无线局域网基本原理

How many times have you needed network or Internet access at home and wished you could work in a different room, or even outside, without having to run a long Ethernet cable? How many times have you been in a public spot, such as an airport or hotel, and realized you needed to send a quick e-mail? How many hours have you wasted sitting in conference rooms between meetings while your e-mails pile up?
If you are like thousands of other corporate network users, telecommuters, business travelers, and home users, the answer is more than once. Network users take heed: 802.11-based WLANs hold the answer for you. 802.11-based networks provide the much sought-after mobility and bandwidth that network users have been asking for.
WLANs are not a new concept. They have been around for decades. The 802.11 standard was ratified in 1997, so why is it that WLANs are really starting to take off now? The answers are bandwidth and cost. Early wireless networks, such as Aloha, ARDIS, and Ricochet, offered data rates of less than 1 Mbps. The 802.11 standard offers vendor interoperable rates as high as 2 Mbps. The ratification of 802.11b in 1999 raised the bar to 11 Mbps, competing against
wired 10 Mbps Ethernet rates. The 802.11a and 802.11g standards offer data rates as high as 54 Mbps, giving wired Fast Ethernet a run for its money.
As early implementers of WLANs, vertical industries such as retail stores, healthcare providers, and manufacturers see the value that WLANs and wireless applications bring.
Many of these industries rely on WLANs as core components of their business. As a result,vendors are rising to meet the challenge of delivering cost-effective WLAN solutions for these customers. Vendors can increase their manufacturing volumes and lower their costs and as a result their prices, making WLAN hardware available to consumer and enterprise customers at reasonable prices.
Although 802.11 networks are a LAN topology, they present new challenges to network administrators who are accustomed to a world of wires and wired-based network technologies such as 802.3 Ethernet networks. Issues like site surveys, security, QoS, and network-device mobility require attention that a network administrator might not have the experience to give.
The premise to this book is to discuss the aspects of 802.11 in terms that IT and network engineers can relate to. This book is a reference guide to operating and troubleshooting 802.11 networks and also serves as the first stepping stone to bridge the gap between wired and wireless networks.