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局域网技术毕业设计外文文献翻译院 系:计算机与信息工程学院年级专业:姓 名:学 号:附 件:Local Area Network(LAN)Technologies.pdf指导老师评语: 指导教师签名:年 月 日 国际技术创新和探索工程杂志(IJITEE) 国际标准连续出版物编号: 2278-3075, 卷-1, 章节-6, 2012.11局域网技术Velaga Pavani, Immadisetty L V Chandrika, A.Rama Krishna 通常来说,小型网络被称为LAN(局域网)。局域网是一种允许方便的访问计算机或外围设备的网络。网络意味着多台计算机互联。这些互联的计算机可以有不同的目的以及使用不同的线缆类型。这会带来时间损耗的减少并且因此增加了生产效率。局域网主要依赖于诸如拓扑,传输介质控制(MAC)和传输介质等特性和因素。这提供了保障。局域网有以太网。局域网主要工作在载波侦听多路访问/冲突检测(CSMA / CD)。以太网规范执行与OSI物理层和数据链路层的数据通信相同的功能。有线局域网传输的数据是通过中继器和网桥来进行物理连接的,而无线局域网发送和接收数据是在空气中进行的,没有使用任何的线缆。无线局域网在物理上没有连接。关键词:LAN(局域网),MAC(介质访问控制),CSMA/CD(碰撞检测多路访问冲突检测),FHSS(跳频扩频技术),DSSS(直接序列扩频技术),AP(接入点),NOS(网络操作系统)。I.介绍 网络意味着互联的计算机。这些电脑可以因为不同的目的而相互连接。我们需要使用网络的基本原因是为了资源共享,共享应用软件,提高生产率。小的网络通常被称为局域网(LAN)。局域网是一个允许容易访问其他电脑的网络。局域网主要取决于五个特点。他们的物理距离被限制在2KM以内,带宽要大于1Mbps,廉价的有线介质(同轴电缆或双绞线),数据或硬件可以在用户间共享,为用户所有。更重要的是,由局域网有三个因素决定了它的性质。他们分别是1、拓扑2、传输介质3、介质访问控制技术(MAC)。局域网有以太网和快速以太网。主要工作在载波侦听多路访问/冲突检测(CSMA / CD)。有线局域网传输的数据是通过中继器和网桥来进行物理连接的,而无线局域网发送和接收数据是在空气中进行的,没有使用任何的线缆。无线局域网在物理上没有连接。II.局域网的因素A.局域网拓扑局域网常见的拓扑结构是总线,树形,环形和星形。总线拓扑:在总线上的所有站都通过称为TAP的相应的硬件接口直接接入线性传输介质或总线,全双工操作(在站点和接口之间发送和接收数据允许数据在总线上同时收发),在总线的两端有个终结器,用于避免反射。树形拓扑结构:树的规划开始在一个称为头节点的地方。每个节点都可能有分支。但是存在两个问题,从任一个站点传送的数据可以在所有的站点都收到,其次是需要有一种机制来调节传输。为了解决这些问题,站点之间传输数据被封装在小的块里面被称为帧。每个帧由数据和帧头组成,包含了信息和目的地址。环形拓扑:网络是由一组中继器加入到点到点链路组成一个封闭的循环。中继器是一个简单的在一条链路上以bit方式收发数据到另一条链路的设备。这些链路是单向的。星形拓扑:在星形拓扑结构中,每个站点直接连接到一个共同的中心站点。一般来说,有2种可选的中心站点操作方式。第一种,中心节点以广播的方式运行,在这种情况,虽然在物理上是这个节点还是中心,但是在逻辑上是总线;任何站点发送的数据都可以被其他站点收到,而且在任一时间同时只能一个站点成功发送数据。第二种,中央节点作为一个帧交换装置。入站帧在中心站点缓存,并被发送到目的站点。B.介质访问控制技术(MAC) 为了有序和有效的传输数据而使用一些控制接入传输介质的方法是有必要的。这就是MAC协议的功能,MAC主要取决于两个关键参数。它们就是在哪传及如何传。在哪传涉及到控制方式是集中式还是分布式的。在一个集中式的方案中,一个控制器被设计用于授权认证接入网络。一个站点如果想传输数据则必须得到控制器的允许。如何传是由拓扑和计算因素之间的权衡来决定的,比如花费,性能以及复杂度。MAC控制目的MAC地址源MAC地址链路逻辑控制协议传输单元循环冗余校验 MAC控制:这个字段包含了MAC协议功能所需要的全部信息。 目的MAC地址:这个帧要去往的局域网中的目的物理接入点。 源MAC地址:这个帧来自局域网中的哪个物理接入点。 逻辑链路控制:LLC关心的是在2个站点之间传输的链路层协议数据单元,没有关心中间节点的必要性。我们有两个主要的特性不与其他协议共享。它们是:它必须支持多路访问以及它需要处理MAC层接入链路的一些细节。 循环冗余校验(CRC):循环冗余校验字段(也被称为帧校验序列FCS字段)。这是一种错误检测码。C.传输介质 我们拥有各种类型的组件用于传输数据。不仅是为了传输也有为了扩大网络。有一些组件有如下讨论: 网卡:插入工作站的网络适配器,提供到网络的连接。 布线电缆:用于连接电脑和网络组件。今天主要使用3种电缆类型。它们是双绞线,同轴电缆,光纤。 中继器:中继器是用来延长网络段。它们放大从一个段接收到的输入信号。中继数量是有限制的。中继器也可以用来隔离一个段中发生的错误。一个中继器工作在物理层,简单的重复把数据从一个段传输到另一个段。 网桥:网桥是互联以太网的桥梁。工作在OSI中的数据链路层。OSI的所有高层信息对于网桥来说是不可用的。因此它们不能区分协议之间的不同。它们只是简单的通过所有的网络协议。它们工作在MAC子层有时也被称为媒介访问控制层的桥梁。 路由器:复杂的网络需要一种设备,这种设备不仅知道每个段的地址,也能确定发送数据和过滤广播流量的最佳路径。像这种设备就被称为路由器。一个路由器使用一个表来为一个入站包确定目标地址。路由器的工作是要知道所有网络的地址,连接到其他网络的可能路径以及经过这些路径所需要的开销。 集线器:集线器提供一些逻辑上的端口,使用单一面板组合这些端口,经常用高于这些端口的速率运行。当端口繁忙时可以使用缓存来允许包暂存在端口中。我们有两种不同类型的端口。它们是被动集线器,只是简单的把工作站分配或组合成一段。另一种活跃的集线器包括中继器的功能,从而能够支持更多的连接。III.以太网和快速以太网(CSMA/CD)A.CSMA/CD的描述 总线/树以及星形拓扑最常用的介质访问控制技术是带冲突检测的载波监听多路访问技术(CSMA/CD)。依赖于CSMA/CD来规范网段中的流量。以太网介质是被动的,这意味着它会一直接受主机发送来的电波除非这些电波被物理隔离或者在终端处被停止。早些时候,有一种被称为ALOHA的技术,是一个真正透明的。当一个站点有一个帧需要被发送的时候,它发送了。这个站点之后就开始监听一段等于最大往返传输延时的时间加上一个小的修正时间。如果这个站点在这段时间内收到了确认信息,那么就完成了发送;反之,它将重复发送这个帧。如果多次不能成功收到确认信息,它就放弃本次发送。CSMA/CD的描述: 虽然比ALOHA更有效率,但是依然存在某些低效率的时候:当两帧碰撞,介质依然会传输已经无效的坏帧。与传播时间比较的话,发送这种帧对于容量的浪费是相当大的。如果站点在发送帧之前可以持续监听传输介质的话,那么这种浪费就可以被避免。这带来了使用CSMA/CD的一些规则。它们分别是:1.如果介质是空闲的,那么可以传输,否则停止发送。2.如果介质是繁忙的,要一直监听到介质空闲为止,然后马上发送帧。3.如果在传输过程中检测到冲突,发送一个简短的干扰信号以保证所有的站点都知4.