circuit,out-of-service testing helps verify equipmentoperation and end-to-end transmission qual-ity. Begin by testing the equipment (e.g.,channel banks, multiplexers), and then veri-fying cable connections, timing source selec-tions, and frequency outputs. After qualifyingthe circuit end-to-end, each CSU is loopedback to ensure that it responds to both loop-up and loop-down codes and to verify that thecircuit operates properly at each end.

2. Acceptance Testing. In addition to the testsperformed during installation, stress tests andtimed tests should be performed to ensure thatthe T1 is operating according to the relevantT1 circuit specifications. Stress the equipmentby injecting errors, jitter, and data patterns(either pseudorandom or fixed) into T1 equip-ment. The same procedure may be performedend-to-end to stress the entire T1 circuit. Timedtests with printouts should be performed over a24- or 48-hour period using standard pseudo-random patterns to simulate live data. This isdescribed more fully in Application 2.

5

T1/F

T1 T

est

ing

3. Preventive Maintenance. Once live data istransmitted across the T1 circuit, preventivemaintenance tests are strongly recommended.Preventive maintenance can alert technicians todegrading service before it disrupts normal op-erations, and involves monitoring the live data forBPVs, frame errors, CRC errors, and signal fre-quency measurements that provide informationabout the performance of the T1 circuit. Thesetests should be performed with printouts over a24- or 48-hour period to detect time-specific orintermittent errors. This is described more fullyin Application 1.

4. Fault Isolation. Fault isolation is requiredonce excessive error rates disrupt service. Thiscan be performed using both in-service and out-of-service tests. In-service testing provides gen-eral information, and can be used before out-of-service analysis to localize problems and mini-mize circuit downtime. Monitoring the circuit atvarious points enables technicians to analyze theresults and determine the source of problems.Performing standard out-of-service tests (e.g.,loopback and end-to-end tests) enables techni-cians to stress the equipment, find sources oferrors, and verify proper operation once thetrouble is repaired.

Application 1:In-Service Analysis of

Live Traffic

This application describes using the FIREBERD 6000to evaluate the general performance of a T1/FT1 span. Thisapplication is useful for the following:

• Performing periodic maintenance, and whenlooking for transmission degradations beforethey affect service.

• Analyzing the span for intermittent errors thatare caused by faulty equipment or environmen-tal influences.

• Analyzing the data stream for data-specific errors(i.e., impairments caused by the data itself).

• Analyzing T1/FT1 circuits that can’t be taken out-of-service.

• Localizing the problem and minimizing cir-cuit downtime.

To derive all of these benefits, the FIREBERD 6000may be configured to monitor the T1 circuit from nearlyany T1 access point. Figure 3 shows a typical circuitand possible monitoring locations. Once monitoring

MON

O R

CSU

DSX-1

T1 Network

MONDSX-1

O R

CSU

Figure 3Possible FIREBERDmonitoring locations.

6T1

/FT1

Test

ing

Table 1Common alarm anderror indications.

begins, look for BPVs, bit slips, frame errors, CRCerrors (for ESF framed circuits), associated rates,and alarm conditions. These results are helpful inisolating the cause of the problem. See Appendix B,on page 19, for information on configuring theFIREBERD for in-service analysis.

Check for Proper Operation

Now that the FIREBERD 6000 is monitoringthe T1 span, verify that it’s operating properly andproviding accurate results by observing the two indi-cators on the right side of the front panel. Table 1highlights common error indications, along withpossible reasons and solutions.

Other In-Service Tests

Identifying Activeand Idle Channels

You may also perform installation tests withthe T1/FT1 Interface Module. With the AUTO CHAN-NEL function, you can verify the active and idle chan-nels of a FT1 circuit. To do this, program the DCS-generated idle code into the 41440A’s IDLE codefunction and then select AUTO CHANNEL in the FT1mode menu. This function then scans all 24 channelsfor the idle code and determines which channels areactive and which are idle.

Indicator Reason Solution

GEN CLK No receiver clock detected Check cabling. Verify proper connections toilluminated by the FIREBERD circuitry. a valid T1 circuit. Try new cable.

SYNC FIREBERD isn’t synchronized If performing an end-to-end test with twonot illuminated to the incoming pseudorandom FIREBERDs, verify that both sets are

pattern. transmitting the same data pattern. CheckFRM SYNC, MK, and SP indicators. If theystill don’t illuminate, perform SELF TEST.

SYNC LOST Indicates history of signal loss Check cabling. Verify connections.illuminated when in LIVE mode.

FRM SYNC Signal is unframed, or Verify all control settings (especially thenot illuminated synchronization to the specified framing format) and connections. If unit

framing hasn’t been achieved. still doesn’t achieve synchronization to theframed signal, perform SELF TEST.

7

T1/F

T1 T

est

ing

Monitoring PerformanceReport Messages

The T1/FT1 Interface Module with ANSI T1.403PRM Option (Option 6009) also enables the FIREBERDto transmit and decode ANSI T1.403 PerformanceReport Messages (PRMs). ANSI standard T1.403(1989) is a technical report that addresses ESF per-formance monitoring. In T1.403 mode, in-serviceperformance results can be monitored from end-to-end of the terminated T1 line.

Overhead bits from the extended superframe(ESF) form an 8 kb/s data link channel that providesperformance results. The ESF datalink enables scheduledPRMs, unscheduled priority messages, and command/response messages to be transmitted and received acrossthe T1 line without interfering with the DS0 channels.

ANSI T1.403 PRMs allow both carrier andservice user’s non-intrusive tests, and provide reportson the previous three seconds and the current sec-ond, by using the information contained in the ESFdatalink. PRMs are scheduled messages that providecontinuous T1 signal performance monitoring be-tween terminating devices. They provide informationon CRC error events, severity of frame errors, theoccurrence of BPVs, controlled slips, and whether thefar-end or near-end is in payload loopback.

PRMs are recalculated at each terminating device(e.g., MUX, DSU, CSU, etc.). Therefore, they don’t indi-cate end-to-end performance – they indicate perfor-mance between the user and the next piece of equipment.This analysis is useful for sectionalizing a problem to thetransmit or receive leg of a line.

Most CSUs can be configured to transmit PRMs,but they don’t have the facilities to decode and present thereceived PRM information. This is left up to T1.403compatible test sets like the FIREBERD 6000, or devicessuch as T1 monitor units (MU). The FIREBERD 6000connects to the DS1 through the DSX monitor jack to

analyze the T1 signal, decode the PRMs, and presentperformance information on its front panel. Refer to theT1 ESF PRM Network Testing Application Note for moreinformation on PRMs.

Results Analysis

The FIREBERD 6000 will accumulate all resultssimultaneously. The desired results can be achievedusing the appropriate CATEGORY and RESULTSswitches. Refer to Figure 4, on the next page, to findyour location along the T1 span, and check your locationwith Table 2, on the following page, to find possiblecauses of problems. Every T1 system is different, and maynot be susceptible to the noted cause.

Application 2:Out-of-Service Testing

This application provides performance informa-tion about a T1/FT1 circuit using pseudorandom data. Itis useful for the following:

• Installing T1/FT1 circuits and verifying end-to-end continuity.

