PDH

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 plesio, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are almost, but not quite perfectly, synchronised.

PDH is now being replaced by Synchronous Digital Hierarchy (SDH) equipment in most telecommunications networks.

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 they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other.

Implementation

The European and American versions of the PDH system differ slightly in the detail of their working, but the principles are the same. The European system is described below.

The basic data transfer rate is a data stream of 2.048 Mbit/s (megabits/second). For speech transmission, this is broken down into 30 x 64 Kbit/s (kilobits/second) channels plus 2 x 64 Kbit/s channels used for signalling and synchronisation. Alternatively, the whole 2 Mbit/s (megabits/second) may be used for non speech purposes, for example, data transmission.

The exact data rate of the 2 Mbit/s data stream is controlled by a clock in the equipment generating the data. The exact rate is allowed to vary some percentage (+/- 50 ppm) either side of an exact 2.048 Mbit/s. This means that different 2 Mbit/s data streams can be (probably are) running at slightly different rates to one another.

In order to move multiple 2 Mbit/s data streams from one place to another, they are combined together, or "multiplexed" in groups of four. This is done by taking 1 bit from stream #1, followed by 1 bit from stream #2, then #3, then #4. The transmitting multiplexer also adds additional bits in order to allow the far end receiving multiplexer to decode which bits belong to which 2 meg data stream and so correctly reconstitute the original data streams. These additional bits are called "justification" or "stuffing" bits.

Because each of the four 2 Mbit/s data streams is not necessarily running at the same rate, some compensation has to be made. The transmitting multiplexer combines the four data streams assuming that they are running at their maximum allowed rate. This means that occasionally, (unless the 2 Mbit/s really is running at the maximum rate) the multiplexer will look for the next bit but it will not have arrived. In this case, the multiplexer signals to the receiving multiplexer that a bit is "missing". This allows the receiving multiplexer to correctly reconstruct the original data for each of the four 2 Mbit/s data streams, and at the correct, different plesiochronous, rates.

The resulting data stream from the above process runs at 8,448 Kbit/s (about 8 Mbit/s). Similar techniques are used to combine four x 8 Mbit/s together, giving 34 Mbit/s. Four x 34 Mbit/s, gives 140. Four x 140 gives 565.

565 Mbit/s is the rate typically used to transmit data over a fibre optic system for long distance transport. Recently, telecommunications companies have been replacing their PDH equipment with SDH equipment capable of much higher transmission rates.