Megaco is a call-control protocol that communicates between a gateway controller and a gateway. It evolved from and replaces SGCP (simple gateway control protocol) and MGCP (media gateway control protocol). Megaco addresses the relationship between a media gateway (MG) and a media gateway controller (MGC). An MGC is sometimes called a softswitch or call agent.
Both Megaco and MGCP are relatively low-level devices that instruct MGs to connect streams coming from outside the cell or packet data network onto a packet or cell stream governed by RTP (real-time transport protocol). A Megaco (H.248) connection model is illustrated in Figure 15.8. There are two principal abstractions relating to the model: terminations and contexts. A termination acts as sources and/or sinks for one or more data streams. In a multimedia conference a termination can be multimedia, and it sources and sinks multiple media streams. The media stream parameters, as well as modem, and bearer parameters are encapsulated within the termination.
A context is an association between a collection of terminations. There is a special type of context called the null context, which contain all terminations that are not associated with any other termination. For example, in a decomposed access gateway, all idle lines are represented by terminations in the null context.
Let's look at three context possibilities. (1) A context with just one termination is call waiting. The caller does not hear anyone else. (2) A context with two terminations is a regular phone call. Of course each person is expected to hear the other. (3) An example of more than two terminations is a conference call. Each party hears each and every other one.
The maximum number of terminations in a context is a media gateway (MG) property. MGs that offer only point-to-point connectivity might allow at most two terminations per context. MGs that support multipoint conferences might allow three or more terminations per context.
The attributes of contexts are:
Media Gateway Context
SCN Bearer Channel |
SCN Bearer j" Channel i
SCN Bearer Channel
SCN Bearer i-Channel
SCN Bearer Channel
SCN Bearer Channel
Figure 15.8 An example of a H.248/Megaco connection model. SCN, switched-circuit network. The asterisk in each box in each of the contexts represents the logical association of terminations implied by the context. [Based on Figure 1, RFC 3015 (Ref. 9).]
The topology of a context describes the flow of media between the terminations within a context. In contrast, the mode of a termination (send/receive_) describes the flow of the media at the ingress/egress of the media gateway.
• The priority is used for a context in order to provide the MG with information about a certain precedence handling for a context. The MGC can also use the priority to control autonomously the traffic precedence in the MG in a smooth way in certain situations (e.g., restart), when a lot of context must be handled simultaneously.
• An indicator for an emergency call is also provided to allow a preference handling in the MG.
Megaco uses a series of commands to manipulate terminations, contexts, events, and signals. For example, the add command adds a termination to a context and may be used to create a new context at the same time. Of course we would expect the subtract command to remove a termination from a context and may result in the context being released if no terminations remain.
There is also the modify command used to modify the description of a termination (e.g., the type of voice compression in use). Notify is used to inform the gateway controller if an event occurs on a termination such as a telephone in an off-hook condition, or digits being dialed. There is also a service change command.
Terminations are referenced by a TerminationID, which is an arbitrary schema selected by the MG. TerminationlDs of physical terminations are provisioned by the media gateway. The TerminationlDs may be chosen to have structure. For example, a TerminationID may consist of a trunk group and a trunk within the group (Ref. 9).
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