The following clauses list the main requirements of idle mode operation and give an outline of the technical solution.

The MS normally operates on its home PLMN (HPLMN) or equivalent home PLMN (EHPLMN). However, a visited PLMN (VPLMN) may be selected, e.g., if the MS loses coverage. There are two modes for PLMN selection:

1. Automatic mode - This mode utilizes a list of PLMN/access technology combinations in priority order. The highest priority PLMN/access technology combination which is available and allowable is selected.
2. Manual mode - Here the MS indicates to the user which PLMNs are available. Only when the user makes a manual selection does the MS try to obtain normal service on the VPLMN.

To prevent repeated attempts to have roaming service on a not allowed area (i.e. LA or TA), when the MS is informed that an area is forbidden, the LA or TA is added to a list of "forbidden location areas for roaming" or "forbidden tracking areas for roaming" respectively which is stored in the MS. These lists, if existing, are deleted when the MS is switched off or when the SIM is removed, and periodically (with period in the range 12 to 24 hours). LA area restrictions are always valid for complete location areas independent of possible subdivision into GPRS routing areas. The structure of the routing area identifier (see TS 23.003) supports area restriction on LA basis. To prevent repeated attempts to obtain service on a PLMN through satellite NG-RAN or satellite E-UTRAN access technology, when the MS receives an integrity protected reject message with cause value #78 "PLMNs not allowed to operate at the present UE location" from a satellite NG-RAN cell or a satellite E-UTRAN cell, the MS maintains a list of "PLMNs not allowed to operate at the present UE location" in which it stores the PLMN ID of the rejecting PLMN, the current geographical location, if known by the MS. A timer is started when the PLMN ID of the rejecting PLMN is added to the list of "PLMNs not allowed to operate at the present UE location". If the geographical location exists, a MS implementation specific distance value needs to be stored. An entry in the list is deleted if the timer associated to the entry expires or the MS successfully registers to the PLMN stored in the entry. An entry in the list may be deleted if the current UE location is known, a geographical location is stored for the entry of this PLMN, and the distance to the current UE location is larger than a UE implementation specific value. For details see TS 24.501 and TS 24.301. In automatic PLMN selection mode, if the MS detects a PLMN in satellite NG-RAN access technology which is part of the list of "PLMNs not allowed to operate at the present UE location" the MS shall consider the PLMN as PLMN selection candidate for satellite NG-RAN access technology only if the current MS location is known, a geographical location is stored for the entry of this PLMN, and the distance to the current MS location is larger than a MS implementation specific value. This does not prevent selection of such a PLMN if it is available in another RAT. A timer is started when the PLMN ID of the rejecting PLMN is added to the list of "PLMNs not allowed to operate at the present UE location". In automatic PLMN selection mode, if the MS detects a PLMN in satellite E-UTRAN access technology which is part of the list of "PLMNs not allowed to operate at the present UE location" the MS shall consider the PLMN as PLMN selection candidate for satellite E-UTRAN access technology only if the current MS location is known, a geographical location is stored for the entry of this PLMN, and the distance to the current UE location is larger than a UE implementation specific value. This does not prevent selection of such a PLMN if it is available in another RAT. If a message with cause value #15 (see TS 24.008, TS 24.301 and TS 24.501) is received by an MS, then the MS shall take the following actions depending on the access technology in which the message was received: A VPLMN is added to a list of "forbidden PLMNs" in the SIM and thereafter that VPLMN will not be accessed except for disaster roaming services, by the MS when in automatic mode if a message with cause value "PLMN not allowed" or "Requested service option not authorized in this PLMN" or "Serving network not authorized" is received by an MS in response to an LR request from that VPLMN and:

• the MS is configured to use timer T3245 as defined in TS 24.008, TS 24.301, and TS 24.501;
• the MS is not configured to use timer T3245 and the message is integrity-protected;
• the MS is not configured to use timer T3245, the message is not integrity-protected and the MS does not maintain a list of PLMN-specific attempt counters; or
• the MS is not configured to use timer T3245, the message is not integrity-protected, the MS maintains a list of PLMN-specific attempt counters and the value of the PLMN-specific attempt counter for that VPLMN is equal to the MS implementation specific maximum value as defined in TS 24.008, TS 24.301 and TS 24.501.

