5 Network discovery and selection

24.5023GPPAccess to the 3GPP 5G Core Network (5GCN) via non-3GPP access networksRelease 17TS

5.1 General

The following aspects are included when selecting a 5GC network and routing traffic via the 5GC network:

a) access network discovery procedures as defined in clause 5.2;

b) access network selection procedures as defined in clause 5.3; and

c) access network reselection procedures as defined in clause 5.4.

5.2 Access network discovery procedure

5.2.1 General

If PLMN selection specified in 3GPP TS 23.122 [13] is applicable (e.g., at switch-on, recovery from lack of 3GPP coverage, or user selection of applicable 3GPP access technology), the PLMN selection to select the highest priority PLMN according to these specifications is performed before any access network discovery.

In the access network discovery procedure, the UE can get ANDSP information on available access networks in its vicinity and can use this information when determining the presence of operator preferred access networks. Determination of the presence of access networks requires using radio access specific procedures, which are not further described here.

NOTE: The procedure for the automatic mode WLAN selection by using ANDSP rules as defined in clause 5.3.2.3 does not apply to an N5CW device that is not registered or cannot register via NG-RAN.

5.2.2 Discovering availability of WLAN access networks

The UE may obtain WLAN Selection Policy (WLANSP) rules information by pre-configuration or by downloading the policy information from the PCF as specified in 3GPP TS 23.503 [16]. The policy contains the UE access network discovery and selection related policy information to help the UE in discovering and selecting a WLAN access network (see 3GPP TS 24.526 [17]).

The UE may receive multiple valid WLANSP rules. When the UE is in the home PLMN, the UE uses the valid WLANSP rules from the home PLMN to select an available WLAN. When the UE is roaming and the UE has valid rules from several of the home PLMN, a visited PLMN and a PLMN equivalent to the visited PLMN, the UE uses the WLANSP rules in the following order of decreasing priority:

a) the valid WLANSP rules from the visited PLMN;

b) the valid WLANSP rules from the equivalent PLMN in which the UE last received WLANSP; and

c) the valid WLANSP rules from the home PLMN.

A WLANSP rule is valid if it meets the validity conditions included in the WLANSP rule (if provided).

The UE may apply the techniques specific to the WLAN access technologies to discover available WLAN access networks. Such techniques will not be further described here.

In addition, the UE may obtain information on operator preferred WLAN access networks via ANDSP.

5.3 Access network selection procedure

5.3.1 General

In this release of the specification, only selection of WLAN access network is supported. The ANDSP policy contains WLANSP rules for the UE to select a WLAN access network. Rules for selecting other types of non-3GPP access networks are not specified.

5.3.2 WLAN selection procedure

5.3.2.1 General

The purpose of the WLAN selection procedure is to create a prioritized list of selected WLAN(s).

The UE shall perform WLAN selection based on the user preferences and WLANSP rules. The UE may be provisioned with WLANSP rules from multiple PLMNs. User preferences take precedence over the WLANSP rules.

The user preferences are used to select between the automatic WLAN selection procedure or the manual WLAN selection procedure:

a) if user preferences are present, the UE shall determine the prioritized list of selected WLAN(s) using the manual mode WLAN selection procedure (see clause 5.3.2.2); or

b) if user preferences are not present or if there is no user-preferred WLAN access network available, the UE shall determine the prioritized list of selected WLAN(s) using the automatic mode WLAN selection procedure (see clause 5.3.2.3).

5.3.2.2 Manual mode WLAN selection

The UE creates a prioritized list of available WLAN(s). The creation of the prioritized list is implementation specific.

5.3.2.3 Automatic mode WLAN selection

The UE shall first determine valid WLANSP rules for WLAN selection:

a) if the UE is not roaming over 3GPP access, the UE shall use the valid WLANSP rules from the HPLMN; or

b) if the UE is roaming over 3GPP access, the UE may have valid WLANSP rules from several of the visited PLMN, a PLMN equivalent to the visited PLMN and the home PLMN. The UE uses the WLANSP rules in the following order of decreasing priority:

1) the valid WLANSP rules from the visited PLMN;

2) the valid WLANSP rules from the equivalent PLMN in which the UE last received WLANSP; and

3) the valid WLANSP rules from the home PLMN.

