H.323              GK                     GW                  PBX
terminal                                          
| 
|                    |                      |                   |
|                    |                      |                   |
|-----ARQ----------->|                      |                   |                   
|    (destInfo    *) |                      |                   |
|                    |                      |                   |
|<----ACF----------- |                      |                   |                     
|(  destInfo     ** )|                      |                   |
|(destCallSigAddr***)|                      |                   |
|                    |                      |                   |                     
|------Setup-------->|                      |                   |
|(calledpartyaddress)|-------Setup--------->|                   |
|                    |                      |-----SETUP/SIEZE-->|                     
|      ....          |      ....            |                   |
|                    |                      |<----ANSWER------- |                     
|                    |<------Connect--------|                   |
|<---Connect---------|                      |                   |



Call sequence starts when the H.323 terminal is ready to setup a call after taking destination address input from the user. Not all messages are shown to keep the flow simple.

*destInfo can be alias or E.164 number incase of gateway call. Its simply taken from the user input.

**destInfo is the translated address. GK translates aliases etc. to E.164 number incase of gateway cals. Eg. vrkiran@hss can be translated to 2989. So, the ARQ could have vrkiran@hss and ACF would have 2989.

***ACF will also carry a destCallSignallingAddress which should be used to setup a call signaling channel. GK can decide to route the call through itself by sending its own  address in this field. It can force direct call routing by sending GW's address. H.323 terminal will simply open a H.225 channel with the destCallSignallingAddress supplied by the GK. It will never need to find out if the call is routed throgh GK or not.