Switching to VoIP

This is the Title of the Book, eMatter Edition

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

Asterisk Channels

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; zapata.conf

switchtype=national

context=default

signalling=pri_cpe

group=1

callerid=1-800-534-1550

channel => 1-23

zaptel.conf sets up a T1 span, configures the 24 DS0 channels on the T1, and sets the

framing and coding. Then zapata.conf configures the telephony apparatus for those

channels: national PRI signaling, a custom caller ID string, and the default dial-plan

context. The channels are grouped into a single rollover group (

1). Finally, on the

last line, these settings are applied to Zaptel channels 1 through 23. Within the dial-

plan, these channels will be referenced as

Zap/1, Zap/2, and so on.

E1 circuits will have 31 bearer channels and typically use HDB3 cod-

ing instead of B8ZS. Keep that in mind if you’re working outside

North America.

SIP Channels

Asterisk implements SIP only partially. Though SIP itself describes a total PBX

replacement approach using VoIP, Asterisk employs SIP mainly for connecting IP

phones and for trunking to other systems that also use SIP—like Zaptel. Asterisk

deals with its SIP subsystem in terms of channels: legs of a call. Two channels are

needed to complete a call between two SIP phones. A call between an analog phone

and a SIP phone also requires two channels: one Zaptel channel and one SIP channel.

Asterisk refers to devices