Enhancements on 5G session management (5GSM) handling of network rejection not due to congestion control

1. A method, comprising:

receiving a 5G Session Management (5GSM) reject message from a mobile communication network by a User Equipment (UE), wherein the 5GSM reject message indicates a network rejection not due to congestion control;

associating a back-off timer with a serving Home Public Land Mobile Network (HPLMN) Single-Network Slice Selection Assistance Information (S-NSSAI) or a mapped HPLMN S-NSSAI based on a register of the UE, wherein the serving HPLMN S-NSSAI is a serving part of a complete S-NSSAI and the mapped HPLMN S-NSSAI is a mapped part of the complete S-NSSAI; and

preventing the UE from sending a 5GSM request message for the same HPLMN S-NSSAI that is associated with the back-off timer.

2. The method as claimed in claim 1 , wherein the 5GSM reject message is associated with a Protocol Data Unit (PDU) session, and the serving HPLMN S-NSSAI is obtained from the serving part of the complete S-NSSAI associated with the PDU session when the UE is registered in a HPLMN.

3. The method as claimed in claim 1 , wherein the 5GSM reject message is associated with a Protocol Data Unit (PDU) session, and the mapped HPLMN S-NSSAI is obtained from the mapped part of the complete S-NSSAI associated with the PDU session when the UE is registered in a Visited PLMN (VPLMN).

4. The method as claimed in claim 3 , wherein the serving part comprises a VPLMN S-NSSAI.

5. The method as claimed in claim 1 , wherein the 5GSM reject message is received when the UE is registered in a first PLMN, and the preventing of the UE from sending a 5GSM request message for the same HPLMN S-NSSAI is performed when the UE is registered in a second PLMN.

6. The method as claimed in claim 5 , wherein the first PLMN and the second PLMN are a HPLMN and one Visited PLMN (VPLMN) in an Equivalent PLMN (EPLMN) list, respectively, or are one VPLMN in the EPLMN list and the HPLMN, respectively, or are a first VPLMN and a second VPLMN which is in an EPLMN list of the first VPLMN, respectively.

7. The method as claimed in claim 6 , wherein the 5GSM reject message comprises a re-attempt indicator which indicates that re-attempt in an EPLMN is not allowed, or the 5GSM reject message does not comprise a re-attempt indicator.

8. The method as claimed in claim 1 , wherein the 5GSM reject message is a PDU SESSION ESTABLISHMENT REJECT message or a PDU SESSION MODIFICATION REJECT message, and the 5GSM request message is a PDU SESSION ESTABLISHMENT REQUEST message or a PDU SESSION MODIFICATION REQUEST message.

9. A User Equipment (UE), comprising:

a wireless transceiver, configured to perform wireless transmission and reception to and from a mobile communication network; and

a controller, configured to receive a 5G Session Management (5GSM) reject message indicating a network rejection not due to congestion control from the mobile communication network via the wireless transceiver, associate a back-off timer with a serving Home Public Land Mobile Network (HPLMN) Single-Network Slice Selection Assistance Information (S-NSSAI) or a mapped HPLMN S-NSSAI based on a register of the UE, and prevent the UE from sending a 5GSM request message for the same HPLMN S-NSSAI that is associated with the back-off timer, wherein the serving HPLMN S-NSSAI is a serving part of a complete S-NSSAI and the mapped HPLMN S-NSSAI is a mapped part of the complete S-NSSAI.

10. The UE as claimed in claim 9 , wherein the 5GSM reject message is associated with a Protocol Data Unit (PDU) session, and the serving HPLMN S-NSSAI is obtained from the serving part of the complete S-NSSAI associated with the PDU session when the UE is registered with a HPLMN.

11. The UE as claimed in claim 9 , wherein the 5GSM reject message is associated with a Protocol Data Unit (PDU) session, and the mapped HPLMN S-NSSAI is obtained from the mapped part of the complete S-NSSAI associated with the PDU session when the UE is registered with a Visited PLMN (VPLMN).

12. The UE as claimed in claim 11 , wherein the serving part comprises a VPLMN S-NSSAI.

13. The UE as claimed in claim 9 , wherein the 5GSM reject message is received when the UE is registered in a first PLMN, and the preventing of the UE from sending a 5GSM request message for the same HPLMN S-NSSAI is performed when the UE is registered in a second PLMN.

