Abstract: Digital data networks (such as the ISDN) which provide transmission between selected locations may use time multiplexed slots with predetermined maximum bit-rates. However local area networks (LANs) transmit packets, containing data bits, at a rate which varies frequently and often corresponds to a higher bit-rate than can be handled by individual slots. In the present invention packets arriving at an input port are allocated to a number of queues each for a respective destination, and each queue is allocated a group of slots on a digital data network. The slot contents are assembled into frames in a buffer store and transmitted to the digital network. On reception, the contents of several frames of incoming multiplexed slots are held in a frame store and then assembled into packets in a number of buffers making allowance for arrival in a different slot order and different frames.