Technical data

Chapter 11 325
E1 Analyzer
More Information About E1 Testing
E1 Analyzer
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