ATSSS support in 5g

ATSSS support in 5g

  November 15, 2020    tls123

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Coexistence with Non-3GPP systems: ATSSS

The ATSSS feature enables a multi-access PDU Connectivity Service, which can exchange PDUs between the UE and a data network by simultaneously using one 3GPP access network and one non-3GPP access network and two independent N3/N9 tunnels between the PSA and RAN/AN. The multi-access PDU Connectivity Service is realized by establishing a Multi-Access PDU (MA PDU) Session, i.e. a PDU Session that may have user-plane resources on two access networks, as shown on the figure below, extracted from TR 23.793. 

MA PDU session

These following procedures are defined in the context of this Feature:

-   Access Traffic Steering: it selects an access network for a new data flow and transfers the traffic of this data flow over the selected access network. Access traffic steering is applicable between one 3GPP access and one non-3GPP access.

-  Access Traffic Switching: it moves all traffic of an ongoing data flow from one access network to another access network in a way that maintains the continuity of the data flow. Access traffic switching is applicable between one 3GPP access and one non-3GPP access.

-   Access Traffic Splitting: it splits the traffic of a data flow across multiple access networks. When traffic splitting is applied to a data flow, some traffic of the data flow is transferred via one access and some other traffic of the same data flow is transferred via another access. Access traffic splitting is applicable between one 3GPP access and one non-3GPP access.

Key concepts of ATSSS supported in Release 16 include the following:

-   Multi-access PDU Session is a PDU Session that provides a PDU connectivity service, which can use one access network at a time, or simultaneously one 3GPP access network and one non-3GPP access network and two independent N3/N9 tunnels between the PSA and RAN/AN.

-    After the establishment of a MA PDU Session:

-    When there are user-plane resources on both access networks:

-    The UE applies network-provided policy (i.e. ATSSS rules derived by UE’s serving SMF based on ATSSS policy from serving PCF) and considers local conditions (such as network interface availability, signal loss conditions, user preferences, etc.) for deciding how to distribute the uplink traffic across the two access networks.

-    Similarly, the UPF anchor of the MA PDU Session applies network-provided policy (i.e. N4 rules derived by UE’s serving SMF based on ATSSS policy from serving PCF) and the feedback information received from the UE via the user-plane (such as access network Unavailability or Availability), the UPF then decides how to distribute the downlink traffic across the two N3/N9 tunnels and two access networks.

-    When there are user-plane resources on only one access network, the UE applies the ATSSS rules and considers local conditions for triggering the establishment or activation of the user plane resources over another access.

-  The type of a MA PDU Session may be one of the following types: i.e. IPv4, IPv6, IPv4v6, and Ethernet. The Unstructured type is not supported in Release 16.

-  The ATSSS feature can be supported over 3GPP and non-3GPP accesses, including untrusted and trusted non-3GPP access networks, wireline 5G access networks, etc., as long as a MA PDU Session can be established over the given type of access network

-     Two ATSSS steering functionalities are supported:

-     MPTCP functionality, for TCP traffic, with MPTCP proxy in UPF, by using the MPTCP protocol over the 3GPP and/or the non-3GPP user plane; and

-     ATSSS-LL functionality for all types of traffic, including TCP traffic, UDP traffic, Ethernet traffic, etc. ATSSS-LL functionality is mandatory for MA PDU Session of type Ethernet.

The following presents the example of the ATSSS traffic steering functionality within the UE.

Steering functionalities in an example UE model

-  The Performance Measurement Function (PMF) is supported by UPF and is specific for ATSSS-LL functionality, if enabled.  In Release 16, PMF supports two types of measurements between UE and UPF to assist access selection and they are:

-     UE and UPF make RTT measurements per access when the "Smallest Delay" steering mode is used; and

-     UE reports access availability/unavailability to UPF

The following presents the protocol stacks of the PMF for the user plane measurements over 3GPP and non-3GPP accesses respectively.

