 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: Context ID. The topology (who hears/sees whom). 15.5 MEDIA GATEWAY CONTROLLER AND ITS PROTOCOLS Media Gateway Context Termination SCN Bearer Termination Channel RTP Stream Termination SCN Bearer Channel Context Termination Termination SCN Bearer SCN Bearer Channel Channel Context Termination Termination SCN Bearer SCN Bearer Channel 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 M