In addition to all of this voice communication, the car is also transmitting telemetry data back to the team and to CART (for example, to supply data to the telemetry board).
Each tire on every car also has its own small, 0.25-watt radio to transmit pressure data to the car's onboard data logging system. The tires and the cars transmit in the 900-MHz band, with the car using spread spectrum techniques to improve reliability and reduce problems with interference.
Also, some cars now have an in-car television camera (sometimes more than one) that transmits real-time images back to the television network for broadcast to viewers so they can see the driver's perspective. With all of these radios transmitting, the big problem is finding a set of clear frequencies that keep everyone separated and are also free from outside interference.
This problem gets even worse when the race course is near a city, where the frequencies are already crowded with city services like police, fire and sanitation, as well as industrial radio users.