《Computer Networking：A Top Down Approach》英文教材教学资源（PPT课件讲稿，3rd edition）Chapter 5 Link Layer and LANs

Chapter 5 Computer Networkin Link Layer and LANs 17lAa学ms A note on the use of these ppt slides James F Kurse. keith w Ross Al material copyright 1996-2007 J F Kurose and K.W. Ross, All Rights reserved Computer Networking A Top Down Approach 4th edition Jim Kurose. Keith ross Addison-Wesley, Jul 2007 5: Dat

5: DataLink Layer 5-1 Chapter 5 Link Layer and LANs A note on the use of these ppt slides: All material copyright 1996-2007 J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007

Link Layer 5. 1 Introduction and o5.6 Hubs and switches services o5.7PPP o 5.2 Error detection and correction 6 5.3 Multiple access protocols o 5.4 Link-Layer Addressing o5.5 Ethernet 5: Dat

Link Layer  5.1 Introduction and services  5.2 Error detection and correction  5.3 Multiple access protocols  5.4 Link-Layer Addressing  5.5 Ethernet  5.6 Hubs and switches  5.7 PPP 5: DataLink Layer 5-2

Link Layer: Setting the Context o Two physically connected devices e Host-router, router-router host-host ● Unit of data: frame M lication M l transport TH M network data link network protoco HnH+I M link link Hi HnHtM physical physical hys link frame adapter card 5: Dat

Link Layer: Setting the Context  Two physically connected devices:  Host-router, router-router, host-host  Unit of data: frame 5: DataLink Layer 5-3 application transport network link physical network link physical M M M M Ht HnHt Hl HnHt Hl HnHt M frame phys. link data link protocol adapter card

Link layer Services ◆ framing, link access: Encapsulate datagram into frame, adding header, trailer e Channel access if shared medium MAC addresses used in frame headers to identify source, dest Different from ip address! o Reliable delivery between adjacent nodes We learned how to do this already chapter 3) o Seldom used on low bit error link(fiber, some twisted pair) o Wireless links: high error rates Q: why both link-level and end-end reliability? 5: Dat

Link Layer Services  Framing, link access:  Encapsulate datagram into frame, adding header, trailer  Channel access if shared medium  “MAC” addresses used in frame headers to identify source, dest  Different from IP address!  Reliable delivery between adjacent nodes  We learned how to do this already (chapter 3)!  Seldom used on low bit error link (fiber, some twisted pair)  Wireless links: high error rates  Q: why both link-level and end-end reliability? 5: DataLink Layer 5-4

Link Layer Services(more) o Flow Control o Pacing between adjacent sending and receiving nodes o Error detection o Errors caused by signal attenuation, noise o Receiver detects presence of errors Signals sender for retransmission or drops frame o Error correction: o Receiver identifies and corrects bit error(s] without resorting to retransmission o Half-duplex and full-duplex e With half duplex, nodes at both ends of link can transmit but not at same time 5: Dat

Link Layer Services (more)  Flow Control:  Pacing between adjacent sending and receiving nodes  Error Detection:  Errors caused by signal attenuation, noise.  Receiver detects presence of errors:  Signals sender for retransmission or drops frame  Error Correction:  Receiver identifies and corrects bit error(s) without resorting to retransmission  Half-duplex and full-duplex  With half duplex, nodes at both ends of link can transmit, but not at same time 5: DataLink Layer 5-5

Link Layer o5.1 Introduction and o5.6 Hubs and switches services o5.7PPP 5.2 Error detection and correction o 5.mUltiple access protocols o 5.4 Link-Layer Addressing o5.5 Ethernet 5: Dat

Link Layer  5.1 Introduction and services  5.2 Error detection and correction  5.3Multiple access protocols  5.4 Link-Layer Addressing  5.5 Ethernet  5.6 Hubs and switches  5.7 PPP 5: DataLink Layer 5-6

Error detection EDC= Error Detection and Correction bits (redundancy) D Data protected by error checking, may include header fields Error detection not 100% reliable Protocol may miss some errors, but rarely Larger edc field yields better detection and correction datagram datagram all bits in d N OK detected error d data bits- D EDC D EDC bit-error prone link 5: Dat

Error Detection 5: DataLink Layer 5-7 EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • Protocol may miss some errors, but rarely • Larger EDC field yields better detection and correction

Checksumming: Cyclic Redundancy Check View data bits, D, as a binary number Choose r+1 bit pattern( generator , G o Goal: choose r crc bits, R, such that o exactly divisible(polynomial division by g(modulo 2 e Receiver knows G, divides by G. If non-zero remainder: error detected! e Can detect all burst errors less than r+1 bits o Widely used in practice (ATM, HDLC d bits ← r bits bit D: data bits to be sent R: CRC bits, pattern D* 2 XOR R mathematica formula 5: Dat

Checksumming: Cyclic Redundancy Check  View data bits, D, as a binary number  Choose r+1 bit pattern (generator), G  Goal: choose r CRC bits, R, such that  exactly divisible (polynomial division) by G (modulo 2)  Receiver knows G, divides by G. If non-zero remainder: error detected!  Can detect all burst errors less than r+1 bits  Widely used in practice (ATM, HDLC) 5: DataLink Layer 5-8

CRC EXample Want D2r XORR=nG 10101 00 01110000 equivalently. 001 D2r=nG XORR 000 equivalently 1010 001 if we divide d2 by g 110 want remainder r 000 1100 1001 R: remainder 2 1010 001 011 R 5: Dat

CRC Example Want: D.2 r XOR R = nG equivalently: D.2 r = nG XOR R equivalently: if we divide D.2 r by G, want remainder R 5: DataLink Layer 5-9 R = remainder[ ] D.2 r G

Link Layer o5.1 Introduction and o5.6 Hubs and switches services o5.7PPP o 5.2 Error detection and correction ●5.3 Multiple access protocols o 5.4 Link-Layer Addressing o5.5 Ethernet 5: Dat

Link Layer  5.1 Introduction and services  5.2 Error detection and correction  5.3 Multiple access protocols  5.4 Link-Layer Addressing  5.5 Ethernet  5.6 Hubs and switches  5.7 PPP 5: DataLink Layer 5-10