BPV:

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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.
The receive signal is often much weaker that the transmit signal. Cross talk in the cable pairs can cause the transmit signal to appear on the receive pair and interfere with the low-level receive signal.