Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

The PSTN

|

3

In switched networks, links between endpoints don’t need to be permanent because

they aren’t needed constantly. The only time a link between two endpoints is needed

is when a call is in progress between them; the rest of the time, the link is idle,

unused. Switching is a method whereby links are established and removed as needed,

eliminating the need for a mesh. The PSTN is a switched network.

The PSTN carries each phone call by setting up and tearing down a temporary link,

usually an electrical circuit, between a caller and a callee. The links that carry the

calls may be comprised of copper wires, fiber optics, or radio systems, depending on

the network infrastructure that exists between the caller and callee.

While the phone connections used in most homes are traditionally analog, electro-

mechanical ones, connections between large telephone switches—for instance, calls

between subscribers of competing phone companies—tend to be digital.

Signaling System 7

SS7 is the PSTN’s signaling component: a second network that runs alongside the

PSTN, whose purpose is coordinating communication between switches, telephone

company databases and billing systems, and other parts of the public voice network.

Toll-free calling and long distance call routing are both functions of SS7. While ...