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This question is somewhat related to Is it better to choose a more-crowded WiFi channel or one with a strong competitor? and 2.4GHz WiFi channel selection when all the non-overlapping channels are crowded but these do omit certain aspects which I believe to be relevant for my scenario. Although I describe my setup specifically, I'm mostly interested in the general question of "few strong competitors on the same channel or many weak ones".

I am trying to optimize a medium-sized WiFi installation consisting of 18 AccessPoints. The following restrictions/parameters apply:

  • 5GHz is not possible (coverage when considering wall penetration as well as old hardware)
  • channels 1, 6, 11 must be used
  • all APs have the same activity pattern (strong activity during evening hours, all APs similarly active with ~6-8 connected stations)
  • positions of APs can not be changed
  • received signal strengths between the APs are known to me
  • almost no external interference (microwave ovens, foreign APs, etc.)
  • I would need like 50-100 stations under my control to test the performance myself, which is not possible. Suggestions welcome!
  • the antennas are oriented vertically, to minimize signal propagation between different floors and maximize horizontal coverage. As a side-effect, this obviously increases channel congestion between access points on the same floor.
  • I have actual up-to-date measurements of received signal strengths between access points and other parameters like connected stations

The APs are mounted in hallways. As a result, APs receive each others signals quite strongly and from many different APs. The stations are mostly located in rooms along the hallways and receive only the closest 1-3 APs due to wall thickness.

As with the questions above, I am curious about the most ideal channel selection for all the APs. Using an optimization algorithm with different heuristics, I was able to determine multiple setups.

Setup 1:
Isolated "Islands" where APs have the same channel. If an AP uses the same channel as one of its neighbors, but it is ensured that the neighbor is actually close (with high received signal strength). There are no neighbors on the same channel with very low received signal strength. The overall number of conflicting channels is quite low, but IF there is a conflict, it is a stong one. I feel like this might not be too bad, since stations on a conflicting channel are easier to "see" for APs (less hidden terminal problems) and the overall number of conflicting stations may be lower.

Explanatory graphic: Setup1. Letters are AP identifiers. Colors of APs encode the channel. conflicting neighbors are indicated by a red edge

Setup 2:
Keep APs on the same channel as far away as possible. There are more conflicts in total (and per AP), but the conflicts have lower signal strength. Strong received signals are never on the same channel. This setup is more in line with the general suggestion of "choose the channel where competing signals are not too strong" which I read in some forums. However, I feel like this setup will encounter problems with hidden/exposed terminals and a lot of airtime is wasted due to RTS/CTS messages.

Explanatory graphic: Setup2

I am aware that a definitive answer would require more information, but this is not only about my setup but about understanding how WiFi works in such a scenario (assuming no external interference, which actually applies in my case).

So, do you have suggestions as to which of the setups is preferrable? Are my assumptions about the setups reasonable?


I just realized that I made some simplifications in my previous TL;DR that slightly skewed the intent of the question, sorry for that.

So the revised TL;DR is more of a new way to phrase the question but uses the example given above: For a given access point, should I optimize for a low number of APs causing co-channel interference xOR should I optimize for reduced RSSI of neighboring APs? As this is a tradeoff, solution 2 would actually increase the number of neighboring APs that interfere on the same channel.

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    Wow, that's good some good going with the images explaining the question, i'm excited for the answers this'll hopefully generate! – djsmiley2k Mar 9 '17 at 18:33
  • I am not sure if it is worth worrying about. It depends on the actual interferences which you cannot see. You could try it, but even then the situation changes as clients move around and APs switch frequencies. Investing some money in 5GHz may be more fruitful. – eckes May 5 '17 at 11:58
  • Thanks for your comment! As stated in the list of restrictions: 5GHz is simply not possible in that building. >100 APs would be required (we tested wall penetration). APs will not switch frequencies on their own. Clients are restricted to their individual rooms. Apart from these restrictions, I'm also very much interested in the theoretical aspects. – Slizzered May 5 '17 at 12:06
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+50

WiFi 802.11b/g/n radios can transmit in the 2.4 GHz band with a total of fourteen available channels (not all are legal). The figure below depicts a spectrum analyzer view of the frequency space occupied by these fourteen channels. Please note that within the 2.4 GHz band, only three channels have non-overlapping frequency space: channels one, six and eleven.

frequency map

This means that if you set a wireless router or access point (AP) to channel 6, it actually uses channels 4-8.

Wi-Fi is only half-duplex, so APs being on the same channel will cause medium contention overhead. WiFi uses a listen-before-you-talk technology called the clear channel assessment (CCA) to ensure that only one radio can transmit on the same channel at any given time.

If an AP on channel 6 is transmitting, all nearby access points and clients on channels 4-8 will defer transmissions. The result is that throughput is adversely affected: Nearby APs and clients have to wait much longer to transmit because they have to take their turn. The unnecessary medium contention overhead that occurs because all the APs are on the same channel is called co-channel interference (CCI).

When designing a wireless LAN (WLAN), overlapping cell coverage is necessary to provide for seamless roaming. However, the overlapping coverage cells should not have overlapping frequency space.

Conclusion: Setup 2 is the correct setup, using only channels one, six and eleven.

Sources: source1 and source2.

