MGCP and MEGACO (H.248)

The MGC Protocol, or MGCP, was designed to address the requirements of VoIP networks that are built using “decomposed” VoIP gateways. MGCP is used by external call control elements called MGCs for controlling MGs. Decomposed MGCP-compliant VoIP gateways appear to the outside as a single VoIP gateway. Examples of these gateways are trunking gateways that interface the PSTN and VoIP network, desktop gateways that provide traditional analog 2500-type interfaces to VoIP networks, and access gateways that provide traditional analog or digital PBX interfaces to VoIP networks. MGCP-based VoIP solutions separate call control (signaling) intelligence and media handling. MGCP functions as an internal protocol between the separate components of a decomposed MGCP-compliant VoIP gateway.

MEGACO (H.248) is a standard protocol for interfacing between external call agents (MGCs) and MGs. The standard is the result of a unique collaborative effort between the IETF and ITU standards organizations. H.248 was derived from MGCP, which in turn was derived from the combination of Skinny Gateway Control Protocol (SGCP) and IPDC. SGCP is sometimes known as Skinny Protocol and is currently used by Cisco systems to support their proprietary IP telephone instruments. H.248 is based heavily on MGCP but includes several enhancements. MEGACO offers the following key enhancements over MGCP:

• Supports multimedia and multipoint conferencing-enhanced services

• Improved syntax for more efficient semantic message processing

• TCP and UDP transport options

• Allows text or binary encoding, formalized extension process for enhanced functionality

• Formalized extension process for enhanced functionality

H.248 has the same architecture as MGCP. The commands are similar, but the main difference is that H.248 commands apply to terminations relative to a context rather than to individual connections, as is the case with MGCP. Connections are achieved by placing two or more terminations into a common context. It is the concept of a context that facilitates support of multimedia and conferencing calls. The context can be viewed as a mixing bridge that supports multiple media streams for enhanced multimedia services.

H.248 packages include more details than MGCP packages. H.248 packages define additional properties, statistics, and event and signal information that may occur on terminations. With H.248, the primary mechanism for extension is by means of packages. To accommodate expanded functionality, MEGACO specifies rules for defining new packages.

Even though MGCP was deployed first, H.248 is gaining momentum and is expected to achieve wider industry acceptance as the official standard for decomposed gateway architectures sanctioned by the IETF and ITU. MGCP is currently maintained under the auspices of the PacketCable and the Softswitch Consortium. There are no plans for the MGCP specification to be enhanced by any international standards body. MGCP is like an orphan without a parent, looking for approval from a standards body.

At the time of this writing, only one IP-PBX supplier (Sphere Communications) has announced support of MGCP for signaling to its proprietary IP telephones, and no IP-PBX supplier plans to use H.248 for proprietary voice terminal support. MGCP and H.248 are sometimes supported by external gateway equipment used as system options to support analog communications devices. MGCP and H.248 are supported for networking applications by a limited number of IP-PBX suppliers, although the more dominant interworking protocol for multiple system network configurations is, and is expected to remain, H.323.