This clause applies to the case where multi-access connectivity via both EPC and 5GC is supported in the 5G-RG and network. In this case, multi-access connectivity using ATSSS via both EPC and 5GC may be provided as described in this clause.
Co-existence with NBIFOM is not defined. It is assumed that NBIFOM and the multi-access connectivity described in this clause are not deployed in the same network.
The use of ATSSS with EPS interworking applies to IP-based PDU Session and PDN Connection types.
For this scenario, the general principles for ATSSS as described in TS 23.501, clause 5.32
apply, with the additions provided in this clause.
A Multi-Access PDU Session may be extended with user-plane resources via an associated PDN Connection on 3GPP access in EPC. This enables a scenario where a MA PDU Session can simultaneously be associated with user-plane resources on 3GPP access network connected to EPC and W-5GAN connected to 5GC. Such a PDN Connection in EPS would thus be associated with multi-access capability in 5G-RG and PGW-C+SMF.
To the MME and SGW this is a regular PDN Connection and the support for ATSSS is transparent to MME and SGW.
The 5G-RG may operate in either single-registration mode or dual-registration mode in 3GPP access. Irrespective of whether the 5G-RG operates in single-registration mode or dual-registration mode in 3GPP access, it is assumed that the 5G-RG supports simultaneous registrations for non-3GPP access in 5GC and 3GPP access in EPC.
The multi-access connectivity described in this clause supports simultaneous connectivity using 3GPP access via EPC and W-5GAN access via 5GC.
The ATSSS rules are provided from the PGW-C+SMF to the 5G-RG via SM NAS signalling over 5GC, as described in TS 23.501, clause 5.32.2
. ATSSS rules are not provided via the EPC.
After the establishment of a MA PDU Session and setting up user-plane resources in 3GPP access in EPC and non-3GPP access in 5GC, the 5G-RG distributes the uplink traffic across the two access networks as described in TS 23.501, clause 5.32.1
. Similarly, the PDU Session Anchor UPF performs distribution of downlink traffic across the two access networks as described in TS 23.501, clause 5.32.1
The PMF protocol may be used via any user plane connection, i.e. via 3GPP access in EPC or non-3GPP access in 5GC.
The PCF functionality to support ATSSS, as described in TS 23.501, clause 5.32.1
and TS 23.503
applies also in the case of interworking with EPC.
When the 5G-RG wants to request a new PDN Connection in EPC and wants to use this PDN Connection as user-plane resource associated with a MA PDU Session:
The 5G-RG requests establishment of a new PDN Connection when the 5G-RG is registered via 3GPP access in EPS using PDN Connection Establishment procedure. The 5G-RG provides via PCO to PGW-C+SMF the following information:
An indication that the PDN Connection is requested to be associated with a MA PDU Session
The 5G-RG's ATSSS capabilities as described in TS 23.501, clause 5.32.2 (i.e. whether the 5G-RG is capable of supporting the ATSSS-LL functionality, or the MPTCP functionality, or both)
The MME may select a PGW-C+SMF as described in TS 23.401 and TS 23.502, clause 4.11.0a.4.
The selection of PGW-C+SMF in the correct 5GC slice requires the same mapping between EPC and 5GC slices as required for single-access PDU sessions. In order to select an ATSSS capable PGW-C+SMF it is assumed that the operator deployment ensures that all PGW-C+SMF(s) configured to support the specific APN in this network slice are also capable to support ATSSS. There is however no assumption that all PGW-U+UPFs need to support ATSSS, since PGW-C+SMF can make a selection of PGW-U+UPF taking the multi-access properties into account.
The PGW-C+SMF determines based its capabilities whether the request can be accepted. The PCF decides whether the multi-access connectivity is allowed or not based on operator policy and subscription data, as described in TS 23.502, clause 4.22.2. The PGW-C+SMF provides the following information in the PCO to the 5G-RG:
An indication whether the request for using the PDN Connection for MA-PDU Session is accepted or not.
