The invention relates to a communication system in which transmission takes place according to a format which permits different types of data to be combined and transmitted within a single transmission. The novel feature is that the communication system transmits variable length frames of data in packets, and that a data combining and transmission sub-system (14, 16, 18, 20) is provided so that when a frame of data does not require a complete packet for transmission, the data combining sub-system combines the frame of data with additional data to provide a complete packet. The data combining sub-system comprises input means for receiving the frame of data and the additional data and for combining the frame of data and the additional data to provide a complete packet responsive to a control signal, and control means for providing the control signal.