Chapter 18: Test Equipment Requirements

Every installation team and maintenance technician must have a good T1 bit error rate test (BERT) instrument. In many cases, a single BERT is adequate, but in other cases having one BERT at each end of a facility is an advantage. The primary BERT must have transmit and receive capability. Often, receive-only T1 monitor instruments are useful in conjunction with the primary set. The best BERTs feature two receive ports instead of one. The second port becomes handy for a synchronization troubleshooting situation.

For 98% of troubleshooting, all that is needed is a good BERT. One of the most common test instruments is generically called a T-BERD and is manufactured by TTC. In a really complex case of "finger-pointing" over the exact DS-1 waveform, the only way to solve it is to use a good high-frequency dual trace oscilloscope. With an oscilloscope, use two calibrated high-impedance probes set for A-B, or differential mode. Tie the two probe grounds together at the logic ground of the device under test. That way you can see if there is a DC offset voltage present.

Test Set Details

There are many brands of DS-1 test sets. A typical example is the TTC FireBERD 209. It has buttons and indicators on the front panel. When the user is trying to test from a CO out to a Larus NIU, he might insert the 209 signal at the office repeater and send the so-called Loop-Up Code. Keep in mind that there are several varieties of these codes for loopback purposes. Different products of different vintages use different loop codes to accomplish this function. There are four-bit, five-bit, and six-bit codes. Just because the technician pushes the button for Send Loop Up, he may not know exactly what code he is sending. If it happens to be the wrong code for the Larus unit, it won't be recognized, and that will cause confusion. Some test sets, like the 209, will allow you to walk down through detailed menus until you can see exactly what code bits are sent from that particular set. Just make sure you know.

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