Re: radio net

On 9/6/98 6:20 PM, Steve Schear (schear@lvcm.com)  passed this wisdom:

>> Spread spectrum would have more promise as many stations could be on the
>>air at once on the same frequency thus making life quite confusing for
>>the T-hunters.
>
>I investigated this application several years back and see two practical
>approaches: one adapt a commercial SSB or Ham transciever to use frequency
>hopping spread spectrum, or two build a pirate spread spectrum satellite
>ground station.
>
>Until recently most SSB gear didn't have the RF characteristics to use FH.
>Now there are a number of inexpensive sets which use direct frequency
>synthesis (as opposed to the older, and much slower, phase-locked loop
>approach) and can be driven at hundreds or even thousands of hops per
>second. FH helps solve two problems: first it provides privacy, second it
>can mitigate or eliminate fading (which is highly time-frequency
>correlated). Also, the higher the hop rate, the higher the process gain,
>jam resistance and the lower the probablity of intecept (all other things
>being equal).
>
>It think it was Phil Karn (Qualcomm) who once mused that it would be rather
>straightforward to masquarade a high process gain SS signal on a commercial
>satellite transponder. To it's owners the SS signal would be almost
>invisible, making itself known as only a very slight depression in the
>transponder's gain. Effectively, this could offer an inexpensive covert
>channel for tunneling packets and thwarting traffic analysis.
>
>After the Captain Midnight episode I discussed this possibility with a very
>technically knowledgeable staffer at the FCC and was assured that discovery
>of such signals were beyond (at that time) the ability of commercial and
>national technical (e.g., Lacrosse) means.

 I would suppose the T-hunt aspects of a clandestine network would be 
obviated by piggybacking it into a commercial satellite transponder 
channels ... which brings to mind about how expensive one or two of those 
channeles might be. I remeber in the early days of ham packet radio we 
had several 'wormholes' where hams had obtained through their places of 
employemnt temporary use of unused satellite channels where we were given 
essentially RS232 access and we adapted the packet switches to an async 
backchannel in place of another synch RF path. They did make for some 
intersting network improvements. I guess it always comes down to how do 
you fund such things.


Brian B. Riley --> http://members.macconnect.com/~brianbr
 For PGP Keys  <mailto:brianbr@together.net?subject=Get%20PGP%20Key>

"Success always occurs in private, and failure in full view." 
   -- from somewhere on the Net

On Tue, Sep 15, 1998 at 04:29:06PM -0400, Brian B. Riley wrote:
> On 9/6/98 6:20 PM, Steve Schear (schear@lvcm.com)  passed this wisdom:
> 

> >It think it was Phil Karn (Qualcomm) who once mused that it would be rather
> >straightforward to masquarade a high process gain SS signal on a commercial
> >satellite transponder. To it's owners the SS signal would be almost
> >invisible, making itself known as only a very slight depression in the
> >transponder's gain. Effectively, this could offer an inexpensive covert
> >channel for tunneling packets and thwarting traffic analysis.
> >
> >After the Captain Midnight episode I discussed this possibility with a very
> >technically knowledgeable staffer at the FCC and was assured that discovery
> >of such signals were beyond (at that time) the ability of commercial and
> >national technical (e.g., Lacrosse) means.
> 
>  I would suppose the T-hunt aspects of a clandestine network would be 
> obviated by piggybacking it into a commercial satellite transponder 
> channels ... which brings to mind about how expensive one or two of those 
> channeles might be. I remeber in the early days of ham packet radio we 
> had several 'wormholes' where hams had obtained through their places of 
> employemnt temporary use of unused satellite channels where we were given 
> essentially RS232 access and we adapted the packet switches to an async 
> backchannel in place of another synch RF path. They did make for some 
> intersting network improvements. I guess it always comes down to how do 
> you fund such things.

	At one time I had some involvement with a company renting
satellite space, and the figure of around $1500 to $3000 a month for a
voice channel capable of being used on small VSAT sized dishes was
passed around.  It depends on how much bandwidth and power the channel
uses which in turn depends on how big the dishes are (G/T to be exact).
Bigger dishes mean weaker signals on the satellite and lower charges. I
could probably find out the formula used to price the service...

	I suspect that the cost of equipment and licensing and regulatory
compliance of various sorts might make it unpleasant for loosely knit
groups of private citizens - uplinks require competant installation
and maintainence to keep them from causing interference to other users
and various other problems such as RF radiation hazards under control.

	On the other hand, satellites are crawling with little signals
transmitting streams of data or voice or music to groups of receivers
scattered over wide geographic areas, so the econmics aren't prohibitive
for people who have some real need...

-- 
	Dave Emery N1PRE,  die@die.com  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18

At 11:41 PM -0400 9/15/98, Dave Emery wrote:
>> On 9/6/98 6:20 PM, Steve Schear (schear@lvcm.com)  passed this wisdom:
>
>	I suspect that the cost of equipment and licensing and regulatory
>compliance of various sorts might make it unpleasant for loosely knit
>groups of private citizens - uplinks require competant installation
>and maintainence to keep them from causing interference to other users
>and various other problems such as RF radiation hazards under control.

Because commercial satellite gear is almost always asembled from several
vendors to form a station, and because it can be assembled in many ways and
is invariably done so by professionals, there are no U.S. satellite
equipment regulations for such gear.

>
>	On the other hand, satellites are crawling with little signals
>transmitting streams of data or voice or music to groups of receivers
>scattered over wide geographic areas, so the econmics aren't prohibitive
>for people who have some real need...
>

Surplus gear is pleantiful for those who know where to look. The hardest
part is building the SS mod/demod and that's pretty straight forward for
any competent RF engineer.

If low bandwidth links can suffice, the very large spread codes (such as
those used by GPS) can be used to place the covert channel well below the
noise floor.  GPS uses a whopping 63dB of code gain.  This in turn means a
small earth station with low power (cheap) transmitter can suffice.  I
would be surprised if you could use a backyard or even DirectTV antenna.

--Steve

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