Tributary of Plesiochronous Digital Hierarchy

As bandwidth demand grew the technology called Plesiochronous Digital Hierarchy (PDH) was developed by ITU-T G.702, whereby the basic primary multiplexer 2.048Mb/s trunks were joined together by adding bits (bit stuffing) which synchronised the trunks at each level of the PDH. 2.048Mb/s was called E1 and the hierarchy is based on multiples of 4 E1s.

1. E2, 4 x E1 - 8Mb/s
2. E3, 4 x E2 - 34Mb/s
3. E4, 4 x E3 - 140Mb/s
4. E5, 4 x E4 - 565Mb/s

The E3 tributaries are faster than the E2 tributaries, E2 tributaries are faster than the E1 tributaries and so on. These needs to be synchronised with other tributaries, so extra bits are added called Justification bits. These tell the multiplexers which bits are data and which are spare.

Multiplexers on the same level of the hierarchy remove the spare bits and are synchronised with each other at that level only. Multiplexers on one level operate on a different timing from multiplexers on another level. For instance, the timing between Primary Rate Muxes (combines 30 x 64Kb/s channels into 2.048Mb/s E1) will be different from the timing between 8Mbit muxes (combines up to 4 x 2Mb/s into 8Mb/s).

Data Rate Mux

Inserting and dropping out traffic from different customers can only happen at the level at which the customer is receiving the traffic. This means that if a 140Mb/s fibre is near a particular site and a new customer requires a 2Mb/s link, then a whole set of demultiplexers are required to do this.