外文翻译---无线网络在校园中的应用.docx

无线网络在校园中的应用一、无线网络的概述和IEEE802.11协议1无线局域网（Wireless localarea network，WLAN）概述所谓无线网络，是指无需布线即可实现计算机互连的网络。它是无线通信技术与网络技术相结合的产物。从专业角度讲，无线局域网就是通过无线信道来实现网络设备之间的通信，并实现通信的移动化、个性化和宽带化。通俗地讲，无线局域网就是在不采用网线的情况下，提供以太网互联功能。无线网络的适用范围非常广泛。凡是可以通过布线而建立网络的环境和行业，无线网络也同样能够搭建，而通过传统布线无法解决的环境或行业，却正是无线网络大显身手的地方了。千万不要以为无线网络的保密性太差，恰恰相反，其保密性能比双绞线要安全得多。无线网，是采用空气作传输介质，用电磁波作为载体来传输数据，目前无线网联网费用较高，还不太普及。但由于联网方式灵活方便，是一种很有前途的连网方式。2无线局域网的配置方式（1）对等模式。Ad-hoc模式。这种应用包含多个无线终端和一个服务器，均配有无线网卡，但不连接到接入点和有线网络，而是通过无线网卡进行相互通信。它主要用来在没有基础设施的地方快速而轻松地建无线局域网。 （2）基础结构模式。Infrastructure模式。该模式是目前最常见的一种架构，这种架构包含一个接入点和多个无线终端，接入点通过电缆连线与有线网络连接，通过无线电波与无线终端连接，可以实现无线终端之间的通信，以及无线终端与有线网络之间的通信。通过对这种模式进行复制，可以实现多个接入点相互连接的更大的无线网络。3.IEEE802.11协议简介无线局域网技术（包括IEEE802.11、蓝牙技术和HomeRF等）将是新世纪无线通信领域最有发展前景的重大技术之一。以IEEE（电气和电子工程师协会）为代表的多个研究机构针对不同的应用场合，制定了一系列协议标准，推动了无线局域网的实用化。4IEEE802.11系列协议作为全球公认的局域网权威，IEEE 802工作组建立的标准在局域网领域内得到了广泛应用。这些协议包括802.3以太网协议、802.5令牌环协议和802.3z100BASE-T快速以太网协议等。IEEE于1997年发布了无线局域网领域第一个在国际上被认可的协议802.11协议。1999年9月，IEEE提出802.11b协议，用于对802.11协议进行补充，之后又推出了802.11a、802.11g等一系列协议，从而进一步完善了无线局域网规范。IEEE802.11工作组制订的具体协议如下：（1）IEEE802.11a 802.11a采用正交频分（OFDM）技术调制数据，使用5GHz的频带。OFDM技术将无线信道分成以低数据速率并行传输的分频率，然后再将这些频率一起放回接收端，可提供25Mbit/s的无线ATM接口和10Mbit/s的以太网无线帧结构接口，以及TDD/TDMA的空中接口。在很大程度上可提高传输速度，改进信号质量，克服干扰。物理层速率可达54Mbit/s，传输层可达25Mbit/s，能满足室内及室外的应用。 （2）IEEE802.11b 802.11b也被称为Wi-Fi技术，采用补码键控（CCK）调制方式，使用2.4GHz频带，其对无线局域网通信的最大贡献是可以支持两种速率-5.5Mbit/s和11Mbit/s。多速率机制的介质访问控制可确保当工作站之间距离过长或干扰太大、信噪比低于某个门限值时，传输速率能够从11Mbit/s自动降到5.5Mbit/s，或根据直序扩频技术调整到2Mbit/s和1Mbit/s。在不违反FCC规定的前提下，采用跳频技术无法支持更高的速率，因此需要选择DSSS作为该标准的惟一物理层技术。 （3）IEEE802.11g 2001年11月，在802.11 IEEE会议上形成了802.11g标准草案，目的是在2.4GHz频段实现802.11a的速率要求。该标准将于2003年初获得批准。802.11g采用PBCC或CCK/OFDM调制方式，使用2.4GHz频段，对现有的802.11b系统向下兼容。它既能适应传统的802.11b标准(在2.4GHz频率下提供的数据传输率为11Mbit/s)，也符合802.11a标准(在5GHz频率下提供的数据传输率56Mbit/s)，从而解决了对已有的802.11b设备的兼容。用户还可以配置与802.11a、802.11b以及802.11g均相互兼容的多方式无线局域网，有利于促进无线网络市场的发展。5.目前的几种无线网络技术目前，实现无线网络的技术，有蓝牙无线接入技术、家庭网络的HomeRF以及IEEE802.11连接技术。1)蓝牙技术：Bluetooth（蓝牙）是一种短距的无线通讯技术，电子装置彼此可以透过蓝牙而连接起来，传统的电线在这里就毫无用武之地了。透过芯片上的无线接收器，配有蓝牙技术的电子产品能够在十米的距离内彼此相通，传输速度可以达到10M/s。以往红外线接口的传输技术需要电子装置在视线之内的距离，而现在有了蓝牙技术，这样的麻烦也可以免除了。不过Bluetooth产品致命的缺陷是任何蓝牙产品都离不开Bluetooth芯片、Bluetooth模块较难生产，Bluetooth难于全面测试。这三点是蓝牙产品发展的瓶颈。2)HomeRF技术：HomeRF是由HomeRF工作组开发的，是在家庭区域范围内的任何地方，在PC机和用户电子设备之间实现无线数字通信的开放性工业标准。作为无线技术方案，它代替了需要铺设昂贵传输线的有线家庭网络，为网络中的设备，如笔记本电脑和Internet应用提供了漫游功能。HomeRF工作频段是2.4GHz，支持数据和音频。该协议的网络是对等网，也就是说，网上的每一个节点都是西对独立的，不受中央节点的控制。因此，任何一个节点离开网络都不会影响到网络上其他节点的正常工作。它的另外一个特点是低功耗，很适合笔记本电脑。 3)IEEE802.11 技术：IEEE802.11是IEEE最初制定的一个无线局域网标准，主要用于解决办公室局域网和校园网中用户与用户终端的无线接入，业务主要限于数据存取，速率最高只能达到2Mb/s。由于IEEE802.11在速率和传输距离上都不能满足人们的需要，因此，IEEE小组又相继推出了IEEE802.11b和IEEE802.11a两个新标准。三者之间技术上的主要差别在于MAC子层和物理层。此外还出现了最新802.11g。本文主要讨论以802.11b为基础的无线局域网。6无线局域网的优势 无线局域网的网络速度与以太网相当，一个AP最多可支持多达上百个用户的接入，最大传输范围可达到几十公里，具有以下的鲜明特点： 1)具有高移动性；通信范围不受环境条件的限制，拓宽了网络的传输范围。在有线局域中，两个站点的距离在使用铜缆（粗缆）时被限制在 500m，即使采用单模光纤也只能达到3000m，而无线局域网中两个站点间的距离可达到50km。一般来说，如教室、图书馆、会议室等地方一般是不可能布设太多信息点的，但是随着学生中笔记本电脑的普及和现代化教学的普及，上述场所往往在同一时刻有大量的电脑，而目前的有线校园网没有办法使学生们在这些区域上网。