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The Illustrated Network- P14:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 3 Network Link Technologies 99 Wireless Laptop Wireless Laptop Station Station Wireless Laptop Wireless Laptop Station Station Wireless Laptop Internet Wireless Laptop Station Station BSS with AP BSS without AP ad hoc network Wireless Laptop Wireless Laptop Wireless Laptop Wireless Laptop Station Station Station Station Wireless Laptop Station Internet ESS Wireless Laptop Station FIGURE 3.11 Wireless LAN architectures. Most home networks are built around an access point built into a DSL router gateway. more APs. While a wireless station can belong to more than one BSS through its AP at the same time this is not helpful when the APs are offering different network addresses and perhaps prices for attachment . This collection of Wi-Fi networks is sometimes called the Wi-Fi jungle and will only become worse as wireless services turn up more and more often in parks apartment buildings offices and so on. How do APs and wireless stations sort themselves out in the Wi-Fi jungle If there are APs present each wireless station in IEEE 802.11 needs to associate with an AP before it can send or receive frames. For Internet access the 802.11 frames contain IP packets of course. The network administrator for every AP assigns a Service Set Identifier SSID to the AP as well as the channels frequency ranges that are associated with the AP The AP has a MAC layer address as well often called the BSSID. The AP is required to periodically send out beacon frames each including the AP s SSID and MAC layer address BSSID on its wireless channels. These channels are scanned by the wireless station. Some channels might overlap between multiple APs because the jungle has no central control but hopefully there are other channels that do not. In practice interference between overlapping APs is not a huge problem 100 PART I Networking Basics in the absence of a high volume of traffic. When you view available networks in Windows XP the display is a list of the SSIDs of all APs in range. To
