A CDN (content delivery network) is a fleet of distributed edge servers that cache video close to viewers so most requests never travel back to the origin. Instead of every viewer pulling segments from one central server (slow for distant users and ruinously expensive at scale), the CDN serves popular segments from an edge location near the viewer, cutting latency and offloading the origin.
For OTT the CDN is non-negotiable: it is what makes global delivery to millions of concurrent viewers physically and economically possible. The key metric is cache-hit ratio — the share of requests served from the edge rather than fetched from origin. A high hit ratio keeps both latency and origin egress low; a low one means the origin is doing the CDN's job.
CDNs are billed primarily on egress (bytes served), which makes them the dominant recurring cost in most platforms and the reason cache efficiency, manifest design, and multi-CDN strategy get so much engineering attention.
