Northern Telecom Limited

James A McEachern

Thomas E Moore

Wayne D Grover

Mark Stephen Wight

David John Nicholson

Richard Romagnino

Wayne Ergle

Dino Diperna

Thomas P Flanagan

A transmission system comprises nodes coupled in a ring via two multiplexed transmission paths having opposite transmission directions. The two paths can be time, space, or wavelength division multiplexed. Bidirectional communications between any two nodes are provided in normal operation using both paths around a first, most direct, part of the ring. In the event of a fault affecting such communications, a protection switch is made to maintain bidirectional communications between the two nodes using both paths around a second, remaining, part of the ring. In this manner, each path can simultaneously carry both normal and protection switched signals. In an embodiment of the invention, only the two nodes adjacent a fault need be aware of the fault, each of these nodes effecting a protection switch for signals terminated at the node and effecting a protective loopback for other signals needing protection. The invention enhances the total capacity of the paths for carrying normal and protection switched signals. A network may comprise a plurality of such ring transmission systems intersecting at at least one network node, the network node comprising a node of each ring and a cross connect switch for switching signals therebetween.

Ring transmission system

A method of and apparatus for synchronizing an asynchronous DS1 signal to produce a synchronized signal in the SONET format is described. The asynchronous signal is stuffed in dependence upon a stuff request signal which is produced from a comparison with a threshold level of a phase difference between read and write phases with which the synchronized and asynchronous signals are respectively read from and written into an elastic store. The phase detection and threshold comparison are effected synchronously with the respective signals. To this end, the read address for the store is latched in synchronism with the synchronized frames, and its difference from the write address and the resulting threshold level comparison is effected in synchronism with the write clock for the store and hence with the asynchronous signal. The synchronizer provides a reduced waiting time jitter in the synchronized signal.

Synchronization of asynchronous data signals

In a synchronous network, asynchronous data signals are synchronized using positive/negative stuffing under the control of stuff request signals which are produced in dependence upon the phase difference between the asynchronous and synchronized data signals as compared to respective threshold values. The phase difference is, or the threshold values are, cyclically changed in a manner to produce additional stuffing, not necessitated by the asynchronous frequency difference, whereby the frequency of jitter, due to stuffing, in the synchronized data signal is increased. The increased frequency jitter is filtered out in an already-provided phase locked loop.

An asynchronous data signal is synchronized by stuffing in dependence upon phase differences between the asynchronous signal and a resultant synchronized signal. The synchronized signal comprises tdm frames each including data bits and a stuffing opportunity bit, and a plurality of frames constitute a superframe in which different frames have different numbers of data bits. To compensate for the biassed timing of stuff requests which would otherwise occur relative to the frame timing in the superframes, the threshold value with which the phase difference is compared to produce the stuff requests is modified with a stepped sawtooth waveform having a period equal to that of the superframe.

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Kanata, Canada

James A McEachern