To determine the bit error rate, a device called a bit error rate tester (BERT) is used. Bit error rate testing (BERT) involves generating a known data sequence into a transmission device and examining the received sequence at the same device or at a remote device for errors.

Normally, BERT testing capability is built into another device, such as a ‘sophisticated’ break-out box or a protocol analyzer; however, several vendors manufacture hand-held BERT equipment. Since a BERT generates a sequence of known bits and compares a received bit stream to the transmitted bit stream, it can be used to test both communications equipment and line facilities.

You would employ a BERT in the same manner to determine the bit error rate on a digital circuit, with the BERT used with CSU/DSUs instead of modems. The modem closest to the terminal into a loop can be used to test the modem. Since a modem should always correctly modulate and demodulate data, if the result of the BERT shows even one bit in error, the modem is considered to be defective.

If the distant modem is placed into a digital loop-back mode of operation where its transmitter is tied to its receiver to avoid demodulation and remodulation of data the line quality in terms of its BER can be determined. This is because the data stream from the BERT is looped back by the distant modem without that modem functioning as a modem.

Since a leased line is a pair of two wires, this type of test could be used to determine if the line quality of one pair was better than the other pair. On occasion, due to the engineering of leased lines through telephone company offices and their routing into microwave facilities, it is quite possible that each pair is separated by a considerable bandwidth.

Since some frequencies are more susceptible to atmospheric disturbances than other frequencies, it becomes quite possible to determine that the quality of one pair is better than the other pair. In one situation the author is aware of, an organization that originally could not transmit data on a leased line determined that one wire pair had a low BER while the other pair had a very high BER.

Rather than turn the line over to the communications carrier for repair during the workday, this organization switched their modems from fullduplex to half-duplex mode of operation and continued to use the circuits. Then after business hours they turned the circuit over to the communications carrier for analysis and repair.

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