Hi Gonzo,
Every modern airliner has a Mode S transponder nowadays. It's been a while since I studied the thing, but essentially it's a Mode C with datalink capability. It can send data like aircraft type and can communicate with other Mode S transponders in case of TCAS conflicts (ie. one plane will make the decision to advise a descent while the other will then advise a climb so that the pilots don't do the same thing and still collide).
Still the 0-7 digits though.