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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (250 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the multi-access (MA) PDU session function was introduced, enabling a PDU session to support multiple QoS sessions with different QoS profiles. This foundational capability for multi-homing support was further defined through specific corrections and interworking procedures for multi-homed IPv6 PDU sessions. The release also clarified the establishment of PDU sessions with multiple S-NSSAIs.
- IMS multimedia telephony communication service and supplementary services TS 24.173CR0122
- Message Waiting Indication (MWI) using IP Multimedia (IM) Core Network (CN) subsystem TS 24.606CR0020
- Multiple S-NSSAIs in PDU session establishment TS 24.501CR0171
- Correction for multi-homed IPv6 PDU session TS 24.501CR0234
- Interworking for multi-homed IPv6 PDU session TS 24.501CR0239
- Multiple 5G-GUTIs TS 24.501CR0583
+ 6 more changes
In Release 16, the MA PDU (Multi-Access PDU) session function introduced new procedures for establishment, modification, and release, specifically supporting scenarios like upgrade to a Multiple Access PDU Session. It defined interactions for EPS interworking, both with and without the N26 interface, and added handling for cases where a UE deregisters from one access while maintaining the MA PDU session. The release also specified reject causes, such as for an unstructured PDU session type, and integrated considerations for allowed NSSAI during establishment.
- Multi-Device and Multi-Identity services TS 24.173CR0137
- PDU session ID usage when the UE is a 5G-RG and requests establishment of a PDN connection as a user-plane resource of a MA PDU session TS 24.301CR3326
- MA PDU request in UL NAS TRANSPORT message TS 24.501CR1020
- Update PDU session establishment procedure to support MA PDU session TS 24.501CR1021
- Multiple Access PDU Session TS 24.501CR1122
- Procedure for Multiple Access upgrade of PDU Session TS 24.501CR1269
+ 55 more changes
In Release 17, the MA PDU function was extended to the Evolved Packet Core (EPC), enabling support for MA PDU Sessions with 3GPP access in that system. This release also introduced procedures for the local deactivation of user plane resources for an MA PDU session with a PDN leg and allowed for ATSSS parameters to be provisioned and modified through EPS procedures.
- Using Service Request procedure for removing paging restrictions in EPS for a Multi-USIM UE TS 24.301CR3517
- Leaving procedure and Reject Paging Indication for Multi-USIM UEs in EPS TS 24.301CR3534
- Multi-USIM UE support indications in EPS TS 24.301CR3514
- Support for MA PDU Session with 3GPP access in EPC TS 24.301CR3591
- UUAA: multiple round trips TS 24.301CR3594
- Local deactivation of UP resource for an MA PDU session with PDN leg - 24301 Part TS 24.301CR3657
+ 63 more changes
In Release 18, key enhancements for the MA PDU function included enabling MA PDU session establishment with non-3GPP access connected to the EPC and 3GPP access connected to the 5GCN, as well as supporting MA PDU sessions via a PDN connection. The release also introduced specific procedures for inter-system change to S1 mode for an MA PDU session with an ePDG leg and refined UE handling of the MA PDU session.
- Redundant steering mode is not applicable for ATSSS-LL functionality (impact on TS 24.193) TS 24.193CR0114
- Support MA PDU session establishment with non-3GPP access connected to EPC and 3GPP access connected to 5GCN TS 24.193CR0102
- ATSSS rules TS 24.193CR0103
- MA PDU session via PDN connection TS 24.193CR0107
- Resolving EN related to including the ATSSS rules to the ATSSS_RESPONSE Notify payload TS 24.193CR0130
- Inter system change to S1 mode for MA PDU with ePDG leg TS 24.193CR0132
+ 51 more changes
In Release 19, the MA PDU function was enhanced with new measurement capabilities for ATSSS, including the addition of MA PDU session release measurement, PMF-related measurements, and access state tracking for MA PDU sessions. The release also introduced support for handling multiple multiplexed media packet filter components within a single packet filter and added the inclusion of ATSSS status in related session management messages. Furthermore, new functionalities were specified, such as support for MPQUIC-based proxy operations for ATSSS and the handling of multiplexed media information.
- Enhancement of 5G ProSe capability for multi-hop relays TS 24.501CR6552
- (S)RTP multiplexed media information support TS 24.501CR6576
- Update 5GMM capability for 5G ProSe multi-hop relays TS 24.501CR6692
- Alt-1: Allow multiple multiplexed media packet filter components in one packet filter TS 24.501CR6699
- Inclusion of ATSSS status in related session management messages TS 24.501CR6880
- NW capability for multiple LCS-UPP connections per UE TS 24.501CR6953
+ 45 more changes
Explore further
Broader topics and technologies where MA plays a role.
Defining Specifications
3GPP specifications that define or reference MA, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.050 v1100 | UMTS Network Principles and Architecture | R99 |
| TS 24.173 vj00 | Multimedia Telephony Service and Supplementary Services in IMS | Rel-19 |
| TS 24.193 vj50 | ATSSS Procedures Specification | Rel-19 |
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.406 v810 | Message Waiting Indication (MWI) Protocol | Rel-8 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.606 vj00 | MWI Service Protocol Description | Rel-19 |
| TS 25.222 vj00 | UTRA TDD Multiplexing & Channel Coding | Rel-19 |
| TS 26.090 vj00 | AMR Speech Codec Detailed Mapping Specification | Rel-19 |
| TS 26.190 vj00 | AMR-WB Speech Codec Detailed Mapping | Rel-19 |
| TS 26.290 vj00 | AMR-WB+ Audio Codec Specification | Rel-19 |
| TS 28.552 vk10 | 5G Performance Management Measurements | Rel-20 |
| TS 29.512 vj40 | 5G Session Management Policy Control Service | Rel-19 |
| TS 29.519 vj40 | UDR Usage for Policy & Exposure Data | Rel-19 |
| TR 38.812 vg00 | Study on NOMA for NR | Rel-16 |
| TR 45.914 vj00 | MUROS Feasibility Study for Voice Capacity | Rel-19 |
| TS 46.060 vj00 | GSM Enhanced Full Rate Speech Codec | Rel-19 |