Figure 4-7 Direct call signaling in VS .NET

Creator Data Matrix ECC200 in VS .NET Figure 4-7 Direct call signaling

Figure 4-7 Direct call signaling
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Figure 4-8 Gatekeeper-routed call signaling
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ARQ ACF Setup Setup ARQ ACF Connect Connect
might or might not be the case In the event that the endpoints are connected to different gatekeepers, then each gatekeeper determines independently whether it would like to be in the path of the call signaling Therefore, in the case of two gatekeepers, the signaling may pass via none, one, or both gatekeepers Both Figure 4-7 and Figure 4-8 show Setup and Connect messages These are part of call signaling (Q931), as described later in this chapter The Setup message is the first message in call establishment between endpoints, and the Connect message indicates that a call is accepted by the called endpoint It is sent when the call is answered Of course, a gatekeeper may decide to deny a particular admission request In such an event, it responds to the ARQ with an AdmissionReject (ARJ) message, which includes a reason for the denial Possible reasons include a lack of available bandwidth, an incapability to translate a destination alias to a real address, or an endpoint not being registered Pregranted Admission In order to minimize call setup delay, a gatekeeper can provide an endpoint with admission in advance When the endpoint registers with a gatekeeper, the RCF can include the parameter preGrantedARQ This parameter can provide permission for the endpoint to make or receive a call without asking for explicit permission through the
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ARQ/ACF mechanism The parameter also indicates whether the endpoint should use direct call signaling or gatekeeper-routed call signaling
Bandwidth Change
At any time during a call, an endpoint can request either an increase or a decrease in allocated bandwidth An endpoint itself can change the bit rate of any given channel without approval from the gatekeeper, provided that the total bandwidth does not exceed the limit specified by the gatekeeper Consequently, an endpoint is not required to request a reduction in bandwidth, though it should do so as a means of informing the gatekeeper of reduced bandwidth requirements, thereby allowing the gatekeeper to release certain resources for potential use in other calls If a change is to occur in a call, however, and that change will lead to an aggregate bandwidth in excess of the limit specified by the gatekeeper, then the endpoint must first ask the gatekeeper for permission The endpoint may increase the bandwidth only after confirmation from the gatekeeper A change in bandwidth is requested by the use of a BandwidthRequest (BRQ) message The message indicates the new bandwidth requested and the call to which the bandwidth applies The gatekeeper can either approve the new bandwidth with a BandwidthConfirm (BCF) message or deny the request with a BandwithReject (BRJ) message If the request is denied, then the endpoint must live with whatever bandwidth has already been allocated, perhaps through the use of flow-control mechanisms Not only can an endpoint request a change in bandwidth, but the gatekeeper can also request that an endpoint change its bandwidth limit For example, the gatekeeper might determine that network resource utilization is approaching capacity and it might order an endpoint to reduce its bandwidth usage If the gatekeeper requests a reduction in bandwidth, then the endpoint must comply with the request and must respond with a BCF message Note that changes in bandwidth are closely tied to H245 signaling Recall that H245 specifies the signaling that occurs between endpoints that are involved in a call and relates to the actual media streams that the endpoints share Also recall that H245 defines the use of logical channels that carry the media between the endpoints If a change in bandwidth is to occur, then one or more logical channels will experience a direct impact In fact, a reduction in bandwidth will require an existing logical channel to be closed and reopened with different parameter settings related to the new bandwidth limitation
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