User Manual

Appendix
413
Step
Action
Description
F1
INVITE—User A to Proxy Server
User A sends an INVITE message to a proxy
server. The INVITE request is an invitation to
User B to participate in a call session.
In the INVITE request:
 The IP address of User B is inserted in
the Request-URI field.
 User 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.
 The port on which User B is prepared
to receive the RTP data is specified.
F2
INVITE—Proxy Server to User B
The proxy server maps the SIP URI in the To
field to User B. The proxy server sends the
INVITE message to User B.
F3
180 Ringing—User B to Proxy
Server
User B sends a SIP 180 Ringing response to
the proxy server. The 180 Ringing response
indicates that the user is being alerted.
F4
180 Ringing—Proxy Server to
User A
The proxy server forwards the 180 Ringing
response to User A. User A hears the
ring-back tone indicating that User B is
being alerted.
F5
200 OK—User B to Proxy Server
User B sends a SIP 200 OK response to the
proxy server. The 200 OK response notifies
User A that the connection has been made.
F6
200 OK—Proxy Server to User A
The proxy server forwards the 200 OK
message to User A. The 200 OK response
notifies User A that the connection has
been made.
F7
ACK—User A to Proxy Server
User A sends a SIP ACK to the proxy server,
The ACK confirms that User A has received
the 200 OK response. The call session is