1 with the FIREBERD 4000 and FIREBERD 6000 Overview The demand for high-quality T1 circuits requires consistent maintenance and circuit analysis. To provide clean, error-free transmissions, the personnel who test the performance of T1 circuits demand reliable instruments. The TTC (Telecommunications Techniques Corporation) FIREBERD Communications Analyzers are ideal for T1 installation, acceptance testing, ongoing maintenance, and fault isolation, all of which are essential to providing quality T1 service. This Application Note describes various T1 and Fractional T1(FT1) fundamentals, and the impairments that can degrade transmission service. It then describes using the FIREBERD for in-service monitoring and 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 by common carriers, telephone companies, private networks, and government agencies. T1 circuits are primarily used to transmit multiple voice and data signals that are multiplexed and transmitted over a single communication path. These circuits are found in both point-to-point and network environments, as shown in Figure 1, on the next page. Part A of Figure 1, on the next page, shows a simple point-to-point circuit connecting two pieces of customer premise equipment (CPE) such as channel banks or multiplexers. There are no intelligent devices such as digital cross-connect systems (DCSs) or higher rate multiplexers (e.g., M13) along the transmission link. This figure represents a typical small private network connecting two sites, or pieces of equipment. Part B of Figure 1, on the next page, shows a network configuration that includes channel service units (s), metallic local loops, span repeaters, digital signal patch bays (DSXs), and transmission devices such as DCSs and M13 multiplexers. The acts as a network interface between the customer premises equipment (CPE) and the service provider s T1 network. Test access is often provided through physical connections or loopback features. Span repeaters, placed at least every six thousand feet along the local loop, regenerate the signal in both directions. Office repeaters terminate the local loop and often feed the T1 signal to either a DSX patch bay or an electronic DCS. The DSX patch bay is used for manual signal routing and test access. The DCS enables multiple T1 circuits to be routed through the office electronically, and also provides test access. Before being sent to the long-haul facilities, the T1 circuits are typically multiplexed to an even higher rate. To do this, M13 multiplexers combine 28 T1 signals into one T3 signal (44.7 Mb/s). When the T3 signal reaches the far office, it is demultiplexed so that each T1 signal may be transmitted across the desired loop and into the far-end s CPE. The T1 signal is then transmitted across another local loop, or is transmitted to another central office (CO) by long-haul facilities (e.g., microwave, satellite, fiber optics).
2 DSX-1 CPE O R O R CPE Customer Premises Local Loop Central Office Local Loop Customer Premises A. Point-To-Point Circuit O R DSX-1 DCS 28 M13 Figure 1 T1 circuit environments. Customer Premises Metallic Local Loop Central Office Long-Haul Facilities (Fiber Optic, Satellite, Radio) DSX-1 M13 28 DCS O R Central Office Metallic Local Loop Customer Premises - 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 FT1 Background Many users need the capabilities of a T1 network, but few need a full complement of 24 DS0 channels running into their facility. For these users needing bandwidth of less than 1.544 Mb/s, FT1 allows them to select DS0 channels individually, rather than in groups of 24. For example, 4 channels can be leased for video applications, 2 for voice and 2 for data communications. As a result, users can meet all of their communication needs with FT1 service, instead of under-utilizing a full T1 circuit. Three main choices must be made when implementing FT1 service: bandwidth, Nx64 or Nx56 DS0s, and contiguous vs. noncontiguous channels.
3 Bandwidth First, users must select the amount of bandwidth that s needed. FT1 service typically offers DS0s in groups of 2, 4, 6, 8, and 12 at rates of 56 or 64 kb/s per channel. The most common fractional data rates offered 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 service provider. Nx56 kb/s is typically offered if the service provider uses alternate mark inversion (AMI) line coding, in which only 56 kb/s of the 64 kb/s DS0 is used for customer data. The remaining bandwidth is filled by the forced insertion of ones to maintain ones density. This reduces throughput, but allows fractional service to be provided on a T1 span using AMI. Nx64 kb/s service is offered if the provider has incorporated bipolar eight zero substitution (B8ZS) line coding into their T1 network. B8ZS allows for utilization of the entire FT1 bandwidth by eliminating the forced insertion of ones. problems caused by different routing paths through a digital area cross-connect system (DACS). If the service user s application is data oriented and requires Nx64 kb/s service, the service providers T1 network must support B8ZS line coding. If not, contiguous bandwidth can t be used for the application. Non-contiguous bandwidth refers to individual DS0s that are not adjacent, but are routed together through a DCS. Non-contiguous DS0 service fills alternating timeslots of a T1 frame with a fixed eightbit word, referred to as the idle code. The idle code must contain at least two ones within each eight bit word. If Nx64 needs to be used for voice applications, and the service provider doesn t offer B8ZS line coding, non-contiguous service must be used because it maintains the T1 ones density requirement. However, it is not appropriate for fractional data applications because bandwidth for data must be contiguous (see Figure 2). The T1 span between the customer premises and the service provider office remains physically and electrically the same for FT1 service. FT1 service uses the same coding and framing techniques as standard T1 service. The customer installs T1 multiplexers 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. Contiguous Figure 2 Contiguous or non-contiguous bandwidth. F 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Contiguous or Non-Contiguous DS0s Finally, users must select either contiguous or non-contiguous DS0s. Contiguous bandwidth is DS0s that are routed together end-to-end to support applications greater than 56 or 64 kb/s. Applications such as data and video often require contiguous bandwidth throughout the T1 network, eliminating any delay *Inactive Channels (5-24) are filled with an idle code (00110000) Non-Contiguous F 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 *Inactive Channels (2, 4, 6, 8-24) are filled with an idle code (00110000) Active Channel: Inactive Channel:
4 Causes of T1 Impairments There are four main causes of T1 impairments: 1. Faulty Equipment. Any T1 equipment can cause errors when components fail or operate outside of specification. Errors that may suggest faulty equipment include bipolar violations (BPVs), bit errors, frame errors, jitter, slips, and excess zeros. For example, BPVs can occur due to faulty clock recovery circuitry in span repeaters. These errors occur as the equipment becomes older and begins to drift out of specification. 2. Improper Connections. Transmission errors are created by improper connections or configurations. For example, intermittent errors can occur when component or cable connections are loose, and timing errors can occur when improper or conflicting timing sources are connected together. Dribbling errors are often caused by loose or unconnected shield ground cables and by bridge taps. Further, upon installation, the circuit may not work at all due to mislabeled pins on terminating cable blocks and to crossed wires: transmit-to-transmit instead of transmit-to-receive. These errors are typically discovered upon circuit installation and possibly during circuit acceptance when end-to-end tests are performed. 3. Environmental. Electrical storms, power lines, electrical noise, interference, and crosstalk between transmission links can cause BPVs as well as bit, frame, and CRC errors. Typically, these conditions cause intermittent, bursty errors, which are among the most difficult to locate. Although the cause of these impairments is obvious, the one that s difficult to pinpoint is crosstalk. Crosstalk is sometimes caused by improperly separated cable pairs. Transmit and receive pairs should be between 25 and 100 pairs apart. 4. Data Specific. Data characteristics, such as repetitive patterns, can force equipment to create pattern-dependent jitter and code errors. These errors may not exist when testing the transmission path with standard pseudorandom patterns. For example, span repeaters regenerate the digital signal and recover timing from the bipolar signal. To facilitate timing recovery, there must be a sufficient number of transitions (e.g., no more than 15 consecutive 0s), otherwise, the repeater can t properly recover timing from the signal. Techniques and Measurements To analyze a T1 circuit s performance and to isolate the causes of degraded service, the test instrument must perform these four basic types of measurements. 1. Installation. When installing a T1 circuit, out-of-service testing helps verify equipment operation and end-to-end transmission quality. Begin by testing the equipment (e.g., channel banks, multiplexers), and then verifying cable connections, timing source selections, and frequency outputs. After qualifying the circuit end-to-end, each is looped back to ensure that it responds to both loopup and loop-down codes and to verify that the circuit operates properly at each end. 2. Acceptance Testing. In addition to the tests performed during installation, stress tests and timed tests should be performed to ensure that the T1 is operating according to the relevant T1 circuit specifications. Stress the equipment by injecting errors, jitter, and data patterns (either pseudorandom or fixed) into T1 equipment. The same procedure may be performed end-to-end to stress the entire T1 circuit. Timed tests with printouts should be performed over a 24- or 48-hour period using standard pseudorandom patterns to simulate live data. This is described more fully in Application 2.
