B-16
Cisco Unified IP Phone 7960G and 7940G Administration Guide for Release 8.0 (SIP)
OL-7890-01
Appendix B SIP Call Flows
Call Flow Scenarios for Successful Calls
Step
Action
Description
1.
INVITE—Phone A to Phone B
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:[email protected];
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 User A is ready to receive is specified.
2.
180 Ringing—Phone B to Phone A
Phone B sends a SIP 180 Ringing response to Phone A.
3.
200 OK—Phone B 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 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.
4.
ACK—Phone A to Phone B
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.
A two-way RTP channel is established between Phone A and Phone B.
5.
INVITE—Phone C to Phone B
Phone C sends a SIP INVITE request to Phone B. The request is an invitation
to User B to participate in a call session.
6.
180 Ringing—Phone B to Phone C
Phone B sends a SIP 180 Ringing response to Phone C.
7.
INVITE—Phone B to Phone A
Phone B sends a midcall INVITE to Phone A with new Session Description
Protocol (SDP) attribute parameter.
SDP: a=sendonly
The a= SDP field of the SIP INVITE contains sendonly. This value places
the call on hold.
8.
200 OK—Phone A to Phone B
Phone A sends a SIP 200 OK response to Phone B.
9.
ACK—Phone B to Phone A
Phone B sends a SIP ACK to Phone A. The ACK confirms that Phone B has
received the 200 OK response from Phone A.
The RTP channel between Phone A and Phone B is torn down.