• Isolating T1/FT1 circuit faults by inserting pseu-dorandom patterns and interpreting results.

• Performing acceptance tests, including timedand stress tests.

Errors found through this analysis may be causedby faulty equipment, improper connections, environ-mental influences, or data content. To find these errors,monitor bit errors, average bit error rate (AVG BER), bitslips, error-free seconds (EFS), percent error-free sec-onds (% EFS), etc., results that are all measured simul-taneously. These results will help in isolating the cause ofthe problems.

8T1

/FT1

Test

ing

Figure 4Monitoring Locations.

O R

CSU

DSX-1

T1 Network

O R

CSU

A

C

B

C

DSX-1

MON MON

A

(A) Bipolar BPVs Local problem. Possibly bad cabling connectionsViolations between test set and circuit, corroded “dirty” cable

plugs, or a defective CSU.

(A) or (B) or (C) Receive RCV FREQ Offset Frequencies that are out-of-range may affect jitterFrequency ±75 b/s tolerance and noise margins, or they may causeOffset error bursts and timing slips.

(B) Bit Slips BIT SLIPS Network problem, due to inconsistent timing throughout the network. Check timing recovery in the DCS.Also may be due to incorrect optioning on CSUs andchannel banks.

(B) or (C) Bipolar BPVs Local T1 span problem. Possible faulty repeater, spanViolations, FRA ERR, CRC ERR line noise, crosstalk, poor cabling, or defective DSXFrame Errors, jacks.or CRC Errors

(B) or (C) Bipolar BPVs Local T1 span problem due to marginal timing recoveryViolations, FRA ERR, CRC ERR during periods of excess zeros. Check repeaters,Frame Errors, XS0s multiplexers.CRC Errors,or Excess Zeros

LocationFor Figure 4

Results DisplayedBy FIREBERD

Problem/SolutionResults

Table 2Correlation of

results andproblem causes.

9

T1/F

T1 T

est

ing

Results DisplayedBy FIREBERD

Problem/SolutionResults

(C) Bipolar BPVs, No FRA Local T1 span problem.Violations, ERR, CRC ERRNo Frame Errors,or CRC Errors

(C) No Bipolar No BPVs, FRA ERR, Typically far-end span line problem. SectionalizeViolations, CRC ERR further. Potential for light guide, radio, or ViolationFrame Errors, Monitor Removal (VMR) equipment in network.or CRC Errors

(C) No Bipolar No BPVs, FRA ERR, Typically far-end span line problem, often due to signalViolations, CRC ERR, generator transmissions that don’t meet pulse densityFrame Errors, XS0s specifications. First, verify signal generator operation,CRC Errors, then check repeaters and multiplexers.or Excess Zeros

LocationFor Figure 4

Table 2Correlation ofresults andproblem causes.(Continued)

There are basically two methods of performingout-of-service testing: loopback testing and end-to-endtesting. The FIREBERD’s configuration is similar for bothtypes of testing. The two major differences are: equip-ment needed and the establishment of a loopback, ad-dressed in the following sections.

End-to-End Testing

End-to-end testing is performed with twoFIREBERDs so that analysis may be performed simulta-neously in both directions. Figure 5 shows that basicsetup of an end-to-end test. This method providesadvantages over loopback testing because the directionof errors can be more quickly isolated, and it allows youto determine whether the transmit or receive leg is faulty.

Figure 5Basic setup for anend-to-end test.

MON

O R

CSU

DSX-1

T1 Network

MONDSX-1

O R

CSU

FIREBERD 6000

Tx RxFIREBERD

6000

TxRx

10T1

/FT1

Test

ing

Loopback Testing

Loopback testing is performed with oneFIREBERD. Figure 6 shows the basic setup of a loopbacktest. If CSU loopbacks are established to perform the test,it’s important to realize that the far-end CSU in loopbackaffects the results. By design, most CSUs (like many otherpieces of transmission equipment) remove received BPVs,frame errors, and other errors before transmitting thedata. This affects analysis interpretation because the near-end technician won’t be aware of BPVs on the far-end’smetallic loop, and may draw inconclusive results.

NOTE: If excessive jitter exists on either sideof the T1 circuit, synchronization tothe pseudorandom pattern may beimpossible. Instead, use the end-to-end technique.

A variety of devices may be looped with theT1/FT1 Interface Module, such as the (NIU) smartjack or the CSU. Another loopback used for testing aFT1 circuit is the V.54 loopback. This allows theFIREBERD to loop up only certain channels in a FT1circuit. A CSU loopback loops all 24 channels, elimi-nating the isolation of certain DS0s from a fractionalcircuit. Instead, a V.54 loopback only loops backspecific channels. For example, assume there is aproblem with video conferencing on a circuit thatcontains both video and Private Branch Exchange(PBX) traffic. The PBX traffic must remain in-service,therefore you only want to test your video circuit(channels 1-4). This can be accomplished by settingonly these channels for TX/RX in a FT1 Insert Mode andthen choosing a V.54 loop type. Now a BERT can beperformed on channels 1-4 without disrupting trafficon the remaining DS0s.

Creating a Loopback

There are three methods of creating a loopback:

1. Press the LOOP UP switch on the FIREBERDfront panel (loop-up code 1:4, also referredto as 1000). The LED within the switch will beilluminated while the loop-up code is beingsent. When the FIREBERD detects a success-ful loop, the LED is extinguished and theFIREBERD stops sending the loopcode. Thisprevents the near-end CSU from looping upand locking the FIREBERD out of the spanunder test.

2. Program and send a facility or special, equip-ment-specific loop-up code such as 2:2 (alsoreferred to as 1100) or 1:5 (also referred toas 100000). Use AUX function 33 and thePGRM DATA switch to program and send codes.

3. Manually enable a loopback at the far-end CSU.

After testing is completed, remove the loopbackon the remote CSU by one of three methods:

1. Press the LOOP DOWN switch on the FIREBERDfront panel (loop-down code 1:2, also referredto as 100).

2. Program and send a facility or special, equip-ment-specific loop-down code such as 3:1 (alsoreferred to as 1110) or 1:3 (also referred to as100). Use AUX function 33 and the PGRM DATAswitch to program and send codes.

3. Manually disable a loopback at the far-end CSU.

11

T1/F

T1 T

est

ing

MON

O R

CSU

DSX-1

T1 Network

MONDSX-1

O R

CSU

FIREBERD 6000

Tx Rx

Figure 6Basic setup for a loopback test.

FIREBERD Testing Modes

Use the T1/FT1 Interface Module to configure theFIREBERD to operate in a variety of different testingmodes, such as Full T1 (FULLT1) or FT1. For FT1 testing,the FIREBERD can be configured to test in FRACT1 orFT1INS mode depending on the application.

Fractional T1 (FRACT1) Mode

This mode allows data to be transmitted andreceived at any FT1 rate of Nx64 kb/s or Nx56 kb/s(N = 1 to 24) on any selection of contiguous and non-contiguous timeslots. In this mode, you can configureindividual channels to transmit, receive, or both.

