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Myungchul Kim mckim@icu.ac.kr

Myungchul Kim mckim@icu.ac.kr. Ch 5. Link layer and Local Area Networks from Ch. 5 of Computer Networking by Jim Kurose and Keith W. Ross, 2003. Data link layer.

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Myungchul Kim mckim@icu.ac.kr

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  1. Myungchul Kim mckim@icu.ac.kr Ch 5. Link layer and Local Area Networksfrom Ch. 5 of Computer Networking by Jim Kurose and Keith W. Ross, 2003.

  2. Data link layer • A transmitting node encapsulates the datagram in a link-layer frame and transmits the frame into the link; and a receiving node receives the frame and extracts the datagram. • Error detection, retransmission, flow control, and random access • A single link in the path • A link-layer protocol includes • Framing • Link access: multiple access problem • Reliable delivery • Flow control: frame buffering capacity • Error detection • Error correction • Half-duplex and full-duplex

  3. Adaptors: network interface cards (NICs) • Fig 5.3 • The link interface is responsible for implementing the link-layer protocol

  4. Multiple access protocol • Point-to-point link: PPP, HDLC • Broadcast link: multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. • Fig 5.9

  5. Multiple access protocols: channel partitioning protocols, random access protocols, and taking-turns protocols. • Channel partitioning protocols • TDM, FDM • Fig 5.10

  6. Code Division Multiple Access (CDMA) • Assigns a different code to each node • Allows different nodes to transmit simultaneously and yet have their respective receivers correctly receive a sender’s encoded data bits in spite of interfering transmissions by other node. • Partitions the codespace • Issues: 1. codes must be carefully chosen, 2. the received signal strengths from various senders at a receiver are the same.

  7. Fig 5.11

  8. Random access protocols: slotted ALOHA, ALOHA, CSMA • Slotted ALOHA • Page 440. • Fig 5.13 • At best only 37 percent of the slots do useful work.

  9. CSMA • Listen before speaking: carrier sensing • If someone else begins talking at the same time, stop talking: collision detection. • CSMA vs CSMA/CD • The longer this propagation delay, the larger the chance that a carrier-sensing node is not yet able to sense a transmission that has already begun at another node in the network. • When a node performs collision detection, it will cease transmission as soon as it detects a collision.

  10. Fig 5.15

  11. Fig 5.16

  12. Taking-turns protocol • Polling protocol • Token-passing protocol

  13. LAN addresses and ARP • LAN address, physical address, Ethernet address, MAC address: six bytes in hexadecimal notation • Fig 5.18 • LAN broadcast address: FF-FF-FF-FF-FF-FF

  14. Address resolution protocol (ARP): an IP address to a LAN address • Fig 5.19 • DNS? • ARP query within a broadcast message and plug-and-play using ARP table (Fig 5.20)

  15. Sending a datagram to a node off the LAN • Fig 5.21

  16. Ethernet • Reasons for success • Deployed early • Simple and cheap • Producing competent versions • Use the same frame structure • Fig 5.23 • Data field (46 to 1500 bytes): carries the IP datagram, MTU

  17. Type fields (2bytes): IP, Novell IPX, AppleTalk, ARP, .. • CRC: detect errors • Preamble (8bytes): 10101010(7times) and 10101011 • Unreliable connectionless service vs IP, TCP, UDP • Baseband transmission • Manchester encoding (physical layer) • Fig 5.24

  18. CSMA/CD mechanisms • An adaptor may begin to transmit at any time; that is, no slots are used. • An adaptor never transmits a frame when it senses that some other adaptor is transmitting; that is, it uses carrier sensing. • A transmitting adapter aborts its transmission as soon as it detects that another adaptor is also transmitting; that is, it uses collision detection. • Before attempting a retransmission, an adaptor waits a random time that is typically small compared with the time to transmit a frame.

  19. 10Base2, 10BaseT, 100BaseT, Gigabit Ethernet • Repeater: a physical-layer device acts on individual bits rather than on frames. • Fig 5.25

  20. A hub is a repeater • Fig 5.26

  21. Hubs • The hub simply broadcasts the bit on all the other interfaces. • The same collision domain • Fig 5.27

  22. Limitations • Larger collision domain • Same Ethernet technologies • Max allowable number of nodes in a collision domain

  23. Bridges • Layer-2 devices • Isolated collision domain, different LAN, no limit on the size of LAN • Filtering and forwarding using bridge table • Fig 5.28

  24. Self-learning: a bridge table is build automatically. • Plug-and-play device • Bridges vs Routers • Plug-and-play or not • Layer 2 or 3 • Broadcast • Flat vs hierarchical addressing • Network size

  25. Fig 5.32

  26. Switches • Bridge (a small number of interfaces) switches (dozens of interfaces) • Full-duplex mode • Fig 5.34 • Neither collision detection nor carrier sending • No medium-access protocol

  27. Cut-through switching: if the buffer becomes empty before the entire packet has arrived, the switch can start to transmit the front of the packet while the back of the packet continues to arrive. • Table 5.1

  28. Wireless Links • Ubiquitous computing • IEEE 802.11b: wireless Ethernet, Wi-Fi • 2.4 GHz • 11 Mbps • Physical layer: Direct Sequence Spread Spectrum (DSSS) • MAC layer • 802.11a: 5-6GHz, 54Mbps • 802.11g: 2.4GHZ, 54Mbps • All of the 802.11 standards have the same architecture and use the same MAC protocol

  29. Basic service set (BSS): a cell, Access point (AP) • ad hoc network • Fig 5.36

  30. 802.11 Media access protocol • An explicit ack from back to the sender • Fig 5.38

  31. No collision detection • Costly • A collision still occur at the receiver, why • Hidden terminal problem and fading • Fig 5.39

  32. To avoid collisions (CSMA/CA) • A duration field indicating the length of time that its frame will be transmitting on the channel, network allocation vector (NAV) • RTS and CTS to reserve access to the channel • CTS frame helps avoid both the hidden station problem and the fading problem • The RTS and CTS frames are short.

  33. Fig 5.40

  34. Bluetooth • 2.45GHz • 721-64kbps • 10 – 100 meter • Replacement of cable • Cf. infrared technology

  35. PPP • Data link layer protocol • Packet framing • Transparency • Multiple network-layer protocols • Multiple types of links • Error detection • Connection liveness • Network-layer address negotiation • Simplicity • Not required to implement • Error correction • Flow control • Sequencing • Multipoint links

  36. PPP data framing • Address and control fields not used • Fig 5.41

  37. Byte stuffing • Forbid the upper-layer protocol from sending data containing the flag field bit pattern. • Control escape byte, 01111101 • Fig 5.43

  38. ATM • characteristics • From an application-level API to the physical layer • CBR, VBR, ABR and UBR • Cell: 5 + 48 bytes • Virtual circuits: virtual channel identifier (VCI) • No retransmission on a link-by-link basis • Congestion control only within the ATM ABR • Run over any physical layer

  39. Fig 5.44 and 5.45

  40. Fig 5.47

  41. IP over ATM • Dynamic vs Permanent virtual channel • pp. 503-504 • Fig 5.52

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