MGCP Call Flows

The call agent sends a notification request (RQNT) to each gateway. Because the gateways are residential gateways, the request instructs the gateways to wait for an off-hook transition (event). When the off-hook transition event occurs, the call agent instructs the gateways to supply dial tone (signal). The call agent asks the gateway to monitor for other events as well. By providing a digit map in the request, the call agent can have the gateway collect digits before it notifies the call agent.

The gateways respond to the request. At this point, the gateways and the call agent wait for a triggering event.

A user on Gateway A goes off hook. As instructed by the call agent in its earlier request, the gateway provides dial tone. Because the gateway is provided with a digit map, it begins to collect digits (as they are dialed) until either a match is made or no match is possible. For the remainder of this example, assume that the digits match a digit map entry.

Gateway A sends a notify (NTFY) to the call agent to advise the call agent that a requested event was observed. The notify message identifies the endpoint, the event, and, in this case, the dialed digits.

After confirming that a call is possible based on the dialed digits, the call agent instructs Gateway A to create a connection (CRCX) with its endpoint.

The gateway responds with a session description if it is able to accommodate the connection. The session description identifies at least the IP address and UDP port for use in a subsequent RTP session. The gateway does not have a session description for the remote side of the call, and the connection enters a wait state.

The call agent prepares and sends a connection request to Gateway B. In the request, the call agent provides the session description obtained from Gateway A. The connection request is targeted to a single endpoint if only one endpoint is capable of handling the call, or to any one of a set of endpoints. The call agent also embeds a notification request that instructs the gateway about the signals and events that it should now consider relevant. In this example, where the gateway is residential, the signal requests ringing, and the event is an off-hook transition.

Gateway B responds to the request with its session description. Notice that Gateway B has both session descriptions and recognizes how to establish its RTP sessions.

The call agent relays the session description to Gateway A in a modify connection request (MDCX). This request might contain an encapsulated notify request that describes the relevant signals and events at this stage of the call setup. Now Gateway A and Gateway B have the required session descriptions to establish the RTP sessions over which the audio travels.

At the conclusion of the call, one of the endpoints recognizes an on-hook transition. In the example, the user on Gateway A hangs up. Because the call agent requested a notification of such an event, Gateway A notifies the call agent.

The call agent sends a delete connection (DLCX) request to each gateway.

The gateways delete the connections and respond.