采用无线方式，在有限的信息点上连接无线接入器，就可以轻松从一个信息点扩展到成百上千个信息点的应用。 2)抗干扰性强、网络的保密性好；对于有线局域网中的诸多安全问题，在无线局域网中基本上可以避免。 3)建网容易，管理方便；相对于有线网络，无线局域网的组建、配置和维护较为容易，一般计算机工作人员都可以胜任网络的管理工作。AP无线接入点可以使原来的一个信息点同时接入数十乃至数百个用户设备，布线的投资以及维护成本大大降低。 4)安装便捷；一般在网络建设中，施工周期最长、对周边环境影响最大的，就是网络布线施工工程。在施工过程中，往往需要破墙掘地、穿线架管。而WLAN最大的优势就是免去或减少了网络布线的工作量，一般只要安装一个或多个接入点 (Access Point)设备，就可建立覆盖整个建筑或地区的局域网络。在实验室、体育馆、礼堂等地方是不宜布线的，但校园网的用户却有上网的需求，采用无线局域网，可以简化在这些区域网络实施，提供直径近200米的无线网络覆盖，用户可以在无线所覆盖的区域移动应用。5）使用灵活；在有线网络中，网络设备的安放位置受网络信息点位置的限制。而一旦 WLAN建成后，在无线网的信号覆盖区域内，任何一个位置都可以接入网络。提高教学效率。教师和学生上课的时候不必再往返于图书馆、办公室、教室、宿舍，采用无线方案可以使老师和同学们在上述地方随意的检索图书馆的网上资料、服务器的教案、寝室电脑里的作业。同时，为用户对校园网的其他资源的应用提供了更便利的条件，提高了资源的利用率。6）易于扩展；WLAN有多种配置方式，能够根据需要灵活选择。这样， WLAN就能胜任从只有几个用户的小型局域网到上千用户的大型网络，并且能够提供像“漫游 (Roaming) ”等有线网络无法提供的特性。7无线局域网的硬件设备无线网络的硬件设备主要包括4种，即无线网卡、无线AP、无线路由和无线天线等。（1）无线网卡无线网卡的作用类似于以太网中的网卡，作为无线网络的接口，实现与无线网络的连接。无线网卡根据接口类型的不同，主要分为三种类型，即PCMCIA无线网卡、PCI无线网卡和USB无线网卡。PCMCIA无线网卡仅适用于笔记本电脑，支持热插拔，可以非常方便地实现移动式无线接入。PCI接口无线网卡适用于普通的台式计算机使用。其实PCI接口的无线网卡只是在PCI转接卡上插入一块普通的PC卡。USB接口无线网卡适用于笔记本电脑和台式机，支持热插拨。不过，由于USB网卡对笔记本而言是个累赘，因此，USB网卡通常被用于台式机。（2）无线AP无线接入点或称无线AP（Access Point），其作用类似于以太网中的集线器。它是用于无线网络的无线交换机，也是无线网络的核心。无线AP是移动计算机用户进入有线网络的接入点，主要用于宽带家庭、大楼内部以及园区内部，典型距离覆盖几十米至上百米，目前主要技术为802.11系列。大多数无线AP还带有接入点客户端模式（AP client），可以和其它AP进行无线连接，延展网络的覆盖范围。（3）无线网桥安装于室外的无线AP通常称为无线网桥，主要用于实现室外的无线漫游、无线网络的空中接力，或用于搭建点对点、点对多点的无线连接。（4）无线路由器无线路由器事实上就是无线AP与宽带路由器的结合。借助于无线路由器，可实现无线网络中的Internet连接共享，实现ADSL、Cable Modem和小区宽带的无线共享接入。如果不购置无线路由，就必须在无线网络中设置一台代理服务器才可以实现Internet连接共享。（5）无线天线无线天线有许多种类型，常见的有两种，一种是室内天线，一种是室外天线。室内天线主要有两种，即板状定向天线和柱状全向天线。室外天线的类型比较多，常见的也有两种，即锅状定向天线和棒状全向天线。室外天线的优点是传输距离远。比较适合远距离传输。 二、基于IEEE802.11标准校园无线网络的设计方案1无线局域网络优势讨论本设计的优势在于减少了楼层间的无线干扰，把跨楼层的漫游任务交给了放置在相同房间的无线路由器阵列实现，并且利用了交换机这种原始有线设备提高了网络的安全性和稳定性，布线系统则采用了轻型线与以太网供电系统，增加了网络的灵活性，超五类双绞线的使用，为网络的升级和维护简便性低价性提供了有力的保障。无线局域网在单纯实现局域网的目标时不对交换机做过多的要求，交换机仅作为访问互联网络的安全设备，同时将无线路由器作为各个逻辑区域的交换机与无线中继器，利用内多种安全机制的综合使用，可以有效的保障网络的安全要求，设计布线可以大量使用现有布线体系，同时冗余下来的设备可以再为无线局域网所使用，可以以超低的成本对无线局域网络进行扩充，最大限度的保护了已有投资，该设计采用了成熟的主流802.11g54M技术标准，同时不依赖于具体设备的网络设计可以使得无线局域网络迅速升级到802.11n108M技术标准。网络易于扩展，不同于传统设计的是我们将无线路由器作为小型交换机来充分保障网络的安全性。内外网络的划分与电化教学中心网络控制室作为互联网络唯一的接入点，不但有利于接入用户的综合管理，同时对网络可以进行集中管理，根据即时需求平衡网络负载，实现局域网络整体性能上的提升，这套设计使我们利用所掌握的理论全面分析网络需求，并对各种因素进行综合讨论后得出的折衷方案。2无线局域网需求分析本次无线局域网络设计，以教学大楼为设计目标，模拟建设一个采用在先进技术的无线局域网络，以服务于学生机房的教学、科研以及其他应用。要求无线局域网除了安全稳定之外，并且有良好的扩展性和自由的移动性，同时要求方案就提供的产品不仅要经济，而且建成后的网络还具有维护工作少等特点。同时所提供的设计方案具有推广价值，满足信息化校园网络的标准。教学楼用户容量为500人规模，用户的类型包括学生、远程办公的管理员、临时会议办公人员，有稳定组网、远程接入与临时组网的需求,同时无线局域网络需要接入教育网络与Internet网络。3无线网络技术的规划本次无线局域网络设计环境即是教学楼的五、六层，共有十间网络实验室，用户容量为500，同时拥有会议室、主任室、网络控制室、值班室，同时要求公共区域必须有网络的覆盖，基于接入点的带宽共享机制，根据无线网络设计普遍适用于802.11a及802.11g网络的有效经验法则，是为每位用户4Mbps带宽，可以提供与用户在有线局域网上相同的使用感受。适用于802.11b网络的经验法则是为每为用户分配1Mbps带宽，这可以提供类似与宽带ADSL连接类似的用户使用感受(但11b标准已经无法适应校园网络的应用需求，因此本次设计主要采用802.11g标准)。因此如果要实现每间教室50人以上同时上网的性能需求，则要为每间教室分配至少200M的带宽。由于网络实验室用户密度较大、信号传输环境很差，因此设计较小的蜂窝即接入点采用定向天线进行无线覆盖，仅为保证漫游需求在两房间交界处采用全向天线，接入方式上可以选择固定台式计算机接入、远程接入与移动笔记本接入的方式。