If:

• after a subsequent manual selection of that PLMN, there is a successful LR not for disaster roaming services, then the PLMN is removed from the "forbidden PLMNs" list;
• the MS is configured to use timer T3245 and the timer T3245 expires, then the PLMN is removed from the "forbidden PLMNs" list ; or
• the MS is not configured to use timer T3245 and:
  1. the MS maintains a list of PLMN-specific attempt counters, the value of the PLMN-specific attempt counter for that PLMN is greater than zero and less than the MS implementation specific maximum value, and timer T3247 expires, then the PLMN is removed from the "forbidden PLMNs" list stored in memory as defined in TS 24.301 and TS 24.501; or
  2. the MS does not maintain a list of PLMN-specific attempt counters, the PLMN is stored in the "forbidden PLMNs" list in the SIM, and the timer T3247 expires, then the PLMN is removed from the "forbidden PLMNs" list in the SIM as defined in TS 24.301.

This list is retained when the MS is switched off or the SIM is removed. The HPLMN (if the EHPLMN list is not present or is empty) or an EHPLMN (if the EHPLMN list is present) shall not be stored on the list of "forbidden PLMNs". In A/Gb mode, an ME not supporting SoLSA may consider a cell with the escape PLMN code (see TS 43.073) to be a part of a PLMN belonging to the list of "forbidden PLMNs". Optionally the ME may store in its memory an extension of the "forbidden PLMNs" list. The contents of the extension of the list shall be deleted when the MS is switched off or the SIM is removed. A VPLMN may be stored in the extension of the "forbidden PLMNs" list if a message with cause value "PLMN not allowed" or "Requested service option not authorized in this PLMN" or "Serving network not authorized" is received by an MS in response to an LR request from that VPLMN, and the following is valid:

• the MS is not configured to use timer T3245, the message is not integrity-protected, the MS maintains a list of PLMN-specific attempt counters and the value of the PLMN-specific attempt counter for that VPLMN is less than an MS implementation specific maximum value as defined in TS 24.008, TS 24.301 and TS 24.501.

If a message with cause value "GPRS services not allowed in this PLMN" or "EPS services not allowed in this PLMN" is received by an MS in response to an GPRS attach, routing area update, EPS attach or tracking area update request or received in a network initiated GPRS detach or EPS detach request (see TS 24.008 and TS 24.301) from a VPLMN, that VPLMN is added to a list of "forbidden PLMNs for GPRS service" which is stored in the MS and thereafter that VPLMN will not be accessed by the MS for GPRS service except for disaster roaming services, when in automatic mode. This list is deleted when the MS is switched off or when the SIM is removed. A PLMN is removed from the list of "forbidden PLMNs for GPRS service" if:

• after a subsequent manual selection of that PLMN, there is a successful GPRS attach, routing area update, EPS attach, tracking area update or registration procedure (see TS 24.501);
• the MS is configured to use timer T3245 and timer T3245 expires; or
• the MS is not configured to use timer T3245, the MS maintains a list of PLMN-specific PS-attempt counters as specified in TS 24.008 and TS 24.301, the value of the PLMN-specific PS-attempt counter for that PLMN has a value greater than zero and less than the MS implementation-specific maximum value as defined in clause 5.3.7b in TS 24.301, and T3247 expires.

The maximum number of possible entries in this list is implementation dependant, but must be at least one entry. The HPLMN (if the EHPLMN list is not present or is empty) or an EHPLMN (if the EHPLMN list is present) shall not be stored on the list of "forbidden PLMNs for GPRS service". An MS that is attaching for emergency bearer services or for access to RLOS, or is attached for emergency bearer services or for access to RLOS, may access PLMNs in the list of "forbidden PLMNs" or the list of "forbidden PLMNs for GPRS service". The MS shall not remove any entry from the list of "forbidden PLMNs" or the list of "forbidden PLMNs for GPRS service" as a result of such accesses. An MS that is registered for disaster roaming services, may access PLMNs in the list of "forbidden PLMNs" or the list of "forbidden PLMNs for GPRS service" following the criteria as specified in clause 4.4.3.1.1 and shall not remove any entry from the list of "forbidden PLMNs" or the list of "forbidden PLMNs for GPRS service" as a result of such accesses. A UE capable of S101 mode maintains a list "forbidden PLMNs for attach in S101 mode"; the properties and handling in NAS signalling is defined in clause 5.3.3 of TS 24.301. If the MS is in GAN mode and a "Location not allowed" message is received (see TS 44.318), then the MS may attempt to select another PLMN so that further GAN registrations may again be attempted. The selection of the PLMN either automatically or manually is implementation dependent. If an MS that has disabled its E-UTRA capability re-enables it when PLMN selection is performed, then the MS of which usage setting is "voice centric":

• should, for duration of timer TD, memorize the PLMNs where E-UTRA capability was disabled as PLMNs where voice service was not possible in E-UTRAN. The number of PLMNs where voice service was not possible in E-UTRAN that the MS can store is implementation specific, but it shall be at least one. The value of timer TD is MS implementation specific, but shall not exceed the maximum possible value of background scanning timer T as specified in clause 4.4.3.3.1.
• in automatic PLMN selection, shall not consider PLMNs where voice service was not possible in E-UTRAN as PLMN selection candidates for E-UTRA access technology, unless no other PLMN is available. This does not prevent selection of such a PLMN if it is available in another RAT; and
• shall delete stored information on PLMNs where voice service was not possible in E-UTRAN when the MS is switched off, the USIM is removed, timer TD expires or MS voice domain configuration changes so that E-UTRA capability disabling is no longer necessary.

