The Media Gateway Control Protocol is a first-generation signaling protocol for media gateway groups defined in RFC 2705. MGCP is used by phone companies and VoIP service providers in carrier-grade switching applications such as hosted PBX and IP Centrex (see Chapter 13 for more on this). Though supported in a number of telephony products from Cisco and others, including several early IP phones like the famously popular Cisco 7960, MGCP has been augmented by MEGACO/H.248.
Perhaps the most common place to encounter MGCP is an IP Centrex-hosted PBX service, wherein a group of IP phones at the customer premise are MGCP clients whose calls are routed by a gateway controller (GC). The GC is a high-density server that is connected to the customer premises by an IP transport like a T1. This essentially client-server setup delivers roughly the same kind of telephony functionality as H.323.
MEGACO is a protocol that describes a standard set of signaling functions for groups of media gateway devices that are physically decomposed that's distributed for all us mere mortalson a packet voice network. MEGACO was jointly developed with the IETF, which published RFC 3015, and the ITU-T, which published recommendation H.248: the documents are identical.
MEGACO provides a next -generation framework for building gateway devices, MCUs, and other types of telephony servers, but doesn't make specific provisions for any of them as H.323 does. In this regard, MEGACO is more extensible than H.323, but less so than SIP.
Depending on how they're implemented, MEGACO gateways can offer support for standard PSTN signaling technologies like DTMF, SS7, and ISDN. They can also provide modular or monolithic support of several packet-based network protocols, including TCP/IP, ATM, and frame-relay.
MEGACO fills in some of the telephony blanks in SIP. Since SIP doesn't make a specific recommendation for media conversion applications, MEGACO complements it nicely in this role. Likewise, an H.323 gateway may use MEGACO/H.248 for communication with the PSTN. Because of that, MEGACO and its predecessor MGCP aren't as commonly used for endpoint signaling as H.323 and SIP. In fact, MEGACO provides for dumb endpoints, the same way a CO switch or PBX does. This makes MEGACO more like the old-fashioned PBX (client/server) than SIP or H.323 (distributed).
MEGACO can be a transition piece that helps large networks of legacy telephone equipment migrate to VoIP, one gateway at a time. This gives it some enterprise appeal . But MEGACO was not designed with enterprise systems in mind. It was instead designed for switching in carrier networks, which have rigorous requirements for control of service, revenue generation, and billing for optional telephony features like caller ID.
Within the next 15 to 30 years , MEGACO could possibly displace the global network of last-mile CO switches and gateways that today use a mixture of in- band and SS7. In some areas, it already has. But there's a monumental cost associated with doing such a large-scale migration, and the phone companies must justify such upgrades with revenue. So don't expect to be using MEGACO signaling for trunks between your home phone and your local LEC any time soon.
The strategy that some phone companies have used thus far is to add MEGACO signaling to their CO switches at the same time they add DSL capability, which itself is in far higher demand. This could mean that, eventually, DSL will be the dominant last-mile vehicle for dial-tone services provided by CO switches that run MEGACO internally. Indeed, most phone companies that offer DSL services in a majority of their service areas have already rolled out some kind of last-mile VoIP service, too.
It isn't likely, however, that MEGACO interfacing will be installed in each home and office served by the telco. Rather, SIP, and to a lesser degree SCCP or MGCP, which can be provisioned as trunk media types on a MEGACO gateway, will eventually dominate last-mile signaling. Since SIP phones are already exploding in popularity, it will be far cheaper for the telephone carriers to bring SIP signaling, rather than MEGACO, to your doorstep, because SIP phones are already in great abundance and are cheap.
ICG, Qwest, AT&T, and many other large phone companies have started to offer VoIP dial-tone services in this very fashionSIP or SCCP for the last mile and MEGACO connecting the CO switches.