ATSSS (Access Traffic Steering, Switching and Splitting) is a new 5G functionality that allows traffic to be steered across multiple concurrent access networks, such as 3GPP, trusted non-3GPP, and untrusted non-3GPP, at a finer granularity than a single PDU session. It introduces the concept of multi-access PDU sessions. ATSSS is controlled by the 5G core network's PCF and can utilize higher layer MPTCP or lower layer steering methods to optimize traffic routing. Initial ATSSS deployments are expected to begin in 2021/2022 as 5G core networks and multi-access integration are implemented.

1. Introduction to ATSSS:
Access Traffic Steering, Switching and Splitting
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2. ATSSS
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• We touched on ATSSS briefly in 5G & FMC presentation – In this one we
will touch on some more details

3. Traditional Mobile Operation
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Voice (CS or PS)
Data

4. Data over Wi-Fi
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Voice (CS or PS)
Data

5. Data and Voice over Wi-Fi
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Voice (VoWiFi – PS only)
Data
C
PS voice is just IP Packet Data

6. 5G Enables Multiple Data Paths
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Data
C
Data
Data

7. Voice & Data are Independent of each other
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Data
C
Data
Data
VoNR (PS only)

8. ATSSS: Access Traffic Steering, Switching and Splitting
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• New Access Traffic Steering, Switching and Splitting (ATSSS)
functionality has been designed in Release 16, which allows traffic
steering across multiple accesses at a finer granularities than a PDU
session.
• The ATSSS feature is an optional feature in Release 16 both on the UE
and the 5GC network
• ATSSS introduces the notion of Multi Access PDU session, a PDU
session for which the data traffic can be served over one or more
concurrent accesses (3GPP access, trusted non-3GPP access and
untrusted non-3GPP access).

9. ©3G4G
ORSteering = = Best Network
Selection
Switching
Splitting
=
=
TO /
FROM
AND
= Seamless
Handover
= Network
Aggregation
Improved
End-User
Experience

10. Important Points about ATSSS #1
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• ATSSS needs 5G Core (5GC)
• ATSSS allows the service provider (SP) to configure ATSSS rules and
push them to the device via the 5GC.
• These rules dictate how the device should utilize the 3GPP and Non-
3GPP access networks it may have available, specifically with
respect to sending uplink traffic.
• For downlink traffic, 5GC can provide rules to the UPF (User Plane
Function) which dictate which access network should be used for
which traffic flow.

11. Important Points about ATSSS #2
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• Within the 5GC, control of ATSSS is largely handled by the PCF (Policy
Control Function), whose responsibility it is to provide ATSSS rules to
both the device and the SMF (Session Management Function) based on
service provider policy.
• The device/UPF could be configured with different “Steering
Functionality” options, such as:
• Higher Layer Steering Functionality
• Lower Layer Steering Functionality (can be applied to all higher layer
traffic types)

12. Non-3GPP
Access
Architecture Reference Model for ATSSS Support
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AMF SMF
UPF DN
N11
N4
N6
3GPP
Access
N2
N3
N1
Based on 3GPP TS 23.501 4.2.10-1: Non-roaming and Roaming with Local Breakout architecture for ATSSS support
N3
N2
N1
PCF
N7
UE
MPTCP
functionality
ATSSS-LL
functionality
MPTCP
Proxy
functionality
PMF
MPTCP – Multi-Path TCP Protocol
PMF – Performance Measurement Functionality

13. Higher Layer Steering Functionality
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• MPTCP (Multipoint Transmission
Control Protocol) – carried out above
IP in the protocol stack, allows the
device to communicate with an
MPTCP Proxy within the UPF to
determine how TCP traffic associated
with applications can be split,
switched or steered.

14. Lower Layer Steering Functionality
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• Can be applied to all higher layer traffic types:
• Active / Standby – for a given traffic flow, this will
categorize one of the access networks (3GPP or non-
3GPP) as active and the other as standby. The standby
access network will only be used for the traffic flow if the
active is unavailable.
• Smallest Delay – this steers a given traffic flow to the
access network which has the smallest delay with respect
to Round Trip Time.
• Load Balancing – this splits a traffic flow across both
access networks, based on a percentage split
• Priority Based – this will steer a traffic flow to the highest
priority access network, until that access network is
considered to be congested. At this point, traffic can also
be split towards the low priority access network.

15. KT tests ATSSS
KT & Tessares
successfully test ‘5G
Low Latency Multi-
Radio Access
Technology’ in a
Commercial 5G
Network – Read the
press release here.
(Download slides from SlideShare)
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16. Conclusion
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• Initial priority for the Service Providers (SPs) is to deploy 5G Core (5GC),
starting next year
• Once 5GC is in place, we will start seeing SPs looking to deploy
advanced features like support for trusted & untrusted non-3GPP
Access
• Once they are in place, we will see ATSSS being implemented, tested
and deployed.
• Realistically, we will start seeing some operators use the feature in
2021/22

17. Further Reading on ATSSS
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• Mpirical: 5G ATSSS
(This is the main reference for this video)
• Tessares: 5G ATSSS solution
• 3G4G: True Fixed-Mobile Convergence (FMC) with 5G
• 3GPP TS 23.501 v16.1.0: System Architecture for the 5G System; Stage 2
• Whitepaper: Fixed-Mobile Convergence in the 5G era: From Hybrid
Access to Converged Core - Massimo Condoluci, Stephen H. Johnson,
Vicknesan Ayadurai, Maria A. Lema, Toktam Mahmoodi, Maria A.
Cuevas, and Mischa Dohler (link)
• Operator Watch Blog: How is KT Reducing the Latency of their 5G
Network (link)

18. Thank You
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