MLO

What is Multi-Link Operation (MLO)?

In previous WiFi generations, like WiFi 6 and WiFi 5, a device could only connect to one WiFi band at a time — either 2.4 GHz or 5 GHz. The latest WiFi 6E products have added the 6 GHz band, but still, only one band is available per client, which means one band goes unused, or you might be limited by the slower band.

Multi-Link Operation (MLO) is a feature in WiFi 7 that addresses this issue by allowing devices to send and receive data simultaneously across different frequency bands and channels. With MLO, WiFi 7 can establish multiple links between a device (such as your phone) and a WiFi access point (such as your router), connecting to the 2.4 GHz, 5 GHz, and 6 GHz bands at the same time. This increases throughput, reduces latency, and improves reliability, making it ideal for applications like VR/AR, online gaming, remote work, and cloud computing.

How Does MLO Work?

MLO enhances the MAC layer by employing MLD (Multi-Link Devices) multi-link aggregation, which boosts throughput, reduces latency, and increases reliability. It also utilizes MLD multi-link seamless dynamic switching to achieve load balancing and maintain low latency.

WiFi 7 MLO mainly includes two operating modes: STR Mode and NSTR Mode.

• STR Mode (Simultaneous Transmit and Receive Operation)

STR Mode, or Simultaneous Transceiver Mode (also known as Asynchronous Mode), means that two or more links operate entirely independently without any interference between them.

• NSTR Mode (Nonsimultaneous Transmit and Receive Operation)

NSTR Mode, or Non-Simultaneous Transceiver Mode (also known as Synchronous Mode), means that simultaneous receiving and sending operations are not permitted. At any given time, all links can either receive or send data, but not both simultaneously.

• Others

Besides STR and NSTR modes, WiFi 7 MLO includes other modes. These modes cater to various application scenarios, such as Mesh and EasyMesh, where multiple links are formed by multiple physical IC radios at both ends of the AP-STA connection. Additionally, there are multiple links created by conventional mobile devices with a single IC.

Technical Features of MLO

The primary technical features of MLO can be categorized into two aspects: packet-level features and flow-level features. Packet-level aggregation enhances latency and peak performance, while flow-level routing optimization improves latency and overall throughput.

The main features are:

1. Packet Level Aggregation

• Packets with the same TID (Traffic Identifier) can be transmitted using one or more radios.

• This contributes to enhancing low-latency and peak throughput.

2. Cross-Wake-Up Signaling for Power-Saving

• The AP indicates to a buffering unit on a link that the STA is monitoring.

• The STA can use one link to signal the wake-up of another link.

• The STA can monitor a link in idle mode to receive BSS/TIM information from other links.

3. Fast Link-Transition

• Active links can be dynamically switched to adapt to load and coexistence conditions.

• This is beneficial for 11be single-radio STAs.

4. Multi-Primary Channel Access

• This is necessary for improving latency.

5. Shared Single Session Across Links

• There is a single BA (Block Acknowledgement) session per TID with a shared sequence number space.

• There is single authentication and key derivation for unicast packets.

• Separate group keys are used for broadcast/groupcast packets.