The telephone that Alexander Graham Bell invented in 1876 did not have a touchtone keypad. Nor did it have a rotary dial. In fact, for decades after the invention of the telephone, many callers could not directly dial the person they wanted to speak with. They first had to speak with an operator and ask the operator to connect them with the desired party.
You have probably seen this scene played out on the Andy Griffith Show. Andy picks up the receiver on his phone in the sheriff's office and asks Sara to connect him with Floyd's Barber Shop. Andy didn't have the option of dialing Floyd directly.
This was standard operation for the phone system until a Kansas City undertaker became fed up. Back in 1889, an undertaker named Almond Brown Strowger was losing business because the local Bell Telephone operator had a brother-in-law who was a competing undertaker. So, when someone called the operator wanting to speak with an undertaker, guess who they were connected with? That's right, the Bell Telephone operator's brother-in-law.
Mr. Strowger thought that callers should be able to call him directly, without a meddling operator. This frustration led to his invention of the first telephone switch. This switch, through a series of mechanical relays, could interpret a caller's dialed digits and form a pathway between the calling party's phone and the called party's phone. The Strowger Switch, also known as the Step-by-Step Switch, was adopted by AT&T in 1924 and was in widespread use in the United States into the 1980s.
Today's CO telephone switches are digital, as opposed to the mechanical Strowger Switch, and these switches are connected in a hierarchical manner. Your home phone probably connects to a Class 5 CO (that is, your local CO), which then connects up to a Class 4 CO, and so on. This hierarchical approach of interconnecting telephone offices minimizes the number of interoffice trunks required to connect all of the nation's telephones together into what is called the PSTN.
The structure of the PSTN is composed of three different types of networks: