Hi there - trying to implement a WISP setup - unfortunately another ISP has already started taking over the tallest buildings in the area with exclusive leases for 2-6GHz equipment. Therefore I’m stuck with a roof level option only - I’m thinking WDS but not sure if thats the best option.. I have one main AP which broadcasts and is feeding Internet access and also running as a DHCP server..
Any ideas? 
I’ve got a similar problem, but my problem is that there are no tall buildings in the area I serve! I target underserviced areas in South Africa, i.e. small towns.
What I’m going to do is to have a single AP serve as my backhaul, and several base stations connect to it on IP-level to pull data. Each base station can run RouterOS on something like a RouterBoard 333, with a PPPoE access concentrator running on one of the wireless miniPCIs, and a normal AP-client on the other wireless miniPCI, then simply set up the access concentrator to use the AP-Cliented wireless interface as the gateway.
If there are subordinate base stations to this base station, simply add another MiniPCI card and an antenna that can pump data to the next tower.
Connection between towers run on normal routing, which eliminates the problem of PPPoE-Relay. Of course, this means you have to set up an access concentrator on each base station, each one will need a WISP level 4 licence (But many of the RBs come with them anyway).
The possible problem here is that if you don’t have a RADIUS server or a User Manager running somewhere, each base station will need it’s own user database serving the users under it’s coverage. In my case, I’ve got a Radius server elsewhere in the country to serve all my networks.
Now I haven’t actually started building this network I’m describing - Currently I’m using PPTP running on a Linux Fedora box, so I can use IP/Routing everywhere. But it’s not that nice. Lots of linux-client issues there. I’ll start building this network within the next 7 days, if anyone wants a follow-up or wants to know if it works.
But here’s a question to the Advanced users - Is this a good way to do it? What I’m trying to do is to minimize data flying all over the place between the towers. So WDS is a big no-no for me, but Bridging I don’t think will cut it. Or will it? 
Hope that helps, Thinair.
-Krige
a fellow n00b