A service multiplexer (2) for connecting a plurality of signal sources to a public switched communication network transfers data in the form of a synchronously transferred data cells from a plurality of inputs (4) to an output (6) for connection to the network. The multiplexer (2) includes a bandwidth control unit which receives a bandwidth control signal from the network representative of available bandwidth for transmission of cells to the network and generates feedback control signals for transmission to the signal sources to cause them to alter their data transmission rates in accordance with the available bandwidth. An activity detector coupled to the bandwidth control unit monitors the bandwidth of data received at the inputs. The multiplexer allows ABR (available bit rate)-commandable signal sources to be linked to a single port of a public network operating on an ABR basis to provide an efficient and fair allocation of bandwidth for each source.