每台路由器大约能带动的用户上限为32左右，因此，四台无线路由器可以充分满足一间教室50人同时访问网络的需求，将信号功率上适当减小，具体数值需专业设备进行检测，将无线漫游及信号中继的功能完全转移到路由器及交换机上，最大限度的减少网络实验室区域内的无线辐射，同时可实现网络的轻松维护、控制与管理。在网络的灵活性设计上采用了最先进的以太网供电系统，全面的实现了工作区域的按需规划的目标。小带宽的分配可以采用配线板设计，根据需要灵活分配带宽，并冗余设计网络信息插座。根据无线局域网的工作原理，在多个子频道同时工作的情况下，为保证频道之间不相互干扰，要求两个频道的中心频率间隔不能低于25MHz。在一个蜂窝区(Cell)内，直序扩频技术最多可以提供4个不重叠的频道同时工作。因此在配置时将位于每层楼楼道内的5个无线接入逻辑点的工作子频道分别设为：1频道、6频道和11频道。当用户感觉到有线网络性能下降的时候，也可以选择禁用有线连接，通过配置使用无线USB网卡接入同时存在的无线网络。或申请在子层无线路由器的交换机端口内扩展接入无线接入点。无线局域网络使用DMZ主机、透明代理、链路聚合、数据备份、VLAN划分等方式进行网络容错与安全保护。无线局域网络使用扩展星型拓扑结构，支持目前和将来各种无线网络应用与网络扩展。在干扰避免上，选择蓝牙数据包类型改变、频率分离、模式切换等方法解决蓝牙网络与无线局域网信号的冲突与传输频率的干扰。The Application of Wireless Network in The CampusOne, a wireless network and IEEE802.11 protocolsThe 1 Wireless LAN ( Wireless Localarea network, WLAN ) overviewWireless network without wiring, refers to realize computer interconnection network. It is the wireless communication technology and network technology to combine the product. From a professional point of view, the wireless local area network is through the wireless channel to realize the network communication between devices, and the realization of mobile communications, personal and broadband. Popularly speaking, wireless LAN is not in the line of cases, to provide Ethernet interconnection function. The wireless network is a very broad scope of application. All through the wiring and the establishment of the network environment and industry, also can build a wireless network, and through conventional wiring is unable to solve the environment or industry, but it is the wireless network display ones skill to the full place. Do not think that the wireless network security is poor, on the contrary, the security performance is much safer than twisted pair.Wireless network, air is used as the transmission medium, electromagnetic wave as a carrier to transmit data, the wireless networking cost is higher, also not too popular. But because the networking mode is flexible and convenient, is a promising networking system.The 2Wireless LAN configuration(1) reciprocal mode. Ad-hoc mode. This application contains a plurality of wireless terminals and a server, are equipped with wireless network card, but not connected to the access point and the cable network, but through wireless network card to communicate with each other. It is mainly used in the absence of infrastructure in place quickly and easily build wireless local area network.(2) the infrastructure mode. Infrastructure mode. The pattern is currently one of the most common architecture, this architecture includes an access point and a plurality of wireless terminal, access point via the cable connection and a wired network connection, through radio and wireless terminals are connected, can realize the wireless communication between terminals, and the wireless terminal and the cable network communication between. Based on this model to copy, can achieve a number of access points are connected to each other and greater wireless network.Brief introduction of 3.IEEE802.11 