How many WiFi access points can co-exist in the same space?
February 13, 2013 1:18 PM   Subscribe

Local cable internet company wants to blanket my condo tower with WiFi APs to provide universal WiFi access for their subscribers. Will this degrade reception or throughput for existing private WiFi networks belonging to residents?

I am on the strata council for my residential condo tower. It is a 25-storey building built with modern steel & concrete methods, about 8 suites per floor ranging from 700 to 1000 sq ft in size. This being downtown Vancouver, pretty much every resident already has their own WiFi AP. In the vicinity of my suite alone, I can see over a dozen different SSIDs.

We have been presented with a request from Shaw Cable (one of three or four major providers in my area) to install WiFi access points throughout the building. Access is for subscribers only. Should we (the strata council) let them?

My concern is that residents with their own WiFi APs will suffer from degraded reception / throughput. Aren’t there are a finite number of channels or frequencies? Is there a threshold of density of APs that shouldn’t be exceeded? Do modern APs intelligently co-exist with each other, switching channels automatically? Can they share channels?
posted by wutangclan to Computers & Internet (18 answers total) 1 user marked this as a favorite
Short version: Yes, definitely. Modern routers can be set up to automatically seek the best channel to avoid interference, but there are limits to this, especially if there aren't channels available. Basically, modern APs do try to co-exist with each other, but in this circumstance, they're going to have much harder time doing so. There's very likely to be a nonzero amount of interference with existing APs.
posted by Tomorrowful at 1:25 PM on February 13, 2013

We're covering this right now during my degree.

The answer is three if you're using the most common 2.4GHz spectrum (for 802.11 b, g and n networks). Although there are between 11 and 14 channels depending on your country (I think 11 for the US), the 2.4 GHz wireless signal is very messy and overlaps around 2 channel spaces either side - you can therefore only fit 3 in the spectrum you have at 2.4GHz.

Given that the bulk of devices currently use this range (even though 802.11n allows for 5GHz, many devices do not implement this for cost reasons), many people will have their network's spectrum overlapped by this installation. The overlapping is detrimental when both networks are in use at the same time.

In our class experiments, we found that if only one network was in use, there was a negligible effect on speed, if any. However, if both were used simultaneously, the drop in performance was up to 90%.
posted by fearnothing at 1:27 PM on February 13, 2013

depends on what they want to deploy.

If they are rolling out "true n" with multispacial streaming across wireless "a" and "g", then you should be able to coexist with a minimal amount of problems, as the specification has methods designed to take advantage of crowded spectrum, and improve performance.

If they want to roll out with "g" only, then you are dealing with a pretty crowded spectrum.

Basically here's how "g" works:

there are 3 channels you typically want to use, 1,6 and 11.

Think of the these channels as "hubs", EVERY wireless router is competing for that channel space, EVERY SSID has to timeslice on that frequesncy, so someone streaming an HD movie on channel 11 in their apartment is impacting the availability of bandwidth on channel 11 for every other wireless AP or router in the vicinity (100-200 foot radius).

So, if Shaw wants to put in 100 APs, then you are going to have co-channel interference in the 802.11g (2.4 Ghz) spectrum.

The only way around it is to dynamically assign channels, it's a real hassle, but that's why engineers have jobs.

The best route is to go 5 Ghz (a radios)
posted by roboton666 at 1:28 PM on February 13, 2013

It's possible for several nodes to share the same frequency because Wifi uses a form of CDMA. This is by design. Also because of the frequencies being used, Wifi fades rapidly with range.

I think you're worrying too much.
posted by Chocolate Pickle at 1:29 PM on February 13, 2013

Make them explain how they are going to prevent their network from crowding out existing networks. Check their work.
posted by wierdo at 1:32 PM on February 13, 2013 [2 favorites] of three or four major providers in my area...

Access is for subscribers only.

If they aren't the only provider for residents in your building, and most residents already have private WiFi networks, it sounds like they will spend a lot of time and money for little benefit - and based on the above comments these benefits will definitely not outweigh the costs in terms of connection quality and speed for all networks.

I would suggest that they instead offer free WiFi routers to their existing subscribers within the building, if anyone is even interested. Other than that, no. Don't let them install anything.
posted by trivia genius at 1:39 PM on February 13, 2013

I don't know about the technical aspects, and I am not a lawyer, but it seems the key here is how long of a contract do they want you to sign and what are the outs? Can you give 30 days notice? If you do your research from a technical standpoint and it still is a problem, can you get them to turn it off or uninstall?
posted by JohnnyGunn at 1:51 PM on February 13, 2013 [1 favorite]

Unless they're providing all residents -- not just all subscribers -- with something useful (free wifi for everyone in common areas, for instance), I cannot see why you'd want to bother. If I were a subscriber who already had my own router, I would certainly not be willing to give it up to use a more public one all the time.
posted by jeather at 2:50 PM on February 13, 2013

roboton666, can you elaborate how I can determine:

...if they are rolling out "true n" with multispacial streaming across wireless "a" and "g"...

so that:

....[we can] coexist with a minimal amount of problems

I.e. what is "true n"? Or point me to a succinct reference?

