 to use one wire to connect calls be- tween the two towns. This is shown in Figure 2.6. Carl's speech, turned into a current on a wire, contains the frequencies 100 to 4000 Hz. His speech is left as is. Gretchen's speech, turned into an electrical signal on a wire, also contains the frequencies 100 to 4000 Hz. A simple device called a mixer (operating at 8000 Hz) is applied to her speech signal. This device moves the frequency content of her speech signal, and the frequen- cies found at 100 Hz to 4,000 Hz are moved to the frequencies 8,100 Hz to 12,000 Hz, as shown in Figure 2.7. Switching Switching Center Carl, Gretchen Center and Monica's voices Fort Collins all on one wire Boulder Figure 2.6 People's speech on one wire Telecommunication Networks Because Carl and Gretchen's calls are now made up of different frequency com- ponents, they can be sent on a single wire without interfering with one another. This too is shown in Figure 2.7. We now want to add Monica's speech signal, since she too is making a call from Fort Collins to Boulder. A mixer, this one operating at 16,000 Hz, is applied to her speech signal. This moves the frequency content of Monica's speech to 16,100 Hz to 20,000 Hz, as shown in Figure 2.7. Because Monica's speech signal is now at different frequencies than Carl and Gretchen's speech signals, her signal can be added onto the same wire as Carl and Gretchen's, without interfering. (Again, take a peek at Figure 2.7.) Over in Boulder, we've got a wire with Carl, Gretchen, and Monica's speech on it, and we need to separate this into three signals. First, to get Carl's signal, we use a device called a low-pass filter (LPF). The filter we use only allows the frequencies in 0 to 4000 Hz to pass through; all other frequency components are removed. So, in our example, this filter passes Carl's speech, but stops Gretchen's and Monica's speech cold. This is shown in Figure 2.8. Carl's speech signal 100-4000 Hz one wire Gretchen's speech signal Mixer 100-4000 Hz 8000 Hz Mixer 16,000 Hz Monica's speech s