Ethernet Overview Physical Layer MAC Bridged Ethernet VLAN Link Aggregation XON/XOFF 802.11 SummaryTOC – EthernetOverview Typical Setup Names Operations PerspectiveTOC – Ethernet – OverviewTypical SetupTOC – Ethernet – Overview –TypicalNames 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, gTOC – Ethernet – Overview –NamesOperations Hub: Single Collision DomainMAC Protocol: Wait until silent (carrier sense)TransmitIf collision, wait random time & repeatCSMA/CDTOC – Ethernet – Overview – Operations - HubOperations Switch: No CollisionsMultiple transmissions are possibleSwitch stores packets that wait for same outputTOC – Ethernet – Overview – Operations - SwitchPerspective 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 largeTOC – Ethernet – Overview – PerspectivePhysical LayerTOC – Ethernet – Physical LayerPhysicalTOC – Ethernet – Physical LayerMAC: Media Access Control Frame Multiple AccessTOC – Ethernet –MACFramePREamble: alternating 1/0 combination producing 10Mhz square wave [@ 10Mbps] for 5.6 µsec; used for receiver synchronization;ends with 11 to indicate start of frame. DA, SA: Destination and Source MAC addresses(All 1’s means broadcast)T/L: Length if <= 1500 Æ followed by 802.3 frameType if > 1500. Examples: 2,048: IP (length in IP header) -2,054: ARP (28 byes)TOC – Ethernet – MAC –Frame CRC4PRE8DA6SA6T/L2USER DATA46-1500Multiple Access High-Level View Multiple Access Protocols Random Access Protocols Slotted ALOHA CSMA CSMA/CD Ethernet Multiple AccessTOC – Ethernet – MAC – Multiple AccessHigh-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)TOC – Ethernet – MAC – Multiple Access – High Level ViewMedium 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 collisionsTOC – Ethernet – MAC – Multiple Access – MA ProtocolsRandom 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/CDTOC – Ethernet – MAC – Multiple Access – Random AccessSlotted 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) slotsTOC – Ethernet – MAC – Multiple Access – Slotted ALOHASlotted Aloha: Efficiency What is the maximum fraction of successful transmissions? Suppose N stations have packets to send Each transmits (independently) in slot with probability p Prob. successful transmission by a particular node: p(1 - p)(N-1) by any of N nodes:S= Prob (only one transmits)= Np(1 - p)(N-1) ≈ 1/e = 0.37if p = 1/N (optimal value)TOC – Ethernet – MAC – Multiple Access – Slotted ALOHA Note: (1 – a/N)N≈ e -afor N >> 1CSMA: 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 intervalTOC – Ethernet – MAC – Multiple Access –CSMACSMA: 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 prob.TOC – Ethernet – MAC – Multiple Access –CSMACSMA/CD (collision detection) Overview Timing EthernetTOC – Ethernet – MAC – Multiple Access –CSMA/CDOverview 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 LANsTOC – Ethernet – MAC – Multiple Access – CSMA/CD –OverviewTimingTOC – Ethernet – MAC – Multiple Access – CSMA/CD – TimingEthernet Overview Collision Detection Minimum Frame Size Maximum Frame Size Operations Efficiency Addressing Fast Ethernet Gigabit EthernetTOC – Ethernet – MAC – Multiple Access - EthernetOverview Will discuss “classical” Ethernet primarily Single segments up to 500m; with up to 4 repeaters gives 2500m max length Baseband signals broadcast, Manchester encoding, 32-bit CRC for error detection Max 100 stations/segment, 1024 stations/EthernetTOC – Ethernet – MAC – Multiple Access - Ethernet –OverviewCollision Detection CD circuit operates by looking for voltage exceeding a transmitted voltage Want to ensure that a station does not complete transmission prior to 1st bit arriving at farthest-away station Time to CD can thus take up to 2x{max prop. delay}ABCDtimeTOC – Ethernet – MAC – Multiple Access - Ethernet – CDMinimum Frame Size Speed of light is about 4µs/km in copper So, max Ethernet signal prop time is about 10 µsec, or 20µsec RTT With repeaters, etc. 802.3 requires 51usec, corresponding
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