ations where proximity to a reader is assured, such as in a warehouse or transportation-based supply chain environment. Active devices are commonly seen in such applications as automated toll- taking systems on major freeways. The EZ-Pass system deployed in the northeastern United States is a good example. 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. Access Technologies Access Technologies Figure 6-13 An array of RFID tags Readers are nothing more than RF emitters connected to back-end software analysis systems. The reader's role is to emit an RF signal that activates the tag or tags within its operational area, to transmit a series of simple commands to the tags, and to collect the data returned as a result of the transmitted commands. Because multiple tags may be acti- vated simultaneously-as in a supply chain environment-most readers have anticollision capability that allows them to control a multitag envi- ronment, forcing each tag to take a turn so that simultaneously activated devices can successfully transmit their information without interference from adjacent tags. Tags come in a variety of forms, as shown in Figure 6-13. The wedge- shaped device (1) is designed to be attached in a number of ways to track- able items. The wedge shape makes it possible to guarantee the orientation of the tag in environments where signal strength is weak or impeded by metal in the transmission area. The paper tag (2) is often seen in bookstores, inserted between the pages of books as a theft deter- rent. The tag contains a single bit of information. If the bit is set by the "deactivator" at checkout, the book was properly purchased. If not, it sets off the alarm upon exiting the store. The card (3) is a typical contactless smart card, used in corporate access control environments, hotel room keyless entries, etc. The large disk (4) can be attached 