sidebands of great justice [was: Meet “badBIOS,” the mysterious Mac and PC malware that jumps airgaps]

[David I. Emery]

On Thu, Nov 07, 2013 at 06:56:44PM -0800, Taral wrote:
> On Thu, Nov 7, 2013 at 5:53 PM, coderman <coderman at gmail.com> wrote:
> > how does one constrain the frequency bands used?
> > blurt_cpp_80211/blurt.cc is a bit opaque, and i'd love to try the max
> > rate benchmark with just a 18-20kHz channel...
> 
> Based on a quick glance, it's using OFDM -- you should be able to
> constrain it to the appropriate sidebands. Although for only 2kHz I'm
> not sure how much you get out of OFDM in the first place.

	FWIW OFDM is multicarrier at low symbol rates per carrier
(compared to total information rate in the signal, not just "low" by
some arbitrary standard).

	The lower the symbol rate, the longer the coherence distance,
which gets to be important to reduce ISI (inter symbol interference)
in a multipath environment.   And indeed sending modulated tones over
a speaker to a remote microphone is likely wickedly full of multipath
including multiple paths due to reflections off hard surfaces that
result in some serious delay spread even compared to the relatively
low speed data you can stuff into 2 KHz from 18-20 KHz.  Sound does
not propagate all that fast compared to useful data rates after all.

	So OFDM is good.


-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."