Boulder at the same time. Switching Center Switching Center Figure 2.3 Connections between Fort Collins, CO switching centers Boulder, CO Telecommunication Networks Consider now a number of other nearby cities, say Longmont and Denver. The folks in these towns want to talk to their friends in Boulder and Fort Collins. We could connect all the switching centers together, as shown in Figure 2.4. Alternatively, we could have a "super" switching center, which would be a switching center for the switching centers, as shown in Figure 2.5. Boulder Fort Collins Switching Switching Center Center Switching Switching Center Center Longmont Denver Figure 2.4 Connecting all the switching centers together Boulder Fort Collins Switching Switching Center Center "Super" Switching Center Switching Switching Center Center Longmont Denver Figure 2.5 Connecting switching centers using a "super" switching center Chapter Two 2.1.3 Multiplexing-An Alternative to a Lot of Wire Let's go back to the Fort Collins and Boulder people in Figure 2.3. Let's say we've connected their switching centers SO they can talk to each other. As more people in Fort Collins want to talk to more people in Boulder, more and more wires need to be added between their switching centers. It could get to the point where there are far too many wires running between the switching centers. The skies around Fort Collins could grow dark under the cover of all these wires. This probably wouldn't be true of a smaller town like Fort Collins, but it was true of big cities like New York. Finally, someone said, "Something must be done!" and multiplexing was invented. Multiplexing refers to any scheme that allows many people's calls to share a single wire. (We'll talk more about this in Chapter 11, but a brief introduction now is useful.) First There Was FDM FDM, short for frequency division multiplexing, was the first scheme created to allow people's calls to share a wire. Let's say Carl, Gretchen, and Monica all want to make a call from Fort Collins to Boulder. We only want