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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)LLC (Logical Link Control)

MAC (Media Access Control)MAC (Media Access Control)

IEEE 802 Extension to the OSI Model

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The IEEE Working GroupsThe IEEE Working Groups

802.1

802.2

802.3

802.4

802.5

802.6

802.7

802.8

802.9

Networking Overview and Architecture

Logical Link Control

Ethernet

Token Bus

Token Ring

MANs

Broadband

Fiber Optic

Isochronous 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 Technologies

Three common layer 2 technologies:– Ethernet: non-deterministic– Token Ring: deterministic– FDDI: deterministic