5 Network Discovery and Selection

24.3023GPPAccess to the 3GPP Evolved Packet Core (EPC) via non-3GPP access networksRelease 18Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.0 General

The following aspects are included when selecting an EPC network and routing traffic via the EPC network:

– access network discovery and selection procedures as defined in clause 5.1;

– EPC network selection as defined in clause 5.2; and

– data traffic routing of IP flows as defined in clause 5.4.

If the UE perform reselection of the access network as defined in clause 5.3 and the UE reselects to a different access network, the UE performs the second item and third item of the above bulleted list.

5.1 Access network discovery and selection procedures

5.1.1 General

If PLMN selection specified in 3GPP TS 23.122 [4] 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. Procedures for EPC selection over non-3GPP access are specified in clause 5.2. In particular, for WLAN access, service provider selection function is specified in the WLAN specific procedures in clause 5.2.3.2

In the access network discovery procedure the UE may get from the ANDSF information on available access networks in its vicinity. The UE may obtain this information by querying the ANDSF, and may 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.

The UE determines the presence of several access networks and then selects between them. If a higher priority access network is found connected to the selected service provider or a higher priority service provider, the UE will attempt to attach via that access network.

5.1.2 Access network discovery procedure

5.1.2.1 Triggering the discovery of operator preferred access networks with the ANDSF

The UE may initiate communications with the ANDSF for operator preferred access network discovery:

– when conditions set up within the policies available in the UE are met; or

– when a user requests for manual selection.

NOTE 1: The minimum allowed time interval between two consecutive UE initiated requests towards the ANDSF can be set by operator policies.

NOTE 2: The UE changing of access networks can override the minimum allowed time interval setting.

5.1.2.2 Discovering availability of access networks

The UE may apply the techniques specific to the non-3GPP access technologies to discover available non-3GPP access networks. Such techniques will not be further described here.

In addition, the UE may signal to the ANDSF to obtain information on operator preferred access networks. The discovery of the ANDSF by the UE, the connection to the ANDSF by the UE and the signalling between the UE and the ANDSF are given in clause 6.8.

5.1.3 Access network selection procedure

5.1.3.1 General

The access network selection may be classified as inter-technology or intra-technology.

The UE can use information received from ANDSF for inter-technology access network selection.

If the RAN rules control the WLAN access selection and traffic routing as described in clause 6.10.2, the UE uses the information described in clause 6.10.4 for inter-technology access network selection.

Other mechanisms for inter-technology access network selection are out of scope of this specification.

5.1.3.2 Specific intra-technology access network selection

In this release of the specification the use of the following specific intra-technology access network selection procedures is specified.

5.1.3.2.1 cdma2000^® HRPD access network selection

The access network selection process for cdma2000^® HRPD access networks shall follow 3GPP2 X.S0057 [20].

5.1.3.2.2 WiMAX NAP selection

The access network selection process for WiMAX which encompasses the NAP discovery and access, shall follow the WiMAX Forum Network Architecture Release 1.0 version 1.2 – Stage 3 [25].

5.1.3.2.3 WLAN selection

5.1.3.2.3.1 General

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

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

The UE shall determine the prioritized list of selected WLAN(s):

1) if user preferences are present, in accordance with the manual mode WLAN selection procedure (see clause 5.1.3.2.3.2); and

2) if user preferences are not present, in accordance with the automatic mode WLAN selection procedure (see clause 5.1.3.2.3.3).

The UE shall use the prioritized list of selected WLAN(s) to select the service provider in the procedure in clause 5.2.3.2.

5.1.3.2.3.2 Manual mode WLAN selection

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

5.1.3.2.3.3 Automatic mode WLAN selection

If the ANDSF rules control the WLAN access selection and traffic routing as described in clause 6.10.2, then the selected WLAN(s) are WLAN(s) that fulfil the selection criteria with the highest priority configured in the active ANDSF WLANSP rule.

If the RAN rules control the WLAN access selection and traffic routing as described in clause 6.10.2, then the selected WLAN(s) are WLAN(s) matching WLAN identifiers in an entry of the list of the WLAN identifiers received along with the move-traffic-to-WLAN indication as described in clause 6.10.4.

