Ethernet Overview Physical Layer MAC Bridged Ethernet VLAN Link Aggregation XON XOFF 802 11 Summary TOC Ethernet Overview Typical Setup Names Operations Perspective TOC Ethernet Overview Typical Setup TOC Ethernet Overview Typical 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 g TOC Ethernet Overview Names Operations Hub Single Collision Domain MAC Protocol Wait until silent carrier sense Transmit CSMA CD If collision wait random time repeat TOC Ethernet Overview Operations Hub Operations Switch No Collisions Multiple transmissions are possible Switch stores packets that wait for same output TOC Ethernet Overview Operations Switch 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 large TOC Ethernet Overview Perspective Physical Layer TOC Ethernet Physical Layer Physical TOC Ethernet Physical Layer MAC Media Access Control Frame Multiple Access TOC Ethernet MAC Frame PRE 8 DA 6 SA 6 T L 2 USER DATA 46 1500 CRC 4 PREamble alternating 1 0 combination producing 10Mhz square wave 10Mbps for 5 6 sec 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 followed by 802 3 frame Type if 1500 Examples 2 048 IP length in IP header 2 054 ARP 28 byes TOC Ethernet MAC Frame Multiple Access High Level View Multiple Access Protocols Random Access Protocols Slotted ALOHA CSMA CSMA CD Ethernet Multiple Access TOC Ethernet MAC Multiple Access 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 TOC Ethernet MAC Multiple Access High Level View 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 collisions TOC Ethernet MAC Multiple Access MA Protocols Random Access protocols When node has packet to send Two or more transmitting nodes collision Random access MAC protocol specifies Transmit at full channel data rate R No a priori coordination among nodes How to detect collisions How to recover from collisions Examples of random access MAC protocols Slotted ALOHA CSMA and CSMA CD TOC Ethernet MAC Multiple Access Random Access 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 slots TOC Ethernet MAC Multiple Access Slotted ALOHA Slotted 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 by any of N nodes p 1 p N 1 S Prob only one transmits Np 1 p N 1 1 e 0 37 Note 1 a N N e a for N 1 if p 1 N optimal value TOC Ethernet MAC Multiple Access Slotted ALOHA 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 interval TOC Ethernet MAC Multiple Access CSMA CSMA collisions spatial layout of nodes along ethernet Collisions can occur propagation delay means two nodes may not hear each other s transmission Collision entire packet transmission time wasted Note role of distance and propagation delay in determining collision prob TOC Ethernet MAC Multiple Access CSMA CSMA CD collision detection Overview Timing Ethernet TOC Ethernet MAC Multiple Access CSMA CD 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 LANs TOC Ethernet MAC Multiple Access CSMA CD Overview Timing TOC Ethernet MAC Multiple Access CSMA CD Timing Ethernet Overview Collision Detection Minimum Frame Size Maximum Frame Size Operations Efficiency Addressing Fast Ethernet Gigabit Ethernet TOC Ethernet MAC Multiple Access Ethernet Overview 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 Ethernet TOC Ethernet MAC Multiple Access Ethernet Overview Collision 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 A B time CD TOC Ethernet MAC Multiple Access Ethernet CD Minimum Frame Size Speed of light is about 4 s km in copper So max Ethernet signal prop time is about 10 sec or 20 sec RTT With repeaters etc 802 3 requires 51usec corresponding to 512 bit times Thus minimum frame size is 512 bits 64 bytes also called slot time TOC Ethernet MAC Multiple Access Ethernet Min Frame Maximum Frame Size 1500 byte limitation on maximum frame transmission size We will call this the MTU limits maximum buffers at receiver allows for other stations to send also requires 96 bit Inter Packet Gap IPG TOC Ethernet MAC Multiple Access Ethernet Max Frame Operations When ready line idle await IPG 96 bit times and send while listening CD If CD true send max 48 bit jamming sequence and do exponential backoff Jamming sequence used to inform all stations that a collision has occurred TOC Ethernet MAC Multiple Access Ethernet Operations Operations For retransmission N 1 N 10 choose k at random on U 0 2 N 1 wait k 51 2 sec to retransmit send on idle repeat on another
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