Fractional T1 Insert(FT1INS) Mode

This mode enables the insertion of a BERTpattern on any combination of selected Nx64 kb/s orNx56 kb/s timeslots without disrupting traffic on theremaining timeslots. FT1INS differs from FRACT1operation because it doesn’t disturb live data on theunselected timeslots, whereas in FRACT1 idle codesare sent over these unused timeslots. The AUTOselection is disabled in the FT1INS MODE.

Test Loopback Mode

The FIREBERD can be optioned to behave as arepeater or CSU for end-to-end testing. In the TestLoopback (TLB) mode, the FIREBERD emulates a CSU ora channel bank in digital loopback. All received data isechoed on the transmitter output and the received signalis analyzed by the FIREBERD data receiver. BPVs andB8ZS coding are stripped from the received signal, andthe outgoing signal is re-timed and re-encoded with AMIor B8ZS according to the CODE menu selection.

Line Loopback Mode

In Line Loopback Mode (LLB), the FIREBERDemulates a repeater. All data received is echoed un-changed on the transmitter output. The received data isanalyzed, but no re-coding or error insertion is available.

Performing an end-to-end BER test in both TLBand LLB mode determines the direction of faulty equip-ment. If errors occur in LLB, but not in TLB, then thetransmit leg of FIREBERD No. 1 is faulty. Likewise, ifFIREBERD No. 1 sees errors while FIREBERD No. 2 isin the TLB mode, then the receive leg of FIREBERD No. 1is faulty.

12T1

/FT1

Test

ing

Additional Applications

1. Once transmission level tests have been per-formed, further analysis can be performed by aprotocol analyzer. By setting the T1/FT1 InterfaceModule to operate in a RS-232 mode, access isprovided to either a 64 kb/s DS0 channel (CHANmode) or to the ESF data link (DATLINK mode).The desired channel may then be dropped andinserted to an external device.

2. The FIREBERD can perform payload and datalinkBER tests. Out-of-band Data Link Line Loopback(DL-LLB) loop codes can also be transmitted andrecognized by the FIREBERD on the ESF data link.These codes are transmitted to the far-end equip-ment to place the far-end equipment into a lineloopback. In contrast, out-of-band Data LinkPayload Loopback (DL-PLB) loop codes are trans-mitted and recognized by the FIREBERD on theESF data link. This code is transmitted to the far-end equipment to place the far-end equipmentinto a payload loopback. The CSU loops back thepayload data, reframes the signal, and recalcu-lates the CRCs.

3. Monitoring voice traffic may also be accom-plished with the FIREBERD in VOICE mode.The user may now check the integrity of thecircuit by inserting and monitoring voice traf-fic and monitoring and transmitting signalingbits on individual channels within the T1/FT1bit stream. Using this mode gives a quickcheck of the integrity of the circuit and verifiesproper signaling sequences. This is accom-plished by dropping an analog signal from anexternal device with the handset connector.For example, you may set the signaling bits foron-hook and then verify that proper off-hookbits are being returned. When monitoringABCD signaling bits, the FIREBERD extractsthe ABCD signaling bits from a single DS0 inthe incoming data.

Timing Analysis

Reference T1

The T1/FT1 Interface Module also allows youto determine bit slips, using the REFT1 connector. Bitslips are determined by comparing the timing of theT1 span under test with a reference T1 span on theother side of the network. REFT1 allows you to selectthe input impedance and signal conditioning for theT1 reference signal: the two input impedances areterminating (TERM) and bridge (BRDG). TERM isused when terminating the T1 reference input with100Ω and accepting a relatively unattenuated (+6through -6 dB) T1 reference. BRDG, which allowsmonitoring of reference T1 lines that are alreadyterminated, uses an impedance greater than 1000Ω.These results are helpful in isolating timing problemswithin the network.

Results Analysis

As long as the indicators are illuminated asshown in Figure 7, the FIREBERD will accumulateall results simultaneously. The desired results can beretrieved using the appropriate CATEGORY and RE-SULTS switches. Once the results are known, it’spossible to isolate the cause of degraded service.Every T1 system is different, so it is nearly impossibleto list all of the causes. Table 3 shows various resultcombinations and possible problem causes.

13

T1/F

T1 T

est

ing

Bit Errors BIT ERRs Check the entire span by isolating sections and testing.

Bipolar Violations BPVs Check the last repeatered span before your present loca-Bit Errors BIT ERRs tion. BPVs are present only on metallic loops and are

removed before the data is retransmitted by practicallyevery piece of transmission equipment except repeaters.

Generator Frequency GEN FREQ Frequencies that are out of range may affect jitter Offset Offset ±75 b/s tolerance, temperature tolerance, and noise margins;Receiver Frequency RCV FREQ they may also cause error bursts, cyclic frame losses, Offset Offset ±75 b/s and timing slips.

Frame Sync Loss FRA LOSS Indicates an out-of-frame (OOF) condition, declaredBit Errors BIT ERRs when two out of five framing bits are missed. A RedSynchronization Loss SYN LOSS Alarm is declared when an OOF condition exists for more

than 2.5 seconds and is defined as a locally detected failure.A Yellow Alarm is transmitted in the opposite direction.

Bipolar Violations BPVs If you’re counting clock slips and not frame losses,Clock Slips CLK SLIPS you’re observing a controlled slip typical of DCS andReceive Frequency RCV FREQ other buffering equipment that maintains framing Offset Offset ±75 b/s integrity. Frequency offsets may also cause controlledNo Frame Sync Losses No FRA LOSS slips. Observe the FRA LOSS and RCV FREQSlips BIT SLIPS measurements. Check timing sources and verify that

multiplexing and demultiplexing equipment is operat-ing properly.

Result ProblemResults DisplayedBy FIREBERD

RECEIVER

MK

SP

SYNC

SYNC LOST

LOOP UP

LOOP DOWN

ALM 1

ALM 2

CODE

FRM SYNC

Indicates signal present

Illuminated for framed circuits only

Illuminated for B8ZS only

Figure 7Indicators.

Table 3Correlation ofout-of-serviceresults andproblem causes.

14T1

/FT1

Test

ing

Results DisplayedBy FIREBERDResult Problem

Frame Sync Loss FRA LOSS If you’re measuring frame losses and clock slips, you’reClock Slips CLK SLIPS observing an uncontrolled slip typical of satellite downlinkBit Slips BIT SLIPS receivers and other buffering equipment that doesn’t distin-

guish between frame and data bits. Verify that multiplexingand demultiplexing equipment is operating properly.

Bipolar Violations BPVS Check the last repeatered span before your presentClock Slips CLK SLIPS location. Problems may be caused by faulty clockExcess Zeros XS0s recovery circuitry in repeaters. Sectionalize further.

Bit Error Rate BER Check circuit’s specifications and tariff for BER standards.For some carriers, the BER should not exceed 1 x 10-2 formore than 2.5 seconds. This equals 38,600 bit errors in2.5 seconds for a 1.544 Mb/s circuit.

Errored Seconds ERR SEC Check circuit’s specifications and tariff for error perfor-mance standards. For some carriers, 15 minute intervalswith more than 300 errored seconds may be reported astrouble.

Percent Error-Free % EFS Check circuit’s specifications and tariff for performanceSeconds level standards. A common, acceptable level is 95% EFS

over a 24-hour period.

Table 3Correlation ofout-of-service

results andproblem causes.