 

UE/UPF measurements related protocol stack for 3GPP access and for an MA PDU Session with type IP

In the case of an MA PDU Session with type Ethernet, the protocol stack over 3GPP access is that same as the one in the above figure, but the PMF protocol operates on top of Ethernet, instead of UDP/IP.

 

UE/UPF measurements related protocol stack for non-3GPP access and for an MA PDU Session with type IP

In the case of an MA PDU Session with type Ethernet, the protocol stack over non-3GPP access is that same as the one in the above figure, but the PMF protocol operates on top of Ethernet, instead of UDP/IP.

-   An ATSSS-capable UE may decide to request a MA PDU Session based on the provisioned URSP rules. In particular, the UE should request a MA PDU Session when the UE applies a URSP rule, which triggers the UE to establish a new PDU Session and the Access Type Preference component of the URSP rule indicates "Multi-Access".

-  The 5G QoS model for the Single-Access PDU Session is also applied to the MA PDU Session, i.e. the QoS Flow is the finest granularity of QoS differentiation in the MA PDU Session. One difference compared to the Single-Access PDU Session is that in a MA PDU Session there can be separate user-plane tunnels between the AN and the PSA, each one associated with a different access. The SMF shall provide the same QFI in 3GPP and non-3GPP accesses so that the same QoS is supported in both accesses. Non GBR QoS Flow can be distributed over 3GPP access and non 3GPP access, but GBR QoS Flow is transferred over single access.

- ATSSS is currently not supported when moving to EPC from 5GC, except for the specific case with wireline access integrated to EPC/5GC with 5G-RG; ATSSS with one User Plane leg in E-UTRA/EPC and one User Plane leg in wireline/5GC is supported.

Coexistence with Non-3GPP systems: Wireless and Wireline Convergence Enhancement

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Wireless and Wireline Convergence Enhancement

Wireless and Wireline Convergence Enhancement

  November 15, 2020    tls123

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Support of wireline access network

Support of Trusted Access network

Radio aspects 

Charging aspects

Coexistence with Non-3GPP systems: ATSSS

Support of wireline access network

The architecture for non-roaming is shown in figure 1, where the Wireline Access Gateway Function (W-AGF) is the access node performing the termination of N2 and N3 reference point, termination of access network interface Y4 and all access network specify functionalities, the relay of N1 to/from the UE., QoS enforcement, etc. The customer device, the UE, is replaced by the Residential Gateway which is augmented to support the 5G functionalities required to connect to 5G systems, such as NAS, URSP, PDU session, etc, called 5G-RG. The specification in TS 23.316 defines the modification to system architecture, procedure and flows, Policy and Charging Control for the 5G System in TS 23.501, TS 23.502 and TS 23.503.

The 5G-RG can also be connected via 3GPP Access basically by means of supporting the specification defined for UE. This scenario is called Fixed Wireless Access (FWA). Furthermore the 5G-RG may simultaneously connect to 3GPP Access and to wireline access by using the Single Access PDU session or supporting ATSSS feature. This scenario is called Hybrid scenario, using a terminology common on wireline access network. The ATSSS is supported as specified in TS 23.501, 23.502 and TS 23.503 where UE is replaced by 5G-RG and the Non-3GPP access is specifically referred to wireline access. In this latter case, TS 23.316 has also specified the support of interworking with EPC via 3GPP Access via a MA PDU session with a PDN Connection as user-plane resource associated with a MA PDU Session.

The support of legacy Residential Gateway not supporting 5G capability (FN-RG) is supported via W-AGF terminating the N1 NAS on behalf of UE and acting as a UE in respect the 5G core. 

In the case of Wireline Access Network defined in Broadband Forum the W-AGF functionalities is specified in BBF TR-470, BBF TR-456 and BBF TR-457, the 5G-RG is defined in BBF TR-124issue6 [8]. In the case of Wireline Access network defined in Cablelabs the W-AGF and 5G-RG functionalities are defined in CableLabs WR-TR-5WWC-ARCH.

