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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.
100 Trying—Phone B to Phone A
Phone B sends a SIP 100 Trying response to Phone A. The response indicates
that the INVITE request has been received.
3.
180 Ringing—Phone B to Phone A Phone B sends a SIP 180 Ringing response to Phone A.
4.
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’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.
5.
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.
User B then selects the option to transfer the call to User C.
6.
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
7.
200 OK—Phone A to Phone B
Phone A sends a SIP 200 OK response to Phone B.
8.
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.
User B dials User C.
9.
INVITE—Phone B to Phone C
Phone B sends a SIP INVITE request to Phone C. The request is an invitation
to User C to participate in a call session.
