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Chabot College

Chabot College. ELEC 99.05 Layer 2 - Highlights. Layer 1 Limitations. No way to communicate with upper layers No addressing No logical grouping or organization of bits (framing) No method to control media access. Layer 2. Layer 2 solves these problems. 2. Data Link Sublayers.

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Chabot College

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  1. Chabot College ELEC 99.05 Layer 2 - Highlights

  2. Layer 1 Limitations • No way to communicate with upper layers • No addressing • No logical grouping or organization of bits (framing) • No method to control media access

  3. Layer 2 Layer 2 solves these problems. 2

  4. Data Link Sublayers LLC (Logical Link Control) MAC (Media Access Control) IEEE 802 Extension to the OSI Model

  5. The IEEE Working Groups 802.1 Networking Overview and Architecture 802.2 Logical Link Control Ethernet 802.3 802.4 Token Bus 802.5 Token Ring 802.6 MANs 802.7 Broadband 802.8 Fiber Optic 802.9 Isochronous LAN ...and more!

  6. OSI v IEEE

  7. Logical Link Control (LLC) • Defined by a committee named 802.2 • Is technology independent • Is not used by all networks What is it?

  8. Logical Link Control • Provides independence to the protocols running in the upper and lower layers. • The LLC acts as a managing buffer between the “executive” upper layers and the “shipping department” lower layers. • Communicates up and down.

  9. 802.2 LLC IPX IP APPLE-TALK Layer 3 LLC Layer 2 - LLC Layer 2- MAC &Layer 1 10BaseT Ethernet

  10. 802.2 LLC IP Layer 3 LLC Layer 2 - LLC Layer 2- MAC &Layer 1 Ethernet Token Ring FDDI

  11. Media Access Control (MAC) • Responsible for the actual framing • builds the 1s and 0s to hand off to the physical layer. • Responsible for media access: • Contention • Token Passing • Polling

  12. The MAC Address

  13. MAC Addresses Are Flat

  14. Framing • A message is “framed” at layer two. • Framing provides order, or structure, to the bitstream.

  15. Frame Format

  16. FCS – Frame Checksum • Used to insure that the data has arrived without corruption. • More efficient than sending the data twice and comparing the results. • Necessary to prevent errors.

  17. Media Access Control Media Access Control methods can be divided into two flavors: • Deterministic • Non-deterministic

  18. Deterministic Media Access • aka “Taking Turns” • example: Token Passing used in Token Ring • No collisions

  19. Non-Deterministic • “First Come, First Served” • Example: CSMA/CD used in Ethernet • Carrier Sense Multiple Access with Collision Detection

  20. Layer 2 Technologies Three common layer 2 technologies: • Ethernet: non-deterministic • Token Ring: deterministic • FDDI: deterministic

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