Switching to VoIP

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Chapter 4: Circuit-Switched Telephony

can hold FXO/FXS interface cards for connecting phones, PSTN lines, or PBXs. One

purpose of a channel bank might be connecting a group of digital phones to a PBX

through that PBX’s T1 interface.

BRI

Basic Rate Interface (BRI) is an access signaling standard for multiplexing two voice

channels on a single digital circuit. A third channel is used for call control. This is the

service loosely referred to as an ISDN line, though ISDN itself means far more than

just BRI. In fact, the PRI standard is also a child of ISDN.

BRI uses two channels of voice (called B channels), each occupying 64 kbps of band-

width, while the third channel (called the D channel) occupies 16 kbps for call sig-

naling. The B channels are purely for transmission of payload—that’s voice or data,

while the third channel, the D channel, is used to signal connects and disconnects on

the channel. Generally, digitizing for BRI is performed using a PCM algorithm, like a

T1 or a digital TDM phone.

Point-to-Point Trunking

Besides the trunks that supply a PBX with a dial-tone from the telephone company

CO, it is common to have trunk connections between multiple PBXs. These types of

connections, which are often high-density copper cables if the PBXs are within a few

hundred meters, are called ...