13.1 General

This chapter provides basic information on the synchronous signal structure, and to help the reader become familiar with the new telecommunications terminology that has emerged with arrival of synchronous systems. First, however, it is necessary to give some details about the older plesiochronous networks, and to describe the evolution of the new synchronous networks that replaced them.

More detailed material on synchronous network standards can be obtained from the documents listed in section 13.13.1 near the end of the chapter.

13.2 The Plesiochronous Network

Before the late 1980s, most high-capacity transmission networks were based on a hierarchy of digital multiplexed signals. Lower-rate tributary signals, such as the ITU-T 2.048 Mbps (E1) or North American 1.544 Mbps (DS1) were multiplexed in fixed asynchronous steps into a higher-rate signal for transmission.

Access to the individual tributary signals at each level in the hierarchy, for signal routing and test purposes, was provided by signal crossconnect points at the appropriate level in the multiplexing structure. Notice that because of the asynchronous nature of the multiplexing, gaining access to a lower-level tributary signal for rerouting or test purposes meant demultiplexing the whole line signal structure step-by-step down to the lower-level tributary data rate.

At each multiplexing step, the bit rate of the individual tributary signals was controlled within specified limits, but was not synchronized with the multiplex equipment. Because the tributary bit rates were controlled, this type of multiplexing is often referred to as being plesiochronous