The UE shall then determine the selected WLAN(s) according to the following steps:

a) use the procedures specified in the IEEE 802.11 [19] to discover the available WLANs. The UE may perform ANQP procedures as specified in the IEEE 802.11 [19] or the Hotspot 2.0 [20] to discover the attributes and capabilities of available WLANs. If the UE supports ANQP procedures, the UE may send an ANQP request for lists of service providers (i.e. ANQP-elements "Domain Name", see IEEE 802.11 [19]) and PLMN identities (i.e. ANQP-element "3GPP Cellular Network", see 3GPP TS 24.302 [7] annex H); and

b) if the UE has performed ANQP procedures to discover the attributes and capabilities of available WLANs, compare the attributes and capabilities of the available WLANs with the group of selection criteria of the valid WLANSP rules and construct a prioritized list of available WLANs that fulfill the selection criteria.

1) when there are multiple valid WLANSP rules the UE evaluates the valid WLANSP rules in priority order. The UE evaluates first if an available WLAN access meets the selection criteria of the highest priority valid WLANSP rule. The UE then evaluates if an available WLAN access meets the selection criteria of the next priority valid WLANSP rule;

NOTE 1: Each WLANSP rule can include one or more groups of selection criteria in priority order. If there are multiple highest priority groups of selection criteria in the valid WLANSP rule, it is up to the UE implementation which one to use.

2) if the Home network ind bit is not set to "1" in the group of selection criteria (see 3GPP TS 24.526 [17]), the WLAN(s) that match the group of selection criteria with the highest priority are considered as the most preferred WLANs, the WLAN(s) that match the group of selection criteria with the second highest priority are considered as the second most preferred WLANs;

3) if the Home network ind bit is set to "1" in the group of selection criteria (see 3GPP TS 24.526 [17]), then the UE shall create a list of available WLANs and shall apply the group of selection criteria to all the WLANs in this list. A WLAN is included in this list, if

i) the other selection criteria in the active WLANSP rule are met; and

ii) the UE received a lists of service providers (i.e. ANQP-elements "Domain Name") and PLMN identities (i.e. ANQP-element "3GPP Cellular Network"), and:

I) if the list with PLMNs that can be selected from the WLAN (see 3GPP TS 24.302 [7]) includes:

A) the HPLMN derived from its IMSI; or

B) a PLMN matching an entry in the UE’s list of equivalent PLMNs; or

II) if the domain name list (see IEEE 802.11 [19]) includes:

A) the home domain name derived from its IMSI; or

B) the domain name derived from its list of equivalent PLMNs; and

NOTE 2: If the Home network ind bit is set to "1" in a group of selection criteria then this group of selection criteria is not expected to include the preferred roaming partner list and the preferred SSID list.

NOTE 3: WLAN advertises PLMN(s) towards which the S2a connectivity or the 5G connectivity using trusted non-3GPP access is supported by using the ANQP-element "3GPP Cellular Network" with the PLMN List with S2a Connectivity IE, the PLMN List with trusted 5G connectivity IE or the PLMN List with trusted 5G connectivity-without-NAS IE in the payload (see 3GPP TS 24.302 [7] Annex H). The PLMN List with trusted 5G connectivity-without-NAS IE is only used by N5CW devices. If the UE selects a PLMN over WLAN included in both the PLMN List with S2a Connectivity IE, and the PLMN List with trusted 5G connectivity IE, the UE requests the PLMN with trusted 5G connectivity (see 3GPP TS 23.501 [2] clause 6.3.12.2).

4) The priority of a WLAN in the available WLANs list is set to the WLAN priority defined in the preferredSSIDlist of the matching group of selection criteria. There may be one or more selected WLANs in the list.

5.3A PLMN selection procedures using trusted non-3GPP access

5.3A.1 General

There are two modes of PLMN selection, namely, manual selection and automatic selection.

The UE follows one of the following two procedures defined in clause 5.3.2.2 and clause 5.3.2.3 depending on its implementation. The N5CW device that is not registered or cannot register via NG-RAN performs manual mode WLAN selection procedure as defined in clause 5.3.2.2.

The PLMN selected in accordance with these procedures determines the WLAN that is selected. When the selected WLAN is a trusted non-3GPP IP access and the UE decides to access 5GC via trusted non-3GPP IP access, the UE shall derive a NAI from the identity of the selected PLMN and use the NAI as the identity for authentication and authorization with the PLMN and usage of the WLAN.

The procedures described in this clause 5.3A shall apply to the UE and the N5CW device.