  • Very nice sources, thank you! Are you certain that APs on channels 4-8 will cease operation while channel 6 is transmitting? As I understood it, CCA only considers the actual channel of the AP and signals on nearby channels are treated as noise. So an AP on channel 4 would still transmit, but possibly require a lower transmission rate to account for the higher noise floor. As described, I already use only channels 1,6,11 so this is not too relevant but rather an interesting observation. – Slizzered May 6 '17 at 7:24
  • As I see it, solution2 creates MORE wait-time due to CCA, as more APs and clients can see each other on the same channel (albeit with lower signal stength. But they can still see/hear each other so they will wait for the transmission to be over, right?) – Slizzered May 6 '17 at 7:24
  • You were correct in assuming the circle-colors to represent channels. Interference is marked by red lines (dotted as well as solid!), while non-interference is marked with black lines. I agree with you on making the signal the weakest and reducing overlap. But in my case it is about the tradeoff. I have few (2-4) strong overlaps on each AP versus many (5-6) overlaps with weak signal, but not as weak as they can be ignored (-75dB to -85dB). – Slizzered May 6 '17 at 8:31
  • @Slizzered: If circle-colors mean channels then solution2 makes for the weakest interference and solution1 is simply bad. Your map seems to indicate that when using only 3 channels, no interference at all is topologically impossible. The rule should be avoiding overlapping frequency as much as possible, and if overlaps are unavoidable make them the weakest possible so with luck they might be ignored. You might try to see if you can use more than 3 channels by using the fact that transmitting on channel 1 affects channel 3 less than it does channel 2. – harrymc May 6 '17 at 8:37
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    I am counting 5 interferences in solution1 where each AP at most encounters one other AP on the same channel. Solution2 shows 7 interferences, where APs "D", "H" and "I" have 2 problematic neighbors and AP "F" even 3 neighbors on the same channel. Considering that all APs service 10 clients, according to CCA this should lead to more wait-time when accessing the channel. – Slizzered May 6 '17 at 8:40
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Assuming you control all the APs in question - this may sound crazy, but it may make sense to make all the APs transmit at lower signal strengths. I've had surprisingly good success with this in an environment with heavy wifi congestion.

That way they don't interfere with each other (which means better signal quality in a shorter range), handovers are neater and you make better use of available channels. I'd then go for the second option

I'd urge an empirical approach to the problem - at this point. Walk around, tweak specific APs where your weaker signal strength is an issue. A phone with a wifi scanner is awesome for this. In many cases architecture affects signal quality, and that's something you can't really account for easily without expensive gear

One of the SE sysadmins did a awesome series on the practical aspects of this (part 1 and part 2) which is worth a read.

I've done something similar with a 2 AP + extender setup.

  • I actually read these blog posts before asking the question. Thanks for linking them here, it is a good source of information. And I completely agree. It does not sound crazy to reduce transmit power. Unfortunately, I already did that to get as low as possible (measured with my phone and the "Wifi Analyzer" app) to ensure the necessary wall penetration. So you would still suggest Solution2, although it will have more interfering channels and devices? – Slizzered May 6 '17 at 7:15
  • yup. That's what I'd do here – Journeyman Geek May 6 '17 at 7:19
  • Which brings me exactly to my question: WHY whould that be better? Virtually all sources I could find start by stating that you should select channels to MINIMIZE the number of overlapping channels. – Slizzered May 6 '17 at 7:29
  • Well, the idea is if the APs on the same channel are far apart and relatively weak, the interference between them should be minimized. – Journeyman Geek May 6 '17 at 7:31
  • But I thought that 802.11g had methods like CCA and RTS/CTS that would occupy/reserve the channel. Which would affect relatively weak APs just as well, as long as they still receive a valid signal that tells them that the channel us busy. I would very much agree with you based on pure radio transmission, but I'm concerned with the protocols involved in actual WIFI. – Slizzered May 6 '17 at 7:36
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One other thing that has bearing on your question but isn't defined is the actual physical layout of the access points.

If the antennas are properly configured and aligned in a vertical orientation then access points directly above or directly below a given access point are in a null area for the transmission of that access point. The higher the gain of antenna that you use for this will both increase signal strength in the plane perpendicular to the antenna orientation and reduce interference from floor to floor in the building. You may depending on building orientation be able to eliminate some access points and as a result reduce cross-channel interference of similar frequencies.

The problem I've seen with many deployments is improper antenna orientation because the installer did not understand antennas and signal propagation.

  • Thanks for considering this factor. I will add the appropriate information in my question. Unfortunately, I do have some experience with signal propagation and already set up the antennas as described. So there is not much headroom left (except for buying higher gain antennas). As your answer is more a comment on other improvements instead of an answer to the question of channel competition, I can not select it as the correct answer :( – Slizzered Mar 15 '17 at 10:39
  • That is fine I just knew that I wouldn't be able to put all of that into a comment because of the character limit. – Rowan Hawkins Mar 16 '17 at 1:14
  • Don't forget that a large amount of wireless data is sent by the clients who are roaming around everywhere without any kind of directional antenna. It's not just the access points. Every transmitted Wi-Fi frame requires that a short response be sent back. – Alex Cannon Feb 21 at 5:22

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