Wireless Physical Link
So with 4804 Mbps 5GHz Wi-Fi capability, I should still be able to get 940 Mbps full internet speed. At this point, we need to understand a concept called wireless physical link.
With WiFi, there is an invisible, wireless link or connection between the access point (AP) or router and client device, just like physical ethernet cable link.
If this link’s speed limit was lower than any other speed limit along the way of my client device and internet, this would be the bottleneck. Below is a screen capture from Asus router web interface.
This shows my iPhone 11 Pro established 1201 Mbps link, which is still greater than 940Gbps internet speed I got at router, but a quarter of Asus claimed theoretical max speed of 4804 Mbps because this is 5 GHz band. Let’s understand how is wireless physical link is formed.
From a general user perspective, two main components define wireless physical link throughput, MIMO and Channel Bandwidth.
MIMO
MIMO stands for multiple input multiple output. Conceptually, think the wireless physical link established between router/access point and client device as a roadway.
The roadway with two lanes each direction allows twice more cars to drive through than one lane each direction. This is the same as saying 2×2 MIMO is twice faster than 1×1 MIMO.
Most wireless clients are 2×2 MIMO even WiFi 6 capable with occasional 3×3 devices. This won’t likely to change for awhile due to balance between speed and energy efficiency.
Here bottleneck concept comes in again. Even the router unit is 4×4 MIMO capable, if client is 2×2 MIMO, the weakest link determines the connection. 4 lane roads merging into 2 lanes will create traffic and maximum traffic accommodated here is indeed 2 lanes worth roadway.
With iPhone 11 Pro being 2×2 MIMO device and Ax11000 4804 Mbps rating is for 4×4 MIMO connection, this is already halved i.e. 2402 Mbps theoretical max speed for physical link with iPhone 11 Pro based on MIMO.
Channel Width
The second half of the core factor determinig wireless physical link throughput is the channel width. Conceptually, think this as analogous to width of each road lane. Up to this point, the actual number is not that important but most standard channel width for 5 GHz band is 80 MHz. However, manufactures, especially with Wi-Fi 6, tries to use 160 MHz channel width for the reported throughput measurement.
Using the analogy, if each lane of the road is twice the width, you can technically drive twice a bigger car, which can theoretically carry twice the amount of passengers or delivery items.
Since iPhone 11 Pro is only 80 MHz capable and Ax11000 reported speed is based on 160 MHz, this is another spec that cut the wireless physical link throughput into half i.e. 2402 Mbps now down to 1201 Mbps. This is indeed the number I have.
*Even some of WiFi 5 devices actually capable of 160 MHz channel width option as well but most routers this is not a default so you need to go to the option and set to 160 MHz. Also, there are some routers/access point using default setting of 40 MHz channel width like UniFi AP. You should change it to 80 MHz to get the better performance.
WiFi (in)efficiency
Now we know why the physical wireless link is 1201 Mbps. However, this still is higher than 940 Mbps at my router.
The last piece for determining real speed is Wi-Fi (in)efficiency. The inefficiency in WiFi primarily relates to overhead and it’s quite significant. Usually there is loss of 30-50% of speed. In another word, WiFi transfer can operate at only 50-70% of wireless physical link speed.
To complete the analogy, this is like saying no roads are frictionless. In fact, most roads are bumpy and you can never reach peak performance of your car because road is so bumpy or may be too many curves.
Below is a table showing wireless physical link speed (PHY) expected for different setup with % efficiency.
As illustrated on this table, with 1200 Mbps wireless physical link between the iPhone 11 Pro and Ax11000, the practical maximum speed I could get by my iPhone 11 Pro is somewhere between 600 to 840 Mbps.
Bingo! This is indeed the actual speed of 603 Mbps I got. So Ax11000 in my setup was 50% efficient. Obviously, as a user I had wished my Asus system was more of 70% efficient system, but that’s a different talk.