41to the local NetLink-T1™ in this test mode will be echoed (returned)back to the user device (i.e., characters typed on the keyboard of a ter-minal will appear on the terminal screen).To perform an LL test, follow these steps:1. Activate LL. This may be done in one of three ways:a. Enter Local Loop from the SystemDiagnostics/Statistics menu and toggle the until “LL” appears next to the Local Loop option.b. Activate the “LL” signal on the DTE. If you are not surewhich lead is the “LL” signal, please refer to Appendix D.c. Toggle the front panel switch to the “Local” position.2. Verify that the data terminal equipment is operating properlyand can be used for a test.3. Perform a V.52 BER (bit error rate) test as described inSection 5.3. If the BER test equipment indicates no faults,but the data terminal indicates a fault, follow the manufactur-er’s checkout procedures for the data terminal. Also, checkthe interface cable between the terminal and the NetLink-T1.5.2.2 Operating Remote Digital Loopback (RL)The Remote Digital Loopback (RL) test checks the performance ofboth the local and remote NetLink-T1™, as well as the communicationlink between them. Any characters sent to the remote NetLink-T1™ inthis test mode will be returned back to the originating device (i.e, char-acters typed on the keyboard of the local terminal will appear on thelocal terminal screen after having been passed to the remote NetLink-T1™ and looped back).There are two Remote Loops that can be initiated from theNetLink-T1 unit: (1) V.54 Loop, and; (2)CSU Loop. The user canselect the type of loop that can be initiated from the SystemDiagnostics/Statistics screen or with Switch S2-1. Select “c” RDLType and press the to toggle between the CSU loop andthe V.54 loop. When a loopback is initiated this is the type of loop thatthe unit uses to loop up the remote unit. NOTE: The NetLink-T1 willrespond to both loops regardless of the state of the RDL Type.To perform an RDL test, follow these steps:1. Activate RDL. This may be done in three ways:a. Enter Remote Loop from the System42Diagnostics/Statistics menu and toggle the until “RL” appears next to the Remote Loop option.;b. Activate the “RL” signal on the DTE. If you are not surewhich lead is the “RL” signal, please refer to Appendix D.c. Set the front panel switch to ‘Remote’.2. Perform a bit error rate test (BERT) using the internal V.52generator (as described in Section 5.3), or using a separateBER Tester. If the BER test indicates a fault, and the LocalLine Loopback test was successful for both NetLink™s, youmay have a problem with the twisted pair line connection.5.2.3 CSU LoopThe NetLink-T1™ also responds to central office initiated loop com-mands. The NetLink-T1™ will implement the “loop up” command whenit recognizes the pattern “10000” in the data stream for a minimum of 5seconds. The “loop down” command is implemented by the pattern“100” in the data stream for a minimum of 5 seconds.When operating in ESF framing mode, loopback commands areissued via the Facility Data Link (FDL). The line loop message willcause a loop back before data enters the framer portion of the CSU.The payload loop message will cause the NetLink-T1™ to loop dataafter the framer portion of the CSU.The NetLink-T1™ will respond to Universal Loopback De-activateto clear all central office loops.5.3 BIT ERROR RATE (V.52) DIAGNOSTICSThe NetLink-T1™ offers three V.52 Bit Error Rate (BER) test pat-terns. These test patterns may be invoked along with the LAL andRDL tests to evaluate the unit(s) and the communication links.When a 511, 2047, or QRSS test is invoked, the NetLink-T1™generates a pseudo-random bit pattern of 511 bits, 2047 bits or 220 bits,respectively, using a mathematical polynomial. The receiving NetLink-T1™ then decodes the received bits using the same polynomial. If thereceived bits match the agreed upon pseudo-random pattern, then theNetLink-T1™(s) and the communication link(s) are functioning properly.511 Initiates a built-in 511 bit pseudo-randompattern generator and detector.2047 Initiates a built-in 2047 bit pseudo-randompattern generator and detector.bbaa
