Wi-Fi 7 (802.11be) Multi-Link Operation

This article deals with one of the key feature that added in Wi-Fi 7 called Multi Link Operation (MLO). Wi-Fi 7 which is also called as 802.11be standard known for Extremely High Throughput (EHT). It has brought some key features to improve the Network Throughput (Mbps) and reduce the latency for high density environments and also focuses on enhancing the usage of spectrum efficiently under higher bandwidth applications.

Key Features of Wi-Fi 7 11be includes 320 MHz support, 4096 QAM, Multi-RU OFDMA, Preamble Puncturing, 16 x 16 MU-MIMO, Multi-Link Operation, Enhanced Quality of Service.

I already discussed about why we need Multi Link Operation and its usage for Wi-Fi 7. Go through my previous article to see the motivation for this feature and its impacts in 11be.

Please find the article link below:

MLO Article Link

Significance of MLO:

MLO has the capability to link multiple links together simultaneously or use single link to ensure the seamless traffic flow across the network and leading to lower latency.

Pre-requisite: Same SSID should be configured in all participating MLO links and WPA3 is mandatory.

Main idea of MLO is to aggregate or steer traffic between multiple links without repeating the four-way handshake. This is acheived by using Upper MAC address otherwise called as Multi Link Device (MLD) address.

From the above figure, MLD is a device that is capable of handling multiple links together which presents a single MAC address to the higher layers. This is acheived by using Lower MAC (L-MAC) address and Upper MAC (U-MAC) address.

Note that both AP and STA will have their own U-MAC and L-MAC addresses for the participating MLO links.

Hence, MLD MAC address (U-MAC) of AP and STA will only be used for Authentication, Association and PTK generation in four-way handshake.

Functionalities of L-MAC and U-MAC address:

L-MAC: Used for Link specific functions such as link specific encryption/decryption, AMPDU aggregation/de-aggregation, Block Ack, Power save mode, Rate adaptation, etc,.

U-MAC: Used for common functions across links such as Authentication, Association, Re-association, Security Association (PMK, PTK), AMSDU ggregation/de-aggregation, Sequence number assignment, Rx packet re-ordering.

MLO Frame Exchanges for WPA3:

It is important to note that client connection happens only on one of the active MLO participation link.

Other links will be either operational state transmitting data streams or in power save state depending on the client MLD Operating modes (MLSR, EMLSR, STR MLMR, Non-STR MLMR, EMLMR).

MLO presence is seen as Multi Link Information Element (ML IE) in Beacon, Probe Request/Response, Association Request/Response frames. If this ML IE is present in these frames, then we can consider that the devices (AP/STA) is supporting Wi-Fi 7 MLO feature.

From the above figure 2, we can infer that Multi Link IE has two fields namely Common Info subfield and Basic STA Profile count subfield. This Common Info is responsible for MLD MAC addresses, corresponding Link ID and its MLD Capabilities. Basic STA Profile Count is known for remaining active links in MLO which respresents L-MAC addresses and their Link IDs respectively.

Operating Modes of MLO:

There are many operating modes of MLO. By default, Access Points support Simultaneous Transmit and Receive (STR) MLD. But, Client hardware capabilities may vary and thus operating modes changes with client devices. It can be Multi Link Single Radio (MLSR), Multi Link Multi Radio (MLMR), Enhanced Multi Link Single Radio (EMLSR).

Again MLMR which can be two types namely STR MLMR, Non-STR MLMR and EMLMR.

From the above figure, we can point out the main difference between MLSR and EMLSR and STR/Non-STR MLMR client operating modes.

Client Operating Modes of MLO:

Note: By default, AP supports STR MLD type and doesn't support EMLSR/EMLMR MLD types.

MLSR Mode:

MLSR is for single radio clients which has common Local Oscillators across links, so that center frequency is aligned properly with AP.

It is capable of only one link/radio operational (Tx/Rx) at a time which implies that particular link is in out of power save state. Remaining active links in MLO is considered to be in power save state. We use Power Management bit set to 1 on other active link to switch between the links.

EMLSR Mode:

EMLSR client operating mode which has two seperate Local Osciallators for links each having its own frequency f1 and f2.

EMLSR client device example: Intel 11BE 200

It is capable of listening on two links simultaneously with 1x1 radio chains for a initial control frame exchange (RTS/CTS) and then AP decides on which links the data is going to send with 2x2 radio chain by changing its local osciallators frequency f1=f2, so that center frequency is aligned with AP frequency to receive 2x2 data streams on that particular MLO active link.

STR/Non-STR MLMR Mode:

In STR MLMR supporting client devices which has multiple radios and each radio has its own RF filters for better isolation between radios to transmit/receive the data.

STR MLMR Client device example: Qualcomm Hamilton client

It is capable to transmitting and receiving the data simultaneously on multiple links, since each radio has enough frequency seperation. So, transmit/receive data from multiple links is not a problem in STR mode of operation whereas Non-STR operation only deals with Tx/Tx simultaneously on multiple links, here simultaneous Tx/Rx operation on multi links is not possible due to no isolation between each radios.

EMLMR Mode:

EMLMR mode which has multiple radios greater than 2 and it also helps in listening to multiple links for a initial frame exchanges and dynamically moved between the multiple radios. Non-STR and EMLMR MLD types has significant implementation challenges and hence it is only in theory.

From the above figure, which shows the detailed view of all the different client operating modes and their characteristics and now we got clarity for all the client operating modes and its usage in acheiving the throughput rates, reduced latency for all Wi-Fi 7 devices.

Advantages and Applications of MLO:

By using this feature in Wi-Fi 7 chipsets, can use multiple band radios simultaneously, which helps to increase the data rates and also helps in reducing the latency in data transmissions, which is crucial for real-time applications.

MLO is ideal for emerging applications like VR/AR, online gaming, 4K/8K video streaming, and cloud computing. It ensures that devices can handle high-bandwidth activities without experiencing significant drops in performances.

If anyone has more insights about Multi Link Operation and its usage in wireless technologies, or if there are any points I might have missed, please let me know. Additionally, I encourage you to check my other articles and share your thoughts as well. Your input is highly valued in expanding our understanding of these exciting new wireless technologies in this wireless world.

Thanks for reading!!!

Email ID: esudharsan2001@gmail.com

Sudharsan E.