5G ATSSS
In LTE, the capability to utilize both 3GPP and Non-3GPP access networks has been there from the early days of the technology. As LTE matured, so too did the techniques available for manipulating how data should be split and transferred over each access network, particularly when both access types are available for use. Both MAPCON (Multi APN Connectivity) and IFOM (IP Flow Mobility) addressed these areas in various ways, although widespread adoption of the technologies has been limited.
Step forward 5G’s ATSSS (Access Traffic Steering, Switching and Splitting). This technique, which requires a 5GC (5G Core) network for implementation, provides similar functionality to the aforementioned 4G steering techniques. In particular, ATSSS allows the service provider to configure ATSSS rules and push them to the device via the 5GC. These rules will dictate how the device should utilize the 3GPP and Non-3GPP access networks it may have available, specifically with respect to sending uplink traffic. Conversely, for downlink traffic, the 5GC can provide rules to the UPF (User Plane Function) which dictate which access network should be used for which traffic flow.
Figure 1 - Access Traffic Steering, Switching and Splitting
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. As such, the device/UPF could be configured with different “Steering Functionality” options, such as:
- Higher Layer Steering Functionality:
– MPTCP (Multipoint Transmission Control Protocol) – carried out above IP in the protocol stack, this 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.
- Lower Layer Steering Functionality (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.
From the perspective of the device, local conditions (e.g. network interface availability or signal loss conditions) may also be considered when determining how to split uplink traffic. Likewise, along with ATSSS rules, the UPF may also use feedback obtained from the device to assist with downlink traffic splitting.
At the moment, implementation of a 5GC is on the agenda for many service providers, with Wi-Fi integration to the 5GC a potentially longer-term goal. Time will tell as to whether or not ATSSS will be a feature of this integration.
Release 17 extended the existing modes. It has added steering mode indicators, such as autonomous load-balancing and UE-assisted operation, and threshold values for RTT and packet loss rate. These thresholds let load-balancing and priority-based modes react to link conditions rather than using fixed rules.
Release 18 added two elements. The first is MPQUIC (Multipath QUIC)-based steering functionality, which lets ATSS handle UDP (User Datagram Protocol) traffic using multipath QUIC. The second is a redundant steering mode, which duplicates traffic across both accesses for reliability-sensitive communication.
Release 19 extends MPQUIC-based steering beyond UDP to support both IP and Ethernet traffic in a Multi-Access PDU Session.
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Blog reviewed and updated: September 2026
