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Operation
Flow Control
Flow control and buffering compensate for the rate differences of error correction and data
compression. In combination with internal data buffering, flow control permits portions of a data
link to operate at different speeds. For example, the rate of transfer between the local DTE and
modem can be greater than the VF line speed negotiated between the local and remote modems,
while the modem-to-DTE transfer rate at the remote site can differ from both.
The modem buffers compensate for such differences to a large degree, but flow control comes into
play when data traffic approaches the limits of buffer capacity. Flow control meters incoming data
and can stop it from coming in when there is not enough room available in the modem's internal
buffer. Without flow control, data will be lost if the modem port and the VF line interface don't
operate at the same bps rate.
Flow control can be achieved either by software or by hardware. Software flow control involves
control characters inserted within the transmitted data. Hardware flow control is achieved by
altering the voltage level of various signal lines at the interface between the computer's serial port
and the modem. The modem supports three forms of hardware flow control: one that is compatible
with asynchronous operation (described below), and two that are only for use with synchronous data
compression.
Software Flow Control
Software flow control uses the XON (Control-Q) and the XOFF (Control-S) characters. The
characters are inserted in the transferred data. When using XON/XOFF software flow control,
make sure that only the desired element in the system responds to the flow control characters. If you
are using XON/XOFF to control the local serial port, you may not want the flow control characters
to be passed along to the remote modem with the data.
The \G command is used to determine whether the modem processes the flow control characters
and passes them to the remote modem along with the data, or simply processes them and deletes
them from the data stream.
Hardware Flow Control
Hardware flow control can be either unidirectional or bi-directional. Exactly how you set the flow
control commands depends on your application. Bi-directional hardware flow control is
recommended if your application demands both file uploads and downloads and the transfer
protocol does not support any form of XON/XOFF.
Hardware flow control for asynchronous operation usually uses the signals RTS (request to send)
and CTS (clear to send). Study the commands \G and \Q in the Extended AT Command Set in
Chapter 4 for further information on this topic.
In MNP Reliable Mode or V.42 Mode the protocol itself controls data flow on the modem port.
Retransmissions due to a very noisy phone line may cause the buffer to fill, however, so the use of
flow control on the serial port is still important.
060R122-000
Issue 15
SpectraComm Dual V.34 Modem
Installation and Operation
Asynchronous Operation
3-31
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