Modem-To-Computer Flow Control - Paradyne 3760 User Manual

Advanced Modem Concepts

Modem-to-Computer Flow Control

Often, your computer is able to send data to your modem faster
than the modem can transmit it across the telephone line. As a
result, the modem's buffer begins to fill with data received from
the computer. To prevent the buffer from filling and overflowing,
which would result in data loss and errors, the modem tells the
computer to stop sending data.
When the modem's buffer is almost full (usually 80%), it tells the
computer to stop sending data as illustrated in Figure D-1. Once
the modem has a chance to transmit its buffered data, it tells the
computer to continue sending data.
Figure D-1. Modem-to-Computer Flow Control
To do this, the modem can use either software characters
(XON/XOFF) embedded in the data stream or hardware
signals (CTS/RTS) on the serial connector to control data flow.
The \Q command lets you determine which flow control method
the modem uses to stop data from the computer, as well as
which method the computer uses to stop data from the modem.
In some situations, the computer can stop data flow from the
modem as illustrated in link A—B of computer-to-modem flow
control in Figure D-2. In this instance, if Computer A's buffer
begins to fill, it can stop data flow by sending an XOFF to
Modem B or by lowering its RTS signal to Modem B.
D-4
Issue 1 November 1994