Plesiochronous digital hierarchy (PDH) is an old technology used in telecommunications networks to create high-speed digital lines by multiplexing multiple data streams from low-speed lines. The word plesiochronous comes from the Greek word plsios, meaning almost, and chronos, meaning time; refers to the fact that the individual PDH lines work almost, but not fully synchronized.
Currently, PDH is used as an interface for lower speeds (E1, E3); SDH (Synchronous Digital Hierarchy) or SONET (Synchronous Optical network) technology is used for higher speeds. .
Separate PDH cascades use twisted pair in metal cables, coaxial cables, microwave channels or optical cables to transmit signals
PDH uses the multiplexing method to create higher order lines. The multiplexer on the transmitting side sequentially takes 1 bit from the first stream, 1 bit from the second, etc.; in addition, it adds additional synchronization bits and padding bits. In the European system, the highest discharge always consists of four lower-order currents; in American and Japanese, different coefficients are used between the levels.
The transfer rate is controlled by the clock on the device that sends the data. At 2048 Mbit/s, the speed can vary within ± 50 ppm. . This means that different lines can run at slightly different speeds. Therefore, the nominal speed of the higher-order line should be so high that even with the maximum permissible negative speed deviation, it can transmit data from all streams, even if they operate with the maximum permissible positive speed deviation. If the sub-stream is running at a slower rate, it may happen that when the multiplexer needs to insert another bit, this bit has not yet arrived from the input stream. In this case, the multiplexer informs the receiver that the corresponding bit is missing. This allows the receiving multiplexer to correctly recover the original data from each of the input streams.
In the case of multiplexing E1 (2048 kbit/s) to E2, four 64 kbit/s streams are inserted into the resulting stream to synchronize and signal the fill bits, so that the resulting rate will not be 4 2048 = 8192, but 4 2048 + 4 64 = 8448 kbit / s .
The main disadvantage of PDH is its inelasticity and the difficulty of separating lower-order channels or streams from higher-order high-performance streams. For example, if it is necessary to access a separate 2 Mbit/s circuit in a 565 Mbit/s 5th order system, it is necessary to demultiplex this 5th order with a set of demultiplexers throughout the transmission hierarchy. Explore more about the captivating gaming universe at allslotsonline.casino/en/casinos/AdmiralSharkCasino/ .
