IES-1248-51V Support Notes
4
Protocol (HTTP: RFC 2616) and Simple Mail Transfer Protocol (SMTP: RFC 821).
SIP, along with Media Gateway Control Protocol (MGCP: RFC 2705), and H.323 (an
International Telecommunications Union (ITU) specification), is one of three
commonly used open protocols for VoIP implementations.
SIP User Agent:
User Agents (UA’s) are specified in RFC 3261 as applications such as SIP phones
and software that initiate and receive calls over a SIP network.
SIP Server:
Servers are specified in RFC 3261 as application programs that accept requests,
service requests and send back responses to those requests.
SIP Call Flow:
The following figure describe the SIP messages exchanged for call establishment and
tear down.
Step
Description
1
Invite: User A initiates a call to user B. This message is an invitation for B
to participate in a SIP telephone call.
2
Ringing: User B sends a response to user A and indicates that the phone is
ringing.
3
OK: B sends a OK response after the call is answered.
4
ACK: User A then sends an ACK message to acknowledge that B has
answered the call.
5
Voice: a two-way voice channel is established over real-time protocol (RTP)
and a conversation takes place between user A and B.
6
Bye: User A hangs up.
7
OK: User B replied a OK message confirming receipt of the Bye request
and the call is terminated.
For more information about SIP messages, please see RFC 3261.
All contents copyright (c) 2007 ZyXEL Communications Corporation.