ter all, those billion (or so) Internet users must be reasonably happy with the technology. Let's now examine the Network Layer. Please note that we are now entering the realm of the chained layers, which you will recall are used by all devices in the path-end-used devices as well as the switches or routers themselves. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Protocols The Network Layer The Network Layer, which is the uppermost of the three chained layers, has two key responsibilities: routing and congestion control. We will also briefly discuss switching at this layer, even though many books consider it to be a layer two process. So for the purists in the audience, please bear with me-there's a method to my madness. When the telephone network first started its remarkable growth path at the sunrise of the twentieth century, there was no concept of switch- ing. If a customer wanted to be able to speak with another customer, he or she had to have a phone in his or her house with a dedicated path to that person's home. Another person required another phone and phone line-and you quickly begin to see where this is leading. Figure 2-28 illustrates the problem: The telephone network's success would bring on the next ice age, blocking the sun with all the aerial wire the telephone network would be required to deploy. Consider this simple mathematical model: In order to fully interconnect, or mesh, five customers as shown in Figure 2-29 SO that any one of them can call any other, the telephone company would have to install ten circuits, according to the equation n(n-1)/2, where n is the number of devices that wish to communicate. Extrapolate that out now to the population of even a small city-say, 2,000 people. That boils down to 3,997,999 circuits that would have to be Figure 2-28 Aerial telephone wire (Courtesy Lucent Technologies