What is bandwidth? There are two definitions, both of which are important to Screencast.com account owners.
Bandwidth is simply a measure of the amount of data that can be transmitted through a connection over a given amount of time. Bandwidth, also called data transfer rate, is usually expressed in bits per second (bps).
When a Screencast.com account owner shares content and that content is viewed, they are billed for the bandwidth required to transfer the content throughout a particular billing cycle. Overall, this can be thought of as the cost of sharing content. At the beginning of a billing cycle, the total bandwidth amount starts over.
For a rough calculation of how much bandwidth you might use, assume you have a 10 MB file and it is downloaded once, 10 MB of bandwidth is used. If that same 10 MB file is downloaded seven times, 70 MB of bandwidth is used. Partial downloads only use bandwidth for the part of the file downloaded, so a file’s views multiplied by its size rarely equals its bandwidth consumed.
While it may be difficult to figure out exactly how much bandwidth you will need since you may not know how many people will view your content or how much content you will make available for viewing, Screencast.com makes it easy for you to purchase extra blocks of rollover bandwidth. If all of the extra bandwidth is not consumed in one billing cycle, it will roll over to the next one until it is all used up.
For more information on this subject, watch this helpful video.
Bandwidth is synonymous with capacity. Every machine on the Internet is connected by a cable or another type of connection. This cable has a capacity; it can carry a certain amount of information through it, similar to a hose. This cable may have a high or low capacity. If you compare a garden hose to a fire hose, more water can pass through the fire hose in a minute than a garden hose.
The slowest of these capacities is the bandwidth of the line; it’s the fastest you can communicate between the ends. Think of the cable as a long tube with someone pouring water into it on one end, and someone draining it out the other. You cannot take out more than was put in, and if you pour it in too fast the water will spill (losing data).
For example, a 28.8 kbps dialup modem connection is much slower than a cable modem connection. The cable modem connection can download more data than the 28.8 kbps dialup modem connection can over the same period of time. The cable modem has a higher bandwidth connection than the 28.8 kbps dialup connection does. High bandwidth allows fast transmission of data or high volume transmission just like the fire hose.
File size is measured in bytes. One kilobyte (K or KB) equals about 1,024 bytes. For example, a small image file might be 20K or about 20,000 bytes in size.
Measurements and Conversions
Data transmission measured in bits per second (bps) is called the baud rate or bit rate - it is the measure of bandwidth. It is commonly measured in thousands of bits per second or kilobits per second. The abbreviation for kilobits per second is kbps or simply k. One kilobit equals about 1,000 bits.If a computer can receive 5KB in a second, it would take 4 seconds for it to receive a file 20K in length.
But bandwidth is not measured in bytes—it is measured in bits. 1 byte contains 8 bits. So, if a computer can receive 5KB (5,000 bytes) in one second, it can receive 40,000 bits per second. 5,000 bytes x 8 bits = 40,000 bits per second or 40 kbps.
If a computer connects to the Internet using a 56 kbps dialup modem, in theory, it means that the computer could receive 56,000 bits per second (56 kbps). That would mean that the computer could receive about 7,000 bytes per second. Remember that a byte equals 8 bits, so 56,000 bps/ 8 bits = 7,000 bytes.
So, to receive the 20K image file, the 56 kbps dialup connection would require slightly less than 3 seconds to receive the file (20K/7,000 bytes=2.85 seconds).
Note: A 56 kbps modem cannot actually communicate at 56 kbps. In reality it’s more like 35-45 kbps. A 56 kbps modem connection generally only provides 35-45 kbps of bandwidth.
Computer video files are basically a number of still images called frames combined sequentially into one file. When the file is played on a player such as Windows Media Player, it goes through the video file and displays each frame sequentially in quick succession to create the illusion of motion similar to a video film rolling through a movie projector.
When a file is transferred, frames are continuously delivered from the computer transferring the video to the computer playing it. Each frame is displayed as it is received.
For example, consider a computer connected to the Internet using a dialup modem. If the modem is connected at 40 kbps, it could receive 5,000 bytes (5 KB) of data per second. If each frame of the video was only 5K then the modem could only receive 1 frame per second.
Commercial motion pictures are 24 fps (frames per second), television is 30 frames per second. The more frames per second (fps), the smother the video playback appears to the viewer. So, a 1 fps video is a very slow and choppy video. The number of frames per second (fps) is also called the frame rate.
