Frequency and time in communications

Modern communications systems make use of sophisticated multiplexing techniques in order to achieve efficient spectrum utilization and thereby realize the maximum number of message channels through a particular transmission medium. Synchronization of transmit/receive carrier (or clock) generators is essential in such systems to avoid distortion; also, close tolerances on the accuracy of carrier and pilot frequencies must be imposed to maintain transmission through minimal bandwidth channel selection filters. Typical requirements on both synchronization and accuracy for several categories of commercial communications systems range from several parts in 10
⁶
to a few parts in 10
¹⁰
.