道发生了冲突,然后停止发送。发送干扰信号后,等待一个随机的时间,然后尝试再次发送。(重复步骤1)B.MAC帧前导码帧起始定界符目的地址源地址长度LLC的长度填充帧校验序列 前导码:一种7bit的序列,用来建立同步。 帧起始定界符(SFD):序列是10101011,用来指明帧真正开始了,并且使得接收器可以定位这一个帧的第一个bit。 目的地址(DA):指出了这个帧要去往的目的站点。这个位置可以是独一无二的物理地址,一个组地址或者一个全局地址。16或48位的地址长度是实施决策的选择,而且在一个特定的局域网上的所有站点都要相同。 源地址(SA):指定发送帧的站。 长度:表明了LLC数据的长度。 LLC数据:LLC的数据单元。 填充:添加字节到数据中,保证这个帧有合适的长度可以被操作。帧校验序列(FCS):一种32位的循环冗余校验,对除了前导码、SFD和FCS以外的所有字段进行校验。IV.无线局域网介绍 无线局域网采用射频(RF)技术,是一种有线局域网的替代。无线局域网在空气中收发数据,不适用任何线缆,对数据的连接性和用户可移动性带来了极大的好处。无线局域网提供了所有有线局域网所能提供的功能甚至更好,比如:1.可移动性-无线局域网可以让用户在他们的组织内随时随地享受实时的信息和资源。2.安装快捷简便-安装一个无线局域网系统可以非常的快与简单并且免除布线穿墙等烦扰。3.安装灵活性-无线技术允许网络到达有线网络无法到达的地方。4.可扩展性-配置容易改变而且对等网络适应的范围可以从个别用户支持到成千上万个用户。A.无线局域网技术类型:我们有多种无线技术。其中的一些是- 窄带技术:窄带无线电系统在一个特定的无线电频率发射和接受用户信息。窄带无线电保持无线电信号频率尽可能多的用语传递信息。这种类型技术的缺点是,终端用户必须在每一个部署无线的站点获取FCC许可。 扩频技术:它适用可靠,安全,是任务关键型通信系统。这种技术消耗了更多的带宽,以产生更明显的信号,因而更容易被探测到。但缺点是这种技术在接收机未调谐到正确频率时,扩频信号看起来像背景噪音。 跳频扩频技术:这里采用了在窄带载波的几个频段中以指定的速率和频率跃迁作为避免干扰的一种方式。这适合用于同步;通过维护一个单一信道以及一个非预期接收机来使网络生效,FHSS会产生短时间的脉冲噪声。 直接序列扩频技术:DSSS技术在一个固定范围的频带内采用无线电发射机和接收器来传播数据包,直接序列扩频的接收机拒绝接收宽带噪声和低功耗的窄带信号。 红外技术:红外技术在商业领域用的很少。红外(IR)系统使用非常高的频率来传输数据,它的频率仅低于可见光的电磁频谱。B.无线局域网的运行 无线局域网使用无线电波从一个点到另一个点,而不依赖于任何物理连接进行通信。被传输的数据被叠加(调制)在无线电载波上,以便它可以在接收端被精确的提取出来。在典型的无线局域网配置中,发射机/接收器,被称为接入点(AP)。终端用户可以通过无线局域网适配器接入无线局域网。无线局域网适配器通过天线提供客户端网络操作系统(NOS)和电波之间的接口。按需网络不需要管理或预配置。在这种情况下,每个客户端将只能访问另一个客户端的资源,而不是中央服务器。这种无线局域网的设置有时被称为一个Ad-Hoc网络。C.客户端和接入点(基础设施模式) 安装的接入点允许每个客户端与其他客户端一样能够访问共享资源。访问点使用标准线缆连接到有线网络的固定位置。每个接入点可以容纳众多客户;具体的数目取决于所涉及的传输的数量和性质。这种无线局域网设置有时也被称为基础架构模式。D.多接入点和漫游 接入点有一个有限的传输范围-约100米(328英尺)的室内和300米(984英尺)的户外。在一个非常大的设施中,例如一个仓库,或者在大学校园骂它可能会安装一个以上的接入点。我们的目标是覆盖这些地方的每一寸土地,是的客户端在整个区域都不会丢失连接。客户端在接入点集群中无缝切换的能力被称为漫游。E.无线局域网的应用1.成长型的业务避免使用昂贵的网络布线。2.当员工都在办公室时,可以使用移动网络收发电子邮件,上网。3.仓管员使用无线局域网与中央数据库交换信息,从而提高工作效率。4.网络管理人员通过实施无线局域网提供备份。5.学生可以连接到协同课堂进行讨论以及可以发送互联网电子邮件和上网。F.无线局域网的安全 G.WEP(有线加密协议)是一种数据加密技术,旨在防止无线局域网流量被窃听。WEP允许管理员基于“密钥字符串”定义一组不同的“键”,通过WEP加密算法传递给每个无线网络用户。对于没分配到密钥的任何用户发起的访问都会被拒绝。V.总结 局域网通常被认为是小型的网络。计算机可以为了不同的目的被连接到一起,并且可以使用不同的线缆类型。局域网主要依赖于诸如拓扑,传输介质控制(MAC)和传输介质等特性和因素。这提供了保障。局域网由有线和无线组成。局域网主要工作在载波侦听多路访问/冲突检测(CSMA / CD)。有线局域网传输的数据是通过物理连接来进行的,而无线局域网发送和接收数据是在空气中进行的。参考文献1 LAN Switching, CCIE Routing and Switching Certification Guide, Fourth Edition. 2 Designing Switched LAN networks, CCIE Fundamentals: case studies and Network design, Second Edition. 3 Networking Fundamentals, CCNA Exam cram, Third Edition. 4 Administration of LAN-to-LAN tunnels, Cisco Secure virtual private Networks, second edition. 5 Switched network Management, Case studies and network design of CCIE fundamentals, second edition.7Local Area Network (LAN) Technologies International Journal of Innovative Technology and Exploring Engineering (IJITEE) ISSN: 2278-3075, Volume-1, Issue-6, November 2012 Local Area Network (LAN) Technologies Velaga Pavani, Immadisetty L V Chandrika, A.Rama Krishna 1Abstract Generally, small networks are often called as LAN (Local Area Network).A LAN is a network allowing easy access to the computers or peripherals. Networking means interconnection of computers. These computers can be linked together for different purposes and using a variety of different cabling types. Thus leading to less wastage of time and hence increased productivity.LAN