Transparent XON/XOFF Local Flow Control in Hayes V series Modems
Flow control during error-control modem connections avoids losing data
during error-control protocol retransmissions. Flow control allows the DTE-DCE
interface to run faster than the line speed, providing increased throughput.
This is due to stripping of start and stop bits by the error control protocol
and data compression techniques such as V.42bis. When the interface runs
at high speed, data loss due to modem buffer overrun may occur if flow control
is absent or functioning improperly.
Hayes Vseries modems support the industry standard RTS/CTS hardware flow
control (invoked by the &K3 command) and XON/XOFF character-based flow
control (&K4) schemes. However, in some situations, it is not possible
to use either of these. For example, hardware flow control cannot be used
if:
1. DTE serial port does not support the RTS and/or CTS hardware signals.
2. Corresponding conductors are not present in the interface cable.
3. Intermediate equipment does not properly transfer the RTS and CTS signals.
XON/XOFF flow control is not available if the XON and XOFF characters appear
in the user data or in the control information associated with the file
transfer protocol. This would interfere with the use of these characters
for flow control, resulting in failure of data transfer and possibly locking
up the interface.
Hayes provided a third flow control scheme, which accommodates systems which
cannot use RTS/CTS but which must transfer data containing the XON and XOFF
characters. This scheme, known as Transparent XON/XOFF Local Flow Control,
uses only the Transmit Data and Receive Data circuits. Transparent flow
control, invoked by the &K5 command, allows
the transfer of all 256 possible 8-bit characters over the interface without
interfering with the flow control scheme.
For more information, click on one of the following topics:
Summary (Transparent XON/XOFF)
Transparentization (Transparent XON/XOFF)
Flow Control (Transparent XON/XOFF)
De-Transparentization (Transparent XON/XOFF)
Examples (Transparent XON/XOFF)
Conclusion (Transparent XON/XOFF)
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