Dropouts, dropouts and more dropouts!

Hi Folks,

This is my first posting on this forum and I'm sorry it's rather lengthy. From the title above, it's clear why I'm asking for suggestions. For several months, I had a reliable wireless connection. Just recently, it's all changed. I get a lot of dropouts, particularly when I click to go to a new web page. I carefully monitor other networks in my neighbourhood using WirelessMon. Right at this moment, there are no less than eight networks visible. I'm using channel 6 and there is a network on channel 7 but its signal strength is very low. One of the things I like about WirelessMon is that it gives signal strength in dBm, which allows meaningful comparisons to be made between networks. So, the received signal strength from my router (BT Voyager 2091) hovers around

-47dBm (on ch 6) and the channel 7 network is less than -81dBm. The only other channels that are in use at present are ch 1 and ch 11.

From the information above, it would be reasonable to assume that I should not get interference from other networks. So, there are two possibilities that come to mind. Firstly that there may be interference from other 2.4GHz sources or that the received signal of -47dBm at my desktop network card (Belkin F5D6001) is too low to maintain a reliable connection. Unfortunately, I don't have a Wi-Spy 2.4Ghz sniffer so I can't eliminate the first possibility. And I don't know what signal strength your average PCI card needs for a good connection. Does anyone have an idea? I could, of course, buy a new router (of the MIMO variety) but I can't be sure that it will solve the dropout problem. And maybe there's something else that I'm overlooking.

Suggestions please.


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jaypeecee hath wroth:

Detail is good, but I'm not thrilled with you spending more words describing the title, than simply duplicating it. I have my newsreaders setup to NOT display the title, so it's an extra step for me to re-read the title.

Good. That means that it was working. The trick is to see what has changed.

OK. Time to do a simple test. You didn't mention anything about your wireless client, it's hardware, or it's operating system, so I'll assume a desktop running XP Home. Start -> Run -> cmd ping -t (the IP address of your wireless router) You should see continuous pings, with a very consistent 1-2 msec latency. However, if you get something like this mess: Reply from bytes=32 time=76ms TTL=128 Reply from bytes=32 time=1ms TTL=128 Reply from bytes=32 time=8ms TTL=128 Reply from bytes=32 time=1ms TTL=128 then you are experiencing packet loss and retransmissions. Try this with various wireless clients to eliminate the possibility that the packet loss is being caused by some program (i.e. worm or virus) causing the client to become busy.

WirelessMon is a good program but it doesn't show "hidden" networks that do not broadcast their SSID. They also do not show MIMO routers and a variety of non-802.11 sources. See list at:

Windoze based sniffers are a good start but you'll see the missing SSID's in addition to client radios using Linux based Kismet. Download the Backtrack 2 Linux LiveCD from:

Make sure you have a compatible wireless card. Boot it on your laptop. Run Kismet.

Please note that the direct sequence signal is about 22MHz or 5 channels wide. Therefore, a signal on Ch 6 will occupy Ch 4 - 8. To be sure that you're clear to use Channel 6, Ch 4 - 8 must also be clear. That's also why the recommended non-overlapping channels are

1, 6, and 11 as any channel in between will receive interfernce from both of the other channels.

Too bad it's rather inaccurate. Same with their tx power output readings. It's not the fault of Passmark, but is more of a design problem. The dBm reading is based on the RSSI value of 0-255 or roughly 10*log(RSSI). That gives fairly good resolution at the low end, and large granularity at the high end. In addition, the current all digital wi-fi chips are not very accurate or linear at determining the RSSI. It's certainly good enough for comparing gains and signal strengths for a given radio, but I would not try to use the RSSI for absolute measurements or for comparing wireless clients.

-47 is very strong. -81 is barely detectable. However, the signal strength doesn't matter with interference. Even with a strong signal, interference can cause problems by filling in the gaps between packets with junk and trashing the ACK's.

Wrong. You're not getting interference from networks that you can see with Wirelessmon. It won't show hidden nodes. Turn off SSID broadcast on your router and WirelessMon will not see it.

There are plenty of other possibilities. However, I don't want to list them in detail until you do a few simple tests and updates.

  1. Update the firmware on your unspecified model wireless router to the latest available. Disclosing the maker and model would also be nice.
  2. Disconnect the router. Take it and your wireless desktop to some place where wi-fi cannot penetrate. A basement or closed room without windows will suffice. Run the ping test and use IPerf to test for thruput.

If that works as expected, then your hardware is clean and not the source of the problems. If that fails, then there's something broken in either the router or the client computah. You can determine which by substitution with known working units.

IPerf is a thruput test that will require an additional wired (not wireless) computer to act as a server.

I've posted instruction on how to do the thruput test at least 4 times, so you should be able to find it with Google Groups Search.

I have the original Wi-Spy. It's lack of sensitivity (inherent because of the lack of about 10dB of processing gain for spread spectrum) will only show very strong source of interference. If possible, borrow a real spectrum analyzer and big dish antenna from one of the local ham radio operators.

Yes. The base sensitivity varies with connection speed. Worst case is 54Mbits/sec which usually runs about -70dBm for a rather lousy BER. At least a 10dB fade margin is usually required so the minimum usable signal is about -60dBm. See graph at:

Incidentally, everyone lies about their rx sensitivity numbers so don't treat these numbers as accurate or even close. Many of them are just the chipset sensitivity, without any concern for what goes between the antenna connector and the chip.

Overload level varies radically with chipset. I have some numbers, but I'm not sure they're any better than the manufacturers. My guess is anything over about -20dBm will begin to cause distortion and corresponding errors. Some chipsets overload at -30dBm. Some of the really good outdoor routers can handle up to +10dBm. Besides being a bad guess, the overload point is not an abrupt reference, but rather the start of slow deterioration. It may take 20-30dB more signal before anything worse than a few retransmissions occurr.

MIMO will buy you more speed. It will NOT buy you more range or more interference immunity. There are also two types of MIMO. The beam steering or beam forming variety (RuckusWireless), will have some benifit is that it can insert a null in the direction of the interference source.

Reply to
Jeff Liebermann

Try switching to channel 3, 4, 8, or 9 to see if the problems continue.

Also might just be your wireless card / drivers.

Reply to
Cal Vanize

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