Cisco 7960G Administrator's Manual page 108

Call Flow Scenarios for Successful Calls
Step Action
INVITE—Phone A to Phone B
1.
180 Ringing—Phone B to Phone A
2.
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.
8. INVITE—Phone B to Phone C
Cisco SIP IP Phone Administrator Guide, Release 6.x and 7.x
B-12
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 User 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 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.
Phone B sends a SIP INVITE request to Phone C. The request is an
invitation to User C to participate in a call session.
Appendix B SIP Call Flows