5 3. Preventive Maintenance. Once live data is transmitted across the T1 circuit, preventive maintenance tests are strongly recommended. Preventive maintenance can alert technicians to degrading service before it disrupts normal operations, and involves monitoring the live data for BPVs, frame errors, CRC errors, and signal frequency measurements that provide information about the performance of the T1 circuit. These tests should be performed with printouts over a 24- or 48-hour period to detect time-specific or intermittent errors. This is described more fully in Application 1. 4. Fault Isolation. Fault isolation is required once excessive error rates disrupt service. This can be performed using both in-service and outof-service tests. In-service testing provides general information, and can be used before out-ofservice analysis to localize problems and minimize circuit downtime. Monitoring the circuit at various points enables technicians to analyze the results and determine the source of problems. Performing standard out-of-service tests (e.g., loopback and end-to-end tests) enables technicians to stress the equipment, find sources of errors, and verify proper operation once the trouble is repaired. Application 1: In-Service Analysis of Live Traffic This application describes using the FIREBERD 6000 to evaluate the general performance of a T1/FT1 span. This application is useful for the following: Performing periodic maintenance, and when looking for transmission degradations before they affect service. Analyzing the span for intermittent errors that are caused by faulty equipment or environmental 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 outof-service. Localizing the problem and minimizing circuit downtime. To derive all of these benefits, the FIREBERD 6000 may be configured to monitor the T1 circuit from nearly any T1 access point. Figure 3 shows a typical circuit and possible monitoring locations. Once monitoring Figure 3 Possible FIREBERD monitoring locations. DSX-1 DSX-1 O R T1 Network O R
6 begins, look for BPVs, bit slips, frame errors, CRC errors (for ESF framed circuits), associated rates, and alarm conditions. These results are helpful in isolating the cause of the problem. See Appendix B, on page 19, for information on configuring the FIREBERD for in-service analysis. Check for Proper Operation Now that the FIREBERD 6000 is monitoring the T1 span, verify that it s operating properly and providing accurate results by observing the two indicators on the right side of the front panel. Table 1 highlights common error indications, along with possible reasons and solutions. Other In-Service Tests Identifying Active and Idle Channels You may also perform installation tests with the T1/FT1 Interface Module. With the AUTO CHAN- NEL function, you can verify the active and idle channels of a FT1 circuit. To do this, program the DCSgenerated idle code into the 41440A s IDLE code function and then select AUTO CHANNEL in the FT1 mode menu. This function then scans all 24 channels for the idle code and determines which channels are active and which are idle. Indicator Reason Solution GEN CLK No receiver clock detected Check cabling. Verify proper connections to illuminated by the FIREBERD circuitry. a valid T1 circuit. Try new cable. SYNC FIREBERD isn t synchronized If performing an end-to-end test with two not illuminated to the incoming pseudorandom FIREBERDs, verify that both sets are pattern. transmitting the same data pattern. Check FRM SYNC, MK, and SP indicators. If they still 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 the not illuminated synchronization to the specified framing format) and connections. If unit framing hasn t been achieved. still doesn t achieve synchronization to the framed signal, perform SELF TEST. Table 1 Common alarm and error indications.
7 Monitoring Performance Report Messages The T1/FT1 Interface Module with ANSI T1.403 PRM Option (Option 6009) also enables the FIREBERD to transmit and decode ANSI T1.403 Performance Report Messages (PRMs). ANSI standard T1.403 (1989) is a technical report that addresses ESF performance monitoring. In T1.403 mode, in-service performance results can be monitored from end-toend of the terminated T1 line. Overhead bits from the extended superframe (ESF) form an 8 kb/s data link channel that provides performance results. The ESF datalink enables scheduled PRMs, unscheduled priority messages, and command/ response messages to be transmitted and received across the T1 line without interfering with the DS0 channels. ANSI T1.403 PRMs allow both carrier and service user s non-intrusive tests, and provide reports on the previous three seconds and the current second, by using the information contained in the ESF datalink. PRMs are scheduled messages that provide continuous T1 signal performance monitoring between terminating devices. They provide information on CRC error events, severity of frame errors, the occurrence of BPVs, controlled slips, and whether the far-end or near-end is in payload loopback. PRMs are recalculated at each terminating device (e.g., MUX, DSU,, etc.). Therefore, they don t indicate end-to-end performance they indicate performance between the user and the next piece of equipment. This analysis is useful for sectionalizing a problem to the transmit or receive leg of a line. Most s can be configured to transmit PRMs, but they don t have the facilities to decode and present the received PRM information. This is left up to T1.403 compatible test sets like the FIREBERD 6000, or devices such as T1 monitor units (MU). The FIREBERD 6000 connects to the DS1 through the DSX monitor jack to analyze the T1 signal, decode the PRMs, and present performance information on its front panel. Refer to the T1 ESF PRM Network Testing Application Note for more information on PRMs. Results Analysis The FIREBERD 6000 will accumulate all results simultaneously. The desired results can be achieved using the appropriate CATEGORY and RESULTS switches. Refer to Figure 4, on the next page, to find your location along the T1 span, and check your location with Table 2, on the following page, to find possible causes of problems. Every T1 system is different, and may not be susceptible to the noted cause. Application 2: Out-of-Service Testing This application provides performance information about a T1/FT1 circuit using pseudorandom data. It is useful for the following: Installing T1/FT1 circuits and verifying end-toend continuity. Isolating T1/FT1 circuit faults by inserting pseudorandom patterns and interpreting results. Performing acceptance tests, including timed and stress tests. Errors found through this analysis may be caused by faulty equipment, improper connections, environmental influences, or data content. To find these errors, monitor bit errors, average bit error rate (AVG BER), bit slips, error-free seconds (EFS), percent error-free seconds (% EFS), etc., results that are all measured simultaneously. These results will help in isolating the cause of the problems.
