The plesiochronous digital hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term plesiochronous is derived from Greek plēsios, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronized. Backbone transport networks replaced PDH networks with synchronous digital hierarchy (SDH) or synchronous optical networking (SONET) equipment over the ten years ending around the turn of the millennium (2000), whose floating payloads relaxed the more stringent timing requirements of PDH network technology. The cost in North America was $4.5 billion in 1998 alone, p. 171. PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate. By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute. However, there is no link between watches to guarantee that they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other. Implementation The data rate is controlled by a clock in the equipment generating the data. The rate is allowed to vary by ±50 ppm of 2048 kbit/s (according to ITU-T recommendation). This means that different data streams can (and probably do) run at slightly different rates from one another. In order to transport multiple data streams from one place to another over a common transmission medium, they are multiplexed in groups of four. Because each of the four data streams is not necessarily running at the same rate, some compensation has to be introduced. Typically the multiplexer takes the data from the 4 incoming 2.048 Mbit/s data streams and feeds each into a 2.112 Mbit/s stream via a buffer store leaving a series of fixed gaps in each frame. The data rate is thus 2.112 Mbit/s x (number of bits in a frame – number of gaps)/(number of bits in a frame) This is slightly greater than 2.048 Mbit/s + 50ppm. If an extra gap is added, this is slightly smaller than 2.048 Mbit/s – 50ppm. Thus on average the data rate can be made exactly equal to the incoming rate by adding a gap in some frames and not others. This extra