Session Establishment - Snom ONE IP Technical Manual

SIP Call Process

Session Establishment

UAC
INVITE
1
ACK
5
Call Flow
1. The UAC sends an INVITE message to
2. The UAS receives the request and responds with a 100 Trying.
3. The UAS sends a 180 Ringing response to the UAC when the phone begins
ringing.
4. Once the call is picked up, the UAS send a 200 Ok message to the UAC.
5. The UAC sends an ACK request to confirm the 200 Ok response was received.
Note: The ACK method completes what is known as the three-way handshake—confir-
mation that a session has been successfully established. The INVITE is the only method
where this occurs, and this is due to the large gap of time that often occurs between the
INVITE itself and the 200 OK response (when a user can't find the phone, is running
to the phone, etc.). So the ACK tells the called party that the caller hasn't hung up and
has accepted the call.
INVITE sip:41@pbx.company.com;user=phone SIP/2.0
Via: SIP/2.0/TLS 192.168.0.32:2061;branch=z9hG4bK-
9gg3wzak;rport
From: "Roland Wright" <sip:40@pbx.company.com>;tag=g5ua0i7fz6
Appendix B: SIP Overview
100 Trying
180 Ringing
200 OK
41@pbx.company.com
509
UAS
2
3
4
(UAS).