8.4 GAN Discovery and Registration related procedures

3GPP43.318Generic Access Network (GAN)Release 17Stage 2TS

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

8.4.1 Discovery and Registration for Generic Access

8.4.1.1 General

The Discovery and Registration procedures are applicable only if the MS preference is operating in GAN-only, GAN-preferred or, if no allowable PLMN is available through GERAN/UTRAN/E-UTRAN, in GERAN/UTRAN/E-UTRAN-preferred mode.

Once the MS has established a connection to the generic IP access network, the mobile determines the appropriate GANC-SEGW to connect to, by completing the Discovery Procedure to the Provisioning GANC in the HPLMN of the MS. The Provisioning GANC provides the address of the Default GANC in the HPLMN of the MS, to which the mobile can register.

The MS attempts to register on the Default GANC provided by the Provisioning GANC during the Discovery procedure, by completing the Registration Procedure. The Default GANC may accept the Registration; redirect the MS to another GANC; or Reject the Registration.

8.4.1.2 Security Gateway Identification

The (U)SIM of the MS contains the FQDN (or IP address) of the Provisioning GANC and the associated SEGW or the MS derives this information based on information in the (U)SIM. If the MS does not have any information about other GANCs and associated SEGW stored, then the MS completes the Discovery procedure towards the Provisioning GANC.

As part of the Registration Procedure, the Default GANC can indicate whether this GANC and SEGW address or the address of a GANC that the MS is being redirected to, may be stored by the MS.

The MS may also store Serving GANC information for Serving GANCs with which the MS was able to complete a successful registration procedure. The default GANC is in control of whether the MS is allowed to store Serving GANC information. If there is no GERAN/UTRAN/E-UTRAN coverage in the AP location, the stored Serving GANC information shall be associated with the AP-ID. If there is GERAN/UTRAN/E-UTRAN coverage in the AP location, the stored Serving GANC information shall be associated with the GSM CGI, UTRAN CI or E-UTRAN CI and the LAI or TAI. The stored Serving GANC information is:

– serving SEGW FQDN or IP address following successful registration;

– serving GANC FQDN or IP address following successful registration; and

– optionally, Serving GANC TCP port following successful registration and if returned from the network.

The number of such entries to be stored in the MS is implementation specific. Only the last successfully registered GANC association shall be stored when the Default GANC indicates that the MS is allowed to store these addresses. An MS may preferentially join a generic IP access network point of attachment whose association with a Serving GANC has been stored in memory.

On connecting to the generic IP access network, if the MS has a stored Serving GANC for the AP-ID or the GERAN/UTRAN/E-UTRAN cell, the MS shall attempt to register with the associated Serving GANC in its memory. The GANC may still reject the MS for any reason even though it may have served the MS before. The MS shall delete from its stored list the address of the Serving GANC on receiving a registration reject or if the registration fails for any other reason (e.g. not receiving any response).

If the MS does not receive a response to the Registration Request sent to the Serving GANC (and which is not the Default GANC), it shall re-attempt to register with the Default GANC. If the MS does not receive a response to the registration request sent to the Default GANC, it shall attempt the discovery procedure with the Provisioning GANC in order to obtain a new Default GANC.

In the case when a MS is attempting to register or discover a GANC after failing to register on a GANC, the MS provides in the Registration or Discovery procedure an indication that the MS has attempted to Register on another GANC, the failure reason, and the GANC and SEGW addresses of the failed registration.

When the MS connects to a generic IP access network, for which it does not have a stored Serving GANC in it’s memory, it shall attempt to register with the Default GANC.

8.4.1.3 GANC capabilities

To be populated with GANC specific information that needs to be transferred to the MS on successful registration.

8.4.1.4 MS capabilities

To be populated with GAN specific capabilities of the MS that needs to be transferred to the GANC during registration and the interaction to what is reported to the CN is FFS.

8.4.1.4a Required GAN Services

The MS may request which GAN services it requires from the GANC as part of the Registration procedures.

8.4.1.4b GAN Mode Selection

The MS (i.e., with GAN Iu mode support) transfers its GAN Mode Support information to the GANC during Discovery and Registration procedures; i.e., in the GAN Classmark IE. GAN Mode Support options are GAN A/Gb mode supported, GAN Iu mode supported, or both modes supported. If no GAN Mode Support information is received, the GANC assumes that the MS supports GAN A/Gb mode operation only.

