Slow Access Points

[john]

I have a 6 meg DSL circuit feeding an AP350 access point. Sitting next to the AP, I only get 3 megs of throughput. Any ideas why? AP is set to 11 mips, 128 bit wep, running V12.05. I'm less than 3 feet from it, so range is not the issue. Thanks, John

[scottmac]

What kind of throughput were you expecting to see?

802.11b will max out around five megabits; eleven megabits is the signaling rate, not the throughput.

From that ~5Mbps, subtract the various overheads for wireless, TCP, IP, and some other transmission control traffic (collision avoidance, polling times, etc).

Being three feet away might also be slowing you down a little ... the signal could be "too loud" and catch a little distortion; try moving away some.

Also note that signal strength is not quite as important as signal quality. You could still have some interfereing signals. Check the utility that comes with the NIC drivers that shows signal strength versus signal quality (most wireless NICs provide this utility with their drivers) or download and run NetStumbler ( www.netstumbler.com - it's free).

Run a scan for a while and see which channels are least congested, and if there is any interference. Stay on channels 1, 6, or 11 ... they do not overlap and are least likely to interfere with each other.

Basically, I think you're in the ballpark, you might be able to squeeze out a little more ... give it a shot.

Good Luck

Scott

[john]

Hi Scott,

The 3 feet is actually inside a water tower, well shielded solid metal. I can do this from any location, inside or out, and still see around 3 megs of throughput. I have all Cisco equipment, so compatibility is not the issue, but in fact, I see the same sort of lower-than-expected throughput from other brands. I'm kind of at a loss as to what the signaling rate is vs. throughput? I understand that there's overhead that eats part of the throughput, but 40% or more??? I've been messing with turning off CDP to see if that helps, but the results are inconclusive. Any ideas? Thanks, John

[scottmac]

Due to the nature of wireless communication, and the likelyhood that some part of the transmission will be interfered with (in a normal environment), there is redundant information contained in every frame transmitted.

The idea is that if the part of the frame sees interference, the likelyhood that the same part of the redundant info would get interference is very small.

So between the prime information, and the redundant information, a complete frame can be constructed.

So, even if you get 100% perfect reception (signaling rate), ~half the information (redundant data, headers, etc) is discarded (what's left is your actual throughput).

Given that you are operating in a sealed can, what you are likely seeing is severe multipath interference.

If you can loosly surround your antennas with something like aluminum foil or put the APs in metal buckets,so that the openings are facing each other, such that they see each other's direct signal, but not all of the reflections, you may see an increase in throughput.

The wavelength is ~5.5," so you might be able to find a "sweet spot" in the RF such that the prime signal can overcome the multipath.

You're operating in an unusual environment; you may need to implement some unusual practices to maximize your throughput.

Are there any structures in the general path of the AP and the client? (think: More multipath).

That's about all I can think of for now, if something else comes to mind, I'll re-post.

Good Luck

Scott