On behalf of Bart
RAN is like a door, so you can ACCESS the network.
RAT is how you are doing it.
If on the other side of the ACCESS door would be a nigh club and you would come dressed not according to the dress code(your technology would be different than on the network) bouncer wouldn't let you in.
RAN is a type of a road, where RAT limits what types of vehicles can drive on it, on which side they should drive, etc. e.g. bikes or horse wagons are not allowed on motorways, where trucks are limited on local roads.