Abbrev. for plesiochronous digital hierarchy. An interim set of standards and products for data transmission, which allows the combination of a number of lower-speed channels (tributaries) into a composite signal transmitted on a synchronous higher-speed bearer. In principle, the data rate on the high-speed channel will simply be the aggregate of the data rates on the lower-speed channels; if the data rates on these channels are the same, or are simple multiples of some common basic rate, this aggregate rate will be a multiple of the data rate on the tributary channels. In practice, the data rates on the tributaries will not be exact, and to allow for this the high-speed channel must have an overall data rate slightly greater than the sum of the data rates on the tributaries. Since the high-speed channel is strictly synchronous, it is necessary to insert “stuffing” bits, which are then subsequently discarded. Further, at each point where data enters or leaves the high-speed channel, the composite signal must be completely broken down into its components, data inserted or extracted, and the composite signal re-created. This adds to the complexity and hence the cost of the system. See also SDH.