What is CDN?

A video website stores all its videos on servers in New York. A viewer in Paris presses play. The video starts slowly and pauses to load, even though the servers in New York are fast and not busy.

The problem is distance. Every piece of the video must cross the Atlantic Ocean, and each round trip between Paris and New York takes about 80 ms. No faster server can remove that travel time.

The fix is to keep copies of the video close to the viewers. This lesson explains the system that does that, how it serves each request, and the terms used to describe it.