Plesiochronous digital hierarchy

The Plesiochronous Digital Hierarchy (PDH ) (Greek plesio " almost, almost"; chronos "time" ) is an internationally standardized technique for multiplexing digital data streams that are transmitted over long-distance links. The data streams must be approximately synchronously; common is a deviation of their clock rate of up to 50 ppm. Today this technology is up to 45 Mbit / s almost only used in data transfer rates. Higher data rates are transmitted digital hierarchy (SDH ) with the powerful technology of the Synchronous Multiplex. Basic difference between PDH and SDH are the processes by which the different clock rates are balanced. The PDH technology works with the so-called " stuffing " the SDH technology with pointers.

The stuffing is that the varying bit rates of the data streams are balanced with the help of additional stuffing or padding bits that are inserted or taken out at precisely defined points in the signal. If that's not done, bit slips could occur, which could lead to serious errors at the receiver due to the misinterpretation of entire bit strings. The packing technique has the disadvantage that the access to a stream of the lowest - level multiplexing is only possible if all higher demultiplexes multiplexed data streams and the stuffing bits have been removed. This requires complex hardware solutions and thus costly PDH network components or even coupling intermediate stations, which allow different data rates. When SDH is possible with the help of pointers to directly access the data streams of different multiplexing levels.

The PDH technology can be used in conjunction with any conventional transmission techniques, for example on lines ( twisted pair, coaxial cable, fiber optic cable), microwave links or satellite links.

The networks of most global carriers based on PDH and SDH technology. Due to the increased bit rates ( usual Gbit / s ), the PDH technology often replaced by the SDH technology, but it is particularly in the "last mile " is still used. For example, based ISDN Primary Rate Interface with 30 bearer channels at the bit rate of E1 ( 2 Mbit / s). The bit rate of E3 ( 34 Mbit / s ) is used to connect, for example, company locations over great distances by a corresponding line is rented from a carrier. The bit rates E4 and E5 were used almost exclusively within large carrier networks. The bit rate of E2 achieved only limited distribution.

In Europe, the USA and Japan with several standards have been developed. Therefore, a " repackaging " of the multiplexed data streams is necessary for intercontinental transmission in PDH technology. This requirement does not apply to SDH technology.

The electrical interfaces of cables which transmit PDH - structured data traffic have been standardized by the ITU- T in Recommendation G.703, the multiplex technology in G.732, G.742 and G.751.