commands of MEGACO and MGCP. For example, the Create connection command in MGCP has an equivalent ADD termination command in MEGACO, the Modify connection command in MGCP equates to the MODIFY termination command of MEGACO and the Delete connection command equates to the SUBTRACT termination command of MEGACO. Messages and commands The Media Gateway Controller always manages the media channels available on the Media Gateway itself by managing specific contexts and terminations. When the call state changes, the MGC sends a corresponding message to the gateway and waits for an acknowledgment. Each message is a transport mechanism for transmitting commands, and not the command itself, unlike most other telecommunication protocols. Protocol Commands: Add Move Subtract Notify Modify AuditValue AuditCapabilities ServiceChange Typical MGC and MG message exchange Media Gateway Media Controller Gateway ISUP IAM (Start Call) | ------ ADD REQUEST ------> | | <------- ADD REPLY ------- | | ------ RESPONSE ---------> | | ----- MODIFY REQUEST ----> | | <------ MODIFY REPLY ----- | | ------ RESPONSE ---------> | ISUP ACM (Destination Number Received) | ----- MODIFY REQUEST ----> | | <------ MODIFY REPLY ----- | | ------ RESPONSE ---------> | | ---- MODIFY REQUEST -----> | | <------ MODIFY REPLY ----- | | ------- RESPONSE --------> | ISUP ANM (Ring Back Tone) | ----- MODIFY REQUEST ----> | | <------ MODIFY REPLY ----- | | ------- RESPONSE --------> | CONNECTION PHASE ISUP REL (End call) | --- SUBSTRACT REQUEST ---> | | <---- SUBSTRACT REPLY ---- | | ------- RESPONSE --------> | Message structure Message {Transaction {Action {Context {Command {Termination {Descriptor {Package}}}}}}} By analogy with the OSI model, the hierarchy of interworking levels from the point of view of transmission over the network (Ethernet or ATM) is following. Comparison with MGCP The H.248/Megaco model is more complex than the Media Gateway Control Protocol (MGCP) model and it provides more flexibility when defining media control. For example, in MGCP a call can use an endpoint mode conference to manage the stream mixing, but it cannot achieve the fine grain control of H.248/Megaco in managing the media streams. The H.248/Megaco model simplifies connection setup within the MG and to entities outside the MG. It simplifies the mechanism by which the media gateway controller (MGC) can specify associated media streams as well as specify the direction of media flow. H.248/Megaco is therefore able to provide greater application level support than MGCP. For