Re: [hylafax-users] High Error Rates revisited.

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

On 2004.08.03 02:38 Stephen Carville wrote:
> I now have all four incoming lines being handled by USR modems.  The
> two
> outgoing lines are still attached to a couple of noname modems.  I
> know that
> this is not ideal but that is what I have :-)
>
> I notice that all the modems default to xon/xoff for flow control when
> setup
> using faxaddmodem:
>
> ModemFlowControl:       xonxoff
>
> Is this ideal?  Is there any advantage in switching to rtscts?

Both XON/XOFF and RTS/CTS are simply flow control mechanisms, i.e. 
their sole design is to keep the software from sending too much data to 
the device and overrunning its internal buffer and thus losing data.  
So if one or both of them do that job properly then you can use that 
one.  If one of them doesn't do that job properly then you can't use 
that one.

You may be surprised at how many modems actually have broken flow 
control mechanisms.  Fortunately, most installations are going to use a 
19200 DTE-DCE baud rate (ModemRate in your config file), and most calls 
are going to be 9600-14400 baud.  If you do the math, it would take 
quite a bit of TIFF data to overrun a moderate buffer with that small 
of a difference in speeds.  However, this issue becomes much more 
pronounced when involving V.34-Fax-capable modems, as the modem config 
file is generally using 56K ModemRate.

Furthermore, you may have a hardware design that eliminates the 
possibility to use hardware flow control.  Some serial port cards will 
interfere with RTS/CTS flow control.  Thus you really can't use a modem 
that doesn't support XON/XOFF with such cards.

XON/XOFF uses signals between HylaFAX and the fax firmware in order to 
control the DTE-DCE data flow.  RTC/CTS uses signals between the serial 
port hardware and the serial port driver to control the DTE-DCE data 
flow.  I don't particularly see one of those as being better than the 
other.

Lee.

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