How video streaming works

Watching while it downloads

Years ago, you had to download a whole film before you could watch it. Streaming changed that. The service splits each video into short pieces, a few seconds long. Your app downloads the first few pieces, starts playing, and keeps fetching the next ones in the background. The pieces waiting to be played are called the buffer.

It is like eating at a sushi conveyor belt: you do not wait for the whole meal to be ready, you take each plate as it arrives, and the kitchen keeps more coming.

Squeezing the video: compression

Raw video is enormous. Every second contains many separate pictures, each with millions of tiny dots. To make it small enough to send, services use compression: clever methods that remove information your eyes are unlikely to notice. For example, if most of the picture does not change between two moments, such as a wall behind someone talking, the video only needs to send the parts that moved.

The software that compresses and decompresses video is called a codec. Newer codecs give the same picture quality using less data, which is why newer devices can sometimes stream better on the same connection.

Copies close to you

If everyone in the world fetched a popular show from one building, it would be slow and fragile. Instead, streaming services keep copies on thousands of servers spread around the world, in a system called a content delivery network (CDN). Your app is sent to a copy close to you, often inside or near your own internet provider. That shortens the journey and keeps things fast, even at busy times.

Our article What is a server? explains what these machines are.

Why the picture goes blurry, then sharp

Each video is stored in several versions, from low to high quality. Your app constantly measures how fast the pieces are arriving. If your connection slows, it quietly switches to a smaller, lower-quality version so the video keeps playing. When the connection improves, it moves back up. This is called adaptive streaming, and it is why the first few seconds sometimes look soft.

If the connection drops so far that even the lowest version cannot keep up, the buffer runs empty and you see the dreaded spinning circle.

Getting smoother streaming

• Connect the TV or streaming box to the router with a cable if you can.
• Move the router or add a mesh unit if the TV is far from it. See How Wi-Fi works.
• Pause big downloads, game updates and cloud backups while you watch.
• Check the app's quality setting. On mobile data, a lower setting saves your allowance.
• Restart the TV app or the streaming device if it stays blurry on a fast connection.

Tip: Streaming on mobile data can use a lot of your allowance. Most apps let you download episodes on Wi-Fi to watch later without using data.

How much data does it use?

As a rough guide, standard definition uses well under 1 GB an hour, HD uses a few GB an hour and 4K uses considerably more. Our guide Internet speed explained covers the speeds needed for each.