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毕业外文翻译无线局域网技术最近几年，无线局域网开始在市场中独霸一方。越来越多的机构发现无线局域网是传统有线局域网不可缺少的好帮手，它可以满足人们对移动、布局变动和自组网络的需求，并能覆盖难以铺设有线网络的地域。无线局域网是利用无线传输媒体的局域网。就在前几年，人们还很少使用无线局域网。原因包括成本高、数据率低、职业安全方面的顾虑以及需要许可证。随着这些问题的逐步解决，无线局域网很快就开始流行起来了。无线局域网的应用局域网的扩展在20世纪80年代后期出现的无线局域网早期产品都是作为传统有线局域网替代品而问世的。无线局域网可以节省局域网缆线的安装费用，简化重新布局和其他对网络结构改动的任务。但是，无线局域网的这个动机被以下一系列的事件打消。首先，随着人们越来越清楚地认识到局域网的重要性，建筑师在设计新建筑时就包括了大量用于数据应用的预先埋设好的线路。其次，随着数据传输技术的发展，人们越来越依赖于双绞线连接的局域网。特别是3类和5类非屏蔽双绞线。大多数老建筑中已经铺设了足够的3类电缆，而许多新建筑里则预埋了5类电缆。因此，用无线局域网取代有线局域网的事情从来没有发生过。但是，在有些环境中无线局域网确实起着有线局域网替代品的作用。例如，象生产车间、股票交易所的交易大厅以及仓库这样有大型开阔场地的建筑；没有足够双绞线对，但又禁止打洞铺设新线路的有历史价值的建筑；从经济角度考虑，安装和维护有线局域网划不来的小型办公室。在以上这些情况下，无线局域网向人们提供了一个有效且更具吸引力的选择。其中大多数情况下，拥有无线局域网的机构同时也拥有支持服务器和某些固定工作站的有线局域网。因此，无线局域网通常会链接到同样建筑群内的有线局域网上。所以我们将此类应用领域成为局域网的扩展。建筑物的互连无线局域网技术的另一种用途是邻楼局域网之间的连接，这些局域网可以是无线的也可以是有线的。在这种情况下，两个楼之间采用点对点的无线链接。被链接的设备通常是网桥或路由器。这种点对点的单链路从本质上看不是局域网，但通常我们也把这种应用算作无线局域网。漫游接入漫游接入提供局域网和带有天线的移动数据终端之间的无线链接，如膝上型电脑和笔记本电脑。这种应用的一个例子是从外地出差回来的职员将数据从个人移动电脑传送到办公室的服务器上。漫游接入在某种延伸的环境下也是十分有用的，如在建筑群之外操作的一台电脑或一次商务行为。在以上两种情况下，用户会带着自己的电脑随意走动，并希望可以从不同的位置访问有线局域网上的服务器。自组网络自组网络（ad hoc network）是为了满足某些即时需求而临时而建立的一种对等网络（没有中央服务器）例如，有一群职员，每人带着一台膝上电脑或掌上电脑，会聚在商务会议室或课堂上。这些职员会将他们的电脑链接起来，形成一个临时性的、仅仅在会议期间存在的网络。无线局域网的要求无线局域网必须满足所有局域网的典型要求，包括大容量、近距离的覆盖能力、相连站点间的完全连接性以及广播能力。另外，无线局域网环境还有一些特殊的要求。以下是一些无线局域网最终要的要求：吞吐量：媒体接入控制协议应当尽可能地有效利用无线媒体以达到最大的容量。节点数量：无线局域网可能需要支持分布在多个蜂窝中的上百个节点。连接到主干局域网：在大多数情况下，要求能够与主干有线局域网的站点相互连接。对于有基础设施的无线局域网，很容易通过利用控制模块完成这个任务，控制模块本身就连接着这两种类型的局域网。对于移动用户和自组无线网络来说，可能需要满足这个要求。电池能量消耗：移动工作人员用的是由电池供电的工作站，它需要在使用无线适配器的情况下，电池供电时间足够长。这就是说，要求移动节点不停地监视接入点或者经常要与基站握手的MAC协议是不适用的。通常，无线局域网的实现都具有在不使用网络时减少能量消耗的特殊性能，如睡眠模式。传输健壮性和安全性：除非涉及合理，无线局域网很容易受到干扰并且容易被窃听。无线局域网的设计必须做到即使在噪音较大的环境中也能可靠传输，并且为应用提供某种程度的安全性，以防窃听。并列的网络操作：随着无线局域网变得越来越流行，很可能有两个或者更多无线局域网同时存在于一个区域内，或在局域网之间可能存在干扰的某些区域内运行。这种干扰可能会阻碍MAC算法的正常运行，还可能造成对特定局域网的非法接入。不需要许可证的操作：用户希望购买和运行的是这样的无线局域网产品，它们不需要专门为局域网所使用的频带而申请许可证。切换和漫游：无线局域网中使用的MAC协议应当让移动站点能够从一个蜂窝移动到另一个蜂窝。动态配置：局域网在MAC地址机制和网络管理方面应当允许端系统能够动态且自动地增加、删除和移动位置，并且不打扰到其他用户。无线局域网技术无线局域网通常根据它所采用的传输技术进行分类。目前所有无线局域网产品都可归为以下三个大类之一：红外线（IR）局域网：红外线局域网的一个蜂窝只能限制在一个房间里，因为红外线无法穿过不透明的墙。扩频局域网：这种类型的局域网利用了扩频传输技术。在大多数情况下，这些局域网运行在ISM(个人、科学和医学)波段内，因此，在美国使用这些局域网不需要联邦通信委员会（FCC）发放的许可证。窄带微波：这些局域网运行在微波频率是，但没有使用扩频技术。其中有些产品运行的频率需要FCC的许可证，而其他一些产品则使用了不需要许可的波段。 无线局域网有一个特性是人们乐意接受的，虽然不是必要的，那就是不需要通过麻烦的授权过程就能使用。每个国家的许可证发放制度都不一样，这就使事情变得更加复杂。在美国，FCC在ISM波段内特许了两个不需要许可证的应用：最大功率为1瓦的扩频系统合最大运行功率为0.5瓦的低功率系统。自从FCC开放了这个波段以来，在扩频无线局域网中的应用就越来越普遍。1990年IEEE802.11工作组成立，它的宪章就是要为无线局域网开发MAC协议以及物理媒体规约。无线局域网中最小的模块是基本服务集（Basic Service Set, BSS）,它由一些执行相同MAC协议并争用同一共享媒体完成接入的站点组成。基本服务集可以是孤立的，也可以通过接入点（Access Point, AP）连到主干分发系统（Distribution System, DS）上。接入点的功能相当于网桥。MAC协议可以是完全分布式的，也可以由位于接入点的中央协调功能控制。BBS通常与文献中的蜂窝相对应，而DS则有可能是交换机或有线网络，也可以是无线网络。MAC层的主要任务是在MAC实体之间传送MSDU，这个任务是由分发服务实现的。分发服务的正常运行需要该ESS内所有站点的信息，而这个信息是由与关联（association）相关的服务提供的。在分发服务向站点交付数据或者接收来自站点的数据之前，该站点必须要建立关联。标准基于移动性定义了三种转移类型：无转移：这种类型的站点或者是固定的，或者只在一个BSS的直接通信范围内移动。BSS转移：这种类型的站点移动是在同一ESS内从一个BSS移动到另一个BSS。在这种情况下，该站点的数据交付需要寻址功能，能识别出该站点的新位置。ESS转移：它的定义是指站点从一个ESS的BSS到另一个ESS的BSS移动。只有从某种意义上看该站点是能够移动的，才能支持这种类型的转移。802.11工作组考虑了两类MAC算法建议：分布式接入协议和集中式接入协议。分布式接入协议类似于以太网，采用载波监听机制把传输的决定权分布到所有节点。集中式接入协议由一个集中的决策模块来控制发送。分布式接入协议对于对等工作站形式的自组网络是有意义的，同时也可能对主要是突发性通信量的其他一些无线局域网颇具吸引力。