The MS may support "E-UTRA Disabling for EMM cause #15" as specified in TS 24.301. If the MS supports "E-UTRA Disabling for EMM cause #15" and the "E-UTRA Disabling Allowed for EMM cause #15" parameter as specified in TS 24.368 or TS 31.102 is present and set to enabled:

• the MS shall maintain a list of "PLMNs with E-UTRAN not allowed";
• when the MS disables its E-UTRA capability on a PLMN due to E-UTRAN not allowed, it shall add the PLMN to the "PLMNs with E-UTRAN not allowed" list, and start timer TE if timer TE is not already running;
• the number of PLMNs that the MS can store in the "PLMNs with E-UTRAN not allowed" list is implementation specific, but it shall be at least one;
• the value of timer TE is MS implementation specific, but it shall not exceed the maximum possible value of background scanning timer T (8 hours or 240 hours for MSs supporting EC-GSM-IoT, Category M1 or Category NB1 as defined in TS 36.306)) as specified in clause 4.4.3.3.1;
• in automatic PLMN selection the MS shall not consider PLMNs included in the "PLMNs with E-UTRAN not allowed" list as PLMN selection candidates for E-UTRAN access technology, unless no other PLMN is available. This does not prevent selection of such a PLMN if it is available in another RAT; and
• the MS shall delete stored information in the "PLMNs with E-UTRAN not allowed" list when the MS is switched off, the USIM is removed or timer TE expires.

The MS should maintain a list of PLMNs where the N1 mode capability was disabled because IMS voice was not available and the MS's usage setting was "voice centric" as PLMNs where voice service was not possible in N1 mode. When the MS disables its N1 mode capability because IMS voice was not available and the MS's usage setting was "voice centric":

• the MS should add the identity of the PLMN to the list of PLMNs where voice service was not possible in N1 mode and should start timer TF if timer TF is not already running. The number of PLMNs that the MS can store where voice services is not possible is implementation specific, but it shall be at least one. The value of timer TF is MS implementation specific, but shall not exceed the maximum possible value of background scanning timer T as specified in clause 4.4.3.3.1;
• in automatic PLMN selection the MS shall not consider PLMNs where voice service was not possible in N1 mode as PLMN selection candidates for NG-RAN access technology, unless no other PLMN is available. This does not prevent selection of such a PLMN if it is available in another RAT; and
• the MS shall delete stored information on PLMNs where voice service was not possible in N1 mode when the MS is switched off, the USIM is removed, timer TF expires or the MS's usage setting changes so that N1 mode capability disabling is no longer necessary.

The MS should maintain a list of PLMNs where the N1 mode capability was disabled due to receipt of a reject from the network with 5GMM cause #27 "N1 mode not allowed", as PLMNs where N1 mode is not allowed for 3GPP access. When the MS disables its N1 mode capability due to receipt of a reject from the network with 5GMM cause #27 "N1 mode not allowed":

• the MS should add the identity of the PLMN to the list of PLMNs where N1 mode is not allowed for 3GPP access and should start timer TG if timer TG is not already running. The number of PLMNs that the MS can store where N1 mode is not allowed for 3GPP access is implementation specific, but it shall be at least one. The value of timer TG is MS implementation specific, but shall not exceed the maximum possible value of background scanning timer T as specified in clause 4.4.3.3.1;
• in automatic PLMN selection the MS shall not consider PLMNs where N1 mode is not allowed for 3GPP access as PLMN selection candidates for NG-RAN access technology, unless no other PLMN is available. This does not prevent selection of such a PLMN if it is available in another RAT;
• if the MS is not configured to use timer T3245, the MS maintains a list of PLMN-specific N1 mode attempt counters for 3GPP access as specified in TS 24.501 and T3247 expires, then the MS removes for each PLMN-specific N1 mode attempt counter for 3GPP access that has a value greater than zero and less than the MS implementation-specific maximum value the respective PLMN from the list of PLMNs where N1 mode is not allowed for 3GPP access, as specified in clause 5.3.20.2 in TS 24.501; and
• the MS shall delete stored information on PLMNs where N1 mode is not allowed for 3GPP access when the MS is switched off, the USIM is removed or timer TG expires.

