Technical data

Chapter 18 533
T1 Analyzer
More Information About T1 Testing
T1 Analyzer
Terminology
CRC:
To an observer, the data on a live T1 Circuit appear to be random. Fortunately there
is a way to perform limited testing when the circuit is designed for Extended Super
Frame (ESF) format. A portion of the frame bits are reserved for a Cyclic
Redundancy Checksum (CRC) sequence that can be monitored for performance.
Simply stated, the CRC bits are calculated on the transmit end and inserted as a
pattern on the frame bit. The CRC pattern depends on the pattern of other bits
transmitted by the T1 Circuit. The receiving end also computes this pattern and
compares it with the CRC that was computed and sent by the transmitting end. Since
both ends use the same rules for computing the pattern, the CRC bits will be
identical when all the bits involved in the computation agree. The CRC check
provides good insight into the end-to-end integrity of the T1 Circuit and should be
used in conjunction with other tests that can help determine what the cause of the
CRC failure might be.
BPV:
The electrical signals on a properly functioning T1 circuit conform to the
specification set forth in the standards. The standards specify that the presence of a
voltage indicates a data '1' and the absence of a voltage represents a data '0'. Each
occurrence of a data one produces a voltage for half a bit interval that is the opposite
polarity of the previous bit, hence the name Alternate Mark Inversion (AMI). The
alternating nature of the signal ensures that the average DC voltage is zero, allowing
it be transformer coupled. Transformer coupling ensures a high degree of common
mode rejection to the equipment that processes T1 signals.
Bipolar 8 Zero Substitution (B8ZS) is an exception to AMI that replaces runs of 8
consecutive zeroes with a special code that violates the AMI rules. It allows greater
flexibility of data patterns by enhancing repeater synchronization by increasing
pulse density thereby providing greater throughput.
When the electrical signal does not adhere to the alternating nature of the waveform
specification a BiPolar Violation (BPV) has occurred. This can happen for a number
of reasons, many of which are outside the control of the wireless technician. One
cause may be from electrical noise radiating from florescent lamps, motors, or spark
plug ignition circuits coupling into the copper lines that carry T1 signals. Shielded
cable is often chosen for T1 circuits to minimize electrical interference. This shield
must be grounded to be effective. When the cable is spliced or terminated the shield
on both cables should be connected together. Often the transmit and receive signals
are routed in separate cable bundles.