protocolThe technology of wireless local area network ( including IEEE802.11, Bluetooth technology and HomeRF ) will be the new century the field of wireless communication the most promising one of the most important technologies. IEEE ( Institute of electrical and Electronics Engineers ) as the representative of a number of research institutes for different applications, developed a series of protocols and standards, promote the wireless LAN utility.4.IEEE802.11 series protocolAs a globally recognized as the local authority, the group of IEEE 802standard in establishment of LAN areas has been widely applied. The protocol includes802.3Ethernet, token ring protocol and802.3z100BASE-T802.5Fast Ethernet protocol. In 1997 IEEE released wireless LAN area the first internationally recognized protocol - Protocol 802.11. In 1999September, IEEE proposed802.11b protocol, used for 802.11protocols are added, and then introduced the802.11a,802.11g and a series of agreements, which further improve the wireless LAN standard. The IEEE802.11working group developed a specific protocol is as follows:(1) IEEE802.11a802.11a orthogonal frequency division modulation ( OFDM ) technology data, use the 5GHz frequency band. OFDM technology wireless channel into the low data rate parallel transmission frequency, then the frequencies together back in the receiving end, can provide 25Mbit / s wireless ATM interface and 10Mbit / s Ethernet wireless frame structure interface, and TDD / TDMA air interface. To a large extent can improve the transmission speed, improved the signal quality, overcome the interference. Physical layer speed can reach 54Mbit / s, transport layer up to 25Mbit / s, can satisfy the indoor and outdoor applications.(2) IEEE802.11b802.11b is also called Wi-Fi technology, using the complementary code keying ( CCK ) modulation, using 2.4GHz band, the wireless local area network communication is the greatest contribution can support two rate -5.5Mbit / s and 11Mbit / s. Multi rate mechanism of medium access control ensures that when the distance between workstations too long or interference is too large, the SNR is below a certain threshold, the transmission rate from11Mbit / s to 5.5Mbit / s automatically, or based on direct sequence spread spectrum technology to adjust to 2Mbit / s and 1Mbit / s. Not in violation of the provisions of FCC premise, use of frequency hopping technology cannot support higher rate, therefore need to choose DSSS as the standard only physical layer technology.(3) IEEE802.11gIn2001November,802.11 IEEE conference formed the draft 802.11g standard, aimed at2.4GHz band802.11a rate requirement. The standard will be at the beginning of 2003approved. 