Multimedia applications are made up of individual elements that have widely different information transfer properties. For example, printed text requires the reader to scan sequences of code and reconstruct the code into words, sentences, and paragraphs.
As elements become more visual, the data rate requirements for network multimedia applications span a wide range due to the variety of applications.
Still images most often refer to photographic reproductions of existing objects. There is a famous saying ‘A Picture Paints A Thousand Words‘. A still image can transfer information faster than text, since human beings process visual information much faster than written information.
Let’s consider the data rate requirements for these applications.
At 1200 dots per inch (dpi), an 8½ inch x 11 inch page translates into (8.5 x 1200) x (11 x 1200) = 10200 x 13200 = 134,640,000 dots. If the page is a grey scale image that uses 256 shades of grey, each dot represents one byte of storage space. Therefore, the whole image requires 134,640,000 bytes or 134.6 MB of storage space, uncompressed.
Even with compression, if many such images are expected to be simultaneously transmitted over a network during normal operations, the network must be specifically designed to accommodate such traffic.
Audio is a dynamic element that can be used to enhance multimedia applications in many different ways. The use of sound contributes to a more realistic representation of information. Capturing and converting audio into digital format can result in very large files.
The ultimate size of the captured audio file depends on four factors:
For a recording in stereo, with a sampling rate of 44,000 hertz (44 kHz) and 16 bits used for each sample, a one-minute recording requires over 10 MB of disk storage capacity.
The calculation is as follows:
Video is the element most often associated with the term multimedia. The communications channel capacity and storage requirements for transmitting and storing digital video are the most demanding of the multimedia elements. One minute of high-quality uncompressed video can consume 500 megabytes (MB) of storage space.
Expanding on that famous ‘picture’ expression, ‘A Video Portrays A Thousand Pictures‘. Video is a sequence of frames-still images-played back in rapid succession. In the case of digital video, the screen images displayed are in a digital format made up of many individual dots or pixels. Video graphics array (VGA) monitors can display resolutions of 640 pixels across by 480 pixels down up to ultra video graphics array (UXGA) resolutions of 1600 pixels across by 1200 pixels down.
To digitally display true, photo-realistic color, each pixel requires three bytes of storage-one each to describe the red, green, and blue (RGB) color components of each pixel. Therefore, the 1600 x 1200 pixel display showing full screen video that is uncompressed would require 5,760,000 bytes (1600 pixels x 1200 pixels x 3 bytes/ pixel) of information to display one frame of video. In actuality, most video is transmitted compressed using standards based encoding schemes such as MPEG2 (Motion Picture Entertainment Group) that require a fraction of the digital information required for uncompressed.
The perception of motion occurs when a series of frames are displayed in rapid succession known as the refresh rate. For example, standard analog television displays use a refresh rate of 30 frames per second (25 frames per second in many international regions). Higher resolution applications require analog refresh rates more than double that on the order of 72 frames per second. To achieve this digitally uncompressed over a data network would require a data throughput of 8 bits/ byte x 5760000 bytes/ frame x 72 frames/ second = 3.3 Gbps.
At 3.3Gbps, it is clear that constant digital video traffic would rapidly cause most networks to fail due to congestion, especially if document sharing and text/image file transfer is added and occurring at the same time in real time.
The success of today’s new networking applications can be a double-edged sword for network administrators. State-of-the-art multimedia, Internet and groupware applications can bring the enterprise major advances in productivity and cost savings.
At the same time, these multimedia applications can tax the bandwidth needs of even the most robust networks, applying pressure at the device, the server and the switch, while creating higher capacity demands at the network core.