nique so that they can be distinguished from one another by the receiver, even when there is substantial noise and multipath interference. As a result, CCK-based systems can achieve sub- 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 stantially higher data rates by eliminating the impact of such effects that would otherwise limit its ability to achieve them. It should also be noted that CCK only works with DSSS systems spec- ified in the original 802.11 standard. It does not work with FHSS. 802.11 Physical Layer All 802 standards address themselves to both the physical (PHY) and media access control (MAC) layers. At the PHY layer, IEEE 802.11 iden- tifies three options for wireless LANs: diffused infrared, DSSS, and FHSS. While the infrared PHY operates at a baseband level, the other two operate at 2.4 GHz, part of the ISM band. It can be used for operating wireless LAN devices and does not require an end-user license. All three PHYs specify support for the 1 Mbps and 2 Mbps data rate. 802.11 Media Access Control Layer The 802.11 MAC layer, like CSMA/CD and token passing, presents the rules used to access the wireless medium. The primary services provided by the MAC layer are as follows: Data transfer: Based on a Carrier Sense, Multiple Access with Collision Avoidance (CSMA/CA) algorithm as the media access scheme Association: The establishment of wireless links between wireless clients and access points (APs) Authentication: The process of conclusively verifying a client's identity prior to a wireless client associating with an AP-802.11 devices operate under an Open System in which any wireless client can associate with any AP without verifying credentials. True authentication is possible with the use of the Wired Equivalent Privacy Protocol or WEP, which uses a shared key validation protocol simila