Links&Reading:&Chapter&2&COS&461:&Computer&Networks&Spring&2009&(MW&1:30‐2:50&in&COS&105)&Mike&Freedman&hGp://www.cs.princeton.edu/courses/archive/spring09/cos461/&1!Goals&of&Today’s&Lecture&• Link‐layer&services&– Encoding,&framing,&and&error&detecTon&– Error&correcTon&and&flow&control&• Sharing&a&shared&media&– Channel&parTToning&– Taking&turns&– Random&access&• Ethernet&protocol&– Carrier&sense,&collision&detecTon,&and&random&access&– Frame&structure&– Hubs&and&switches&2!Message,&Segment,&Packet,&and&Frame&3!HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP IP Ethernet interface Ethernet interface SONET interface SONET interface host host router router HTTP message TCP segment IP packet IP packet IP packet Ethernet frame Ethernet frame SONET frameLink&Layer&Protocol&for&Each&Hop&• IP&packet&transferred&over&mulTple&hops&– Each&hop&has&a&link&layer&protocol&– May&be&different&on&different&hops&• Analogy:&trip&from&Princeton&to&Lausanne&– Limo:&Princeton&to&JFK&– Plane:&JFK&to&Geneva&– Train:&Geneva&to&Lausanne&• Refining&the&analogy&– Tourist&==&packet&– Transport&segment&==&communicaTon&link&– TransportaTon&mode&==&link‐layer&protocol&– Travel&agent&==&rouTng&algorithm&4!Adaptors&CommunicaTng&• Link&layer&implemented&in&adaptor&(network&interface&card)&– Ethernet&card,&PCMCIA&card,&802.11&card&• Sending&side:&– Encapsulates&datagram&in&a&frame&– Adds&error&checking&bits,&flow&control,&etc.&• Receiving&side&– Looks&for&errors,&flow&control,&etc.&– Extracts&datagram&and&passes&to&receiving&node&5!sending node frame receiving node datagram frame adapter adapter link layer protocolLink‐Layer&Services&• Encoding&– RepresenTng&the&0s&and&1s&• Framing&– EncapsulaTng&packet&into&frame,&adding&header,&trailer&– Using&MAC&addresses,&rather&than&IP&addresses&• Error&detecTon&– Errors&caused&by&signal&aGenuaTon,&noise.&&– Receiver&detecTng&presence&of&errors&• Error&correcTon&– Receiver&correcTng&errors&without&retransmission&• Flow&control&– Pacing&between&adjacent&sending&and&receiving&nodes&6!Encoding&• Signals&propagate&over&physical&links&– Source&node&encodes&the&bits&into&a&signal&– Receiving&node&decodes&the&signal&back&into&bits&• Simplify&some&electrical&engineering&details&– Assume&two&discrete&signals,&high&and&low&– E.g.,&could&correspond&to&two&different&voltages&• Simple&approach&– High&for&a&1,&low&for&a&0&7!0 0 1 1 0 0 1 1 0 0 0 1 1 1 1 1 0 0Problem&With&Simple&Approach&• Long&strings&of&0s&or&1s&introduce&problems&– No&transiTons&from&low‐to‐high,&or&high‐to‐low&• Receiver&keeps&average&of&signal&it&has&received&– Uses&the&average&to&disTnguish&between&high&and&low&– Long&flat&strings&make&receiver&sensiTve&to&small&change&• TransiTons&also&necessary&for&clock&recovery&– Receiver&uses&transiTons&to&derive&its&own&clock&– Long&flat&strings&do¬&produce&any&transiTons&– Can&lead&to&clock&drid&at&the&receiver&• AlternaTves&(see&SecTon&2.2)&– Non‐return&to&zero&inverted:&&TransiTon&for&1,&None&for&0&– Manchester&encoding:&&clock&XOR&NRZ:&&LH&(0),&HL&(1)&&8!Framing&• Break&sequence&of&bits&into&a&frame&– Typically&implemented&by&the&network&adaptor&• SenTnel‐based&– Delineate&frame&with&special&paGern&(e.g.,&01111110)&– Problem:&what&if&special&paGerns&occurs&within&frame?&– SoluTon:&escaping&the&special&characters&• E. g.,&sender&always&inserts&a&0&ader&five&1s&• …&and&receiver&always&removes&a&0&appearing&ader&five&1s&– Similar&to&escaping&special&characters&in&C&programs&9!01111110! 01111110!Frame contents!Framing&(ConTnued)&• Counter‐based&– Include&the&payload&length&in&the&header&– …&instead&of&pumng&a&senTnel&at&the&end&– Problem:&what&if&the&count&field&gets&corrupted?&• Causes&receiver&to&think&the&frame&ends&at&a&different&place&– SoluTon:&catch&later&when&doing&error&detecTon&• And&wait&for&the&next&senTnel&for&the&start&of&a&new&frame&• Clock‐based&– Make&each&frame&a&fixed&size&– No&ambiguity&about&start&and&end&of&frame&– But,&may&be&wasteful&&10!Error&DetecTon&• Errors&are&unavoidable&– Electrical&interference,&thermal&noise,&etc.&• Error&detecTon&– Transmit&extra&(redundant)&informaTon&&– Use&redundant&informaTon&to&detect&errors&– Extreme&case:&send&two&copies&of&the&data&– Trade‐off:&accuracy&vs.&overhead&• Techniques&for&detecTng&errors&– Parity&checking&– Checksum&– Cyclic&Redundancy&Check&(CRC)&11!Error&DetecTon&Techniques&• Parity&check&– Add&an&extra&bit&to&a&7‐bit&code&– Odd&parity:&ensure&an&odd&number&of&1s&• E.g.,&0101011&becomes&01010111&– Even&parity:&ensure&an&even&number&of&1s&• E.g.,&0101011&becomes&01010110&• Checksum&– Treat&data&as&a&sequence&of&16‐bit&words&– Compute&a&sum&of&all&16‐bit&words,&with&no&carries&– Transmit&the&sum&along&with&the&packet&• Cyclic&Redundancy&Check&(CRC)&– See&SecTon&2.4.3&12!Point‐to‐Point&vs.&Broadcast&Media&• Point‐to‐point&– PPP&for&dial‐up&access&– Point‐to‐point&link&between&Ethernet&switch&and&host&• Broadcast&(shared&wire&or&medium)&– TradiTonal&Ethernet&– 802.11&wireless&LAN&13!MulTple&Access&Protocol&• Single&shared&broadcast&channel&– Avoid&having&mulTple&nodes&speaking&at&once&– Otherwise,&collisions&lead&to&garbled&data&• MulTple&access&protocol&– Distributed&algorithm&for&sharing&the&channel&– Algorithm&determines&which&node&can&transmit&• Classes&of&techniques&– Channel&parTToning:÷&channel&into&pieces&– Taking&turns:&passing&a&token&for&the&right&to&transmit&– Random&access:&allow&collisions,&and&then&recover&14!Channel&ParTToning:&TDMA&TDMA:&Tme&division&mulTple&access&&• Access&to&channel&in&"rounds"&&– Each&staTon&gets&fixed&length&slot&in&each&round&• Time‐slot&length&is&packet&transmission&Tme&&– Unused&slots&go&idle&&• Example:&6‐staTon&LAN&with&slots&1,&3,&and&4&15!Channel&ParTToning:&FDMA&FDMA:&frequency&division&mulTple&access&&•
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