mainly depends on the characteristics and the factors such as Topology, Medium Access Control (MAC) and Transmission Medium. This provides the security. LAN has Ethernet. LAN mainly works on the Carrier-Sense Multiple Access with Collision Detection (CSMA/CD). The Ethernet specification performs the same functions as the OSI physical and Data Link Layer of data communications. Wired LANs transmit the data but these are physically connected through repeaters and bridges and Wireless LANs transmit and receive data over air, without the use of any cable. These are physically not connected. Keywords- LAN (Local Area Network), MAC (Medium Access Control), CSMA/CD (Collision-Sense Multiple Access with Collision Detection), FHSS (Frequency-Hopping Spread Spectrum technology), DSSS (Direct-Sequence Spread Spectrum Technology), AP (Access Point), NOS (Network Operating System). I. INTRODUCTIONNetworking means interconnection of computers. These computers can be linked together for different purposes. The basic reasons we need to be networked are to share resources, to share application software, increase productivity. Small networks are often called Local Area Networks (LAN). A LAN is a network allowing easy access to the other computers. LAN mainly depends on five characteristics. They are physically limited distance that is about less than 2Km, high bandwidth that is more than 1mbps, inexpensive cable media (coax or twisted pair), data and hardware sharing between users, owned by the user. Mainly, LAN has mainly three factors that determine its nature. They are-1.Topology 2.Transmission Media 3.Medium Access Control Technique (MAC).LAN has Ethernet and Fast Ethernet .LAN mainly works on the carrier-sense multiple access with collision detection (CSMA/CD). The Ethernet specification performs the same functions as the OSI physical and Data Link Layer of data communications. Wired LANs transmit the data but these are physically connected through repeaters and bridges and wireless LANs transmit and receive data over the air, without the use of any cable. Manuscript received on November, 2012 Velaga Pavani, Electronics and Computer Engineering, K L University, Guntur Dt, Andhra Pradesh, India. Immadisetty L V Chandrika, Electronics and Computer Engineering, K L University, Guntur, Dt, Andhra Pradesh, India. A.Rama Krishna, Electronics and Computer Engineering, K L University, Krishna Dt, Andhra Pradesh, India. II. FACTORS OF LAN A. LAN Topologies- The common topologies for LAN are bus, tree, ring and star. Bus Topology:-For the bus all stations are attach through appropriate hardware interfaces called TAP directly to the linear transmission medium or bus, Full-duplex operation (sending and receiving between the station and the tap permits data to be transmitted onto the bus and received from the bus. At each end of the bus is a terminator, to avoid the reflection. Tree Topology:-The tree layout begins at a point known as head-end. Each of these may be have branches. But there are 2 problems, they are- transmission from any one station can be received by all other stations and secondly a mechanism is needed to regulate the transmission. To solve