Page 221 - Asterisk™: The Future of Telephony

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 As-
terisk. 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 possible (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 it and connect via SIP
instead.

UNISTIM

Support for Nortel’s proprietary VoIP protocol, UNISTIM, means that Asterisk is 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 work well enough to put into production, but the fact that somebody took the
trouble to do this 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.
We’ve all seen optical illusions; likewise, voice-compression algorithms take advantage
of our tendency to interpret what we believe we should hear, rather than what we

† Cisco has recently announced that it will be migrating toward SIP in its future products.

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