5.3A.2 PLMN solicitation

The UE shall determine which PLMNs are available from each WLAN on the list of available WLANs constructed using the WLAN selection procedure described in clause 5.3.2 using the following procedures:

i) the UE selects a WLAN from the list of selected WLAN(s) constructed using the WLAN selection procedure described in clause 5.3.2;

NOTE 1: An N5CW device that is not registered or cannot register via NG-RAN uses only the manual mode WLAN selection procedure described in clause 5.3.2.

ii) if both the WLAN selected in step i) and the UE support ANQP specified in IEEE Std 802.11 [19] and if the UE did not obtain a list of realms using ANQP in clause 5.3.2.3 item 1, the UE shall send an ANQP request for a list of realms (i.e. ANQP-elements "NAI Realm") and/or PLMN identities (i.e. ANQP-element "3GPP Cellular Network"); and

NOTE 2: The UE uses procedures defined in IEEE Std 802.11 [19] to determine if the WLAN supports ANQP and to send the ANQP request for ANQP-elements "NAI Realm" and/or "3GPP Cellular Network", as specified in IEEE Std 802.11 [19].

iii) if either the WLAN selected in step i) or the UE does not support ANQP (see IEEE Std 802.11 [19]) or the UE does not receive a list of realms in item ii), an EAP-Request/Identity is received and the EAP-Request/Identity does not include one or more of realms and/or PLMN identities (encoded in accordance with IETF RFC 4284 [31]), the UE supports IEEE 802.1x authentication (see IEEE Std 802.1X™ [30]), the UE shall request a list of realms and/or PLMN identities interworking with that WLAN by sending the EAP-Response/Identity message including as identity the alternative NAI; and

iv) the UE repeats this procedure for all WLANs from the available list of WLANs as constructed using the WLAN selection procedure described in clause 5.3.2.

NOTE 3: The list with realms and/or PLMN identities received in accordance with procedures in IETF RFC 4284 [31], is of limited size and might not contain all the realms and/or PLMN identities available via the WLAN.

The UE shall convert any received PLMN identities into realms of the PLMNs using the rules defined in clause 19 and clause 28 of 3GPP TS 23.003 [8]. The N5CW device shall convert any received PLMN identities into realms of the PLMNs using the rules defined in clause 28 of 3GPP TS 23.003 [8].

5.3A.3 Manual PLMN selection mode procedure

The UE indicates to the user the PLMNs which are available via the WLAN. The UE may obtain the PLMNs available for WLAN access using procedures as described in clause 5.3A.2. The UE selects the PLMN based on the user preference.

5.3A.4 Automatic mode PLMN selection procedure

5.3A.4.1 General

The purpose of this procedure is to:

– select a PLMN over WLAN; and

– construct a NAI for use with authentication signalling with the selected PLMN in order for the UE to be authorised to use the WLAN.

Until the highest priority PLMN is found, the UE shall verify if a PLMN available over a WLAN of the selected WLAN(s) is the highest priority PLMN:

1) using the PLMNs which are available for WLAN as described in clause 5.3A.2, the UE uses the realms of the PLMN in the remaining steps of this clause;

2) if the UE is registered over 3GPP access, the realm of the RPLMN of the 3GPP access is included in the list of realms created in clause 5.3A.2 and the realm of the RPLMN of the 3GPP access does not match a realm converted from any PLMN ID in the list of "forbidden PLMNs for non-3GPP access to 5GCN", the UE shall select the RPLMN of the 3GPP access;

3) if the UE is registered over 3GPP access, the realm of the RPLMN of the 3GPP access is not included in the list of realms created in clause 5.3A.2, the PLMN is in the "N3AN node selection information" (see 3GPP TS 24.526 [17]) and the PLMN is not in the list of "forbidden PLMNs for non-3GPP access to 5GCN" then the UE shall select the RPLMN of the 3GPP access and performs N3AN node selection with the RPLMN as defined in clause 7.2;

4) if the condition in steps 2) and 3) are not satisfied, the UE shall select a PLMN in the following order:

i) if the UE used the procedures in IETF RFC 4284 [31] (see clause 5.3A.2) to obtain a list of realms, then the UE is only required to select the realm of the HPLMN (if available);

ii) if the UE can determine the country it is located in (see clause 7.2.3) and the UE determines it is located in the home country, the UE follows the procedures in clause 5.3A.4.2;

iii) if the UE can determine the country it is located in (see clause 7.2.3) and the UE determines it is located in a visited country, the UE determines whether it is mandatory to select a PLMN in the visited country.

If the UE determines that it is not mandatory to select a PLMN in the visited country, the UE shall follow the procedures in clause 5.3A.4.2;

If the UE determines that it is mandatory to select a PLMN in the visited country, the UE shall select, in priority order, a PLMN from the list of realms created in clause 5.3A.2, if:

I) the PLMN is in the User Controlled PLMN Selector list (see 3GPP TS 31.102 [35]); or

II) the PLMN is in the Operator Controlled PLMN Selector list (see 3GPP TS 31.102 [35]).

