Full Mode S requires a Mode S transponder in the a/c and Mode S sensors and automation on the ground. The Mode S sennsor on the ground has a dozon or so transmission types to work with ATCRBS transponders, mode S transponders and military stuff. The thing that is special about Mode S is that interogations can be made selectively (where the "S" come from) to individual aircraft so you don't have to spend your time interrogating and getting rid of replies from all the airplanes you don't want to see to get at the one you want.
The data link part is kind of true. there are specifications for a reply that has more than the Mode C data that can be used as a data link, but isn't right now in any consistent fashion. One of the things this data link could be used for is for Automatic dependent Surveillance where the aircraft knows where it is and tells everyone else where it is. this is NOT TCAS, and in fact is independent of TCAS and doesn't interoperate with TCAS.
Anyone for dueling standards?