the local loop. There are about 20,000 end offices in Canada and the United States. 2.2.2 Long Distance Calls Connecting the Call Let's say that Carl's mom, Mona, who lives in the cold of Montreal, Canada, wants to call Carl in Colorado and see if he's eating well (yes, Mom, I'm eating well). How would the telephone system connect this call? We'll use Figure 2.11 as our guide. First, Mona's Are you eating well? sounds are turned into an analog electrical signal by her telephone. This electrical signal is sent along copper wire to the end office (or class 5 switching center). The end office, realizing that this isn't a local call, does two things: (1) it mixes Mona's signal with other people's voice signals (that are also not local calls), using multiplexing; then, (2) it takes the mix of Mona's speech signal and the other people's speech signals and sends this mix to a bigger switching center, called a Class 4 switching center, or toll center, shown in Figure 2.11. The toll center is con- nected to three sets of things. Multiplexing of Mona's speech and other speech Class 5 Class 4 Class 3 Toll Center Primary Class 2 Class End Office Center Class 5 End Office Class 1 Class 5 End Office Class 2 Class 2 Class 4 Class 5 Toll Center Class 2 End Office Class 4 Toll Center Class 5 Class 4 End Office Toll Center Class 3 Primary Class 4 Center Figure 2.11 Toll Center Class 3 Primary Class 4 A long-distance call in Class 5 Center Toll Center End Office Class 3 Primary Center Telecommunication Networks First, it's connected to a number of end offices (A, B, C, and D in Figure 2.11), each end office like the one that Mona's call came from. If Mona's call was intended for a phone that was connected to end office C for example, then Mona's speech would be sent to end office C and from there to the intended phone. The second thing the Class 4 switching center (toll center) is connected to is other Class 4 switching centers (BB in Figure 2.11). If the call is intended for a phone connected to end office E, then it would likely 