Wi-Fi 7 Core: MLO Multi-Link

In the long history of Wi-Fi technology iteration, the arrival of Wi-Fi 7 (802.11be) is far more than just a simple speed upgrade; its core innovation, MLO (Multi-Link Operation), has sparked a disruptive revolution in MAC layer architecture. Unlike the superficial optimization of traditional "dual-band concurrency," MLO completely breaks the tight coupling relationship of "one MAC corresponding to one PHY" that has existed for over 20 years, introducing the entirely new logical entity MLD (Multi-Link Device), enabling a qualitative leap in the efficiency, compatibility, and flexibility of wireless connections. This article will start from the essence of the protocol to dissect the technical core and practical value of MLO.

"Layered Collaboration" Revolution at the MAC Layer

The core breakthrough of MLO lies in the reconstruction of the MAC layer, which is divided into upper and lower layers that work together, ensuring compatibility with the upper layer system and achieving efficient scheduling of multiple links.

1. Universal Reference Model: Transparent Multi-Link Management

For upper layers such as the operating system or LLC layer, devices supporting MLO are no different from ordinary Wi-Fi devices—thanks to the "single MAC-SAP (MAC Service Access Point)" design. The entire MLD device only presents one MLD MAC address and one data interface to the outside world. Upper layers do not need to be aware of the number of underlying links and switching logic, perfectly realizing the "transparency" of technology upgrades without requiring modifications to the existing software ecosystem.

2. Upper-layer MAC: The intelligent brain of a multi-link network

The MAC sublayer above the MLD is the decision-making core of the MLO, responsible for handling common functions independent of specific links, and ensuring consistency in multi-link collaboration.

Unified serial number management：All data packets on all links share the same sequence number space (SNS), which completely solves the out-of-order problem in multi-link transmission and makes reordering at the receiving end more efficient.
Global encryption and decryption：Security is achieved by using a unified key (PTKSA), which avoids the key synchronization problem caused by separate encryption of multiple links;
Fragmentation and Reassembly and Traffic Scheduling：It can not only perform unified fragmentation and reassembly of large data packets, but also distribute traffic of different priorities to appropriate links through the TID-to-Link Mapping (TTLM) mechanism to achieve load balancing.

3. Lower-layer MAC: The "executive limbs" of the link”

Each physical link (e.g., 2.4GHz, 5GHz, 6GHz) is equipped with an independent lower-layer MAC entity, specifically responsible for direct interaction with the physical layer (PHY), focusing on link-level operations:

Channel access control：The EDCA mechanism completes the channel contention and backoff processes.
Link control interaction：RTS/CTS handshake, ACK reply, etc. must be completed independently on the corresponding link to ensure the real-time performance of the link transmission;
Link management and maintenance：Process Beacon frames and link-specific action frames to ensure the stable operation of a single link.

Equipment differences: MLO implementation logic of AP and terminal

MLD is not a "one-size-fits-all" design. The differences in architecture between AP (router side) and Non-AP (mobile phones, computers and other terminals) are adapted to different usage scenarios.

1. AP MLD: Compatible with both new and old hybrid architectures

As the core hub of the wireless network, the AP needs to serve both Wi-Fi 7 MLD terminals and traditional terminals of Wi-Fi 6 and earlier, therefore it adopts a "dual-path" architecture:

MLD path: For Wi-Fi 7 terminals, data is uniformly encrypted and scheduled by the upper-layer MAC before being distributed to each affiliated AP, thereby improving the transmission rate through multi-link collaboration;
Legacy Path: For traditional terminals, each affiliated AP retains an independent Non-MLD upper-layer MAC function, which can directly and independently serve old devices without going through the complex MLD processing, ensuring both compatibility and transmission efficiency.

2.Non-AP MLD: A Lightweight Architecture Focused on Power Saving and Roaming

The core requirements of terminal devices are low power consumption and seamless roaming, therefore Non-AP MLD designs have a clear focus:

Single authentication entity: The entire device only requires one IEEE 802.1X authentication requester. A single authentication (4-way handshake) can generate keys (PTK) for all links, eliminating the need for repeated authentication across frequency bands and greatly simplifying the connection process;
Lightweight collaboration: The upper-layer MAC uniformly manages the core logic of all links, while the lower-layer MAC focuses on link-level execution. While ensuring the advantages of multiple links, it minimizes power consumption and adapts to the usage scenarios of mobile devices.

