A processor-controlled packet-store-and-forward data-communication system is described and performance figures are presented. The philosophy of the system is characterised by the use of a route-search packet to establish the availability of both a route through the network and the destination terminal before the message data is accepted by the network. Subsequent message-data packets are constrained to follow the path established by the route search and are maintained in correct sequence by a comprehensive acknowledgment-and-signalling procedure. This also provides the facility of error correction by retransmission, and is the basis of a convenient method for the control of buffer storage within the network. The exchange implementation comprises a control computer operating in conjunction with link terminal units that are dedicated to the control of the input and output links of the exchange and to packet assembly and checking functions. Each of these units consists of a small autonomous microcomputer module controlled by a changeable microprogram. They can thus be readily adapted to accommodate the characteristics of the particular links and error-control facilities employed.