TDM data communications network wherein each data frame is divided into respective lower rate sub-frames for respective sets of substations, and main station and substation for use therein

Telecommunications passive optical networks are known which employ a TDMA frame structure for transmission between a main station and a number of substations. Due to increased capacity demands and still further service demands, there is a need for higher bit rates over such networks. The invention provides a high bit rate passive optical network wherein the substations operate at only a fraction of the network bit rate, and so are hardly more complex than in lower bit rate networks. That is achieved by formation by the main station of sub-frames of the data intended for transmission to a number of the substations, and bit interleaving the sub-frames to form a data frame which is transmitted over the optical fiber cable. Each substation is adjusted to only listen to the sub-frame to which it is assigned. In one embodiment the substations are automatically adjusted to only listen to a particular phase in the interleaved bit stream, which is identified by phase adjustment data transmitted by the main station in a housekeeping time slot of the data frame.