Very confused. I’m using inSSiDer, WiFi Overview 360 and Wifi Analyzer apps to try to determine the best wifi channel to use. Assuming all channels are in use and there are about 20 different users in close proximity…
Q1: Signal Strength
If all channels are in equal use, do you select a channel where others that use it have the lowest signal strength? I’m guessing that a higher signal strength means that it’s from a router from your closest neighbour and you want to avoid that. Is that the case? Does lower signal strength mean the person using it is further away and therefore less likely to interfere with your signal? If 2 people are on channel 1 and have high signal strength and 2 people are on channel 11 and have low signal strength, do you choose channel 11? Assume 6 cannot be used.
Q2: Signal Strength Weighted Against Number of People
What is the impact of signal strength compared to number of people using channel? If 1 person is on channel 1 with very high signal strength and 2 people are on channel 11 with low signal strength, do you choose channel 11 over 1? Assume 6 cannot be used?
Q3: Channel Overlap
If, for example, I had 11 routers sitting on top of each other, each set to a different channel, I’m guessing channel 1 and 11 would be the least interferred. Is that right? People go on about the fact that only channel 1, 6 or 11 should be used (in US) as they don’t overlap but surely that statement is wrong. Yes, if only 3 people were using Wifi, the best channels to use would be 1, 6 and 11 as they don’t overlap but in practice, there are about 20 people using the channels in the same area. What do you do in that scenerio. Do you go for the shoulder channels if all channels are being used equally with equal strength?
Q4: Auto Channel Select
Do routers set to auto channel select only choose between 1, 6 and 11? How do they make the choice? Do they select by choosing channel with lowest other routers and do they factor in signal strengths. My router auto chooses channel 1 but my closest neighbour is on that channel. Is that correct.
Q5: Baby Monitors and Wireless Phones
What channels do 2.4 GHz baby monitors and wireless phones use? No useful info on the internet for them. Do inSSiDer, etc. pick them up?
Q6: I have access to channel 13. All other things being equal, am I better to use this over channel 11 as it is further away from the other channels?
Those questions have puzzled me for a number of years and Google is not helping!! Would really be grateful if someone could give sensible answers to one or all of them. Thanks, Mike.
Solution:
Whew, thats a lot of questions.
First things first, It appears you are interested in wireless technology. You should checkout the Cisco Wireless Network Administrators Certification Book. It will answer a lot of these questions. (You can get the previous version for cheap)
Signal Strength
Yes, you want to choose the channel with the least amount of sideband overlap. If all ISM bands have AP’s already on them, choose the channel with the lowest competing signal strength.
Number of People
Signal strength will usually trump AP occupancy. This isn’t too much of an issue.
Channel Overlap
If you are in a location where all 13 channels are equally taken, then your ideal solution would be to move to the 5ghz UNI Bands. If you don’t have the option to move to 5ghz, then you should go with the channel with the lowest competing signal strength.
Many wireless administrators recommend staggering your AP layout in a triangular fashion alternating between 1, 6 and 13. That way you are able to keep AP’s on the same channel as far away as possible from each other.
Take a look at the following image. 802.11b and 802.11g have slightly different wireless signatures. 802.11b looks more like a bell, where as 802.11g has a more rectangular shape.
Basically the transmission occurs on the actual channel but the transmission ‘over flows’ onto nearby frequencies. The same thing happens in your car when you can still hear a radio station if you tune 1 frequency up or down.
Auto Channel Select
Auto Channel Select is junk. The only wireless routers that I’ve seen somewhat intelligently are the open mesh access points. Every other AP I’ve run into, perform terribly in their channel selection. Most recently I saw 3 ubiquity AP’s set to auto negotiate, and every one of them chose the exact same channel, despite there being 23 other open 5ghz frequencies.
Never trust auto channel select, The fact that the AP’s jump around, make it impossible to troubleshoot why 5 minutes ago the network was great, and now it is performing horribly.
Baby Monitors
The 2.4 ghz frequency is known as the ISM band. (Industrial Scientific Medical) meaning that anyone can use it without a license (along as you stay within certain limits specified by the FCC).
If you ever get your hands on a spectrum analyzer (some high end ubiquity wireless bridges have software spectrum analyzers built in), turn on a microwave and then look at the wave patterns. You will see the microwave absolutely drowning out the access point. http://www.ekahau.com/userData/ekahau/wifi-design/screenshots/spectrum-analyzer-big_microwave_oven.png
Here is a video showing what a baby monitor does to wireless networks
The baby monitor will not show up on normal software because they do not broadcast an SSID. (Other than the fact that your noise floor will be way higher). They will show up in spectrum analyzers.
Channel 13
Feel free to use channel 13 as long as you keep the transmissions indoors and less than 1 watt. You will likely get less sideband interference.
Bonus
What is Channel 14
The US only originally allowed for channels 1 – 11. An addendum was added that allows for the usage of channel 12 and 13 in low power, indoor only settings.
Channel 14 is allowed in Japan, but only if using 802.11b. I know some security experts who setup a hidden AP on channel 14 during pen tests because it is much less likely to show up on a wireless scan.
Wikipedia has some good information on the different frequencies