I have been made the point man on this issue by the strata council, but it's all a bit over my head.

posted by wutangclan at 2:55 PM on February 13, 2013

Yes, the more access points you have in a given area, the more interference there will be between them. From a practical standpoint you will see this reflected mainly in reduced effective range from each AP. That said, I wouldn't expect this installation to dramatically increase the overall number of visible APs from any one point in your building and think the overall saturation burden would be about the same as an additional unit or two on each floor got their own wifi router to install, which you currently would not regulate anyway. Inter-floor penetration of wifi through concrete is relatively poor. For perspective, at my office where I'm sitting this very instant, my wifi analyzer can detect no less than 30 APs all fighting for different channels and our in-office wifi works well to a distance of about 50 feet, even through walls.

I presume this is to support their ShawOpen initiative? Overall I think it's a strange request as each of their customers in your building already have a wifi router built in to their cable modems...
posted by barc0001 at 3:13 PM on February 13, 2013


I don't know exactly what "true n" roboton might be referring to - perhaps he means 802.11n access points which include both 2.4 and 5GHz capability. There is NO equipment that implements ALL of the specification of 802.11n, not even Cisco has produced actual hardware that does any more than 75% of the peak theoretical speed for example. The kit they propose to install in your building will not be the Cisco kit mentioned either, it will be even further from the full spec.

Even if they implemented "true n", this would not solve the base problem, which is that the vast majority of devices do not have a choice between 2.4 and 5GHz, they have to use 2.4. Therefore the access points this company puts in will have to serve the 2.4 band, and that means overlapping everyone's spectrum.
posted by fearnothing at 3:28 PM on February 13, 2013

...if they are rolling out "true n" with multispacial streaming across wireless "a" and "g"... ....[we can] coexist with a minimal amount of problems

That is a problematic statement. 802.11 n, a, and g are different protocols and there is no meaning to the concept of rolling out 802.11n "across a and g". I suspect roboton666 means "the 5 GHz frequency band" instead of "a" and "the 2.4 GHz frequency band" instead of "g", but I don't think that completely solves the problem.

As he and others point out, all access points on the same channel that are within range of each other compete for time on the channel. Since there are only three non-overlapping channels (1, 6, and 11) in the 2.4 GHz band and since a lot of user equipment only works in the 2.4 GHz band, it is very likely that the cable company APs will impact the performance of many residents' home WiFi networks.

Even if the cable company uses dynamic channel selection and power control, a lot of their traffic is still going to end up in the 2.4 GHz band and hence will impact other networks.


Now, when I say "impact," what exactly does that mean? There are two main effects that an interfering network can have on other networks.

If the interfering network is running the same protocol (that is to say a, b, g, or n) as the target network, the main effect is that both networks will have slower maximum data throughput as they have to share time on the radio channel. The impact of this time sharing gets worse as more access points are sharing the channel, and as the total amount of network traffic offered* by each access point grows.

If you have five or ten APs sharing a channel and the traffic offered by each AP is quite low, there might not be much slowdown at all. If you have two or three APs sharing a channel, and each AP is offering a lot of traffic, it is quite likely that all three networks will slow each other down significantly.

The other (and usually worse) effect happens when the interfering network is running a protocol that the target network either doesn't know about or isn't running in "compatibility mode" with [or vice-versa]. The transmissions using the newer protocol appear as noise/interference to the equipment using the older protocol. In addition to reducing the total network speed available, this also reduces the range of the affected APs.


It's an interesting problem overall. It will definitely "impact" other Wi-Fi networks, but it is very hard to say to what extent the networks will be affected. I would not be upset if someone installed a system like this in my apartment building, but then I would know how to mitigate any problems it might cause for me. I also wonder what the benefit is of installing this system in the first place. If it is for cable Internet subscribers only, wouldn't the subscribers be able to use their own cable modems and/or the Wi-Fi access points already running off of them?

* "Offered" traffic is the amount of traffic that the system would like to send or receive. It tends to exceed the amount of traffic actually sent and received when the link is slow or the channel is busy.
posted by Juffo-Wup at 3:36 PM on February 13, 2013

I question what the upside of this is for your condo association, or for Shaw, but if they are offering it to you, chances are they think they know how to do it technically.

