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1、数据通信基础数据通信基础第第14章章 (局域网局域网) Local Area Networks20120326BITLiuJKPage 2 第第14章内容提要章内容提要14.1 Traditional Ethernet 传统以太网Project 802 802协议Ethernet 以太网Bridged Ethernet 网桥式以太网Switched Ethernet 交换式以太网14.2 Fast Ethernet 快速以太网14.3 Gigabit Ethernet 千兆以太网20120326BITLiuJKPage 3 Chapter BriefFrom this chapter on,
2、we come to the field of computer communication. Local Area Network (LAN) was primarily used for computer groups within an office.But gradually the concept of LAN is changed to communication systems within limited areas.In this chapter we discuss the protocols used in LAN, including Project 802, Ethe
3、rnet, Fast Ethernet, Gigabit Ethernet.20120326BITLiuJKPage 4 Definition of LAN & WANA Local Area Network (LAN) 局域网 is a data communication system that allows a number of independent devices to communicate directly with each other in a limited geographic area.A Wide Area Network (WAN) 广域网 is a networ
4、k that uses a technology that can span a large geographical distance.20120326BITLiuJKPage 5 14.1 Traditional EthernetProject 802LLC and MACPacket Data Unit (PDU) 协议数据单元协议数据单元Frame Format 帧格式帧格式Ethernet Bridged Ethernet Switched Ethernet20120326BITLiuJKPage 6 Project 802The protocols in LAN was defin
5、ed by IEEE in 1985. This is a group of protocols, denoted by 802.X protocol.Project 802 protocol covers the lower two layers of OSI model, i.e. Physical and Data Link Layer.There are two sublayers for the data link layer: Logical Link Control (LLC) layer 逻辑链路控制子层 and Medium Access Control (MAC) laye
6、r 介质访问控制子层.20120326BITLiuJKPage 7 OSI Model & Project 80220120326BITLiuJKPage 8 Protocols in Project 80220120326BITLiuJKPage 9 LLC and MAC LayersLLC layer is the upper sublayer of data link layer. LLC layer deals with functions of end-user: logical addresses 逻辑编址, control information, and data.MAC l
7、ayer is the lower sublayer of the data link layer. MAC layer deals with the contention of the shared media. It contains physical address 物理编址, synchronization, flow and error control.20120326BITLiuJKPage 10 PDU Format in LLCThe data link layer protocols for project 802 are based on HDLC, which was d
8、iscussed in chapter 11.The Address field contains two parts: Destination Service Access Point (DSAP) 目的服务访问点 and Source Service Access Point (SSAP). 源服务访问点The other two fields are Control and Information.There are no Flag and CRC fields in LLC format.The Control field is the same with HDLC.20120326B
9、ITLiuJKPage 11 PDU Format20120326BITLiuJKPage 12 PDU Control Field20120326BITLiuJKPage 13 IEEE 802.3IEEE 802.3 protocol was developed by Xerox, DEC and Intel. It was called Ethernet, using the name of ether.The access method in Ethernet is CSMA/CD. these three names are used interchangeably.IEEE 802
10、.3 defines two categories: Baseband and Broadband.20120326BITLiuJKPage 14 First Ethernet Drawing20120326BITLiuJKPage 15 Address in MACMAC address in Ethernet consists of SIX octets (48 bits). This is the only address for the Network Interface Card (NIC) 网卡.The first three bytes indicate the manufact
11、uring factory and the last three bytes indicate different cards.How to find the MAC address?20120326BITLiuJKPage 16 MAC Address6-byte MAC addresses must be unique they are burned onto a network card during manufacture.Each card manufacturer has been allocated a range of addresses it may use. There a
12、re 248 = 281,474,976,710,656 possible MAC addresses available (about 50,000 for each person alive today)One bit of byte 1 has special significance it allows multicasting20120326BITLiuJKPage 17 MAC Frame Format20120326BITLiuJKPage 18 MAC Frame Format DetailPreamble 前导符 and SFD (start frame delimiter,
13、 起始帧分界符) are used as Flag field in HDLC.Destination Address and Source Address are all MAC addresses.There is one 2-byte extra field of length/type of PDU.The Information field comes from the upper sublayer of LLC. The minimum and maximum length of 802.3 PDU are 46 to 1500 bytes.At last the CRC fiel
14、d is 4 bytes long.20120326BITLiuJKPage 19 Collision DetectionThe maximum time for a collision to be detected is 2T, where T is the propagation time through 2.5 km cable and 4 repeatersFor a 10Mb/s LAN this corresponds to 51.2s and a packet size of 64 Bytes.ABABABABt=0, packet starts from At=T-dT, pa
