So just to stir the pot, perhaps all is not what it may be claimed regarding 9/11 perhaps 8-|

THe Control tower does not send any iplight plans to the plane  If the pilots of the plane want to change a flight plan they do include that change in voice message to the "tower" but the flight plan is entered by hand or from the company servers and then checked by both the ppilot flying and by te pilot monitoring.

Yes planes do have a transponder.  It does send information about its current position to anyone recieving.  This data can be read freely on sites such as flightaware.com/  THe information transmitted by planes consist of its position and altitude.  The information of planes within a close radius of the plane is read by onboard equipment which calculates the track of the surrounding planes, compares them with the location and track of the aircraft and calculates if a collission possible,.  If there is a warning is shown on the flight display.  More advanced units also compute the best way to avoid the collission and give it in verbal announcement form.  Such as TCAS pull up pull up turn right. or TCAS descend descend turn left. TCAS has nothing to do with flight plans.  
Security workers ina ll countries have decided that there is little risk to the tracking information being desplayed because someone only has to look out the window of their house to find if a plane is around.  The TCAS does not fly the plane but gives avoidance warnings...just like a passenger shouting out when they see a car approaching on the wrong side of the road.