these problems, stations transmit data in small blocks called frames. Each frame consists of data and frame header that contains the information and the destination address. Ring Topology:-The network consists of a set of repeaters joined by point-to-point links in a closed loop. The repeater is a simple device of receiving data on one link and transmitting them, bit by bit, on the other link. The links are unidirectional. Star Topology:-In star topology, each station is directly connected to a common central node. In general, there are 2 alternatives for the operation of the central node. Firstly, the central node to operate in a broadcast fashion. In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may transmit successfully. Secondly, the central node acts as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station. B. Medium Access Control (MAC): Some means of controlling access to the transmission medium is needed to provide for an orderly and efficient use of that capacity. This is the function of medium access control (MAC) protocol. Mainly, MAC depends on 2 key parameters. They are-Where and How. Where refers to or whether control is centralized or distributed fashion. In a centralized scheme, a controller is designated that has the authority to grant access to the network. A station wishing to transmit must until it receives permissions from the controller. How is determined by the topology and is a trade-off among competing factors such as-including cost, performance and complexity. MAC control Destination MAC address Source MAC address LLC PDU CRC MAC Control This field contains any protocol information needed for the functioning of the MAC protocol. Destination MAC Address - The destination physical attachment point on the LAn for this frame. Source MAC Address The source physical attachment point on the LAN for this frame. Logical Link Control LLC is concerned with the transmission of a link-level protocol data unit (PDU) between two stations, without the necessity of an intermediate node. We have mainly 2 characters not shared by any other protocols. They are: It must support the multi-access and It is relieved of some details of link access by the MAC layer. Cyclic Redundancy Check (CRC) The cyclic redundancy check field (also known as the frame check sequence, FCS, field). This is an error-detecting code. C. Transmission Media: For the transmission of the data we have carious types of components. Not only for the transmission but also for the expanding the network. Some of the components are discussed following: Network Adapter Cards - A network adapter card plugs into the workstations, providing the connection to the network. Cabling Cables are used to interconnect computers and network components together. These are 3 main cable types used today. They are- twisted pair, coax, and fiber optic. Repeaters Repeaters extend the network segments. They amplify the incoming signal received from one segment. These are limit on numbers. These also allow isolation of segments in the event of failures or fault conditions. A repeater works at the physical layer by simply repeating all data from one segment to another. Bridges