Megaco/H.248: Media Gateway Control Protocol

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Megaco/H.248: Media Gateway Control Protocol

Megaco/H.248, the Media Gateway Control Protocol, is for control of elements in a physically decomposed multimedia gateway, which enables separation of call control from media conversion. The Media Gateway Control Protocol (Megaco) is a result of joint efforts of the IETF and the ITU-T Study Group 16. Therefore, the IETF defined Megaco is the same as ITU-T Recommendation H.248.

Megaco/H.248 addresses the relationship between the Media Gateway (MG), which converts circuit-switched voice to packet-based traffic, and the Media Gateway Controller (MGC, sometimes called a call agent or softswitch, which dictates the service logic of that traffic). Megaco/H.248 instructs an MG to connect streams coming from outside a packet or cell data network onto a packet or cell stream such as the Real-Time Transport Protocol (RTP). Megaco/H.248 is essentially quite similar to MGCP from an architectural standpoint and the controller-to-gateway relationship, but Megaco/H.248 supports a broader range of networks, such as ATM.

There are two basic components in Megaco/H.248: terminations and contexts. Terminations represent streams entering or leaving the MG (for example, analog telephone lines, RTP streams, or MP3 streams). Terminations have properties, such as the maximum size of a jitter buffer, which can be inspected and modified by the MGC.

Terminations may be placed into contexts, which are defined as when two or more termination streams are mixed and connected together. The normal, "active" context might have a physical termination (say, one DS0 in a DS3) and one ephemeral one (the RTP stream connecting the gateway to the network). Contexts are created and released by the MG under command of the MGC. A context is created by adding the first termination, and it is released by removing (subtracting) the last termination.

A termination may have more than one stream, and therefore a context may be a multistream context. Audio, video, and data streams may exist in a context among several terminations.

Protocol Structure - Megaco/H.248 (Media Gateway Control Protocol)

All Megaco/H.248 messages are in the format of ASN.1 text messages. Megaco/H.248 uses a series of commands to manipulate terminations, contexts, events, and signals. The following is a list of the commands:

1. Add. - The Add command adds a termination to a context. The Add command on the first Termination in a Context is used to create a Context.
2. Modify - The Modify command modifies the properties, events and signals of a termination.
3. Subtract - The Subtract command disconnects a Termination from its Context and returns statistics on the Termination's participation in the Context. The Subtract command on the last Termination in a Context deletes the Context.
4. Move - The Move command atomically moves a Termination to another context.
5. AuditValue - The AuditValue command returns the current state of properties, events, signals and statistics of Terminations.
6. AuditCapabilities - The AuditCapabilities command returns all the possible values for Termination properties, events and signals allowed by the Media Gateway.
7. Notify - The Notify command allows the Media Gateway to inform the Media Gateway Controller of the occurrence of events in the Media Gateway.
8. ServiceChange - The ServiceChange Command allows the Media Gateway to notify the Media Gateway Controller that a Termination or group of Terminations is about to be taken out of service or has just been returned to service. ServiceChange is also used by the MG to announce its availability to an MGC (registration), and to notify the MGC of impending or completed restart of the MG. The MGC may announce a handover to the MG by sending it ServiceChange command. The MGC may also use ServiceChange to instruct the MG to take a Termination or group of Terminations in or out of service.

All of these commands are sent from the MGC to the MG, although ServiceChange can also be sent by the MG. The Notify command, with which the MG informs the MGC that one of the events the MGC was interested in has occurred, is sent by the MG to the MGC.