In order to enable isochronous data to be transmitted over communications systems which may introduce timing jitter each packet of isochronous data is provided with a timestamp which indicates to a receiver the time at which a packet should be processed. The receiver notes the time the first packet of a data stream arrive and adds a given offset time which is greater than or equal to the maximum jitter to produce a time t1. Each succeeding packet is processed at a time t=t1+(Tn−T1), where T1 is the timestamp in the first data packet and Tn is the timestamp in the current packet. Thus the processing at the receiver is dependent on the relative times of arrival of the packets rather than the absolute times defined by the timestamps.