Main impacts on the system by the WWC for wireline support are the following:

-    W-AGF: the access network function which performs the termination of N2 and N3 reference point, termination of access network interface Y4 and all access network specify functionalities, the relay of N1 to/from the UE. QoS enforcement, etc. When the W-AGF facing the FN-RG the W-AGF is supporting the termination of N1 NAS and performs the interworking between 5GC and the legacy wireline access network.

-    5G-RG: end user device replacing the UE which supports 5G capabilities (NAS protocol and procedure, USRP, IMSI, ATSSS) and extension of wireline access layer specific functionalities defined by Broadband forum and CableLabs. The 5G-RG may also support UE capability when connects via 3GPP Access.

-    FN-RG: end user device replacing the UE which does not support 5G capabilities.

-    Global Line Identifier (GLI): in case of wireline access based on BBF specifications this parameter uniquely identifies the line at which the 5G-RG in connected to within an operator domain.

-    Global Cable identifier (GCI): in case of wireline access based on CableLabs specification this parameter uniquely identifies the line at which the 5G-RG in connected to within an operator domain.

-    SUPI for FN-RG based on GCI and GLI.

-    All procedures defined in TS 23.502 have been modified to introduce the new network elements. The procedures are focused mainly on the part of specification that required improvements and to point out the access network interaction involving the W-AGF, 5G-RG and FN-RG to allow the Broadband Forum and CableLabs to develop the specifications under their responsibility.

-    IPTV support: The specification TS 23.316 in clauses 4.9.1 and 7.7.1 defines the support of IPTV via the support of multicast over unicast PDU session by using IGMP/MLD message send by STB via 5G-RG on PDU session and managed by UPF for adding the requiring 5G-RG to a multicast group and replicating the traffic received on N6 interface to the PDU session. The SMF is improved to control the support of IPTV by the UPF acting as PSA using PDR, FAR, QER, URR. This includes control of which IGMP and MLD requests the UPF is to accept or to deny.

-    QoS: the QoS model for wireline network is based on a subscription maximum aggregate bitrate including both GBR and Non-GBR traffic, hence the new parameter RG Total Maximum Bit Rate (RG-TMBR) has been defined. The RG-TMBR limits the aggregate bit rate that can be expected to be provided across all GBR and Non-GBR QoS Flows of a RG. The RG-TMBR is a parameter provided to the W-AGF by the AMF based on the value of the Subscribed RG-TMBR retrieved from UDM. The QoS control on wireline access network (i.e scheduling, rate limiting and traffic class management) is based on the line characteristic included in user subscription, for example different priority of service, different traffic class support by line of the single user, etc, for such reason the new parameter RG Level Wireline Access Characteristics (RG-LWAC) has been introduced. The format and content of RG LWAC is specified by BBF and it is transparently provided by UDM to AMF which may provide to the W-AGF at the time of the RG registration

-    mobility restriction based on GLI and GCI

-    support of BBF interaction with the Access Configuration System (ACS) to support the provisioning of configuration and remote management of 5G-RG as described in BBF TR-069 [12] or in BBF TR-369.

 Non- roaming architecture for 5G Core Network for 5G-RG with Wireline 5G Access network and NG RAN

Non- roaming architecture for 5G Core Network for FN-RG with Wireline 5G Access network and NG RAN

Support of Trusted Access network

The support of Trusted Network addresses the scenario where the Non-3GPP access network has a tighter relationship with 5GC in respect the untrusted scenario. However how the network is considered Trusted or Untrusted is not in the scope of this WID. The architecture for non-roaming is shown in figure 3, where the Trusted Non-3GPP Access Network (TNAN) is the access node performing the termination of N2 and N3 reference point, termination of access network interface, relay of N1 to/from the UE. From 3GPP point of view the TNAN network is composed by the TNGF and the Trusted Non-3GPP Access Point (TNAP) which are interconnected via the reference point Ta. However the detailed definition of TNAN and of Ta is beyond the WID scope. The reference point between the UE and the TNG, the NWt, is specified leveraging the IKEv2 defined for Untrusted. The main difference in contrast to Untrusted is in registration procedure, where it is assumed that EAP-5G can be carried between UE and TNAP directly on access layers, such on IEEE 802.11x and between TNAP and TNGF via Ta and not as part of IKEv2 establishment.  From other the point of view of other procedures, such as session management, the same procedure specified for Untrusted Non-3GPP access network can be used with basically the TNGF replacing the N3IWF, and modification that IKEv2 Child SA establishment is requested by TNGF and not by UE side.

