Cisco 7960G Administrator's Manual page 136

Call Flow Scenarios for Successful Calls
Step Action
INVITE—Phone A to Phone B
1.
180 Ringing—Phone B to
2.
Phone A
200 OK—Phone B to Phone A
3.
ACK—Phone A to Phone B
4.
A two-way RTP channel is established between Phone A and Phone B.
5. INVITE—Phone B to Phone A
200 OK—Phone A to Phone B
6.
ACK—Phone B to Phone A
7.
The RTP channel between Phone A and Phone B is torn down. A is put on hold.
Cisco SIP IP Phone Administrator Guide, Release 6.x and 7.x
B-40
Description
Phone A sends a SIP INVITE request to Phone B. The request is an invitation to
User B to participate in a call session. In the INVITE request:
The phone number of User B is inserted in the Request-URI field in the form
•
of a SIP URL. The SIP URL identifies the address of User B and takes a form
similar to an e-mail address (user@host, where user is the telephone number
and host is either a domain name or a numeric network address). For example,
the Request-URI field in the INVITE request to User B appears as "INVITE
sip:555-0199@companyb.com; user=phone." The "user=phone" parameter
distinquishes that the Request-URI address is a telephone number rather than
a username.
Phone A is identified as the call session initiator in the From field.
•
A unique numeric identifier is assigned to the call and is inserted in the
•
Call-ID field.
• The transaction number within a single call leg is identified in the CSeq field.
The media capability A is ready to receive is specified.
•
Phone B sends a SIP 180 Ringing response to Phone A.
Phone B sends a SIP 200 OK response to Phone A. The response notifies Phone A
that the connection has been made.
If Phone B supports the media capability advertised in the INVITE message sent
by Phone A, it advertises the intersection of its own and Phone A's media
capability in the 200 OK response. If Phone B does not support the media
capability advertised by Phone A, it sends back a 400 Bad Request response with
a 304 Warning header field.
Phone A sends a SIP ACK to Phone B. The ACK confirms that Phone A has
received the 200 OK response from Phone B.
The ACK might contain a message body with the final session description to be
used by Phone B. If the message body of the ACK is empty, Phone B uses the
session description in the INVITE request.
Phone B sends a midcall INVITE to Phone A with new SDP session parameters
(IP address), which are used to place the call on hold.
Call_ID=1
SDP: c=IN IP4 0.0.0.0
The c= SDP field of the SIP INVITE contains an 0.0.0.0. This places the call in
hold.
Phone A sends a SIP 200 OK response to Phone B.
Phone B sends a SIP ACK to Phone A. The ACK confirms that Phone B has
received the 200 OK response from Phone A.
Appendix B SIP Call Flows