无线网络 - 外文翻译 - 外文文献 - 英文文献 - 无线局域网技术

技术传输距离短和信号穿透能力差的特点同样也是不可克服的。

2.2 Wi-Fi技术的优势

近几年，WAP 的数量呈迅猛增长，无线网络的方便与高效使其得到迅速的普及。无论是无线城市的建设，还是企事业单位局域网的开通，还是手机的 Wi-Fi 功能，都与Wi-Fi 技术自身的优势是分不开的： 1）较广的局域网覆盖范围

基于蓝牙技术的电波覆盖范围非常小，半径大约只有 15m，而 Wi-Fi 的半径则可达100 m，可以覆盖整栋办公大楼； 2）传输速度快

Wi-Fi 技术传输速度非常快，可以达到 11Mbps（802.11b）或者 54Mbps （802.11a），适合高速数据传输的业务； 3）无需布线

Wi-Fi 最主要的优势在于不需要布线，可以不受布线条件的限制，因此非常适合移动办公用户的需要。在机场、车站、咖啡店、图书馆等人员较密集地方设置“热点” ，并通过高速线路将因特网接入上述场所。用户只要将支持无线 LAN的笔记本电脑或PDA拿到该区域内，即可高速接入因特网； 4）健康安全

IEEE802.11规定的发射功率不可超过 100 毫瓦，实际发射功率约 60～70毫瓦，而手机的发射功率约 200毫瓦～1瓦间，手持式对讲机高达5瓦。与后者相比，Wi-Fi 产品的辐射更小。

3 结论

每一种无线通信方式都有其独特之处，超宽带技术传输速率高，Wi-Fi 技术覆盖范围广。随着无线通信技术的发展，人们对高速短距离无线通信的要求越来越高，超宽带技术、Wi-Fi 技术都有着广泛的发展前景。

WIRELESS LAN

In just the past few years, wireless LANs have come to occupy a significant niche in

the local area network 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 networking, 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 networks. 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 network 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 networking 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. Cross-Building Interconnect

Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a point-to-point wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single point-to-point 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 computer or notepad computer. 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 computer 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 computers and may wish access to the servers on a wired LAN from various locations. Ad Hoc Networking

An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting. Wireless LAN Requirements

A wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs:

Throughput. The medium access control protocol should make as efficient use as

possible of the wireless medium to maximize capacity.

Number of nodes. Wireless LANs may need to support hundreds of nodes across multiple cells.

Connection to backbone LAN. In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wireless LANs, this is easily accomplished through the use of control modules that connect to both types of LANs. There may also need to be accommodation for mobile users and ad hoc wireless networks.

Service area. A typical coverage area for a wireless LAN may be up to a 300 to 1000 foot diameter.

Battery power consumption. Mobile workers use battery-powered workstations that need to have a long battery life when used with wireless adapters. This suggests that a MAC protocol that requires mobile nodes to constantlymonitor access points or to engage in frequent handshakes with a base stationis inappropriate.

Transmission robustness and security. Unless properly designed, a wireless LAN may be interference-prone and easily eavesdropped upon. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.

Collocated network operation. As wireless LANs become more popular, it is quite likely for two of them to operate in the same area or in some area where interference between the LANs is possible. Such interference may thwart the normal operation of a MAC algorithm and may allow unauthorized access to a particular LAN.

License-free operation. Users would prefer to buy and operate wireless LAN products without having to secure a license for the frequency band used by the LAN.

HandoWroaming. The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another.

Dynamic configuration. The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users. Physical Medium Specification

Three physical media are defined in the current 802.11 standard:

Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm.