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To obtain the fastest data connection for a ZyXEL, a number of steps should be
taken:

a) Lock the serial port speed to the fastest/highest speed supported by the 
   host computer.
b) Turn off "auto-baud detect", "dial speed-matching", or any similar parameter.
c) Turn on hardware flow control by enabling RTS/CTS flow control in the 
   communications program.
d) Turn off software flow control by disabling XON/XOFF flow control in the 
   communications program.
e) Turn off DSR/DTR hardware flow control, as this is used for null modem 
   connections.  (Recommended if the communications program supports this
   feature.)  
f) External modem owners must use a modem cable that is wired for hardware 
   flow control.

Notes:  1) Again, the modem cable MUST be wired correctly.  Apparently, some 
           modem cables, even from the factory, are not wired properly for 
           hardware flow control. 

Alan Brown (alan@papaioea.manawatu.gen.nz) adds:
I find that it's generally a good idea to switch the modem to CCITT
DSR (AT &S1), even though no programs I use support this for flow
control.

In CCITT mode, DSR comes up before the CONNECT message (actually as
soon as a link standard is established, even though error correction
may still need to be negotiated), and goes down after NO CARRIER