Ch05 data link

MẠNG MÁY TÍNH (COMPUTER NETWORKING) J.F Kurose and K.W Ross Computer Networking A Top Down Approach 6th edition, Addision Wesley, 2013 Chapter 5: DATA LINK LAYER Introduction and Services Error detection and correction Elementary Data Link Protocols Sliding Window Protocols Multiple access protocols Link-layer Addressing PPP 1.3 Link layer: Introduction  Some terminology:  Host and router are nodes  Communication channels that connect adjacent nodes along communication path are links   Wired links  Wireless links  LANs Packet is a frame, ancapsulates datagram Data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 1.4  Datagram transferred by Transportation analogy different link protocols over different links   Trip from Princeton to Lausanne ex: Ethernet on first link, fram relay on intermediate links, 802.11 on last link  Each link protocol provides Limo: Princeton to JFK  Plane: JFK to Geneva  Train: Geneve to Lausanne  Tourist = datagram different services   ex: may or may not provide rdt over link  Transport segment = communication link  Transportation mode = link layer protocol  Travel agent = routing algorithm 1.5 Link layer services (1/2)  Framing, link access:  Encapsulate datagram into frame, adding header, trailer  Channel access if share medium  “MAC” addresses used in frame headers to identify source, destination  Different from IP address!  Reliable delivery between adjacent nodes:  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? 1.6  Framing (1/3) A character stream (a) Without errors (b) With one error 1.7  Framing (2/3) (a) A frame delimited by flag bytes (b) Four examples of byte sequences before and after stuffing 1.8  Framing (3/3) Bit stuffing (a) The original data (b) The data as they appear on the line (c) The data as they are stored in receiver’s memory after destuffing 1.9 Link layer services (2/2)  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 retransmision  Half-duplex and full-duplex  With half-duplex, nodes at both ends of link can transmit, but not at the same time 1.10 “Taking Turns” MAC Protocols 1.47 “Taking Turns” MAC Protocols 1.48 Summary of MAC protocols 1.49 Introduction and Services Error detection and correction Elementary Data Link Protocols Sliding Window Protocols Multiple access protocols Link-layer Addressing PPP 1.50 MAC addresses and ARP   32-bit IP address:  network-layer address  used to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address:  function: get frame from one interface to another physicallyconnected interface (same network)  48 bit MAC address (for most LANs)  burned in NIC ROM, also sometimes software settable 1.51 LAN addresses and ARP  MAC address allocation administered by IEEE  manufacturer buys portion of MAC address space (to assure uniqueness)  analogy:  (a) MAC address: like Social Security Number  (b) IP address: like postal address  MAC flat address ➜ portability  can move LAN card from one LAN to another  IP hierarchical address NOT portable  address depends on IP subnet to which node is attached 1.52 LAN addresses and ARP 1.53 ARP: Address Resolution Protocol 1.54 ARP protocol: routing in Same LAN (network) 1.55 Addressing: routing to Another LAN 1.56 Addressing: routing to another LAN 1.57 Introduction and Services Error detection and correction Elementary Data Link Protocols Sliding Window Protocols Multiple access protocols Link-layer Addressing PPP 1.58 PPP – Point to Point Protocol The PPP full frame format for unnumbered mode operation 59 1.59 PPP – Point to Point Protocol (2) A simplified phase diagram for bring a line up and down 60 1.60 PPP – Point to Point Protocol (3) The LCP frame types 61 1.61