Switching to VoIP

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Chapter 4: Circuit-Switched Telephony

IXCs

Interexchange carriers are network operators that provide connections between

PSTN network carriers. A network connection between a dedicated LD service pro-

vider and a regional Bell operating company may pass through an IXC. Many LD

carriers are considered IXCs, too.

ISPs

Internet service providers, though not a part of the PSTN, are often operated by the

same companies that provide PSTN services. Lots of large LD carriers, RBOCs, and

CLECs are ISPs, too.

Components of the PSTN

The Public Switched Telephone Network and its signaling counterpart, SS7, con-

nect, monitor, bill, and disconnect calls. At the edge of the PSTN are large, main-

frame-like switches call exchanges, or central offices (COs). The role of each CO

switch is to connect calls between channels on that switch and, when necessary, to

connect calls to channels on other switches in the PSTN.

Calls travel along temporary pathways through the voice network. Though tempo-

rary, these pathways are end-to-end circuits, the root of the catchall moniker for leg-

acy call management: circuit switching.

The Central Office

The CO is the building where the local exchange switch resides. A CO’s switch may

serve telephone service subscribers in a very narrow geographic area—such as a sin-

gle ...