Performance of a System of Mutually Synchronized Clocks

This paper is concerned with mutual synchronization--a scheme for synchronizing a nationwide network of clocks for an integrated digital transmission and switching communication system. The clock at a switching center in such a system determines the rate of flow of data bits through the switch and to the output links. Buffer stores can make allowances for small, temporary differences in the received rate (determined by a distant clock and delay change) and the local switch clock rate. However, if a frequency difference persists then a buffer will occasionally overflow (or underflow) causing deletion (or repetition) of data bits from the output stream. Either such error is referred to as a slip. In this paper a synchronous network is one in which there are either 110 slips or, alternatively, in which the slip rate does not exceed some given rate. Mutual synchronization was conceived about 10 years ago 1 as an 2449 2450 T H E BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1971 alternative to a master-slave synchronous timekeeping plan for large telephone networks. This new plan aimed at eliminating the possible need to reorganize a master-slave network in the event of failure of a timing link or the master clock. Previous analyses have been largely concerned with questions of stability, 2 equilibrium frequency, 3 dynamic response, 4 ' 5 and control strategies. 0 Questions relating to the system's ability to meet specific performance objectives, e.g., no slips, have been largely ignored.