The description of the telecommunications network in the previous section emphasized the physical components of the network, namely, station equipment, transmission facilities, and switching systems. In that context, the network was referred to as the facilities network.

It is also important to consider the manner in which this network provides the various telecommunications; services. From this perspective, the network may be thought of as a set of traffic networks sharing common facilities.

For example, the PSTN/ which provides public switched telephone network services, is the largest and best-known traffic network. Many other traffic networks provide a variety of special services such as private-line voice and data and audio and video program services.

Each traffic network is designed to meet a particular set of requirements related to transmission performance, reliability, maintenance, and the ability to handle the expected traffic volume.

The following sections describe some of these traffic networks and show how they use common elements and, in some cases, share them.