Google Video Quality Report.
YouTube has a global network.
There are multiple copies of YouTube videos in our servers all around the world, so we can start the video stream from as close to you as possible.
pick the shortest, most direct route.
When you click play, YouTube carries the video data through its system to your Internet Service Provider (ISP) via the most efficient path possible. Sometimes, exceptional circumstances mean we may need to use a less-direct path.
We can’t do it alone.
When your ISP receives your video from YouTube, they begin the important job of carrying it across their network to your home. They must ensure there’s enough capacity where they receive the data from YouTube. Otherwise, your video streaming quality will suffer.
Finally, the video must travel into your home.
In addition to congestion in your ISP’s network, your video performance can also be affected by the size of the ISP’s connection into your home, your wi-fi setup, and other in-home factors such as the number of connected devices.
Congestion on any portion of the end-to-end path will affect your video streaming quality, especially at peak times.
This can cause frequent interruptions and fuzzy picture quality. When everything works well, you should get a smooth HD video experience.