Chapter 14 - Wi-Fi
Wi-Fi is not a terribly complicated technology. It takes a modem (like we use to talk over the phone network) and instead of going over the phone network it transmits over radio. Here we can see I've upgraded my home network to include Wi-Fi. I've added a "Wireless Bridge" which takes the bits that come in over its interface to the Wi-Fi modem and bridges them onto its interface to Ethernet network.
This new configuration of Modem -> Firewall -> Router -> Switch -> Wireless bridge is what most ISP customers use today. It's such a common and successful configuration that most ISPs give their customers a single box called a "combination wi-fi modem router" that includes all the circuitry and software for all five of those previously separate boxes. Here in the US, one of these combination boxes costs around $100-300. Usually the ISP will rent it to you for an extra $10 a month on your bill.
These wireless bridges go by another name sometimes, Wireless Access Points (WAP). Though Cardi B's song WAP has taken over the meaning of the acronym since it was released in 2020.
Anyways, the point here is that Wi-Fi created a big problem for network security.
Up until the introduction of Wi-Fi, almost all networks operated under the assumption that communication across the network was secure because the signals only passed through wires that were physically secure inside locked offices and houses. Although getting your network tapped on the outside was possible (think like a hollywood style "phone tap" you'd see in a spy movie), for most people it was fine to just assume that the connection wasn't tampered with and all the ISPs making up the Internet mesh were not seriously interested in spying on your connections.
Because Wi-Fi runs over radio, all communications sent over Wi-Fi get broadcasted in a way that can be listened in on from hundreds of meters around when using a strong enough antenna. This led to a need to develop an encryption code to prevent people from listening in on any data being sent across the Wi-Fi connection. The first attempt at this encryption code was termed Wired Equivalent Privacy (WEP), because it intended to allow you to shout your data all over the nearby radio spectrum while still giving you the privacy you would have had if you were using a wired connection.
WEP has been replaced by several newer encryption standards. Today, most networks use WPA2 (Wi-Fi Protected Access version 2) which utilizes AES encryption. This is not a talk about encryption, but the point is that when you connect to a password-protected Wi-Fi network all your data gets securely encrypted by your device (laptop or smartphone) before being blasted on the radio, and then it gets decrypted by the WAP/Wireless Bridge before the bridge forwards it on to the Ethernet network.
So with this Wi-Fi tech, if I want to send you "hi" it goes like this:
- My program sends "hi" to the Operating System's socket API.
- The OS puts "hi" into a UDP/IP packet format.
- The OS puts the UDP/IP packet into an Ethernet frame.
- The OS hands the Ethernet frame over to a Wi-Fi modem driver program. The Driver program is specific to the hardware. Linux builds most drivers into the kernel, but some drivers can come as loadable kernel modules.
- The Driver encrypts the data using AES encryption.
- The Driver delivers the encrypted data as binary to the physical Wi-Fi modem inside your laptop, which is normally attached as an expansion card but there are also USB Wi-Fi modems you can buy. If you're really old-school, you can get a Serial Wi-Fi modem.
- The physical Wi-Fi modem inside your laptop or phone modulates the signal so it will go across radio properly, then pushes it as voltage over to an antenna.
- The antenna does what antennas do and causes the voltage to leave the antenna and splatter all around as radio waves.
- The wireless bridge's antenna catches some of those waves which turn back into voltage.
- The wireless bridge's modem demodulates the signal back into binary data.
- The wireless bridge decrypts the binary data back into an Ethernet frame.
- The wireless bridge forwards the Ethernet frame onto the Ethernet network.
- Then all the regular networking stuff we've already talked about happens.
It's a lot of work to send just 16 bits of binary data (01101000 (h) 01101001 (i)), but that's the price we pay for technology.
This is what a Wi-Fi modem expansion card for a desktop PC looks like.
This is what they look like for laptops (the white and black wire there are antennas, one to send and one to receive)
This is what a Serial wifi modem looks like
This is what USB ones look like. The tiny ones have really awful tiny antennas basically curled up inside them, but they do work. I use that design a lot on Raspberry Pi computers