Mesh Channel Selection

There's been a lot of discussion about how to select wireless channels in JBad. As anyone reading along will notice, it's getting really crowded up on the water tower (nine downlinks) to the point where it's important to worry about airtime on the radios:

A standard wireless router has 11 available wireless channels. Of these 11, only three are completely non-overlapping (ie: can talk at the same time without interfering with each other).

In our implementation, no two nodes can communicate unless they're on the same channel. In short, this is because if they could, eventually everything would end up on the same channel, which we don't want. In terms of OLSR this is important, because if two nodes are on separate overlapping channels, they will interfere on each other's transmissions if they try to talk at the same time, but will ignore each other's RTS CTS messages, so they won't ever coordinate. As a result, you get crappy throughput if you have two busy nodes on, channels 5 and 6 right next to each other.

In our implementation most of the downlinks on the water tower were channel 6. We then deployed a pair on channel 5, and despite good signal were seeing an ETX of 4.5 instead of the < 1.1 that we would expect for a link of this quality. Switching the channel to 6 brought the ETX down to 1.05.

Take Home:

Wireless channels must be either the same or non-overalpping for any nodes in your network that can hear each other.