Apparatus and method for timing distribution over an asynchronous ring

A local area network, configured as a token-passing ring, for communicating packets of information between nodes attached to the ring, at least one of the nodes being designated slave for the purpose of receiving information regarding the frequency of the clock at the master node, the slave node

• characterized by:
• means for transmitting an identifiable packet onto the ring;
• means for receiving said identifiable packet after it has traversed the ring;
• means responsive to a clock at the slave node for measuring the time interval between the event that the identifiable packet is transmitted and the event that it is received in order to measure ring latency;
• means for receiving a measurement of ring latency made at the master node;
• means jointly responsive to the ring latency measurement made at the master node and to the ring latency measurement made at the slave node for modifying the frequency of the clock at the slave node in a manner that decreases the difference between the latency measurements.

A method for timing synchronization in an asynchronous ring network that interconnects first and second stations;

• each station having a timing signal generator, a transmitter for entering data onto the network, and a receiver for receiving data from the network, the ring network circulating data around the ring in a time interval known as "ring latency", the method comprising the steps of:
• measuring ring latency at the first station;
• transmitting the measure if ring latency to the second station;
• measuring ring latency at the second station;
• calculating an error signal, at the second station, as the difference between measurements of ring latency made by said first and second stations;
• and
• modifying the frequency of the timing signal generator, at the second station, in a manner that decreases the magnitude of the error signal.

A method for establishing frequency synchronization between clocks at two different geographic locations, each location having access to a common transmission media, and each location having apparatus, including its clock, for independently measuring signal propagation delay over the transmission media, the method including the steps of:

• measuring the propagation delay over said transmission media at each location:
• communicating the measurement of the propagation delay made at one location to the other location;
• and
• changing the frequency of the clock at said other location in accordance with the difference between the measurements of propagation delay made at each location.