1、1啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊啊(fm.DocWiki CISCO / zslcn编译注)二 Introduction to LAN Protocols 介绍 LAN协议http:/ page was last modified on 17 December 2009, at 21:54二 Introduction to LAN Protocols 介绍 LAN(局域网)协议From DocWikiJump to: navigation, searchThis article introduces the various media-access method
2、s, transmission methods, topologies, and devices used in a local-area network (LAN). Topics addressed focus on the methods and devices used in Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, and Fiber Distributed Data Interface (FDDI). Subsequent articles in LAN Protocols (Part 2) address specific proto
3、cols in more detail. 本文介绍了用于局域网(LAN)的各种媒体访问方法,传送方法,拓扑,和设备。所讲的题目集中在用于以太网/IEEE 802.3， 令牌环/IEEE 802.5，和 FDDI(光纤分布式数据接口)。在手册第二部分 LAN协议各篇文章中将更详细地讲述具体的协议。Figure 1: Three LAN Implementations Are Used Most Commonly illustrates the basic layout of these three implementations. 图 1 三种最普遍使用的 LAN实现 举例说明了这三种实现的基本
4、布置。Contentshide1 Figure: Three LAN Implementations Are Used Most Commonly 图 1.：三种最常见的局域网实现2 What Is a LAN? 什么是局域网？联网技术手册 指导目录 ContentsPart 1 Internetworking Basics 互联网络联网基本原理Part 2 LAN Technologies 局域网技术part 3 WAN Technologies 广域网技术part 4 Internet Protocols 互联网协议part 5 Bridging and Switching 桥接和交换pa
5、rt 6 Routing 路由part 7 Network Management 网络管理part 8 Voice/Data Integration Technologies 声音/数据集成技术part 9 Wireless Technologies 无线技术part 10 Cable Access Technologies 有线接入技术part 11 Dial-up Technology 拨号技术part 12 Security Technologies 安全技术part 13 Quality of Service Networking 联网服务质量part 14 Network Cachi
6、ng Technologies 网络缓存技术part 15 IBM Network Management IBM网络管理part 16 Multiservice Access Technologies 多业务接入技术23 LAN Protocols and the OSI Reference Model 局域网协议和 OSI参考模型3.1 Figure: Popular LAN Protocols Mapped to the OSI Reference Model 图 3.1：流行的局域网协议与 OSI参考模型的映射关系4 LAN Media-Access Methods 局域网介质访问方法5
7、 LAN Transmission Methods 局域网传送方法6 LAN Topologies 局域网拓扑6.1 Figure: Some Networks Implement a Local Bus Topology图 6.1：某些网络实现本地总线拓扑6.2 Figure: Some Networks Implement a Logical Ring Topology 图 6.2：某些网络实现本地逻辑环网拓扑6.3 Figure: A Logical Tree Topology Can Contain Multiple Nodes 图 6.3：一个逻辑树形拓扑可以包含多个节点7 LAN
8、Devices 局域网设备7.1 Figure: A Repeater Connects Two Network Segments 图 7.1：一个中继器连接两段网络段7.2 Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN 图 7.2：多个局域网扩展器可以通过广域网连接到主机路由器8 Review Questions 复习问答9 For More Information 参阅更多信息1. Figure: Three LAN Implementations Are Used Most Comm
9、only图 1 三种最普遍使用的 LAN实现2. What Is a LAN? LAN 是什么？A LAN is a high-speed data network that covers a relatively small geographic area. It typically connects workstations, personal computers, printers, servers, and other devices. LANs offer computer users many advantages, including shared access to devic
10、es and applications, file exchange between connected users, and communication between users via electronic mail and other applications. LAN是一个高速数据网络,它覆盖了相对较小的地理区域。它通常连接工作站 s,PC,打印机,服务器,和其它设备。3对计算机用户而言,LANs 有很多优点,包括共享所访问的设备 s和应用 s,已连接用户间的文件交换,以及用户间经由电子邮件和其它(包括多媒体)应用 s的通信。3. LAN Protocols and the OSI
11、Reference Model LAN协议 s及其 OSI参考模型LAN protocols function at the lowest two layers of the OSI reference model, as discussed in article Internetworking Basics between the physical layer and the data link layer. LAN协议是在 OSI参考模型的最低两层上起作用,如在第一章互联网络联网的基本原理中所讨论的,在物理层和数据链路层之间。Figure: Popular LAN Protocols Ma
12、pped to the OSI Reference Model illustrates how several popular LAN protocols map to the OSI reference model. Figure: Popular LAN Protocols Mapped to the OSI Reference Model4. LAN Media-Access Methods LAN介质访问方法Media contention occurs when two or more network devices have data to send at the same tim
13、e. Because multiple devices cannot talk on the network simultaneously, some type of method must be used to allow one device access to the network media at a time. This is done in two main ways: carrier sense multiple access collision detect (CSMA/CD) and token passing. 介质抢占发生在两个或更多的设备,要在同一时间发送数据情况下。
14、由于多个设备不能在网络上同时交谈,必须使用某种方法以允许在同一时间内只有一个设备访问网络介质。这可以用两种主要的方法来完成: CSMA/CD(载波感知多重访问/冲突检出)和令牌传送。In networks using CSMA/CD technology such as Ethernet, network devices contend for the network media. When a device has data to send, it first listens to see if any other device is currently using the network.