15、cket almost at Bt=T, B starts to Txt=2T, noise burst Rx at AFigure 14.16 A network with and without a bridgeIn (a), all frames from all nodes are sent to all other nodes creating unnecessary traffic on the network.A bridge is a layer 2 device. It does not always pass frames acrossto the other segmen
16、t, but listens to source MAC addresses from frames on each side, and builds up a bridging table. It only passes a frame across if its destination is on the other side.In this way, the number of collisions is significantly reduced, since mostframes are meant for nodes on the same side of the bridge.T
17、hus two collision domains 冲突域冲突域 are created.20120326Page 20 BITLiuJKFigure 14.17 Collision domains in a non-bridged and bridged networkIn (b), unnecessary network traffic is reduced by a factor of 420120326Page 21 BITLiuJK20120326BITLiuJKPage 22 ImplementationNotation of the Ethernet implementation
18、:SPEED Baseband Transmission Media or Distance10Base5 means 10 Mbps, Baseband media and 500 meters transmission distance.10Base5 Thick Ethernet10Base2 Thin Ethernet10BaseT Twisted-Pair Ethernet20120326BITLiuJKPage 23 WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 199810Base520120326BITLiuJKPage 24
19、 WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998Transceiver20120326BITLiuJKPage 25 WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 199810Base220120326BITLiuJKPage 26 WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 199810Base-T20120326BITLiuJKPage 27 Switch vs HubIn traditional Ethernet, all
20、stations connect to a central hub and they share the capacity of the hub because of the contention. Normally the full speed of the network can not be achieved.In switched Ethernet, the switch will allocate a dedicated path for any two stations. So theoretically the speed of each station can be full
21、speed.20120326BITLiuJKPage 28 Full-duplex Switched EthernetFigure 14.18 Switched EthernetA hub or repeater passes all frames without address checking layer 1 devicesA bridge checks addresses, but only allows a single data path from one port to another at any given timeA switch checks addresses, and
22、allows multiple data paths at the same time, creating very small collision domains. Switches are now commonplace 20120326Page 29 BITLiuJK20120326BITLiuJKPage 30 14.2 Fast Ethernet 快速以太网快速以太网MAC SublayerPhysical LayerPhysical Layer Implementation 20120326BITLiuJKPage 31 Fast EthernetFast Ethernet ope
23、rates at 100 Mbps.Different signal formats are used for 10 and 100 Mbps Ethernet, so these two protocols can coexist in one network.The increase in speed means the decrease of transmission distance within one segment of Ethernet.100Base-TX is often used together with traditional Ethernet.20120326BIT
24、LiuJKPage 32 Fast Ethernet physical layer20120326BITLiuJKPage 33 Fast Ethernet ImplementationUse Fiber Use four pairs of category 5 UTP20120326BITLiuJKPage 34 100Base-TX Implementation20120326BITLiuJKPage 35 100Base-FX Implementation20120326BITLiuJKPage 36 100Base-T4 Implementation20120326BITLiuJKPa
25、ge 37 14.3 Gigabit Ethernet 千兆以太网千兆以太网MAC SublayerPhysical LayerPhysical Layer Implementation 20120326BITLiuJKPage 38 Gigabit EthernetGigabit Ethernet operates at 1000 Mbps or 1 Gbps.Normally a pair of optical fiber will be needed for transmission. This protocol is called 1000Base-X.An alternative i
26、mplementation is 4 pairs of category 5 UTP, which is called 1000Base-T.20120326BITLiuJKPage 39 Gigabit Ethernet Physical Layer20120326BITLiuJKPage 40 Gigabit Ethernet Implementation20120326BITLiuJKPage 41 1000Base-T Implementation20120326BITLiuJKPage 42 1000Base-X Implementation20120326BITLiuJKPage
27、43 第第14章小结章小结802协议是局域网 LAN 的标准。以太网是使用最广泛的LAN协议。802协议的数据链路层分为两个子层：逻辑链路控制 (LLC) 子层介质访问控制 (MAC) 子层802.3, 以太网 and CSMA/CD 指的是同一件事情的不同方面。20120326BITLiuJKPage 44 第第14章小结章小结(续续)10-Mbps 以太网的最小帧长是64个字节；最大帧长是1518个字节（包含帧头帧尾）。10Base5, 10Base2, 10Base-T 是10 Mbps 以太网的实现方式。网桥可以隔离以太网LAN中的冲突域。交换式以太网、快速以太网和千兆以太网都是以太网的实现方式，数据速率分别是 10, 100, 1000 Mbps.20120326BITLiuJKPage 45 ExercisesPage 356-35982236
