[MLB-WIRELESS] [Fwd: [Oz-ISP] Technical question re wireless transmission 802.11b]

[sanbar]

Is someone able to help Ross out and explain a bit of RF/networking
theory?
- Barry

-----Forwarded Message-----

> From: Ross Wheeler <ross at home.albury.net.au>
> To: aussie-isp at aussie.net, isp-australia at isp-australia.com
> Subject: [Oz-ISP] Technical question re wireless transmission 802.11b
> Date: 28 Mar 2003 10:11:49 +1100
> 
> 
> Does anyone here happen to know much about the RF-level characteristics of
> 802.11b?
> 
> Specifically, if I have two cards, (a) and (b), both "capable" of 11
> megabits per second, but because of signal levels are only actually
> operating at 2 megabits/second, is the whole datarate slowed down to
> 2Mbps, or is the data rate actually 11Mbps but for only 20% of the time?
> 
> I ask this because I'm trying to understand (interpret) some power
> readings.
> 
> If I pull a 10 megabyte file (80 megabits) from (a) to (b) and do so in 41
> seconds, that's 1,951,219 bits per second. Since the cards are running at
> 2 Mbps, its (close enough) to 97% of the "possible" throughput.
> 
> If I monitor the output using a powermeter, I'm seeing +13dbm (ie, 20
> milliwatts) output from a 100mW card.
> 
> Since we've got the card transmitting pretty well solidly (97%), I would
> expect to be seeing 97% of the full output power (since this is an
> averaging power meter), but I'm seeing roughly 20% of full (rated) power.
> 
> If the card were transmitting at 11 megabits/second, but only for 11/2 of
> the time, the figures would sit close enough for me to be happy with.
> 
> But it doesn't make sense to me that the cards would fall back in this
> regard - if the signal was sufficently poor that the cards had to drop
> back in speed, then keeping the same symbol rate makes no sense, I'd
> certainly expect it to slow the symbol rate down and therefore have to
> transmit for the whole 97% of the time (see above time/data figures).
> 
> Anyone know any definitive information sources? I've not found anything so
> far that actually states it one way or the other....
> 
> TIA
> RossW
> 
> ----
> email "unsubscribe aussie-isp" to majordomo at aussie.net to be removed.
-- 
barry park
melbourne wireless node bca http://melbourne.wireless.org.au
community wireless project http://wireless.bur.st
-=all your http_get are belong to us=-


-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 232 bytes
Desc: This is a digitally signed message part
URL: <http://lists.wireless.org.au/pipermail/melbwireless/attachments/20030328/6e31ec40/attachment.sig>