15、 If not, it starts sending its data. After finishing its transmission, it listens again to see if a collision occurred. A collision occurs when two devices send data simultaneously. When a collision happens, each device waits a random length of time before resending its data. In most cases, a collis
16、ion will not occur again between the two devices. Because of this type of network contention, the busier a network becomes, the more collisions occur. This is why performance of Ethernet degrades rapidly as the number of devices on a single network increases. 在使用 CSMA/CD技术,如以太网,的网络中,网络设备为网络介质而争夺。当一个
17、设备有数据要发送,它就要侦听是否有其它设备当前在使用网络。如果没有,它就开始发送数据。在完成发送后,它再次侦听是否有竞争发生。当两个设备同时发送数据时,竞争就发生了。此时,每个设备在发送它的数据前先等待一段随机时间间隔。在大多数的情况下,两个设备间的竞争不会再发生。然而这类网络越忙碌,竞争就越多,这就是为什么以太网在单一网络中随着设备数量的增加而性能迅速退化。4In token-passing networks such as Token Ring and FDDI, a special network frame called a token is passed around the net
18、work from device to device. When a device has data to send, it must wait until it has the token and then sends its data. When the data transmission is complete, the token is released so that other devices may use the network media. The main advantage of token-passing networks is that they are determ
19、inistic. In other words, it is easy to calculate the maximum time that will pass before a device has the opportunity to send data. This explains the popularity of token-passing networks in some real-time environments such as factories, where machinery must be capable of communicating at a determinab
20、le interval. 在令牌传送网络,如令牌环和 FDDI,一种特殊的帧叫令牌帧,围绕网络,从一个设备到另一个设备,被传递着。当一个设备有数据要传送,它必须等待令牌到来,然后发送数据,完成后,释放令牌,以便其它设备使用。令牌传递网络是确定性的。换句话说,在设备有机会发送数据前,它易于计算传递的最长时间。这就解释了令牌传递网络 s在某些实时环境中为何流行,如在工厂里,装置必须能够在可确定的时间区间内进行通信。For CSMA/CD networks, switches segment the network into multiple collision domains. This redu
21、ces the number of devices per network segment that must contend for the media. By creating smaller collision domains, the performance of a network can be increased significantly without requiring addressing changes. 网络交换机将网络分段成几个区段。对于 CSMA/CD,这就减少了一个区段中必须为使用媒体而竞争的设备数目。通过创建较小的竞争区段,一个网络的性能可以显著地增加而不需要改
22、变寻址；(并可引进全双工)。Normally CSMA/CD networks are half-duplex, meaning that while a device sends information, it cannot receive at the time. While that device is talking, it is incapable of also listening for other traffic. This is much like a walkie-talkie. When one person wants to talk, he presses the t
23、ransmit button and begins speaking. While he is talking, no one else on the same frequency can talk. When the sending person is finished, he releases the transmit button and the frequency is available to others. 通常 CSMA/CD是半双工的,即当一个设备发送信息时它无法同时接收信息。它在说话时也无法侦听其它信流。这很像步话机。When switches are introduced,
24、 full-duplex operation is possible. Full-duplex works much like a telephone-you can listen as well as talk at the same time. When a network device is attached directly to the port of a network switch, the two devices may be capable of operating in full-duplex mode. In full-duplex mode, performance c
25、an be increased, but not quite as much as some like to claim. A 100-Mbps Ethernet segment is capable of transmitting 200 Mbps of data, but only 100 Mbps can travel in one direction at a time. Because most data connections are asymmetric (with more data traveling in one direction than the other), the
26、 gain is not as great as many claim. However, full-duplex operation does increase the throughput of most applications because the network media is no longer shared. Two devices on a full-duplex connection can send data as soon as it is ready. 当引进交换机以后,全双工就可能了。全双工十分像电话,可以同时听和说。当一个网络设备直接加到一个网络交换机接口时,两
