| | Compression : Applications, Advantages and Disadvantages | 
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The three most common applications for which data compression is used are :
Software is often distributed on a set of floppy disks. Some packages can use in excess of 30 floppy disks. The software must be installed onto a hard disk before it can be used. Software manufacturers usually compress the files that make up a package on the floppy disks. When the software is installed onto a hard disk the files are decompressed by the installation software.
| Advantages | Software can therefore be put onto fewer disks reducing distribution costs. |
| Fewer disks means that the user does not need to swap the disks as often making installation more convenient. | |
| Disadvantages | Because the software is compressed it can not be run directly off the floppy disks. It must be installed first. |
Files stored on hard disks can also be compressed. Some utilites will compress and decompress individual files whilst others can compress an entire hard disk automatically as files are saved and loaded.
| Advantages | Files take up less space so you can store more files on a hard disk. |
| If a file takes up less space to save then the cost of storing the file is reduced. | |
| Disadvantages | You must buy a compression program to compress or decompress files. | The compression program will use up some of the computer's memory, leaving less available for other programs. |
Compressed files may load more quickly or more slowly than files that are not compressed. This will depend upon how well compressed a file is and the speed of different components of your computer hardware.
Telephone lines will only allow the transmission of data at very slow speeds (e.g. 3Kb per second). To transmit a picture the size of an A4 page can take several minutes. If data that is to be transmitted can be compressed before it is sent then there can be considerable advantages :
| Advantages | Faster transmission time. |
| Reduced transmission costs. | |
| Disadvantages | Compression can only be used if both the transmitting and receiving modems support the same compression procedure. |
There are a variety of agreed standards for compressing data that is sent over telephone lines. Some modem manufacturers claim compression ratios of 1:4 (i.e. data can be reduced to a quarter of its original size before being transmitted). Such high compression rates are rarely achieved.
GCSE ICT Companion 04 - (C) P Meakin 2004