El 06/03/2010 10:03, Dave AA6YQ escribió:
>>>I don’t have a definition, Rein; I agree with you that the Wikipedia
entry is not authoritative. The fact that part 97 references spread
spectrum without defining it is one of the root causes of this
controversy, leaving us to make “individual decisions” in the absence
of decision criteria. Transparency (ability for anyone to copy without
a private key) and spreading factor are clearly important factors, but
to what does the spreading factor apply? Information content?
Bandwidth of the signal being spread? Mike N4QLB claims in a post on
the ROS reflector that “it’s not spread spectrum if the resulting
bandwidth is 3 khz”. Is that true? If so, why 3 khz, as opposed to,
say, 3.1 khz?
Bandwidth of the RESULTING signal. Depends mostly on the chipping
frequency and waveform (DSSS) or spread of center frequencies and dwell
time (FHSS)
independent of the message.
Ideally, it should not be discernible from noise, but the FHSS scheme
ROS uses does not reduce spectral power density to such low levels in a
3 kHz bandwidth.
The chipping should be many times faster than the baseband bandwidth,
with no specific, fixed figure. All real systems have finite bandwidth
to be realizable, and ROS is no exception. The fact that the bandwidth
is that of a SSB voice channel is imposed by the available hardware
(radios).
The QRM potential does not come from the bandwidth alone, but also from
the resultant spectral power density achieved in such a bandwidth. No SS
system can achieve the ideal limits due to hardware and administrative
constraints, even 802.11 systems.
What is at a stake is also the blurred, fuzzy limits that real systems
have nowadays in practice. I do believe that there should be room for
such systems, as much as there is room for Olivia wide modes or MT63,
and there are specific frequencies for such modes.
So far, even using a different paradigm, ROS does not achieve such a
robustness in practice, because of what it still lacks. But that is
something else, as is the possible presence of backdoors.
I do recommend a personal firewall, I am using an old one by Kerio, that
fields and denies all non acceptable background communications, even
when I do not have a direct connection to the Internet, as it "rings the
bell" whenever such an unauthorized attempt happens. Not only that, it
has rules to effectively deny any further attempts. Nothing is 100%
safe, but it does less harm that way.
Anyone can check the spectrum of the baseband signal using the proper
tool. I used Spectran and a loopback cable for simplicity on my PC.
Others have used Spectrum Lab, believe even Visual Analyzer could help
as well.
73,
Jose, CO2JA