Address and Identification: MLO's "Communication Language System"

The core of multi-link collaboration is to clarify "who is communicating" and "which link is being used for communication". MLO constructs clear communication rules through a dual address system and Link ID identification.

1. Dual MAC addresses: "Dual identity" for devices and links

Each MLD device has two types of MAC addresses, each with its own function:

MLD MAC address: Represents the overall identity of the device and is used for authentication, association, and encryption key generation. It is the device's "global identifier" in the network.
Link MAC address: Corresponds to the radio frequency entity of a specific link, used to identify the sender (TA) and receiver (RA) of data frames, ensuring the accuracy of link-level interactions such as underlying ACK replies - after all, actual data transmission occurs on a specific link and must be located through the Link MAC address.

2. Address translation rules: Ensure consistency between upper and lower layers.

To balance the continuity of upper-layer sessions with the transmission requirements of lower-layer links, MLO has established strict address translation logic:

Management Frames: During the MLD establishment phase, the (Re)Association request/response frame body carries the MLD MAC address, while the frame header still uses the current link MAC address to ensure link connectivity;
Data frames: When sending, the upper layer does not need to modify the address. The lower layer replaces the TA with the Link MAC address of the sending link during encapsulation. When receiving, the lower layer driver will map the source address back to the MLD MAC address to ensure that the upper layer application is unaware of the link switch.

3.Link ID: A unique identifier for a link.

AP MLD assigns a unique Link ID (0-14) to each member AP. Its core feature is that it is "unique and static"—as long as the AP is not removed, the Link ID remains unchanged even when switching channels. In scenarios such as traffic scheduling negotiation, Block Ack establishment, and power-saving settings, the Link ID bitmap is used to refer to the link, which significantly reduces signaling overhead and improves interaction efficiency.

Physical Limitations and Solutions: The Synergistic Approach to STR and NSTR

MLO's multi-link parallel capability is limited by hardware conditions and is divided into two modes: STR and NSTR. Wi-Fi 7 has overcome the physical bottleneck through targeted design.

1.STR mode: Interference-free "parallel transmission"

When the device hardware isolation is sufficient (such as in the 2.4GHz and 5GHz bands), it can support STR (Simultaneous Transmit and Receive) mode—when one link is sending data, the other link can receive it synchronously without interference. Wi-Fi 7 requires that multi-link APs (MLDs) must support STR (except for special mobile hotspot modes) to ensure the AP's high-performance scheduling capabilities and fully leverage the parallel advantages of multiple links.

2.NSTR Mode: Synchronization and Coordination in Response to Interference

When frequency bands are too close together (e.g., 5.2GHz and 5.8GHz), devices may enter NSTR (Not Simultaneously Receive/Receive) mode due to internal interference—when one link transmits, the other link will be interfered with and unable to receive, creating an "NSTR dead zone." To solve this problem, Wi-Fi 7 introduces a "synchronous operation" mechanism:

Synchronous transmission: Both links must start transmitting data simultaneously;
Synchronization termination: The data packet transmission termination time must be strictly aligned, with an error not exceeding 8µs. This design avoids interference from the transmission signal of another link when one link has finished transmitting and receiving the ACK, ensuring that multiple links can still improve the transmission rate in NSTR mode.

Wi-Fi 7's MLO technology is not simply a matter of "multi-link overlay," but rather, through MAC layer reconstruction, flexible device architecture, clear communication rules, and targeted physical layer optimization, it fundamentally improves the speed, stability, and compatibility of wireless connections. It meets the high-speed, low-latency demands of scenarios such as high-definition video, VR/AR, and industrial IoT, while backward compatibility protects users' existing equipment investments, and its power-saving design on the terminal side adapts to the usage needs of mobile devices. With the widespread adoption of Wi-Fi 7 devices, MLO will become the core technology foundation supporting wireless communication in the era of the Internet of Things, making a "seamless, high-speed, and stable" wireless experience the norm.