8 A DSX-1 B DSX-1 A O R T1 Network O R C C Figure 4 Monitoring Locations. Location For Figure 4 Results Results Displayed By FIREBERD Problem/Solution (A) Bipolar BPVs Local problem. Possibly bad cabling connections Violations between test set and circuit, corroded dirty cable plugs, or a defective. (A) or (B) or (C) Receive RCV FREQ Offset Frequencies that are out-of-range may affect jitter Frequency ±75 b/s tolerance and noise margins, or they may cause Offset error bursts and timing slips. (B) Bit Slips BIT SLIPS Network problem, due to inconsistent timing through out the network. Check timing recovery in the DCS. Also may be due to incorrect optioning on s and channel banks. (B) or (C) Bipolar BPVs Local T1 span problem. Possible faulty repeater, span Violations, FRA ERR, CRC ERR line noise, crosstalk, poor cabling, or defective DSX Frame Errors, jacks. or CRC Errors Table 2 Correlation of results and problem causes. (B) or (C) Bipolar BPVs Local T1 span problem due to marginal timing recovery Violations, FRA ERR, CRC ERR during periods of excess zeros. Check repeaters, Frame Errors, XS0s multiplexers. CRC Errors, or Excess Zeros
9 Location For Figure 4 Results Results Displayed By FIREBERD Problem/Solution (C) Bipolar BPVs, No FRA Local T1 span problem. Violations, ERR, CRC ERR No Frame Errors, or CRC Errors (C) No Bipolar No BPVs, FRA ERR, Typically far-end span line problem. Sectionalize Violations, CRC ERR further. Potential for light guide, radio, or Violation Frame 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 signal Violations, CRC ERR, generator transmissions that don t meet pulse density Frame Errors, XS0s specifications. First, verify signal generator operation, CRC Errors, then check repeaters and multiplexers. or Excess Zeros Table 2 Correlation of results and problem causes. (Continued) There are basically two methods of performing out-of-service testing: loopback testing and end-to-end testing. The FIREBERD s configuration is similar for both types of testing. The two major differences are: equipment needed and the establishment of a loopback, addressed in the following sections. End-to-End Testing End-to-end testing is performed with two FIREBERDs so that analysis may be performed simultaneously in both directions. Figure 5 shows that basic setup of an end-to-end test. This method provides advantages over loopback testing because the direction of errors can be more quickly isolated, and it allows you to determine whether the transmit or receive leg is faulty. Figure 5 Basic setup for an end-to-end test. DSX-1 DSX-1 O R T1 Network O R Tx Rx FIREBERD 6000 Rx Tx FIREBERD 6000
10 Loopback Testing Loopback testing is performed with one FIREBERD. Figure 6 shows the basic setup of a loopback test. If loopbacks are established to perform the test, it s important to realize that the far-end in loopback affects the results. By design, most s (like many other pieces of transmission equipment) remove received BPVs, frame errors, and other errors before transmitting the data. This affects analysis interpretation because the nearend technician won t be aware of BPVs on the far-end s metallic loop, and may draw inconclusive results. NOTE: If excessive jitter exists on either side of the T1 circuit, synchronization to the pseudorandom pattern may be impossible. Instead, use the end-toend technique. A variety of devices may be looped with the T1/FT1 Interface Module, such as the (NIU) smart jack or the. Another loopback used for testing a FT1 circuit is the V.54 loopback. This allows the FIREBERD to loop up only certain channels in a FT1 circuit. A loopback loops all 24 channels, eliminating the isolation of certain DS0s from a fractional circuit. Instead, a V.54 loopback only loops back specific channels. For example, assume there is a problem with video conferencing on a circuit that contains 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 setting only these channels for TX/RX in a FT1 Insert Mode and then choosing a V.54 loop type. Now a BERT can be performed on channels 1-4 without disrupting traffic on the remaining DS0s. Creating a Loopback There are three methods of creating a loopback: 1. Press the LOOP UP switch on the FIREBERD front panel (loop-up code 1:4, also referred to as 1000). The LED within the switch will be illuminated while the loop-up code is being sent. When the FIREBERD detects a successful loop, the LED is extinguished and the FIREBERD stops sending the loopcode. This prevents the near-end from looping up and locking the FIREBERD out of the span under test. 2. Program and send a facility or special, equipment-specific loop-up code such as 2:2 (also referred to as 1100) or 1:5 (also referred to as 100000). Use AUX function 33 and the PGRM DATA switch to program and send codes. 3. Manually enable a loopback at the far-end. After testing is completed, remove the loopback on the remote by one of three methods: 1. Press the LOOP DOWN switch on the FIREBERD front panel (loop-down code 1:2, also referred to as 100). 2. Program and send a facility or special, equipment-specific loop-down code such as 3:1 (also referred to as 1110) or 1:3 (also referred to as 100). Use AUX function 33 and the PGRM DATA switch to program and send codes. 3. Manually disable a loopback at the far-end.
11 O R DSX-1 T1 Network DSX-1 O R Tx Rx FIREBERD 6000 Figure 6 Basic setup for a loopback test. FIREBERD Testing Modes Use the T1/FT1 Interface Module to configure the FIREBERD to operate in a variety of different testing modes, such as Full T1 (FULLT1) or FT1. For FT1 testing, the FIREBERD can be configured to test in FRACT1 or FT1INS mode depending on the application. Fractional T1 (FRACT1) Mode This mode allows data to be transmitted and received at any FT1 rate of Nx64 kb/s or Nx56 kb/s (N = 1 to 24) on any selection of contiguous and noncontiguous timeslots. In this mode, you can configure individual channels to transmit, receive, or both. Fractional T1 Insert (FT1INS) Mode This mode enables the insertion of a BERT pattern on any combination of selected Nx64 kb/s or Nx56 kb/s timeslots without disrupting traffic on the remaining timeslots. FT1INS differs from FRACT1 operation because it doesn t disturb live data on the unselected timeslots, whereas in FRACT1 idle codes are sent over these unused timeslots. The AUTO selection is disabled in the FT1INS MODE. Test Loopback Mode The FIREBERD can be optioned to behave as a repeater or for end-to-end testing. In the Test Loopback (TLB) mode, the FIREBERD emulates a or a channel bank in digital loopback. All received data is echoed on the transmitter output and the received signal is analyzed by the FIREBERD data receiver. BPVs and B8ZS coding are stripped from the received signal, and the outgoing signal is re-timed and re-encoded with AMI or B8ZS according to the CODE menu selection. Line Loopback Mode In Line Loopback Mode (LLB), the FIREBERD emulates a repeater. All data received is echoed unchanged on the transmitter output. The received data is analyzed, but no re-coding or error insertion is available. Performing an end-to-end BER test in both TLB and LLB mode determines the direction of faulty equipment. If errors occur in LLB, but not in TLB, then the transmit leg of FIREBERD No. 1 is faulty. Likewise, if FIREBERD No. 1 sees errors while FIREBERD No. 2 is in the TLB mode, then the receive leg of FIREBERD No. 1 is faulty.