27、个设备就能运行在全双工模式下。尽管全双工能提高性能,但没有声称的那么多。两个全双工连接的设备一旦准备好就能立刻发送。Token-passing networks such as Token Ring can also benefit from network switches. In large networks, the delay between turns to transmit may be significant because the token is passed around the network. 令牌传递网络也能从网络交换机中得益。在大型网络中，轮转发送由于令牌围绕着网络
28、传递而可能延迟显著。(交换机可显著降低令牌传递延迟。)5. LAN Transmission Methods LAN发送方法LAN data transmissions fall into three classifications: unicast, multicast, and broadcast. In each type of transmission, a single packet is sent to one or more nodes. LAN数据发送分成三类: 单播,组播,和广播。在每类传送中,单个数据包被送到一个或更多节点。5In a unicast transmissio
29、n, a single packet is sent from the source to a destination on a network. First, the source node addresses the packet by using the address of the destination node. The package is then sent onto the network, and finally, the network passes the packet to its destination.单播传送是在一个网络中将单个数据包从信源送到信宿。首先,源节点
30、使用目的节点地址作为数据包的地址；然后,数据包被送到网络上；最后,网络传递数据包到目的地。A multicast transmission consists of a single data packet that is copied and sent to a specific subset of nodes on the network. First, the source node addresses the packet by using a multicast address. The packet is then sent into the network, which makes
31、 copies of the packet and sends a copy to each node that is part of the multicast address. 一个组播传送是将单一数据包复制并送往网络的一组指定节点组成的子集。首先,源节点使用组播地址作为数据包地址；然后,数据包被送往网络,实现数据包复制,并将拷贝送到属于组播地址范围内的每个节点上。A broadcast transmission consists of a single data packet that is copied and sent to all nodes on the network. In
32、these types of transmissions, the source node addresses the packet by using the broadcast address. The packet is then sent on to the network, which makes copies of the packet and sends a copy to every node on the network. 一个广播传送是将单一数据包复制并送到网络的所有节点上。源节点使用广播地址作为数据包地址,然后将包送向网络,实现数据包复制,并将拷贝送到网络的每个节点上。6.
33、 LAN Topologies LAN拓扑LAN topologies define the manner in which network devices are organized. Four common LAN topologies exist: bus, ring, star, and tree. These topologies are logical architectures, but the actual devices need not be physically organized in these configurations. Logical bus and ring
34、 topologies, for example, are commonly organized physically as a star. A bus topology is a linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations. Of the three most widely used LAN implementations, Ethernet/IEEE
35、802.3 networks-including 100BaseT-implement a bus topology, which is illustrated in Figure 6.1: Some Networks Implement a Local Bus Topology.LAN拓扑定义了网络设备逻辑上如何组织起来的方法。这些拓扑是逻辑结构,实际的设备在物理上并不需要按照这些配置来组织。有四种 LAN拓扑：总线,环形,星形,和树形。一个总线拓扑是一种线性 LAN结构,从网络站发送,沿着介质的长度方向传播,并为所有其它站点所接收。三种最广泛使用的 LAN实现之一,以太网/IEEE 802
36、.3网络-包括 100BaseT-实现了一种总线拓扑,它举例说明在下图 6.1中。Figure 6.1: Some Networks Implement a Local Bus TopologyA ring topology is a LAN architecture that consists of a series of devices connected to one another by unidirectional transmission links to form a single closed loop. Both Token Ring/IEEE 802.5 and FDDI
37、networks implement a ring topology. 环形拓扑是一种 LAN结构,它是由一串设备按单向传送链路彼此连接构成单个封闭回路。令牌环/IEEE 802.5和 FDDI网络实现了一种环形拓扑。图 2-4说明了一种逻辑环形拓扑。,Figure: Some Networks Implement a Logical Ring Topology depicts a logical ring topology. Figure: Some Networks Implement a Logical Ring Topology6A star topology is a LAN arc
38、hitecture in which the endpoints on a network are connected to a common central hub, or switch, by dedicated links. Logical bus and ring topologies are often implemented physically in a star topology, which is illustrated in the following figure. 星形拓扑是一种 LAN结构,其中网络的各端点都通过专有链路 s连向一个公共的中心集线器,或交换机。逻辑总线
