Chabot College
ELEC 99.05
Layer 2 - Highlights
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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
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Layer 2
Layer 2 solves these problems.
2
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Data Link Sublayers
LLC (Logical Link Control)
MAC (Media Access Control)
IEEE 802 Extension to the OSI Model
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The IEEE Working Groups
802.1 Networking Overview and Architecture
802.2 Logical Link Control
802.3 Ethernet
802.4 Token Bus
802.5 Token Ring
802.6 MANs
802.7 Broadband
802.8 Fiber Optic
802.9 Isochronous LAN
...and more!
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OSI v IEEE
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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 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 LLC
Layer 3
IPX
IP
APPLETALK
Layer 2 - LLC
LLC
Layer 2- MAC
&Layer 1
10BaseT Ethernet
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802.2 LLC
IP
Layer 3
LLC
Layer 2 - LLC
Layer 2- MAC
&Layer 1
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Ethernet
Token
Ring
FDDI
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 Address
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MAC Addresses Are Flat
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Framing
• A message is “framed” at layer two.
• Framing provides order, or structure, to
the bitstream.
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Frame Format
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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.
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Media Access Control
Media Access Control methods can be
divided into two flavors:
– Deterministic
– Non-deterministic
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Deterministic Media Access
• aka “Taking Turns”
– example: Token Passing used in Token
Ring
– No collisions
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Non-Deterministic
• “First Come, First Served”
– Example: CSMA/CD used in Ethernet
– Carrier Sense Multiple Access with
Collision Detection
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Layer 2 Technologies
Three common layer 2 technologies:
– Ethernet: non-deterministic
– Token Ring: deterministic
– FDDI: deterministic
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