In a communications system a transmitter is arranged to transmit asynchronous transfer mode (ATM) cells in which synchronous transfer mode (STM) channels are assembled. In order to minimize the bandwidth required at lease some of the ATM cells include one or more switch command(s) (12) that indicate a change in the activation state of one or more STM channels (11-0 to 11-n).The switch commands (12) and STM channels (11) fill the ATM cell from opposite ends of the cell.Activate switch commands comprise two bytes, a high byte including a pointer (18) pointing to the start of a block of STM channels (11) and a low byte that includes a pointer (19) that points to the channel within a block of STM channels (11) to which the switch command applies.