【正文】 畢業(yè)設(shè)計 (論文 )外文資料翻譯 學(xué)生姓名 學(xué) 號 系 部 管理系 專 業(yè) 外文出處 指導(dǎo)教師 WIRELESS LAN In just the past few years, wireless LANs have e to occupy a significant niche in the local area work market. Increasingly, organizations are finding that wireless LANs are an indispensable adjunct to traditional wired LANs, as they satisfy requirements for mobility, relocation, ad hoc working, and coverage of locationsdifficult to wire. As the name suggests, a wireless LAN is one that makes use of a wireless transmission medium. Until relatively recently, wireless LANs were little used。 the reasons for this included high prices, low data rates, occupational safety concerns, and licensing requirements. As these problems have been addressed, the popularity of wireless LANs has grown rapidly. In this section, we first look at the requirements for and advantages of wireless LANs, and then preview the key approaches to wireless LAN implementation. Wireless LANs Applications There are four application areas for wireless LANs: LAN extension, crossbuilding interconnect, nomadic access, and ad hoc works. Let us consider each of these in turn. LAN Extension Early wireless LAN products, introduced in the late 1980s, were marketed as substitutes for traditional wired LANs. A wireless LAN saves the cost of the installation of LAN cabling and eases the task of relocation and other modifications to work structure. However, this motivation for wireless LANs was overtaken by events. First, as awareness of the need for LAN became greater, architects designed new buildings to include extensive prewiring for data applications. Second, with advances in data transmission technology, there has been an increasing reliance on twisted pair cabling for LANs and, in particular, Category 3 unshielded twisted pair. Most older building are already wired with an abundance of Category 3 cable. Thus, the use of a wireless LAN to replace wired LANs has not happened to any great extent. However, in a number of environments, there is a role for the wireless LAN as an alternative to a wired LAN. Examples include buildings with large open areas, such as manufacturing plants, stock exchange trading floors, and warehouses。 historical buildings with insufficient twisted pair and in which drilling holes for new wiring is prohibited。 and small offices where installation and maintenance of wired LANs is not economical. In all of these cases, a wireless LAN provides an effective and more attractive alternative. In most of these cases, an organization will also have a wired LAN to support servers and some stationary workstations. For example, a manufacturing facility typically has an office area that is separate from the factory floor but which must be linked to it for working purposes. Therefore, typically, a wireless LAN will be linked into a wired LAN on the same premises. Thus, this application area is referred to as LAN extension. CrossBuilding Interconnect Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a pointtopoint wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single pointtopoint link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN. Nomadic Access Nomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop puter or notepad puter. One example of the utility of such a connection is to enable an employee returning from a trip to transfer data from a personal portable puter to a server in the office. Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may move around with their portable puters and may wish access to the servers on a wired LAN from various locations. Ad Hoc Networking An ad hoc work is a peertopeer work (no centralized serv