only as the devices re- quire them. Thus, more devices can be served on the same line. These stat muxes also have buffers SO that, when statis- tics are wrong and more devices are transmitting than statistically anticipated, the buffer will hold the information until the circuit is free to transmit it. CSU/DSU Hardware Lines for digital transmission require hardware that is some- what like a modem called a DSU (Data Service Unit) or CSU (Channel Service Unit). Often the DSU and CSU are built into a single unit. The CSU/DSU maintains a steady current on the line to control the quality of the signal being sent. More Transmission Hardware Other types of hardware come into play for transmitting tele- communications signals. For devices that are nearby, the signal may travel over the medium without any help. For relatively longer distances the sig- nal begins to attenuate (diminish) and must be boosted or amplified by what is called a repeater. All communications circuits also have devices called trans- formers, as do other electrical circuits. The transformer provides a variety of functions, including stepping up and stepping down voltage, managing of line impedance and electrical isolation via magnetic coupling, an important safety item. Other types of hardware that send communications signals through the air are the microwave dish and satellite with its tran- sponders. These satellites are in geo-stationary orbits appearing to hover at a fixed point 22,000 miles over the Equator. In reality they are orbiting at a speed in excess of 8,000 mph in sync with the Earth's rotation. The Telecom Handbook * 243 TRANSMISSION A microwave dish sends a signal to another dish or bounces the signal off a satellite. The signal is then received by another dish. A microwave dish may be several feet in diameter or the size of a three-story building. (Do not stand in front of, or even nearby, a dish if it is transmitting. It may reorganize some of your cells. The receiving dishes are harmless.) Now that we have the transmission medium