12 Additional Applications 1. Once transmission level tests have been performed, further analysis can be performed by a protocol analyzer. By setting the T1/FT1 Interface Module to operate in a RS-232 mode, access is provided to either a 64 kb/s DS0 channel (CHAN mode) or to the ESF data link (DATLINK mode). The desired channel may then be dropped and inserted to an external device. 2. The FIREBERD can perform payload and datalink BER tests. Out-of-band Data Link Line Loopback (DL-LLB) loop codes can also be transmitted and recognized by the FIREBERD on the ESF data link. These codes are transmitted to the far-end equipment to place the far-end equipment into a line loopback. In contrast, out-of-band Data Link Payload Loopback (DL-PLB) loop codes are transmitted and recognized by the FIREBERD on the ESF data link. This code is transmitted to the farend equipment to place the far-end equipment into a payload loopback. The loops back the payload data, reframes the signal, and recalculates the CRCs. 3. Monitoring voice traffic may also be accomplished with the FIREBERD in VOICE mode. The user may now check the integrity of the circuit by inserting and monitoring voice traffic and monitoring and transmitting signaling bits on individual channels within the T1/FT1 bit stream. Using this mode gives a quick check of the integrity of the circuit and verifies proper signaling sequences. This is accomplished by dropping an analog signal from an external device with the handset connector. For example, you may set the signaling bits for on-hook and then verify that proper off-hook bits are being returned. When monitoring ABCD signaling bits, the FIREBERD extracts the ABCD signaling bits from a single DS0 in the incoming data. Timing Analysis Reference T1 The T1/FT1 Interface Module also allows you to determine bit slips, using the REFT1 connector. Bit slips are determined by comparing the timing of the T1 span under test with a reference T1 span on the other side of the network. REFT1 allows you to select the input impedance and signal conditioning for the T1 reference signal: the two input impedances are terminating (TERM) and bridge (BRDG). TERM is used when terminating the T1 reference input with 100Ω and accepting a relatively unattenuated (+6 through -6 db) T1 reference. BRDG, which allows monitoring of reference T1 lines that are already terminated, uses an impedance greater than 1000Ω. These results are helpful in isolating timing problems within the network. Results Analysis As long as the indicators are illuminated as shown in Figure 7, the FIREBERD will accumulate all results simultaneously. The desired results can be retrieved using the appropriate CATEGORY and RE- SULTS switches. Once the results are known, it s possible to isolate the cause of degraded service. Every T1 system is different, so it is nearly impossible to list all of the causes. Table 3 shows various result combinations and possible problem causes.
13 RECEIVER MK SP SYNC FRM SYNC CODE Illuminated for framed circuits only Figure 7 Indicators. SYNC LOST Indicates signal present ALM 1 ALM 2 LOOP UP LOOP DOWN Illuminated for B8ZS only Result Results Displayed By FIREBERD Problem 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 practically every piece of transmission equipment except repeaters. Table 3 Correlation of out-of-service results and problem causes. 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, declared Bit Errors BIT ERRs when two out of five framing bits are missed. A Red Synchronization 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 and Receive Frequency RCV FREQ other buffering equipment that maintains framing Offset Offset ±75 b/s integrity. Frequency offsets may also cause controlled No Frame Sync Losses No FRA LOSS slips. Observe the FRA LOSS and RCV FREQ Slips BIT SLIPS measurements. Check timing sources and verify that multiplexing and demultiplexing equipment is operating properly.
14 Result Results Displayed By FIREBERD Problem Frame Sync Loss FRA LOSS If you re measuring frame losses and clock slips, you re Clock Slips CLK SLIPS observing an uncontrolled slip typical of satellite downlink Bit Slips BIT SLIPS receivers and other buffering equipment that doesn t distinguish between frame and data bits. Verify that multiplexing and demultiplexing equipment is operating properly. Bipolar Violations BPVS Check the last repeatered span before your present Clock Slips CLK SLIPS location. Problems may be caused by faulty clock Excess 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 for more than 2.5 seconds. This equals 38,600 bit errors in 2.5 seconds for a 1.544 Mb/s circuit. Table 3 Correlation of out-of-service results and problem causes. (Continued) Errored Seconds ERR SEC Check circuit s specifications and tariff for error performance standards. For some carriers, 15 minute intervals with more than 300 errored seconds may be reported as trouble. Percent Error-Free % EFS Check circuit s specifications and tariff for performance Seconds level standards. A common, acceptable level is 95% EFS over a 24-hour period. Fault Isolation To localize problems, break the T1 circuit into manageable sections, one step at a time, as shown in Figure 8. In Stage 1, verify that errors are occurring somewhere in the circuit. In Stage 2, test a section of the circuit and circle in on the source (or sources) of errors. If errors still occur at Stage 2, at least one of the sources is between your location and the far- end. Test the span section between Stage 1 and Stage 2 to make sure you haven t missed any sources of errors. Before determining that the circuit is good, test it one more time endto-end for verification.
15 O R DSX-1 T1 Network DSX-1 O R Tx Rx FIREBERD 6000 Rx Tx FIREBERD 6000 Rx Tx FIREBERD 6000 Stage 1 Stage 2 Stage 3 FIREBERD 6000 Figure 8 Sectionalizing a T1 circuit. Testing Options To fully evaluate the performance of your circuits, there are three additional testing options you can perform. These are Live Data Emulation, Stress Testing, and Transmission Delay Analysis. Live Data Emulation When transmitting the quasi-random signal source (QRSS) pattern, the FIREBERD emulates digitally-encoded voice signals across all 24 channels of the T1 circuit. The QRSS pattern is a 2 20-1 pseudorandom pattern with 14-zero suppression. Other pseudorandom patterns may be used to stress clock recovery and other T1 equipment circuitry. Stress Testing The live data emulation procedure doesn t necessarily stress the T1 circuit. Whether or not errors were found using the QRSS pattern, the T1 circuit should be tested from one end to the other with various stress patterns to see how the T1 circuit reacts. Equipment that s operating on the borderline of specification may prove faulty when stressed with one of the patterns listed in Table 4. This table lists stress patterns that may also be used for out-of-service testing, and the purpose for using each pattern. If errors occur using these stress patterns, use the isolation technique described previously in Fault Isolation to break the circuit into sections and pinpoint the cause to a specific span section or piece of equipment.
