Speed considerations

There are three (different) data transfer speeds between two computers connected with modems:

• the speed between the computer and modem that initiates the call (the port speed)

• the speed between the two modems (the line speed)

• the speed between the modem and the computer that answers the call

The port speed can be configured using the Serial Manager. If you use high-speed modems or modems that provide data compression, the port speed is usually set higher than the line speed to take account of data compression. Typically, when transmitting text, a four-fold compression ratio can be achieved. It is generally recommended that you set the port speed to two times the line speed for binary transfers, and four times the line speed for text transfers.

If you do not have a high-speed modem, the simplest method of selecting the port speed is by observing system performance (using sar(1M)) with different port speeds. You should start at the line speed of the modem and try successively higher speeds.