Switching to VoIP

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Chapter 4: Circuit-Switched Telephony

PRI/T1

The Primary Rate Interface is a digital access–signaling standard that allows 23 digi-

tally encoded voice channels, called DS0s, to be carried across a T1 circuit between

the CO and an enterprise PBX. These channels can be partitioned by an administra-

tor using a DSU/CSU device, so that as few as a single one can be used for voice.

Channels not designated for voice tend to be used for data networking or not used at

all. PRI service is sold by the telephone companies as a high-density dial-tone trunk

solution. The 24th channel is called the carrier channel or the signaling channel, and

it carries call-management signals over the T1. T1 is an integrated services digital

network signaling standard and stems from ITU-T recommendation Q.931. For

more about T1, see the later section titled “Time Division Multiplexing.”

Digital signaling standards differ from analog ones because they don’t

rely on electrical signals; instead, they use signals passed through a bit

stream.

SONET and DS3

Synchronous optical network STS-1 (SONET) and DS3 (sometimes called T3

because it is a high density of T1-style paths) are methods of providing high-band-

width-signaling pathways between switches on the PSTN. These links can be used

Figure 4-4. SS7 is accessed alongside ...