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: An apparatus for testing optical fibre links in a data communication network includes communication equipment (13) for generating and receiving traffic over one or more fibre links X at a wavelength .lambda.2, optical test instrumentation (10) for generating test signals at a wavelength .lambda.1, different from .lambda.2, and circuitry (11) for superimposing a test signal from the test means onto a traffic signal from the communication equipment (13).