Bandwidth Throttling
Setting up a limit on maximum bandwidth that may be used to deliver a CDN asset is called bandwidth throttling. Bandwidth throttling can be used to reduce bandwidth and data transfer costs incurred by a CDN client.
Universal CDN allows to limit the transfer rate:
- For all assets of a specific zone by static zone configuration.
- For a specific asset via cdn_bw URL parameter.
For additional information regarding the cdn_bw parameter, please refer to Limit Access Using a Secret Key.
- For video files automatically.
In case of multiple limits simultaneously set, automatic video throttling has the lowest priority, and URL parameter has the highest.
Bandwidth throttling via static zone configuration
Follow the steps outlined below to set the bandwidth limit for assets of a CDN zone via the Universal CDN Control Panel:
1. Log in to the Universal CDN Control Panel at my.ucdn.com.
2. Navigate to the My Zones tab located in the top left corner.
3. Click on the cogwheel icon 
next to your CDN zone.
4. Click on the Download speed tab.
5. Check the box next to “Limit files download speed”.
6. Specify the limit of the downloading speed by adding a constant rate.
7. Click on the Save Changes button.
Limiting the download speed through the Control Panel for a CDN zone will be overridden by the value of the rate set with the cdn_bw parameter (if such has been included with the secret key feature) for this CDN zone.
Bandwidth throttling via cdn_bw URL parameter
Download speed of assets on each CDN zone can be limited to a predefined rate by including the ‘cdn_bw‘ parameter combined with a secret key.
Please refer to Limit Access Using a Secret Key for more information on how to restrict the assets’ download speed by including the ‘cdn_bw‘ parameter.
Automatic bandwidth throttling for video files
In a MPEG-4–compliant container, every movie contains a “moov” atom. The “moov” atom, also referred to as the movie atom, defines the timescale, duration, and display characteristics of the movie.
Universal CDN offers automatic bandwidth throttling using metadata about video and audio bitrates encoded in the “moov” atom section of MP4 files.
With automatic bandwidth throttling enabled, a short period of a movie is delivered to a user at full speed. It allows a media player to buffer the beginning of the media file before it starts playing and ensures that the required movie metainformation is downloaded first. The rest of the file is delivered at a speed high enough for smooth playback, which is automatically calculated using movie bitrate data.
Automatic bandwidth throttling has two variables set by a client:
- The number of seconds of a movie delivered at full speed.
- Bandwidth allowance above the bitrate of a movie (e.g., 20% above the bitrate).
Requirements for video files
For progressive download and streaming, the “moov” atom must be positioned at the beginning of the file. This ensures that the required movie information is downloaded first, enabling playback to start right away.
To enable Automatic bandwidth throttling for video files for one or more of your CDN zones, please Contact Us.
Limit Access Using a Secret Key