Digital Data System: Network Synchronization

The network for the DDS consists of an interconnected set of digital transmission facilities. At any node of the network, if the average rate of transmission bits leaving a node is not exactly equal to the average rate entering the node, errors occur. Such errors are defined as slips, which means that bits are arbitrarily deleted if the input rate exceeds the output rate, or that bits are repeated or inserted arbitrarily if the input rate is slower than the output rate. The best performance is realized if no slips occur. This requires that every node in the system be synchronized to the identical average frequency and be capable of absorbing delay fluctuations. The transmission media comprise radio, coaxial cable, and twisted pairs. Each of these admits to small but troublesome variation in delay because of thermal and other effects. The rates of change of such delay variations are equivalent to small frequency disturbances. To be free from slip under these conditions requires two elements: (i) reproduction of identical average frequencies at every node, and (ii) storage of sufficient data at each point of entry to accommodate the 879