如果一个局域网的配置是由许多互连的无线站点和以某种形式连接到主干有线局域网的基站组成，则采用集中式接入控制是自然而然的事情。当某些数据是时间敏感的或者是高优先级的时，这种方法特别有用。IEEE802.11的最终结果是一个称为分布式基础无线MAC（Distributed Foundation Wireless MAC,DFWMAC）的算法，它提供了一个分布式接入控制机制，并在顶端具有可选的集中式控制。MAC层的低端子层是分布式协调功能（Distributed Coordination Function , DCF）.DCF采用争用算法向所有通信量提供接入。正常的异步通信量直接使用DCF。点协调功能（Point Coordination Function, PCF）是一个集中式MAC算法，用于提供无争用服务。分布式协调功能DCF子层使用一种简单的CSMA（载波监听多点接入）算法。如果站点有一个MAC帧要发送，则先监听媒体。如果媒体空闲，站点可以发送。否则，该站点必须等待直到当前的发送结束。DCF不包括冲突检测功能（CSMA/CD），因为在无线网络中进行冲突检测是不实际的。媒体上信号变动范围很大，所以如果正在传输的站点接收到微弱信号，它无法区分这是噪声还是因为自己的传输而带来的影响。为了保证算法的平稳和公平运行，DCF包含了一组等价于优先级策略的时延。我们首先考虑一个称为帧间间隔（InterFrame Space,IFS）时延。采用IFS后CSMA的接入规则如下：1。有帧要传输的站点先监听媒体。如果媒体是空闲的，等待IFS长的一段时间，再看媒体是否空闲，如果是空闲，立即发送。2。如果媒体是忙的（或是一开始就发现忙，或是在IFS空闲时间内发现媒体忙），则推迟传输，并继续监听媒体直到当前的传输结束。3。一旦当前的传输结束，站点再延迟IFS一段时间。如果媒体在这段时间内都是空闲的，则站点采用二进制指数退避策略等待一段时间后再监听媒体，如果媒体依然是空闲的，则可以传输。在退避期间，如果媒体又变忙了，退避定时器暂停，并在媒体变空闲后恢复计时。点协调功能PCF是在DCF之上实现的另一种接入方式。其操作由中央轮询主控器（点协调器）的轮询构成。点协调在发布轮询时采用PIFS。因为PIFS比DIFS小，所以点协调器在发布轮询和接收响应时能获取媒体并封锁所有的异步通信量。点协调器不断地发布轮询，并永远封锁所有异步通信量。为了避免这种情况，定义了一个称为超帧（superframe）的时间间隔。在超帧时间的开始部分，点协调器以循环方式向所有配置成轮询的站点发布轮询。然后，在余下的超帧时间里，点协调器空闲，允许异步通信量有一段争用接入的时间。在超帧开始时，点协调器可以在给定时间内获得控制权和发布轮询，这由选项决定。由于响应站点发出的帧的长度是变化的，所以这个时间间隔也是变化的。超帧剩余的时间用于基于争用的接入。在超帧末尾，点协调器泳PIFS时间争用媒体接入权。如果媒体是空闲的，点协调器可以立刻接入，然后又是一个全超帧期。不过，媒体在超帧末尾有可能是忙的。在这种情况下，点协调器必须等待直到媒体空闲并获得接入。其结果是下一个循环中相应缩短的超帧期。WIRELESS LANIn 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 ApplicationsThere 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 ExtensionEarly 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 InterconnectAnother 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 AccessNomadic 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 NetworkingAn 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 RequirementsA 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 SpecificationThree 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.Direct-sequence spread spectrum operating in the 2.4-GHz ISM band. Up to 7 channels, each with a data rate of 1 Mbps or 2 Mbps, can be used.Frequency-hopping spread spectrum operating in the 2.4-GHz ISM band. The details of this option are for further study.Wireless LAN TechnologyWireless LANs are generally categorized according to the transmission techniquethat is used. All current wireless LAN products fall into one of the following categories:Infrared (IR) LANs. An individual cell of an IR LAN is limited to a single room, as infrared light does not penetrate opaque walls.Spread Spectrum LANs. This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate in the ISM (Industrial, Scientific, and Medical) bands, so that no FCC licensing is required for their use in the U.S.Narrowband Microwave. These LANs operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands.A set of wireless LAN standards has been developed by the IEEE 802.11 committee. The terminology and some of the specific features of 802.11 are unique to this standard and are not reflected in all commercial products. However, it is useful to be familiar with the standard as its features are representative of required wireless LAN capabilities.The smallest building block of a wireless LAN is a basic service set (BSS), which consists of some