Optical fibre cables provide high speed serial data links. DS-512 is a known protocol for sending data over an optical fibre cable. According to this protocol, transmissions comprise frames of data, each frame consisting of 512 words. Each word is a ten bit payload word which is expanded to twelve bits prior to transmission in order to embed clock information in the word for extraction by the receiver. Words of like rank in each frame (e.g., the fifty-fourth word in each frame) comprise data from one channel (e.g., one telephone connection). The first word of each frame (i.e., channel 0) is a framing word. The framing word comprises a set pattern, which in the standard for 12 bit code is unique and is the hexadecimal number "782". A receiver uses the framing word to ensure it stays in synchronisation with the transmitter during a transmission.
In the case of an alarm situation (e.g., a loss of synchronisation or a failure of one of the transmitter or receiver), the transmitting end may flood the channel data with specific codes, overwriting the payload data words of the channels.
Typically, two fibres provide a single DS-512 link with each fibre providing a data path in one direction; this allows for the facilities connected to each end of the cable to act as both transmitter and receiver simultaneously.
This invention seeks to provide an improved manner of utilising a serial link, which link may be a DS-512 link.