ZyXEL P-2802HWL-I1 User Guide - Page 130
SIP Call Progression, SIP Servers
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Chapter 10 Voice 10.2.1.2 SIP Service Domain The SIP service domain of the VoIP service provider is the domain name in a SIP URI. For example, if the SIP address is [email protected], then "VoIP-provider.com" is the SIP service domain. 10.2.2 SIP Call Progression The following figure displays the basic steps in the setup and tear down of a SIP call. A calls B. Table 46 SIP Call Progression A B 1. INVITE 2. Ringing 3. OK 4. ACK 6. BYE 5.Dialogue (voice traffic) 7. OK A sends a SIP INVITE request to B. This message is an invitation for B to participate in a SIP telephone call. 6 B sends a response indicating that the telephone is ringing. 7 B sends an OK response after the call is answered. 8 A then sends an ACK message to acknowledge that B has answered the call. 9 Now A and B exchange voice media (talk). 10 After talking, A hangs up and sends a BYE request. 11 B replies with an OK response confirming receipt of the BYE request and the call is terminated. 10.2.3 SIP Servers SIP is a client-server protocol. A SIP client is an application program or device that sends SIP requests. A SIP server responds to the SIP requests. When you use SIP to make a VoIP call, it originates at a client and terminates at a server. A SIP client could be a computer or a SIP phone. One device can act as both a SIP client and a SIP server. 10.2.3.1 SIP User Agent A SIP user agent can make and receive VoIP telephone calls. This means that SIP can be used for peer-to-peer communications even though it is a client-server protocol. In the following figure, either A or B can act as a SIP user agent client to initiate a call. A and B can also both act as a SIP user agent to receive the call. 130 P-2802H(W)(L)-I Series User's Guide