Re: Optimal throughput speed on a 802.11b wlan

On 14 Aug 2005 18:47:16 -0700, mukeige@xxxxxxxxxxx wrote:

>(...)
>modem throughput is around 15-20mbps. Here at home I get up/download
>speeds of around 3.5-3.8 mbps pretty consistantly. I'm wondering that
>with 11b shouldn't I be able to get 7-8 mbps throughput at a distance
>of 4km?

Nope. An 802.11b 11Mbit/sec connection will yield about 4Mbit/sec
thruput. By juggling the packet timing, I've squeezed about
5Mbits/sec out of such a connection.

>As an experiment,I bought 2 used Buffalo WBR-G54 routers and loaded
>them with Sveasoft Alchemy firmware. I set them in AP mode, upped the
>transmission power to 50-100mw and put them in place of the 2 Buffalo
>WLM2-L11G AP's. Doing this throughput actually went down to about 2mbps
>in both 11b and 11g mode.
>I'd like to get more speed somehow. Any suggestions?

Did you try it a normal (35mw) power output before you started messing
with the tx power?

If you're going to play point to point link, turn *OFF* the 802.11b
compatibility mode in the WBR-G54 router. If it hears an 802.11b
signal, it will slow down.

This is stolen from an Atheros PDF at:
http://www.atheros.com/pt/athe ros_range_whitepaper.pdf
with some additions and corrections by me.

Non-overlapping Modulation Max Max Max
Channels ------- | Link TCP UDP
| | | | |
802.11b 3 CCK 11 5.9 7.1
802.11g (with
802.11b) 3 OFDM/CCK 54 14.4 19.5
802.11g only 3 OFDM 54 24.4 30.5
802.11g turbo 1 OFDM 108 42.9 54.8
802.11a 13 OFDM 54 24.4 30.5
802.11a turbo 6 OFDM 108 42.9 54.8

If 802.11b works, 802.11g should work better. My only guess is that
something is wrong with the RF connections to the antennas. Possibly,
one of the new radios is defective in some way.

Playing with the numbers and run a sanity check:
http://www.terabeam.com/support/calculations/som.php
TX power = +15dBm
TX coax loss = 2dBm (5 meters LMR-400 + connectors)
TX antenna gain = 19dBi
Distance = 4km = 2.5 miles
RX antenna gain = 24dBi
RX coax loss = 2dBm (5 meters LMR-400 + connectors)
RX sensitivity = -79dBm (for 24Mbits/sec connection)
fade margin = ????
I get a fade margin of 20.8dB which is about the minimum that can be
made to work reliably. It should work according to my numbers. With
a 24Mbit/sec OFDM connection, you should get about 12Mbits/sec
thruput, which should be adequate. Try setting the access point speed
to 24Mbits/sec OFDM instead of "auto".



--
Jeff Liebermann jeffl@xxxxxxxxxxxxxxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
AE6KS 831-336-2558
.

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