August 06, 2003
Cellular vs. Meshed
I was working a couple of weeks ago with Pebble linux, and the possibility of setting it up as turn-key node.
I got to the stage where you can turn on a PC running this distro, and it will automatically mesh with neighbouring nodes, sharing route information, setting up a wireless mesh with no user intervention!
However, reading this got me thinking if meshing is the right way to go.....
Basically the above article outlines some of the problems SOWN have found in running a mesh. Throughput is crap because of interference between neighbouring nodes, and the limitations inherent in WiFi - it's half-duplex (can't talk and listen at the same time) - so bandwidth is halved over each hop. If your net connection is 6 hops away, this means you're only gonna get 5Mb/2x2x2x2x2 = 19Kbyte/sec ie. not much at all.
The alternative to meshing, is of course cellular networks. It's no coincidence that this is how the mobile networks are designed, the difficulties involved with WiFi networking arguably make cellular networks even more appropriate.
Under a cellular network we'd divide the city up into cells, and look into getting an access point in each one of these cells.
The picture above (taken from SOWN's topology document) shows how the cells are set up to minimise interference (neigbouring access points use different, non-overlapping frequencies). In some ways this could make things easier, we can use 'dumb APs' more easily, but to have a backbone, we need at least 2 radios at each site. In a cellular network it's very important that the access point has good line-of-sight to the entire cell, so there less of a focus on setting up a node at home.
If we're going to follow this route, we need to settle on how we're going to build each access point, and get talking with people who literally have friends in high places.