14. The UE as claimed in claim 13 , wherein the first PLMN and the second PLMN are a HPLMN and one Visited PLMN (VPLMN) in an Equivalent PLMN (EPLMN) list, respectively, or are one VPLMN in the EPLMN list and the HPLMN, respectively, or are a first VPLMN and a second VPLMN which is in an EPLMN list of the first VPLMN, respectively.

15. The UE as claimed in claim 14 , wherein the 5GSM reject message comprises a re-attempt indicator which indicates that re-attempt in an EPLMN is not allowed, or the 5GSM reject message does not comprise a re-attempt indicator.

16. The UE as claimed in claim 9 , wherein the 5GSM reject message is a PDU SESSION ESTABLISHMENT REJECT message or a PDU SESSION MODIFICATION REJECT message, and the 5GSM request message is a PDU SESSION ESTABLISHMENT REQUEST message or a PDU SESSION MODIFICATION REQUEST message.

17. A method, comprising:

sending a 5G Session Management (5GSM) request message to a mobile communication network by a User Equipment (UE), wherein the 5GSM request message is associated with a serving Home Public Land Mobile Network (HPLMN) Single-Network Slice Selection Assistance Information (S-NSSAI) or a mapped HPLMN S-NSSAI based on a register of the UE, wherein the serving HPLMN S-NSSAI is a serving part of a complete S-NSSAI, the mapped HPLMN S-NSSAI is a mapped part of the complete S-NSSAI, and the serving HPLMN S-NSSAI or the mapped HPLMN S-NSSAI is associated with a back-off timer;

receiving a 5GSM reject message from the mobile communication network, wherein the 5GSM reject message indicates a network rejection not due to congestion control; and

preventing the UE from sending another 5GSM request message for the same HPLMN S-NSSAI in response to the 5GSM reject message.

18. The method as claimed in claim 17 , further comprising:

in response to the 5GSM reject message comprising a 5GSM cause value of #50 “PDU session type IPv4 only allowed”, #51 “PDU session type IPv6 only allowed”, #57 “PDU session type IPv4v6 only allowed”, #58 “PDU session type Unstructured only allowed”, or #61 “PDU session type Ethernet only allowed”, preventing the UE from sending another 5GSM request message for the same HPLMN S-NSSAI until any of the following conditions is fulfilled:

the UE is registered to a new PLMN which was not in a list of equivalent PLMNs at the time when the 5GSM reject message was received;

the UE is registered to a new PLMN which was in a list of equivalent PLMNs at the time when the 5GSM reject message was received, and either the 5GSM reject message does not include a re-attempt indicator or the re-attempt indicator included in the 5GSM reject message indicates that re-attempt in an equivalent PLMN is allowed;

a PDU session type which is used to access to the HPLMN S-NSSAI is changed by the UE which subsequently requests a new PDU session type;

the UE is switched off; and

a Universal Subscriber Identity Module (USIM) is removed or an entry in a list of subscriber data for a current Stand-alone Non-Public Network (SNPN) is updated.

19. The method as claimed in claim 17 , further comprising:

in response to the 5GSM reject message comprising a 5GSM cause value of #68 “not supported SSC mode”, preventing the UE from sending another 5GSM request message for the same HPLMN S-NSSAI until any of the following conditions is fulfilled:

the UE is registered to a new PLMN which was not in a list of equivalent PLMNs at the time when the 5GSM reject message was received;

a Session and Service Continuity (SSC) mode which is used to access to the DNN and the HPLMN S-NSSAI is changed by the UE which subsequently requests a new SSC mode or no SSC mode;

the UE is switched off; and

a Universal Subscriber Identity Module (USIM) is removed or an entry in a list of subscriber data for a current Stand-alone Non-Public Network (SNPN) is updated.

20. The method as claimed in claim 17 , wherein the 5GSM request message is a PDU SESSION ESTABLISHMENT REQUEST message or a PDU SESSION MODIFICATION REQUEST message, and the 5GSM reject message is a PDU SESSION ESTABLISHMENT REJECT message or a PDU SESSION MODIFICATION REJECT message.