Error Control (&Mn) - US Robotics COURIER User Manual

COURIER HIGH SPEED MODEMS
ERROR CONTROL (&Mn)
The Courier first attempts a connection using V.42 (LAPM) error
control and, if that doesn't succeed, it attempts an MNP
connection. If that doesn't succeed, the Courier tries to connect
without error control. (The process takes much less time than it
takes to key in this paragraph.)
The exception to the above is that HST modems, at high speeds,
do not use V.42 error control. They use USRobotics' proprietary
HST technique, an MNP-type of protocol adapted for its high
speed/low speed asymmetrical modulation.
Error control is possible at speeds of 1200 bps and higher. For
more detailed information, see Appendix A. The following
options are available.
&MO Normal mode, no error control. Because of the nature of
phone line channels, this is never recommended for calls
above 2400 bps.
&Ml This setting is exclusive of the modems' error control and
is used only for Synchronous mode. See Chapter 7,
Synchronous Operations, for instructions.
&M2 Reserved.
&M3 Reserved.
&M4 Normal/ ARQ mode. This setting is the default. If the
remote modem doesn't recognize the Courier's error con-
trol signals-V.42, HST or MNP-the modem operates in
Normal mode, as though it were set to &MO.
NOTE: V.32 bis modems reverting to Normal mode
transfer data at high speeds without the reliability of error
control. To avoid this, V.32 bis and V.32 modems, local
and remote, should always be set for error control. HST
modems, if unable to establish an error control
connection, drop to 2400 bps.
&MS ARQ mode. If the remote modem doesn't recognize the
error-control request-V.42, HST or MNP-the Courier
hangs up.
Interface Controls 4-5