802.11g using PBCC or CCK / OFDM modulation mode, using the2.4GHz band, the existing802.11b system compatible. It can adapt to the traditional 802.11b ( in2.4GHz frequency provides data transmission rate of11Mbit / s ), also in line with the 802.11a standard ( in5GHz frequency provides data transmission rate 56Mbit / s ), so as to solve the existing802.11b equipment compatibility. The user can also be configured with 802.11a,802.11b and 802.11g are mutually compatible multi mode wireless local area network, promote the development of the market of wireless network.The 5and present several wireless network technologyAt present, the realization of wireless network technology, Bluetooth wireless access technology, the HomeRF and IEEE802.11 connected to the home network technology.1) the Bluetooth Technology: Bluetooth ( Bluetooth ) is a short distance of wireless communication technology, electronic devices can connect through Bluetooth instead of conventional wires, here is no play. Through the wireless chip receiver with Bluetooth technology, electronic products can be ten meters away from each other within the same, the speed can reach10M / s. Previous infrared interface transmission technology needs of electronic device in sight distance, and now with Bluetooth technology, this also can avoid the trouble. But Bluetooth products fatal defect is any Bluetooth products are inseparable from the Bluetooth chip, Bluetooth module is difficult to produce, Bluetooth is difficult to be fully tested. The three point is the bottleneck of the development of Bluetooth products.2) HomeRF: HomeRF by HomeRF working group development, is in a home area anywhere within the range, in the PC machine and the user electronic device between wireless digital communications open industry standard. As wireless technology scheme, it replaced the need to build costly transmission line wired home networking, network equipment, such as a notebook computer and Internet application provides roaming function. HomeRF working frequency is 2.4GHz, support for data and audio. The network protocol is a peer to peer network, that is to say, each node of the Internet is west of independence, from the central node control. Therefore, any one node to leave the network will not affect the normal work of other nodes on the network. Its another characteristic is low power consumption, it is suitable for notebook computer.3)IEEE802.11: IEEE802.11IEEE technology originally developed by a wireless LAN standard, mainly used to solve the office LAN and campus network user and the wireless user terminal access, business is mainly limited to data access, only the highest rate reached 2Mb / s.As a result of IEEE802.11rate and transmission distance are unable to meet the needs of the people, therefore, IEEE group also launched IEEE802.11b and IEEE802.11a two new standards. Between the three technology is the main difference between MAC layer and physical layer. In addition also appeared in the latest 802.11g. This paper mainly discusses the802.11b based wireless lan.The 6wireless LANThe wireless LAN network speed and Ethernet, a AP can support up to hundreds of user access, the maximum transmission range can reach several