System information

MGCP is defined in RFC 3435.
*
It was designed to make the end devices (such as
phones) as simple as possible, and have all the call logic and processing handled by
media gateways and call agents. Unlike SIP, MGCP uses a centralized model. MGCP
phones cannot directly call other MGCP phones; they must always go through some
type of controller.
Asterisk supports MGCP through the chan_mgcp.so module, and the endpoints are
defined in the configuration file mgcp.conf. Since Asterisk provides only basic call agent
services, it cannot emulate an MGCP phone (to register to another MGCP controller
as a user agent, for example).
If you have some MGCP phones lying around, you will be able to use them with
Asterisk. If you are planning to put MGCP phones into production on an Asterisk
system, keep in mind that the community has moved on to more popular protocols,
and you will therefore need to budget your software support needs accordingly. If pos-
sible (for example, with Cisco phones), you should upgrade MGCP phones to SIP.
Proprietary Protocols
Finally, let’s take a look at two proprietary protocols that are supported in Asterisk.
Skinny/SCCP
The Skinny Client Control Protocol (SCCP) is proprietary to Cisco VoIP equipment. It
is the default protocol for endpoints on a Cisco Call Manager PBX.
†
Skinny is supported
in Asterisk, but if you are connecting Cisco phones to Asterisk, it is generally recom-
mended that you obtain SIP images for any phones that support this and connect via
SIP instead.
UNISTIM
Asterisk’s Support for Nortel’s proprietary VoIP protocol, UNISTIM, makes it the first
PBX in history to natively support proprietary IP terminals from the two biggest players
in VoIP: Nortel and Cisco. UNISTIM support is totally experimental and does not yet
work well enough to put it into production, but the fact that somebody has taken the
trouble to implement it demonstrates the power of the Asterisk platform.
Codecs
Codecs are generally understood to be various mathematical models used to digitally
encode (and compress) analog audio information. Many of these models take into ac-
count the human brain’s ability to form an impression from incomplete information.
* RFC 3435 obsoletes RFC 2705.
† Cisco has recently announced that it will be migrating toward SIP in its future products.
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