The MS in NB-S1 mode may maintain a list of "PLMNs with NB-IoT not allowed" where the NB-IoT capability was disabled due to receipt of a reject from the network with EMM cause #15 "no suitable cells in tracking area" and an Extended EMM cause IE with value "NB-IoT not allowed", as PLMNs where NB-S1 mode is not allowed. When the MS disables its NB-IoT capability due to receipt of a reject from the network with EMM cause #15 "no suitable cells in tracking area" and an Extended EMM cause IE with value "NB-IoT not allowed":

• the MS may add the identity of the PLMN to the list of "PLMNs with NB-IoT not allowed" and start timer TH if timer TH is not already running. The number of PLMNs that the MS can store in the "PLMNs with NB-IoT not allowed" list is implementation specific, but it shall be at least one. The value of timer TH is MS implementation specific, but shall not exceed the maximum possible value of background scanning timer T as specified in clause 4.4.3.3.1;
• in automatic PLMN selection the MS shall not consider PLMNs included in the "PLMNs with NB-IoT not allowed" list as PLMN selection candidates for NB-IoT access technology, unless no other PLMN is available. This does not prevent selection of such a PLMN if it is available in another RAT; and
• the MS shall delete stored information in the "PLMNs with NB-IoT not allowed" list when the MS is switched off, the USIM is removed or timer TH expires.

If the MS supports CSG, it is provisioned with an Allowed CSG list and an Operator CSG list and associated PLMN identities. Both lists can be retrieved either from the USIM if the lists are available in the USIM, or as described in TS 24.285 if the lists are not available in the USIM. These lists have zero or more entries. There are two modes of CSG selection:

• Automatic mode: This mode utilizes the Allowed CSG list and the Operator CSG list. After a PLMN is selected, the MS camps on a cell in that PLMN only if the cell is either not a CSG cell or it is a CSG cell with a CSG identity that is in the Allowed CSG list or in the Operator CSG List. The idle mode procedures of NAS are not impacted by this mode. Upon switch on the MS is in automatic mode.
• Manual mode: In this mode, the MS indicates to the user a list of available CSGs and the associated PLMNs. Based on configuration by the HPLMN, the list of CSGs provided to the user for a certain PLMN is either:
  • not restricted by the Allowed CSG list and the Operator CSG List stored in the MS; or
  • restricted to entries in the Operator CSG List only.
  After the user makes a selection, the MS camps on a cell with the selected CSG identity and may attempt to register with the associated PLMN (see TS 24.008 and TS 24.301).

The permitted CSG list is a combination of Operator CSG list and the Allowed CSG list. NAS shall provide the permitted CSG list to the AS. If the contents of the permitted CSG list have changed, NAS shall provide an updated permitted CSG list to the AS. If an integrity protected message with cause value #25 (see TS 24.008 and TS 24.301) is received by an MS for a CSG ID and associated PLMN identity present in the Operator CSG list, then for an implementation dependent time which is not shorter than 60 minutes, or until the MS is switched off, or the SIM/USIM is removed, or the Operator CSG list is updated:

1. The NAS shall not include this CSG ID and the associated PLMN identity in the permitted CSG list provided to the AS; and
2. In CSG manual mode selection, the MS shall not indicate to the user that this CSG ID and the associated PLMN identity is in the Operator CSG List stored in the MS.

If the MS supports ProSe communications and needs to perform PLMN selection for ProSe communications as specified in TS 24.334 or TS 24.554, then the MS shall proceed as follows: If the PLMN selected for ProSe communications is a VPLMN, the MS shall not periodically scan for higher priority PLMNs during the duration of ProSe communications. The solution to prevent potential ping-pong between the RPLMN and the PLMN selected for ProSe communications is MS implementation specific.

If the MS supports V2X communication over E-UTRA-PC5 or NR-PC5 and needs to perform PLMN selection for V2X communication over PC5 as specified in TS 24.386 or TS 24.587, then the MS shall proceed as follows: If the PLMN selected for V2X communication over PC5 is a VPLMN, the MS shall not periodically scan for higher priority PLMNs during the duration of V2X communication over PC5. The solution to prevent potential ping-pong between the RPLMN and the PLMN selected for V2X communication over PC5 is MS implementation specific.

