T1
I have used, for the most part, Acterna’s 224, 117 and the 2310 T-Berds to perform these tests. The most common test to verify signal quality is to loopback the NIU at the customer premise. Test patterns are then sent to determine the condition of the T1 span. It’s important to understand the specific parameters that are being tested with each pattern.
The Quasi-Random Signal Source (QRSS) or just Quasi as most techs call it, is the most commonly used pattern in T1 testing. QRSS simulates the ideal data condition, so it contains exactly 50% ones and 50% zeros. Although it does provide a measure of how the T1 performs, the drawback with QRSS is that it doesn’t test the loop under adverse data conditions.
ANSI T1.403 states that a T1 signal must contain no more than 15 consecutive zeros, and at least 12.5% ones density. This means that an average of one bit out of every eight bits is a one. These lower limits were established to ensure there are there are enough pulses being sent over the local loop to enable repeaters to recover timing and maintain synchronization. Since QRSS doesn’t approach these conditions, the 3 in 24 test pattern was developed. 3 in 24 is used to stress the timing recovery capabilities of the local T1 loop.
To ensure that the local loop meets ANSI T1-403 specs patterns must be run in the following combination:
QRSS for 15 minutes
ALL ONES(AMI) for 5 minutes or ALL ZEROS(B8ZS) for 5 minutes
3 in 24 for 5 minutes
Acterna developed the MULTIPAT pattern to automatically run these patterns. In addition to stressing timing recovery and maximizing power, this test is also useful in identifying common span faults such as one-side opens, grounds and shorts.
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