How do you know that UE is Registered in IMS?
January 28, 2020

Yes, this is an interesting question, as per the VoLTE call procedure the UE MUST register in IMS to can get Radio access bearer or QCI=5 to can make the registration and IMS signaling. 

So we can go through the below steps: 

  1. UE should be registered first at the LTE network and has the default bearer for VoLTE APN.
  2. UE sends a REGISTER message to the home network (to S-CSCF via P-CSCF and I-CSCF) to perform SIP registration with public user identity:
    REGISTER FROM:<PhoneNumber>@ims… P:ASSERTED_ID Cell_ID
  3. UE receives SIP “401 Unauthorized” response from the IMS core (P-CSCF):
    401 Challenge Random challenge (RAND)
  4. UE sends the 2nd REGISTER with the calculated response (RES) based on a shared secret and previously received RAND.
  5. UE receives a SIP 200 OK response from the S-CSCF routed back via all the CSCFs.

So here you can make sure that UE is registered in IMS by receiving 401 Unauthorized with RAND and SIP 200 OK.

For more info about VoLTE, Please visit this link.

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