rms of Use as given at the website. Access Technologies Chapter 6 Figure 6-16 Central switch "authority" commands end points to "hop" as instructed them to their targets. To that end, a pair of industrious entrepreneurs filed patent number 2,292,387 titled, "Secret Communications System." The inventors were orchestra composer George Antheil and electrical engineer Hedy K. Markey, better known as early film star Hedy Lamarr. The technique described in their patent application is remark- ably simple: A central "authority" (the base station, shown schemati- cally in Figure 6-16) communicates with the two communicating endpoint devices, instructing them to "hop" randomly from frequency to frequency at randomly selected times and for random intervals, typically on the order of 5 ms. Or less. Only the base station (think of an orchestra conductor) and the two devices know when and where to jump-and for how long. To an outsider looking in-that is, any device wishing to eaves- drop on the conversation-the hopping process appears completely ran- dom. It isn't, however: The base station knows precisely what it is doing and when it is doing it, so the hopping behavior is actually pseudoran- dom, and the technique is often referred to as a "pseudorandom hopping code." This technique is most commonly used in CDMA cellular systems, and is (as you might expect) used extensively in secure military com- munications systems. Direct Sequence Spread Spectrum (DSSS) DSSS, sometimes called "noise-modulated spread spectrum," is a very different technique than its FHSS cousin. In frequency hopping, a 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 "conversation" jumps from frequency to frequency on a seemingly ran- dom basis. In noise modulation, the actual signal is combined with a carefully crafted "noise" signal that disgu