Plesiochronous Transmission

Digital data and voice transmission is based on a 2.048Mbit/s bearer consisting of 30 time division multiplexed (TDM) voice channels, each running at 64Kbps (known as E1 and described by the CCITT G.703 specification). At the E1 level, timing is controlled to an accuracy of 1 in 1011 by synchronising to a master Caesium clock.

Increasing traffic over the past decade has demanded that more and more of these basic E1 bearers be multiplexed together to provide increased capacity. During this time rates have increased through 8, 34, and 140Mbit/s. The highest capacity commonly encountered today for inter-city fibre optic links is 565Mbit/s, with each link carrying 7,680 base channels, and now even this is insufficient.

A typical Plesiochronous Drop & Insert

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.

Multiplexing such bearers (known as tributaries in SDH speak) to a higher aggregate rate (e.g. 4 x 8Mbit/s to 1 x 34Mbit/s) requires the padding of each tributary by adding bits such that their combined rate together with the addition of control bits matches the final aggregate rate. Plesiochronous transmission is now often referred to as plesiochronous digital hierarchy (PDH).