Compressing files is the processes of making them smaller in terms of the number of bytes of data that they contain. Compression can be described as lossless, where the a compressed file is exactly the same as the original file, when it is uncompressed or lossy, where data that is not needed in the file is stripped out and is no longer available. The zip convention uses lossless compression when a folder is converted to a zip folder. All Zip programs are lossless. Converting uncompressed audio files to a compressed audio format such as WMA (Windows Media Audio) uses lossy compression. Data representing sound that is beyond the range of human hearing is removed from the file during the conversion process. The resulting audio file can be played, and the removed data will not be noticed by a listener.
Storage space, such as that provided by computer hard drives, comes at a price. Compressing data files allows you to store more files in the storage space you have available. Lossless compression, used in zip file technology, will typically reduce a file to 50 percent of its original size. However, you will not see a difference in the file size if your zip files are already in a compressed format, such as MP3 audio files or PDF (Portable Document Format) text-only files.
Bandwidth and Transfer Speed
The download process uses network bandwidth whenever you download a file, such as an MP3 audio file, from a server on the Internet. Bandwidth is the speed at which the network transfers data and is measured in Mbps (megabits per second). Compressed files contain fewer "bits" of data than uncompressed files, and, as a consequence, use less bandwidth when you download them. This means that the transfer speed, that is to say the time it takes for your file to download, is quicker. It will take 10 seconds to download a file if you have bandwidth of 1Mbps available, and you are downloading a file that is 10Mb (megabits) in size. It will only take 5 seconds to download the file if the file is compressed to 5Mb.
The costs of storing your data are reduced by compressing your files for storage because you can store more files in your available storage space when they are compressed. You will need to buy a second 250MB drive if you have 500MB (megabytes) of uncompressed data and a 250MB hard drive on which to store it. You will not need to buy the extra hard drive if you compress the data files to 50 percent of their uncompressed size. This saving can be applied to the costs of maintaining an Internet connection. Many contracts with Internet Service Providers (ISP) include charges for the amount of data that you download. Download compressed files, and you are downloading much less data than you would be if you were downloading uncompressed files. Your Internet download charges will be less as a consequence.