Why are the channels (78) in the channel subsets (13) using 6MHz spacing?

Question: Why are the channels (78) in the channel subsets (13) using 6MHz spacing?

 

Answer:

For the channel hopping algorithm to work, it is necesaary to have a prime number of channels. Because the bridge doesn't just go to the next channel in the subset, it skips 2-5 channels and by using a prime number of channels we guarantee that the entire channel subset will be covered no matter how many channels are skipped. It was found that 13 is a nice prime number of channels per subset, as the algorithm worked well with this number.