The provisioning GANC may use the received GAN Mode Support information to assign the MS to an appropriate default GANC (e.g., if separate GAN A/Gb mode and GAN Iu mode GANCs are deployed in the network) or to an appropriate TCP port on the default GANC (e.g., if separate TCP ports are used for GAN A/Gb mode and GAN Iu mode service). During registration, the GAN Iu mode capable GANC shall also indicate the GAN mode to use for the current session in the GAN Mode Indicator IE; this allows the MS to determine the GAN Iu mode capability of the Home PLMN.

The following table enumerates the discovery handling for the various combinations of MS and Home PLMN GAN mode capabilities.

Table 1: GAN Mode Selection procedures associated with GAN Discovery

	Home PLMN GAN Mode Capabilities
MS GAN Mode Capabilities	A/Gb only	Iu only	Both
A/Gb only	GANC: Handle as normal GAN A/Gb mode discovery MS: Proceed with registration	GANC: No GAN Mode Support information provided or GAN A/Gb mode (only) indicated by MS, therefore Reject (Unspecified) MS: Retry on next power-on	GANC: No GAN Mode Support information provided or GAN A/Gb mode (only) indicated by MS, therefore handle as normal GAN A/Gb mode discovery. Assign MS to GAN A/Gb-capable GANC. MS: Proceed with registration
Iu only	GANC: Handle as normal GAN A/Gb mode discovery MS: Proceed with registration	GANC: GAN Iu Mode Support (only) indicated by MS, therefore accept and assign MS to GAN Iu mode capable GANC. MS: Proceed with registration	GANC: GAN Iu Mode Support (only) indicated by MS, therefore accept and assign MS to GAN Iu mode capable GANC. MS: Proceed with registration
Both	GANC: Handle as normal GAN A/Gb discovery MS: Proceed with registration	GANC: Support for both modes indicated by MS, therefore accept and assign MS to GAN Iu mode capable GANC. MS: Proceed with registration	GANC: Support for both modes indicated by MS, therefore accept and assign MS to GAN A/Gb mode capable GANC or GAN Iu mode capable GANC (Note 1). MS: Proceed with registration

Note 1: The GANC’s choice of GAN Iu mode versus GAN A/Gb mode may be based on other information received in the GAN discovery message from the MS, information stored in the GANC, and on operator policy.

The default or serving GANC may use the received GAN Mode Support information to redirect the MS to a different GANC or a different TCP port on the current GANC. The GAN Iu mode capable GANC shall also indicate the GAN mode to use for the current session in the GAN Mode Indicator IE.

The following table enumerates the registration handling for the various combinations of MS and Default/Serving GANC GAN mode capabilities.

Table 2: GAN Mode Selection procedures associated with GAN Registration

	Default/Serving GANC GAN Mode Capabilities
MS GAN Mode Capabilities	A/Gb only	Iu only	Both
A/Gb only	GANC: Handle as normal GAN A/Gb mode registration MS: Proceed per GAN A/Gb mode procedures	GANC: No GAN Mode Support information provided or GAN A/Gb mode (only) indicated by MS, therefore Reject (Invalid GANC) MS: Attempt registration with Default GANC or re-discovery (per GAN A/Gb mode procedures)	GANC: No GAN Mode Support information provided or GAN A/Gb mode (only) indicated by MS, therefore handle as normal GAN A/Gb mode registration. If required, redirect MS to GAN A/Gb mode capable GANC. MS: Proceed per GAN A/Gb mode procedures
Iu only	GANC: Handle as normal GAN A/Gb mode registration MS: No GAN Mode Selection provided by GANC, therefore Deregister and treat as register reject (Invalid GANC)	GANC: GAN Iu Mode Support (only) indicated by MS, therefore accept and send GAN Mode Indicator = GAN Iu mode MS: Proceed per GAN Iu mode procedures	GANC: GAN Iu Mode Support (only) indicated by MS, therefore accept and send GAN Mode Indicator = GAN Iu mode. MS: Proceed per GAN Iu mode procedures
Both	GANC: Handle as normal GAN A/Gb mode registration MS: No GAN Mode Selection provided by GANC, therefore proceed per GAN A/Gb mode procedures	GANC: Support for both modes indicated by MS, therefore accept and send GAN Mode Indicator = GAN Iu mode MS: Proceed per GAN Iu mode procedures	GANC: Support for both modes indicated by MS, therefore accept and send GAN Mode Indicator = GAN Iu mode or GAN A/Gb mode (Note 1). If required, redirect MS to GAN Iu mode or GAN A/Gb mode capable GANC. MS: Proceed per GAN Iu mode or GAN A/Gb mode procedures

Note 1: The GANC’s choice of GAN Iu mode versus GAN A/Gb mode may be based on other information received in the GAN registration message from the MS, information stored in the GANC, and on operator policy.

