edium. Later designs, such as IBM's Token Ring, used a con- tiguous 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. Figure 5-16 A bus-based Figure 5-17 A ring-based 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. Premises Technologies Premises Technologies Figure 5-18 A star-wired 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 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. Premises Technologies Chapter 5 emits nominal 300 Hz, 10 volt pulses, which reflect back