The UE determines the selected WLAN(s) according to the following steps:

1) the UE shall construct prioritized list of available WLANs as follows:

a) if the ANDSF rules control the WLAN access selection and traffic routing as described in clause 6.10.2, the UE shall use the procedures specified in IEEE Std 802.11 [57] to discover the available WLANs. The UE may perform ANQP procedures as specified in IEEE Std 802.11 [57] to discover the attributes and capabilities of available WLANs. The UE shall compare the attributes and capabilities of the available WLANs with the highest priority selection criterion that has not been used yet in the active WLANSP rule, and construct a prioritized list of available WLANs that fulfil the selection criteria. If there are multiple highest priority selection criteria, it is up to the UE implementation which one to use. In particular, if:

– the group of selection criteria include the HomeNetworkIndication and it is set to "1" (see 3GPP TS 24.312 [13]); and

– the HomeNetworkPreference:

i) does not include 3GPP_RPLMN_Preferred; or

ii) includes 3GPP_RPLMN_Preferred and it is set to "0" (see 3GPP TS 24.312 [13]);

then a WLAN is included, if:

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

– the domain name list (see IEEE Std 802.11 [57]) includes:

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

ii) any realm in the EquivalentHomeSPs as specified in 3GPP TS 24.312 [13].

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

b) if the RAN rules control the WLAN access selection and traffic routing as described in clause 6.10.2, the UE shall use the procedures specified in IEEE Std 802.11 [57] to discover available WLANs. The UE shall construct a prioritized list of available WLANs and populate it with each discovered WLAN which matches all WLAN identifiers included in an entry of the list of the WLAN identifiers received along with the move-traffic-to-WLAN indication as described in clause 6.10.4. The priority of a discovered WLAN in the prioritized list of available WLANs is decided by the UE in an implementation specific way;

2) if the ANDSF rules control the WLAN access selection and traffic routing as described in clause 6.10.2, and if the following conditions are fulfilled:

– the UE supports the PDN connection establishment over WLAN using the applicable S2a procedures specified in 3GPP TS 23.402 [6];

– the "S2a connection preference" indicator exists and indicates that PDN connection establishment over WLAN using the applicable S2a procedures specified in 3GPP TS 23.402 [6] is preferred; and

– one or more WLANs in the list constructed in step 1) is a trusted non-3GPP IP access network;

then the UE considers the WLANs that have the highest priority and indicate the HPLMN or RPLMN in the PLMN list with S2a connectivity IE (see annex H) as the selected WLAN(s).

Otherwise, the UE considers the WLAN(s) that has or have the highest priority as the selected WLAN(s). And

NOTE 1: WLAN advertises PLMN(s) towards which the S2a connectivity is supported using ANQP-element "3GPP Cellular Network" with the PLMN List with S2a Connectivity IE in the payload, according to annex H.

NOTE 2: Advertising S2a connectivity over a WLAN using EAP signalling is not supported in this version of the specification.

3) if the ANDSF rules control the WLAN access selection and traffic routing as described in clause 6.10.2, if there are no WLAN(s selected in step 2), the UE may repeat the procedure from step 1) taking into consideration selection criteria with lower priority from the active WLANSP rule.

NOTE 3: UE implementation can optimize the steps described above, e.g. by combining the ANQP procedures.

5.2 EPC network selection over non-3GPP access

5.2.1 General

The following EPC network selection procedures over non-3GPP access are defined:

1) WiMAX specific;

2) EPC network selection via cdma2000^® HRPD access is given in 3GPP TS 23.122 [4] with any exceptions detailed in clause 5.3.4;

3) WLAN specific procedures in clause 5 apply: the procedures detail selecting one or more WLANs and (subsequently) selecting one service provider offering services via the WLAN (see clause 5.2.3.2). When the operator of the WLAN requires authentication and the authentication succeeds (see clause 6.4 and 6.5.1), the UE follows the procedures defined for connecting with the EPC. When the UE is connected to EPC through WLAN access, the tunnel is set-up with the ePDG (as described in clause 7 of this document) using a root NAI as defined in 3GPP TS 23.003 [3] or with the HA (as described in 3GPP TS 24.303 [11]); and

4) generic EPC network selection for other access technologies not listed above.

The UE performs the appropriate EPC selection procedure over non-3GPP access when the non-3GPP radio becomes enabled. If the UE needs to establish emergency session over untrusted access, the UE shall select an ePDG that supports emergency services as described in clause 7.2.1 and 3GPP TS 23.402 [6].

