Sec 16.7 - 534 The Silicon Web: Physics for the Internet...

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534 The Silicon Web: Physics for the Internet Age of data to pass through a very large number of nodes without experiencing degradation. Only having this capability allows the use of high levels of network redundancy, and the high reliability that comes with high redundancy. Even in the absence of catastrophic failures of the network, it is vital to have this type of robustness. Hardware can malfunction; human error can wreak havoc (think of Homer Simpson), etc. The packet-switching scheme allows us to build a reliable sys- tem with unreliable components. As Baran said in 2001, “You can get any reliability you want—far, far greater than the reliability of the components.” In his early design, he decided “to have a half-second delay maximum from one point to another. Then at every one of these nodes we would keep a carbon copy of the message until we were sure it got through intact to the next node. So the carbon copy with error detection allowed repeated transmission of any message block, now called a packet.” Baran still marvels at the advantages of this system that he uncovered using his computer simula- tions, “Packet switching had all these wonderful properties that weren’t invented—they were discovered.” 16.7 WIRELESS MOBILE CELL PHONE NETWORKS One of the biggest impacts of digital communications on people’s lives was the recent rapid rise of cell phone use. Cell, or cellular, phone systems are an example of wireless networks, and are especially interesting in that they are also mobile—you can carry the phone with you. The main physics idea behind cellular phone systems is the fact that the strength of radio waves falls off with increasing distance. Although this may be a disadvantage if you want to communicate directly by radio over a long distance, it has a great beneF t: separate stations broadcasting radio waves will not interfere with each other if they are far enough apart. This is illustrated in Figure 16.21 . The broadcasting station labeled S1 sends out radio waves that are easily picked up by people at locations labeled A and B. These waves decrease in strength with increasing distance, so that people at C, D, or E cannot detect them. Likewise, the station S2 sends out waves that can be detected at D and E, but not at A, B, or C. The advantage of this situation is that each broadcasting station can use the same set of frequencies to broadcast to a local group of listeners, without interfering with other groups of listeners. In this simple example, which avoids interference between stations S1 and S2, a person at C is not able to communicate. This problem can be remedied by using a slightly more involved strategy, described next.
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Sec 16.7 - 534 The Silicon Web: Physics for the Internet...

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