can I rule our interference?

I had a problem a while back, solved it. It wasn't interference. But i'm wondering generally.. what the symptoms of interference are, and if I can ever rule it out.

For example, suppose I change to each channel and sitll the same problem, would that rule out interference or could it just mean there's interference on all channels.

Interference is when you connect but had low responsiveness right?

What about signal? If I connect but have a good signal like -45db amplitude reported by inSSIDer, and 5 bars reported by Windows. But let's say for some reason it's unresponsive. Would that , the 5 bars or the -45db, mean definitely not interference?

Reply to
tommy alimo
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If you ping the wireless router, instead of a nice constant and consistent 1-3msec latency, it will hop around radically in value, with the occasional added bonus of a time out. Something like this is what interference looks like:

C:\> ping -t Pinging with 32 bytes of data: Reply from bytes=32 time=2ms TTL=64 Reply from bytes=32 time=2ms TTL=64 Reply from bytes=32 time=20ms TTL=64 Reply from bytes=32 time=105ms TTL=64 Reply from bytes=32 time=15ms TTL=64 Reply from bytes=32 time=75ms TTL=64 Reply from bytes=32 time=5ms TTL=64 Reply from bytes=32 time=2ms TTL=64 The variations are the result of retransmissions and retries inspired by lost packets.

Yes, you can rule it out if your wireless router and client are both located in an RF shielded room, basement, dungeon, or some location where sources of RF are unlikely. Otherwise, interference is a prevalent problem.

There are only 3 channels (1, 6, and 11) so that's not going to be effective. Many wireless routers set their operating channels to "auto" by default, which makes channel selection a crap shoot. Many MIMO (802.11n) routers use a 40Mhz wide channel instead of the usual

20Mhz resulting in the entire band being one channel.

Lack of responsiveness is one symptom. Erratic and slow downloads are another.

That doesn't work. You can have an absurdly strong signal and it will still be susceptible to interference. There's no FM "capture effect' in 802.11a/b/g. A fairly weak interference signal can drastically reduce the thruput from a fairly strong signal.

So, what problem are you trying to solve?

Reply to
Jeff Liebermann

On my program it says strength and data quality. There are three basic channels.


Reply to

but if I don't get that symptom with the pings, then there can be no (significant) interference problem?

I don't actually have any connection problem it's just a general question so I know for the future and understand the situation better.

inSSIDer and xirrus wifi inspector show my router on channel 3, and I can select any from 1 to 11, so what do you mean?

it's all very nice if of neighbourign routers, one is on 1, and one is on 11, then i go on 6. But a bunch are on 1, and a bunch are on 6, and a bunch are on 11.

So what the heck, why not just choose 3?

And also, let's suppose there is just one router and it's on 1, wouldn't a good use of space for me to put mine on channel 5? channels 1 and 5 don't overlap do they?

1 2 3 4 5

+- 20Mhz either side.. channel 1 = 2412Mhz Channel 2 = 2417Mhz channel 3 = 2422Mhz channel 4 = 2427Mhz Channel 5 = 2432Mhz

Each does a 20Mhz range, 10Mhz each direction

Channel 1 = 2402Mhz-2422Mhz

Channel 5 = 2422Mhz-2442Mhz

No overlap.

Why does it have to be one router on channel 1, another router 5Mhz away from the edge of its spectrum? thus on channel 6. Why not channels 1 and 5.. no gap, but no overlap either.

and would channel 1 be like up to 2421.9999 and Channel 5 be from

2422.000001 ? Even if not, 2422 is only one discrete number that'd overlap on a spectrum with 20 of them, so is it so bad to have just one frequency 2422Mhz overlapping?

I'm just trying to understand wireless issues a bit better so I know what's going on a bit more.

btw xirrus wifi inspector works nicely showing WPA and WPA2.

Reply to
tommy alimo

Theoretically, interference could still cause a throughput impact, even if you aren't getting packet retransmissions hence the variable latency scenario that Jeff shows above. Remember that 802.11 does CSMA/CA where CA = "Collision AVOIDANCE" - the transmitter will check the channel before sending, and if there is noise on it above a given threshold (the Clear Channel Assessment [CCA] - typically -85dBm or so), it will defer.

One or the other.

He meant that there are only 3 *nonoverlapping* channels: 1, 6, and 11. If you have set your router on channel 3, then your signal will overlap with other cells that are on *both* channels 1 and 6. So your performance should be worse than if you had picked either 1 or 6.

Because your signal is now interfering with (/being interfered by) *two* bunches of APs, the bunch on 1 and the bunch on 6.

Yes they do.

Here's a little white paper that you may enjoy reading:

Channel Deployment Issues for 2.4-GHz 802.11 WLANs

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Reply to
Aaron Leonard

Sometimes it's not about how many AP's are on a channel but rather how strong they appear to be, and programs like inSSIDer can help show you that. I'd rather share a channel with a dozen very weak AP's versus a channel with a couple of very strong AP's.

Reply to
Char Jackson

Variable latency is just one of the symptoms of interference. It's like a runny nose and a cold. If you have a cold, it's highly likely that you have a runny nose. However, if you have a runny nose, it can be caused by a cold, flu, hay fever, allergy, etc. It really depends on the source of the interference. For example, a periodic interference source will usually produce very consistent latency figures. Instead of the usual 1-2 msec latency, you'll see a much larger number, with little variation.

Because the ping packets are small, they don't test for all forms of interference. A much better indication is a thruput test. I suggest you download and use Jperf:

If you get nice stable upload and download speeds through the wireless link, at a speed you would consider normal, then you do not have any interference. If it varies radically, you have interference.

Good question. One of the common questions I see is "My wireless was working just fine until a few days ago. Now, it's erratic, disconnects often, slow, and useless. I didn't change anything". Usually, that means the neighbors have dragged home a new wireless router or wireless toy, and are creating interference. Being able to recognize the difference between interference and simple misconfiguration is useful.

Aaron Leonard answered that question effectively. The occupied bandwidth (how wide the signal appears) is roughly 22MHz for

802.11a/b/g. At 5MHz per channel, that's a bit over 4 channels wide. With 11 channels available, that's about 3 usable channels that do NOT overlap each over.

Note that the degree of interference from adjacent channels depends heavily on the distance between you and the source of interference. If BOTH radios were in the same room, you might need more isolation than if they were located in separate buildings.

Because you receive interference from others on both channels 1 and channel 6. On a statistical basis, you'll be worse off than picking either 1 or 6. In addition, you will be generating interference to users of both 1 and 6, thus doubling the number of irate neighbors banging on your door at odd hours.

The channels are 5MHz wide. However, your signal is 22MHz wide.

Cool. You might also try:

Unfortunately, it doesn't do a good job of identifying encryption type, but does everything else well enough. To see interference caused by non-802.11 devices, such as microwave ovens, you'll need a spectrum analyzer.

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