第16单元 高速局域网,Chapter 16 High Speed LANs,William Stallings Data and Computer Communications 7th Edition,2,Introduction,Range of technologies Fast and Gigabit Ethernet Fibre Channel High Speed Wireless LANs (see Chapter 17),3,16.1 Why High Speed LANs?,Office LANs used to provide basic connectivity Connecting PCs and terminals to mainframes and midrange systems that ran corporate applications Providing workgroup connectivity at departmental level Traffic patterns light Emphasis on file transfer and electronic mail Speed and power of PCs has risen Graphics-intensive applications and GUIs MIS organizations recognize LANs as essential Began with client/server computing Now dominant architecture in business environment Intranetworks Frequent transfer of large volumes of data,4,Applications Requiring High Speed LANs,Centralized server farms中央服务器集群 User needs to draw huge amounts of data from multiple centralized servers E.g. Color publishing Servers contain tens of gigabytes of image data Downloaded to imaging workstations Power workgroups超级工作组 Small number of cooperating users Draw massive data files across network E.g. Software development group testing new software version or computer-aided design (CAD) running simulations High-speed local backbone高速本地干线 Processing demand grows LANs proliferate at site High-speed interconnection is necessary,5,16.2 Ethernet (CSMA/CD) 以太网,Carriers Sense Multiple Access with Collision Detection Xerox Ethernet Xerox/DEC/Intel 3com IEEE 802.3,6,16.2.1 IEEE802.3 Medium Access Control,Random Access Stations access medium randomly Contention Stations content for time on medium,7,ALOHA,Packet Radio When station has frame, it sends Station listens (for max round trip time)plus small increment If ACK, fine. If not, retransmit If no ACK after repeated transmissions, give up Frame check sequence (as in HDLC) If frame OK and address matches receiver, send ACK Frame may be damaged by noise or by another station transmitting at the same time (collision) Any overlap of frames causes collision Max utilization 18%,8,Slotted ALOHA,Time in uniform slots equal to frame transmission time时隙长度等于发送时间 Need central clock (or other sync mechanism) Transmission begins at slot boundary Frames either miss or overlap totally Max utilization 37%,9,CSMA,Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Wait reasonable time (round trip plus ACK contention) No ACK then retransmit Max utilization depends on propagation time (medium length) and frame length Longer frame and shorter propagation gives better utilization,10,Nonpersistent CSMA,If medium is idle, transmit; otherwise, go to 2 If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1 Random delays reduces probability of collisions Consider two stations become ready to transmit at same time While another transmission is in progress If both stations delay same time before retrying, both will attempt to transmit at same time Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting Nonpersistent stations deferential,11,1-persistent CSMA,To avoid idle channel time, 1-persistent protocol used Station wishing to transmit listens and obeys following: If medium idle, transmit; otherwise, go to step 2 If medium busy, listen until idle; then transmit immediately 1-persistent stations selfish If two or more stations waiting, collision guaranteed Gets sorted out after collision,12,P-persistent CSMA,Compromise that attempts to reduce collisions Like nonpersistent And reduce idle time Like1-persistent Rules: If medium idle, transmit with probability p, and delay one time unit with probability (1 p) Time unit typically maximum propagation delay If medium busy, listen until idle and repeat step 1 If transmission is delayed one time unit, repeat step 1 What is an effective value of p?,13,Value of p?,Avoid instability under heavy load n stations waiting to send End of transmission, expected number of stations attempting to transmit is number of stations ready times probability of transmitting np If np 1on average there will be a collision Repeated attempts to transmit almost guaranteeing more collisions Retries compete with new transmissions Eventually, all stations trying to send Continuous collisions; zero throughput So np 1 for expected peaks of n If heavy load expected, p small However, as p made smaller, stations wait longer At low loads, this gives very long delays,14,CSMA Picture HERE,NEEDS EDITING fig 16.1,15,CSMA/CD,With CSMA, collision occupies medium for duration of transmission Stations listen whilst transmitting If medium idle, transmit, otherwise, step 2 If busy, listen for idle, then transmit If collision detected, jam then cease transmission After