The Megaco H.248 Signaling Protocol
In June of 2000, the IETF Megaco working group and the ITU-T study group 16 jointly standardized a new protocol to deal with the elements of a physically decomposed multimedia gateway. Whereas MGCP enjoyed limited deployment and was best suited for circuit to packet control in North America, Megaco / H.248 extends the model to include support for more advanced services like multimedia conferencing, and encompasses technologies used in countries world wide.
As with the other decomposed gateway models, Megaco relies on the MGC to control the MG using the H.248 protocol, but will communicate with other MGCs, endpoint devices, and Gateways through SIP or H.323 making it complimentary to those protocols. Megaco/H.248 can be found in Network Access Servers, Central Office Switches, Cable Modems, trunking, residential, and access Gateways, PBXs, and IP phones both hard and soft.
With development starting in 1998, the standard has its roots based strongly on MGCP and the preceding SGCP and IPDC protocols. The adoption of the new MDCP protocol added support for binary encoding, thus allowing the media description language to use H.245 for an H.323 entity, as well as SDP for SIP devices. By requiring MGCs to support both UDP and TCP, Megaco/H.248 becomes a transport independent protocol, allowing the media gateway as the initiator of the connection to choose between TCP and UDP.
The Connection Model
The Megaco/H.248 connection model describes the logical entities on the media gateways that are controlled by the media gateway controller. A termination consists of one or more streams of media appearing on the MG (a connection) either logically or physically, and can be persistent (circuit based) or ephemeral (RTP streams set up and torn down within the course of a call). A Root Termination exits on the MG that describes such properties as service state, and is not a source of media flow.
A Context associates collections of Terminations and describes the media switching and mixing parameters as well as topology of the associations when more than two terminations exist. A connection is made when two or more terminations are placed in the same context. A context can have many terminations, but a termination can only exist in one context at a time. A Context is created at the detection of the first termination, and ends when the last one is torn down.
Packages for Megaco/H.248
Packages are used to define properties as they relate to a specific application. While the original H.248 protocol defines a base set of packages, vendors, developers and standards committees can create their own and have them registered with the IANA. As of Megaco/H.248, properties for any given function should be defined in one package only, allowing packages to extend other packages in a modular fashion. This allows for a base set of packages that can be extended by smaller packets defining the parameters necessary to a specific application.
Megaco/H.248 has been proven to be a viable connection model for multimedia applications and since its standardization in 2000 has enjoyed broad support in the development community. Ongoing development of the protocol by standards comities and third party institutions has led to Megaco/H.248 version 2.
With the inclusion of binary encoding, it is said that Megaco has become more difficult to implement, and that the protocol does not lend itself well to smaller scale implementations.
Nevertheless, the beat goes on. Some of the newer features in version 2 include the ability to audit properties, events, signals and statistics, and a cap on the number of transaction pending messages allowed to avoid congestion. A new ServiceChange command has also been added to indicate a change in the capabilities of an MG.
