Wireless+Networking+in+the+Developing+World_Part4 - Chapter...

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Ethernet cable, a wireless link, or any other kind of physical network. The cloud symbol is commonly used to stand in for “The Internet”, and represents any number of intervening IP networks. Neither Alice nor Bob need to be concerned with how those networks operate, as long as the routers forward IP traf c towards the ultimate destination. If it weren ± t for Internet protocols and the cooperation of everyone on the net, this kind of communication would be impossible. Designing the physical network It may seem odd to talk about the “physical” network when building wireless networks. After all, where is the physical part of the network? In wireless networks, the physical medium we use for communication is obviously elec- tromagnetic energy. But in the context of this chapter, the physical network refers to the mundane topic of where to put things. How do you arrange the equipment so that you can reach your wireless clients? Whether they ll an of ce building or stretch across many miles, wireless networks are naturally arranged in these three logical con gurations: point-to-point links , point- to-multipoint links , and multipoint-to-multipoint clouds . While different parts of your network can take advantage of all three of these con gurations, any individual link will fall into one of these topologies. Point-to-point Point-to-point links typically provide an Internet connection where such ac- cess isn ± t otherwise available. One side of a point-to-point link will have an Internet connection, while the other uses the link to reach the Internet. For example, a university may have a fast frame relay or VSAT connection in the middle of campus, but cannot afford such a connection for an important building just off campus. If the main building has an unobstructed view of the remote site, a point-to-point connection can be used to link the two together. This can augment or even replace existing dial-up links. With proper anten- nas and clear line of sight, reliable point-to-point links in excess of thirty kilo- meters are possible. Point to point link VSAT Figure 3.14: A point-to-point link allows a remote site to share a central Internet connection. Chapter 3: Network Design 51
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Of course, once a single point-to-point connection has been made, more can be used to extend the network even further. If the remote building in our ex- ample is at the top of a tall hill, it may be able to see other important locations that can ± t be seen directly from the central campus. By installing another point-to-point link at the remote site, another node can join the network and make use of the central Internet connection. Point-to-point links don ± t necessarily have to involve Internet access. Suppose you have to physically drive to a remote weather monitoring station, high in the hills, in order to collect the data which it records over time. You could connect the site with a point-to-point link, allowing data collection and monitoring to happen in realtime, without the need to actually travel to the site. Wireless
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Wireless+Networking+in+the+Developing+World_Part4 - Chapter...

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