Switching to VoIP

This is the Title of the Book, eMatter Edition

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

70

|

Chapter 4: Circuit-Switched Telephony

Foreign exchange office (FXO) refers to the signaling used by telephones or PBX sys-

tems that are subscribers of the CO switch. These switches are either COs or PBXs.

An FXO interface is on a phone (or PBX) that is connected to a port at the CO.

If you remember one thing about FXO/FXS, remember this: FXO

interfaces use FXS signaling to communicate with FXS interfaces and

vice versa.

Loop Start Signaling

In order to establish an analog phone user’s intent to begin a call, electromechanical

signaling is used. Really, this is just a fancy way of saying that, at the outset of a call,

a phone is lifted from the hook—the local loop is closed, and current begins to flow.

This current becomes the sound of the dial-tone in your handset’s transducer. The

method of closing the loop at the hook switch to start the call is called loop start. It’s

by far the most common method. Less-common variations on this theme include

ground start, ear and mouth, and several more. Usually, these methods are used with

PBX trunks and not residential phones—which are almost always loop start devices.

Channel-Associated Signaling

A digital cousin of FXS and FXO uses an in-band or “robbed-bit” approach—this is

called channel-associated signaling, or CAS. Digital telephones