Is the VoLTE APN used even without having a call?

4G LTE
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1

Hi Everyone, one quick question…

Is the VoLTE APN used even without having a call?

2

Hi. Yes, it’s always in use for IMS signaling (like re-registration or getting SMS).

3

Yep.

I found an answer related to this:

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)

I’m trying to find 3GPP reference.

4

Hi Yes. It is of my expertise and let me elaborately tell you what happens around the VoLTE APN.

  1. The VoLTE APN, by name, “IMS” gets downloaded in the LTE attach (in the S6a Update Location Answer from the HSS) for subscribers provisioned with VoLTE service; apart from the traditional operator specific internet APN.

  2. When the VoLTE is enabled in the UE, GTP Create Session Request is initiated towards the PGW, which in turn creates a Gx session with the IMS PCRF and SIP REGISTER with the IMS P-CSCF

  3. It then maintains a dedicated IPSec Tunnel association with the P-CSCF.

  4. Once a call is made, it will be done using SIP INCITE over the IPSec Tunnel.

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