tens of kilometers, has the following characteristics:1) having high mobility;Communication range is not affected by environmental conditions, to broaden the range of transmission network. In the wired local area, two stations distance in the use of copper ( thick ) was restricted to the 500m, even with a single-mode optical fiber can reach 3000m, and wireless local area network in two inter-site distance can reach50km. In general, such as classrooms, library, conference rooms and other places it is generally not possible laid too much information at the point, but as a student of notebook computer popularization and modernization of teaching, the places are often at the same time a large number of computer, and the current wired campus network had no way to keep the students in these areas to get on the internet. Using wireless means, in the limited information point is connected with wireless access device, can easily from an information point spread to hundreds of thousands of information at the point of application.2) strong anti-interference, network security;For wired LAN many security problems in WLAN, basically avoided.3) easy network-construction, convenient management;Relative to the cable network, wireless LAN, configuration and maintenance easier, general computer staff can do network management. AP wireless access points can be made the original a point of information at the same time access to dozens and even hundreds of user equipment, wiring investment and maintenance cost is greatly reduced.4) easy to install;Generally in network construction, the construction cycle is the longest, the largest impact on the surrounding environment, is the construction of network cabling project. During the construction process, often need to break through the wall to dig, threading tube frame. WLAN is the biggest advantage of replacing or reducing network wiring work, usually as long as the installation of one or more access points ( Access Point ) equipment, can be established to cover the entire building or area local area network. In the lab, gymnasium, auditorium and other places is not the wiring, but the campus network users have access needs, using the wireless local area network, can be simplified in these area network implementation, with a diameter of nearly 200 meters of the wireless network coverage, the user can be in the wireless area covered mobile application.5) the use of flexible;In a wired network, network equipment placement by network information point position limit. Once the WLAN is completed, in the wireless network signal coverage area, any one location can access network. To improve the teaching efficiency. The teacher and the students when the class does not need to return to office, library, classrooms, dormitories, using wireless solution can make teachers and students in the local random search library online information, the server plans, dormitory computer operating. At the same time, fo