4.7 NAS over non-3GPP access

24.5013GPPNon-Access-Stratum (NAS) protocol for 5G System (5GS)Release 17Stage 3TS

4.7.1 General

From the UE’s NAS perspective, in general the procedures and messages defined for 5GMM and 5GSM are used over non-3GPP access as over 3GPP access. However, a number of aspects are different as described in the following subclauses.

4.7.2 5GS mobility management aspects

4.7.2.1 General

The mobility management procedures defined over 3GPP access are re-used over non-3GPP access with the following exceptions:

a) the registration status, and the 5GMM parameters of the UE’s 3GPP access and non-3GPP access 5GMM state machine instances are independent in each of these accesses and can be different;

b) single-registration mode and dual-registration mode do not apply for 5GMM over non-3GPP access;

c) the RPLMN over non-3GPP access can be different from the RPLMN over 3GPP access. The MCC of the RPLMN over 3GPP access and the MCC of the RPLMN over the non-3GPP access can also be different;

d) the registration for 3GPP access and for non-3GPP access are performed separately. Like for 3GPP access, an access stratum connection exists before the UE can perform the registration procedure for non-3GPP access. As at registration over non-3GPP access the UE is allocated a registration area, which is associated with a single TAI, list management of registration areas is not required, and registration updating due to registration area change with the registered PLMN is not performed. Furthermore, the periodic registration update procedure is also not performed. New registration at change of PLMN is required;

e) the 5GMM over non-3GPP access in the UE considers that the N1 NAS signalling connection is established when the lower layers indicate that the access stratum connection is established succcessfully;

f) the UE-initiated service request procedure via non-3GPP access is supported. Upon indication from the lower layers of non-3GPP access, that the access stratum connection is established between the UE and the network, the UE in 5GMM-REGISTERED state and in 5GMM-IDLE mode over non-3GPP access shall initiate the service request procedure via non-3GPP access. The UE may indicate with the service request message the PDU session(s) associated with non-3GPP access to re-establish user-plane resources for which the UE has pending user data to be sent;

g) paging procedure is not performed via non-3GPP access;

h) service area restrictions do not apply for non-3GPP access other than the wireline access;

i) the establishment cause for non-3GPP access is determined according to subclause 4.7.2.2;

j) eCall inactivity procedure is not performed via non-3GPP access;

k) local area data network (LADN) does not apply for non-3GPP access;

l) the Allowed PDU session IE shall not be included in the REGISTRATION REQUEST message or the SERVICE REQUEST message sent over non-3GPP access;

m) DRX parameters do not apply for non-3GPP access;

n) Mobile initiated connection only mode (MICO) does not apply for non-3GPP access;

o) CIoT 5GS optimizations do not apply for non-3GPP access;

p) unified access control does not apply for non-3GPP access;

q) UE radio capability signalling optimisation (RACS) does not apply for non-3GPP access;

r) Closed access group (CAG) does not apply for non-3GPP access; and

s) the N1 NAS signalling connection release, paging indication for voice services and reject paging request do not apply for non-3GPP access. The Paging restriction IE shall not be included in the REGISTRATION REQUEST message, the SERVICE REQUEST message or the CONTROL PLANE SERVICE REQUEST message sent over non-3GPP access. The AMF shall not delete any stored paging restriction preferences for the UE and shall not stop restricting paging when receiving REGISTRATION REQUEST message, SERVICE REQUEST message or CONTROL PLANE SERVICE REQUEST message over non-3GPP access.

4.7.2.2 Establishment cause for non-3GPP access

When establishment of an N1 NAS signalling connection over non-3GPP access is initiated, the UE shall:

a) determine one or more access identities to be associated with the establishment of the N1 NAS signalling connection as specified in subclause 4.5.2 and table 4.5.2.1;

b) select the establishment cause for non-3GPP access from the determined one or more access identities and the event which triggered initiation of the N1 NAS signalling connection over non-3GPP access by checking the rules specified in table 4.7.2.2.1; and

c) provide the selected establishment cause for non-3GPP access to the lower layers.

While an MMTEL voice call is ongoing:

– any:

1) service request procedure; or

2) registration procedure;

initiated in 5GMM-IDLE mode is mapped to "MO MMTel voice call" type access attempt.

While an MMTEL video call is ongoing and no MMTEL voice call is ongoing:

– any:

1) service request procedure; or

2) registration procedure;

initiated in 5GMM-IDLE mode is mapped to "MO MMTel video call" type access attempt.

