Data transfer speed

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

Data transfer speed

When you initiate a dial-up connection, there are three separate components that determine the speed of the connection:

• The local interface speed is the speed of the connection between your computer and your modem, also called the DTE rate.

• The connection speed is the speed at which data is transmitted between modems over the communication lines, also called the DCE rate.

• The remote interface speed is the speed at which data is transmitted between the receiving modem and computer.

The actual speed at which data is transmitted between computers depends on all three rates. When you buy a modem, the manufacturer normally specifies the maximum connection speed. The maximum port speed that can be set in Phone and Modem Options represents the maximum local interface speed that communications programs can use to send data to the modem. The remote interface speed may also limit transmission speeds. For example, an online service provider must support the same 56 Kbps protocol used by your modem for you to receive high speed transmissions with a 56 Kbps modem. (See Attaining fast speeds with a 56 Kbps modem.)

Most high-speed modems today fix their local interface speed at a given value instead of changing it according to the connection speed. The local interface speed is then normally higher than the actual transmission of data over the phone line. For example, a V.34 modem that supports 33.6 Kbps may have its maximum port speed set for 115.2 Kbps.

This difference, called speed buffering, allows a modem to seem faster than the actual connection speed. Using this, a modem can compress the data and send it in fewer bits. Since there are fewer bits, it takes less time to transmit than the uncompressed data. The V.34 modem may then obtain throughput rates of 33.6 Kbps even though the line is carrying fewer actual bits than that.