16 Pattern Description 2047 2047-bit (2 11-1) pseudorandom pattern that generates a maximum of 10 sequential zeros and 11 sequential ones. Used to test DDS circuits and other circuits operating at 56 kb/s. 2 15-1 32,767-bit pseudorandom pattern that generates a maximum of 14 sequential zeros and 15 sequential 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. 2 20-1 1,048,575-bit pseudorandom pattern that generates a maximum of 19 sequential zeros and 20 sequential ones. Used on T1 applications to stress circuits with excess zeros. Pattern cannot be used to test asynchronous circuits. 2 23-1 8,388,607-bit pseudorandom pattern that generates a maximum of 22 sequential zeros and 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 the 12.5% ones density requirement for T1-type circuits. This pattern cannot be used to test asynchronous circuits. 3 in 24 Fixed pattern that generates 3 Marks separated by 3 Spaces and 15 consecutive Spaces in every 24 bits transmitted. The pattern generated appears as: 1000 1000 1000 0000 0000 0000. It s used to test the excess zeros requirement for T1-type circuits. This pattern can not be used to test asynchronous circuits. T1-1 Long User 72-octet pattern. Useful for stress testing of the repeater preamplifier and automatic line build-out (ALBO) circuitry. Detects marginal equipment using rapid transitions from a low ones density to a high ones density. Table 4 Pseudorandom patterns for stress testing. T1-2 Long User 96-octet hex pattern. Used for empirical stress testing of T1 circuits and equipment. T1-3 Variation on a Long User 54-octet hex pattern. Used for empirical stress testing of T1 circuits and equipment.
17 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 T1-mW DDS-1 DDS-2 DDS-3 DDS-4 DDS-5 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 defaults its USER 1 pattern to the T1-6 stress pattern. 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. 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 signal recovery. Cannot be used in asynchronous timing mode. Long User Pattern of 100 octets of 01111110, followed by 100 octets of all zeros. Simulation of an HDLC packet frame. Cannot be used in asynchronous timing mode. Fixed pattern 01001100. Minimum stress test of a DDS circuit. Cannot be used in asynchronous timing mode. Fixed pattern 00000010. Moderate stress on DDS clock recovery circuits. Cannot be used in asynchronous timing mode. Long User Pattern, consisting of DDS patterns 1-4. A quick test for those wishing to test a circuit with 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 in multipoint applications. Table 4 Pseudorandom patterns for stress testing. (Continued)
18 Transmission Delay Analysis It s helpful to measure the round-trip transmission delay of a long-haul T1 circuit. For example, voice echoes, protocol errors due to time-outs, and wander, are all impairments that may indicate changes in a T1 circuit s transmission distance. For terrestrial T1 circuits, some carriers guarantee one-way absolute delay of no greater than 100 ms. Refer to your circuit s specifications and tariff for details. The FIREBERD s delay measurement is helpful in determining changes to the T1 circuit s transmission path. Utilizing the FIREBERD s round-trip delay function on individual DS0s of a FT1 circuit can help you determine which DS0s are being carried along different routes. The route of various DS0s and their associated delay is important for speed-sensitive applications such as video and data. To measure transmission delay, use either the QRSS or 2 23-1 pattern. Also, use the MENU switch to select the AUXILIARY function. With the keypad, enter the number 31 and press ENTER. This displays the delay function. Then, use the softkeys under each of the following selections: 1. Press START to access the start-of-measurement signal sources. 2. Press the MORE key twice to display additional selections. 3. Press GPATT to select the generator pattern sync pulse. 4. Press the key to return to the previous menu level. 5. Press STOP to access the end-of-measurement signal sources. 6. Press the MORE key twice to display additional selections. 7. Press RPATT to select the receiver pattern sync pulse. The delay measurement circuitry is armed immediately upon completion of the setup, and the DELAY result (available in the SIGNAL category of the ANALYSIS RESULTS switches and displays) is blanked until the measurement is complete. To initiate or repeat this measurement, press the RESTART switch. This setup measures round-trip delays up to 0.67 seconds. See the FIREBERD 6000 Reference Manual for more information. Conclusion Transmission testing does not have to be complicated. A T1 transmission test instrument should perform four basic types of measurements: Installation Acceptance testing Preventive maintenance Fault isolation The FIREBERD communications analyzers enable you to perform installation and acceptance tests, ongoing preventive maintenance, and fault isolation on the entire range of T1 and FT1 services. In-service and out-of-service testing are easy to perform by service personnel at all levels.
19 Appendix A: T1/FT1 Interface Module (Model 41440A) Menu Tree INTERFACE: FT1/T1 CONFIG MODE ERRINS INTERFACE: FT1/T1 IDLE LOOP RCVBYT MORE MODE: FULLT1 FT1 FRACT1 FT1INS MODE: FT1 FULLT1 FRACT1 FT1INS MODE: FT1 VOICE TLB LLB MODE: FT1 ESFDL RS232 T1.403 LOOP: RESP:NONE TYPE RESPND MORE MORE CH01:TX/RX 01X64/01X64 CH#UP CH#DN TX/ RX CLRALL 56/64 CH01:TX/RX 12X64/12X64 VOICE TX:01 RX:01 CH#UP CH#DN TX/ RX SEL< CH#UP CH#DN AUTO CLRALL 56/64 SIG RS232 TX:01<RX:01<64K SEL< CH#UP CH#DN T1.403 PRM EMUL: CARR CUST CARR (1...24) (24...1) MORE (24...1) (1...24) (1...24) SIG TX:01 ABCD:1010 OFF/ON HELP RS232 TX:01<RX:01<64K 56/64 DATLINK LOOP: FAC1 FAC2 DL-LLB DL-PLB PRGM LOOPCODE RESP:NONE AUTO NONE V.54 MORE MORE PRGM DN: 1000 UP/DN HELP INSERT DELETE INTERFACE: FT1/T1 CONFIG MODE ERRINS INTERFACE: FT1/T1 IDLE LOOP RCVBYT Enter 8 bit binary idle code pattern MORE CONFIG: ESF, AMI, TERM CONFIG:STD, 0dB, TERM ERRINS: LOGIC OFF IDLE: 01111111 RCVBYTE CHANNEL: 01 FRAME CODE INPUT MORE RESULT LBO REFT1 OFF SINGLE RATE BPV LOGIC L+BPV 1FRAME 2FRAME 3FRAME 4FRAME HELP CH#UP CH#DN MORE (1...24) (24...1) OFF ESF D4 SLC AMI B8ZS TERM BRIDGE DSX STD LIV 0dB -7.5dB -15dB TERM BRIDGE MORE Enter 8 bit binary idle code pattern NOTE: SHIFT Key can be used to reverse the direction of CHAN#. Entering of a new page will reset to increment direction.