39、拓扑和环形拓扑被用于星形拓扑的物理实现，图 2-5作了举例说明。A tree topology is a LAN architecture that is identical to the bus topology, except that branches with multiple nodes are possible in this case. 树形拓扑是一种与总线拓扑一样的拓扑,唯其可以带有多节点的分叉。图 2-5举例说明了一种逻辑树形拓扑。Figure: A Logical Tree Topology Can Contain Multiple Nodes illustrates a
40、logical tree topology. Figure: A Logical Tree Topology Can Contain Multiple Nodes7. LAN Devices LAN设备Devices commonly used in LANs include repeaters, hubs, LAN extenders, bridges, LAN switches, and routers. 通常用于 LAN的设备包括中继器,集线器,LAN 扩展器,网桥,交换机,和路由器。Note: Repeaters, hubs, and LAN extenders are discuss
41、ed briefly in this section. The function and operation of bridges, switches, and routers are discussed generally in “Bridging and Switching Basics“ and “Routing Basics.“7中继器,集线器,和 LAN扩展器在本节中作简单的讨论。网桥,交换机,和路由器在第四章,桥接和交换的基本原理,作一般性的讨论。A repeater is a physical layer device used to interconnect the media
42、 segments of an extended network. A repeater essentially enables a series of cable segments to be treated as a single cable. Repeaters receive signals from one network segment and amplify, retime, and retransmit those signals to another network segment. These actions prevent signal deterioration cau
43、sed by long cable lengths and large numbers of connected devices. Repeaters are incapable of performing complex filtering and other traffic processing. In addition, all electrical signals, including electrical disturbances and other errors, are repeated and amplified. The total number of repeaters a
44、nd network segments that can be connected is limited due to timing and other issues. 中继器是物理层的设备,用于一个扩展网络介质段的互连.中继器本质上使一连串缆线段处理成像一条缆线。中继器从一个网络段接收信号,放大,再定时,再发送这些信号到另一个网络段。通过这些作用,阻止了信号由于缆线太长和连接设备太多而引起的退化。中继器不可能执行复杂的过滤和其他的信流处理。另外,所有电气信号,包括电气干扰和其他错误,都被中继和放大。用中继器连接起来的网络段总数和中继器总数,由于时间同步引进其他方面的问题,是有限制的。Figu
45、re: A Repeater Connects Two Network Segments illustrates a repeater connecting two network segments. 下图 举例说明了一个中继器连接了两段网络段。Figure: A Repeater Connects Two Network SegmentsA hub is a physical layer device that connects multiple user stations, each via a dedicated cable. Electrical interconnections ar
46、e established inside the hub. Hubs are used to create a physical star network while maintaining the logical bus or ring configuration of the LAN. In some respects, a hub functions as a multiport repeater. 集线器是物理层设备,它连接着多个用户站,每一个都连上一条专用线缆。电气交换建立在集线器内部。集线器用于建立一个物理的星形网络,维持着 LAN的逻辑总线或环网的配置。在某些方面,一个集线器的功
47、能,就像是一个多口中继器。A LAN extender is a remote-access multilayer switch that connects to a host router. LAN extenders forward traffic from all the standard network layer protocols (such as IP, IPX, and AppleTalk) and filter traffic based on the MAC address or network layer protocol type. LAN extenders scal
48、e well because the host router filters out unwanted broadcasts and multicasts. However, LAN extenders are not capable of segmenting traffic or creating security firewalls.LAN扩展器是一个远端访问的多层交换机,它在逻辑上连接着一个主机路由器。LAN扩展器根据所有标准网络层协议(如 IP,IPX,和 AppleTalk)转发信流,过滤出基于 MAC地址或网络层协议类型的信流。LAN 扩展器能很好地缩放,因为主机路由器能过滤出不
49、需要的广播和组播。然而,不能分段信流或建立安全的防火墙。Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN illustrates multiple LAN extenders connected to the host router through a WAN. 下图 举例说明了通过一个 WAN连接了主机路由器的多个 LAN扩展器。Figure: Multiple LAN Extenders Can Connect to the Host Router Through a WAN 多个 LAN扩展器可8以通过一个广域网连接到主机路由器8. Review Questions 复习问答 sQ - Describe the type of media access used by Ethernet. 叙述以太网所使用的介质访问的类型。A - Eth
