Multi-Access PDU Session

Description

A Multi-Access PDU Session (MA PDU Session) is a type of Protocol Data Unit (PDU) Session in the 5G System that can be established over multiple access networks. Specifically, it can use one access network at a time (either 3GPP or non-3GPP) or simultaneously use one 3GPP access network (e.g., NR or LTE) and one non-3GPP access network (e.g., Wi-Fi or wireline). This capability is defined in 3GPP TS 23.501 and related specifications such as TS 29.512 (N4 interface) and TS 29.561 (Interworking with Non-3GPP Networks). The MA PDU Session is a cornerstone of 5G's support for access traffic steering, switching, and splitting (ATSSS), which allows the network to dynamically route traffic across available accesses based on operator policies, link quality, and service requirements.

The architecture of an MA PDU Session involves several key network functions. The SMF (Session Management Function) manages the session and enforces ATSSS policies. The UPF (User Plane Function) handles the user plane traffic and may perform measurements to support traffic steering. For non-3GPP access, the N3IWF (Non-3GPP InterWorking Function) or TNGF (Trusted Non-3GPP Gateway Function) provides connectivity. The UE (User Equipment) must support the ATSSS feature and the Multi-Access PDU Session procedures, including the ability to maintain simultaneous connectivity over both accesses. The AMF (Access and Mobility Management Function) coordinates registration and mobility across both accesses.

The MA PDU Session can be established with different modes of operation. In the single-access mode, the session uses only one access at a time, but can switch between accesses based on triggers such as signal strength or policy. In the multi-access mode, both accesses are active simultaneously, and traffic can be split at the IP flow level or even within a flow using MPTCP (Multipath TCP) or QUIC. This allows for improved throughput, reliability, and seamless handover between accesses. The ATSSS rules are provided by the SMF to the UE and UPF, specifying how to steer traffic (e.g., active-standby, smallest delay, load balancing).

The MA PDU Session offers several benefits. It enhances user experience by providing seamless connectivity when moving between 3GPP and non-3GPP accesses, without dropping sessions. It enables operators to leverage both licensed and unlicensed spectrum, improving capacity and coverage. It also supports new use cases such as Fixed Wireless Access (FWA) and enhanced mobile broadband (eMBB) with multi-access aggregation. Furthermore, it simplifies network operations by unifying session management across heterogeneous accesses, as opposed to maintaining separate sessions for each access.

However, deploying MA PDU Sessions requires careful consideration of policy and charging, as well as support in the UE and network. 3GPP has specified the necessary signaling in TS 24.501 (NAS) and TS 24.193 (ATSSS). The feature has evolved since its introduction in Release 16, with subsequent releases refining procedures and adding support for additional access types and steering modes. As 5G evolves towards 6G, the concept of multi-access sessions is expected to expand further, integrating satellite and other non-terrestrial networks.

Purpose & Motivation

The MA PDU Session was introduced in Release 16 to address the growing need for seamless integration of multiple access technologies in 5G networks. Prior to this, 3GPP systems supported interworking between 3GPP and non-3GPP accesses but typically through separate PDU Sessions that required complex handover procedures and could result in service interruption. The MA PDU Session provides a unified session that can span both access types, enabling smoother mobility and better resource utilization.

The primary motivation was to enable operators to leverage both licensed and unlicensed spectrum effectively, improving capacity and coverage. With the explosion of data traffic, operators needed ways to offload traffic to Wi-Fi or other non-3GPP accesses without sacrificing session continuity. The MA PDU Session allows for dynamic traffic steering, which can optimize network performance and user experience. Additionally, it supports use cases like Fixed Wireless Access where a single session can be maintained across different access technologies.

Another key driver was the desire to simplify the core network architecture. Instead of maintaining separate session management contexts for each access, a single MA PDU Session can be managed by a single SMF, reducing signaling overhead and complexity. This also facilitates policy and charging consistency across accesses. The MA PDU Session thus represents a significant step towards a truly converged network where access technologies are transparent to the user and services are delivered seamlessly.