number of stations executing the same MAC protocol and competing for access to the same shared medium. A basic service set may be isolated, or it may connect to a backbone distribution system through an access point. The access point functions as a bridge. The MAC protocol may be fully distributed or controlled by a central coordination function housed in the access point. The basic service set generally corresponds to what is referred to as a cell in the literature. An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. Typically, the distribution system is a wired backbone LAN. The extended service set appears as a single logical LAN to the logical link control (LLC) level. The standard defines three types of stations, based on mobility:No-transition. A station of this type is either stationary or moves only within the direct communication range of the communicating stations of a single BSS.BSS-transition. This is defined as a station movement from one BSS to another BSS within the same ESS. In this case, delivery of data to the station requires that the addressing capability be able to recognize the new location of the station.ESS-transition. This is defined as a station movement from a BSS in one ESS to a BSS within another ESS. This case is supported only in the sense that the station can move. Maintenance of upper-layer connections supported by 802.11 cannot be guaranteed. In fact, disruption of service is likely to occur. details of this option are for further study.The 802.11 working group considered two types of proposals for a MAC algorithm: distributed-access protocols which, like CSMAICD, distributed the decision to transmit over all the nodes using a carrier-sense mechanism; and centralized access protocols, which involve regulation of transmission by a centralized decision maker. A distributed access protocol makes sense of an ad hoc network of peer workstations and may also be attractive in other wireless LAN configurations that consist primarily of bursty traffic. A centralized access protocol is natural for configurations in which a number of wireless stations are interconnected with each other and with some sort of base station that attaches to a backbone wired LAN; it is especially useful if some of the data is time-sensitive or high priority.The end result of the 802.11 is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed access-control mechanism with an optional centralized control built on top of that. Figure 13.20 illustrates the architecture. The lower sublayer of the MAC layer is the distributed coordination function (DCF). DCF uses a contention algorithm to provide access to all traffic. Ordinary asynchronous traffic directly uses DCF. The point coordination function (PCF) is a centralized MAC algorithm used to provide contention-free service. PCF is built on top of DCF and exploits features of DCF to assure access for its users. Let us consider these two sublayers in turn.Distributed Coordination FunctionThe DCF sublayer makes use of a simple CSMA algorithm. If a station has a MAC frame to transmit, it listens to the medium. If the medium is idle, the station may transmit; otherwise, the station must wait until the current