Understanding "Auto" channel selection

[pfm]

The 2.4Ghz band around my townhome community needless to say is pretty crowded. I use InSSIDer and have experimented with channel selection but have never really understood how Auto channel selection feature on the router works. I have an ASUS RT-N66U. See attached screen shot from InSSIDer.

In Auto mode it always picks Ch 6. Other combo modem gateways I have used have typically chosen Ch 11. This router picks Ch 6 even though it has slightly more cochannel and overlapping network than Ch 11. And no router has ever picked Ch 1 which as you can see from the pic is practically abondoned. Perhaps everyone's routers are set to Auto and there is something about the channel selection algorithm that runs away from Ch 1 ?

Another thing I have not understood is how often does this auto channel selection check to see if it is on the best channel ? Once per reboot ? Every few minutes ?

Finally, what do you think would be the best channel for me ?

 

[sm00thpapa]

Choose CH 1 for your area. Remember if all channels are crowded choose the channel with the least traffic and make sure you are at least -20 dbm from the closets router traffic. This is on the iNSSIDer website as well.

 

[abailey]

I agree, channel 1 looks best. Remember that there are only 3 non-overlapping channels. 1, 6, and 11. Because of this channel 6 is almost always the worst one to use. Any channel from 2-10 will overlap channel 6 (so 9 channels). For channel 1 you have 1-5 that overlap (5 channels) and for channel 11 you have 7-11 that overlap (5 channels). This gets really crazy when someone decides to try to use 40Mhz wide channels on 2.4ghz instead of the standard 20Mhz wide. Its bad enough that they would be using 2 of the 3 non-overlapping channels, but if they set their radio to use something else besides the 1,6,and11 channels, they could effectively take up the entire spectrum as they could overlap all the channels.

 

[pfm]

funny thing is when I switch to Ch 1, inssider suggests moving to Ch 6 (?!)

 

[sm00thpapa]

Just leave it on CH 1 and you should be fine.

 

[stevech]

funny thing is when I switch to Ch 1, inssider suggests moving to Ch 6 (?!)

Unlicensed bands... changes from hour to hour, day to day, month to month as neighbors change systems, usage habits, etc.

It's not the number of nearby SSIDs you detect, it's if any one or more are very busy, heavily used, streaming HD TV on WiFi, etc. And that's sporadic.

Virtually no WiFi analyzer tools that are free will dwell a while and tell you if the SSIDs are heavily used (lots of packets flowing constantly) at the moment.

 

[pfm]

stevech,
you have touched two very good points - channel usage, and whether the auto channel is good as its cracked up to be for how frequently it samples the band.

I suppose something like the heatmap from NetSurveyor would be more useful in that case -

 

[azazel1024]

I agree, channel 1 looks best. Remember that there are only 3 non-overlapping channels. 1, 6, and 11. Because of this channel 6 is almost always the worst one to use. Any channel from 2-10 will overlap channel 6 (so 9 channels). For channel 1 you have 1-5 that overlap (5 channels) and for channel 11 you have 7-11 that overlap (5 channels). This gets really crazy when someone decides to try to use 40Mhz wide channels on 2.4ghz instead of the standard 20Mhz wide. Its bad enough that they would be using 2 of the 3 non-overlapping channels, but if they set their radio to use something else besides the 1,6,and11 channels, they could effectively take up the entire spectrum as they could overlap all the channels.

This depends a little. If you are talking 20MHz versus 20MHz, yes. If someone is set to 40MHz though, that isn't entirely true. If you look at the exact frequencies used, a 40MHz one set on channel 11 is actually using 7+11 and if you set it to 1 it is actually using 1+5. So you can technically have a 20MHz channel 1 or 2 set and it won't interfere with a 40MHz 7+11 or vice versa, a 40MHz 1+5 won't interfere with a 20MHz channel 10 or 11.

That said...Yeah, if you stick a 40MHz somewhere in the middle...bad stuff.

As for auto, it depends on the firmware. I don't know how all firmware works, obviously, but I know a lot simply looks at surrounding beacons to identify SSIDs and their RSSI (Received Signal Strength Indicator) and if the RSSI is above a certainly level it'll select a different non-overlapping channel. If all channels have RSSIs above a certain level, then it probably goes with a default setting within the firmware (possibly/probably channel 6). If no RSSI is above a certain level, it probably also goes with a default channel (again, probably 6).

In most cases, interference is minimal if the RSSI is at least 20dBm below your wifi network.

Issue here though is, if you move your client device further from the router, your RSSI drops...so if you are 20ft and 2 rooms over, that neighboring network that was at -65dBm might be at -50dBm and yours might have dropped from -32dBm to -55dBm...making the neighboring network stronger!

Of course no way for your router to determine that, so auto is always a bad idea because it isn't like your router can actually figure any of that out. Which is where things like InSSIDer come in as well as site surveying, which in a house should be pretty simple. You walk around to a few locations and check signal strengths of yours and the other networks in several places. Write em down if you need to.

Probably/possibly also check your network throughput at the same time, though that is chancier because that depends on how active the other networks are, which probably varies throughout the day.

 

[stevech]

stevech,
you have touched two very good points - channel usage, and whether the auto channel is good as its cracked up to be for how frequently it samples the band.

I suppose something like the heatmap from NetSurveyor would be more useful in that case -

Does it measure busyness, aka channel utilization? Heat map might meas rssi.

 

[pfm]

yeah they call it channel "usage" but looks like its more about signal strength than traffic volume.

 

[sfx2000]

might want to look here - as most AP/Router implementations implement this...

http://wireless.kernel.org/en/users/Documentation/acs

sfx

 

[j3iii]

This was an informative thread as i got a new network tool (or 2) to download and play with and also a way better understanding about the channels. I have just purchased a nighthawk and flashed it and now set my WNDR4500 for an AP. and now my network is slower but with threads such as this one i will get it on track soon. So thanks for all the informative info on channels