(Continued)

Fault Isolation

To localize problems, break the T1 circuit intomanageable sections, one step at a time, as shown inFigure 8. In Stage 1, verify that errors are occurringsomewhere in the circuit. In Stage 2, test a section of thecircuit and “circle in” on the source (or sources) of

errors. If errors still occur at Stage 2, at least one of thesources is between your location and the far- end. Testthe span section between Stage 1 and Stage 2 to make sureyou haven’t missed any sources of errors. Before deter-mining that the circuit is good, test it one more time end-to-end for verification.

15

T1/F

T1 T

est

ing

MON

O R

CSU

DSX-1

T1 Network

MONDSX-1

O R

CSU

FIREBERD 6000

Tx RxFIREBERD

6000

TxRxFIREBERD

6000

FIREBERD 6000TxRx

Stage 1 Stage 2 Stage 3

Figure 8Sectionalizinga T1 circuit.

Stress Testing

The live data emulation procedure doesn’t nec-essarily stress the T1 circuit. Whether or not errors werefound using the QRSS pattern, the T1 circuit should betested from one end to the other with various stresspatterns to see how the T1 circuit reacts. Equipmentthat’s operating on the borderline of specificationmay prove faulty when stressed with one of thepatterns listed in Table 4. This table lists stresspatterns that may also be used for out-of-servicetesting, and the purpose for using each pattern. Iferrors occur using these stress patterns, use theisolation technique described previously in Fault Isola-tion to break the circuit into sections and pinpoint thecause to a specific span section or piece of equipment.

Testing Options

To fully evaluate the performance of your circuits,there are three additional testing options you can per-form. These are Live Data Emulation, Stress Testing, andTransmission Delay Analysis.

Live Data Emulation

When transmitting the quasi-random signal source(QRSS) pattern, the FIREBERD emulates digitally-en-coded voice signals across all 24 channels of the T1circuit. The QRSS pattern is a 220-1 pseudorandom pat-tern with 14-zero suppression. Other pseudorandompatterns may be used to stress clock recovery and otherT1 equipment circuitry.

16T1

/FT1

Test

ing

Table 4Pseudorandom

patterns forstress testing.

Pattern Description

2047 2047-bit (211-1) pseudorandom pattern that generates a maximum of 10 sequential zerosand 11 sequential ones. Used to test DDS circuits and other circuits operating at 56 kb/s.

215-1 32,767-bit pseudorandom pattern that generates a maximum of 14 sequential zeros and 15sequential ones. Compatible with CCITT Recommendations O.151 (at 64, 1544, 2048, 3152,and 6312 kb/s) and G.703. Provides the maximum number of zeros allowed for framed,non-B8ZS testing.

220-1 1,048,575-bit pseudorandom pattern that generates a maximum of 19 sequential zerosand 20 sequential ones. Used on T1 applications to stress circuits with excess zeros.Pattern cannot be used to test asynchronous circuits.

223-1 8,388,607-bit pseudorandom pattern that generates a maximum of 22 sequential zerosand 23 sequential ones. This pattern cannot be used to test asynchronous circuits.

MARK Provides a fixed Mark only (All Ones) test pattern. The pattern is used as a keep alive,idle, or Red Alarm pattern in some circuits.

1:7 Fixed pattern that generates 1 Mark for every 7 Spaces. The pattern is used to stress the12.5% ones density requirement for T1-type circuits. This pattern cannot be used to testasynchronous circuits.

3 in 24 Fixed pattern that generates 3 Marks separated by 3 Spaces and 15 consecutive Spaces inevery 24 bits transmitted. The pattern generated appears as: 1000 1000 1000 0000 00000000. It’s used to test the excess zeros requirement for T1-type circuits. This pattern cannot be used to test asynchronous circuits.

T1-1 Long User 72-octet pattern. Useful for stress testing of the repeater preamplifier andautomatic line build-out (ALBO) circuitry. Detects marginal equipment using rapidtransitions from a low ones density to a high ones density.

T1-2 Long User 96-octet hex pattern. Used for empirical stress testing of T1 circuits andequipment.

T1-3 Variation on a Long User 54-octet hex pattern. Used for empirical stress testing of T1circuits and equipment.

17

T1/F

T1 T

est

ing

Table 4Pseudorandompatterns forstress testing.(Continued)

Pattern Description

T1-4 Long User 120-octet hex pattern. Used for empirical stress testing of T1 circuits and equipment.

T1-5 Long User 53-octet hex pattern. Used for empirical stress testing of T1 circuits and equipment.

T1-6 Long User 55-octet hex pattern. Used for empirical stress testing of T1 circuits and equipment.Not available in asynchronous timing mode.

DALY Test pattern commonly used in T1 installations and turn-ups. Long User 55-octet hex test pattern.Same as T1-6, except that byte 7 is 80 instead of 00. For this reason, the FIREBERD 4000 defaultsits USER 1 pattern to the T1-6 stress pattern.

T1-mW T1 Milliwatt. Long User Pattern digitized 1004 Hz tone with a 0 dBm0 level on one DS0 channel.This pattern length is 238 bytes. Standard tone (µ-law) used in Voice Frequency (VF) testing.

DDS-1 Long User Pattern of 100 octets with all ones (11111111), followed by 100 octets with all zeros(00000000). Stresses any DDS circuit’s minimum and maximum power requirements for signalrecovery. Cannot be used in asynchronous timing mode.

DDS-2 Long User Pattern of 100 octets of 01111110, followed by 100 octets of all zeros. Simulation of anHDLC packet frame. Cannot be used in asynchronous timing mode.

DDS-3 Fixed pattern 01001100. Minimum stress test of a DDS circuit. Cannot be used in asynchronoustiming mode.

DDS-4 Fixed pattern 00000010. Moderate stress on DDS clock recovery circuits. Cannot be used inasynchronous timing mode.

DDS-5 Long User Pattern, consisting of DDS patterns 1-4. A quick test for those wishing to test a circuitwith DDS patterns 1-4.

DDS-6 Long User Pattern, a 7-octet fixed pattern of 1111 1110 followed by 1 octet of 1111 1111.Simulates a DDS signal transition from IDLE mode to DATA mode. Detects marginal equipment inmultipoint applications.

18T1

/FT1

Test

ing

Transmission Delay Analysis

It’s helpful to measure the round-trip transmis-sion delay of a long-haul T1 circuit. For example, voiceechoes, protocol errors due to time-outs, and wander,are all impairments that may indicate changes in a T1circuit’s transmission distance. For terrestrial T1 cir-cuits, some carriers guarantee one-way absolute delay ofno greater than 100 ms. Refer to your circuit’s specifica-tions and tariff for details.

