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Data compression

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Data compression

Data compression enables information to be transmitted beyond the actual connection speed. Data, particularly text and graphics, usually contain repeated sequences of identical information. Data compression works by replacing many characters of repeated information with a few characters and transmitting only one copy of repeated sequences of data.

Common methods for compressing data include MNP 3, MNP 4, MNP 5, and V.42bis.

The effectiveness of compression varies, depending on the data. Text and graphics may often be compressed considerably. Compression for data that is already compressed may be less efficient because the modem is trying to compress data that is already compressed. V.42bis handles compressed data better than the other protocols.

For any error correction or data compression protocol to work, modems on both ends of the connection must support the protocol. Once two modems are connected, they automatically negotiate to determine the best mutual protocol. For example, if V.42bis fails, the backup is MNP 5, and if it fails, the backup is "no compression."

To achieve maximum rates when transferring data, hardware flow control is required, and the maximum port speed must be set for two to four times the rated speed of the modem. This is usually configured correctly when you install a modem. If you know which type of compression is supported by your modem, you can calculate this value by multiplying the modem speed by the ratio shown above. For example, for a 14.4 kilobit per second (Kbps) modem with V.42bis compression, you would perform the following calculation.

14.4 Kbps x 4 = 57.6 Kbps

This value is the maximum rate that data may be transferred by the modem. The actual throughput depends on the data.

Software data compression

Communication software, such as Network Connections, may support data compression. For example, using a 56 Kbps V.90 modem, you can enable software compression, and experience an average throughput of 44 Kbps. Tests show that software compression can result in higher data transfer rates than hardware compression.

If you use software compression, you may want to check the settings. For best results with software compression, you can check your modem's properties for the following:

  • Enable modem error correction by setting Data Protocol to Standard EC or Forced EC.

  • Disable hardware compression by setting Compression to Disabled.

  • Enable the software compression provided by your communication software.

See Change data connection preferences to change this setting for a modem.