An MS may have a "regionally restricted service" where it can only obtain service on certain areas (i.e. LAs or TAs). If such an MS attempts to camp on a cell of an area for which it does not have service entitlement, when it does an LR request, it will receive a message with cause value #12 (see TS 24.008, TS 24.301 and TS 24.501). In this case, the MS shall take the following actions depending on the access technology in which the message was received: In A/Gb mode, a cell may be reserved for SoLSA exclusive access (see TS 24.008 and TS 44.060). An MS is only allowed to camp normally on such a cell if it has a Localised Service Area subscription to the cell. Other MS may enter the limited service state. The MS operating in SNPN access mode shall maintain one or more lists of "5GS forbidden tracking areas for regional provision of service", each associated with an SNPN and, if the MS supports access to an SNPN using credentials from a credentials holder, entry of the "list of subscriber data" or the PLMN subscription. The MS shall use the list of "5GS forbidden tracking areas for regional provision of service" associated with the selected SNPN and, if the MS supports access to an SNPN using credentials from a credentials holder, the selected entry of the "list of subscriber data" or the selected PLMN subscription. If the MS selects a new SNPN, the MS shall keep the list of "5GS forbidden tracking areas for regional provision of service" associated with the previously selected SNPN and, if the MS supports access to an SNPN using credentials from a credentials holder, the previously selected entry of the "list of subscriber data" or the previously selected PLMN subscription. If the number of the lists to be kept is higher than supported, the MS shall delete the oldest stored list of "5GS forbidden tracking areas for regional provision of service". The MS shall delete all lists of "5GS forbidden tracking areas for regional provision of service", when the MS is switched off. The MS shall delete the list of "5GS forbidden tracking areas for regional provision of service" associated with an SNPN:

1. when the entry with the subscribed SNPN identifying the SNPN in the "list of subscriber data" is updated;
2. when the USIM is removed if:
   • EAP based primary authentication and key agreement procedure using EAP-AKA'; or
   • 5G AKA based primary authentication and key agreement procedure;
   was performed in the selected SNPN; or
3. if the MS supports access to an SNPN using credentials from a credentials holder, when the list of "5GS forbidden tracking areas for regional provision of service" is associated with:
   • the entry of the "list of subscriber data" and the entry of the "list of subscriber data" is updated; or
   • the PLMN subscription and USIM is removed.

If the MS is moving in a border area between registration areas, it might repeatedly change between cells of different registration areas. Each change of registration area would require an LR, which would cause a heavy signalling load and increase the risk of a paging message being lost. The access stratum shall provide a mechanism to limit this effect.

There are a number of situations in which the MS is unable to obtain normal service from a PLMN or SNPN. These include: (In automatic PLMN selection mode, items a, c and f would normally cause a new PLMN selection, but even in this case, the situation may arise when no PLMNs are available and allowable for use). (In automatic SNPN selection mode, items a, c, d, and f would normally cause a new SNPN selection if there are two or more entries in the "list of subscriber data", but even in this case, the situation may arise when no SNPNs are available and allowable for use). For the items a to f, if the MS does not operate in SNPN access mode, the MS attempts to camp on an acceptable cell, irrespective of its PLMN identity, so that emergency calls or access to RLOS can be made if necessary, with the exception that an MS operating in NB-S1 mode, shall never attempt to make emergency calls or to access RLOS. When in the limited service state with a valid SIM, the MS shall search for available and allowable PLMNs in the manner described in clause 4.4.3.1 and when indicated in the SIM also as described in clause 4.4.3.4. For an MS that is not in eCall only mode, with the exception of performing GPRS attach or EPS attach for emergency bearer services, performing an initial registration for emergency services, or performing EPS attach for access to RLOS, no LR requests are made until a valid SIM is present and either a suitable cell is found or a manual network reselection is performed. For an MS in eCall only mode, no LR requests are made except for performing EPS attach for emergency bearer services or an initial registration for emergency services. When performing GPRS attach or EPS attach for emergency bearer services, an initial registration for emergency services, or performing EPS attach for access to RLOS, the PLMN of the current serving cell is considered as the selected PLMN for the duration the MS is attached for emergency bearer services, registered for emergency services, or attached for access to RLOS. In the limited service state the presence of the MS need not be known to the PLMN on whose cell it has camped. If the MS is enabled for SNPN, the MS needs to make an emergency call, there is no available PLMN supporting emergency services and the MS determines that there is an available SNPN supporting emergency services (based on broadcasted information of SNPN support for emergency services), the MS may start operating in SNPN access mode and attempt to camp on a cell of the SNPN supporting emergency services. After an emergency call is released, the MS should stop operating in SNPN access mode and perform PLMN selection. If the MS is enabled for SNPN and wants to perform configuration of SNPN subscription parameters in PLMN via the user plane, but there is no available PLMN for normal services, either because of no available PLMN or all available PLMNs being in forbidden PLMN list due to LR failure, the MS may start operating in SNPN access mode (if the MS is configured with default UE credentials) and perform SNPN selection as per subclause 4.9.3.1.3 or 4.9.3.1.4. After the onboarding services in SNPN are complete, the MS may stop operating in SNPN access mode and perform PLMN selection. For the items a, c, d and f, if the MS operates in SNPN access mode and the MS has a valid entry in the "list of subscriber data", the MS shall search for available and allowable SNPNs in the manner described in clause 4.9.3.1. For the item b, if the MS operates in SNPN access mode, the MS:

• attempts to camp on an acceptable cell so that emergency calls can be made if supported and necessary; and
• may perform SNPN selection procedure for onboarding services in SNPN if the MS is configured with the default UE credentials for primary authentication.