Bridges interconnect Ethernet segments. These work at the Data Link layer of the OSI layer. All information contained in the higher levels of the OSI model is unavailable to them; therefore they do not distinguish between one protocol to another protocol. They just simply pass all protocols along the network. These work at the MAC sub layer and are sometimes referred to as Medium Access Control Layer Bridges. Routers A network this complex needs a device which not only knows the address of each segment, but also determine the best path for sending data and filtering broadcast traffic to the local segment. Such a device is called a router. A router uses a table to determine the destination address for incoming data. The working of the router is to know all the network addresses, to connect to other networks, the possible path between those routers, and cost of sending data over those paths. Hubs Hub provides a number of ports, which are logically, combined using a single backplane, which often runs at a much higher data rate than that of the ports. Ports can be buffered, to allow packets to be held in case the hub or port is busy. We have 2 different types of ports. They are-Passive hubs are simply splitters or combiners that group workstations into a single segment. And Active hubs include a repeater function and are thus capable of supporting many more connections. III.ETHERNET&FAST ETHERNET (CSMA/CD) The most commonly used medium access control technique for bus/tree and star topologies is carrier-sense multiple access with collision detection (CSMA/CD).Relies on CSMA/CD to regulate traffic on the main segment. Ethernet media is passive which means it draws power from the computer and thus will not fail unless the media is physically cut or improperly terminated. Earlier, the techniques, known as ALOHA or it sometimes called, is a true free-for-all. Whenever a station has a frame to send, it does so. The station then listens for an amount of time equal to maximum possible round-trip propagation delay on the network plus a small fixed time increment. If the station hears an acknowledgement during this time, fine; otherwise, it re-sends the frame. If the station fails to receive an acknowledgement after repeated transmissions, it gives up. A. Description of CSMA/CD: CSMA, although more sufficient than ALOHA or ALOHA, still has one glaring inefficiency: When two frames collide, the medium remains unusable for the duration of transmission of both damaged frames. For long frames, compared to propagation time, the amount 2of wasted capacity can be considerable. This waste can be reduced if a station continues to listen to the medium while transmitting. This leads to the have some rules in the working of CSMA/CD. They are 1. If the medium is idle, transmit; otherwise go to step 2. 2. If the medium is busy, continue to listen until the channel is idle, and then transmit immediately. 3. If the collision is detected during transmission, transmit a brief jamming signal to assure that all stations know that there has been a collision and then cease the transmission. 