The telephone network

The terminology used in wide area networking is extensive, and for those of us who were baptized in data networks in the 1980s and 1990s, much of this terminology appears archaic, with its roots firmly planted in telephony. Nevertheless, it is worth revisiting the source of this terminology briefly, since these terms frequently appear in material concerning the migration from circuit-switched to packet-switched networking.

Telephone networks were originally designed as a rigid hierarchy, as illustrated in Figure 7.1, and many of the components of this model have alternate names, as shown, sometimes adding to the confusion. At the core of the network are a handful of regional centers. High-capacity trunks distribute call traffic through progressively smaller switching offices, until we reach the Central Office (CO). Between the larger switching offices, coaxial cable, microwave, and fiber-optic cables are used. Cable distribution is defined by a tree topology; as we near the subscriber, large, multipair copper is spliced and joined to form many smaller bundles (e.g., 900-pair to 3 300-pair to 6 150-pair to 18 50-pair and so on). From the central office, 2-pair unshielded copper cable is delivered to individual subscriber premises over what is termed the local loop.

Figure 7.1: Structure of the telephone network and associated terminology.

Point of Presence (PoP)

A Point of Presence (PoP) is a provider location where DCE equipment is deployed to terminate user access from a variety...