8.4.1.5 Discovery Procedure

8.4.1.5.1 Normal Case

When an MS supporting GAN first attempts to connect to a GAN, the MS needs to identify the Default GANC. Each GAN capable MS can be configured with the FQDN (or IP address) of the Provisioning GANC and the associated SEGW or the MS can derive this FQDN based on information in the (U)SIM (see 3GPP TS 23.003 [43]). The MS first connects to a Provisioning GANC-SEGW and GANC in the HPLMN of the MS, by establishing a secure IPsec tunnel and a TCP connection using the provisioned or derived addresses. The MS obtains the FQDN or IP address of the Default GANC in the HPLMN and the associated SEGW, through the Discovery procedure.

If no GERAN/UTRAN/E-UTRAN coverage is available when an MS connects to the GANC for GAN service, then the GANC cannot necessarily determine the location of the MS for the purposes of assigning the MS to the correct serving GANC (e.g., to enable handover and location-based services). The GANC shall permit the operator to determine the service policy in this case; e.g. the operator could provide service to the user with certain limitations (possibly with a user interface indication on the MS).

NOTE: When the MS initiates the Discovery/Registration procedures and no GERAN/UTRAN/E-UTRAN coverage is available, the GANC may have insufficient information to correctly route subsequent emergency calls.

Figure 10: Discovery procedure

In the description below it is assumed that the MS has a mode selection of GAN-only or GAN-preferred or GERAN/UTRAN/E-UTRAN-preferred and that the MS has already connected to the generic IP access network.

NOTE: It is implementation specific what signal level should be deemed as sufficient for triggering the GAN Discovery and Registration procedures.

1. If the MS has a provisioned or derived FQDN of the Provisioning SEGW, it performs a DNS query (via the generic IP access network interface) to resolve the FQDN to an IP address. If the MS has a provisioned IP address for the Provisioning SEGW, the DNS step is omitted.

2. The DNS Server returns a response including the IP Address of the Provisioning SEGW.

3. The MS establishes a secure tunnel to the Provisioning SEGW.

4. If the MS has a provisioned or derived FQDN of the Provisioning GANC, it performs a DNS query (via the secure tunnel) to resolve the FQDN to an IP address. If the MS has a provisioned IP address for the Provisioning GANC, the DNS step will be omitted.

5. The DNS Server returns a response including the IP Address of the Provisioning GANC.

6. The MS sets up a TCP connection to a well-defined port on the Provisioning GANC. It then queries the Provisioning GANC for the Default GANC, using GA-RC DISCOVERY REQUEST. The message contains:

– If the MS is camped on a GERAN/UTRAN/E-UTRAN cell, the current camping GERAN/UTRAN/E-UTRAN cell ID.

– The last LAI or TAI where the MS successfully registered.

– Generic IP access network attachment point information: AP-ID, as defined in annex C.

– MS Identity: IMSI.

– GAN Classmark: Including GAN Mode Support information indicating GAN A/Gb mode supported, GAN Iu mode supported or both modes supported.

7. The Provisioning GANC returns the GA-RC DISCOVERY ACCEPT message, using the information provided by the MS (e.g. the CGI), to provide the FQDN or IP address of the Default GANC and its associated Default SEGW. This is done so the MS is directed to a "local" Default GANC in the HPLMN to optimize network performance.

8. If the Provisioning GANC cannot accept the GA-RC DISCOVERY REQUEST message, it returns a GA-RC DISCOVERY REJECT message indicating the reject cause.

9. The secure IPsec tunnel to the Provisioning SEGW is released. It shall also be possible to reuse the same IPsec tunnel for GAN Registration procedures. In this case the IPsec tunnel is not released.

8.4.1.6 Registration procedure

8.4.1.6.1 Normal case

Following the Discovery procedure the MS establishes a secure tunnel with the secure gateway of the Default GANC, provided by the Provisioning GANC in the Discovery procedure, and attempts to register with the Default GANC. The Default GANC may become the Serving GANC for that connection by accepting the registration, or the Default GANC may redirect a MS performing registration to a different Serving GANC.

