Terminology
CRC
To an observer, the data on a live E1 Circuit appears to be random. Fortunately there is a way to perform limited testing when the circuit is designed for Cyclic Redundancy Checksum (CRC) format. A portion of the frame bits is reserved for a 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 E1 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 E1 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 E1 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 E1 signals.
High Density Bipolar 3 (HDB3) is an exception to AMI that replaces runs of 4 consecutive zeroes with a special code that violates the AMI rules. It allows greater flexibility of data pattern 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 E1 signals. Shielded cable is often chosen for E1 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.
Frame
Pulses streaming in an E1 circuit would be meaningless if there were no way to organize the pulses into a meaningful structure.