Switching to VoIP

This is the Title of the Book, eMatter Edition

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

Limits of Traditional Telephony

|

5

several simultaneous conversations between parties in the same office can occur

without making use of the PSTN at all. One job of the PBX is to determine how to

“route” calls—that is, how to ascertain whether the calling party is trying to reach

another person within the same office or trying to reach somebody via the PSTN.

Most PBX vendors refer to the call-routing scheme as the dial-plan.

Single-line phones, key systems, and PBX systems all connect to the PSTN—but for

various reasons. Single-line phones and key systems connect calls to the PSTN even if

they are from one phone to another in the same office, but a PBX connects calls to

the PSTN only if they are bound for an outside organization.

Despite their various capabilities, POTS, KTSs, and PBXs are all based on the same

circuit-switching, electromagnetic-signaling technologies. Even POTS’ higher-capac-

ity digital cousins, ISDN and T1, which are able to squeeze many simultaneous

phone calls onto a single copper loop, are members of the same technology family.

All the standards that govern these traditional telephony systems stem from the

International Telecommunications Union (ITU), and many have been unchanged for

decades because they are incredibly reliable. They’d have to be reliable in order ...