ZyXEL Communications prestige 2002 series 사용자 설명서

Prestige 2002 Series User’s Guide

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Chapter 5 Introduction to VoIP

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

1 A sends a SIP INVITE request to B. This message is an invitation for B to participate in a 

SIP telephone call. 

2 B sends a response indicating that the telephone is ringing.

3 B sends an OK response after the call is answered. 

4 A then sends an ACK message to acknowledge that B has answered the call. 

5 Now A and B exchange voice media (talk). 

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

7 B 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. 

A SIP user agent server 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 server to receive the call.

Table 9   SIP Call Progression

1. INVITE

2. Ringing

3. OK

4. ACK 

5.Dialogue (voice traffic)

6. BYE

7. OK