Flow control

Flow control (or handshaking) is a method used by various devices to cope when one device is sending data to the other faster than it is being processed. The receiving device can ask the sending device to pause while it catches up. For example, if a printer's buffer is full of data that it still has to print, it tells the computer to stop sending data until it is ready to accept more.

There are two methods of flow control which can be configured using the Serial Manager:

• hardware flow control (such as RTS/CTS) consists of two control signal lines between the DTE and DCE that are used to control the flow of data between the devices.

• software flow control (such as XON/XOFF) uses special control characters embedded in the data stream. This method will not work with binary data as the control characters themselves could be data.

Flow control is usually necessary because there can be different data transfer speeds between two computers connected with modems.

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.