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
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!

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 LLC 10BaseT Ethernet Layer 3 Layer 2 - LLC
APPLE-TALK
Layer 3
LLC
Layer 2 - LLC
Layer 2- MAC &Layer 1
10BaseT Ethernet

10. 802.2 LLC IP LLC Layer 3 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