Plesiochronous digital hierarchy

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The Plesiochronous Digital Hierarchy ( PDH ) (Greek plesio “fast, almost”; chronos “time”) is an internationally standardized technology for multiplexing digital data streams that are transmitted over long-haul routes. The data streams must be approximately synchronous ; A deviation of their clock rate of a maximum of 50  ppm is common . Today this technology is almost only used at data transmission rates of up to 45 Mbit / s. Higher data transmission rates are transmitted with the more powerful multiplex technology of the synchronous digital hierarchy (SDH). The fundamental difference between PDH and SDH are the methods with which the different clock rates are compensated. The PDH technique works with the so-called "stuffing technique", the SDH technique with pointers .

The stuffing technique consists of using additional stuffing or filling bits , which are inserted or removed at precisely defined points in the signal, to compensate for the varying bit rates of the data streams. If this is not done, bit slips could occur, which could lead to serious errors at the receiver due to the misinterpretation of entire bit sequences. The stuffing technique has the disadvantage that access to a data stream of the lowest multiplex level is only possible after all higher multiplexed data streams have been demultiplexed and the stuffing bits removed. This requires complex hardware solutions and thus cost-intensive PDH network components or coupling intermediate stations that enable different data rates. With the SDH , direct access to the data streams of the various multiplex levels is possible with the help of pointers.

The PDH technology can be used in connection with all common transmission technologies, for example on lines ( twin wire , coaxial cable , fiber optic cable ), radio links or satellite links .

The networks of most global carriers are based on PDH and SDH technology. Due to the increased bit rates (usually GBit / s), PDH technology has often been replaced by SDH technology, but it is still used, especially in the “last mile” . For example, based ISDN - primary rate interface with 30 data channels (64 kbit / s = 1920 kbit / s) bit rate on the E1 (2 Mbit / s). The bit rate E3 (34 Mbit / s) is used, for example, to connect company locations over long distances by renting a corresponding line from a carrier. The bit rates E4 and E5 were used almost exclusively within large carrier networks. The E2 bit rate achieved only limited use.

Different standards have been developed in Europe , the USA and Japan . This is why it is necessary to “repack” the multiplexed data streams for intercontinental transmission in PDH technology. This need does not exist with SDH technology.

level Europe North America Japan
Mbit / s ppm Frame length Frame duration designation Mbit / s designation Mbit / s designation
1 2.048 ± 50 256 bit 125.0 µs E1, PCM30 1.544 T1 1.544 J1
2 8.448 ± 30 848 bit 100.37 µs E2, PCM120 6.312 T2 6.312 J2
3 34.368 ± 20 1536 bit 44.6927 µs E3, PCM480 44.736 T3 32.064 J3
4th 139.264 ± 15 2928 bit 21.024 µs E4, PCM1920 274.176 T4 97.728 J4
5 564,992 ± 15 2688 bit 4.7575 µs E5, PCM7680 397,200 J5

The electrical interfaces of lines that 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.

See also