What is Flow Control? Print

In modem communications there are two speeds used to describe the process of transmitting data between the connected devices. The first is called the DTE (Data Terminal Equipments) speed. The DTE speed refers to the speed at which the computer communicates with the modem. The second is called the DCE (Data Communication Equipment) speed. The DCE speed refers to the speed at which the two modems communicate. This difference is important for the discussion of data compression and flow control.

Your modem provides both error correction and data compression features. Data compression allows your modem to effectively transfer files at speeds greater than the established DCE speed. However in order to compress data, the modem must receive the data at a faster rate than it is sending it over the telephone line. This is why it is generally suggested that the 'baud rate' setting in your communication software be configured to a speed greater than the modems maximum DCE speed. For example, it is suggested that you set the baud rate to 38,400 bps or greater with a 56,000 bps modem.

Since data is being sent to the modem at a faster rate than the modem is transmitting data, you could expect data loss to occur. To stop this, local flow control between the computer and modem is used to tell the computer when to start and stop sending data. There are two types of local flow control. They are:

XON/XOFF (Software) Flow Control

XON/XOFF flow control relies on embedding special characters as part of the data stream to tell the computer when to start and stop sending data. The XOFF character (13H) is used to tell the computer to stop sending data. The XON character (11H) tells the computer to begin sending data again. XON/XOFF flow control is enabled in most modems with the AT&K4 command.

RTS/CTS (Hardware) Flow Control

RTS/CTS flow control is achieved by raising and lowering the voltage levels on the RTS and CTS signal lines of the RS-232 serial interface between the modem and the computer. Since this form of flow control is established with the modem hardware, this form of flow control is sometimes referred to as hardware flow control. RTS/CTS flow control is generally preferred over XON/XOFF flow control with high seed modems. RTS/CTS flow control is enabled with the AT&K3 command. This is the default setting.