Plesiochronous Digital Hierarchy
Encyclopedia
The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications network
s 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, synchronised.
PDH is typically being replaced by Synchronous Digital Hierarchy (SDH) or Synchronous optical networking (SONET) 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.
The data rate is controlled by a clock in the equipment generating the data. The rate is allowed to vary by ±50 ppm of 2.048 Mbit/s. This means that different data streams can be (probably are) running at slightly different rates to one another.
In order to move multiple data streams from one place to another, they are 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 data stream, and so correctly reconstitute the original data streams. These additional bits are called "justification" or "stuffing" bits
.
Because each of the four data streams is not necessarily running at the same rate, some compensation has to be introduced. 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 × 8 Mbit/s together, plus bit stuffing
, giving 34 Mbit/s. Four × 34 Mbit/s, gives 140. Four × 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.
2.048 Mbit/s 8.448 Mbit/s 34.368 Mbit/s 139.264 Mbit/s
Multiplex levels: Uses Positive justification to adapt frequency differences Overheads: CRC Defects: LOS, LOF, AIS
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
s to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term plesiochronous
Plesiochronous
The term Plesiochronous is derived from the Greek plesio, meaning near, and chronos, time, and refers to the fact that plesiochronous systems run in a state where different parts of the system are almost, but not quite perfectly, synchronised....
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, synchronised.
PDH is typically being replaced by Synchronous Digital Hierarchy (SDH) or Synchronous optical networking (SONET) 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 basic data transfer rate is a data stream of 2048 kbit/s. For speech transmission, this is broken down into thirty 64 kbit/s channels plus two 64 kbit/s channels used for signalling and synchronisation. Alternatively, the entire bandwidth may be used for non-speech purposes, for example, data transmission.The data rate is controlled by a clock in the equipment generating the data. The rate is allowed to vary by ±50 ppm of 2.048 Mbit/s. This means that different data streams can be (probably are) running at slightly different rates to one another.
In order to move multiple data streams from one place to another, they are 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
Multiplexer
In electronics, a multiplexer is a device that selects one of several analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output...
also adds additional bits in order to allow the far end receiving multiplexer to decode which bits belong to which data stream, and so correctly reconstitute the original data streams. These additional bits are called "justification" or "stuffing" bits
Bit stuffing
In data transmission and telecommunication, bit stuffing is the insertion of noninformation bits into data...
.
Because each of the four data streams is not necessarily running at the same rate, some compensation has to be introduced. 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 × 8 Mbit/s together, plus bit stuffing
Bit stuffing
In data transmission and telecommunication, bit stuffing is the insertion of noninformation bits into data...
, giving 34 Mbit/s. Four × 34 Mbit/s, gives 140. Four × 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.
2.048 Mbit/s 8.448 Mbit/s 34.368 Mbit/s 139.264 Mbit/s
Multiplex levels: Uses Positive justification to adapt frequency differences Overheads: CRC Defects: LOS, LOF, AIS
See also
- Synchronous Digital Hierarchy
- List of device bandwidths
- T-carrierT-carrierIn telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
and E-carrierE-carrierIn digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
system - Digital multiplex hierarchyDigital multiplex hierarchyIn telecommunications, a digital multiplex hierarchy is a hierarchy consisting of an ordered repetition of tandem digital multiplexers that produce signals of successively higher data rates at each level of the hierarchy....