The Media Gateway Control Protocol (MGCP) specifies communication
between call control elements and telephony gateways.
It is a text-based protocol. Media gateways are telephony
gateways that convert circuit-switched voice signals to data
packets for multiservice packet networks. The Internet
Engineering Task Force (IETF) created MGCP to address
some of the perceived shortcomings of H.323. See Figure 2-18.
The main purpose of MGCP is to place control of call signaling
and processing intelligence in call agents or media gateway
controllers. (Call agents and media gateway controllers are
synonymous with and similar to the gatekeeper functions in
H.323 and are also called soft switches.) A new version of
MGCP, released in August 2000, is called Megaco or H.248.
Although Megaco was created for the same purpose, Voiceover-
IP, it differs from MGCP because it supports a broader
range of networks and devices such as ATM, Remote Access
Servers, Multi-Protocol Label Switching routers (MPLS),
Digital Subscriber Line Access Multiplexers (DSLAMs), and
more.
Because Megaco is very new, interoperability testing is
ongoing. It appears to answer many of the deficiencies of
H.323 and will become very important as we move toward
Voice-over-IP networks. It is somewhat unclear at this time if
Megaco will replace MGCP or just supplement it. Megaco is
more suited for media applications than MGCP, but MGCP
may be a better choice for nonmedia-centric applications, such
as MPLS-based session control.