Call Control Signaling H225
In H.323 networks, call control procedures are based on International Telecommunication Union (ITU) Recommendation H.225, which specifies the use and support of Q.931 signaling messages. A reliable call control channel is created across an IP network on TCP port 1720. This port initiates the Q.931 call control messages between two endpoints for the purpose of connecting, maintaining, and disconnecting calls.
The actual call control and keepalive messages move to ephemeral ports after initial call setup. But 1720 is the well-known port for H.323 calls. H.225 also specifies the use of Q.932 messages for supplementary services.
The following Q.931 and Q.932 messages are the most commonly used signaling messages in H.323 networks:
Setup—A forward message sent by the calling H.323 entity in an attempt to establish connection to the called H.323 entity. This message is sent on the well-known H.225 TCP port 1720. Call Proceeding—A backward message sent from the called entity to the calling entity to advise that call establishment procedures were initiated.
Alerting—A backward message sent from the called entity to advise that called party ringing was initiated.
Connect—A backward message sent from the called entity to the calling entity indicating that the called party answered the call. The connect message can contain the transport UDP/IP address for H.245 control signaling.
Release Complete—Sent by the endpoint initiating the disconnect, which indicates that the call is being released. You can send this message only if the call signaling channel is open or active. Facility—A Q.932 message used to request or acknowledge supplementary services. It also is used to indicate whether a call should be directed or should go through a gatekeeper.
Figure 10-7 illustrates the signaling messages for call setup. Interaction with the gatekeeper is limited to RAS messages for call permission and, possibly, on status messages.
Figure 10-7. Call Setup Signaling Messages
Figure 10-7. Call Setup Signaling Messages
You can route the call signaling channel in an H.323 network in two ways: through Direct Endpoint Call Signaling and GKRCS. In the Direct Endpoint Call Signaling method, call signaling messages are sent directly between the two endpoints, as illustrated in Figure 10-8.
Figure 10-8. Direct Endpoint Call Signaling
Figure 10-8. Direct Endpoint Call Signaling
In the GKRCS method, call signaling messages between the endpoints are routed through the gatekeeper, as illustrated in Figure 10-9.
Figure 10-9. Gatekeeper Routed Call Signaling
Figure 10-9. Gatekeeper Routed Call Signaling
Continue reading here: Figure 1011 Gatekeeper Routed Call Signaling Same Gatekeeper
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