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SIP DOES NOT provide QoS support.
QoS is coupled with SIP through the notion of preconditions.
Objective is to ensure that resources are made available before the phone rings.
Invitations might indicate in SDP that QoS assurance is mandatory.
Call setup should only proceed after satisfying the preconditions
SIP extended method (COMET) indicates the success or failure of the preconditions.
Posted by sagitraz
To do this, SIP sets up a temporary voice path to send QOS setup along the correct path. Once the voice path is assured, it initiates the actual call with a special king of re-invite.
1. The Invite has one line added to the SDP section. “a=qos: mandatory”
2. Because of the QOS request, the UAS sends a 183 (call progress report) along with QOS requirements in the SDP section
3. The UAC acknowledges that these QOS requirements are acceptable with a special Ack called a PRACK. PRACK says, “I agree with the preconditions, but we can’t complete the call until we are sure they can be met.”
4. A voice path is established, but not between headsets. The voice path goes from QOS requesting software on the UAC to the QOS software in the UAS
5. If QOS is successful, a special re-invite called COMET is issued using the same Call-ID as the initial Invite. COMET says, “QOS request successful, complete the call.” (This is often just a final ACK)
6. The rest of the call proceeds as normal
Posted by snjysanjay05
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