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Berkeley ELENG 122 - Lecture 22

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Lecture 22EthernetEthernetOverviewTypical SetupNamesOperationsOperationsPerspectivePhysical LayerPhysicalMAC: Media Access ControlFrameFrameMultiple AccessHigh-Level ViewMedium Access ProtocolsRandom Access protocolsSlotted AlohaSlotted Aloha: EfficiencyCSMA: Carrier Sense Multiple AccessCSMA: collisionsCSMA/CD (collision detection)OverviewTimingEthernetOverviewCollision DetectionMinimum Frame SizeMaximum Frame SizeOperationsOperationsOperations: Capture EffectEfficiencyEfficiencyEfficiency - AnalysisEfficiency - AnalysisAddressingAddressing - MulticastFast EthernetGigabit Ethernet IEEE 802.3{z,ab} 1000 Mb/sLecture 22EthernetEECS 122University of CaliforniaBerkeley2EECS122 - Contents – Index -Ethernet Overview Physical Layer MAC Learning and Spanning Tree VLAN Link Aggregation XON/XOFF 802.11ETHERNET3EECS122 - Contents – Index -Overview Typical Setup Names Operations PerspectiveETHERNET - Overview4EECS122 - Contents – Index -Typical SetupETHERNET – Overview -Typical5EECS122 - Contents – Index -Names Structure[rate][modulation][media or distance] 10Base5 (10Mbps, baseband, coax, 500m) 10Base-T (10Mbps, baseband, twisted pair) 100Base-TX (100Mbps, baseband, 2 pair) 100Base-FX (100Mbps, baseband, fiber) 1000Base-CX for two pairs balanced copper cabling  1000Base-LX for long wavelength optical transmission  1000Base-SX for short wavelength optical transmission. Wireless: Wi-Fi = 802.11 Versions: a, b, gETHERNET – Overview - Names6EECS122 - Contents – Index -Operations Hub: Single Collision DomainMAC Protocol: Wait until silent (carrier sense)TransmitIf collision, wait random time & repeatCSMA/CDETHERNET – Overview –Operations Æ7EECS122 - Contents – Index -Operations Switch: No CollisionsMultiple transmissions are possibleSwitch stores packets that wait for same outputETHERNET – Overview Å Operations8EECS122 - Contents – Index -Perspective Ethernet is wildly successful, partly due to low cost (compare with FDDI or Token Ring--- see text book) Some issues: nondeterministic service no priorities min frame size may be largeETHERNET Å Overview - Perspective9EECS122 - Contents – Index -Physical LayerETHERNET –Physical Æ10EECS122 - Contents – Index -PhysicalETHERNET Å Physical11EECS122 - Contents – Index -MAC: Media Access Control Frame Multiple AccessETHERNET -MAC12EECS122 - Contents – Index -Frame7 byte preamble: alternating 1/0 combination producing 10Mhz square wave [@ 10Mbps] for 5.6 µsec; used for receiver synchronization 1 byte SFD (start of frame delimiter) 10101011ETHERNET –MAC–Frame Æ13EECS122 - Contents – Index -FrameLength/Type field: Type (Ethernet) indicates type of data contained in payload issue: what is the length? Length field (802.3) type info follows frame header  So, is it the type or length? “Ethernet”: types have values above 2048 (RFC894 for IP) 802.3: length (RFC1042 for IP) If length, next headers are LLC & SNAP (for IP) LLC (3 bytes): DSAP, SSAP, CTL SNAP (5 bytes): org code, type (above)ETHERNET –MACÅ Frame14EECS122 - Contents – Index -Multiple Access High-Level View Multiple Access Protocols Random Access Protocols Slotted ALOHA CSMA/CDETHERNET –MA15EECS122 - Contents – Index -High-Level View Goal: share a communication medium among multiple hosts connected to it Problem: arbitrate between connected hosts Solution goals: High resource utilization Avoid starvation Simplicity (non-decentralized algorithms)ETHERNET –MAÅ High Level16EECS122 - Contents – Index -Medium Access Protocols Channel partitioning Divide channel into smaller “pieces” (e.g., time slots, frequency) Allocate a piece to node for exclusive use Random access Allow collisions “recover” from collisions “Taking-turns” Tightly coordinate shared access to avoid collisionsETHERNET –MAÅ Protocols17EECS122 - Contents – Index -Random Access protocols When node has packet to send Transmit at full channel data rate R. No a priori coordination among nodes Two or more transmitting nodes -> “collision”, Random access MAC protocol specifies:  How to detect collisions How to recover from collisions Examples of random access MAC protocols: Slotted ALOHA CSMA and CSMA/CDETHERNET –MAÅ Random18EECS122 - Contents – Index -Slotted Aloha Time 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) slotsETHERNET –MA– S.Aloha Æ19EECS122 - Contents – Index -Slotted Aloha: Efficiency What 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 (very approximated analysis!): by a particular 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 = 0.37ETHERNET –MAÅ S.Aloha20EECS122 - Contents – Index -CSMA: Carrier Sense Multiple Access CS (Carrier Sense) means that each node can distinguish between an idle and a busy link Sender 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 intervalETHERNET –MA–CSMA Æ21EECS122 - Contents – Index -CSMA: collisionsspatial layout of nodes along ethernetCollisions can occur:propagation delay means two nodes may not hear each other’s transmissionCollision:entire packet transmission time wastedNote:role of distance and propagation delay in determining collision prob.ETHERNET –MAÅ CSMA22EECS122 - Contents – Index -CSMA/CD (collision detection) Overview Timing EthernetETHERNET –MA–CSMA/CD23EECS122 - Contents – Index -Overview Collisions detected within short time Colliding transmissions aborted, reducing channel wastage  Easy in wired LANs: measure signal strengths, compare transmitted, received signals Difficult in wireless LANsETHERNET –MA–CSMA/CD- Overview24EECS122 - Contents – Index -TimingETHERNET –MA–CSMA/CD- Timing25EECS122 - Contents – Index -Ethernet


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