This is a technique that allows signals that are almost, but not exactly, synchronous to be multiplexed together. The technique relies on using a ‘justification word’ (JW) which sometimes carries real data from the tributary and at other times dummy or other data. For example consider the multiplexing of the video signal. In each fixed-length frame of the 140Mbit/s multiplex, the video signal is allocated 292 6-bit words plus one 6-bit justification word. When the JW is carrying video data, the multiplex will be demanding data at a rate higher than the incoming rate (53.28Mbit/s against a nominal 53.2Mbit/s). An input buffer, used to temporarily store the samples to be multiplexed, will gradually empty. When its occupancy hits a pre-determined threshold, the video data is no longer allocated to the JW. This will be accompanied by appropriate signalling carried by separate control words in the frame. This causes the input buffer to re-fill (as the output will be demanding words slower than they are being supplied) until, at another buffer occupancy threshold, video data is again allocated to the JW. In this way the video rate carried in the multiplex is made, on average, equal to the actual incoming video data-rate.