Abstract: A method and apparatus is provided for detecting a frame alignment word in a data stream. The apparatus comprises a storage circuit (9) arranged to receive a data stream and pass each bit of the data stream serially through each location of the storage circuit. A template pattern means (10), arranged to generate a template pattern, is connected to the storage circuit (9) and arranged in bit groups, each group generating an output signal when it identifies a group of bits corresponding to its template pattern. A decoder circuit (B) is arranged to receive the output signals from the groups and generate a decoder output signal when a specified number of groups match the template pattern indicating that the frame alignment word has been detected.

Abstract: In highspeed digital communication systems a number of tributary data streams may be multiplexed into a single main data stream having a higher aggregate data rate. This main data stream contains frame alignment information to achieve correct demultiplexing. In addition pulse justification (pulse stuffing) time slots are provided, catering for the differences in phase and frequency between the individual tributary data and the main data stream, to control the remote oscillators in the demultiplexers. In the event of a data stream failure it is necessary to transmit an alarm indication signal over the failed data stream however the pulse justification equipment attempts to force the bit rate to zero. To overcome this, in the prior art, the data stream is replaced by one generated from a standby oscillator.