ZyXEL Communications P-2608HWL-Dx Series User Manual

P-2608HWL-Dx Series User’s Guide

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Chapter 11 SIP

The SIP service domain of the VoIP service provider (the company that lets you make phone 
calls over the Internet) is the domain name in a SIP URI. For example, if the SIP address is 

1122334455@VoIP-provider.com

, then “VoIP-provider.com” is the SIP service domain.

The following figure displays the basic steps in the setup and tear down of a SIP call. A calls 
B.  

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). 

10After talking, A hangs up and sends a BYE request. 

11B replies with an OK response confirming receipt of the BYE request and the call is 

terminated.

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. 

Table 53   SIP Call Progression

1. INVITE

2. Ringing

3. OK

4. ACK 

5.Dialogue (voice traffic)

6. BYE

7. OK