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Chabot CollegeChabot College

ELEC 99.05ELEC 99.05Layer 2 - HighlightsLayer 2 - Highlights

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Layer 1 LimitationsLayer 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

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Layer 2Layer 2

Layer 2 solves these problems.

2

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Data Link SublayersData Link Sublayers

LLC (Logical Link Control)MAC (Media Access Control)

IEEE 802 Extension to the OSI Model

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The IEEE Working GroupsThe IEEE Working Groups802.1802.2802.3802.4802.5802.6802.7802.8802.9

Networking Overview and ArchitectureLogical Link ControlEthernetToken BusToken RingMANsBroadbandFiber OpticIsochronous LAN

...and more!

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OSI v IEEEOSI v IEEE

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Logical Link Control (LLC)Logical Link Control (LLC)

• Defined by a committee named 802.2• Is technology independent• Is not used by all networks

What is it?

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Logical Link ControlLogical 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.

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802.2 LLC802.2 LLC

IPX IP APPLE-TALK

LLC

10BaseT Ethernet

Layer 3Layer 3

Layer 2 - LLCLayer 2 - LLC

Layer 2- MAC Layer 2- MAC &Layer 1&Layer 1

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802.2 LLC802.2 LLC

Ethernet Token Ring

FDDI

LLC

IPLayer 3Layer 3

Layer 2 - LLCLayer 2 - LLC

Layer 2- MAC Layer 2- MAC &Layer 1&Layer 1

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Media Access Control (MAC)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

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The MAC AddressThe MAC Address

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MAC Addresses Are FlatMAC Addresses Are Flat

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FramingFraming

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

to the bitstream.

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Frame FormatFrame Format

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FCS – Frame ChecksumFCS – 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.

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Media Access ControlMedia Access Control

Media Access Control methods can be divided into two flavors:– Deterministic– Non-deterministic

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Deterministic Media AccessDeterministic Media Access

• aka “Taking Turns”– example: Token Passing used in Token

Ring– No collisions

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Non-DeterministicNon-Deterministic

• “First Come, First Served”– Example: CSMA/CD used in Ethernet– Carrier Sense Multiple Access with

Collision Detection

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Layer 2 TechnologiesLayer 2 TechnologiesThree common layer 2 technologies:– Ethernet: non-deterministic– Token Ring: deterministic– FDDI: deterministic