transmission is complete before transmitting. The DCF does not include a collision-detection function (i.e., CSMAICD) because collision detection is not practical on a wireless network. The dynamic range of the signals on the medium is very large, so that a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission. To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme. Let us start by considering a singledelay known as an interframe space (IFS). In fact, there are three different IFS values, but the algorithm is best explained by initially ignoring this detail. Using an IFS, the rules for CSMA access are as follows:I. A station with a frame to transmit senses the medium. If the medium is idle, the station waits to see if the medium remains idle for a time equal to IFS, and, if this is so, the station may immediately transmit.2. If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over.3. Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off using a binary exponential backoff scheme and again senses the medium. If the medium is still idle, the station may transmit.Point Coordination FunctionPCF is an alternative access method implemented on top of the DCF. The operation consists of polling with the centralized polling master (point coordinator). The point coordinator makes use of PIFS when issuing polls. Because PIFS is smaller than DIFS, the point coordinator can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses.As an extreme, consider the following possible scenario. A wireless network is configured so that a number of stations with time-sensitive traffic are controlled by the point coordinator while remaining traffic, using CSMA, contends for access.The point coordinator could issue polls in a round-robin fashion to all stations configured for polling. When a poll is issued, the polled station may respond using SIFS. If the point coordinator receives a response, it issues another poll using PIFS. If no response is received during the expected turnaround time, the coordinator issues a poll. If the discipline of the preceding paragraph were implemented, the point coordinator would lock out all asynchronous traffic by repeatedly issuing polls. To prevent this situation, an interval known as the superframe is defined. During the first part of this interval, the point coordinator issues polls in a round-robin fashion to all stations configured for polling. The point coordinator then idles for the remainder of the superframe, allowing a contention period for asynchronous access.At the beginning of a superframe, the point coordinator may optionally seize control and issue polls fora give period of time. This interval varies because of the variable frame size issued by responding stations. The remainder of the superframe is available for contention-based access. At the end of the superframe interval, the point coordinator contends for access to the medium using PIFS. If the medium is idle, the point coordinator gains immediate access, and a full superframe period follows. However, the medium may be busy at the end of a superframe. In this case, the point coordinator must wait until the me
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