Telephone System

• The power supply , processor , softwate and interface cards are normally referred to as the KSU , or Key Service Unit . 
• The telephones and telephone lines ( numbers from the phone company ) connect to the KSU either by plugging directly into the KSU , or more commonly by connecting to a System Interface , called a 66 block . 

• Show the entire telephone network , including a home connection , cell phone towers , long distance exchanges and transcontinental connections .