信息工程网络双语 (10).ppt

William Stallings Data and Computer Communications 7th EditionChapter 16 High Speed LANsChapter 16 High-speed LANs16.1 The emergence of high-speed LANs 16.2 Ethernet 16.3 Token ring 16.4 Fibre channel16.2 Ethernet (CSMA/CD) 以太网• IEEE 802.3 Medium Access Control 介质介入控制 • IEEE 802.3 10-Mbps Specifications 规约(Ethernet) • IEEE 802.3 100-Mbps Specifications(Fast Ethernet) • Gigabit Ethernet 10^9 bps • 10- Gbps EthernetIEEE802.3 Medium Access Control 介质接入控制• Xerox – Ethernet 施乐 • Bob Metcalfe was working at Xerox PARC in 1973 when he co- invented Ethernet, a standard for connecting computers over short distances, with David Boggs.• Metcalfe founded 3Com. • 发表《以太网：局域计算机网络 的分布式包交换技术》的文章 Robert Metcalfe (1946~)• Carriers Sense Multiple Access with Collision Detection (CSMA/CD)带冲突检测的载波侦听多点接入• Random Access or Contention 随机接入或竞争 — Stations access medium randomly随机接入 —Stations content for time on medium竞争 • Collision（冲突）: two frames from different station interfere with each other on the channel at the receiver so that neither gets through1. ALOHA• Pure ALOHA 纯ALOHA —first deployed in 1970 —When station has frame, it sends —listens for (max round trip time) plus small increment —ACK, fine. If not, retransmit. —Max utilization 18% • Slotted ALOHA 时隙ALOHA —slots equal to frame transmission time —Transmission begins at slot 在时隙开始处 传输 —Max utilization 37%Random Access:Norman Abramson Pure ALOHA (1)In pure ALOHA, frames are transmitted at completely arbitrary(任意的) times.Shaded slots indicate collision (阴影的时隙表示冲突)Pure ALOHA (2)Vulnerable period for the shaded frame.易受攻击的时间碰撞Slotted ALOHA时隙ALOHAPure ALOHA and Slotted ALOHAThroughput versus offered traffic for ALOHA systems.2. CSMA p503• Carrier sense multiple access 载波侦听多址接入• First listen for clear medium 首先侦听，等待空闲的信道 • If idle, transmit 如果信道空闲，就发送数据。

—Wait reasonable time (round trip plus ACK contention) for ACK —No ACK then retransmit • If Busy, wait 如果信道忙，就等待 —1-persistent CSMA • If busy, listen for idle then transmit immediately —p-persistent —non-persistent 非坚持Random Access:Non-persistent CSMA p5031. If medium is idle, transmit; otherwise, go to 2 2. If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1• Random delays reduces probability of collisions • Capacity is wasted because medium will remain idle following end of transmission1-persistent CSMA p504•To avoid idle channel time, 1-persistent protocol used • 1. If medium idle, transmit; otherwise, go to step 2 2. If medium busy, listen until idle; then transmit immediately•1-persistent stations selfish •If two or more stations waiting, collision guaranteedp-persistent CSMA p504•Attempts to reduce collisions and reduce idle time1. If medium idle, transmit with probability p, and delay one time unit with probability (1 – p)1. If medium busy, listen until idle and repeat step 1 2. If transmission is delayed one time unit, repeat step 1 •What is an effective value of p?Persistent and Nonpersistent CSMAComparison of the channel utilization versus load for various random access protocols.3. CSMA/CD Rules p5051. If medium idle, transmit 2. If busy, listen for idle, then transmit 3. If collision detected, cease transmission and transmit a jamming signal to notify other stations 阻塞信号 4. After jam, back off for a random time then start again (repeat from step 1) 退避一段随机时 间Random Access:Binary exponential back off (二进制 指数退避）CSMA/CD OperationIEEE 802.3 Frame Format计算机通过适配器 和局域网进行通信 硬件地址至局域网适配器 （网卡）串行通信CPU 和 存储器生成发送的数据 处理收到的数据把帧发送到局域网 从局域网接收帧计算机IP 地址并行 通信适配器（网卡）• 网络接口板又称为通信适配器(adapter)或网络接口卡 NIC (Network Interface Card)，或“网卡”。

• 适配器的重要功能： —进行串行/并行转换 —对数据进行缓存 —在计算机的操作系统安装设备驱动程序 —实现以太网协议 10Mbps Specification (Ethernet)10Base510Base210Base-T10Base-FPMediumCoaxialCoaxialUTP850nmfiber SignalingBaseband BasebandBasebandManchester TopologyBusBusStarStar Maximum 500 185 100 500 length (m) Nodes10030-33100Mbps (Fast Ethernet)100Base-TX 100Base-FX 100Base-T42 pair, STP 2 optical fiber 4 pair, cat 3,4,52 pair, Cat 5 UTPMLT-3 4B5B,NRZI 8B6T,NRZCollision Detection• Collision detected if cable signal greater than single station signal • Signal attenuated over distance • Limit distance to 500m (10Base5) or 200m (10Base2) • For twisted pair (star-topology) activity on more than one port is collisionEthernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.16.3 Token Ring (802.5)• Developed from IBM's commercial token ring • Because of IBM's presence, token ring has gained broad acceptance • Never achieved popularity of Ethernet • Currently, large installed base of token ring productsRing Operation• Each repeater connects to two others via unidirectional transmission links • Data transferred bit by bit from one repe。