The FIREBERD’s delay measurement is helpful indetermining changes to the T1 circuit’s transmissionpath. Utilizing the FIREBERD’s round-trip delay functionon individual DS0s of a FT1 circuit can help you deter-mine which DS0s are being carried along differentroutes. The route of various DS0s and their associateddelay is important for speed-sensitive applications suchas video and data. To measure transmission delay, useeither the QRSS or 223-1 pattern. Also, use the MENUswitch to select the AUXILIARY function. With the keypad,enter the number 31 and press ENTER. This displays thedelay function. Then, use the softkeys under each of thefollowing selections:

1. Press START to access the start-of-measurementsignal sources.

2. Press the MORE key twice to display addi-tional selections.

3. Press GPATT to select the generator patternsync pulse.

4. Press the ∆ key to return to the previous menulevel.

5. Press STOP to access the end-of-measurementsignal sources.

6. Press the MORE key twice to display addi-tional selections.

7. Press RPATT to select the receiver patternsync pulse.

The delay measurement circuitry is armed imme-diately upon completion of the setup, and the DELAYresult (available in the SIGNAL category of the ANALYSISRESULTS switches and displays) is blanked until themeasurement is complete. To initiate or repeat thismeasurement, press the RESTART switch.

This setup measures round-trip delays up to0.67 seconds. See the FIREBERD 6000 ReferenceManual for more information.

Conclusion

Transmission testing does not have to be compli-cated. A T1 transmission test instrument should performfour basic types of measurements:

• Installation

• Acceptance testing

• Preventive maintenance

• Fault isolation

The FIREBERD communications analyzers en-able you to perform installation and acceptance tests,ongoing preventive maintenance, and fault isolationon the entire range of T1 and FT1 services. In-serviceand out-of-service testing are easy to perform byservice personnel at all levels.

19

T1/F

T1 T

est

ing

Appendix A:T1/FT1 Interface Module (Model 41440A) Menu Tree

AUTO CLRALL 56/64 SIG

4FRAME1FRAME 2FRAME 3FRAME

BPV LOGIC L+BPV

ERRINS: LOGIC OFF

OFF SINGLE RATE

MODE: FT1

VOICE TLB LLB

MORE

MODE: FT1

ESFDL RS232 T1.403

INTERFACE: FT1/T1

CONFIG MODE ERRINS

NOTE: SHIFT Key can be used to reverse the direction of CHAN#.

Entering of a new page will reset to increment direction.

INTERFACE: FT1/T1

IDLE LOOP RCVBYT

MORE

MODE: FT1

FULLT1 FRACT1 FT1INS

CH01:TX/RX 12X64/12X64

CH#UP CH#DN TX/ RX

MODE: FT1

FULLT1 FRACT1 FT1INS

SIG TX:01 ABCD:1010

OFF/ON HELP

LOOP:CSU RESP:NONE

TYPE RESPND

INTERFACE: FT1/T1

CONFIG MODE ERRINS

MORE

INTERFACE: FT1/T1

IDLE LOOP RCVBYT

MORE

IDLE: 01111111

HELP

(24...1)

RCVBYTE CHANNEL: 01

CH#UP CH#DN

(1...24)

CONFIG:STD, 0dB, TERM

RESULT LBO REFT1

0dB-7.5dB-15dB

TERMBRIDGE

CONFIG: ESF, AMI, TERM

FRAME CODE INPUT

OFFESFD4SLC

AMIB8ZS

TERMBRIDGE

DSX MON

MORE

CH01:TX/RX 01X64/01X64

CH#UP CH#DN TX/ RX

CLRALL 56/64

(1...24)(1...24)

STDLIV

MORE

Enter 8 bit binary

idle code pattern

Enter 8 bit binary

idle code pattern

RS232 TX:01<RX:01<64K

56/64 DATLINK

T1.403 PRM EMUL: CARR

CUST CARR

RS232 TX:01<RX:01<64K

SEL< CH#UP CH#DN

MORE

VOICE TX:01 RX:01

SEL< CH#UP CH#DN

LOOP: CSU

CSU FAC1 FAC2

DL-LLB DL-PLB PRGM

V.54

MORE MORE

(1...24) (24...1)

LOOPCODE RESP:NONE

AUTO NONE

HELP

PRGM DN: 1000

UP/DN INSERT DELETE

(24...1)

MORE

20T1

/FT1

Test

ing

MARK

1:1

63

511

2047

2047R

2 -1

2 -1(A)

QRSS

PRGM

FOX

USER

2 -1

SYNTH

INTF

BNC

SYNC

ASYNC

RECOVD

GEN CLK

TIMING MODE

DATA

ERROR INSERT

SELF LOOP

10 -3

15

20

23

SYNTH FREQ

INTF SETUP

TEST INTERVAL

CHAR FORMAT

JITTER

PRINT EVENT

AUXILIARY

RECALL/STORE

VOLUME AUX FUNC IN USE MENU

MORE

ERROR

PERFORMANCE

TIME

SIGNAL

T-CARRIER

ALARM

CATEGORY

RESULT

RESULTS

CONTROLS

OFF/ON

PRINTER

ANALYSIS RESULTS

ANALYSIS MODE

CONTINUOUS

SINGLE

DISPLAY HOLD

RESTART

COMMUNICATIONS ANALYZER

POWER

OFF ON

RECEIVER

MK

SP

SYNC

SYNC LOST

LOOP UP

LOOP DOWN

ALM 1

ALM 2

CODE

FRM SYNC

7 8 9

4 5 6

1 2 3

0 . SHIFT ENTER

FIREBERD MC6000

JITTER MENU GRAPH LIST

% MASK: BPV Rate 54.0 3.1 E-07

3

4 6 7

1 5

2

Front Panel Setup

The FIREBERD 6000 is shown in Figure 9. Thenumbers in the figure correspond to the front-panelswitches that control each configuration activity, asdescribed in Table 5. This section describes only front-panel switches that are used for in-service monitoring.

Configure the FIREBERD before cabling to theT1 circuit to prevent T1 circuit disturbances and toobtain accurate measurements. If you perform thisanalysis frequently on similar circuits, use the STORE/RECALL function to store all front panel and interfaceconfigurations. See the FIREBERD 6000 ReferenceManual for more information on this function.

Appendix B:Configuring the FIREBERD for

T1 In-Service Analysis of Live Traffic

Figure 9FIREBERD 6000 front panel.

21

T1/F

T1 T

est

ing

Table 5Setting up anin-service test.

Location InFigure 9 Switch Activity

1 SELF LOOP The SELF LOOP switch should be in the OFF position (the red LEDwithin the switch should NOT be illuminated.

2 MENU/INTF SETUP Interface setup is accomplished by pressing the MENU switch toselect the INTF SETUP function and then by selecting displayedparameters with the three softkeys and the MORE key beneath thedisplay. The ∆ key on the keypad may be used to move to theprevious menu level. After selecting the INTF SETUP function, pressthe softkey under each of the following selections:

a. T1/FT1 to select the T1/FT1 interface.

b. CONFIG to access the configuration menu.

c. FRAME to select the appropriate framing of yourT1 circuit.

d. CODE to select the appropriate coding of your circuit.

e. INPUT to select receiver termination mode, then:

NOTE: This next selection is dependent upon theFIREBERD’s cabling location.

DSXMON - if cabling to a resistor-isolated monitor jack.

BRIDGE - if cabling to wire-wrap pins.

f. RESULT to set it to LIV. This eliminates unnecessaryresults for monitoring live traffic such as bit errorsor CRCs.

g. LBO to set the cable loss for the T1 output signal level.0 dB is recommended.

h. REFT1 to set the input impedance to BRIDGE. (Used formonitoring, exhibits an impedance of 100Ω).

i. MODE to access the MODE menu.

j. FULLT1 to select and monitor the full T1 bandwidth.

k. ENTER to return to the main menu.

