Wi-Fi 7 MLO in action: EMLSR

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.

In this blog we will take a deeper look at one of the aspects of EMLSR (Enhanced Multi-link Single Radio).

EMLSR 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 devices in congested environments without drastic increase in complexity.

The setup used to run the test is given below:

Let us take a look at the relevant PCAP screenshots:

 Association Response:

Action frame: The action frame is used to enable EMLSR mode. The data transfer happening after this frame can happen in the EMLSR mode. The receiver should listen for RTS on both the channels and be ready to receive data in the channel the sender chooses to send.  

Now let us take a look at the Wireshark I/O graphs.

The Wireshark I/O graph with packets per 1 second interval is shown above. The red line represents 5 GHz and the blue line represents 6 GHz. Initially the data was getting transmitted only in 6 GHz. When we injected 6 GHz noise the action frame got triggered and then data was getting transmitted in both the links. Look closely at the graph to see the action frame represented using a square shape. The graph shows as if both the links are transmitting data simultaneously once the action frame is triggered.

How can this be possible when EMLSR is able to transmit only in one channel at a time?

To answer this question we need more granular details. The graph below shows Wireshark I/O with packets per 1 ms interval. Here again the blue line represents 6 GHz and the red line represents 5 GHz. In this graph we can see the rapid switching between 5 GHz and 6 GHz. We can also see that only one link is transmitting data once EMLSR is triggered by the action frame (that little square again !).

This rapid switching helps EMLSR function better in noisy environments and provide a better user experience.  One of the advantages is reduced latency due to the rapid switching to a less congested link. This helps in improving user experience of time sensitive traffic like video calls, XR and online gaming. 

Let us take a closer look at the graph again. Here the red and blue lines are more clearly visible.

To get data like this we will need a multi channel time synchronized Wi-Fi 7 sniffer with high accuracy.  

Next Steps:

In the future blogs we will cover how EMLSR helps to reduce latency with some tests in real environments.