Description
The MAC Protocol Data Unit (MPDU) is a fundamental concept in data communication protocols, representing the packet or frame that is processed and transmitted by the Medium Access Control (MAC) layer. In the 3GPP architecture, particularly for LTE and 5G NR, the MAC layer is a sublayer of the Layer 2 protocol stack (alongside RLC and PDCP) in both the UE and the base station (eNB/gNB). The MPDU is the unit of data that the MAC layer delivers to the physical layer for transmission over the air interface, or receives from the physical layer for processing. It encapsulates higher-layer data (MAC SDUs, which are RLC PDUs) by adding a MAC header that contains control information necessary for the receiving MAC entity to correctly process the packet.
Architecturally, an MPDU is constructed by the transmitting MAC entity. The MAC layer receives Service Data Units (SDUs) from the Radio Link Control (RLC) layer via logical channels. The MAC layer performs functions such as logical channel prioritization, multiplexing of SDUs from different logical channels into a single transport block, and adding the MAC header. The MAC header consists of one or more subheaders, each corresponding to a MAC SDU or a MAC control element (CE). Each subheader includes fields such as the Logical Channel ID (LCID), which identifies the logical channel or type of control information, and the Length (L) field indicating the size of the corresponding SDU. MAC Control Elements are used for in-band control signaling, such as scheduling requests, buffer status reports, or timing advance commands. After the header and SDUs/CEs, padding may be added to align the MPDU to a required size, and a CRC (Cyclic Redundancy Check) may be attached for error detection at the physical layer.
How it works involves the MAC layer at the transmitter assembling an MPDU based on the scheduling grant received from the physical layer (via DCI on the PDCCH/MPDCCH). The scheduler in the base station determines the size of the transport block (which carries one MPDU) and the resources allocated. The MAC entity selects which logical channels to serve, segments or concatenates RLC PDUs if necessary (though segmentation is primarily an RLC function in LTE/NR), and builds the MPDU with the appropriate header. This MPDU is then passed to the physical layer as part of a transport block, which undergoes channel coding, modulation, and mapping to physical resources. At the receiver, the physical layer decodes the transport block and delivers the MPDU to the MAC layer. The receiving MAC entity parses the MAC header using the LCID values to demultiplex the SDUs and deliver them to the correct RLC entities or process any MAC CEs. The MPDU is thus the vehicle for all user plane data and critical MAC-layer control signaling across the radio interface, enabling efficient multiplexing, priority handling, and reliable data transfer.
Purpose & Motivation
The concept of the MPDU exists as a fundamental part of layered protocol design, providing a standardized format for data exchange at the MAC layer. In wireless systems like 3GPP LTE and NR, the MAC layer is responsible for critical functions such as mapping between logical channels (which represent different types of data flows, e.g., voice, video, signaling) and transport channels (the physical resources), scheduling, and priority handling. The MPDU structure, with its header containing LCIDs, allows the MAC layer to efficiently multiplex data from multiple logical channels (e.g., SRB1 for RRC signaling, DRBs for user data) into a single transport block for transmission, and to demultiplex them at the receiver. This solves the problem of managing multiple concurrent data streams with different QoS requirements over a shared radio resource.
Historically, the MAC PDU format has evolved through 3GPP releases to support new features and efficiencies. In early 3GPP systems (UMTS), the MAC layer also used PDUs, but the structure and functionalities were adapted for CDMA-based transport channels. With the move to OFDMA-based LTE in Release 8, the MAC PDU structure was redesigned to be more flexible and efficient for packet-switched, all-IP traffic. The inclusion of MAC Control Elements within the MPDU allows for low-latency, in-band control signaling without requiring dedicated resources, which is crucial for dynamic scheduling and mobility management.
The MPDU's purpose is to provide a well-defined interface between the MAC and physical layers, ensuring that the rich set of MAC functions (multiplexing, HARQ, scheduling) can be performed reliably. It addresses the need for efficient use of radio resources by allowing data from different sources to be packed together, reducing overhead compared to sending each logical channel's data in separate physical transmissions. The design motivations include minimizing header overhead through efficient subheader structures, supporting variable-sized SDUs, and enabling fast processing in both base stations and UEs, which is essential for high-data-rate, low-latency communications in modern cellular networks.
Key Features
- Contains a MAC header with one or more subheaders identified by LCID
- Carries multiplexed MAC Service Data Units (SDUs) from one or more logical channels
- Can include MAC Control Elements for layer-specific signaling (e.g., BSR, PHR)
- Supports padding to meet transport block size requirements
- Forms the payload of a transport block delivered to the physical layer
- Enables efficient priority handling and logical channel prioritization (LCP)
Evolution Across Releases
Referenced in the context of management and orchestration specifications (e.g., 28.403). The fundamental MAC PDU structure for the radio interface was established in earlier releases (Rel-8 for LTE). In Rel-13 and later, the MPDU concept remains consistent, with enhancements in MAC functionality (e.g., for MTC, dual connectivity) but not necessarily changes to the core PDU format itself.
Defining Specifications
| Specification | Title |
|---|---|
| TS 28.403 | 3GPP TS 28.403 |