3 ANALYSIS MODE Choose one of the two analysis modes:CONTINUOUS - Continuous test.SINGLE - Timed test. Set the test length using the TEST

INTERVAL function of the MENU switch.

22T1

/FT1

Test

ing

Location InFigure 9

Switch Activity

h. Press the LBO softkey and select theappropriate output level (0dB, -7.5dB,or -15dB).

i. Return to the main interface menu.

If you perform this analysis frequently on similarcircuits, use the STORE/RECALL function to store allfront panel and interface configurations. See theFIREBERD 6000 Reference Manual for more informa-tion on this function.

Check for Proper Operation

Now that the FIREBERD is monitoring the T1span, verify that it’s operating properly and providingaccurate results by observing the frame synchroniza-tion (FRM SYNC) and pattern synchronization (PATSYNC) indicators on the right side of the front panel.

The FIREBERD is now ready to monitor a T1circuit. The only remaining operation required iscabling the FIREBERD to the T1 circuit.

Table 5Setting up an

in-service test.(Continued)

4 MENU/TEST INTERVAL Use the TEST INTERVAL function of the MENU switch to set the BERblock length. When the ANALYSIS MODE switch is in the SINGLEposition, use the TEST INTERVAL function to set the time interval.

5 GEN CLK Use the GEN CLK switch to select INTF timing. The FIREBERDrecovers timing from the T1 signal.

6 MENU/PRINT EVENT Use the PRINT EVENT function of the MENU switch to set theconditions under which a results printout will be generated (choosesetting for a specific time interval or on the occurrence of an errorcondition).

7 MENU/RECALL /STORE If desired, use the RECALL/STORE function of the MENU switch tosave this configuration (and up to 10 FIREBERD test configurations)for future retrieval.

A Typical PerformanceReport Message Test

A typical ANSI T1.403 test is presented below:

1. Configure the T1/FT1 Interface Module.

a. Select the appropriate framing (ESF),line coding, etc., for your application.

b. Press the MODE softkey to access theMODE menu.

c. Press the MORE softkey until T1.403 ap-pears in the display, then select the modeby pressing the softkey below it.

d. Press the CUST softkey to select customergenerated PRMs.

e. Press the ∆ key to return to the previousmenu.

f. Press the CONFIG softkey.

g Press the INPUT softkey and select theDSX MON input impedance level.

23

T1/F

T1 T

est

ing

Appendix C:Configuring the FIREBERD 6000 for a

FT1 Out-of-Service Test

To verify that the FIREBERD 6000 is set up prop-erly before cabling to the circuit, perform a self-loop testand check the indicators.

NOTE: The GEN CLK switch must be set to theSYNTH position for proper self-loopoperation.

Front Panel Setup

The FIREBERD 6000 is shown in Figure 10. Thenumbers in the figure correspond to the front-panelswitches that control each configuration activity, as de-scribed in Table 6. This section describes only front-panel switches that are used for an out-of-service test.

MARK

1:1

63

511

2047

2047R

2 -1

2 -1(A)

QRSS

PRGM

FOX

USER

2 -1

SYNTH

INTF

BNC

SYNC

ASYNC

RECOVD

GEN CLK

TIMING MODE

DATA

ERROR INSERT

SELF LOOP

10 -3

15

20

23

SYNTH FREQ

INTF SETUP

TEST INTERVAL

CHAR FORMAT

JITTER

PRINT EVENT

AUXILIARY

RECALL/STORE

VOLUME AUX FUNC IN USE MENU

MORE

ERROR

PERFORMANCE

TIME

SIGNAL

T-CARRIER

ALARM

CATEGORY

RESULT

RESULTS

CONTROLS

OFF/ON

PRINTER

ANALYSIS RESULTS

ANALYSIS MODE

CONTINUOUS

SINGLE

DISPLAY HOLD

RESTART

COMMUNICATIONS ANALYZER

POWER

OFF ON

RECEIVER

MK

SP

SYNC

SYNC LOST

LOOP UP

LOOP DOWN

ALM 1

ALM 2

CODE

FRM SYNC

7 8 9

4 5 6

1 2 3

0 . SHIFT ENTER

FIREBERD MC6000

JITTER MENU GRAPH LIST

% MASK: BPV Rate 54.0 3.1 E-07

1 6

7 9 10

2 3 4 5

8

Figure 10FIREBERD 6000front panel.

24T1

/FT1

Test

ing

Table 6Setting up a FT1out-of-service test.

Switch ActivityLocation InFigure 10

1 DATA Use the DATA switch to select the QRSS pattern. Other patterns may beused later to stress the T1 circuit.

2 ERROR INSERT The ERROR INSERT switch should be in the OFF position (the red LEDwithin the switch should not be illuminated).

3 SELF LOOP The SELF LOOP switch should be in the OFF position (the red LED withinthe switch should not be illuminated).

4 GEN CLK Use the GEN CLK switch to select one of three sources:

INTF: The FIREBERD recovers timing from the T1 signal. Use if thenear-end mux or channel bank is slave to another clock source (e.g.,DCS networks).

SYNTH: Timing is generated from the FIREBERD’s internal synthe-sizer. Select the particular frequency with the SYNTH FREQ functionof the MENU switch. This is a required setting to perform SELF LOOP.

BNC: The FIREBERD 6000 uses an external clock source via the rearpanel’s BNC connector.

5 MENU/SYNTH FREQ When the GEN CLK switch is in the SYNTH position, set the synthesizerfrequency by using the MENU switch to select the SYNTH FREQ function.After the SYNTH FREQ LED is illuminated, press the MORE key until1544 kHz is displayed. Press the softkey beneath the display thatcorresponds to 1544 kHz.

6 ANALYSIS MODE Choose one of the following analysis modes:

CONTINUOUS: Continuous test

SINGLE: Block or timed test. Set the length of a timed test with theTEST INTERVAL function of the MENU switch.

7 MENU/TEST INTERVAL Use the TEST INTERVAL function of the MENU switch to set the BERblock length. When the ANALYSIS mode switch is in the SINGLEposition, set the time interval with the TEST INTERVAL function.

25

T1/F

T1 T

est

ing

Location InFigure 10

8 MENU/INTERFACE After selecting the INTF SETUP function, press the softkey under each ofSETUP the following selections:

a. T1/FT1 to access the T1/FT1 interface.

b. CONFIG to access the configuration menu.

c. FRAME to select the proper framing.

d. CODE to select the proper line coding.

e. INPUT to set receiver termination to TERM (selects 100Ω ofinput impedance).

f. RESULT to set to STD.

g. LBO to select the transmitter’s line build out. 0 dB is therecommended setting.

h. REFT1 to set to BRIDGE (exhibits an input impedance of 100Ωor greater). ∆ key to return to the previous menu.

i. MODE to access the MODE menu.

j. FT1INS to select and monitor any selection of time slots at a rateof 64 or 56k. Use the CH# UP/DOWN softkeys, the 56/64 and theTX/RX softkeys, to configure each channel individually.

k. ERRINS to select a form of error insertion or it may be turned offwith the OFF key. ∆ key to return to the previous menu.

l. MORE to access more of the configuration menu.

m. LOOP to select the device to loopback.

n. ENTER (on the keypad) to return to the main menu.

