[Dshield] DI-624 encryption - Run it if you have it!

John Holmblad jholmblad at aol.com
Wed Feb 25 18:45:56 GMT 2004


re: your comment on using frequency hopping for additional security:

Although the 802.11 standard supports several physical layer standards, 
Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread 
Spectrum (FHSS), infrared, and Orthogonal Frequency Division 
Multiplexing (OFDM - this method was added to the standard as a part of 
both 802.11a and 802.11g in ~2001 to support higher data rates up to  54 
mbps, see: http://www.wave-report.com/tutorials/OFDM.htm)  virtually all 
of the commercial products of which I am aware use either  the DSSS 
method (802.11 and 802.11b) or the OFDM method (802.11a) or both DHSS 
and OFDM (802.11g). With the DHSS method the spreading sequence (an 11 
bit barker code) is standardized and is the same for all radios. Thus 
spread spectrum is not used for message privacy but as a means for 
maintaining satisfactory signal to noise (S/N) ratio  while sharing the 
channel in unlicensed radio bands where there may be other signals 
(cordless telephones, microwave ovens, etc.) on specific frequencies in 
the same band. If different transmitter-receiver pairs were using 
different and more complex spreading codes from those used in the case 
of DSSS, then such a multiplexing method would be referred to as Code 
Division Multiple Access or CDMA, like we see in some of the mobile 
wireless networks, especially in the US.

I would be interested to learn if there are commercial products deployed 
using FHSS. However I should point out that even with FHSS as defined 
within the 802.11 standard, the hopping sequence is fixed by region of 
the world  and does not provide a means to assure any kind of message 

I suspect that with the introduction of 802.11b which increased the 
channel data rate to 11 mbps versus the 802.11 FHSS maximum data rate of 
4.5 mbps that, in the end, 802.11b and consequently DSSS won out 
commercially over 802.11 FHSS. Better still, in terms of throughput at 
least, are the OFDM based standards, 802.11a and 802.11g, both of  
which, as mentioned above, support the much higher 54 mbps maximum data 


Best Regards,


John Holmblad


Televerage International


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