Routers and clients that support the latest WLAN specification, IEEE 802.11be or Wi-Fi 7, are expected to be released later this year in 2023.
This specification includes notable features such as
- 4096 QAM
- 320 MHz
- Multi-link Operation (MLO)
- Multi-User Resource Unit (MRU).
While the first two features increase peak rates, the latter two features primarily improve latency in congested environments. MLO can also increase peak rates and will likely be marketed in retail environments. In this blog, we will focus on MLO from the Wi-Fi client perspective, as we believe clients will define what happens in the real world.
Multi-link Operation (MLO)
Multi-link Operation (MLO) is a feature that enables multiple links (on different channels on the same band or different bands) to be associated with a single Wi-Fi client for transmission and reception with the AP. Although link aggregation is possible, the current interest in MLO is mainly from the perspective of clients and APs using the most suitable link(s) to avoid congestion and provide better latency.
There are various types of Multi-link Operations that may be supported by client devices (Note: APs will have to support STR MLMR)
- MLSR: Multi-link Single Radio
- MLMR NSTR: Multi-link Multi Radio Non-simultaneous Transmit and Receive
- MLMR STR: Multi-link Multi Radio Simultaneous Transmit and Receive
- EMLSR: Enhanced Multi-link Single Radio
In the case of MLSR, the client has only a single radio but can still support multiple links. A change in the link is possible after a fixed static time once the current transmission is complete. However, this additional time is needed as the time to change the channel by the client is large. As the congestion on links changes very fast, this kind of change would not be helpful.
These devices do support simultaneous transmission/reception on multiple links but are not capable of doing a transmission one on link and receive on the other link at the same time. These kinds of devices would need coordination of transmission between the links (called synchronous MLO) and complex to achieve.
Clients with support for this feature are capable of asynchronous transmission and reception on both the links simultaneously. This is the right kind of client to reap the benefits of MLO both in terms of higher throughput and higher latency. As it can be guessed, this would add complexity to the implementation. Components would also become expensive to reduce what is called in-device coexistence (IDC) interference.
This feature requires clients to listen on two channels but actual transmission happens only on one of the channels. By listening, we mean the device can do Clear Channel Assessment (CCA) and receive a control frame (Ex: RTS) which would indicate the link where the next transmission is going to happen. This mode gets the benefit of dynamic switching of links without adding too much complexity. Since a large number of clients in the market already have two transceivers (2×2 MIMO), this would be a relatively small change to make. Many client vendors prefer this as it gives some of the latency benefits of STR MLMR device in congested environments without the drastic increase in complexity.
Even though simulations show the benefit of MLO (Multi-link operation) in terms of reducing latency, with the variety of clients going to be deployed (including 802.11 a/b/g/n/ac/ax), we would need to wait and see the benefits of MLO in the real world. However we think EMLSR would be the first of the modes to be deployed and tested in the market due to reduced complexity and cost.
Alethea’s flagship product, WiCheck, is utilized by leading OEM/ODM companies to examine AP/routers and wi-fi infrastructure. Alethea is presently focused on enhancing WiCheck to test the latest Wi-Fi 7 capabilities, including MLO. Stay tuned for further updates.
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