Within the context of Trusted Non-3GP network, also the scenario of devices not supporting NAS connected via WLAN is specified. The role of TNGF is replaced the Trusted WLAN Interworking Function (TWIF) with the main difference that TWIF terminates the N1 NAS interface and it play the role of UE in respect the 5GC.

The specification is addressed in TS 23.501, TS 23.502 and TS 23.503

Non-roaming architecture for 5G Core Network with trusted non-3GPP access

Radio aspects 

The objective includes:

•  The description and enhancement of NG protocols to support the interface between the Trusted Non-3GPP Access Network and the 5GC;

•   The description and enhancement of NG protocols to support the interface between the Wireline 5G Access Network and the 5GC.

- from TS 29.413 and TS 38.413.

General radio aspects

-    Introduce the Trusted Non-3GPP Gateway Function (TNGF), Trusted WLAN Interworking Function (TWIF) to support the Trusted Non-3GPP Access, and Wireline Access Gateway Function (W-AGF) to support Wireline Access in TS 29.413 and TS 38.413.

-    Add the Global TNGF ID in the applicable NGAP messages between the TNGF and the AMF; add the Global TWIF ID in the applicable NGAP messages between the TWIF and the AMF; add the Global W-AGF ID in the applicable NGAP messages between the W-AGF and the AMF.

-    Add the selected PLMN Identity for trusted non-3GPP access and wireline access in Initial UE Message for Key derivation.

-    Add procedural texts that the Security Key IE may include KTNGF, or KTWIF, or KWAGF in TS 29.413.

 

Supporting the Trusted Non-3GPP Access with the 5GC – specific aspects

-    Add TNGF Identity Information, TWIF Identity Information in the UPLINK NAS TRANSPORT message containing a list of identifiers of NG-U terminations at TNGF/TWIF for UPF selection.

-    Add TNGF related and TWIF related User Location Information in the User Location Information IE.

 

Supporting the Wireline Access connectivity with the 5GC – specific aspects

-    Add W-AGF Identity Information in the UPLINK NAS TRANSPORT message containing a list of identifiers of NG-U terminations at W-AGF for UPF selection.

-    Add W-AGF related User Location Information in the User Location Information IE.

-    Add procedural texts to clarify the UE-AMBR is not used for wireline access in TS 29.413.

-    Add RG Level Wireline Access Characteristics in INITIAL CONTEXT SETUP REQUEST messages stored in the UE context by the W-AGF, indicating the wireline access technology specific QoS information corresponding to a specific wireline access subscription.

-    Add the Authenticated Indication in INITIAL UE MESSAGE to indicate that the FN-RG has been authenticated by the wireline 5G access network.

Charging aspects

The Wireless and Wireline Convergence for 5G system architecture (5WWC)is specified in TS 23.501, TS 23.502, TS 23.503 and TS 23.316. The enhancement to charging aspect for 5WWC is considered as part of this series specifications for this 5WWC.

Following charging scenarios are included in charging aspect of 5WWC as following:

-    UE Connects to 5G Core via Trusted Non-3GPP access

-    5G-RG connects to 5G Core via NR-RAN and via W-5GAN

-    FN-RG connects via W-5GAN.

The specifications related to 5WWC charging include TS 32.255, TS 32.291 and TS 32.298. The subscriber’s identifiers and PEI in 5G-RG and FN RG scenarios specified in TS 23.501 and TS 23.361 are used in charging information. The procedures and related triggers in 5WWC charging scenarios are also specified in charging aspect for 5WWC. The related changes to OpenAPI are specified in TS 32.291.

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