Digital multiplexing

Multiplexing plays a key role in the construction of transmission networks. It consists in combining or grouping incidental or primary data rates to form a higher or resulting data rate that is easier to transmit and manage in the transmission network. It is obviously impossible to transmit all 64 kbit/s signals in the telephone network without multiplexing! To manage complexity, networks are generally built in the form of planes, specialized in a range of data rates, with each plane managed autonomously. Each plane accepts signals from the lower plane via a multiplexer, as well as signals directly in its own bitrate range.

It is shown that, in most cases, the solution using a multiplexer is less costly than the solution requiring direct transmission of each of the primary signals.

In this document, the transmission network is divided into two parts:

• the access network or local connection network near the subscriber;

• the broadband transport network, comprising the main interconnection network (sometimes called the "backbone"), the sector network and the main local networks.

• Multiplexing levels are described in 2 . A distinction is made between :

  • the low-speed level up to 2 Mbit/s (or 1.5 for the US network), which includes rate adaptation in the 64 kbit/s channel, basic access to ISDN (Integrated Services Digital Network), multiplexing of 64 kbit/s channels in 2 or 1.5 Mbit/s digital ducts;

  • the high-speed level from 2 Mbit/s, which can be found in plesiochronous or synchronous form [the European Synchronous Digital Hierarchy (SDH) and the US SONET hierarchy].

• The multiplexing functions are presented in paragraph 3