If no match is found in either of the lists, the UE may perform N3AN node selection as defined in clause 7.2.

The UE shall construct a NAI for authentication with the highest priority PLMN as follows:

1) if the PLMN selected was selected from:

i) a list of realms obtained using IETF RFC 4284 [31]; or

ii) a list of PLMNs obtained from the PLMN List IE (see annex H of 3GPP TS 24.302 [7]), and the PLMN was neither present in the PLMN List with S2a Connectivity IE, in the PLMN List with trusted 5G Connectivity IE nor the PLMN List with trusted 5G connectivity-without-NAS IE;

then the UE constructs a NAI as specified in clause 5.2.3.2.3 of 3GPP TS 24.302 [7] for the case when the NAI is used for access via non-3GPP access to EPC and in accordance to the rules of 3GPP TS 23.003 [8] and the UE proceeds processing as defined in 3GPP TS 24.302 [7];

2) if the PLMN selected was selected from a list of PLMNs obtained from the PLMN List with trusted 5G Connectivity IE or the PLMN List with trusted 5G connectivity-without-NAS IE (see annex H of 3GPP TS 24.302 [7]) then the UE constructs a NAI as specified in:

i) clause 28.7.6 of 3GPP TS 23.003 [8] if the selected type of trusted connectivity is 5G connectivity using trusted non-3GPP access; or

ii) clause 28.7.7 of 3GPP TS 23.003 [8] if the selected type of trusted connectivity is 5G connectivity without NAS using trusted non-3GPP access; or

3) if the PLMN selected was selected from a list of PLMNs obtained from the PLMN List with S2a Connectivity IE (see annex H of 3GPP TS 24.302 [7]) for the case when the NAI is used for access via trusted non-3GPP access to EPC, then the UE constructs a NAI as specified in clause 5.2.3.2.3 of 3GPP TS 24.302 [7] and the UE proceeds processing as defined in 3GPP TS 24.302 [7].

NOTE 1: UE implementations can optimize the steps described above, e.g. by combining the ANQP procedures described in clause 5.3A.2 with the ANQP procedures in clause 5.3.2.3.

NOTE 2: Selecting a WLAN from multiple WLANs advertising support for the selected PLMN is UE implementation specific.

NOTE 3: The N5CW device which is not registered or cannot register via NG-RAN only uses the PLMN List with trusted 5G connectivity-without-NAS IE, and the PLMN List with trusted 5G connectivity-without-NAS IE is only used by the N5CW devices.

5.3A.4.2 Attempting to select HPLMN or equivalent HPLMN

If the realm of the HPLMN is included in the list of realms created in clause 5.3A.2 then the UE shall select the HPLMN.

If the realm of the HPLMN is not included in the list of realms created in clause 5.3A.2, but a realm of an equivalent HPLMN is included, then the UE shall select the equivalent HPLMN.

If neither realm is included in the list of realms created in clause 5.3A.2, then the UE aborts its attempt to use trusted non-3GPP IP access.

5.3A.4.3 Void

5.3B PLMN selection procedures using wireline access

Roaming support for wireline access is not defined in the present version of the present document.

The 5G-RG, the W-AGF acting on behalf of the FN-RG and the W-AGF acting on behalf of the N5GC device shall consider that the HPLMN is available on each wireline access network and shall select HPLMN on the wireline access network.

5.4 Access network reselection procedure

5.4.1 General

The access network reselection procedure can be triggered based on the user’s request or the operator’s policy. Such operator policy for supporting network reselection can be provided by the ANDSP or can be pre-provisioned in the UE.

The access network reselection procedure can also be triggered by the UE during periodical re-evaluation of ANDSP policies (see clause 6.4.2), or if the ‘active’ rule becomes invalid (conditions no longer fulfilled), or other manufacturer specific trigger.

NOTE: How frequently the UE performs the discovery and reselection procedure is UE implementation specific.

5.4.2 WLAN reselection procedure

For WLAN access network reselection, the UE configured with a WLANSP rule shall use the access network selection procedure as specified in clause 5.3.2. The UE first uses WLAN Selection Policy (WLANSP) to determine the active WLANSP rule. The UE selects the highest priority and valid WLANSP rule as the active WLANSP rule.

The access network reselection procedure can be in automatic mode or manual mode. The manual mode reselection shall follow the behaviour described in clause 5.3.2.3 and the automatic mode reselection shall follow the behaviour described in clause 5.3.2.4.