2358 CHAPTER 136: H.323 C ONFIGURATIONFigure 681 H.323 networkH.323 FundamentalsGatekeeper Discovery When an endpoint wants to establish a call with another endpoint, it first looks fora gatekeeper with which it can register for services. This process is calledgatekeeper discovery. An RAS signaling message exchange occurs betweenendpoints and the gatekeeper. The calling endpoint unicasts or multicasts aGatekeeper_Request (GRQ) message. After receiving the request, the gatekeeperreplies with a Gatekeeper_Confirm (GCF) or Gatekeeper_Reject (GRJ) message.Registration When the calling endpoint receives a GCF message from the gatekeeper, it sends aRegistration_Request (RRQ) message, requesting for joining the zone governed bythe gatekeeper. The gatekeeper replies with a Registration_Confirm (RCF) orRegistration_Reject (RRJ) message to accept or reject the registration request.After registration, either the endpoint or the gatekeeper sends anUnregister_Request (URQ) message. However, it is up to the gatekeeper todetermine whether to cancel registration, while the endpoint can only replies withan Unregister_Confirm (UCF) message to cancel registration.Address Translation If the calling endpoint knows the alias, but not the call signaling address of thecalled endpoint, it sends a Location_Request (LRQ) message to the gatekeeper.Admission Control If the address of the called endpoint is available, the calling endpoint sends anAdmission_Request (ARQ) message, based on which the gatekeeper decideswhether to allow this endpoint to join a call process. This is how the gatekeepercontrols admission. In the ARQ message sent to the gatekeeper, the callingendpoint may ask for direct call signaling (see Figure 682) or gatekeeper-routedcall signaling (see Figure 683). The gatekeeper finalizes the way of sending callsignaling to the called endpoint and notifies the calling endpoint of it via anAdmission_Confirm (ACF) message.IP networkTelephone TelephoneGateway GatewayGatekeeper
