Wi-Fi, mobile and what changed

The model so far assumed wires. Most devices no longer use them, and wireless breaks assumptions that were safe for decades.

Why Wi-Fi cannot detect collisions

Wired Ethernet uses CSMA/CD: listen, send, and if you hear a collision, stop and retry after a random wait.

A radio cannot do that. Its own transmission drowns out everything else, so it cannot hear a collision while causing one. Wi-Fi uses CSMA/CA instead: listen, wait a random time, then send, and expect an acknowledgement. No acknowledgement means it was lost.

That is why Wi-Fi acknowledges every frame at the link layer and Ethernet does not.

The hidden terminal

A and C can both reach the access point B, but cannot hear each other. Both sense a clear channel, both transmit, and both collide at B.

The fix is RTS/CTS: A asks B for permission, B answers with a clear-to-send that C can hear, and C stays quiet. It costs two extra frames per transmission, so it is used for large frames only.

Mobile networks

A phone moving between towers must not drop its connection. Handover transfers the session to the next tower mid-call. Each generation raised capacity and lowered latency:

What is coming

IPv6 has 128-bit addresses, ending the shortage that NAT has been papering over for twenty years. Adoption is slow because NAT works well enough.

HTTP/3 runs over QUIC, which runs over UDP, and reimplements reliability there. The reason is head-of-line blocking: in HTTP/2 over TCP, one lost packet stalls every stream on the connection. QUIC keeps streams independent.