Optimal throughput speed on a 802.11b wlan

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- mukeige
  
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posted
18 years ago

Mon, Aug 15, 2005 1:47 AM

I'm connected to the internet via optical fiber. A friend let me put the line and modem inside his garage and a 19dbi parabola on the roof to connect to my home 4km away. In the garage the modem is wired to a Buffalo WHR2-G54 router, the router wired to a Buffalo WLM2-L11G (802.11b only, the kind with cards inside) AP. The AP is connected to the parabola with 5 meters of some kind of pretty fat coaxial (sorry I don't know the specs). At home I have a 24dbi parabola connected to a Buffalo WLM2-L11G AP again connected through 5 meters of fat coaxial. There is clear line of sight as we are both on top of hills. At the 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? 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?

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- Jeff Liebermann
  
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posted
18 years ago

Mon, Aug 15, 2005 2:18 AM

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.

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:

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ros_range_whitepaper.pdf with some additions and corrections by me.

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:

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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".

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- mukeige
  
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posted
18 years ago

Mon, Aug 15, 2005 3:43 AM

I really appreciate the response and the numbers. I really don't understand enough to do the calculations myself. No, I guess I didn't try Alchemy in normal power output. The Buffalo APs are connected up now, but I'll try your suggestions about the power and settings in the alchemized WBR-G54s when it stops raining and thundering outside (I probably shouldn't be on line now but have been pretty lucky up to now in weather like this).

You mentioned perhaps a problem with connectors or radios. Buffalo uses mostly MC lucent connectors I think. Whenever I plug in a pigtail, I always worry whether the plug is sitting properly in the jack. These connectors don't seem so well designed. And radios in the WBR-G54, are the radios on the Mini PCI card or the main board of the unit? I have a couple of extra Mini PCI cards I could trade out and check if one's defective.

Thanks again for your time.