PDU Multi-Access PDU

Description

The PDU Multi-Access PDU (MA PDU) is a protocol data unit designed to operate over multiple underlying access networks concurrently, such as 3GPP access (e.g., 5G NR) and non-3GPP access (e.g., Wi-Fi). It is a key enabler for the Access Traffic Steering, Switching and Splitting (ATSSS) feature defined in 3GPP, which allows the network and user equipment to intelligently distribute traffic flows across multiple available accesses. The MA PDU functions at a layer above the individual access-specific PDCP (Packet Data Convergence Protocol) or equivalent layers, providing a unified session layer that can aggregate, steer, or switch traffic based on policies, link quality, and application requirements.

Architecturally, the MA PDU is managed by the Multi-Access PDU Session anchor, which is a user plane function (UPF) in the 5G Core network. This anchor terminates the MA PDU session and is responsible for the integration of traffic from the multiple accesses. In the user equipment (UE), a corresponding Multi-Access (MA) protocol layer, often realized as an adaptation layer or within the ATSSS framework, handles the construction and processing of MA PDUs. This layer interacts with lower layers specific to each access technology, receiving data from the application layer and deciding, based on ATSSS rules, which access network(s) to use for transmission, potentially splitting a single flow.

The operation involves the MA layer applying steering modes—such as active-standby, smallest delay, load balancing, or priority-based—to direct traffic. For splitting, it may segment upper-layer packets into multiple subflows, each assigned to a different access, with sequencing and reassembly mechanisms to handle out-of-order delivery. The MA PDU itself carries necessary control information, potentially including sequence numbers and access network identifiers, to facilitate correct reassembly and monitoring at the receiving end. Its role is critical for enhancing user experience through improved reliability, higher aggregate bandwidth, and seamless mobility between heterogeneous networks, forming a core component of 5G's convergence vision.

Purpose & Motivation

The MA PDU was created to address the growing need for seamless and efficient utilization of multiple network access technologies available to a device. Historically, devices could connect to different networks (e.g., cellular and Wi-Fi) but typically used them in a mutually exclusive manner per data session, with switching often causing session breaks. This limited the ability to leverage combined resources for better performance or reliability. The proliferation of heterogeneous access networks and the demand for ultra-reliable, high-bandwidth services in 5G necessitated a more integrated approach.

The primary problem it solves is the rigid binding of a PDU session to a single access type. By introducing the MA PDU, 3GPP enables a single PDU session to be established over multiple accesses simultaneously. This allows for traffic steering, switching, and splitting based on dynamic network conditions, policies, and application needs. It was motivated by use cases requiring enhanced mobility (seamless handover between 3GPP and non-3GPP), improved reliability through redundancy, and increased throughput via aggregation. The technology addresses limitations of previous single-access sessions and non-integrated multi-connectivity solutions, providing a standardized, network-controlled framework for multi-access management.