Principles of Synchronous Digital Hierarchy

133Signal Impairments, Error Detection, and Correction

by the parity check are a good measure of transmission errors. More impor-

tantly, these indications enable performancemonitoring without taking the

circuit out of service.

In practical situations, we hardly ever need to know the exact number of

bit errors happening in the system. All we are interested in is the general

performance of the circuit. We need to know whether the errors are within

the tolerable limits for a particular application or for a particular customer.

The actual number of bit errors (BER) also tells us the same story. Thus, for

making a decision about the action to be taken, parity check gives a reason-

ably good indication of circuit performance.

6.7.3 Through CRC

One ...