ACKETS IN A PACKET NETWORK to the gateways under their command. There is no mechanism defined in MGCP for synchronizing "call agents." It is, in essence, a master/slave protocol where the gateways are expected to execute commands sent by the "call agents." In the MGCP protocol an assumption is made that the connection model consists of constructs that are basic end-points and connections. End-points are sources or sinks of data and could be physical or virtual. The following are two examples of end-points: 1. An interface on a gateway that terminates a trunk connected to a PSTN switch (e.g., local or toll-connecting, etc.). A gateway that terminates trunks is called a trunk gateway. 2. An interface on a gateway that terminates an analog POTS (plain old telephone service) connection to a telephone, a key system, PABX, and SO on. A gateway that terminates residential POTS lines (to telephones) is called a residential gateway. An example of a virtual end-point is an audio source in an audio-content server. Creation of physical end-points requires a hardware installation, while creation of virtual end-points can be done in software [20]. 15.5.4 Megaco or ITU-T Rec. H.248 (Ref. 13) 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