WAN speed good, LAN speed bad

paulmac110 used Ask the Experts™
Hi - I set up a wireless LAN for a client which has been working well for a year.It is distributed using 2 EnGenius 2611p outdoor WiFi access points and is building to building about 200 yards apart.
Both units are in access point mode and using WDS.

The WAN speed is about 25Mb/s incoming but when distributed out through the wireless access points it is currently dropping to 0.5 Mb/s.

I have the 2 outdoor access points on different channels, is this right? I originally set them up on the same channel but was advised to change them by a network engineer who happened to be visiting during the time I was installing.

Could anyone recommend a software tool that might indicate at which point the network is being strangulated?
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I can recommend some fairly high-end analysis tools (OpenView, Tivoli, etc), but probably not something you want to pay for and set up for a point problem.

First you need to determine if the problem is in the LAN or with the wireless AP.  Start by testing the LAN.  If possible, I would pull the wire from the AP and connect a laptop to it for speed test.  If fine, we need to look at what is going on with the wireless.  If slow, work back toward the edge router to find where speed drops off, then you can work on troubleshooting for the specific device.

If problem is in the wireless, I think I'd check for interference sources first given that both AP's are experiencing the issue.  Change channels (yes, having them on different channels is correct IF they don't talk to each other...but if you are extending your MAN by using the APs as a wireless hop between buildings put both on the same channel).  It is possible that one of the devices is having an issue and actually creating interference.  Turn one off to test the other, then vice-versa.
AkinsdNetwork Administrator

Is there anything blocking the line of sight between the APs?

Signal loss can occur if the path between the APs are obstructed.

Another possibility is if another access point was installed nearby that may interfere with your straight path.

I would recommend doing a visual inspection first if you haven't already done that.
> Both units are in access point mode and using WDS.

Are both APs hard wired to the LAN and talking only to clients wirelessly?
If that's the case, I'm not sure why AP WDS is needed; if they are connecting the 2 segments of the LAN together, it would be better to use Bridge WDS and have them talk only to each other, in my opinion.

> I have the 2 outdoor access points on different channels, is this right?
If they're talking to each other they would *have* to be on the same channel.

If they're not talking to each other, then different channels would maximize throughput, assuming available wired bandwidth is at least double the wireless connected speed (since because of frame and encoding overhead, actual wireless throughput is only about half the reported connection speed anyway), and that they're at least 5 channels apart so they don't overlap and interfere with each other, and that there are no other wireless stations nearby within 5 channels of either one, causing interference.

To check for WiFi interference sources, you can install a program like MetaGeek's  inSSIDer on a laptop with a WiFi card and walk around the coverage area with it to see how many other stations there are and on what channel[s]. To check for non-WiFi interference sources, you would need a spectrum analyzer... you can spend $4000 and up on a wideband spectrum analyzer (least expensive 100Hz to 6GHz easily-portable analyzers start around $10,000), or there are narrow band USB devices that leverage the CPU and display on a laptop to bring the cost down to a couple hundred dollars. e.g. the 2.4x and Chanalyzer software from http://www.metageek.net/products/wi-spy/ (I do not recommend the cheaper 2.4i, because it doesn't work with the chanalyzer software and it doesn't have the RP-SMA connection for a directional antenna...   and I personally have not felt the need to pay the extra money for the chanalyzer pro software; the regular version has shown me everything I need, so far)... but look for WiFi sources, first.
Anything within 5 channels will interfere... well, if they're the same SSID they will take turns if they're on the *same* channel, because of carrier sense multiple access (CSMA) which is part of the base 802.11 protocol, but overlapping adjacent channels don't use CSMA to prevent stepping on each other whether they're the same SSID or not.
2.4GHz WiFi Channel MapThose blocks really show only some of the overlap, but not how the energy tapers off near the edge of each channel... e.g. at the points channels 1 and 4 overlap, they're about 25 dBr weaker than the middle of the channels, and though they're spec'd as being 22MHz wide, they're actually nearly 60 MHz wide, but the orthogonal frequency division multiplexing (OFDM) modulation rolls off the signal so it's about 40 dBr (or 10,000x) weaker by the time they're 30MHz off the center of the frequency... e.g. http://www.cisco.com/en/US/i/100001-200000/120001-130000/121001-122000/121146.jpg and the overlap is considered to be eliminated when the signal strength has dropped more than 30dB.

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