What is SIP signalling in QCI table?Why it is needed in LTE?

Question

QCI 5 is mandatory for IMS SIP Signalling? What If Network assign different QCI (Say 6 or 8) for IMS Default Bearer? Will SIP Signalling still flow?

Answer 1

To establish IMS signalling a separate bearer is created with QCI = 5 as per standard and to exchange data QCI=1 bearer is established. QCI 5 is a non-GBR bearer and used for IMS pdn connectivity. In network multiple PDNs are deployed to provide different services such as internet APN, IMS APN etc.
VoLTE is one of the critical requirement that's why a separate data and signalling bearer have been defined in standard.
This time I don't have technical insights which derives these two QCI =5 and QCI =1 bearers values. However it might be based on various factors such as packet delay budget, packet loss error rate, priority etc.

Answer 2

Every QCI is associated with a Priority level.
Priority level 0.5 is the highest priority assigned to "Mission Critical delay sensitive signalling".This is assigned to QCI 69.
QCI 5 has priority 1 which is used for IMS signalling.
QCI 1 has priority 2 which is used for VoLTE.

The main idea of having the priority levels is to discard the traffic associated with the lowest priority level traffic when there is a congestion.