Closing Logical Channels and Ending a Session Closing a logical channel in VS .NET

Paint Data Matrix in VS .NET Closing Logical Channels and Ending a Session Closing a logical channel

Closing Logical Channels and Ending a Session Closing a logical channel
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involves the sending of a CloseLogicalChannel message In the case of a successful closure, the far end should send the response message Close LogicalChannelAck
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H323
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Figure 4-16 Opening bidirectional logical channels
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Open Logical Channel { Forward Logical Channel Number Forward Logical Channel Parameters { Data Type Session ID RTP Payload Type, etc } Reverse Logical Channel Parameters { Transport Address Media type RTP Payload Type, etc } }
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b Open Logical Channel Ack{ Forward Logical Channel Number Reverse Logical Channel Parameters { Reverse Logical Channel Number Transport Address RTP Payload Type, etc } } c Open Logical Channel Confirm { Forward Logical Channel Number }
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Media Exchange
Not all parameters are shown
In general, a logical channel can be closed only by the entity that created it in the first place For example, in the case of a unidirectional channel, only the sending entity can close the channel The receiving endpoint in a unidirectional channel can, however, request the sending endpoint to close the channel It does so by sending the RequestChannelClose message, indicating the channel that the endpoint would like to have closed If the sending entity is willing to grant the request, then it responds with a positive acknowledgment and then proceeds to close the channel
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H323
H323
One exception should be noted regarding the closure of a bidirectional channel When an endpoint closes a forward logical channel, it also closes the reverse logical channel when the two are part of a bidirectional channel Once all logical channels in a session are closed, then the session itself is terminated when an endpoint sends an EndSession command message The receiving endpoint responds with an EndSession command message Once an entity has sent this message, it must not send any more H245 messages related to the session Figure 4-17 provides an example of channel closure where one entity requests that the other close the channel, followed by the channel closure
Fast Connect Procedure
Having reviewed RAS, Q931, and H245 signaling, it is possible to construct a call flow that incorporates more of the signaling required for call
Figure 4-17 Closing a logical channel
Terminal
Terminal
a Request Channel Close Forward Logical Channel Number Reason b Request Channel Close Ack Forward Logical Channel Number c Close Logical Channel Forward Logical Channel Number source reason d Close Logical Channel Ack Forward Logical Channel Number e End Session f End Session
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H323
4
establishment between two endpoints Figure 4-18 shows an example of such a call establishment and can be considered a more complete version of the example depicted in Figure 4-11 One can see that many messages are required even for such a simple call (hence the term slow start in the caption) Furthermore, when we consider the fact that the figure does not show a Capabilities Exchange or possible master-slave determination, it becomes clear that even more messaging can be required
Gatekeeper
Figure 4-18 Combining RAS, Q931, and H245 signaling (slow start)
a b c d e f g h i j k l m n o p q r s DCF ACF
Terminal
Terminal
Gatekeeper
Setup Call Proceeding ARQ ACF Alerting Connect OLC (bidirectional) OLC Ack OLC Confirm Media Exchange CLC CLC Ack End Session End Session Release Complete DRQ DRQ DCF
OLC = Open Logical Channel
CLC = Close Logical Channel
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H323
H323
If one imagines a scenario with gatekeeper-routed call signaling and the need for the Facility message, as shown in Figure 4-13, plus the possibility of gatekeeper-routed H245 control signaling, it is clear that the number of messages can be greater still Call establishment becomes cumbersome and is certainly not conducive to fast call setup Considering the fact that fast call setup is a requirement for carrier-grade operation, the situation is lamentable Thankfully, the designers of H323 recognized these issues and defined a procedure that reduces signaling overhead and speeds things up considerably Appropriately, the procedure is known as the Fast-Connect procedure The Fast-Connect procedure involves setting up media streams as quickly as possible To achieve this goal, the Setup message can contain a faststart element within the User-to-User information element The faststart element is actually one or more OpenLogicalChannel request messages containing all the information that would normally be contained in such a request The faststart element also includes reverse logical channel parameters if the calling endpoint expects to receive media from the called endpoint If the called endpoint also supports the procedure, then it can return a faststart element in one of the Call-Proceeding, Alerting, Progress, or Connect messages That faststart element is basically another OpenLogicalChannel message, which appears like a request to open a bidirectional logical channel The included choices of media formats to send and receive are chosen from those offered in the faststart element of the incoming Setup message The calling endpoint has effectively offered the called endpoint a number of choices for forward and reverse logical channels, and the called endpoint has indicated those choices that it prefers The logical channels are now considered open as if they had been opened according to the procedures of H245 The use of faststart is depicted in Figure 4-19 Note that the faststart element from the called party to the calling party may be sent in any message up to and including the Connect message If the element has not been included in any of the messages, then the calling endpoint shall assume that the called endpoint either cannot or does not want to support Fast-Connect In such a case, the standard H245 methods must be used The use of the Fast-Connect procedure means that H245 information is carried within the Q931 messages and there is no separate H245 control channel Therefore, bringing a call to a conclusion is also faster The call is released simply by the sending of the Q931 Release Complete message When used with the Fast-Connect procedure, this has the effect of closing
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H323
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