NOTE: The UE can perform the appropriate EPC selection procedure over non-3GPP access based on other implementation-specific triggers, e.g. regaining non-3GPP access network coverage or connectivity.

The UE can utilize information received from ANDSF to which EPCs an access network is connected as described in 3GPP TS 24.312 [13]. Additionally, any technology specific means can be employed to acquire such information, but these are out of scope of this specification.

5.2.2 Generic EPC network selection procedure over non-3GPP access

5.2.2.1 Identification of the EPC

The identification of EPC shall be based on one of the following:

– PLMN-Id (i.e. pair of MCC+MNC), as specified in 3GPP TS 23.003 [3]; or

– Home/Visited Network Realm/Domain, as specified in 3GPP TS 23.003 [3].

5.2.2.2 EPC network selection

5.2.2.2.1 UE selection modes

Two modes of EPC network selection are defined, manual and automatic. The UE shall select the EPC network according to the selected operating mode.

5.2.2.2.2 Manual EPC network selection

The UE shall present the list of available EPC networks, to which connectivity is provided through the selected non-3GPP access network, to the user. If UE’s HPLMN or PLMNs equivalent to it are in this list, they shall be shown in the highest ranking order. The ordering of the rest of entries in the list is implementation dependent. If available, the UE should display names and/or realms/domains.

If multiple equivalent HPLMNs are available, then the display order among them is UE implementation specific.

5.2.2.2.3 Automatic EPC network selection

The UE may use locally stored data for selecting between EPC networks available for connectivity via the currently selected non-3GPP access network.

The UE shall select a PLMN according to the PLMN selection procedures of the selected non-3GPP access network.

Additional criteria are out of scope of this specification and remain implementation specific.

5.2.3 Access technology specific EPC network selection procedures

5.2.3.1 EPC network selection procedures for WiMAX

5.2.3.1.1 Identification of the EPC by the WiMAX access network

With WiMAX as a non-3GPP access network, the WiMAX NSP is mapped onto the EPC network operator. The NSP indication can be provided to the UE in accordance to WiMAX Forum Network Architecture Release 1.0 version 1.2 [25]. The WiMAX access network should advertise the NSP identity of the EPC in the MCC, MNC format.

5.2.3.1.2 EPC network selection

5.2.3.1.2.1 UE selection modes

There are two modes of network selection, namely, manual network selection and automatic network selection. The UE shall follow one of the following two procedures depending on its operating mode.

5.2.3.1.2.2 Manual EPC network selection

The manual network selection for WiMAX access shall follow the WiMAX Forum Network Architecture Release 1.0 version 1.2 – Stage 3 [25] with the following exceptions and additions:

– When presenting the list of available networks for user selection, the UE shall provide the network name of the related MCC + MNC pair. If that is not possible, the UE shall provide the MCC + MNC pair; and

– If the UE is unable to register to the user selected NSP, further UE action is implementation dependent.

5.2.3.1.2.3 Automatic EPC network selection

The automatic network selection for WiMAX access shall follow the WiMAX Forum Network Architecture Release 1.0 version 1.2 – Stage 3 [25] without any exceptions or additions.

5.2.3.2 EPC network selection procedures for WLAN

5.2.3.2.1 UE selection modes

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

The UE follows one of the following two procedures defined in clause 5.2.3.2.2 and 5.2.3.2.3 depending on its implementation.

The service provider selected in accordance with these procedures determines the WLAN that is selected. When the selected WLAN is a trusted WLAN IP access and the UE decides to access EPC via S2a using trusted WLAN IP access, the UE shall derive a NAI from the identity of the selected service provider and use the NAI as the identity for authentication and authorization with the service provider and usage of the WLAN (see clause 6.4).

