TD 92685EN
28 June 2012 / Ver. A
Troubleshooting Guide
Ascom i62 VoWiFi Handset
25
3. The VoWiFi Handset as
At this point serverB sends a 180 RINGING message back to user1's SIP server serverA and in
turn serverA relays the message back to user1, as shown in figure 9:
Figure 9. Ringing
The user1 handset will now probably produce some kind of audible call pending tone that
confirms user1 that the called party has been reached and their handset is ringing. If user2
answers, an 200 OK message is generated by serverB and forwarded to user1 via serverA.
The call message flow as defined by the SIP standard is finally completed when the user1
handset sends the user2 handset an acknowledgement (ACK) as shown in figure 10.
After the ACK, SIP signaling is concluded and:
• A media session is established whereby a flow of RTP voice packets b
etween the
user1
and user2 handsets is established as shown in figure 10. The voice session continues
until one of the parties terminates the call.
• A SIP dialog is established between user1 and user2. The SIP dialog
is
the critical
component in the media session because it is associated with all requests and responses
that are made and received during the session. The dialog is identified by a Call Identity,
a tag value associated with the user-1, the initiator of the call, and a tag value associated
with user-2, the recipient of th
e call and th
ese values are used in all requests and
responses.
Figure 10. Call Acknowledgement an
d Voice Session Establishment
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