Ethernet Packet Size/Cable LengthCollision DetectionCollision Detection: ImplicationsMinimum Packet SizeSummary: Minimum Packet SizeHigher Speed Ethernet802.3u Fast Ethernet802.3z Gigabit EthernetRepeater/Hub/SwitchInternetworkingRepeatersBuilding Larger LANs:BridgesEthernet SwitchesMore Efficient EncodingLonger Distance EthernetWireless Ethernet (802.11)Switching: Why?Big Ethernet NetworkScalability Issues with Switched EthernetStructure of A Generic Communication SwitchAddressing and Look-upDatagram: Layer 2 (e.g., Ethernet)Datagram: Layer 3 (e.g., IP)Datagram: Layer 3 (e.g., IP)Datagram: Layer 3 (e.g., IP)Datagram: Layer 3 (e.g., IP)Virtual CircuitTechniques: Source RoutingControl PlaneExamples: Cisco GSR - 12416Examples: Juniper M160Examples: Cisco 7600Examples: Cisco Catalyst 6500Examples: Extreme - SummitExamples: Foundry - ServerIronSwitching: CharacteristicsArchitectures: GenericArchitectures: First GenerationArchitectures: Second GenerationArchitectures: Third GenerationArchitectures: Input FunctionsArchitectures: Output FunctionsInput QueuesHead-of-line BlockingArchitectures: Output QueuedArchitectures: Virtual Output BuffersArchitectures: Combined IN/OUTSwitching: Summary115-441 Computer NetworksEthernet and Switch DesignProfessor Hui [email protected] ZhangEthernet Packet Size/Cable Length Ethernet specifies a maximum packet size, why? Ethernet specifies a minimum packet size, why? Ethernet specifies a maximum cable length, why?3Hui ZhangCollision DetectionABCTime4Hui ZhangCollision Detection: Implications All nodes must be able to detect the collision. Any node can be sender The implication is that either we must have a short wires, or long packets. Or a combination of bothABC5Hui ZhangMinimum Packet Size Host 1 Host 2a) Time = t; Host 1 starts to send framepropagation delay (d)propagation delay (d)Host 1 Host 2b) Time = t + d; Host 2 starts to send a frame just before it hears fromhost 1’s framepropagation delay (d)Host 1 Host 2c) Time = t + 2*d; Host 1 hears Host 2’s frame Ædetects collisionLAN length = (min_frame_size)*(light_speed)/(2*link_rate) = = (8*64b)*(2*108mps)/(2*107bps) = 5.12 km6Hui ZhangSummary: Minimum Packet Size Why put a minimum packet size? Give a host enough time to detect collisions A host should be able to detect collision before it finishes thetransmission of a packet In Ethernet, minimum packet size = 64 bytes (two 6-byte addresses, 2-byte type, 4-byte CRC, and 46 bytes of data) If host has less than 46 bytes to send, the adaptor pads (adds) bytes to make it 46 bytes7Hui ZhangHigher Speed Ethernet What need to be changed in the Ethernet protocol to make it run 100Mbps?8Hui Zhang802.3u Fast Ethernet Apply original CSMA/CD medium access protocol at 100Mbps Must change either minimum frame or maximum diameter: change diameter Requires 2 UTP5 pairs 4 UTP3 pairs 1 fiber pair No more “shared wire” connectivity. Hubs and switches only9Hui Zhang802.3z Gigabit Ethernet Same frame format and size as Ethernet. This is what makes it Ethernet Full duplex point-to-point links in the backbone are likely the most common use. Added flow control to deal with congestion Choice of a range of fiber and copper transmission media. Defining “jumbo frames” for higher efficiency.10Hui ZhangRepeater/Hub/Switch Why do we need the following devices? Repeaters Hubs Switches What are the differences between them?11Hui ZhangInternetworking There are many different devices for interconnecting networks.EthernetRouterEthernetEthernetToken-ringBridgeRepeaterATM12Hui ZhangRepeaters – Used to interconnect multiple Ethernet segments– Merely extends the baseband cable– Amplifies all signals including collisions Repeater13Hui ZhangBuilding Larger LANs:Bridges Bridges connect multiple IEEE 802 LANs at layer 2. Only forward packets to the right port Reduce collision domain compared with single LAN In contrast, hubs rebroadcast packets. Implications: Performance Distance Type of networkshost host host host hosthosthost hosthosthostEthernetToken RingBridgeEthernethosthost14Hui ZhangEthernet Switches Bridges make it possible to increase LAN capacity. Packets are no longer broadcasted - they are only forwarded on selected links Ethernet switch is a special case of a bridge: each bridge port is connected to a single host. Can make the link full duplex (really simple protocol!)– Full duplex vs. half duplex vs simplex– What are the original CSMA/CD Ethernet? Simplifies the protocol and hardware used (only two stations on the link) – no longer full CSMA/CD Can have different port speeds on the same switch– Unlike in a hub, packets can be stored (what is the benefit?)– An alternative is to use cut through switching (what is the benefit?)15Hui ZhangMore Efficient Encoding Ethernet: Manchester Fast Ethernet: 4B/5B Borrowed from FDDI Gigabit Ethernet: 8B/10B Borrowed from HIPPI 10Giga Ethernet: 64/66B Borrowed from SONET Issues Clock recovery DC balance Clock rate vs. bit rate16Hui ZhangLonger Distance Ethernet GbE and 10GbE can run over one of the wavelength of a DWDM link, whose distance can be extended by optical amplification Can we have a national Ethernet network?17Hui ZhangWireless Ethernet (802.11) Why CSMA/CD is used in Ethernet but not in 802.11?18Hui ZhangSwitching: Why? Direct vs. Switched Networks: Direct Network Limitations: Distance (coordination delay; propagation limitation) Number of hosts (collisions; shared bandwidth; address tables) Single link technology (cannot mix optical, wireless, …) Internetworking Distance Performance Multiple types of links and networksSingle linkn linksSwitchesDirect Switched19Hui ZhangBig Ethernet Network With switching and optical Ethernet Distance is no longer an issue How big Ethernet network can we build? “big” in terms of Geographical reach Number of hostsLAN 2Bridge 2LAN 5LAN 3LAN 1LAN 4Bridge 5Bridge 4Bridge 3dBridge 120Hui ZhangScalability Issues with Switched Ethernet Broadcast Still broadcast in switched Ethernet network? Size of forwarding table Not being able to utilize all links Only links on spanning tree can forward packets21Hui ZhangStructure of A Generic Communication SwitchLine CardsSwitch FabricControl ProcessorLine Cards Switch
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