20 Appendix B: Configuring the FIREBERD for T1 In-Service Analysis of Live Traffic Configure the FIREBERD before cabling to the T1 circuit to prevent T1 circuit disturbances and to obtain accurate measurements. If you perform this analysis frequently on similar circuits, use the STORE/ RECALL function to store all front panel and interface configurations. See the FIREBERD 6000 Reference Manual for more information on this function. Front Panel Setup The FIREBERD 6000 is shown in Figure 9. The numbers in the figure correspond to the front-panel switches that control each configuration activity, as described in Table 5. This section describes only frontpanel switches that are used for in-service monitoring. 1 5 3 FIREBERD MC6000 COMMUNICATIONS ANALYZER RECEIVER MK MARK 1:1 63 511 2047 JITTER MENU GRAPH LIST % MASK: BPV Rate 54.0 3.1 E-07 RESTART DISPLAY HOLD SP SYNC SYNC LOST 2047R 15 2-1 20 2-1 23 2-1(A) QRSS PRGM FOX USER DATA SYNTH INTF BNC GEN CLK 7 8 9 4 5 6 1 2 3 MORE SYNTH FREQ INTF SETUP TEST INTERVAL CHAR FORMAT JITTER ANALYSIS RESULTS ERROR PERFORMANCE TIME SIGNAL T-CARRIER ALARM CONTINUOUS SINGLE ANALYSIS MODE PRINTER RESULTS FRM SYNC CODE ALM 1 ALM 2 LOOP UP LOOP DOWN 10-3 ERROR INSERT SYNC ASYNC RECOVD 0. SHIFT ENTER PRINT EVENT RECALL/STORE AUXILIARY CATEGORY CONTROLS SELF LOOP TIMING MODE VOLUME AUX FUNC IN USE MENU RESULT OFF/ON POWER OFF ON 4 6 7 2 Figure 9 FIREBERD 6000 front panel.
21 Location In Figure 9 Switch Activity 1 SELF LOOP The SELF LOOP switch should be in the OFF position (the red LED within the switch should NOT be illuminated. 2 MENU/INTF SETUP Interface setup is accomplished by pressing the MENU switch to select the INTF SETUP function and then by selecting displayed parameters with the three softkeys and the MORE key beneath the display. The key on the keypad may be used to move to the previous menu level. After selecting the INTF SETUP function, press the 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 your T1 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 the FIREBERD s cabling location. DSX - if cabling to a resistor-isolated monitor jack. BRIDGE - if cabling to wire-wrap pins. f. RESULT to set it to LIV. This eliminates unnecessary results for monitoring live traffic such as bit errors or 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 for monitoring, 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. Table 5 Setting up an in-service test.
22 Location In Figure 9 Switch Activity 4 MENU/TEST INTERVAL Use the TEST INTERVAL function of the MENU switch to set the BER block length. When the ANALYSIS MODE switch is in the SINGLE position, use the TEST INTERVAL function to set the time interval. 5 GEN CLK Use the GEN CLK switch to select INTF timing. The FIREBERD recovers timing from the T1 signal. 6 MENU/PRINT EVENT Use the PRINT EVENT function of the MENU switch to set the conditions under which a results printout will be generated (choose setting for a specific time interval or on the occurrence of an error condition). Table 5 Setting up an in-service test. (Continued) 7 MENU/RECALL /STORE If desired, use the RECALL/STORE function of the MENU switch to save this configuration (and up to 10 FIREBERD test configurations) for future retrieval. A Typical Performance Report 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 the MODE menu. c. Press the MORE softkey until T1.403 appears in the display, then select the mode by pressing the softkey below it. d. Press the CUST softkey to select customer generated PRMs. e. Press the key to return to the previous menu. f. Press the CONFIG softkey. g Press the INPUT softkey and select the DSX input impedance level. h. Press the LBO softkey and select the appropriate output level (0dB, -7.5dB, or -15dB). i. Return to the main interface menu. If you perform this analysis frequently on similar circuits, use the STORE/RECALL function to store all front panel and interface configurations. See the FIREBERD 6000 Reference Manual for more information on this function. Check for Proper Operation Now that the FIREBERD is monitoring the T1 span, verify that it s operating properly and providing accurate results by observing the frame synchronization (FRM SYNC) and pattern synchronization (PAT SYNC) indicators on the right side of the front panel. The FIREBERD is now ready to monitor a T1 circuit. The only remaining operation required is cabling the FIREBERD to the T1 circuit.
23 Appendix C: Configuring the FIREBERD 6000 for a FT1 Out-of-Service Test To verify that the FIREBERD 6000 is set up properly before cabling to the circuit, perform a self-loop test and check the indicators. NOTE: The GEN CLK switch must be set to the SYNTH position for proper self-loop operation. Front Panel Setup The FIREBERD 6000 is shown in Figure 10. The numbers in the figure correspond to the front-panel switches that control each configuration activity, as described in Table 6. This section describes only frontpanel switches that are used for an out-of-service test. 1 2 3 4 5 6 FIREBERD MC6000 COMMUNICATIONS ANALYZER RECEIVER MK MARK 1:1 63 511 2047 JITTER MENU GRAPH LIST % MASK: BPV Rate 54.0 3.1 E-07 RESTART DISPLAY HOLD SP SYNC SYNC LOST 2047R 15 2-1 20 2-1 23 2-1(A) QRSS PRGM FOX USER DATA SYNTH INTF BNC GEN CLK 7 8 9 4 5 6 1 2 3 MORE SYNTH FREQ INTF SETUP TEST INTERVAL CHAR FORMAT JITTER ANALYSIS RESULTS ERROR PERFORMANCE TIME SIGNAL T-CARRIER ALARM CONTINUOUS SINGLE ANALYSIS MODE PRINTER RESULTS FRM SYNC CODE ALM 1 ALM 2 LOOP UP LOOP DOWN 10-3 ERROR INSERT SYNC ASYNC RECOVD 0. SHIFT ENTER PRINT EVENT RECALL/STORE AUXILIARY CATEGORY CONTROLS SELF LOOP TIMING MODE VOLUME AUX FUNC IN USE MENU RESULT OFF/ON POWER OFF ON 7 9 10 8 Figure 10 FIREBERD 6000 front panel.