4. After transmitting the jamming signal, wait a random amount of time, then attempt to transmit again. (Repeat the step 1). B. MAC FRAME Preamble A 7- octet pattern of alternating 0s and 1s used by the receiver to establish bit synchronization. Start Frame Delimiter (SFD) The sequence 10101011, which indicates the actual start of the frame and which enables the receiver to locate first bit of the frame. Destination Address (DA) Specifies the station(s) for which the frame is intended. It may be unique physical address, a group address, or a global address. The choice of the 16- or- 48- bit address length is a implementation decision, and must be the same for all stations on a particular LAN. Source Address (SA) Specifies the station that sent the frame. Length Length of the LLC data field. LLC Data Data unit supplied by LLC. Pad Octets added to ensure that frame is along enough for proper CD operation. Frame Check Sequence (FCS) A 32- bit cyclic redundancy check, based on all fields except the preamble, the SFD and the FCS. IV. INTROCTION TO WIRELESS LAN A wireless local area network (LAN) utilizes radio frequency (RF) as an alternative for a wired LAN. Wireless LANs transmit and receive data over the air, without the use of any cable, combining the benefits of data connectivity and the user mobility. Wireless LANs provide all the functionality of wired LANs with benefits such as 1. Mobility Wireless LANs can provide users with access to real-time information and resources anywhere in their organization. 2. Installation Speed and Simplicity Installing a wireless LAN system can be fast and easy and eliminates the need to pull cable through walls and ceilings. 3. Installation Flexibility Wireless technology allows the network to go where wire cannot go. 4. Scalability Configurations are easily changed and range from peer-to-peer networks suitable for a small number of users to full infrastructure networks of thousands of users. A.Types of Wireless LAN Technology: We have various types of wireless technologies. Some of them are- Narrowband Technology A narrowband radio system transmits and receives user information on a specific radio frequency. Narrowband radio keeps the radio signal frequency as narrow as possible just to pass the information. The drawback to this type of technology is that the end-user must obtain an FCC license for each site where it is employed. Spread Spectrum Technology This uses in reliable, secure, mission-critical communications systems. In this technology more bandwidth is consumed to produce a louder and thus easier ti detect broadcast signal. But the drawback to this technology is when the receiver is not tuned to the right frequency, a spread-spectrum signal looks like background noise. Frequency-hopping Spread Spectrum Technology (FHSS) This uses a narrowband carrier that hops among several frequencies at a specific rate and sequence as a way of avoiding interference. This is properly synchronized; the net effect is to maintain a single logical channel and to an unintended receiver, FHSS appears to be short-duration impulse noise. Direct Sequence Spread Spectrum Technology (DSSS) DSSS technology uses in a radio transmitter to spread data
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