Why plug in a cable?

One channel, everyone hears

Picture four stations, each with a handheld radio, all on the same channel. Whatever one station says, every radio in the room hears. While one talks, the other three have to listen, so the rule is to listen first and talk only when the channel is quiet.

Two stations that press at the same moment garble each other, and neither call gets through. Each stops and waits a random moment before trying again; the shorter wait goes first, and the other hears the channel busy and holds off. One channel works, but every station waits for every other.

A line each

Now give every station its own line to a dispatch desk in the middle. The desk notes which line each station speaks from, and passes a call only down the line it is for. Two calls on different lines happen at once without touching, and a station alone on its line can talk and listen at the same time.

The first Ethernet was the shared channel. A switched network is the desk.

Why a cable, then

On Wi-Fi, every device in range shares the air on its channel, taking turns, and joining takes a network name and a password. On a switched cable, each board has a link of its own to the switch.

To the software on an ESP32 the two are the same kind of thing, a network interface, and the same IP traffic can ride on either. The board side, with its pins and its code, is Ethernet instead of Wi-Fi in the ESP32 book.

What the radio room gets wrong

The radio helps picture early Ethernet sharing one channel. Real Ethernet sends encoded electrical signals over cable, not voices through the air, and a modern switch forwards frames by their MAC addresses, not by call signs. From lesson 6 the radios are put away and the bench is the real hardware.

Common mistakes

• Thinking Ethernet is a kind of radio. The radio room is a picture of sharing one channel. Ethernet is a cable link, and Wi-Fi is a different link.
• Taking a cable for Internet access. A cable to a switch is a link on the local network. Reaching the Internet needs IP routing beyond it.
• Assuming nobody on a cable ever waits. On a full-duplex link to a switch nobody collides. On a shared channel, or behind a hub, the stations still take turns.