9 MENU/PRINT EVENT Use the PRINT EVENT function to set the conditions under which aresults printout will be generated at specific time intervals or on theoccurrence of an error condition.

10 MENU/RECALL/ Use the RECALL/STORE function to save this configurationSTORE (and up to 10 FIREBERD test configurations) for future retrieval.

Switch Activity

Table 6Setting up a FT1out-of-servicetest. (Continued)

26T1

/FT1

Test

ing

If you perform this analysis frequently onsimilar circuits, use the STORE/RECALL function tostore all front panel and interface configurations. Seethe FIREBERD 6000 Reference Manual for moreinformation on this function.

Since the FIREBERD is now configured for an out-of-service test, make sure that the transmit and receivecables are properly aligned as shown in Figure 11.

Check for Proper Operation

Now that the FIREBERD is monitoring the T1span, verify that it’s operating properly and providingaccurate results by observing the FRM SYNC and PATSYNC indicators on the right side of the front panel.

(Tx) (Rx)

T R T1 R1

CSU

FIREBERD 6000

Tx Rx

3

Tx Rx

FIREBERD 6000

4

MON

FIREBERD 6000

Tx Rx

1

FIREBERD 6000

Tx Rx

CSU2

T1 Span Equipment

Figure 11RX and TX cablealignment.

27

T1/F

T1 T

est

ing

Appendix D:Overview of Jitter Testing

This appendix provides an overview of T1/FT1testing with the FIREBERD 6000’s Jitter options.

NOTE: The jitter options are available with theFIREBERD 6000 only. All referencesto the FIREBERD in this appendix arefor the FIREBERD 6000 only.

Jitter is the deviation in time between whenthe pulse transitions occur, when they ideally shouldoccur, and when the digital decoding gear expectsthem to occur. It is unintentional phase modulation,having both an amplitude and frequency component.The jitter’s amplitude is the magnitude of the phasedeviation; its frequency is a measure of how rapidlythe phase changes. Continued and increased jittercan result in errors, bit slips, and total impairment ofthe digital line.

Jitter is a composite of many wave forms, notspecifically sinusoidal. Figure 12 is an example of thevarious displaced jitter pulses. In this case, eight snap-shots, each taken at a fixed interval between the mark-ers y and y1, form a three-dimensional compositediagram. This figure compares the differences in theposition of the pulse over time. It reflects an earlyarrival time at snapshot 2, while 3 through 7 are late.The figure also reflects a developing cyclic pattern.

There are three units by which jitter can bemeasured: time, phase angle, and digit period. Themost convenient is the digit period known as the unitinterval (UI), which is also the unit that the FIREBERDmeasures. UIs are not measured over a single pulse,but instead as they gradually accumulate over a num-ber of pulses. One UI is equivalent to each of thefollowing: 360 degrees, 648 nanoseconds, 100 per-cent, one bit, and one time slot.

8

76

5

4

3

2

Elapsed Time

The firstobservedpulse

The amount of displacement, which isthe jitter, is measured in amplitude (U.I.)and frequency (Hz)

Subsequent Pulses in this caseare displaced in accordance witha pattern which approximates aSine Wave

The expectedpulse position

A theoreticalview point

y y1

1 Figure 12Jitter pulses.

28T1

/FT1

Test

ing

1 1 0 0 0 0 0 0 1 0

1 1 0 0 0 0 0 0 0 1

(A) Data Sent

(B) Jittered T1

(C) Decision Points

(D) Recovered Clock

(D) Recovered Data

— Threshold

+ Threshold

Two Bit Errors

Figure 13Received T1 signaland clock signals.

Electromagnetic interference (EMI), far-end/near-end crosstalk, imperfections in regenerators,and the bit stuffing/de-stuffing process can all pro-duce jitter. A form of jitter called wander occurs withphase variations of less than 10 Hz. T1 bit stuffing,environmental impairments, clocking, and transmissionpath delay variation are normal causes of wander.

Jitter causes errors because equipment can’taccurately determine the decision threshold. A deci-sion threshold is used to distinguish between a 1 or a0, or the presence or absence of a pulse. Using mid-bit sampling, if the pulse is shifted by more than 50percent of its width (0.25 UI), it will be sampledincorrectly. The validity of the pulse is affected by theamount of jitter. This is illustrated by Figure 13.

Jitter analysis is essential to isolating prob-lems on T1/FT1 links. Specific devices within a T1network, particularly when faulty, tend to introducejitter at unidentifiable frequencies. Jitter masks havebeen developed by organizations such as AT&T,Bellcore, and ITU-T, to ensure that transmission equip-ment vendors conform to jitter tolerance specifica-tions. Jitter tolerance specifies the amount of jitterthat can be tolerated at an input before errors start tooccur. Performing a spectrum analysis, while simul-taneously monitoring for degradation in error perfor-mance (bit errors, timing slips, etc.), can link asymptom to its cause. Figure 14 depicts a spectrumanalysis plot.

Jitte

r Am

plitu

de(U

nit I

nter

vals

Pea

k-to

-Pea

k)

Jitter Frequency (Hz)

28.0

10.08.0

5.0

2.0

1.0

0.50.40.3

0.2

0.1

0.0

01 2 10 100 200 300 500 1,000 4K 8K 10K 32K 40K 50K

Figure 14Widely used

jitter masks.

29

T1/F

T1 T

est

ing

The three standard ways of measuring jitter are:tolerance measurements, intrinsic jitter measurements,and transfer characteristic measurements. When per-forming a tolerance test, the FIREBERD’s jitter amplitudeand frequency on the transmitted test pattern is increasedin steps between 10 Hz and 40 kHz until the onset of biterrors occurs. If the mask limits are exceeded at everyfrequency tested, then the tested equipment has adequatejitter tolerance. Intrinsic jitter testing looks at the jitteroutput without a jitter input signal from the FIREBERD.The jitter output is plotted for frequencies between 40 kHzand 10 MHz. During a transfer characteristic test, jitteris injected at amplitudes of 50/75/100% of the MASK forwhich the equipment is specified, allowing you to see atwhat frequencies the mask is exceeded. These resultsallow you to isolate devices in the network that magnifythe jitter they receive in the transmitted signal. Sinusoi-dal jitter modulation should be used when testing forcompliance with specifications. Waveforms such assquare and triangle-shaped waves contain harmonicsthat can exceed the mask, resulting in incorrect specifi-cation results.

In-Service Analysis

In-service analysis allows users to test livetraffic without introducing disturbances on the lines.This testing method permits users to monitor infor-mation about the performance of the T1 system.When monitoring jitter in-service, a spectral analysisof the live data provides a more useful and accurateanalysis than an out-of-service test. Figure 15depicts in-service monitor points.

