5G NAS – Registration Reject Cause

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In 5G NR, Registration is a NAS layer or 5G Mobility Management (5GMM) process and it is very much similar to the Attach Process in LTE except some of the new IEs added or renamed mainly due to 5G Core network structure. Within 5G Core AMF is manage the registration procedure, which could be successful or could be reject due to different reasons.

  • Case#A Registration Accept: UE sends Registration Request over N2 interface to AMF. After reciveing Registration Request AMF start a timer T3510  and UE and AMF does performs NAS:Authentication and NAS:Security procedures. After successful Authentication and Security procedure AMF send Registration Accept, stop the Timer T3510 and starts another Timer T3550. Once UE recieved the Registration Accept and likes all the parameter, it replys with Registration Complete and AMF stops timer T3550.

5G Registration Accept Call flow

  • Case#B Registration Reject: There can be a case where AMF does not like a specific Registration Request and even after successful NAS:Authentication and NAS:Security, AMF can reject the Registration with Registration Reject Message to UE specifying the proper Cause value discussed here in this post.

Registration Reject Reason in 5G

Following is the list of  various reject cause and the expected UE behavior based on 3GPP specification TS 24.501. We put captured only high level and most critical with ‘Expected UE behavior’. For more detialed information please refer the speicifcation.

  • Cause #3 – Illegal UE
    • Cause #3  Illegal UE in 5GMM is sent to the UE when the network refuses service to the UE either because an identity of the UE is not acceptable to the network or because the UE does not pass the authentication check. UE does following actions upon recieving this cause.
      • The UE shall consider the USIM as invalid for 5GS services until switching off or the UICC containing the USIM is removed
      • UE shall delete the list of equivalent PLMNs and enter the state 5GMM-DEREGISTERED
      • USIM shall be considered as invalid also for non-EPS services until switching off or the UICC containing the USIM is removed
  • Cause #6 – Illegal ME
    • Cuase #6 Illegal ME  is sent to the UE if the ME used is not acceptable to the network, e.g. blacklisted. UE does following actions upon recieving this cause.
      • The UE shall consider the USIM as invalid for 5GS services until switching off or the UICC containing the USIM is removed
      • UE shall delete the list of equivalent PLMNs and enter the state 5GMM-DEREGISTERED
      • USIM shall be considered as invalid also for non-EPS services until switching off or the UICC containing the USIM is removed
  • Cause #7 – 5GS services not allowed
    • This 5GMM cause is sent to the UE when it is not allowed to operate 5GS services
      • The UE shall consider the USIM as invalid for 5GS services until switching off or the UICC containing the USIM is removed
      • UE shall delete the list of equivalent PLMNs and enter the state 5GMM-DEREGISTERED
      • USIM shall be considered as invalid also for non-EPS services until switching off or the UICC containing the USIM is removed
  • Cause #11 – PLMN not allowed
    • This 5GMM cause is sent to the UE if it requests service, or if the network initiates a de-registration request, in a PLMN where the UE, by subscription or due to operator determined barring, is not allowed to operate.
      • UE shall delete the list of equivalent PLMNs and reset the registration attempt counter and store the PLMN identity in the “forbidden PLMN list”.
      • UE shall enter state 5GMM-DEREGISTERED, PLMN-SEARCH and perform a PLMN selection
  • Cause #12 – Tracking area not allowed
    • This 5GMM cause is sent to the UE if it requests service, or if the network initiates a de-registration request, in a tracking area where the HPLMN determines that the UE, by subscription, is not allowed to operate.
    • If 5GMM cause #12 is sent to a roaming subscriber the subscriber is denied service even if other PLMNs are available on which registration was possible.
      • UE shall delete 5G-GUTI, last visited registered TAI, TAI list and ngKSI
      • UE shall reset the registration attempt counter
      • UE shall store the current TAI in the list of “5GS forbidden tracking areas for regional provision of service” and enter the state 5GMM-DEREGISTERED, LIMITED-SERVICE
  • Cause #13 – Roaming not allowed in this tracking area
    • This 5GMM cause is sent to a UE which requests service, or if the network initiates a de-registration request, in a tracking area of a PLMN which by subscription offers roaming to that UE but not in that tracking area.
      • UE shall delete 5G-GUTI, last visited registered TAI, TAI list and ngKSI. Additionally, the UE shall delete the list of equivalent PLMNs
      • UE shall reset the registration attempt counter
      • UE shall store the current TAI in the list of “5GS forbidden tracking areas for roaming” and enter the state 5GMM-DEREGISTERED, LIMITED-SERVICE or optionally 5GMM-DEREGISTERED.PLMN-SEARCH
  • Cause #15 – No suitable cells in tracking area
    • This 5GMM cause is sent to the UE if it requests service, or if the network initiates a de-registration request, in a tracking area where the UE, by subscription, is not allowed to operate, but when it should find another allowed tracking area in the same PLMN or an equivalent PLMN.
    • Cause #15 and cause #12 differ in the fact that cause #12 does not trigger the UE to search for another allowed tracking area on the same PLMN.
      • UE shall delete 5G-GUTI, last visited registered TAI, TAI list and ngKSI. Additionally, the UE shall delete the list of equivalent PLMNs
      • UE shall reset the registration attempt counter
      • UE shall store the current TAI in the list of “5GS forbidden tracking areas for roaming” and enter the state 5GMM-DEREGISTERED, LIMITED-SERVICE or optionally 5GMM-DEREGISTERED.PLMN-SEARCH
  • Cause #22 – Congestion
    • If the T3346 value IE is present in the REGISTRATION REJECT message and the value indicates that this timer is neither zero nor deactivated,
      • UE shall abort the initial registration procedure and enter state 5GMM-DEREGISTERED, ATTEMPTING-REGISTRATION
      • stop timer T3346 if it is running
      • stays in the current serving cell and applies the normal cell reselection process. The initial registration procedure is started if still needed when timer T3346 expires or is stopped.
    • Else
      • Consider it as abnormal case
  • Cause #27 – N1 mode not allowed
    • This 5GMM cause is sent to the UE if it requests service, or if the network initiates a de-registration request, in a PLMN where the UE by subscription, is not allowed to operate in N1 mode.
      • UE shall delete any 5G-GUTI, last visited registered TAI, TAI list and ngKSI
      • UE shallreset the registration attempt counter and shall enter the state 5GMM-NULL
      • UE shall disable the N1 mode capability for both 3GPP access and non-3GPP access
  • Cause #72 – Non-3GPP access to 5GCN not allowed
    • This 5GMM cause is sent to the UE if it requests accessing 5GCN over non-3GPP access in a PLMN, where the UE by subscription, is not allowed to access 5GCN over non-3GPP access.
      • UE shall delete 5G-GUTI, last visited registered TAI, TAI list and ngKSI
      • UE shall reset the registration attempt counter and enter the state 5GMM-DEREGISTERED
  • Cause #73 – Serving network not authorized
    • UE shall reset the registration attempt counter
    • UE shall store the PLMN identity in the “forbidden PLMN list”
      UE shall enter state 5GMM-DEREGISTERED, PLMN-SEARCH

Reference

  • 3GPP TS 24.5015G; Non-Access-Stratum (NAS) protocol for 5G System (5GS)

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