CS 640: Introduction to Computer NetworksMultiple Access ProtocolsDesirable MAC PropertiesContention-Based ProtocolsEthernet Physical LayerEthernet Frame FormatEthernet host sideSender-side: MAC ProtocolCSMA/CD Algorithm OverviewCollision DetectionCollision Detection: ImplicationsMinimum Packet SizeCSMA/CD: Some DetailsLAN PropertiesBuilding Larger LANs: BridgesBasic Bridge Functionality“Transparent” BridgesAddress Lookup/Forwarding ExampleLearningSpanning Tree BridgesSpanning Tree Protocol OverviewSpanning Tree AlgorithmSlide 23Slide 24Spanning Tree Algorithm ExampleEthernet SwitchesA Word about “Taking Turn” ProtocolsToken-Passing ProtocolsCS 640: Introduction to Computer NetworksAditya AkellaLecture 7 -Ethernet, Bridges, Learning and Spanning TreeMultiple Access Protocols•Prevent two or more nodes from transmitting at the same time over a broadcast channel.–If they do, we have a collision, and receivers will not be able to interpret the signal•Several classes of multiple access protocols.–Partitioning the channel, e.g. frequency-division or time division multiplexing–Taking turns, e.g. token-based, reservation-based protocols, polling based–Contention based protocols, e.g. Aloha, EthernetDesirable MAC PropertiesBroadcast channel of capacity R bps.•1 node throughput = R bps•N nodes throughput = R/N bps, on average•Decentralized•Simple, inexpensiveContention-Based Protocols•Idea: access the channel in a “random” fashion - when collisions occur, recover.–Each node transmits at highest rate of R bps–Collision: two or more nodes transmitting at the same time•Each node retransmits until collided packet gets through–Key: don’t retransmit right away•Wait a random interval of time first•Examples–Aloha–Ethernet – focus todayEthernet Physical Layer•10Base2 standard based on thin coax 200m–Nodes are connected using thin coax cables and BNC “T” connectors in a bus topology–Thick coax no longer used•10BaseT uses twisted pair and hubs 100m–Stations can be connected to each other or to hubs–Hub acts as a concentrator•Dumb device•The two designs have the same protocol properties.–Key: electrical connectivity between all nodes–Deployment is differenthost host host hosthost host host hostHubHostEthernet Frame Format•Preamble marks the beginning of the frame.–Also provides synchronization•Source and destination are 48 bit IEEE MAC addresses.–Flat address space–Hardwired into the network interface•Type field is a demultiplexing field.–What network layer (layer 3) should receive this packet?•Max frame size = 1500B; min = 46B–Need padding to meet min requirement•CRC for error checking.Preamble Type PadDest Source Data CRC8 6 6 2 4Ethernet host side•Transceiver: detects when the medium is idle and transmits the signal when host wants to send–Connected to “Ethernet adaptor”–Sits on the host•Any host signal broadcast to everybody–But transceiver accepts frames addressed to itself–Also frames sent to broadcast address–All frames, if in promiscuous mode•When transmitting, all hosts on the same segment, or connected to the same hub, compete for medium–Said to “share same collision domain”–Bad for efficiency!Sender-side: MAC Protocol•Carrier-sense multiple access with collision detection (CSMA/CD). –MA = multiple access–CS = carrier sense–CD = collision detectionCSMA/CD Algorithm Overview•Sense for carrier.–“Medium idle”?•If medium busy, wait until idle.–Sending would force a collision and waste time•Send packet and sense for collision.•If no collision detected, consider packet delivered.•Otherwise, abort immediately, perform exponential back of and send packet again.–Start to send after a random time picked from an interval–Length of the interval increases with every collision, retransmission attemptCollision DetectionTimeA B10bit times500 bit timesCollision Detection: Implications •All nodes must be able to detect the collision.–Any node can be sender•=> Must either have short wires, long packets, or both•If A starts at t, and wirelength is d secs, –In the worst case, A may detect collision at t+2dWill have to send for 2d secs.d depends on max length of ethernet cableA Bd secsMinimum Packet Size•Give a host enough time to detect a collision.•In Ethernet, the minimum packet size is 64 bytes.–18 bytes of header and 46 data bytes–If the host has less than 46 bytes to send, the adaptor pads bytes to increase the length to 46 bytes•What is the relationship between the minimum packet size and the size of LAN?•How did they pick the minimum packet size?LAN size = (min frame size) * light speed / (2 * bandwidth)CSMA/CD: Some Details•When a sender detects a collision, it sends a “jam signal”.–Make sure that all nodes are aware of the collision–Length of the jam signal is 32 bit times–Permits early abort - don’t waste max transmission time•Exponential backoff operates in multiples of 512 bit times.–RTT= 256bit times backoff time > Longer than a roundtrip time–Guarantees that nodes that back off will notice the earlier retransmission before starting to send•Successive frames are separated by an “inter-frame” gap.–to allow devices to prepare for reception of the next frame–Set to 9.6 sec or 96 bit timesLAN Properties•Exploit physical proximity.–Often a limitation on the physical distance –E.g. to detect collisions in a contention based network•Relies on single administrative control and some level of trust.–Broadcasting packets to everybody and hoping everybody (other than the receiver) will ignore the packet•Broadcast: nodes can send messages that can be heard by all nodes on the network.–Almost essential for network administration–Can also be used for applications, e.g. video conferencing•But broadcast fundamentally does not scale.Building Larger LANs: Bridges•Hubs are physical level devices–Don’t isolate collision domains broadcast issues•At layer 2, bridges connect multiple IEEE 802 LANs–Separate a single LAN into multiple smaller collision domains•Reduce collision domain sizehost host host host hosthost host host host hosthosthostBridgeBasic Bridge Functionality•Bridges are full fledged packet switches•Frame comes in on an interface–Switch looks at destination LAN address–Determines port on which host connected–Only forward packets
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