Archived from groups: alt.internet.wireless (
More info?)
On Thu, 24 Jun 2004 16:49:44 GMT, "gary" <pleasenospam@sbcglobal.net>
wrote:
>I'll bet that b/g sales for this year far outstrip pure 802.11 sales,
>whether you measure in dollars, or units moved.
Of course. 802.11b/g probably outsell all other technologies in unit
volume and probably dollars sold. I was just commenting on the
possibility that there were some other "minor" technologies that you
may be ignoring.
>> Some minor drivel:
>> 1. When direct sequence spread spectrum meets frequency hopping,
>> frequency hopping always wins. That's one reason that FH is still
>> being sold.
>Depends on what you mean by "win". What characteristics are you saying are
>optimized? Range? Insensitivity to interference? Insensitivity to echo?
>Maximum sustainable bitrate? Mobility support? I'm not a hardware or DSP
>engineer, so I won't presume to argue the details. But I do know more than
>one wifi engineer who would debate this claim, especially when stated as a
>sweeping generalization.
Well, we're a bit off the topic, but...
In general (there are exceptions), if you place an FH (frequency
hopper) and a DS (direct sequence) in the same airspace (about 3ft
apart), the FH will slow down somewhat while the DS radio will come to
a screeching halt. I've seen several reports claiming either minimal
interference or none betweeen 802.11b (DSSS) and Bluetooth (FHSS). My
testing shows that Bluetooth continues to shovel data while 802.11b
performance drops like a rock. I've also done the same test using
Breezecom SA-10/AP-10 FH radios versus Linksys WAP11 access points for
WISP (wireless ISP) service. FHSS keeps working while DSSS just
stops.
Doing the same tests with OFDM modulation (802.11g) vs Bluetooth
resulted in some rather interesting oddities. As long as the access
point (DWL-900AP+v2) could be forced to stay in the 802.11g OFDM mode
(or the proprietary pre-802.11g 22Mbit/sec mode), the Bluetooth
interference to 802.11g was rather minimal. However, the basic
algorithm for 802.11g is that when faced with an "unknown" signal, it
assumes that it's an 802.11b radio and reverts from 802.11g ODFM to
802.11b DSSS operation. As soon as it switches to DSSS, Bluetooth
causes sustantial interference, and DSSS traffic stops.
See:
http://rfdesign.com/images/archive/0900McCune90.pdf
I know it's 900MHz, not 2.4GHz, but the technology is the same.
Here's another:
http://www.toolworx.com/pdf/DSSS%20vs%20FHSS.pdf
Here's a quote from above:
"In the face of noise in the channel and loss of bandwidth,
DS may abruptly stop working, whereas FH will continue to
degrade gradually to no bandwidth."
If you want more details, test results (if I can find them),
explanations of what I think is happening, and predictions of the
imminent demise of wireless as we know it, just ask.
>> 2. Metricom/Ricochet/Aerie, Bluetooth, and Zigbee are all frequency
>> hoppers.
>
>Again, these don't appear to apply to the survey in question, as none of
>them are based on 802.11.
I know. I just tossed it in to demonstrate the FHSS isn't totally
obsolete and not used. Good luck on your survey.
--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D 831-336-2558
Santa Cruz CA 95060 AE6KS
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