First of all I should say that I'm no expert by any means. So take this with a grain of salt.

A little while back, I've been "thinking load" and wrote a whole lot down regarding a network. A lot of questions I've just been throwing out there. Including probably some ridiculous thoughts. Might have been a bit overwhelming, hence no one ever replied to it.
Having said that, I also wrote down a few thoughts about wireless. Don't know how much help this will be.

Anyway, I want to throw in my 2 cents worth here and hope it will get you a step closer to your solution.

OK, before we touch the wireless, I'll make a few other comments first.
Regarding the idea of using a computer rather then a dsl router. I would say, don't do it. Unless that modem is so old that it's not actually capable of delivering the speed you get out of your line I would simple continue using it. If it's that old or if it's not working properly, replace the modem with a newer one. Don't use a computer for that. The dsl modem is quite capable of delivering that speed. 512kbps is really no drama for that device.

Please forgive my ignorance but I don't know where you are. So this paragraph my be redundant.
I assume you have looked into other internet option, another service provider. If there is no other available, have you checked if it is possible to switch to a business contract for dsl? That might give you better performance and better service. But as I said, it might depend on where you are, what is available to you, and therefore also what it would cost.

In any case, I would recommend to look into a proxy server. That might help to free up some of the internet traffic, since the proxy will cache frequently visited sites and therefore not everyone needs to always download it from the net but rather gets it via the proxy server. In the Open Source world Squid is a common recommendation.
Squid homepage / @Wikipedia
Additionally it might also be worth to have a look at monowall. This is also an often recommended Open Source software. A firewall, and something you can use for traffic shaping / throttling / (prioritising as well, if I remember correctly). With that you then could make sure that no one blocks the whole internet by watching a youtube video. Though, due to the limited overall speed, youtube or similar content might be a bit slow if you throttle the connection for everyone, as in one might need to pause the video and let it pre-load before watching. But that shouldn't be a problem really. Overall, it should give you a better / smoother internet experience since no single device can block the whole thing.
monowall homepage / @Wikipedia
Since I don't have a personal need for either of those at home I can not give any feedback from personal experience. It's just what I see recommended a lot and it seems like you could profit from these.

If possible have a central switch for all your network, not daisy chaining it. At least, I assume you would have something like a central file server? For the internet it wouldn't be necessary, but for moving data to/from a file server it's much nicer if nothing is daisy chained. But it also reduces the inconvenience that a single broken cable might cut of multiple people from the network.
While you are at it, have you thought about providing an ethernet cable for everyone with a laptop? Usually when I suggest this people argue that this is exactly the point of a laptop, to be flexible and mobile. And that is correct. But while I'm stationary at a desk I might as well plug in a ethernet cable and enjoy the benefits of it. E.g network speed when I need it to move larger files, less potential interference, etc. And when I need to be mobile I still can unplug the ethernet, the power, the mouse, grab my laptop and walk into that meeting or what ever else.

So, the wireless.
Since you want it, I assume you already have thought about it and concluded that you actually do need wireless, right? But are you really sure you need it in every spot of the building? Is it possible to put people who desperately need wireless in places with good coverage? Yes, yes, I know, the ever increasing number of smart phones and tablets. I don't know your Base policy about personal internet usage. But if all those phones and tablets are privately owned and personal internet use is discouraged, then there shouldn't be a problem with not having them connected, right?
If all devices are legitimate then how many wireless connections do you actually have? You say up to 15 people. But I kinda doubt that everyone would work with a laptop and only using wireless? Plus potential phones, etc. The number of wireless devices could easily exceed the number of people, but it could also only be a handful.
In any case, even if you would have 30 devices, 6 wireless access points seems like a lot. With 30 devices that's an average of 5 devices per wireless AP. I have easily more on mine at home, an 8 year old AP and the cheapest I could get at this time.
So, the problem is purely coverage. This is were the fun begins.

Provided that you really do need every spot on Base covered, this is what I would do.
First of all, are the 2 APs you have the same? If not find out which one is the weaker. so turn of one, and see how far away you can get with a laptop or phone before the signal drops out. Repeat with the other AP in the same spot. Then take the weaker, put it in the middle of a room (away from walls, microwaves, anything metal). Then try with multiple devices how far you can get away from the AP. With the one that drops out first you can start discovering dead spots, good and bad places where to install an AP. By taking the 2 weakest devices you get the worst possible scenario.
Now you start putting the AP in different spots, where you think you could place it. With your weakest mobile device you can walk around and discover dead spots and how far away you get acceptable coverage. Pray that this isn't a 17" laptop or you might get some unexpected exercise. See what happens when you move the AP around just a little bit. It might be too close to a wall with cables or steal construction inside which blocks the signal, a microwave, toaster, or any other object with metal might be in the way. Moving it a little bit could potentially increase or decrease the signal dramatically. You can repeat that with the other AP, if you want, to see what difference in coverage it makes.

During this process it might be handy to have a rough sketch (or multiple copies of it) of your offices to take notes in it, marking the covered area, dead spots identified, etc.

From that research you then can identify what you really need. Depending on where you need coverage, where you can tolerate dead spots, where you actually can install an AP, etc. It might take you a little bit but the work should be worth it.

PS: It's long past bed time. I hope what I wrote still makes sense and will be helpful.

Hi Steve, I read your post a few weeks ago which is why I decided to ask my question here at YWAM IT.

Providing wireless internet access is the single goal of our base network. The second floor of the building is set up like a dormitory, and about half of our YWAM staff live there year-round. The staff and DTS/SBS students living at the base split the cost of the internet connection and use it as their only internet access. We do not have a file server and all the computers are personally owned laptops. Our current wireless setup does not use any security and does not require a password.

It might be easier to think of our situation more like a coffee shop. We simply want to offer wireless internet access to anyone in the building, prevent any one person from using all the resources, and ask everyone who uses it regularly to pitch in and help pay for the cost.

The DSL connection that we currently have is a business connection, which is the only option available to us since the building is owned by YWAM. There are faster connection speeds available, but since we are being charged business rates, it is very expensive. We are planning to move from the 512kbps plan to the 1Mbps plan since the prices just came down a bit. At least it's faster than the 64kbps connection we had just two years ago!

You said not to replace the DSL router with a computer, but then you recommended using squid and monowall. Aren't both of these programs that need to be installed on a computer? My idea of replacing the DSL router with a computer would be to add the caching and traffic shaping features you talked about. Are there DSL routers that have those features built in, or would a computer be the best way to effectively split the internet connection among many users preventing any one person from using up all the bandwidth? Also, would a computer react more quickly to an interruption in internet access, resetting the connection faster than a DSL router box?

The only reason I am thinking about 6 access points is simply to blanket the entire building with good signal strength. 4 APs might possibly work, but 2 is definitely not doing it for us.

I need help determining the best upgrade path, and where our money would be best spent in order to give our unreliable internet connection maximum up-time and serve it to as many people as possible as fairly as possible. I also need to know the right way to set up multiple access points around the building that may possibly overlap their coverage by a little bit.

If I were you I would try as much as possible to avoid a computer based option. The idea of using a wireless router and access point with dd-WRT loaded is a good option, and you can set up quality of service with it as well. We used a similar set-up at the last YWAM Centre I was at to cover our large main building and four other buildings that we connected to using direction antennas.

Here are a few resources that I found that could help you with setting something like that up:

• Load DD-WRT on a Router
• Set up Quality of Service on your DD-WRT router
• More DD-WRT QoS Information
• DD-WRT
• Asus Routers that Support DD-WRT, including the RT-N16 that you mentioned