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OSI data link layer

OSI data link layer. Application. HTTP, FTP, TFTP, SMTP etc. Data stream. Presentation. Application. Session. Transport. Transport. TCP, UDP. Segment. Network. Internet. IP. Packet. Data link. Ethernet, WAN technologies. Frame. Network Access. Physical. Bits.

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OSI data link layer

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  1. OSI data link layer Jaringan Komputer_D3 TT

  2. Application HTTP, FTP, TFTP, SMTP etc Data stream Presentation Application Session Transport Transport TCP, UDP Segment Network Internet IP Packet Data link Ethernet, WAN technologies Frame Network Access Physical Bits OSI data link layer • OSI model layer 2 • TCP/IP model part of Network Access layer Jaringan Komputer_D3 TT

  3. Data link layer topics • Data Link layer protocols • Preparing data for transmission • Media access control methods • Logical network topologies • Encapsulating packets into frames • Layer 2 frame structure and header and trailer fields Jaringan Komputer_D3 TT

  4. Functions of data link layer • Encapsulates packets by adding a frame header and trailer including appropriate addressing. • Controls access to the transmission medium. Jaringan Komputer_D3 TT

  5. Hops • There may be a different layer 2 protocol in use on each hop of a journey. • Different media, different types of link, different bandwidths, LAN/WAN affect the choice of protocol. • Different protocols have different frames. • The router removes the old frame and adds a new header and trailer for the next hop. Jaringan Komputer_D3 TT

  6. Sets up the frame header and trailer to encapsulate the packet.Identifies network layer protocol. Network Adds layer 2 address.Marks frame start and end. Physical Sublayers Logical link control Data link Media access control Jaringan Komputer_D3 TT

  7. Standards Institute of Electrical and Electronics Engineers • IEEE 802.2 Logical link control • IEEE 802.3 Ethernet • IEEE 802.5 Token ring • IEEE 802.11 Wi-fi International Telecommunication Union (ITU) • Various WAN standards: HDLC, ISDN, Frame relay Jaringan Komputer_D3 TT

  8. Point to point link • Only two devices on the network • Full duplex: both can send at the same time, no problem with media access • Half duplex: data can only travel one way at a time so one device can send at a time. Simple media access control. Jaringan Komputer_D3 TT

  9. Shared medium Physical bus Star with hub • Needs media access control. • If there is no control there will be many collisions and the frames will be destroyed. Jaringan Komputer_D3 TT

  10. Controlled media access • Predictable, deterministic. • Each device is given a time when it may send, and it most not send at any other time. • High overhead. • No collisions. • Token passing – each host in turn gets the token and is allowed to send. • E.g. token ring, FDDI Jaringan Komputer_D3 TT

  11. Contention based media access • Non-deterministic, first come first served. • Each device “listens” and sends when the medium seems to be clear. • Low overhead. • Collisions occur. • Need a way of re-sending lost frames. • Becomes inefficient on large networks. • E.g. traditional Ethernet. Jaringan Komputer_D3 TT

  12. Variation on contention based • Traditional Ethernet uses CSMA/CD (collision detection): collisions are allowed and detected, frames sent again. • Wi-fi uses CSMA/CA (collision avoidance): when the medium is clear, host sends signal to say it is about to use the medium. It then sends. Jaringan Komputer_D3 TT

  13. Different environments • Fragile environment e.g. satellite link – frames are likely to be lost – need large overhead of control mechanisms to make sure data arrives. • Protected environment e.g. modern LAN – frames not often lost – do not need such elaborate control mechanisms • Therefore need different layer 2 protocols Jaringan Komputer_D3 TT

  14. Addressing needs • Point to point link – only one possible destination. Minimal addressing. • Multi-access network – need full addressing system. • Therefore need different layer 2 protocols. Jaringan Komputer_D3 TT

  15. Layer 2 frame format • All protocols have the same general form but there are variations. Jaringan Komputer_D3 TT

  16. PPP frame • Point to point links. Minimal addressing. Control mechanisms. Start Minimal address Check and stop Packet Jaringan Komputer_D3 TT

  17. Ethernet frame • Multi-access links. Full addressing. No control field. • Same for all Ethernet types/bandwidths. Timing and start Addresses48 bits each Check and stop Layer 3 protocol Packet Jaringan Komputer_D3 TT

  18. 802.11 Wi-Fi • LAN wireless protocol • Fragile environment – lots of interference, risk of lost frames, contention. • Every transmission needs to be acknowledged. • No acknowledgement – re-send frame. • Lots of control mechanisms in frame. Jaringan Komputer_D3 TT

  19. End to end • PC sends packet to server • Packet header has PC IP address and source and server IP address as destination. • Frame header has PC MAC address as source and router MAC address as destination. Jaringan Komputer_D3 TT

  20. Within a LAN • Hubs and switches link hosts Jaringan Komputer_D3 TT

  21. Between networks • Routers link networks together and act as gateways between them. Jaringan Komputer_D3 TT

  22. Hub • Frame comes in. Hub regenerates it and forwards it through all ports except incoming port. • Shared medium, shared bandwidth. Hosts are in the same collision domain. • Cheap. • For small LANs only. Jaringan Komputer_D3 TT

  23. Switch • Frame comes in. Switch regenerates it and forwards it to destination only. • Segments network into separate collision domains. • More expensive but better performance than hub Jaringan Komputer_D3 TT

  24. Thank You Ref: S Ward Abingdon and Witney College Jaringan Komputer_D3 TT

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