iPhone 16 supports tri-band Wi-Fi 7 and Multi-Link Operation (MLO). More specifically, the Enhanced Multilink Single‐Radio (EMLSR) mode. The client connects using 2 different Wi-Fi bands, only actively uses 1 of them and listens on both bands simultaneously. Let’s enable it on an access point and verify that it works.
We have a 320 MHz channel configured on the 6 GHz radio. This is for experimental purposes only. Please use narrower channel in production to avoid adjacent channel interference with other 6 GHz access points.
From client perspective, the iPhone connects using 160 MHz channel width as that is the maximum it supports.
The tri-band SSID is announced in 2.4 GHz, 5 GHz and 6 GHz bands. It is up to the client device to choose the preferred band. MLO-capable Wi-Fi 7 clients can also enable the MLO feature.
Although iPhone 16 supports MLO, the phone itself doesn’t indicate if MLO is active or not. So our only option is to monitor it from the access point’s side. This is a consumer access point and it doesn’t provide a huge amount of detail. I am hoping to retest this with a proper enterprise-grade AP when I can.
Single band at a time
From the Association Request sent by the iPhone to the access point, we can see that it advertises support for only one band at a time.
⏬ Feel free to download the Association Request Wi-Fi frame and dig deeper.
6 GHz and 2.4 GHz EMLSR MLO mode
With the default settings of the TP-Link Deco BE85 Wi-Fi 7 access point in place, the iPhone establishes MLO using 2.4 GHz and 6 GHz. It actively uses 6 GHz. 2.4 GHz is there for backup purposes.
The iPhone uses its 160 MHz wide channel capability and actively pushes all data using the 6 GHz channel 69 as I am trying to demonstrate below using Oscium WiPry Clarity spectrum analyser. Check the “waterfall diagram” that shows the top 160 MHz of the 320 MHz channel being busy processing the data transmission.
The 2.4 GHz link just sits there in the background, unused. Using the same method, I verified that there is no spectrum utilisation whatsoever on the 2.4 GHz channel.
5 GHz and 2.4 GHz EMLSR MLO mode
When we change Preferred Wi-Fi Band setting to 5 GHz, the iPhone establishes 5 GHz active MLO link and 2.4 GHz as backup.
6 GHz and 5 GHz EMLSR MLO mode
Now, how do we force MLO using the two modern bands? For the purposes of the demo, I simply disable 2.4 GHz radio on the access point.
The phone establishes 6 GHz active data connection and uses the 5 GHz band as a backup. How can I be so sure? I watched the spectrum and generated nearly 900 Mbps of data over the wireless link. While the 6 GHz channel shows high utilisation, the 5 GHz channel shows no signs of use.
On the iPhone, we see active channels 69 in the 6 GHz band. That matches what I’ve just seen using the spectrum analyser.
How to trigger MLO band change?
Now, I connect the iPhone using 5 GHz channel. I am going to saturate the channel with other client’s traffic. My hope is that high channel utilisation makes the iPhone stop using the 5 GHz channel and switch to the backup 2.4 GHz channel.
And the result? It correctly detected and reported high channel utilisation, but the MLO band change did not happen.
So channel utilisation on its own did not do the trick for me. Perhaps the algorithm penalises and tries to avoid the 2.4 GHz band which is typically in a much worse condition than 5 GHz? Or high channel utilisation must persist for a longer period of time? Time will tell.