24 Location In Figure 10 Switch Activity 1 DATA Use the DATA switch to select the QRSS pattern. Other patterns may be used later to stress the T1 circuit. 2 ERROR INSERT The ERROR INSERT switch should be in the OFF position (the red LED within the switch should not be illuminated). 3 SELF LOOP The SELF LOOP switch should be in the OFF position (the red LED within the 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 the near-end mux or channel bank is slave to another clock source (e.g., DCS networks). SYNTH: Timing is generated from the FIREBERD s internal synthesizer. Select the particular frequency with the SYNTH FREQ function of the MENU switch. This is a required setting to perform SELF LOOP. BNC: The FIREBERD 6000 uses an external clock source via the rear panel s BNC connector. 5 MENU/SYNTH FREQ When the GEN CLK switch is in the SYNTH position, set the synthesizer frequency by using the MENU switch to select the SYNTH FREQ function. After the SYNTH FREQ LED is illuminated, press the MORE key until 1544 khz is displayed. Press the softkey beneath the display that corresponds 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 the TEST INTERVAL function of the MENU switch. 7 MENU/TEST INTERVAL Use the TEST INTERVAL function of the MENU switch to set the BER block length. When the ANALYSIS mode switch is in the SINGLE position, set the time interval with the TEST INTERVAL function. Table 6 Setting up a FT1 out-of-service test.
25 Location In Figure 10 Switch Activity 8 MENU/INTERFACE After selecting the INTF SETUP function, press the softkey under each of SETUP 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 of input impedance). f. RESULT to set to STD. g. LBO to select the transmitter s line build out. 0 db is the recommended setting. h. REFT1 to set to BRIDGE (exhibits an input impedance of 100Ω or 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 rate of 64 or 56k. Use the CH# UP/DOWN softkeys, the 56/64 and the TX/RX softkeys, to configure each channel individually. k. ERRINS to select a form of error insertion or it may be turned off with 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 a results printout will be generated at specific time intervals or on the occurrence of an error condition. 10 MENU/RECALL/ Use the RECALL/STORE function to save this configuration STORE (and up to 10 FIREBERD test configurations) for future retrieval. Table 6 Setting up a FT1 out-of-service test. (Continued)
26 If you perform this analysis frequently on similar circuits, use the STORE/RECALL function to store all front panel and interface configurations. See the FIREBERD 6000 Reference Manual for more information on this function. Since the FIREBERD is now configured for an outof-service test, make sure that the transmit and receive cables are properly aligned as shown in Figure 11. Check for Proper Operation Now that the FIREBERD is monitoring the T1 span, verify that it s operating properly and providing accurate results by observing the FRM SYNC and PAT SYNC indicators on the right side of the front panel. 1 2 Tx Rx FIREBERD 6000 T1 Span Equipment Tx Rx FIREBERD 6000 3 4 FIREBERD 6000 Tx Rx Tx Rx FIREBERD 6000 T R T1 R1 (Tx) (Rx) Figure 11 RX and TX cable alignment.
27 Appendix D: Overview of Jitter Testing This appendix provides an overview of T1/FT1 testing with the FIREBERD 6000 s Jitter options. NOTE: The jitter options are available with the FIREBERD 6000 only. All references to the FIREBERD in this appendix are for the FIREBERD 6000 only. Jitter is the deviation in time between when the pulse transitions occur, when they ideally should occur, and when the digital decoding gear expects them to occur. It is unintentional phase modulation, having both an amplitude and frequency component. The jitter s amplitude is the magnitude of the phase deviation; its frequency is a measure of how rapidly the phase changes. Continued and increased jitter can result in errors, bit slips, and total impairment of the digital line. Jitter is a composite of many wave forms, not specifically sinusoidal. Figure 12 is an example of the various displaced jitter pulses. In this case, eight snapshots, each taken at a fixed interval between the markers y and y 1, form a three-dimensional composite diagram. This figure compares the differences in the position of the pulse over time. It reflects an early arrival 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 be measured: time, phase angle, and digit period. The most convenient is the digit period known as the unit interval (UI), which is also the unit that the FIREBERD measures. UIs are not measured over a single pulse, but instead as they gradually accumulate over a number of pulses. One UI is equivalent to each of the following: 360 degrees, 648 nanoseconds, 100 percent, one bit, and one time slot. The amount of displacement, which is the jitter, is measured in amplitude (U.I.) and frequency (Hz) y y1 Subsequent Pulses in this case are displaced in accordance with a pattern which approximates a Sine Wave 6 7 8 5 4 3 Elapsed Time The expected pulse position 2 1 Figure 12 Jitter pulses. The first observed pulse A theoretical view point
28 (A) Data Sent (B) Jittered T1 (C) Decision Points (D) Recovered Clock (D) Recovered Data 28.0 1 1 0 0 0 0 0 0 1 0 + Threshold Threshold 1 1 0 0 0 0 0 0 0 1 Figure 13 Received T1 signal and clock signals. Two Bit Errors Electromagnetic interference (EMI), far-end/ near-end crosstalk, imperfections in regenerators, and the bit stuffing/de-stuffing process can all produce jitter. A form of jitter called wander occurs with phase variations of less than 10 Hz. T1 bit stuffing, environmental impairments, clocking, and transmission path delay variation are normal causes of wander. Jitter causes errors because equipment can t accurately determine the decision threshold. A decision threshold is used to distinguish between a 1 or a 0, or the presence or absence of a pulse. Using midbit sampling, if the pulse is shifted by more than 50 percent of its width (0.25 UI), it will be sampled incorrectly. The validity of the pulse is affected by the amount of jitter. This is illustrated by Figure 13. Jitter analysis is essential to isolating problems on T1/FT1 links. Specific devices within a T1 network, particularly when faulty, tend to introduce jitter at unidentifiable frequencies. Jitter masks have been developed by organizations such as AT&T, Bellcore, and ITU-T, to ensure that transmission equipment vendors conform to jitter tolerance specifications. Jitter tolerance specifies the amount of jitter that can be tolerated at an input before errors start to occur. Performing a spectrum analysis, while simultaneously monitoring for degradation in error performance (bit errors, timing slips, etc.), can link a symptom to its cause. Figure 14 depicts a spectrum analysis plot. Jitter Amplitude (Unit Intervals Peak-to-Peak) 10.0 8.0 5.0 2.0 1.0 0.5 0.4 0.3 0.2 Figure 14 Widely used jitter masks. 0.1 0.0 0 1 2 10 100 200300 500 1,000 4K 8K10K 32K 40K50K Jitter Frequency (Hz)
