Multi-Link Operation (MLO) – One Wi-Fi, Many Radios
MLO is the fresh stuff – the kind of thing that changes how radios talk. Defined in EHT (802.11be), this thing lets Wi-Fi devices chat over multiple links – same band, different bands, same time, different time. You get the idea. It's like the difference between juggling one ball and spinning five at once. Smarter, faster, more chaotic if you mess it up.
What is an MLD?
MLD = Multi-Link Device. There’s two kinds you care about:
- AP MLD: The boss. It manages multiple APs on different channels.
- non-AP MLD: The client. That’s your phone, your laptop – it joins up with the AP MLD and can do cool multi-link stuff… if it’s built right.
One non-AP MLD can only be linked to one AP MLD at a time. The client always starts the dance by associating.
Upper MAC Does the Heavy Lifting
The MLD’s upper MAC (yes, there’s one upper, many lowers) has a bunch of jobs:
- Association & Auth – Gets the whole relationship started.
- Security – Handles PTK/GTK/IGTK/BIGTK, including PMKSA stuff per link.
- Sequence Numbers – Keeps track of frame counters across links. One 48-bit PN for unicast with PTKSA. Same SN logic for group frames.
- TTLM (TID-to-Link Mapping) – Chooses which link gets which traffic stream. Important stuff.
- Aggregation – Reassembles MPDUs that came from different links.
- In-order Delivery – Makes sure packets show up in the right order. Even if they came from different radios.
How MLO Gets Set Up
When you wanna join an AP MLD, your non-AP MLD needs to include the Basic Multi-Link element in its Association Request. No excuses. That element says “I’m multi-link capable and here’s my setup.”
Also, all APs under the same AP MLD need to advertise the same RSNE – except for a few fields (AKM Suite List and MFPR bit can differ). They all need to share at least one AKM suite though.
Wanna change stuff later? There’s a Reconfiguration Multi-Link element for that. Use it to add or remove links as needed. Flexibility, baby.
Link Management – Who Talks Where?
TID-to-Link Mapping (TTLM)
Decides what stream (TID) goes over which link. It can be set manually, negotiated, or adjusted on the fly. The AP MLD should be smart about this – consider client radio types, if it's single-radio, power constraints, etc.
STR vs NSTR – Simultaneous or Not?
If your device can transmit and receive on multiple links at the same time, that’s STR. If not, it’s NSTR. Your MLD tells the AP MLD this by flipping bits in the NSTR Indication Bitmap in the Basic Multi-Link element:
- 0 = STR
- 1 = NSTR
This matters for scheduling – the AP can’t expect you to TX and RX on two links at once if you’re NSTR.
Block Ack in MLO
When two MLDs agree on Block Ack, there’s one shared scoreboard context per TID – but each link can run in full-state or partial-state mode. Keeps it tidy but flexible.
MLO Power Management – Don’t Burn Your Radios
Multi-link needs multi-power-smarts:
- Listen Interval – Says how often at least one STA in the MLD wakes up to catch beacons.
- Max Idle – This timeout is applied at the MLD level. As long as the whole MLD isn’t idle past that, you’re good.
- More Data Subfield – If the AP has more data for you on a specific link, it marks this bit in the frame. No guessing needed.
Why MLO is a Gamechanger
- One STA, multiple radios, multiple links.
- Different bands? Sure. Same band? Also fine.
- Reorder packets, secure per-link, map TID to link, merge frames. All good.
- MLO isn’t just about speed. It’s about flexibility, resiliency, and spectral efficiency.
MLO isn’t optional for high-performance Wi-Fi anymore. If your device can’t do it, it’s already falling behind. And if you're designing for it – get that upper MAC tight, that TTLM logic solid, and make sure your NSTR/STR flag is dead honest.