In telecommunication, a digital multiplex hierarchy is a hierarchy consisting of an ordered repetition of tandemdigital multiplexers that produce signals of successively higher data rates at each level of the hierarchy.
Digital multiplexing hierarchies may be implemented in many different configurations depending on (a) the number of channels desired, (b) the signalingsystem to be used, and (c) the bit rate allowed by the communications media.
Some currently available digital multiplexers have been designated as Dl-, DS-, or M-series, all of which operate at T-carrier rates.
In the design of digital multiplex hierarchies, care must be exercised to ensure interoperability of the multiplexers used in the hierarchy.
The use of synchronous digital transmission by PTOs in their backbone fibre-optic and radio network will put in place the enabling technology that will support many new broad-band data services demanded by the new breed of computer user.
Unlike E1 2.048Mbit/s bearers, higher rate bearers in the hierarchy are operated plesiochronously, with tolerances on an absolute bit-rate ranging from 30ppm (parts per million) at 8Mbit/s to 15ppm at 140Mbit/s.
The introduction of synchronous digital transmission in the form of SDH will eventually revolutionise all aspects of public datacommunication from individual leased lines through to trunk networks.
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