29 The three standard ways of measuring jitter are: tolerance measurements, intrinsic jitter measurements, and transfer characteristic measurements. When performing a tolerance test, the FIREBERD s jitter amplitude and frequency on the transmitted test pattern is increased in steps between 10 Hz and 40 khz until the onset of bit errors occurs. If the mask limits are exceeded at every frequency tested, then the tested equipment has adequate jitter tolerance. Intrinsic jitter testing looks at the jitter output without a jitter input signal from the FIREBERD. The jitter output is plotted for frequencies between 40 khz and 10 MHz. During a transfer characteristic test, jitter is injected at amplitudes of 50/75/100% of the MASK for which the equipment is specified, allowing you to see at what frequencies the mask is exceeded. These results allow you to isolate devices in the network that magnify the jitter they receive in the transmitted signal. Sinusoidal jitter modulation should be used when testing for compliance with specifications. Waveforms such as square and triangle-shaped waves contain harmonics that can exceed the mask, resulting in incorrect specification results. Figure 15 In-service monitor points. In-Service Analysis In-service analysis allows users to test live traffic without introducing disturbances on the lines. This testing method permits users to monitor information about the performance of the T1 system. When monitoring jitter in-service, a spectral analysis of the live data provides a more useful and accurate analysis than an out-of-service test. Figure 15 depicts in-service monitor points. The Wideband Jitter Option (Option 6001) enables the FIREBERD to analyze wideband jitter and look at wideband results, so the T1 that is monitored for alarms and other indications of faulty equipment can be monitored for jitter levels. This analysis of the entire spectrum gives results such as jitter hits (JTR HITs), maximum jitter (MAX JTR) and 1 SEC JTR to see where the timing jitter actually exceeds the user-selectable threshold. The 1 SEC JTR, MAX JTR, and JTR HITS results are found in the SIGNAL category. Wideband jitter analysis can be performed in a variety of ways. Filtering out frequencies less than 8 khz can further identify the problem. This allows only the high frequencies to pass, giving further insight into problems associated with faulty or marginal span repeaters. Wideband results measure maximum peak-to-peak jitter in 40 frequency bands from 10 Hz to 40 khz and displays the resulting UIs peak-to-peak. This measurement may be done in one of two ways: FIREBERD 6000 T1 MUX Smart Jack T1 Repeater Office Rep. Office Rep. DSX-1 T1 NETWORK Customer Premise Local Loop Local Exchange IXC
30 1. Continuously measure 1 of the 40 available frequency bands and constantly update the jitter amplitude at that frequency or, 2. Continuously sweep through all 40 frequency bands. When the sweep is enabled, each jitter value shown in the 40 frequency bands is compared to the measured value of the corresponding frequency. The maximum value is saved until the test restart occurs. The Jitter Spectral Analysis Option (Option 6002) enables the FIREBERD to perform spectral analysis (S/A). In addition to a spectral analysis, S/A and %MASK results can be viewed in the results window. The S/A measures the maximum peak-topeak UI over (N) discrete frequencies. N varies, based on a T1 circuit. The S/A option may be used for both in-service and out-of-service testing. 3. Press S/A to access the spectrum analysis results. a. SWEEP determines the range of the spectrum analysis. b. ON - if set ON, the analysis will cover the entire 10 Hz-40 khz range. Selecting OFF enables you to select a frequency band. c. FREQ - if you want to look at one specific band in the 10Hz to 40 khz range. SWEEP should be set to OFF if choosing a specific band. d. PEAK - to find peaks in the spectrum. Used only when SWEEP is enabled. e. PEAKUP - finds the next higher frequency jitter peak. f. PEAKDOWN - finds the next lower frequency jitter peak. S/A Plot A jitter S/A should be performed during the installation of a network. This provides a basis for comparison of any future degradations, to this initial jitter analysis. Deviations can then be isolated, subsequently assisting in identifying the problem. Below is an 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 the JITTER function. Use the following softkeys: 1. Press HITS to set the decision threshold to count a jitter hit. The range is between 0.1 and 6.5 UI. 2. Press FILTER to determine the range over which the results should be complied. a. FULL softkey for the entire range. b. >8 khz to isolate frequencies of 8 khz or higher. 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 external waveform, or a combination of both) to produce T1 jitter. Internal timing, the recovered clock, and an external clock source may be used with the jitter generator. Out-of-service testing enables users to accurately assess the quality of a T1 line. The data test pattern used is extremely important when testing jitter. When out-ofservice testing is performed, QRSS is typically used because it represents live data. Fixed patterns, such as 1:7 or 3 in 24, should be used to provide greater stress of clock-recovery circuits and to simulate marginal data content conditions. When testing for jitter tolerance, the error rate should be monitored while input jitter is varied. The FIREBERD may be used to generate jitter onto the T1 span, while performing a BERT test.
31 Individual pieces of network equipment, such as muxes, may be tested for acceptance by performing a back-to-back test with a second FIREBERD. If more jitter is seen on the receive leg than the transmit leg, then the equipment is amplifying instead of deleting jitter to the signal. Likewise if the jitter received is less than that transmitted by the FIREBERD, the equipment is attenuating jitter from the signal. This kind of jitter test is further described in the example below. For a tolerance test, FIREBERD No. 1 should be in loopback mode. 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 mask amplitude. c. XFR50% or XFR75% to add jitter at a percentage of the MASK level. FIREBERD No. 2 FIREBERD No. 1 Scroll through the MENU selections and choose the JITTER function. Use the following softkeys: 1. Press MASK to select the specification of the equipment 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 the FIREBERD. b. SINE for an accurate specificationcompliance result. c. EXT if the external BNC connector (labeled Jitter MOD IN) is the modulation source. If an additional waveform is mixed with the BNC signal, use a SINE wave for an accurate specification-compliance result. 4. Press FREQ to set the frequency band you want to test. a. SWEEP - to generate jitter at all 40 frequency bands. b. HZ - to generate jitter at a specific frequency band. c. to return to the previous menu level. The second FIREBERD can be optioned to work in a thru mode to obtain wideband jitter results. Follow the procedures for out-of-service testing. The FIREBERD should be in a Line Loopback (LLB) Mode, if you are using the 41440A Interface. Then, follow the jitter setup procedures for an intrinsic jitter test. Two of the most common problems caused by T1 jitter are bit errors, or an increase in BER, and timing slips. Bit errors occur when the receiving circuits incorrectly sample the incoming pulse. Refer to the table below for some frequently found results, their causes, and suggested solutions. Results Pattern slips, framing errors, and frame loss. Signifies a controlled slip. Usu- ally a network timing problem. Pattern slips, no framing errors, no frame loss. Problem/Solution Signifies an uncontrolled slip. Usually caused by the overflow of a buffer within an M13 mux. Refer to the FIREBERD 6000 Reference Manual for more information on the FIREBERD jitter options. 1995 Telecommunications Techniques Corporation. Telecommunications Techniques Corporation, TTC, and FIREBERD are registered trademarks of Telecommunications Techniques Corporation. All other trademarks and registered trademarks are property of their respective companies.
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DS M 34 O R Behind Successful Networks You'll Find TTC 20400 Observation Drive, Germantown, Maryland 20876 Tel. (800) 638-2049 (301) 353-1550 (MD) FAX (301) 353-0731 www.ttc.com FB T1/FT1 AN - 10/97
FIREBERD 35 X-1 ON DSX-1 O R Application Note T1 Network