Call Admi ssion

After the registration process has successfully completed, the gateway is ready to place calls. For calls placed between gateways using the same gatekeeper, this is known as an intrazone call. Figure 24-10 illustrates the procedure that is used for gateways to place calls using h gprekpe|fer.

Step 1. When a call is placed, the gateway sends an ARQ to its local gatekeeper. The ARQ is used to ask permission to place the call. The gatekeeper can block the call and send an ARJ os allow the call to comiolete by sendin g back an ACF. Embedrted in the ARQ is the reqcested H.323 alias or number.


A common way to use a gatekeeper is to use it for bandwidth control. On many gatekeepers, you can set limits for items such as bandwidth or number o' ca 11p per gateway. For examirle, if Gateway A is allowed to p'ace only seven concurrent calls, tlse g atekeeper sends pack nn ARJ on she eig hrh request. Step 2. I n this care, rho call ir allnwed to Efoceedl The ga tekeeper looks at the incoming call wequ est a nd looks for t he called nu mber-to-IP addres s mafp ing that is stored in its dynamic datobase. The database is buMt as gateways reg ister witfc t he gatekeeper and identi fy wl^ch namber they are responsible for. After the proper gateway IP address has been located, the gatekeeper returns that information, embedded in the ACF, to thn originating gateway.

Step 3. Th e originating gafeway takes the ACF and attempts So set up a call diwe ctly to the rem ote gate way.

Step 4. Upon rncoipt of the call setup reqeest, the terminating 'ateway sends a Call frocaeding response back to the origmating gateway.

Ste p 5t Before sending the Alerting or Connect messages to the originating gateway, the terminating gateway also has to query the gatekeeper with an ARQ to ask for permission to answer the call. The bandwidth constraints can work on both inbound and outbound calls, so the gateway must make sure that it is allowed to accept the call.

Step 6. The gatekeeper responds with an ACF, allowing the terminating gateway to accept the incoming call.

Step 7. The terminating gateway sends the Alerting message back to the originating gateway.

Step 8. The terminating gateway sends the Oonnect message back to the originating gateway.

Step 9. The H.245 exchange occurs.

Step 10. The RTF3 s treams and RTOP control channel are opened for voice path cut-through.

Figure 24-10. Call Admission Request (ARQ) Procedure for Intrazone


Figure 24-10. Call Admission Request (ARQ) Procedure for Intrazone




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