Eli5:Why is it that cell phones , with their tiny antenaes are able to download files at LTE speeds inside concrete buildings from cell towers kilometers away yet getting wifi to work behind a corner is an exercise in frustration?
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All wireless communication suffers from something called “attenuation”. Attenuation means a reduction in signal strength. Many things can cause attenuation, but physical objects are one of the more common causes of attenuation when we’re talking about electromagnetic waves.
A physical object can be anything: a wall, a person, a mountain, you name it. Our world is full of them, and they all attenuate the radio waves we use for wifi and cellular communications, not to mention broadcast television signals and radio stations.
An interesting characteristic of radio waves though, is that waves of different frequencies are attenuated different amounts by different physical objects. As a general rule, the higher the frequency, the greater the attenuation a radio wave will suffer from a given object.
WiFi uses 2.4 GHz (2,400 MHz) and 5 GHz (5,000 MHz) bands exclusively. There are no lower frequency bands for it to choose from. Both of these bands are easily attenuated by common building materials, so the walls in your home attenuate your WiFi signal easily. Per the rule of thumb I mentioned earlier, the higher frequency 5 GHz band is more easily attenuated than the lower 2.4 GHz band.
Cellular devices have multiple bands to choose from. Some examples are 700 MHz, 850 MHz, 1,900 MHz (1.9 GHz), and 2,100 MHz (2.1 GHz). Newer wireless technologies like 5G utilize frequencies as high as 52.6 GHz. When you are outside, the device will use a higher frequency band for maximum performance, while switching to lower bands for better indoor performance.
Because they are working on two very different technologies.
They use different waves of radio frequencies. Cell Phones operate in terms of Mega Hertz, Wifi Operates in terms of Giga Hertz.
Since wifi uses a much higher frequency, when you have a good wifi connection you can transfer way more information much faster than cellular is capable of.
The drawback is that because wifi uses this much faster frequency, it can not travel as far and is blocked relatively easily.
Cellular systems operate at a lower frequency, meaning they can travel further, but have a lower max speed of information transfer.
To give even more context radio, like your car radio, is an even Lower frequency, and can travel dozens to hundreds of miles, but can only move enough information to reliably play audio (and maybe some text information about the song).
TL:DR The higher frequency you go, the faster you can transfer information, but the shorter distance it travels.
It is not that hard to get wifi to work over several kilometers either, just as long as there is no corners. Actually the longer distances allow the signals to go parallel to the walls so they can go through the least amount of concrete and rebar as possible. In addition to this most taller buildings have the majority of concrete and rebar in its center columns thus allowing signals through the outer walls but not from one side of the building to the next. If you walk around the corner of the building you are probably no longer connected to the same cell tower. People do not notice this because there are way more cell towers then most are aware of, they hide them well. While a typical office building need about four wifi access points per floor to provide good access it typically connects to over 10 cell towers within half a kilometer radius. So no matter where you are in relation to the columns of the building you are likely in sight of one of them.
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