Questions about overlapping wifi access points

Question

I have a few questions about wifi.

Say we have a small campus with a set of wifi access points in most buildings, one each for channels 1, 6, and 11. It's not exactly that clean, but that's the general idea anyway. However, these are mainly cheap consumer wifi routers that have been re-configured to work more like access points. They are all set up with the same SSID.

For the most part, the buildings are far enough apart that APs do not overlap. Where buildings are closer together it's usually because they're smaller or less-used buildings that won't need to use all three non-overlapping channels. Where a building is much larger we'll move the access points around to try to get good coverage throughout the building.

This configuration seems to be working, but I have some questions and concerns:

• The APs don't do any kind of load balancing or sharing connections. I'm worried about students in crowded common areas being forced to choose the closest/strongest AP even though it might be overloaded when one just a little weaker on a different channel might serve them better. We have an iPod touch program on campus, where every student has one. Those devices only list each SSID once when choosing a network, so students can't adjust for this kind of thing manually.
• The older dorms, for example, are concrete block and need a lot of work to get good coverage. It's weird how sometimes you might get a strong signal from an AP and at others almost none at all. What will happen if I start adding additional APs to provide stronger coverage in some of these sometime-covered areas where all three non-overlapping channels are in use? Should I look into getting stronger radios/antennas on existing APs near those locations instead? How would a stronger radio at the AP help if the signal sent from the laptop or iPod is too weak to reach it? And if it won't help, why do they sell them?
• There are other places where signals on the same channel (and, as mentioned earlier, the same SSID) will overlap somewhat. I'm unsure about what happens when you connect in one of these places where the signals overlap. I know from experience that I am able to use a wireless device there, but could I be creating duplicate traffic on the network if both APs that are in range pick up on the transmissions from my device? Even if it's not duplicate traffic, could my wireless device be generating interference (and collisions) on the AP that I'm not using, causing slow-downs at that location? What can I do to minimize these?
• Would changing the SSIDs to be unique everywhere even help with overlapping channels, since it's still noise on the channel?
• I'm especially worried about common areas, where students sometimes gather in numbers and there may be a lot of devices active at once. Is there anything I can do to help coverage in these spaces, especially since they tend to already be a little crowded from a channel-perspective?

Is there anything else obvious I'm overlooking that might improve things? It was a real mess when I started here and I've already done a lot of work getting things working better:

• The initial reconfiguration from router to access point so that things like dhcp and dns requests go on to our main server.
• Set up power timers so the APs restart every night (routing tables on the cheap APs fill up over time in a larger network)
• Use a wifi-analyzer app on my iPod ($2, but highly recommended) to tweak channels for less overlap and help work out new placements.
• Added a few additional APs (still cheap consumer routers) to some of the least covered locations in the dorms.

What else could I do to make these work better?

Finally, I'm unlikely to be able to get buy-in from higher up in updating these access points to nicer managed models. The college had a bad experience going that route, and they are convinced that the consumer class devices are as good or even better. Even if that weren't the case, I have more pressing needs in my budget. But we are looking at replacing a number of the existing access points with ones that are 802.11n capable in anticipation of Apple eventually releasing an 802.11n iPod, so I'd appreciate suggestions for good consumer level 802.11n routers.

Answer 1

From past experience working with these sort of networks, I can offer the following:

• No, I don't believe your standard consumer-grade APs will have any sort of load-balancing. there are options out there, like Cisco's wireless solution (for the right $$$) that can do things like this.

• In areas that are hard to cover, you may need to look at different antenna configurations or simply adding more APs. This will require a bit of work to determine the right balance, as adding too many will affect performance as well. You may want to get some wireless mapping software to help you determine the signal strength in these areas, as well as overlap. A stronger radio will not help if it cannot 'hear' the Iphone; for this you will need a more sensitive radio / antenna.