For the item l, the MS shall search for available and allowable PLMNs in the manner described in clause 4.4.3.1 and when indicated in the SIM also as described in clause 4.4.3.4. When in the limited service state, with the exception of performing an initial registration for emergency services, no LR requests are made until a valid entry of the "list of subscriber data" is present and either a suitable cell is found or a manual network reselection is performed. In the limited service state, the presence of the MS need not be known to the SNPN on whose cell it has camped. If the MS needs to make an emergency call, the MS supports accessing a PLMN, and there is no available SNPN supporting emergency services and the MS determines that there is an available PLMN supporting emergency services (based on broadcasted information of PLMN support for emergency services), the MS shall stop operating in SNPN access mode and attempt to camp on a cell of the PLMN supporting emergency services so that emergency calls can be made. After an emergency call is released, the MS may re-start operating in SNPN access mode and perform SNPN selection. If the MS is enabled for SNPN and wants to select an SNPN for onboarding services in SNPN, but finds no suitable SNPN as per subclause 4.9.3.1.3 or 4.9.3.1.4, the MS may stop operating in SNPN access mode, and perform PLMN selection (if the MS is configured with PLMN credentials in USIM). The MS may perform PLMN selection as per clause 4.4. After the configuration of SNPN subscription parameters in PLMN via the user plane is complete, the MS may start operating in SNPN access mode and perform SNPN selection. There are also other conditions under which only emergency calls or access to RLOS may be made if the MS does not operate in SNPN access mode. These are shown in Table 2 in clause 5. ProSe communications can be initiated in case of PLMN if necessary (see TS 24.334 or TS 24.554) when in the limited service state due to items a) or c) or f). V2X communication over PC5 can be initiated if necessary (see TS 24.386 or TS 24.587) when in the limited service state due to items a) or c) or f).

In CTS mode only or in automatic mode with CTS preferred, the CTS MS normally operates on a CTS fixed part on which the mobile station is already enrolled. If the CTS MS loses CTS coverage in these modes, it shall attempt periodically to select again a CTS fixed part. To select a CTS fixed part, the CTS MS shall listen to the CTSBCH frequencies of all the fixed parts on which the MS is currently enrolled. If the CTS MS is moving in a border area between one area with CTS coverage and one without it, it might repeatedly require CTS attachments and LU on the PLMN. To prevent this, the criteria C1_CTS and C2_CTS (defined in TS 45.008, clause 11.1) are used. To attach to a CTS FP, the C1_CTS criterion shall be greater than zero. When the C2_CTS criterion falls below zero, the CTS MS shall consider itself to be no more under CTS coverage.

NAS behaviour can be operator configurable using parameters in the USIM (see TS 31.102) or in the ME (see TS 24.368). For parameters available in both the USIM and the ME, precedence is specified in TS 31.102, clause 5.2.29.

The MS may support CAG. If the MS supports CAG, the MS can be provisioned by the network with a "CAG information list", consisting of zero or more entries, each containing:

1. a PLMN ID;
2. an "Allowed CAG list". The "Allowed CAG list" contains zero or more CAG-IDs; and
3. an optional "indication that the MS is only allowed to access 5GS via CAG cells".

The "CAG information list" provisioned by the network is stored in the non-volatile memory of the ME, as specified in TS 24.501 Annex C. In addition, the MS can also be pre-configured with a "CAG information list" stored in the USIM (see TS 31.102). The "Allowed CAG list" included in the entry for the HPLMN or EHPLMN in "CAG information list" stored in the USIM can contain a range of CAG-IDs. 3GPP TS 24.501 Annex C specifies condition under which the "CAG information list" stored in the ME is deleted. Additionally, when a USIM is inserted, if:

