ng each signal for T/3 seconds in every T-second interval. The output of this switch, again shown in Figure 2.9, consists of one piece of Carl's speech, then one piece of Gretchen's speech, then one piece of Monica's speech in every T-second interval. In this way, a part of everybody's speech sample gets onto one wire. These speech samples are now sharing time on the wire, and hence the name time-division multiplexing. Telecommunication Networks Carl's digital speech Carl, Gretchen & Monica's Gretchen's digital speech digital speech "The Big Switch" Gretchen Monica Monica's digital speech Figure 2.9 How three signals share one wire in TDM 2.2 POTS: Plain Old Telephone System Enough of the basics. Let me now introduce you to a telecommunication network currently in use. In fact, it's the most frequently used telecommunication network in the world. It's called POTS, short for Plain Old Telephone System. We'll be consider- ing the phone connections exclusively in Canada and the United States, but keep in mind that similar systems exist worldwide. 2.2.1 Local Calls Let's say Gretchen, at home in Fort Collins, decides to call Carl, who is hard at work at CSU writing this book. Here's how the call gets from Gretchen to Carl. First, Gretchen's phone turns her sounds into an analog electrical signal, as explained in Section 1.2.2. This analog electrical signal is sent along a Class 5 Switching Center copper wire (called a twisted-pair cable) to the Gretchen at home (end office) switching center called the Class 5 switching center, or end office (Figure 2.10). Carl at the office Figure 2.10 Connecting a local call: the local loop Chapter Two The switching center knows Gretchen's call is a local call to Carl's office (based on a 7-digit number she initially dialed), and it sends Gretchen's speech signal down the copper wire that connects to Carl's office. This signal then enters Carl's phone, which turns the electrical signal back into Gretchen's speech sounds, and there we have it. This part of the phone system is called 