With a higher bandwidth connection, more frames per second could be received. With a 128 kbps ISDN connection for example, 32 5K frames could be delivered per second.
But, a 5K image or frame is not very big. A small 320x200, 16 bit JPEG file can easily be 20K in size. So, for the modem connected with only 40 kbps of bandwidth, it would take 4 seconds to receive only one frame of the video! At that rate, the video would degrade into a slide show, and not be a video at all.
This is the reason why many videos that you see online or on news sites are very small, and why a dialup Internet connection just does not have enough bandwidth to enjoy a very rich multimedia experience.
As static image files are compressed using various compression algorithms the video and audio data in transferred files is compressed. This reduces the number of bytes in each frame thus reducing the bandwidth requirement to deliver the video. While data compression helps considerably, another step is taken to reduce bandwidth requirements.
Video files are a number of still images called frames combined sequentially into one file. Each frame is displayed at some given number of frames per second (fps) to create the illusion of movement. But many times there is no movement or change in the video between one frame and the next.
For example, a video demonstration of an application may show the opening of a new window and then not change for several minutes while the audio narration explains the application. If nothing changes, there is no reason to send a new frame of video data. The player can just display the same frame. This reduces bandwidth requirements.
Consider a video that is a demonstration of an application. Perhaps, all that is changing in the video is that the cursor moves around the application as the author of the video points out different areas in the application. Instead of sending the entire frame, only the changes to the new frame are sent. The mouse pointer is very small and the number of bytes of video data that represents it is minimal, so very little video data needs to be transmitted to reflect the change between frames. Sending only the part of the frame that has changed can also greatly reduce bandwidth requirements.
Changes from one frame to the next, increases the bandwidth requirements of the video. The more movement occurs, the more area of the screen is changed, resulting in larger amounts of video data that must be sent to update to the next frame. If the entire screen changed from one frame to the next, the entire frame would have to be sent.
There are two types of video frames: key frames and delta frames. Key frames contain all of the pixels that comprise the complete frame. Delta frames only contain what changed from the previous frame. Key frames are placed in the video at regular intervals, either every so many seconds or so many frames. Windows Media Encoder, for example, defaults to 1 key frame every 10 frames. It looks something like this:
Key frame | delta frame | delta | delta | delta | key | etc…
If there is no change from one frame to the next, delta frames can contain 0 bytes of data. If the only change from one frame to the next is the movement of the mouse pointer, the delta frame would contain very little data. If the entire screen had changed, the delta frame would be as large as a key frame, as it would have to contain bytes of data representing every pixel in the frame. Although high key frame rates increase file size, they improve seeking.
Frame rate may or may not have much effect on the bandwidth requirements of the video. If there is a lot of change between frames, then the size of each frame is larger and more data must be transmitted for each frame. In this case, higher capture frame rates require increased bandwidth. But, if there is little or no change between frames, then little or no video data is transmitted for each frame. So, depending on the content of the video, increasing the frame rate may have little or great effect on how much bandwidth is required.
Transferred video is displayed as it is received so the video can start to display the data before the entire file has been transmitted. If for some reason the data is slowed or interrupted, the video will stop playing. Network congestion and other problems are fairly common, and to help ameliorate the interruption of the data transfer, buffering is implemented.
Buffering works by storing a portion of the video locally, and then playing the video by retrieving data from the local buffer. Before the video starts playing, the player downloads some amount of the video and stores it locally. Generally this is not a large portion of the video, usually 10 seconds or so. It then plays the video from this local buffer while continually downloading more of the video to keep the buffer full.
If the network becomes congested, or if the transfer is interrupted for some reason, the player can continue playing from the buffer, and hopefully the interruption will be corrected before the buffer is depleted and the video stops playing.
Buffering can also help with encoding videos that contain spikes of high bandwidth. This can occur if something in the video suddenly requires more bandwidth. For example, in a video demonstrating an application, the only movement might be the mouse pointer moving across the screen for many frames. If the author of the video clicks a button in the application that causes a new window to open, the entire frame might change, requiring a large block of data needing to be transmitted to update the next frame. This causes a spike in the required bandwidth.Modern media encoders take into account the extra time afforded by the buffer. While the extra data caused by the increase in bandwidth is being delivered, the video can be played from the buffer, thereby not interrupting playback. Increasing the amount of buffering time can make the difference between a failed or a successful encoding process.