Why Wi-Fi 7 Multi Link Operation?

This arcticle mainly focuses on the functionality and usage of Wi-Fi 7 Multi Link Operational radios rather than using a Band steering/Band Balancing existing Wi-Fi 6 feature.

As we all know Wi-Fi 7 contributes immense support in terms of throughput data rates called as Extremely High Throughput (EHT) due to various factors namely Multi Link Operation (MLO) radios, Multiple Resource Units (MRU) allocation, 320MHz Bandwidth support, Channel puncturing and 4K Quadrature Amplitude Modulation (QAM) etc,. This enhances the network particularly in terms of low latency and higher Bandwidth applications such as 4K/8K video streaming, gaming etc,.

What is Band Steering and how it compared with MLO?

Band steering is the feature where client connect to single band either 2.4GHz or 5GHz or 6GHz radio depending on the Band balancing threshold that we keep on the particular radios. If the threshold is exceeded, client will steer to other participating radios in the AP by sending BSS Transition Management (BTM) request to other radios which comes under 802.11v standards.

Prerequisite: Same SSID should be broadcasted on all the radios in an AP, so that client can steer from one band to other.

From the above figure, you would have understood the Band Balancing feature. Our main aim is not to overload any single Band radios on that AP and make sure that your Client devices (Mobiles, laptops) also supporting all the Bands that AP is supported.

When switching from one band to the other band, Client has to go through authentication, association, four-way handshake every time to obtain its own Pairwise Transient Key (PTK). This is because MAC address is changed when client is switching bands.

Note: In my first article, I explained how PTK is generated.

PTK = PSF(ANounce, SNounce, PMK, MAC address of AP & STA)

PSF - Pseudo random Function

PMK - Pairwise Master Key

MAC - Medium Access Control

This causes the network traffic disruption and slows down the network communication in the network.

To address this challenge, Multi-Link Operation (MLO) was introduced in Wi-Fi 7 (802.11be) devices.

How MLO solves this problem?

MLO has the capability to link multiple radios (2.4G/5G/6G) together simultaneously and ensure the seamless traffic flow across the network and leading to low 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,

Thanks for reading!!

Sudharsan E.