5.2.3.2.1A Service provider solicitation

The UE shall determine which service providers are available from the available list of WLANs as constructed using the WLAN selection procedure described in clause 5.1.3.2.3 using following procedures:

i) the UE selects a WLAN from the list of selected WLAN(s) constructed using the WLAN selection procedure described in clause 5.1.3.2.3;

ii) if the WLAN selected in step i):

a) supports ANQP specified in IEEE Std 802.11 [57] and if the UE did not obtain a list of realms using ANQP in clause 5.1.3.2.3.3 item 1, the UE sends 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 1: The UE uses procedures defined in IEEE Std 802.11 [57] to determine if the WLAN supports ANQP and to send the ANQP query request for ANQP-elements "NAI Realm" and/or "3GPP Cellular Network", as specified in IEEE Std 802.11 [57].

b) does not support ANQP (see IEEE Std 802.11 [57]) or the UE does not receive a list of realms in item a), an EAP-Request/Identity is received and the EAP-request/Identity does not include one or more of realms and/or PLMN identities of service providers (encoded in accordance with IETF RFC 4284 [60]), the UE supports IEEE 802.1x authentication (see IEEE Std 802.1X™-2010 [61]), the UE shall request a list of realms and/or PLMN identities of service providers interworking with that WLAN by sending the EAP-Response/Identity message including as identity the alternative NAI; and

iii) the UE repeats this procedure for all WLANs from the available list of WLANs as constructed using the WLAN selection procedure described in clause 5.1.3.2.3.

NOTE 2: The list with realms and/or PLMN identities of service providers received in accordance with procedures in IETF RFC 4284 [60], is of limited size and might not contain all the realms and/or PLMN identities of service providers available via the WLAN.

The UE shall convert any received PLMN identities into PLMN realms using the rules defined in 3GPP TS 23.003 [3].

5.2.3.2.2 Manual Service Provider selection mode procedure

The UE indicates to the user the service providers which are available for WLAN. The UE may obtain the service providers available for WLAN using procedures as described in clause 5.2.3.2.1A. The UE will select the service provider based on the user preference.

5.2.3.2.3 Automatic mode service provider selection procedure

The purpose of this procedure is to:

– select a service provider over WLAN; and

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

If the RAN rules control the WLAN access selection and traffic routing as described in clause 6.10.2:

– if the RPLMN or an equivalent PLMN (see 3GPP TS 24.301 [10] or 3GPP TS 24.008 [46]) is available as described in clause 5.2.3.2.1A via a WLAN from the selected WLAN(s) constructed using the WLAN selection procedure described in clause 5.1.3.2.3, the highest priority service provider is the RPLMN or an equivalent PLMN (see 3GPP TS 24.301 [10] or 3GPP TS 24.008 [46]);

– if the RPLMN and an equivalent PLMN (see 3GPP TS 24.301 [10] or 3GPP TS 24.008 [46]) are not available as described in clause 5.2.3.2.1A via a WLAN from the selected WLAN(s) constructed using the WLAN selection procedure described in clause 5.1.3.2.3 and Home PLMN or an EHPLMN is available, the highest priority service provider is Home PLMN or an EHPLMN; and

– if the RPLMN, an equivalent PLMN (see 3GPP TS 24.301 [10] or 3GPP TS 24.008 [46]), Home PLMN and an EHPLMN are not available as described in clause 52.3.2.1A via a WLAN from the selected WLAN(s) constructed using the WLAN selection procedure described in clause 5.1.3.2.3, the highest priority service provider is a PLMN selected in an implementation-dependent way.

If the ANDSF rules control the WLAN access selection and traffic routing as described in clause 6.10.2, a service provider is the highest priority service provider if the service provider is available via a WLAN from the selected WLAN(s) constructed using the WLAN selection procedure described in clause 5.1.3.2.3 and if:

i) the service provider is selected in item 3; or

ii) the conditions in item 3 are not met, and:

– the service provider is the HPLMN;

– the service provider is an equivalent home service provider (i.e. the service provider’s realm matches a realm in the EquivalentHomeSPs as specified in 3GPP TS 24.312 [13]); or

– no WLAN of the selected WLAN(s) provides access to a higher priority service provider.

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

1) Void

2) Using the service providers which are available for WLAN as described in clause 5.2.3.2.1A, the UE uses the PLMN realms as the service provider realms in the remaining steps of this clause.

3) If the following conditions are fulfilled:

– the "3GPP RPLMN preferred" indicator is configured to prioritize 3GPP RPLMN; and

– the realm of the RPLMN or the realm of an equivalent visited service provider included in the EquivalentVisitedSPs as specified in 3GPP TS 24.312 [13] is included in the list of realms created in clause 5.2.3.2.1A, step ii);

then the UE shall select the RPLMN or the equivalent visited service provider. The RPLMN shall be selected with higher priority than the equivalent visited service provider. If the RPLMN is selected, the UE shall convert the RPLMN identity into selected PLMN realm using the rules defined in 3GPP TS 23.003 [3] and use it as the service provider realms in the remaining steps of this clause.