If the 5G-RG has indicated that it is capable of supporting the MPTCP functionality and the PGW-C+SMF accepts to activate the MPTCP functionality, then the network provides MPTCP proxy information to the 5G-RG, as described in TS 23.501, clause 5.32.2.
UE Measurement Assistance Information (as described in TS 23.501, clause 5.32.2).
After the PDN Connection establishment:
If the 5G-RG registers to 5GC and wants to add W-5GAN user-plane resources, then the 5G-RG shall send a PDU Session Establishment Request over this access containing a "MA PDU Request" indication as described in TS 23.501, clause 5.32.2.
Adding the PDU Session connectivity and user plane resources over W-5GAN in 5GS allows the PGW-C+SMF to provide ATSSS rules to the UE.
When the 5G-RG wants to request a new MA PDU Session in 5GC/W-5GAN, the description in TS 23.501, clause 5.32.2
, applies. After the MA PDU Session establishment in 5GS/W-5GAN, the description in TS 23.501, clause 5.32.2
, applies with the following additions:
If the 5G-RG is registered to EPC and wants to add user-plane resources on 3GPP access over EPC, then the 5G-RG shall send a PDN Connection Establishment Request over this access containing a "handover" indication and include a "MA PDU Request" indication in the PCO.
When the 5G-RG deregisters from the EPC access (but remains registered on the 5GC access), the MME will notify the PGW-C+SMF that the PDN Connection is released, as described in TS 23.401. The SMF can then notify the UPF that the access type has become unavailable.
A 5G-RG that has an established MA-PDU session over non-3GPP access in 5GC and 3GPP access in EPS, may be able to use EN-DC for the 3GPP access leg.
Depending on the RAT types supported by the 5G-RG, the PDN connection may also be handed over to 3GPP access in 5GC. For a 5G-RG supporting both E-UTRAN/EPC access and NG-RAN/5GC access, the user plane resources for 3GPP access may be moved between E-UTRAN/EPC access and NG-RAN/5GC access as described in TS 23.501, clause 5.17.2
. The PDU Session and User Plane resources active over W-5GAN are not affected by such inter 3GPP access RAT change.
The general principles for QoS support with ATSSS as described in TS 23.501, clause 5.32.4
, applies, with the clarifications provided in this clause.
With an MA PDU Session associated to a PDN Connection on EPS there may be separate user-plane tunnels between the AN and the PGW-U+UPF, one associated with 3GPP access in EPC and one associated with W-5GAN in 5GS.
As described in TS 23.502, clause 188.8.131.52
and Annex C, the PGW-C+SMF maps the 5G QoS information received from PCC to EPS QoS parameters. This mapping is e.g. based on operator configuration and may result in that multiple QoS flows are mapped to a single EPS bearer. The PGW-C+SMF applies the appropriate QoS signalling in each access, e.g. to manage dedicated bearers in the access associated with EPC and QoS flows in the access associated with 5GC. The PGW-C+SMF also provides N4 rules to UPF for performing QoS enforcement and for mapping downlink traffic to appropriate GTP-U tunnels.
As described in TS 23.501, clause 5.32.4
, for a GBR QoS flow, the QoS profile is provided to a single access network at a given time. GBR QoS flows (and associated MBR, GBR) are thus only enforced in either the access associated to EPC or the access associated to 5GC. In order to maintain consistency between QoS information received via AS and NAS layers in each system, the PGW-C+SMF only provides the GBR QoS information to the 5G-RG for the access where the GBR traffic is enforced.
The 5G-RG shall treat the uplink traffic sent via EPC according to the EPS QoS information received in EPC (e.g. UL TFTs) and the uplink traffic sent via 5GC according to the 5G QoS rules received in 5GS. The 5G-RG thus need to determine what access to use (3GPP and on-3GPP) before applying the uplink QoS treatment.
The UPF shall treat the downlink traffic according to the N4 rules (QER, etc.) received from PGW-C+SMF.