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nity to use the shared medium; and all have some kind of software called a network operating system (NOS) that controls the environment. All LANs have the same basic components as shown in Figure 5-14. A collection of devices such as PCs, servers, and printers serve as the interface between the human user and the shared medium. Each of these machines hosts a device called a network interface card (NIC), which provides the physical connectivity between the user device and the shared medium. The NIC is either installed inside the system or, less commonly, as an external device. In laptop machines, the NIC is either a PC card that plugs into a slot in the machine, shown in Figure 5-15, or, more commonly today, built into the system. In many machines today Wi-Fi is also built in; it s basically a wireless version of Ethernet.
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Figure 5-14 Typical LAN components
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Figure 5-15 A PC card network interface card (NIC)
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The NIC device implements the access protocol that devices wishing to access the shared medium use on their particular LAN. These access schemes will be discussed shortly. The NIC also provides the connectivity required to attach a user device to the shared network. Topologically, LANs differ greatly. The earliest LANs used a bus architecture shown in Figure 5-16 so-called because they were literally a long run of twisted-pair wire or coaxial cable to which stations were periodically attached. Attachment was easy; in fact, early coax systems relied on a device called a vampire tap, which poked a hole in the insulation surrounding the center conductor in order to suck the digital blood from the shared medium. Later designs, such as IBM s Token Ring, used a contiguous ring architecture like that shown in Figure 5-17. Both have their advantages and will be discussed later in this chapter. Later designs combined the best of both topologies to create star-wired LANs (Figure 5-18), also discussed later.
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Figure 5-16 A bus-based LAN
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Figure 5-17 A ring-based LAN
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Figure 5-18 A star-wired LAN
LAN Access Schemes
LANs have traditionally fallen into two primary categories characterized by the manner in which they access the shared transmission medium (shared among all the devices on the LAN). The first, and most common, is called contention, and the second group is called distributed polling. I tend to refer to contention-based LANs as the Berkeley Method, whereas I view distributed-polling LANs as users of the Harvard Method. I ll explain in a moment.
Contention-Based LANs
Dear Mr. Metcalf: We re not sure how to break this to you, but we have discovered that your claim of a patent for the invention of Ethernet must be denied after the fact due to its existence prior to the date of your claim of invention. There is a small freshwater fish, Gymnarchus Niloticus, that uses an interesting technique for locating mates, food, and simply communicating with peers. The fish s body is polarized, with a cathode on its head and an anode on its tail. Using special electric cells in its body similar to those employed by the electric eel or the California electric ray, Gymnarchus
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emits nominal 300 Hz, 10 volt pulses, which reflect back and inform the fish about its immediate environment. In the event that two Gymnarchus are in the same area, their emissions interfere with one another (intersymbol interference ), rendering their detection mechanisms ineffective. But, being the clever creatures that they are, Gymnarchus has designed a technique to deal with this problem. When the two fish hear each other s transmissions, they both immediately stop pulsing. Each fish waits a measurably random period of time while continuing to swim, after which they begin to transmit again, but this time at slightly different frequencies to avoid interference. We hope that you understand that under the circumstances we cannot in good conscience grant this patent. Sincerely yours, U.S. Patent Office
I don t think a fish can hold a patent, but if it could, Niloticus would hold the patent for a widely used technique called Carrier Sense, Multiple Access with Collision Detection. Please read on. Perhaps the best-known, contention-based medium access scheme is Ethernet, a product developed by 3Com founder and Xerox PARC veteran Bob Metcalfe. In contention-based LANs, devices attached to the network vie for access using the technological equivalent of gladiatorial combat. If it feels good, do it is a good way to describe the manner in which they share access (hence the Berkeley Method). If a station wants to transmit, it simply does so, knowing that the possibility exists that the transmitted signal may collide with the signal generated by another station that transmits at the same time. Even though the transmissions are electrical and are occurring on a LAN, there is still some delay between the time that both stations transmit and the time that they both realize that someone else has transmitted. This realization is called a collision, and it results in the destruction of both transmitted messages as shown in Figure 5-19. In the event that a collision occurs as the result of simultaneous transmission, both stations back off by immediately stopping their transmissions, wait a random amount of time, and try again. This technique has the wonderful name of truncated binary exponential backoff. It s one of those phrases you just have to commit to memory because it sounds so good when you casually let it roll off the tongue in conversation. Ultimately, each station will get a turn to transmit, although how long they may have to wait is based on how busy the LAN is. Contentionbased systems are characterized by what is known as unbounded delay,
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