Reason #1 — One bad apple can spoil the whole…

Transfer rates are individual for each device and are also different for up and down traffic. The most common reason for poor Wi-Fi bandwidth is low data transfer rate, experienced by more than 40% of the homes in our study.

34.9% homes have reduced Wi-Fi bandwidth due to weak coverage or interference by analogue radio noise. Lower data rates are more robust than higher ones, so it is a common strategy by routers to downgrade transfer rate when conditions get tough. This maintains some connectivity for the device in question but comes at a significant cost to the Wi-Fi bandwidth for all devices.

Additionally, 10.8% of the homes suffer from what we call “bad apples” — devices use 802.11b as rate set for no apparent logical reason, even though they are perfectly capable of communicating using a faster version of the protocol. We believe this behaviour might be caused by a device-side wi-fi chipset software bug, but have not been able to verify.

In many cases of low transfer rates we see significant data transfers using 5,5 Mbps on the 2.4GHz radio, across routers types and providers. These are all devices that normally use 130 Mbps. In Figure 2 we have made a histogram of the average transfer rate traffic is sent for a number of Norwegian households. Keep in mind the maximal transfer rate is around 100 Mbps for the 2.4 GHz band when overhead is disregarded. The average rate for the average household is less than half of the optimal rate.

The result of this is devastating. When accounting for typical protocol overhead, a device using 5,5Mbps instead of 130 Mbps get reduces Wi-Fi bandwidth from around 80 Mbps to max 3 Mbps. And as both directions use the same radio, this capacity needs to be shared between down and up traffic.

One device with low transfer rate getting low bandwidth is bad, but understandable. What is less intuitive is that the bandwidth is also reduced for all the other devices on the same network, no matter what transfer rates they have. This is an effect of the Distributed Coordination Function (DCF) commonly applied on Wi-Fi routers. The physical reason is that low transfer rate device spend 25x as much airtime on the radio frequency to transfer the data. This means it effectively blocks everyone else.

So what typically happen is some family member using the Wi-Fi far away from the router, and bandwidth for everyone else suddenly drops next to nothing. The Wi-Fi bandwidth for all devices is determined by the worst performing device!

New top range routers that support Airtime Fairness algorithms will reduce this collateral damage, but Airtime Fairness also introduce new inefficiencies in static resource allocation (i.e. effectively splitting bandwidth potential in equal share for each active device).

So the best — and cheapest — approach is just to avoid low transfer rates altogether.

Want to know the second reason for why Wi-Fi sucks?