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234                                           Universal Network Terminal TAU-72.IP/TAU-36.IP 
8.2  Call Algorithm Involving SIP Proxy Server 
This section describes a connection initiation scenario between two gateways involving SIP proxy server. In 
this case, caller gateway (Subscriber A) should know subscriber's permanent address and proxy server IP address. SIP 
proxy server processes messages received from Subscriber A, discovers Subscriber B, prompts the communication 
session and performs router functions for two gateways. 
 
Fig. 16—Call algorithm involving SIP proxy server 
Algorithm description: 
Subscriber A and Subscriber B register at SIP server. 
1.  Subscriber A and Subscriber B register at SIP server. 
2.  SIP server prompts for authorization. 
3.  Subscriber A and Subscriber B register at SIP server with authorization. 
4.  SIP server responses on successful registration. 
5.  Subscriber A rings up Subscriber B. 
6.  SIP server requests authentication. 
7.  Subscriber A gateway confirms received authorization request command. 
8.  Subscriber A rings up Subscriber B.