Data Compression

In order to transmit data at a speed greater than 600 bits per second (bps), it is necessary for modems to collect bits of information together and transmit them via a more complicated sound. This allows the transmission of many bits of data at the same time. Computers are capable of transmitting information to modems much faster than the modems are able to transmit the same information over a phone line. This gives the modem time to group bits together and apply compression algorithms to them. Two common compression protocols by which this is accomplished are MNP-5, which has a compression ratio of 2:1, and v.42bis, which has a 4:1 compression ratio. MNP-5, however, produces a great deal of overhead in its compression, so it actually makes the transmission of pre-compressed (ZIP, for example) files much longer. V.42bis can sense if compression is unnecessary, so can speed up transfer of pre-compressed files as well. If V.42bis is unavailable, it is best to disable MNP-5 data compression when transferring precompressed files