Switching to VoIP

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

Voice Channels

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Transport

The T1 carrier versus VoIP

The T1’s 24 DS0 channels are each never-ending streams of digitized voice informa-

tion. In reality though, the T1 circuit itself is one big stream of binary digits that uses

TDM to divide the T1 into those 24 DS0 channels. Each is assigned a time slice of

the big stream, and each time slice is further divided into frames, as shown in

Figure 6-1. All voice T1s use the same amount of bandwidth no matter how many

calls are in progress—roughly 1.54 mbps. Trunking with T1s is very stable and pre-

dictable as a result.

VoIP frees the system builder from requirements traditionally imposed on the lower

OSI layers of the network. In a T1, the transport and data link layers are defined

together as a bundled carrier, and you have to use the G.711 PCM codec on all the

channels, yielding 24 simultaneous voice channels in the available bandwidth. VoIP

lets you pick and choose the codec, packet interval, and transport technologies you

want and thus gives you ultimate control. Using the G.729A codec and a T1, you

could conceivably trunk hundreds of calls at once.

Unlike G.711 traffic on a T1, VoIP’s “carrier” is TCP/IP. So VoIP can traverse Ether-

net, T1s, DSL lines, cable internet lines, POTS lines, frame relay networks, virtual

private networks ...