odd thing..

[admin]

Nothing is different. K2 and K3 are supported only so that they don't return error messages. They all do the same thing.

Every modem manufacturer treated these differently. AT&K1 is RTS/CTS, unidirectional (which makes no sense to me). Here is the actual list of Hayes commands for AT&K:

AT&K0 - No flow control
AT&K1 - Enables unidirectional RTS/CTS hardware flow control.
AT&K2 - Enables unidirectional XON/XOFF
AT&K3 - Enables bi-directional RTS/CTS hardware flow control.
AT&K4 - Enables bi-directional XON/OFF
AT&K5 - Enables transparent XON/OFF
AT&K6 - Enables both RTS/CTS and XON/OFF
AT&K7 - Enables bi-directional DTR/DSR hardware flow control.
AT&K8 - Enables unidirectional DTR/DSR hardware flow control

I only support either no flow control or bi-directional flow control using AT&K1, AT&K2, or AT&K3. I don't see a need for XON/XOFF flow control (I have never had anyone even ask about it).