• no "CAG information list" is stored in the non-volatile memory of the ME; or
• the SUPI from the USIM does not match the SUPI stored together with the "CAG information list" in the non-volatile memory of the ME;

and the MS has a "CAG information list" stored in the USIM (see TS 31.102), the MS shall store the "CAG information list" from the USIM into the ME, as specified in TS 24.501 Annex C. If an entry in the "CAG information list" stored in the USIM includes an "Allowed CAG list" which contains a range of CAG-IDs, the range of CAG-IDs can be replaced with individual CAG-IDs matching the range up to ME implementation. If the MS supports CAG and a PLMN is selected as described in clause 4.4.3.1.1, the automatic CAG selection is performed as part of clause 4.4.3.1.1. If the MS supports CAG and a PLMN is selected as described in clause 4.4.3.1.2, the manual CAG selection is performed as part of clause 4.4.3.1.2. The NAS shall provide the AS with a "CAG information list", if available. If the contents of the "CAG information list" have changed, the NAS shall provide an updated "CAG information list" to the AS. If an entry in the "CAG information list" includes an "Allowed CAG list" which contains a range of CAG-IDs, whether the NAS provides the AS the range of CAG-IDs or individual CAG-IDs matching the range is up to ME implementation. The "indication that the MS is only allowed to access 5GS via CAG cells" is not applicable in EPS.

An MS may be enabled for SNPN. An MS enabled for SNPN may operate in SNPN access mode. An MS may support onboarding services in SNPN. An MS enabled for SNPN may support access to an SNPN using credentials from a credentials holder. With the exception of onboarding services in SNPN, the MS operating in SNPN access mode selects:

1. an SNPN for which it is configured with a subscriber identifier and credentials; or
2. if the MS supports access to an SNPN using credentials from a credentials holder, an SNPN which supports access using credentials from a credentials holder.

The MS can have several sets of subscriber identifiers, credentials, SNPN identities, and other parameters related to SNPN selection (see clause 4.9.3.0). There are two modes for SNPN selection:

1. Automatic SNPN selection mode.
2. Manual SNPN selection mode.

For onboarding services in SNPN, the MS operating in SNPN access mode selects an SNPN indicating that onboarding is allowed. There are two modes for SNPN selection for onboarding services in SNPN:

1. Automatic SNPN selection mode.
2. Manual SNPN selection mode.

The MS may support Minimization of service interruption (MINT). MINT is not applicable in SNPNs. For a PLMN that provides disaster roaming services, if one of the CAG ID(s) broadcasted by a CAG cell for the PLMN is present in the "Allowed CAG list" included in the entry for the PLMN in the "CAG information list", then the UE may attempt to access the PLMN on the CAG cell for disaster roaming services. If the MS supports MINT, the MS can be provisioned by the network with:

1. an indication of whether disaster roaming is enabled in the UE, provided by the HPLMN;
2. a "list of PLMN(s) to be used in disaster condition" provided by the HPLMN, consisting of zero or more entries, each containing a PLMN ID. The PLMNs are listed in order of decreasing priority, with the first PLMN being the highest priority PLMN;
3. one or more "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN, consisting of zero or more entries, each containing a PLMN ID. The PLMNs are listed in order of decreasing priority, with the first PLMN being the highest priority PLMN;
4. a disaster roaming wait range consisting of a minimum wait time and a maximum wait time;
5. a disaster return wait range consisting of a minimum wait time and a maximum wait time; and
6. an indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN', provided by the HPLMN.

The network may provide the "list of PLMN(s) to be used in disaster condition", the disaster roaming wait range and the disaster return wait range to the UE during a successful registration procedure or a generic UE configuration update procedure. The network may also provide the disaster return wait range to the UE during a network initiated de-registration procedure, an unsuccessful registration procedure or an unsuccessful service request procedure. The HPLMN may also provide an indication of whether disaster roaming is enabled in the UE and an indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN' during a UE parameters update procedure. The indication of whether disaster roaming is enabled in the UE, the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN', one or more instances of "list of PLMN(s) to be used in disaster condition" each stored with the PLMN identity of the PLMN that provided it, the disaster roaming wait range and the disaster return wait range provisioned by the network are stored in the non-volatile memory of the ME, as specified in TS 24.501 Annex C. In addition, the MS can also be pre-configured with an indication of whether disaster roaming is enabled in the UE, the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN', a "list of PLMN(s) to be used in disaster condition" provided by the HPLMN, a disaster roaming wait range and a disaster return wait range stored in the USIM (see TS 31.102). 3GPP TS 24.501 Annex C specifies the conditions under which the indication of whether disaster roaming is enabled in the UE, the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN', the one or more "lists of PLMN(s) to be used in disaster condition", the disaster roaming wait range and the disaster return wait range stored in the ME are deleted. Additionally:

