Figure 4-12 Gatekeeper routed/direct endpoint call signaling in VS .NET

Encoding Data Matrix 2d barcode in VS .NET Figure 4-12 Gatekeeper routed/direct endpoint call signaling

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endpoints are connected to different gatekeepers, then it is very possible that one gatekeeper will choose to route the call signaling itself, while the other gatekeeper will not Such a scenario is depicted in Figure 4-12, where the gatekeeper of the calling endpoint chooses to route the call signaling itself, while the gatekeeper of the called endpoint does not want to be in the path of call-signaling messages
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Figure 4-12 Gatekeeper routed/direct endpoint call signaling
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ARQ ACF Setup Call Proceeding Setup Call Proceeding ARQ ACF Alerting Alerting Connect Connect H245 Establishment Media Exchange H245 Release Release Complete Release Complete DRQ DCF DRQ DCF
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ARQ/ACF message exchange is required only if admission has not been pregranted
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Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
H323
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In the first ACF message, the gatekeeper of the calling endpoint indicates that call signaling should be sent to the gatekeeper The gatekeeper includes a call-signaling address of itself in the ACF Note that the endpoint might have received pregranted admission from the gatekeeper In such a case, and if the gatekeeper wants to be in the path of call signaling, then the pregranted admission would specify that call signaling should be routed via the gatekeeper The endpoint sends a Setup message to the gatekeeper and the gatekeeper responds immediately with Call Proceeding The gatekeeper then sends a Setup message to the destination endpoint using the call-signaling address of that endpoint If the called endpoint wants to handle the call, it first responds with a Call-Proceeding message and then sends an ARQ to its gatekeeper In this case, the gatekeeper responds with an ACF indicating direct call signaling as opposed to gatekeeper-routed call signaling Again, the ARQ/ACF procedure is not necessary if the endpoint has pregranted admission The called endpoint alerts the user and sends an Alert message to the gatekeeper of the calling endpoint That gatekeeper forwards the Alerting message to the calling endpoint Once the called party accepts the call, the called endpoint sends a Connect message, which follows the same path as the Alert message At this point in the example, H245 messages are exchanged in order to set up the media streams between the endpoints Note that an H245 message exchange can occur earlier depending on the options implemented in the two endpoints In the example, the H245 messages are routed directly from endpoint to endpoint However, they could, like call-signaling messages, be sent via one or both gatekeepers, depending on whether each gatekeeper wants to be in the path of the H245 messages It is important to remember that the actual RTP streams will pass directly from endpoint to endpoint This is regardless of whether a gatekeeper is in the path of call signaling and/or H245 control signaling It is not required that the media streams follow the same path as the H245 control messages
A Basic Call with Gatekeeper-Routed Call Signaling
The signaling process is slightly more complex when the gatekeeper controlling the called endpoint wants to be in the path of call-signaling messages Such a scenario is shown in Figure 4-13, where both gatekeepers are
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Figure 4-13 Gatekeeper-routed call signaling
Terminal Gatekeeper Gatekeeper
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ARQ ACF Setup Call Proceeding Setup Call Proceeding ARQ ARJ Facility Release Complete Setup Call Proceeding Setup Call Proceeding ARQ ARJ Connect Connect Connect H245 Establishment Media Exchange H245 Release Release Complete Release Complete Release Complete ARQ/ACF message exchange is required only if admission has not been pregranted
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H323
involved in call signaling Again, in the example, it is assumed that H245 control messages are passed directly from endpoint to endpoint rather than via one or both gatekeepers Upon receipt of the Setup message from the calling endpoint s gatekeeper, the called endpoint requests permission from its own gatekeeper to handle the call (assuming that admission has not been pregranted) Since the gatekeeper wants to handle the call signaling, it returns an ARJ, with cause code of routeCallToGatekeeper This action causes the called endpoint to send a Facility message to the gatekeeper of the calling endpoint The Facility reason indicates that the call should be routed to the gatekeeper, and the call-signaling address of the gatekeeper is also included so that the calling gatekeeper knows where to send the call signaling Note that a pregranted admission can include an indication that call signaling should be routed via a gatekeeper If the called endpoint has pregranted admission with the requirement to route the call to the gatekeeper, then the endpoint will send the Facility message directly, without the need for the ARQ/ACF message exchange The gatekeeper on the originating side releases the call to the endpoint and attempts to establish the call to the gatekeeper on the terminating side instead Upon receipt of the Setup message, the gatekeeper on the terminating side returns a Call-Proceeding message and forwards the Setup message to the called endpoint The called endpoint immediately returns a Call-Proceeding message If admission has not been pregranted, the called endpoint will again request permission to handle the call (ARQ) Once the user is alerted, the Alerting message (not shown) is passed from the called endpoint, via the two gatekeepers to the calling endpoint The same path is followed by the Connect message when the called user answers Once media have been exchanged, the Release Complete message is passed from one endpoint to the other via the two gatekeepers Each endpoint then disengages using the DRQ/DCF message exchange with its gatekeeper (not shown)
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