EE 122: Lecture 6High Level ViewMedium Access ProtocolsRandom Access protocolsSlotted AlohaSlotted Aloha EfficiencyCSMA: Carrier Sense Multiple AccessCSMA collisionsCSMA/CD (Collision Detection)CSMA/CD collision detectionEthernetEthernet Frame StructureEthernet Frame Structure (more)Ethernet’s CSMA/CDEthernet’s CSMA/CD (more)Minimum Packet SizeMinimum Packet Size (more)Ethernet Technologies: 10Base210BaseT and 100BaseT10BaseT and 100BaseT (more)Gbit EthernetInterconnecting LANsEE 122: Lecture 6Ion [email protected] 13, 2001(* this talk is based in part on the on-line slides of J. Kurose & K. Rose)[email protected] 2High Level ViewGoal: share a communication medium among multiple hosts connected to itProblem: arbitrate between connected hostsSolution goals:-High resource utilization-Avoid starvation-Simplicity (non-decentralized algorithms)[email protected] 3Medium Access ProtocolsChannel partitioning-Divide channel into smaller “pieces” (time slots, frequency)-Allocate piece to node for exclusive useRandom access-Allow collisions-“recover” from collisions“Taking-turns”-Tightly coordinate shared access to avoid [email protected] 4Random Access protocolsWhen node has packet to send-transmit at full channel data rate R.-no a priori coordination among nodesTwo or more transmitting nodes -> “collision”,Random access MAC protocol specifies: -how to detect collisions-how to recover from collisionsExamples of random access MAC protocols:-slotted ALOHA-CSMA and CSMA/[email protected] 5Slotted AlohaTime is divided into equal size slots (= packet transmission time)Node with new arriving pkt: transmit at beginning of next slot If collision: retransmit pkt in future slots with probability p, until successful.Success (S), Collision (C), Empty (E) [email protected] 6Slotted Aloha EfficiencyWhat is the maximum fraction of successful transmissions? Suppose N stations have packets to send-each transmits in slot with probability p-prob. successful transmission S is: by a single node: S = p (1-p)(N-1) by any of N nodes S = Prob (only one transmits) = N p (1-p)(N-1) <= 1/e = [email protected] 7CSMA: Carrier Sense Multiple AccessCS (Carrier Sense) means that each node can distinguish between an idle and a busy linkSender operations:-If channel sensed idle: transmit entire packet-If channel sensed busy, defer transmission •Persistent CSMA: retry immediately with probability p when channel becomes idle•Non-persistent CSMA: retry after a random time [email protected] 8CSMA collisionsCollisions can occur:propagation delay means two nodes may not hear each other’s transmissionCollision:entire packet transmission time wastedspatial layout of nodes along ethernetNote:role of distance and propagation delay in determining collision [email protected] 9CSMA/CD (Collision Detection)Collisions detected within short timeColliding transmissions aborted, reducing channel wastage Easy in wired LANs: measure signal strengths, compare transmitted, received signalsDifficult in wireless LANs: receiver shut off while [email protected] 10CSMA/CD collision [email protected] 11EthernetDominant LAN technologyCSMA/CD protocolCheap $20 for [email protected] 12Ethernet Frame StructureSending adapter encapsulates IP datagramPreamble: -7 bytes with pattern 10101010 followed by one byte with pattern 10101011-Used to synchronize receiver, sender clock [email protected] 13Ethernet Frame Structure (more)Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not matchType: 2 bytes, indicates the higher layer protocol-E.g., IP, Novell IPX, AppleTalkCRC: 4 bytes, checked at receiver, if error is detected, the frame is simply droppedData payload: maximum 1500 bytes, minimum 46 [email protected] 14Ethernet’s CSMA/CDSense channel, if idle-If detect another transmission•Abort, send jam signal•Delay, and try again-Else•Send frameReceiver accepts:-Frames addresses to its own address-Frames addressed to the broadcast address (broadcast)-Frames addressed to a multicast address, if it was instructed to listen to that address-All frames (promiscuous mode)[email protected] 15Ethernet’s CSMA/CD (more)Jam signal: make sure all other transmitters are aware of collision; 48 bits; Exponential back-off-Goal: adapt retransmission attempts to estimated current load-Heavy load: random wait will be longer-First collision: choose K from {0,1}; delay is K x 512 bit transmission times-After second collision: choose K from {0,1,2,3}…-After ten or more collisions, choose K from {0,1,2,3,4,…,1023}[email protected] 16Minimum Packet SizeWhy put a minimum packet size?Give a host enough time to detect collisionsIn 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 bytesWhat is the relationship between minimum packet size and the length of the [email protected] 17Minimum Packet Size (more)propagation delay (d)a) Time = t; Host 1 starts to send frameHost 1 Host 2propagation 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 detects collisionLAN length = (min_frame_size)*(light_speed)/(2*bandwidth) = = (8*64b)*(2.5*108mps)/(2*107 bps) = 6400m [email protected] 18Ethernet Technologies: 10Base210: 10Mbps; 2: under 200 meters max cable lengthThin coaxial cable in a bus topologyRepeaters used to connect up to multiple segmentsRepeater repeats bits it hears on one interface to its other interfaces: physical layer device [email protected] 1910BaseT and 100BaseT10/100 Mbps rate; latter called “fast ethernet”T stands for Twisted PairHub to which nodes are connected by twisted pair, thus “star topology”CSMA/CD implemented at [email protected] 2010BaseT and 100BaseT (more)Max distance from node to Hub is 100 metersHub can disconnect “jabbering adapterHub can gather monitoring information, statistics for display to LAN
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