西南师范大学：《计算机网络》课程教学资源（PPT课件讲稿，英文版）Chapter 5 Data Link Layer

Chapter 5 Data link Layer Computer Networking A Top Down Approach Featuring the Internet The powerpoint slides are based on the material 2nd edition provided by Jim Kurose, Keith Ros J F Kurose and K.W. Ross Addison-Wesley, Juf 2002 5: DataLink Layer 5a-1

5: DataLink Layer 5a-1 Chapter 5 Data Link Layer Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002. The PowerPoint Slides are based on the material provided by J.F Kurose and K.W. Ross

Chapter 5: The data link layer Our goals O understand principles behind data link layer services: o error detection correction o sharing a broadcast channel: multiple access o link layer addressing o reliable data transfer, flow control: done! D instantiation and implementation of various link lyer technologies 5: DataLink Layer 5a-2

5: DataLink Layer 5a-2 Chapter 5: The Data Link Layer Our goals:  understand principles behind data link layer services:  error detection, correction  sharing a broadcast channel: multiple access  link layer addressing  reliable data transfer, flow control: done!  instantiation and implementation of various link layer technologies

Chapter 5 outline 0 5. 1 Introduction and 0 5.6 Hubs bridges, and services switches 0 5.2 Error detection g 5.7 Wireless links and and correction LANS 0 5. 3 Multiple access 05. 8 PPP protocols 059ATM 0 5.4 LAN addresses o 5.10 Frame Relay and Arp 0 5.5 Ethernet 5: DataLink Layer 5a-3

5: DataLink Layer 5a-3 Chapter 5 outline  5.1 Introduction and services  5.2 Error detection and correction  5.3Multiple access protocols  5.4 LAN addresses and ARP  5.5 Ethernet  5.6 Hubs, bridges, and switches  5.7 Wireless links and LANs  5.8 PPP  5.9 ATM  5.10 Frame Relay

Link Layer: Introduction link" Some terminology: a hosts and routers are nodes (bridges and switches too 0 communication channels that connect adjacent nodes along communication path are links o wired links o wireless links LANS 口2 PDU is a frame encapsulates datagram data-link layer has responsibility of transferring datagram from one node to ad jacent node over a link 5: DataLink Layer 5a-4

5: DataLink Layer 5a-4 Link Layer: Introduction Some terminology:  hosts and routers are nodes (bridges and switches too)  communication channels that connect adjacent nodes along communication path are links  wired links  wireless links  LANs  2-PDU is a frame, encapsulates datagram “link” data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

Link layer: context o Datagram transferred by transportation analo different link protocols o trip from princeton to over different links: Lausanne o limo: Princeton to JFK o e.g., Ethernet on first link, frame relay on o plane: JFK to Geneva intermediate links 802 11 o train: Geneva to lausanne on last link 口 fourist= datagram D Each link protoco O transport segment provides different communication link services 口 transportation mode e. g, may or may not provide rdt over link link layer protocol 口 travel agent= routing algorithm 5: DataLink Layer 5a-5

5: DataLink Layer 5a-5 Link layer: context  Datagram transferred by different link protocols over different links:  e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link  Each link protocol provides different services  e.g., may or may not provide rdt over link transportation analogy  trip from Princeton to Lausanne  limo: Princeton to JFK  plane: JFK to Geneva  train: Geneva to Lausanne  tourist = datagram  transport segment = communication link  transportation mode = link layer protocol  travel agent = routing algorithm

Link Layer Services 口 Framing, link access: encapsulate datagram into frame adding header trailer channel access if shared medium o physical addresses' used in frame headers to identify source. dest different from IP address D Reliable delivery between adjacent nodes o 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: DataLink Layer 5a-6

5: DataLink Layer 5a-6 Link Layer Services  Framing, link access:  encapsulate datagram into frame, adding header, trailer  channel access if shared medium  ‘physical 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?

Link Layer Services(more g Flow Control: o pacing between ad jacent sending and receiving nodes 口 Error Detection: o errors caused by signal attenuation, noise o receiver detects presence of errors: signals sender for retransmission or drops frame 口 Error Correction o receiver identifies and corrects bit error(s without resorting to retransmission o Half-duplex and full-duplex o with half duplex, nodes at both ends of link can transmit but not at same time 5: DataLink Layer 5a-7

5: DataLink Layer 5a-7 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

Adaptors Communicating datagram roving sending link layer protocol node node frame frame adapter adapter o link layer implemented in o receiving side adaptor"(aka NIC) looks for errors, rdt flow o Ethernet card, PCmcI control etc card. 802.11 card o extracts datagram, passes 口 sending side to roving node o encapsulates datagram in D adapter is sem a frame autonomous o adds error checking bits, o link& physical layers rdt, flow control, etc 5: DataLink Layer 5a-8

5: DataLink Layer 5a-8 Adaptors Communicating  link layer implemented in “adaptor” (aka NIC)  Ethernet card, PCMCI card, 802.11 card  sending side:  encapsulates datagram in a frame  adds error checking bits, rdt, flow control, etc.  receiving side  looks for errors, rdt, flow control, etc  extracts datagram, passes to rcving node  adapter is semi￾autonomous  link & physical layers sending node frame rcving node datagram frame adapter adapter link layer protocol

Chapter 5 outline g 5.1 Introduction and 口56Hubs, bridges,and services switches 0 5.2 Error detection g 5.7 Wireless links and and correction LANS 0 5. 3 Multiple access 05. 8 PPP protocols 059ATM 0 5.4 LAN addresses o 5.10 Frame Relay and Arp 0 5.5 Ethernet 5: DataLink Layer 5a-9

5: DataLink Layer 5a-9 Chapter 5 outline  5.1 Introduction and services  5.2 Error detection and correction  5.3Multiple access protocols  5.4 LAN addresses and ARP  5.5 Ethernet  5.6 Hubs, bridges, and switches  5.7 Wireless links and LANs  5.8 PPP  5.9 ATM  5.10 Frame Relay

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 a bits in D N ○K ? error ← d data bits D EDC D EDC o bit-error prone link 0 5: DataLink Lo 5a-10

5: DataLink Layer 5a-10 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