Master and Secondary Clock in Telecommunications Networks

Allan Risley

Abstract: Telecommunication networks of time division switches, interconnected by digital transmission are being put into place. At each switch, each incoming hit stream is brought into its own buffer. Then the clock in the switch "reads" each buffer to re-establish phase. Care must be taken to keep frequency differences between various clocks from becoming too large, other-wise, buffers will under/over flow at an unacceptably high rate. Based on empirically defined data transmission requirements, one major network has determined that fractional frequency inequality between switches should he no worse than 1.7x10^-9, A network needs near frequency equality between its own switches, and also between its switches and other networks with which it interfaces. Frequency accuracy - per se - is not required, but as a practical matter, the best way to achieve needed frequency equality is for each network have master clock with an accuracy which is at least as good as 1 x 10^-10. To be certain that this accuracy is achieved the master clock in each network should be based on a signal from a cesium source. Cesium sources for the purpose of providing master clock, fall into two categories: 1) Cesium standard on site. 2) Cesium based signal distributed via some transmission medium. Concerns of systems designers are that: A) The master signal he tied to a properly functioning cesium standard. B) If master signal is lost., the secondary clocks he of sufficiently low aging that they free-run at least. a week before 5 - their accuracy degrades below 1X10^-9. The accuracy of a properly functioning commercial Cs beam standard is no worse than about 1x10^-11. The master/secondary system must be designed such that the accuracy of the, master can be verified. The paper discusses the relationship between the master and the secondary clocks. The quest ions of master clock accuracy and precision and the free-running capability of the secondary clocks will be examined.
Published in: Proceedings of the 16th Annual Precise Time and Time Interval Systems and Applications Meeting
November 27 - 29, 1984
NASA Goddard Space Flight Center
Greenbelt, Maryland
Pages: 403 - 410
Cite this article: Risley, Allan, "Master and Secondary Clock in Telecommunications Networks," Proceedings of the 16th Annual Precise Time and Time Interval Systems and Applications Meeting, Greenbelt, Maryland, November 1984, pp. 403-410.
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