Lecture 22 Ethernet EECS 122 University of California Berkeley Ethernet Overview Physical Layer MAC Learning and Spanning Tree VLAN Link Aggregation XON XOFF 802 11 EECS122 Contents Index ETHERNET 2 Overview Typical Setup Names Operations Perspective EECS122 Contents Index ETHERNET Overview 3 Typical Setup EECS122 Contents Index ETHERNET Overview Typical 4 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 EECS122 Contents Index ETHERNET Overview Names 5 Operations Hub Single Collision Domain MAC Protocol Wait until silent carrier sense Transmit CSMA CD If collision wait random time repeat EECS122 Contents Index ETHERNET Overview Operations 6 Operations Switch No Collisions Multiple transmissions are possible Switch stores packets that wait for same output EECS122 Contents Index ETHERNET Overview Operations 7 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 EECS122 Contents Index ETHERNET Overview Perspective 8 Physical Layer EECS122 Contents Index ETHERNET Physical 9 Physical EECS122 Contents Index ETHERNET Physical 10 MAC Media Access Control Frame Multiple Access EECS122 Contents Index ETHERNET MAC 11 Frame 7 byte preamble alternating 1 0 combination producing 10Mhz square wave 10Mbps for 5 6 sec used for receiver synchronization 1 byte SFD start of frame delimiter 10101011 EECS122 Contents Index ETHERNET MAC Frame 12 Frame Length 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 EECS122 Contents Index ETHERNET MAC Frame 13 Multiple Access High Level View Multiple Access Protocols Random Access Protocols Slotted ALOHA CSMA CD EECS122 Contents Index ETHERNET MA 14 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 EECS122 Contents Index ETHERNET MA High Level 15 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 EECS122 Contents Index ETHERNET MA Protocols 16 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 EECS122 Contents Index ETHERNET MA Random 17 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 EECS122 Contents Index ETHERNET MA S Aloha 18 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 by any of N nodes S p 1 p N 1 S Prob only one transmits N p 1 p N 1 EECS122 Contents Index 1 e 0 37 ETHERNET MA S Aloha 19 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 EECS122 Contents Index ETHERNET MA CSMA 20 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 EECS122 Contents Index ETHERNET MA CSMA 21 CSMA CD collision detection Overview Timing Ethernet EECS122 Contents Index ETHERNET MA CSMA CD 22 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 EECS122 Contents Index ETHERNET MA CSMA CD Overview 23 Timing EECS122 Contents Index ETHERNET MA CSMA CD Timing 24 Ethernet Overview Collision Detection Minimum Frame Size Maximum Frame Size Operations Efficiency Addressing Fast Ethernet Gigabit Ethernet EECS122 Contents Index ETHERNET MA CSMA CD Ethernet 25 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 EECS122 Contents Index ETHERNET MA CSMA CD Ethernet Overview 26 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 EECS122 Contents Index ETHERNET MA CSMA CD Ethernet CD 27 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 EECS122 Contents Index ETHERNET MA CSMA CD Ethernet Minimum 28 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
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