Yealink SIP-T54S - Page 1119

Appendix
1093
6. User A mixes the RTP channels and establishes a conference between User B and User C.
F9. INVITE(sendonly)
F10. INVITE (sendonly)
F15. INVITE C
F11. 200 OK
F12. 200 OK
F1. INVITE B
F2. INVITE B
F3. 180 Ringing
F4. 180 Ringing
F5. 200 OK
F6. 200 OK
F7. ACK
F8. ACK
Session1 established between User A and User B is active
F13. ACK
F14. ACK
Session 1 established between User A and User B is hold
F16. INVITE C
F18. 180 Ringing
F17. 180 Ringing
F19. 200 OK
F20. 200 OK
F21. ACK
F22. ACK
Both calls are active, come into three-party conference
User A
Proxy Server
User B User C
Initiate
three party
conference
Step
Action
Description
F1
INVITE—User A to Proxy Server
User A sends the 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