OT Re: Before I buy something: any suggestions?

From: Eric Johanson (ericj_at_cubesearch.com)
Date: 2002-07-27 21:55:05 UTC

On Sat, 27 Jul 2002, Tom Mercelis wrote:

> Hello,
> I want to turn my linux box into a Wireless Access Point. It has one
> Free ISA slot and several PCI slots.
> The (at this moment only one) linux client pc only has a free PCI slots.
> I also have an iMac which one day has to be removed from the wired
> network and will need to have access to the wireless network.
> I've been looking the last days on several sites about this subject, and
> there seems to be such a variety of choises. But some of the technical
> data I don't understand:
> What does it mean when a card has RF output power 15 dBm

It's the output power. It can be converted to milliwatts; 15dBm = 30mw, 20dBm = ~100mw. The higher the power, the longer you can go (provided you don't have much noise from other 2.4ghz stuff).

> , and a sensitivity of -85 dBm @ 11 Mbps,

The more sensitive cards can pick a signal out from the noise better. Again, improves your distance. Think of RF as talking to someone. If you are at a rock concert, you need to talk much louder to be heard. Also, someone with better hearing may be able to understand you better than someone with worse hearing. It's the same concept with RF.

> and about antennas: what does xx dBi mean?

Dbi = isotropic Db level. Higher the number, higher the gain. Keep in mind that you'll lose coverage area, depending on the type of antenna. I have a 24dbi parabolic dish (directional antenna) that has a 7 deg. beam width. You don't want that, unless you are building p2p links. I suspect you won't need an antenna at all, but you might depending on the setup of your hostap-PC.

> The distance between the computers will be at most 30 meters (100 feet)
> through the walls of my house.

Well, that depends on what's in the walls & the amount of noise. I hope you don't have a 2.4gzh cordless phone, those will kill 802.11b. Do some tests with two cards in iBSS (adhoc) mode.

> I don't feel like bungling with antennas, so wich product should I chose
> that meets all these requirements?

Well, if you want to run hostap, you might end up needing an antenna. A standard PC power supply is very noisy (on RF). Having the card INSIDE the pc case will limit your range by quite a bit. Also, even with the card's antenna sticking out the back, it may end up being un-seeable from the 'front' of the case. A small omni/sector antenna to get you away from the PC will help quite a bit.

As for products:

Be careful selecting a prism2 based card. While there are many out there, most of them have really, really bad receive sensitivity. I can tell you to stay away from the dlink DWL650 (I've not used the + or H models), and 3com airconnect cards. But I have yet to have played with a prism card that works better than my lucent silver card or cisco 350.

The problem: Lucent cards aren't support as AP's under hostap.

The solution (!?): Use lucent cards in your clients, but find a higher-power prism2 based card for your AP. 200mw cards are out, but that's probably overkill for your needs. Zocomax (sp) I hear makes a great prism2.5 based 100mw card with an external antenna connector. I've not used one, but I've heard good results. I'm not sure about hostap mode on this card. Maybe somebody else will share some details on this list.


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