WiFi can be quite robust, if it is planned and implemented properly. At my work, people can wander all over the building with their wireless headsets and roam from AP to AP and stay on a telephone call. But that's one single network with Cisco equipment.

The overlapping bands thing is both a detriment and a positive. It is true that the channels overlap, but that's only for APs within range of each other. APs that aren't all that near each other can use the "overlapping" channels and this just makes the signal a little noisier. But that *can* be a benefit, if this allows the APs to be laid out such that any conflicting networks are pretty far away.

Think of an 11 story building with 1 AP per floor. Going up the floors, you'd assign them with channels something like 6,10,3,7,11,4,8,1,5,9. (or something like that, there is an algorithm for figuring that out) You trade a little extra noise for a lot of different channels. On any floor, the obvious signal is the one on that floor. The closest signals are so far away that they are little more than background noise. Seeing an SSID is not the same as being able to actually connect and get throughput on that AP. Combine that with directional antennas and you have a system that's pretty clean.
posted by gjc at 4:14 PM on February 13, 2013

You need to do a building survey to determine how many people would turn off their current wireless access points and switch to using the communal access points. If a significant number of current aps are going off, then it might even have a net benefit. (Personally, I wouldn't touch it with a ten foot pole, but someone who only uses the network to get to the outside world, instead of connecting between devices, might go for it.)
posted by anaelith at 4:15 PM on February 13, 2013

Firstly, most wireless devices that the residents will be using will be on the 2.4GHz band, so what they install will be transmitting on that frequency. No sane company would install 5GHz only equipment - even if it existed!

Thus it will be transmitting on the same channels as everyone else, of which there are 3 non-overlapping ones (1, 6, 11.) In terms of effect, the competition reduces the range of the existing access points, and reduces the transmission speed of the access points when they're all transmitting (so, at peak time for network usage in your condo.)

The most affected people will be those using the oldest equipment, which has the worst signal range, and transmission speed in the first place. The oldest standard (802.11b) uses CSMA which only allows one station (computer) to transmit at once on a channel, and 802.11g isn't much better. The people using the oldest equipment are also the people least likely to be able to diagnose or fix the problem - possibly even being forced into becoming customers for the subscriber?
posted by Ashlyth at 4:38 PM on February 13, 2013

I presume this is to support their ShawOpen initiative?

Seems like it's now called Shaw Go, but yep, that's the one.
posted by wutangclan at 4:47 PM on February 13, 2013

Lots of ways to look at this. One way:

The optimal situation, from a wireless performance point of view, is that there would be one properly engineered, centrally managed wireless network in your building using multiple access points. In this situation, the channel-selection and transmission power of each access point would be coordinated to minimize interference and maximize throughput.

Unfortunately, that isn't an option in your situation. You already have people operating their own access points. Many of those router/access points are probably limited to the 2.4GHz range, which, as people have noted, only has, at best, 3 non-overlapping channels and interference from microwaves. Some may support 5 GHz either as an alternative or addition to 2.5GHz operation, unfortunately, many devices are limited to 2.4GHz. Some personal WiFi routers/access points have good channel auto selection, but there is only so much it can do in a noisy environment.

Further, more "sophisticated" users may think they can do a better job and pick their own channel, at the same time, they might choose to jack up the transmit power and select channel bonding in pursuit of better wireless performance. Unfortunately, this just ends up making things worse for everyone.

In think it is still possible that a well engineered network could be added to your environment without making the situation significantly worse, particularly if they build out a 5GHz network, and it could even make things better over time if people end up retiring their own access points in favor of the ISP's.

I have two suggestions:

1) Get references for people who specialize in the engineering of WiFi networks, ideally WiFi networks in dense urban environments. Approach them about doing a review of whatever survey and plan the cable provider proposes. I don't know how much it will run, but they should be able to give you a thumbs up or thumbs down based on whatever the cable provider provides. Either it will be thorough enough to contain all the info the consultant needs, or it won't, which is a red flag of its own.

2) Ask the provider to provide a detailed proposal for the network they want to install for the purpose of review by the independent consultant. If they want you and the consultant to sign an NDA, thats probably fine, as long as it only covers the details of the network engineering and business forecasts. It shouldn't prevent you from sharing your opinion of their service levels and engagement style.

In addition, ask them how many subscribers they currently have in your building and what their working projections are for additional subscribers in order to pay back the cost of installing the network. Based on their past experience in other buildings, what % of existing customers will retire their WiFi access points? What are their plans for maintaining and upgrading the network?
posted by Good Brain at 5:14 PM on February 13, 2013

I am on long island where cablevision has optimum wifi. They started the whole wifi for its subscribers thing.

Maybe you can have shaw just put Ap's in jus tthe common areas? Do you have say a gym or rec room?
posted by majortom1981 at 4:39 AM on February 14, 2013

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