The Wideband Jitter Option (Option 6001) en-ables the FIREBERD to analyze wideband jitter and lookat wideband results, so the T1 that is monitored foralarms and other indications of faulty equipment can bemonitored for jitter levels. This analysis of the entirespectrum gives results such as jitter hits (JTR HITs),maximum jitter (MAX JTR) and 1 SEC JTR to see wherethe timing jitter actually exceeds the user-selectablethreshold. The 1 SEC JTR, MAX JTR, and JTR HITS re-sults are found in the SIGNAL category. Wideband jitteranalysis can be performed in a variety of ways. Filteringout frequencies less than 8 kHz can further identify theproblem. This allows only the high frequencies to pass,giving further insight into problems associated withfaulty or marginal span repeaters. Wideband resultsmeasure maximum peak-to-peak jitter in 40 frequencybands from 10 Hz to 40 kHz and displays the resulting UIspeak-to-peak. This measurement may be done in one oftwo ways:

Figure 15In-service monitor points.

DSX-1

MON

OfficeRep.

MON

OfficeRep.

SmartJack

CSUT1MUX

MONMON

FIREBERD6000

T1 Repeater

LocalLoop

Local Exchange IXCCustomer Premise

T1 NETWORK

30T1

/FT1

Test

ing

1. Continuously measure 1 of the 40 available fre-quency bands and constantly update the jitteramplitude at that frequency or,

2. Continuously sweep through all 40 frequencybands. When the sweep is enabled, each jittervalue shown in the 40 frequency bands is com-pared to the measured value of the correspond-ing frequency. The maximum value is saved untilthe test restart occurs.

The Jitter Spectral Analysis Option (Option6002) enables the FIREBERD to perform spectralanalysis (S/A). In addition to a spectral analysis, S/Aand %MASK results can be viewed in the resultswindow. The S/A measures the maximum peak-to-peak UI over (N) discrete frequencies. N varies,based on a T1 circuit. The S/A option may be used forboth in-service and out-of-service testing.

S/A Plot

A jitter S/A should be performed during theinstallation of a network. This provides a basis forcomparison of any future degradations, to this initialjitter analysis. Deviations can then be isolated, subse-quently assisting in identifying the problem. Below isan example of a typical intrinsic jitter test:

The FIREBERD should be in a Line Loopback(LLB) Mode, if you’re using the 41440A Interface.Scroll through the MENU selections and choose theJITTER function. Use the following softkeys:

1. Press HITS to set the decision threshold to counta jitter hit. The range is between 0.1 and 6.5 UI.

2. Press FILTER to determine the range over whichthe results should be complied.

a. FULL softkey for the entire range.

b. >8 kHz to isolate frequencies of 8 kHz orhigher.

3. Press S/A to access the spectrum analysis results.

a. SWEEP determines the range of the spec-trum analysis.

b. ON - if set ON, the analysis will cover theentire 10 Hz-40 kHz range. Selecting OFFenables you to select a frequency band.

c. FREQ - if you want to look at one specificband in the 10Hz to 40 kHz range. SWEEPshould be set to OFF if choosing a specificband.

d. PEAK - to find peaks in the spectrum. Usedonly when SWEEP is enabled.

e. PEAKUP - finds the next higher frequencyjitter peak.

f. PEAKDOWN - finds the next lower fre-quency jitter peak.

Out-of-Service Analysis

The DS1 Jitter Generation Option (Option 6003)enables the FIREBERD to phase-modulate a T1 signal(using an internally generated waveform, an externalwaveform, or a combination of both) to produce T1jitter. Internal timing, the recovered clock, and an exter-nal clock source may be used with the jitter generator.

Out-of-service testing enables users to accuratelyassess the quality of a T1 line. The data test pattern usedis extremely important when testing jitter. When out-of-service testing is performed, QRSS is typically used be-cause it represents live data. Fixed patterns, such as 1:7or 3 in 24, should be used to provide greater stress ofclock-recovery circuits and to simulate marginal datacontent conditions.

When testing for jitter tolerance, the error rateshould be monitored while input jitter is varied. TheFIREBERD may be used to generate jitter onto the T1span, while performing a BERT test.

31

T1/F

T1 T

est

ing

Individual pieces of network equipment, such asmuxes, may be tested for acceptance by performing aback-to-back test with a second FIREBERD. If more jitteris seen on the receive leg than the transmit leg, then theequipment is amplifying instead of deleting jitter to thesignal. Likewise if the jitter received is less than thattransmitted by the FIREBERD, the equipment is attenuat-ing jitter from the signal. This kind of jitter test is furtherdescribed in the example below. For a tolerance test,FIREBERD No. 1 should be in loopback mode.

FIREBERD No. 1

Scroll through the MENU selections and choosethe JITTER function. Use the following softkeys:

1. Press MASK to select the specification of theequipment under test.

∆ (Up Arrow).

2. Press GEN to access the jitter generation menu.

3. Press MOD to select a modulation source.

a. ON if the modulation source is theFIREBERD.

b. SINE for an accurate specification-compliance result.

c. EXT if the external BNC connector (la-beled Jitter MOD IN) is the modulationsource. If an additional waveform ismixed with the BNC signal, use a SINEwave for an accurate specification-com-pliance result.

4. Press FREQ to set the frequency band you wantto test.

a. SWEEP - to generate jitter at all 40 fre-quency bands.

b. HZ - to generate jitter at a specific fre-quency band.

c. ∆ to return to the previous menu level.

5. Press AMP to set the amplitude of jitter inserted.

a. UIp-p - to add jitter at a specific amplitude.

b. MASK - to add jitter at the pre-chosen maskamplitude.

c. XFR50% or XFR75% to add jitter at apercentage of the MASK level.

FIREBERD No. 2

The second FIREBERD can be optioned towork in a thru mode to obtain wideband jitter results.Follow the procedures for out-of-service testing. TheFIREBERD should be in a Line Loopback (LLB) Mode,if you are using the 41440A Interface. Then, followthe jitter setup procedures for an intrinsic jitter test.

Two of the most common problems caused byT1 jitter are bit errors, or an increase in BER, and timingslips. Bit errors occur when the receiving circuits incor-rectly sample the incoming pulse.

Refer to the table below for some frequentlyfound results, their causes, and suggested solutions.

Results Problem/Solution

Pattern slips, Signifies an uncontrolled slip.framing errors, Usually caused by the overflowand frame loss. of a buffer within an M13 mux.

Pattern slips, Signifies a controlled slip. Usu-no framing errors, ally a network timing problem.no frame loss.

Refer to the FIREBERD 6000 Reference Manualfor more information on the FIREBERD jitter options.

© 1995 Telecommunications Techniques Corporation.Telecommunications Techniques Corporation, TTC, and FIREBERDare registered trademarks of Telecommunications TechniquesCorporation. All other trademarks and registered trademarks areproperty of their respective companies.

32T1

/FT1

Test

ing

Notes

33

T1/F

T1 T

est

ing

34T1

/FT1

Test

ing

20400 Observation Drive, Germantown, Maryland 20876Tel. (800) 638-2049 • (301) 353-1550 (MD) • FAX (301) 353-0731

www.ttc.com

FB T1/FT1 AN - 10/97

M

O R

CSU

DS

Behind Successful NetworksYou'll Find TTC

35

T1/F

T1 T

est

ing

FIREBERD

T1/F

T1 T

est

ing

Ap

pli

cati

on

No

te

MONSX-1

T1 Network

MONDSX-1

O R

CSU