Cable TV is currently a high capacity analog signal. Since each station needs a frequency of 6M, 50 cable channels are multiplexed into a signal of 300M hertz. Cable systems frequently use optical fiber to junction boxes and coaxial cables into residences. These coaxial cables have a much higher capacity than the twisted copper wire phone system. With the exception of a few experimental interactive systems, cable systems are one-way communication systems. The amount of interaction in the experimental interactive systems is limited. Because cable systems broadcast a fixed set channels into each home, they have no need for exchanges found in telephone systems. Also, because each household has a fixed number of channels the cable companies do not need an elaborate billing system as that for long distance telephone traffic.