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Berkeley ELENG 122 - Ethernet - Links, Hubs, Switches

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Ethernet Links Hubs Switches EE 122 Intro to Communication Networks Fall 2006 MW 4 5 30 in Donner 155 Vern Paxson TAs Dilip Antony Joseph and Sukun Kim http inst eecs berkeley edu ee122 Materials with thanks to Jennifer Rexford Ion Stoica and colleagues at Princeton and UC Berkeley 1 Announcements Office hours 329 Soda Regular slot moving to Weds 3 4PM half hour later Extra office hours Monday Oct 16 1 30 3 30PM Also by appointment but not this Thursday Friday 2 1 Goals of Today s Lecture Ethernet single segment Carrier sense collision detection and random access Frame structure Ethernet spanning multiple segments Repeaters and hubs Bridges and switches Cut through switching Self learning plug and play Spanning trees Virtual LANs VLANs The spectrum of interconnections Hubs vs switches vs routers 3 Ethernet CSMA CD Protocol Carrier sense wait for link to be idle Collision detection listen while transmitting No collision transmission is complete Collision abort transmission send jam signal Random access exponential back off After collision wait a random time before trying again After mth collision choose K randomly from 0 2m 1 and wait for K 512 bit times before trying again The wired LAN technology Hugely successful 3 10 100 1000 10000 Mbps 4 2 CSMA CD Collision Detection 5 Limitations on Ethernet Length B A latency d Latency depends on physical length of link Time to propagate a packet from one end to the other Suppose A sends a packet at time t And B sees an idle line at a time just before t d so B happily starts transmitting a packet B detects a collision and sends jamming signal But A can t see collision until t 2d 6 3 Limitations on Ethernet Length B A latency d A needs to wait for time 2d to detect collision So A should keep transmitting during this period and keep an eye out for a possible collision Imposes restrictions on Ethernet For 10 Mbps Maximum length of the wire 2 500 meters Minimum length of the packet 512 bits 64 bytes 512 bits 51 2 sec at 10 Mbit sec For light in vacuum 51 2 sec 15 000 meters vs 5 000 meters round trip to wait for collision 7 Ethernet Frame Structure Sending adapter encapsulates packet in frame Preamble synchronization Seven bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver sender clock rates Type indicates the higher layer protocol Usually IP but also Novell IPX AppleTalk CRC cyclic redundancy check Receiver checks simply drops frames with errors 8 4 Ethernet Frame Structure Continued Addresses 48 bit source and destination MAC addresses Receiver s adaptor passes frame to network level protocol If destination address matches the adaptor s Or the destination address is the broadcast address ff ff ff ff ff ff Or the destination address is a multicast group receiver belongs to Or the adaptor is in promiscuous mode Addresses are globally unique Assigned by NIC vendors top three octets specify vendor During any given week 500 vendor codes seen at LBNL Data Maximum 1 500 bytes Minimum 46 bytes 14 bytes header 4 byte trailer 512 bits 9 Unreliable Connectionless Service Connectionless No handshaking between sending and receiving adapter Unreliable Receiving adapter doesn t send ACKs or NACKs Packets passed to network layer can have gaps Gaps will be filled if application is using TCP Otherwise application will see the gaps 10 5 Benefits of Ethernet Easy to administer and maintain Inexpensive Increasingly higher speed Evolved from shared media to switches Changes everything except the frame format A good general lesson for evolving the Internet The right interface service model can often accommodate unanticipated changes In fact Ethernet framing used for wildly different technologies e g 802 11 wireless 11 Shuttling Data at Different Layers Different devices switch different things Physical layer electrical signals repeaters and hubs Link layer frames bridges and switches Network layer packets routers Application gateway Transport gateway Router Frame Packet TCP header header header User data Bridge switch Repeater hub 12 6 Physical Layer Repeaters Distance limitation in local area networks Electrical signal becomes weaker as it travels Imposes a limit on the length of a LAN In addition to limit imposed by collision detection Repeaters join LANs together Analog electronic device Continuously monitors electrical signals on each LAN Transmits an amplified copy Repeater 13 Physical Layer Hubs Joins multiple input lines electrically Do not necessarily amplify the signal Very similar to repeaters Also operates at the physical layer hub hub hub hub 14 7 Limitations of Repeaters and Hubs One large collision domain Every bit is sent everywhere So aggregate throughput is limited E g three departments each get 10 Mbps independently and then if connect via a hub must share 10 Mbps Cannot support multiple LAN technologies Repeaters hubs do not buffer or interpret frames So can t interconnect between different rates or formats E g no mixing 10 Mbps Ethernet 100 Mbps Ethernet Limitations on maximum nodes and distances Does not circumvent limitations of shared media E g still cannot go beyond 2500 meters on Ethernet 15 Link Layer Bridges Connects two or more LANs at the link layer Extracts destination address from the frame Looks up the destination in a table Forwards the frame to the appropriate LAN segment Each segment is its own collision domain host host host host host host host host Bridge host host host host 16 8 Link Layer Switches Typically connects individual computers Essentially the same as a bridge though connecting hosts not LANs In a point to point fashion Like bridges support concurrent communication Host A can talk to C while B talks to D B A C switch D 17 Dedicated Access and Full Duplex Dedicated access Host has direct connection to the switch rather than a shared LAN connection Full duplex Each connection can send in both directions At the same time otherwise half duplex Host sending to switch and host receiving from switch Completely avoids collisions Each connection is a bidirectional point to point link No need for carrier sense collision detection and so on 18 9 Bridges Switches Traffic Isolation Breaks subnet into LAN segments Filters packets Frame only forwarded to the necessary segments Segments become separate collision domains switch bridge collision domain hub collision domain hub hub collision domain 19 5 Minute Break Questions Before We Proceed 20 10 Advantages Over Hubs Repeaters Only


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Berkeley ELENG 122 - Ethernet - Links, Hubs, Switches

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