While an SMSoIP is ongoing, no MMTEL video call is ongoing and no MMTEL voice call is ongoing:

– any:

1) service request procedure; or

2) registration procedure;

initiated in 5GMM-IDLE mode is mapped to "MO SMS over IP" type access attempt.

If the access attempt matches more than one rule, the establishment cause for non-3GPP access of the lowest rule number shall be used.

Table 4.7.2.2.1: Mapping table for determination of establishment cause for non-3GPP access

Rule #

Access identities

Type of access attempt

Requirements to be met

Establishment cause for non-3GPP access

1

1

Any

Any

mps-PriorityAccess

2

2

Any

Any

mcs-PriorityAccess

3

11, 15

Any

Any

highPriorityAccess

4

12,13,14,

Any

Any

highPriorityAccess

5

0

Emergency

UE is attempting access for an emergency session (NOTE 1)

emergency

UE NAS initiated 5GMM specific procedures

Access attempt is for MO signalling

mo-Signalling

UE NAS initiated 5GMM connection management procedures or 5GMM NAS transport procedure

Access attempt is for MO data

mo-Data

MO SMS over NAS or MO SMS over IP

Access attempt is for MO SMS over NAS or MO SMS over IP

mo-SMS

MO MMTel voice call

Access attempt is for MO MMTel voice call

mo-VoiceCall

MO MMTel video call

Access attempt is for MO MMTel video call

mo-Videocall

NOTE 1: This includes 5GMM specific procedures while the service is ongoing and 5GMM connection management procedures required to establish a PDU session with request type = "initial emergency request" or "existing emergency PDU session", or to re-establish user-plane resources for such a PDU session.

NOTE 2: See subclause 4.5.2, table 4.5.2.1 for use of the access identities of 0, 1, 2, and 11-15.

4.7.3 5GS session management aspects

The session management procedures defined over 3GPP access are re-used over non-3GPP access with the following exceptions:

– Serving PLMN rate control does not apply for non-3GPP access.

– Small data rate control does not apply for non-3GPP access.

– Handling of 5GSM cause value #82 "maximum data rate per UE for user-plane integrity protection is too low" does not apply for non-3GPP access.

4.7.4 Limited service state over non-3GPP access

There are a number of situations in which the UE is unable to obtain normal service from a PLMN over non-3GPP access and the UE enters the limited service state over non-3GPP access. These include:

a) no USIM in the ME;

b) an "illegal UE" or "illegal ME" is received when registration, network-initiated de-registration or service request is performed (any USIM in the ME is then considered "invalid");

c) a "5GS services not allowed" is received when a registration, network-initiated de-registration or service request is performed;

d) a "PLMN not allowed" is received when registration, network-initiated de-registration or service request is performed;

e) a "Tracking area not allowed" is received when a registration, network-initiated de-registration or service request is performed;

f) a "Roaming not allowed in this tracking area" is received when a registration, network-initiated de-registration or service request is performed;

g) void; or

h) a "Serving network not authorized" is received when a registration or service request is performed.

In limited service state with a valid USIM in the UE, the network selection is performed as defined in 3GPP TS 24.502 [18].

With the exception of performing initial registration for emergency services, no registration requests are made until a valid USIM is present. For registration for emergency services, the PLMN of the current N3IWF or TNGF is considered as the selected PLMN for the duration the UE is registered for emergency services.

4.7.5 NAS signalling using trusted WLAN access network

A trusted WLAN interworking function (TWIF) provides functionalities for a non-5G capable over WLAN (N5CW) device to access 5GCN, including:

a) NAS signalling over N1 NAS signalling connection with AMF; and

b) PDU session establishment, modification and release on behalf of the N5CW device, over N2 connection with the AMF.

The TWIF registers on behalf of the N5CW device to an AMF according to subclause 5.5.1.3 by populating the parameters for the registration by using implementation specific default values which are the same for N5CW devices.

The TWIF may request to establish a PDU session as specified in subclause 6.4.1.2 on behalf of the N5CW device upon receipt of an IP configuration request from the N5CW device by populating either all the required parameters or part of the required parameters for the PDU session establishment by using implementation specific default values from the TWIF’s configuration. Only one PDU session is supported when N5CW device accessing 5GC via the TWIF.

NOTE 1: If part of the required parameters for the PDU session establishment is provided by the TWIF, the remaining of the required parameters are determined by the AMF or the SMF based on the N5CW device’s subscription information.

Upon loss of the IP address of the N5CW device, the TWIF acting on behalf of the N5CW device shall initiate the UE-requested PDU session release procedure as defined in subclause 6.4.3.

NOTE 2: The established PDU session on behalf of the N5CW device can be modified by the TWIF or the network.