1. when a USIM is inserted:
   1. if:
      1. no indication of whether disaster roaming is enabled in the UE is stored in the non-volatile memory of the ME; or
      2. the SUPI from the USIM does not match the SUPI stored together with the indication of whether disaster roaming is enabled in the UE in the non-volatile memory of the ME;
         and the MS has an indication of whether disaster roaming is enabled in the UE stored in the USIM (see TS 31.102), the MS shall store the indication of whether disaster roaming is enabled in the UE from the USIM into the ME, as specified in TS 24.501 Annex C;
   2. if:
      1. no disaster roaming wait range is stored in the non-volatile memory of the ME; or
      2. the SUPI from the USIM does not match the SUPI stored together with the disaster roaming wait range in the non-volatile memory of the ME;
      and the MS has a disaster roaming wait range stored in the USIM (see TS 31.102), the MS shall store the disaster roaming wait range from the USIM into the ME, as specified in TS 24.501 Annex C; and
   3. if:
      1. no disaster return wait range is stored in the non-volatile memory of the ME; or
      2. the SUPI from the USIM does not match the SUPI stored together with the disaster return wait range in the non-volatile memory of the ME;
      and the MS has a disaster return wait range stored in the USIM (see TS 31.102), the MS shall store the disaster return wait range from the USIM into the ME, as specified in TS 24.501 Annex C;
   4. if:
      1. the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN' is stored in the non-volatile memory of the ME; or
      2. the SUPI from the USIM does not match the SUPI stored together with the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN' in the non-volatile memory of the ME;
      and the MS has an indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN' stored in the USIM (see TS 31.102), the MS shall store the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN' from the USIM into the ME, as specified in TS 24.501 Annex C; and
   5. if:
      1. no "list of PLMN(s) to be used in disaster condition" provided by the HPLMN is stored in the non-volatile memory of the ME; or
      2. the SUPI from the USIM does not match the SUPI stored together with the "list of PLMN(s) to be used in disaster condition" provided by the HPLMN in the non-volatile memory of the ME;
         and the MS has a "list of PLMN(s) to be used in disaster condition" provided by the HPLMN stored in the USIM (see TS 31.102), the MS shall store the "list of PLMN(s) to be used in disaster condition" provided by HPLMN from the USIM into the ME, as specified in TS 24.501 Annex C;
2. when the ME receives a USAT REFRESH command indicating that:
   1. the indication of whether disaster roaming is enabled in the UE stored in the USIM has been updated, the MS shall store the indication of whether disaster roaming is enabled in the UE from the USIM into the ME, as specified in TS 24.501 Annex C;
   2. the disaster roaming wait range stored in the USIM has been updated, the MS shall store the disaster roaming wait range from the USIM into the ME, as specified in TS 24.501 Annex C;
   3. the disaster return wait range stored in the USIM has been updated, the MS shall store the disaster return wait range from the USIM into the ME, as specified in TS 24.501 Annex C;
   4. the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN' stored in the USIM has been updated, the MS shall store the indication of 'applicability of "lists of PLMN(s) to be used in disaster condition" provided by a VPLMN' from the USIM into the ME, as specified in TS 24.501 Annex C; or
   5. the "list of PLMN(s) to be used in disaster condition" provided by the HPLMN stored in the USIM has been updated, the MS shall store the "list of PLMN(s) to be used in disaster condition" provided by the HPLMN from the USIM into the ME, as specified in TS 24.501 Annex C; or

If the MS does not have an indication of whether disaster roaming is enabled in the UE stored in the ME, or the indication of whether disaster roaming is enabled in the UE stored in the ME is set to "Disaster roaming is disabled in the UE", disaster roaming is disabled at the MS. In this case, the MS shall not perform disaster roaming. Upon selecting a PLMN for disaster roaming, if there is a disaster roaming wait range stored in the ME, the MS shall generate a random number within the disaster roaming wait range and start a timer set to the generated random number. While the timer is running, the MS shall not initiate registration with the exception of performing an initial registration for emergency services, in the selected PLMN. If performing an initial registration for emergency services in the selected PLMN, the MS shall keep the timer running. Upon expiration of the timer, if the MS does not have an emergency PDU session, the MS may initiate registration, if still camped on the selected PLMN. Upon expiration of the timer, if the MS has an emergency PDU session, the MS shall initiate registration after release of the emergency PDU session, if still camped on the selected PLMN. Upon determining that a disaster condition has ended and selecting the PLMN previously with disaster condition, if there is a disaster return wait range stored in the ME, the MS shall generate a random number within the disaster return wait range and start a timer set to the generated random number. While the timer is running, the MS shall not initiate registration with the exception of performing an initial registration for emergency services, in the selected PLMN. If performing an initial registration for emergency services in the selected PLMN, the MS shall keep the timer running. Upon expiration of the timer, if the MS does not have an emergency PDU session, the MS may initiate registration, if still camped on the selected PLMN. Upon expiration of the timer, if the MS has an emergency PDU session, the MS shall initiate registration after release of the emergency PDU session, if still camped on the selected PLMN.