• I believe the client has a lot to do with how this scenario will perform. Normally I think they will simply pick the strongest signal, all things being equal. Multiple APs on the same channel will interfere with each other, but you will probably need to do testing to see how bad the situation really is. Drops and collisions are expected in wifi.

• When/If budget allows, you might upgrade to some commercial grade routers. These shouldn't need nightly reboots, and will probably perform better under higher loads. You may also want to look into 802.11n if you aren't already. (iPhone 4S supports the 2,4 GHz 802.11n, the iPhone 5 adds support for the 5GHz band of 802.11n)

hope that helps somewhat... ?

(in a previous life I did support around 50 Cisco APs in a hospital setting... we never had much problem with overlapping or slowness, as long as the channels were spaced out as they could be)

Answer 2

First I add that some of these replies need clarification.

There are two kinds of overlap, one is channel overlap where the frequencies overlap, and the second being signal overlap.

You MUST have signal overlap for all devices to have coverage in all areas, or even most devices in most areas.

Secondly, there are various schools of thought for frequency overlap and some manufacturers even suggest putting all APs on a common channel. In the case of roaming IP phones this case becomes even stronger as a phone may hop across APs while in a call. This of course depends much on the hardwae of the phones and antenna placement and design.

Let us assume that we had a large open area that we wanted wifi coverage in. Now lets take a pole and place it in the middle of the area. Now we place 4 directional 90 degree antennas on the pole, each 90 degrees from the other . In this situation one may make a strong case for having all APs on the same channel to facilitate roaming. In theory there is little signal overlap but all frequencies overlap.

Now we have an open area with walls on four sides. and place an AP on each of the four walls. The signals WILL overlap from each of the 90 degree antennas , so we may want to consider using separate non overlapping channels on each AP , however there are only 3 non overlapping channels. 1, 6 , and 11. So instead we do the best we can in North America this might be 1, 4, 7, and 11 , each AP having SOME necessary frequency overlap. Of course in a perfect world this might be better accomplished with three APS in a triangular configuration.

In my home I have toyed with APs on Same channel and separate channels and in the end I see little coverage difference., I do see however that some devices such as wireless IP phones can more easily hop to another AP while in a phone call. I see that in most areas I do not have more than 2 overlapping signals and each on channel 4 at present. As I sit here I can launch wifi seeker on my android and see either of the 2 available APs and even connect to either. This of course is easier to test with separate SSIDs but more practical to use common SSIDs fopr everyday use.

Answer 3

I set up and maintain a set of wireless APs for a mobile IP phone system in our factory. In order to facilitate roaming, the AP radios have to overlap enough to get a good signal from one of them at all times in the transition zone. So long as the AP radios are on well-separated channels they work well.

This IP phone network fully overlaps a separate wireless data network. The installation, which I wasn't consulted on, placed each phone AP one foot from each data AP because it was convenient for wiring. Each AP is at least half the channel range away from its "partner" so there is no frequency overlap.

This hasn't created a problem. (Caveat: They're Symbol APs which are rather expensive and may not compare well to sub-$100 APs.)

Answer 4

• The older dorms, for example, are concrete block and need a lot of work to get good coverage. It's weird how sometimes you might get a strong signal from an AP and at others almost none at all. What will happen if I start adding additional APs to provide stronger coverage in some of these sometime-covered areas where all three non-overlapping channels are in use? Should I look into getting stronger radios/antennas on existing APs near those locations instead? How would a stronger radio at the AP help if the signal sent from the laptop or iPod is too weak to reach it? And if it won't help, why do they sell them?

I want to add that for this point, at least, I've since learned that the solution offered by commercial vendors like Cisco or Aruba is to actually run the access points at reduced power rather than increased. This allows you to place the access points a lot closer together (more density) while still avoiding radio interference among access points, resulting in much more consistent coverage. The controller may even automatically lower the power for you to the optimum level. Of course, that means paying for and placing a lot more access points. Higher-powered radios work better in more open spaces, where when coupled with higher sensitivity antenna they can cover a larger area.