Switching to VoIP

This is the Title of the Book, eMatter Edition

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

Dial-Tone Trunks

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Cisco’s media gateways are Cisco routers with VoIP firmware. They have modular

expansion slots that allow them to connect T1s, optical connections, POTS lines,

and Ethernet interfaces for IP-based trunking. In a Cisco CallManager setup, all

PSTN trunking to and from the softPBX is facilitated by these media gateways, as

shown in Figure 12-3.

Avaya’s S8300 and S8700 softPBX chassis can connect to the PSTN in a more direct

manner, using an onboard POTS or PRI interface. While the Avaya solution can cer-

tainly make use of a media gateway, say for trunking PSTN traffic back and forth to

the softPBX over Ethernet, most Avaya chassis in the field are hosting PSTN trunks

with onboard interface hardware. (You might have cause to use a gateway for PSTN

trunks if the demarc is a great distance from the softPBX and you can use your IP

network to overcome that distance.) (See Figure 12-4.)

An Asterisk server can also use either method. Digium’s T1 and FXO/FXS interface

cards give the Asterisk PC the ability to directly connect to PSTN trunks, while off-

board gateway equipment lets you take a more distributed route. Figure 12-4 illus-

trates how an Asterisk server can use either approach. The dial-plan programming on

the Asterisk server must be altered to support offboard gateways, of course, and

those gateways must be programmed to route calls to the Asterisk server. For this

reason, the preferred solution is to use onboard interfacing.

Channelized or Split-Use T1s

So far, we’ve talked about using T1s for voice dial-tone service from the CO, and

we’ve talked about T1s for data: an IP point-to-point link for example. We’ve even

covered getting your voice dial-tone service by way of VoIP on such a link. But what

we haven’t talked about is split-use service, which allows you to use a T1 for both.

Since a T1 circuit has 24 channels, most local phone companies will gladly group

those channels into appropriately-sized pipes: one for Internet access service and the

Figure 12-3. Cisco CallManager connects to the PSTN through a PRI-equipped media gateway

router

PSTN

CallManager

Media Gateway

T1 / PRI

PRI Interface

Ethernet

Interface

Ethernet

Switch