ATSSS support in 5g
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.
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| 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.
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