4) if the condition in step 3) is not satisfied, the UE shall select a service provider in the following order:

i) HPLMN matching a realm in the list of realms received in step ii) as described in clause 5.2.3.2.1A;

ii) realm found both in the list of realms received in step ii) as described in clause 5.2.3.2.1A and in the EquivalentHomeSPs as specified in 3GPP TS 24.312 [13]; and

iii) realm found both in the list of realms received in step ii) as described in clause 5.2.3.2.1A and in the PSPL as specified in 3GPP TS 24.312 [13] with the priority higher than any other service provider’s priority indicated as available via the WLAN.

If a UE used the procedures in IETF RFC 4284 [60] (see clause 5.2.3.2.1A) to obtain a list of service providers, then the UE is only required to select the HPLMN (if available) or an available equivalent home service provider.

NOTE 1: A UE using procedures in IETF RFC 4284 [60] to obtain a list of service providers is only required to select the HPLMN (if available) or an available equivalent home service provider. If the UE selects another service provider, the UE could be roaming even though the HPLMN or equivalent home service provider is available at the access point.

The UE shall select the WLAN providing access to the highest priority service provider.

If a highest priority service provider could not be determined, the UE proceeds in implementation-dependent way.

The UE shall construct a NAI for authentication with the highest priority service provider as described in 3GPP TS 23.003 [3]. Specifically, the UE constructs the:

a) root NAI corresponding to the HPLMN, if the highest priority service provider is the HPLMN advertised using a PLMN identity;

b) decorated NAI with double decoration including the realm of the highest priority service provider and the realm of the RPLMN, if the highest priority service provider is an equivalent visited service provider; or

c) decorated NAI including the realm of the highest priority service provider, otherwise.

NOTE 2: UE implementation can optimize the steps described above, e.g. by combining the ANQP procedures described in clause 5.2.3.2.1A with the ANQP procedures in clause 5.1.3.2.3.3.

5.3 Access Network reselection

5.3.1 General

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

5.3.2 UE procedures

The UE may retrieve information from ANDSF, which includes available access network and operator’s policy as specified in clause 6.8.2.

The information which is retrieved from the ANDSF shall not impact the PLMN selection and reselection procedures specified in 3GPP TS 23.122 [4]. For WLAN access, the UE configured with a WLANSP rule specified in 3GPP TS 24.312 [13], shall use the access network selection procedure and a PLMN selection procedure defined in this document which are different from and shall not be used in conjunction with the procedures for I-WLAN access specified in 3GPP TS 24.234 [9].

The network reselection procedure can be in automatic mode or manual mode dependent on UE configuration settings. For WiMAX access, the manual mode reselection shall follow the behaviour described in clause 5.2.3.1.2.2 and the automatic mode reselection shall follow the behaviour described in clause 5.2.3.1.2.3.

If the RAN rules control the WLAN access selection and traffic routing as described in clause 6.10.2, if the UE receives move-traffic-to-WLAN indication, along with the list of the WLAN identifiers as described in clause 6.10.4, the UE shall perform the procedure in clause 6.10.4.

5.3.3 EPC procedures

The ANDSF shall send available access network(s) and operator’s policy to the UE in response to the UE’s request or based on the network triggers as specified in clause 6.8.2.

5.3.4 Periodic EPC network reselection attempts

In automatic mode, when UE is not in its HPLMN or one of its equivalent HPLMNs, the UE shall make a periodic attempt to return to its HPLMN or one of its equivalent HPLMNs. For this purpose the timer value given in the EF_HPPLMN as defined in 3GPP TS 31.102 [45] shall be used with the following exceptions:-

– For UE accessing the EPC via cdma2000^® HRPD access networks, the UE’s search for a more preferred system shall abide by the parameters and procedures defined in 3GPP2 C.S0016 [23a].

– For UE accessing the EPC via WiMAX access networks, the time period between periodic network searches is implementation specific.

– For UE accessing the EPC via any other non-3GPP access networks, unless the UE has availability to EF_HPPLMN, the time period between periodic network searches is implementation specific but shall not be less than 30 minutes.