The Channel and Its Limit
The ladder gave us many storages. But steps do not touch: data must travel between them, and it travels through channels. This second refinement of the real machine is small to state and enormous in consequence — the single law at its end explains more slow systems than any other sentence in engineering.
Start with the question
Which is "faster": a courier pigeon carrying a memory card with a terabyte of photos across town, or your internet connection sending the same photos?
The honest answer is: the question is broken — and learning exactly how it is broken is this lesson. "Fast" secretly means two different things, and until you split them, half of all reasoning about machines, networks and systems is fog.
Two numbers, not one: throughput and latency
Every channel — a wire between processor and memory, a cable to the disk, a fiber across the ocean — is described by two independent quantities:
Throughput (bandwidth): how much data the channel moves per second once it is flowing. A river's width.
Latency: how long you wait for the first answer after asking. The river's length.