A Telephone-Based Time Dissemination System

Telephone time dissemination can serve a broad spectrum of users who require time accuracy in the millisecond range and who are reluctant to use radio time dissemination services. A system which uses a simple protocol for telephone loop-delay correction is presented. The protocol uses ASCII characters transmitted by the frequency shift keying techniques of standard 300 baud modems. The protocol is designed for the transfer of the correct time from a master clock to a slave clock. The slave clock places the telephone call, puts the master clock into a prompt echo mode, measures the loop delay time, and applies half the loop delay time as a correction to the time it receives from the master clock. The precision and accuracy of this protocol are discussed and measurements are presented on the precision of time transfer using Leitch CSD-5300 units as master and slave clocks. The capabilities of the full system, and the Leitch CSD 5300 units which are used at each distribution node, are discussed as possible solutions to questions that are often addressed or misaddressed to Precise Time research groups.