A frame without a cable

The same frame, no cable

An Ethernet cable carries a frame from one board to one switch port. Wi-Fi carries the same kind of frame, with the same kind of 48-bit MAC address, through the air. A radio wave spreads in every direction, and every radio near enough on that channel receives it. The address says who keeps the frame; it does not steer the wave.

Energy, decoded, for me

Receiving is not one thing. A radio can sense the energy and still fail to read the frame, because the signal was too weak for the rate it was sent at: the faster the rate, the stronger the signal it needs. A radio that reads the frame then checks the address and drops it if it is not the addressee. Each float on the bench shows the three as lamps. Press the sensor and the access point reads its frame, while the laptop and the phone, further away, sense only energy.

The signal is a number in dBm, an estimate from a model of distance and loss, on a meter. Nothing about a ripple's height is a signal strength.

The access point

The access point is a station too, and the bridge between the air and the wire: a frame for a server elsewhere reaches it over the air and leaves by its uplink. A home router is usually an access point and a router in one box.

This course stops at a board holding an IP address. What uses the address is TCP and UDP; the code is in the ESP32 book, starting with station mode.

What the tank gets wrong

The ripple tank shows that waves spread, reflect and overlap. Wi-Fi uses electromagnetic waves that travel in three dimensions at nearly the speed of light, through empty space, and the tank does not reproduce Wi-Fi's channel rules or its exact signal strength.

Common mistakes

• Thinking a frame goes only to the device it is addressed to. Every radio in range on that channel receives the energy, which is why encryption matters (lesson 11).
• Taking "in range" for "decoded". A frame can arrive too weak for its rate. The radio senses something and reads nothing.
• Treating the access point as the internet. It bridges the air to a wired network. Whether that network reaches anywhere is a question for IP and the router.