Interface unit for transferring digital data between time-domain multiplexed communication links

An apparatus for transferring digital data between time-domain multiplexed communication links of a telephone system which forms part of a computer network. A time-domain multiplexed communication link interface unit for transferring digital data between a first channel on a first time-domain multiplexed communication link and a second channel on a second time-domain multiplexed communication link of a multipoint communication network is disclosed. The multipoint communication network includes a plurality of network units with at least two of the network units comprising said time-domain multiplexed communication link interface units. The network units are coupled to telephone system communication links. The network units transmit and receive digital data in a frame mode in which the data is organized into frames on the communication links.