tieth century and are generally owned Electronics Engineers (IEEE), with which we shall be and operated by the descendants of long-distance greatly concerned in this chapter, generally divides the telephone providers or their competitors, where data link layer into an upper logical link control layer and present. Almost all long-distance telecommunication a lower medium access control (MAC) layer. In this today is carried over fiber-optic cables, commonly at chapter, we are almost exclusively interested in what OC-12 (622Mbps), OC-48 (2.5 Gbps), or goes on in the link layer and the physical (PHY) layer. OC-192 (10 Gbps) rates; OC-768 (40 Gbps) is in The logical link control layer is focused mainly on the the early stages of deployment in 2004. WAN requirements of the higher layers, and in practice the connections cross the oceans and continents and carry logical link control layer used for many WLAN technol- the voice and data commerce of the world. The In- logies is the same as the one used for wired technologies. ternet initially evolved separately from WANs, but its We do not discuss it further. However, the MAC layer is explosive growth in the late 1990s resulted in intimately involved in the efficient use of the wireless a complex comingling of the traditional WAN and medium, and it is of great interest for any user of LAN communities, businesses, and standards bodies. a wireless link. Wireless local area networks CHAPTER 5.1 client computer client computer Application Layer Application Layer Presentation Layer Presentation Layer Session Layer Session Layer Transport Layer network router Transport Layer Network Layer access point Network Layer Network Layer Data Link Layer Data Link Layer Data Link Layer Data Link Layer Data Link Layer Physical Layer Physical Layer Physical Layer Physical Layer Physical Layer wireless link wired link Figure 5.1-1 The OSI protocol stack; the configuration shown is typical of an 802.11 infrastructure network. 5.1.2 WLANs from LANs Wireless stations are mobile, and access to th