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IES-1248-51V Support Notes

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.

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