GANC redirection may be based on information provided by the MS during the Registration procedure, operator chosen policy or network load balancing.

The GAN Registration procedure serves the following functions:

– Ensures the MS is registered to the appropriate GANC entity i.e. with use of the redirection process;

– Informs the GANC that the MS is now connected through a generic IP access network and is available at a particular IP address. The GANC maintains the registration context for the purposes of (for example) mobile-terminated calling;

– Provides the MS with the operating parameters associated with the GAN service, including the GAN mode to use for the session, either A/Gb or Iu. The "System Information" message content that is applicable to the GAN cell is delivered to the MS during the GAN registration process. This enables the MS to switch to GAN mode, and following the Registration procedure trigger NAS procedures with the core network (such as Location/Routing Area Update, mobile originated calls, mobile terminated calls, etc.); and

– Enables the MS to request which GAN services are required.

NOTE: The GA-RC REGISTER REDIRECT message may contain: a single Serving SEGW and GANC address or a list of PLMN identities and associated Serving SEGW and GANC addresses; and an Indication of whether GANC address(es) can be stored in the MS for future use.

Figure 11: Registration procedure

1. If the MS was provided the FQDN of the Default or Serving SEGW, the MS shall perform a DNS query (via the generic IP access network interface) to resolve the FQDN to an IP address. If the MS has a provisioned IP address for the SEGW, the DNS step is omitted.

2. The DNS Server returns a response.

3. The MS shall then set up a secure IPsec tunnel to the SEGW. This step may be omitted if an IPsec tunnel is being reused from an earlier Discovery or Registration.

4. If the MS was provided the FQDN of the Default or Serving GANC, the MS shall then perform a DNS query (via the secure tunnel) to resolve the FQDN to an IP address. If the MS has an IP address for the GANC, the DNS step is omitted.

5. The DNS Server returns a response.

6. The MS then sets up a TCP connection to a TCP port on the GANC. The TCP port can either be a well-known port or one that has been earlier received from the network during Discovery or Registration. The MS shall attempt to register on the GANC by transmitting the GA-RC REGISTER REQUEST. The message includes:

– If the MS is camped on a GERAN/UTRAN/E-UTRAN cell, the MS includes the current camping GERAN/UTRAN/E-UTRAN cell ID.

– If the MS is camped on a UTRAN cell, the MS includes the UARFCN of the current serving cell.

– The last LAI or TAI where the MS successfully registered.

– Generic IP access network attachment point information: AP-ID, as defined in annex C.

– MS Identity: IMSI.

– MS Capability Information.

– GAN Services Required

– GAN Classmark: Including GAN Mode Support information indicating GAN A/Gb mode supported, GAN Iu mode supported or both modes supported, and an indication of whether or not the MS supports RFC 4867 [54] bandwidth-efficient mode.

7. If the GANC accepts the registration attempt it shall respond with a GA-RC REGISTER ACCEPT. The message contains:

– GAN specific system information (e.g.):

– GAN Mode Indicator: GAN A/Gb mode or GAN Iu mode.

– Cell description of the GAN cell:

– If GAN A/Gb mode selected: The BCCH ARFCN, PLMN colour code, and base-station colour code corresponding to the GAN cell.

– If GAN Iu mode selected: The UTRA ARFCN (UARFCN) and Primary Scrambling Code (PSC) corresponding to the GAN cell.

– Location-area identification comprising the mobile country code, mobile network code, and location area code corresponding to the GANC cell.

– Cell identity identifying the cell within the location area corresponding to the GAN cell.

– Applicable system timer values (e.g., for the application-level keep-alive message transmission interval, see clause 8.4.4).

– GAN Capability Information.

In this case the TCP connection and the secure IPsec tunnel are not released and are maintained as long as the MS is registered to this GANC.

8. Alternatively, the GANC may reject the request. In this case, it shall respond with a GA-RC REGISTER REJECT indicating the reject cause. The TCP connection and the secure IPsec tunnel are released and the MS shall act as defined in clause 8.4.1.3.2.

9. Alternatively, if the GANC wishes to redirect the MS to (another) Serving GANC, it shall respond with a GA-RC REGISTER REDIRECT providing the FQDN or IP address of the target Serving GANC and the associated SEGW. In this case the TCP connection is released and the secure IPsec tunnel is optionally released depending on if the network indicates that the same IPsec tunnel can be reused for the next registration.

