No. 4 ESS: Network Clock Synchronization

The No. 4 ESS is a digital toll switching system, whose time-division network routes standard 8-bit PCM signals.1 Digital interfacing of the network to T-carrier facilities is provided by the Digital Interface Frame (DIF), which converts and concatenates a number of T1 facilities into higher-speed serial bit streams for the switch, and vice versa. Thus, the basic timing for the switching network is the 8-kHz frame 1109 rate typical of T-carrier facilities. Timing for the network is provided by the Network Clock (NCLK) frame, which distributes a 16.384-MHz square-wave pulse train to each of the network frames. The 8-kHz framing information is transmitted as a missing pulse in the 16.384MHz signal once every 125 /xs. Synchronization of the network consists of controlling the frequency of the NCLK such that No. 4 ESS offices that are digitally connected run as close as possible to the same frequency. This is done by phase-locking the clock oscillators to the externally supplied Bell System Reference Frequency (BSRF) or to a T1 line from another No. 4 ESS. Thus, the No. 4 ESS is part of an overall system timekeeping plan consisting of a master-slave hierarchical timing structure. 2 The BSRF is the master timing source, distributed throughout the country to clusters of digitally interconnected No. 4 ESS switches. One switch in each cluster is designated as a master and is phase-locked to the BSRF, while the other switches are slaved in a tree-like structure to the master via timing carried in the digital interconnections.