One of the universal complaints in the broadband world is that WiFi networks operate poorly. So today I thought I’d talk a bit about how WiFi functions. I think it’s probably different than what most people expect.
Most people know that there are two frequencies used for WiFi today – 2.4 GHz and 5 GHz. The 2.4 GHz band covers 80 megahertz of total bandwidth and is divided into 11 channels in the US. That may sound like a lot, but one 802.11 connection requires five consecutive channels. In practical terms this means that almost all WiFi gear in the US is preset to only offer channels 1, 6, and 11 and that means that only three non-overlapping transmissions can occur at the same time. The WiFi in Japan covers a wider spectrum footprint, up to channel 14, meaning they can use four non-overlapping signals simultaneously.
In practical use if you can see three or more WiFi networks you are experiencing interference, meaning that more than one network is trying to use the same channel at the same time. It is the nature of this interference that causes the most problems with WiFi performance. When two signals are both trying to use the same channel, the WiFi standard causes all competing devices to go quiet for a short period of time, and then both restart and try to grab an open channel. If the two signals continue to interfere with each other, the delay time between restarts increases exponentially in a phenomenon called backoff. As there are more and more collisions between competing networks, the backoff increases and the performance of all devices trying to use the spectrum decays. Your data is transmitted in short bursts each time you make a connection and before the restart cycle repeats.
If you’ve ever been in a hotel where you can see ten or more other WiFi signals, the reason for slow speeds is that there are huge conflicts between competing devices. People generally assume that the hotel has a poor Internet connection, but they could have a fast connection and the slo speeds are due to so many devices trying to connect simultaneously. Each WiFi device is rapidly turning on and off repeatedly trying to get open access to a channel. Your device will grab a channel for a short time and then get kicked off due to interference. Congestion has become so bad on the 2.4 GHz band that AT&T and Comcast no longer use 2.4 GHz for video or voice. Almost all smartphone makers no longer recommend using their smartphones at 2.4 GHz.
WiFi has improved dramatically with the introduction of the 5 GHz spectrum. In North America this spectrum swath has 24 non-overlapping channels. However, more than half of these channels are reserved for weather and military radar. However, this still provides a lot more potential paths to add to the three paths provided by the 2.4 GHz spectrum. Unfortunately the 5 GHz band shares the same WiFi characteristics as the 2.4 GHz spectrum and has the identical interference issues. But with more open channels there is still an increased chance of finding a free channel to use.
And interference between devices is not the only culprit of poor WiFi speeds. The network configuration can also contribute to poor performance. Some of the biggest sources of interference are range extenders or mesh networks that are used to try to get better signals. Range extenders listen to all WiFi transmissions and then retransmit them at a higher power level, and usually using a different channel. This creates even more WiFi signals in the intermediate environment competing for an open channel. When you can see your neighbor’s WiFi network, if they are using range extenders they might be always trying to use most of the available WiFi channels.
In a lot of the US we now also see a lot of public hotspots. For example, Comcast is in my neighborhood and I can walk and maintain a WiFi signal is most places from WiFi public signals that are transmitted from every Comcast home WiFi router. These public signals are always on, meaning that the WiFi router is using at least one channel at all times.
Probably the biggest new culprit for poor WiFi performance comes from our quest for greater speeds. The 802.11ac standard operates by merging together a lot of WiFi channels, and divides the whole WiFi spectrum into just two 160 MHz-wide channels. This means that only two devices using this 802.11ac can use up all of your home WiFi bandwidth. This standard was intended to be used to operate in short high-bandwidth bursts, but as people use this for gaming or watching 4K video the channels stay occupied all of the time.
Unfortunately the demands for WiFi are only increasing. The cellular carriers are still pestering the FCC to allow LTE-U, which would using WiFi to complete cellular calls. There are currently tests underway of the technology. We can also expect an increasing demand for WiFi from IoT devices. While most WiFi devices won’t use spectrum continuously, they still place demands on the channels and cause interference. There are also increasing use of devices that are always on, such as video surveillance cameras or smart home controllers like the Amazon Echo. A lot of experts look out five or ten years and expect WiFi to be unusable in a lot of places.