8.4.1.6.2 Abnormal cases

If the Serving GANC rejects the Register request and does not provide redirection to another Serving GANC, the MS shall re-attempt Registration to the Default GANC including a cause indicating the failed registration attempt and the Serving GANC and SEGW with which the Register request failed. The MS should also delete all stored information about this Serving GANC.

If the Default GANC rejects a Registration Request and is unable to provide redirection to suitable Serving GANC, the MS may re-attempt the Discovery procedure to the Provisioning GANC (including a cause indicating the failed registration attempt and the Default GANC provided in the last Discovery procedure). The MS should also delete all stored information about the Default GANC.

8.4.2 De-Registration

The GA-RC De-Registration procedure allows the MS to explicitly inform the GANC that it is leaving GAN mode (e.g. when it detaches from the generic IP access network), by sending a GA-RC DEREGISTER message to the GANC, allowing the GANC to free resources that it assigned to the MS. The GANC also supports "implicit GAN deregistration", when the TCP connection to the MS is abruptly lost.

The GANC can also autonomously release the MS registration context, and send a GA-RC DEREGISTER message to the MS. Alternatively, the GANC can implicitly deregister the MS by closing the TCP connection with the MS.

NOTE: At power-down the GA-RC sublayer of the MS ensures that the MS explicitly detaches from the network, where possible, before completing the GA-RC De-Registration procedure.

Figure 12: De-Registration initiated by the MS

1. The MS sends the GA-RC DEREGISTER to the GANC, which removes the MS context in the GANC.

Figure 13: De-Registration initiated by the GANC

1. The GANC sends the GA-RC DEREGISTER to the MS.

8.4.3 Registration Update

The GA-RC Registration Update procedure allows the MS to update information in the GANC by sending a GA-RC REGISTER UPDATE UPLINK message to the GANC carrying the updated information. This message is sent as a result of the following:

– Detecting the availability of GERAN/UTRAN/E-UTRAN coverage after reporting no coverage during GAN registration.

– Changes to the identity of the serving GERAN/UTRAN/E-UTRAN cell while in GERAN/UTRAN/E-UTRAN mode.

– Changes to the UARFCN of the serving UTRAN cell while in UTRAN mode.

– Establishment of a new serving cell as a result of inter-RAT handover from GERAN to UTRAN or E-UTRAN to UTRAN.

– Changes to the generic IP access network point of attachment.

This may result in the MS being redirected to another serving GANC, or being denied service e.g. due to operator policy.

The GAN Registration Update procedure also allows the GANC to update the GAN system information in the MS, if needed, by sending a GA-RC REGISTER UPDATE DOWNLINK message to the MS carrying the updated information.

Figure 14: Registration Update Uplink

1. When the MS detects any of the conditions listed above, it shall send the GA-RC REGISTER UPDATE UPLINK to the GANC with the updated information. Whenever the generic IP access network point of attachment changes, the MS shall send a GA-RC REGISTER UPDATE UPLINK to the GANC with the updated generic IP access network point of attachment information. If the MS requires to update the GANC with a new list of GAN Services required, then the MS sends GA-RC REGISTER UPDATE UPLINK message to the GANC including the new GAN Services Required list.

2. The GANC may optionally send the GA-RC REGISTER REDIRECT when it wants to redirect the MS based on updated information.

3. The GANC may also optionally deregister the MS on receiving an update by sending GA-RC DEREGISTER to the MS.

Figure 15: Registration Update Downlink

1. The GANC sends GA-RC REGISTER UPDATE DOWNLINK with the updated system information.

8.4.4 Keep Alive

The Keep Alive process is a mechanism between the peer GA-RC entities to indicate that the MS is still registered to the GANC. Using periodic transmissions of the GA-RC KEEP ALIVE message the MS in turn determines that the GANC is still available using the currently established lower layer connection.

Figure 16: Keep Alive procedure

1. The MS sends GA-RC KEEP ALIVE to the GANC.

8.4.5 Cell Broadcast Information

The Cell Broadcast Information is a mechanism between the peer GA-RC entities, allowing the GANC to pass the MS information relating to the Cell Broadcast Services. The MS includes GAN Service Required information in the GA-RC REGISTER REQUEST and GA-RC REGISTER UPDATE UPLINK messages passed to the GANC, indicating that the MS requires the Cell Broadcast Service. The GANC then passes the required information to the MS in the GA-RC CELL BROADCAST INFO message.

Figure 16a: Cell Broadcast Information

1. The GANC sends the CELL BROADCAST INFO message to the MS, including information required by the MS.