When default bearer of IMS signaling is released in VoLTE call?

Hi All,
For VoLTE, when call end, only dedicated bearer of voice traffic is released?
When is default bearer of IMS signaling is released?

Everything is released after RRC connection release.

Only UE IP address not.

My understanding is that initially when the UE is switched on, a default bearer with QCI 9 is established that would be used for all the signalling.
Later on, another default bearer with QCI 5 is established with the IMS CN.
When a VoLTE call is being setup, a dedicated bearer with QCI 1 is setup for the voice call.

1 Like

I think UE can release default IMS bearer.
Because VoLTE UE also in idle mode, before send/receive SIP, it has to initiate RRC conn.

It is stored in MME/Core network. I mean to say everything is released at radio level.

Yes right, in this case QCI-5 is always maintained.

This is not like that.

Why not?
I see VoLTE UE receive paging, initiate rrc mt-access, then reply sip-invite.

How do you know that it is VoLTE UE before receiving paging?

From some VoLTE call procedure, if UE in idle, UE must initiate RRC mo-data or mt-access before SIP.

Actually the QCI5 is used for the default bearer of IMS signalling (IMS control plane) so it must be always activated together with the “internet” default bearer (in your case QCI9). If it was deactivated you cannot register towards IMS network neither do calls in VoLTE. The bearer that is activated only with calls is a dedicated bearer with QCI1 that is used to transport RTP packets (media packets) of the user. During the call you should have 3 bearers:

  • default bearer with QCI9 (apn “internet”) - used for internet data
  • default bearer with QCI5 (apn “ims”) - used for IMS signalling
  • dedicated bearer with QCI1 (over the apn “ims”) - used for media (audio)

You read GSMA IR.92 that explains these concepts.


Yes, I know.
Thats why I said, QCI-5 is always maintained as long as there is RRC connetion.

Very well :+1:.
For VoNR is it same like VoLTE?
If anyone having practical experience…
Does we require there as well qci5?

Conclusion is qci5 is kept together with rrc connection.
Can not say always kept.

Please do not confuse AS & NAS concepts! RRC Connection release is just for Uu conn b/w UE & NodeB, while Bearers are NAS level, stored in Core NW.

All the packets do flow b/w NodeB & UE, but the AS context (RRC Connection) is handled at NodeB, irrespective of active EBI/PDNs.

PDN activity though can cause RRC Connection release after inactivity and RRC Connection Setup in case of MT PS Paging or MO Data.


Well said. This was one of my biggest confusions during my initial LTE years. There is a logical connection to LTE that is active even though RRC connection is relased. When MT data is available, SGW maps that data to corresponding logical channel id and if the UE is not available with the radio connection, it will be paged.

Yes, they are at different points.
But RRC release & Bearer release are always together.

Not exactly.
There seems to be misunderstanding between RB release with EBI release, those are separate and handled differently.
DRBs (RRC) are mapped to PDN/EBI (NAS), but DRB release doesn’t mean PDN/EBI is released.
The context for PDN remains in CN and when DRBs are reestablished, PDN/EBI can be resumed there.


In 4G, we do not have PDN context inactive, right?
Only 5G?

That is RRC Inactive state, again different than PDN/PDU.