1. Chapter 4
Data Link Layer

2. INFORMATION SYSTEMS ASSOCIATION
ISA Club Meeting
January 26, 2017
6:00pm
CI 1006
Come grab free pizza and help us brainstorm upcoming events and the future direction of ISA

3. Announcements and Outline
Recap
Physical Layer
Circuits
Media
Digital Transmission (Digital Data)
Analog Transmission (Digital Data)
Digital Transmission (Analog Data)
Outline
Data Link Layer
Media Access Control
Error Control
Data Link Protocols 3

4. Network Layers
Computer 1
Computer 2

5. Data Link Layer - Introduction
Responsible for moving messages from
one device to another
Controls the way messages are sent on media
Organizes physical layer bit streams into
coherent messages for the network layer
Major functions of a data link layer protocol
Media Access Control
Error Control
Message Delineation
Network Layer
Data Link Layer
Physical Layer 5

6. Data Link Layer – Ethernet Address6

7. Media Access Control (MAC)
Controlling when and what computer transmit
Why used:
When to use:
Two possible approaches
Controlled access
Contention based access 7

8. Controlled Access
Controlling access to shared resources Commonly used by mainframes (or its front end processor) Also used by some LAN protocols 8

9. Contention
Collisions ? Common Use: Problematic? 9

10. Relative Performance Depends on network conditions Cross-over point
With high traffic, _________ access is better because of the high collisions of __________.
With low traffic, the higher overhead of _________ access makes __________ more efficient.
Cross-over point
Cross-over point used to be 20 devices, but very much depends on devices, protocols, traffic, etc. 10

11. Error Control
Handling of network errors caused by problems in transmission
Network errors
Human errors
Categories of Network Errors
Corrupted
Lost data 11

12. Error Control (Cont.)
Error Rate Burst Error 12

13. Sources of Errors Line noise and distortion – major cause
More likely on electrical media
Undesirable electrical signal
Introduced by equipment and natural disturbances
Degrades performance of a circuit
Manifestation
Extra bits
“Flipped” bits
Missing bits 13

14. Sources of Errors
White Noise
Impulse Noise 14

15. Sources of Errors
Cross Talk
Echo 15

16. Sources of Errors Source Examples
What do you think is a major cause of impulse noise? 16

17. Sources of Errors and Prevention
Source of Error
What causes it
How to prevent it
Line Outages
Faulty equipment, Storms, Accidents (circuit fails)
White Noise
Movement of electrons (thermal energy)
Increase signal strength (increase SNR)
Impulse Noise
Sudden increases in electricity
(e.g., lightning, power surges)
Shield or move the wires
Cross-talk
Multiplexer guard bands are too small or wires too close together
Increase the guard bands, or
move or shield the wires
Echo
Poor connections (causing signal to be reflected back to the source)
Fix the connections, or
tune equipment
Attenuation
Gradual decrease in signal over distance (weakening of a signal)
Use repeaters or amplifiers
Intermodulation Noise
Signals from several circuits combine
Move or shield the wires
Jitter
Analog signals change (small changes in amp., freq., and phase)
Tune equipment
Harmonic Distortion
Amplifier changes phase (does not correctly amplify its input signal)
More important
mostly on analog 17

18. Major Functions of Error Control
Error prevention
Error detection
Parity checks
Cyclic Redundancy Check (CRC)
Error correction 18

19. Error Detection
Sender calculates an Error Detection Value (EDV) and transmits it along with data
Receiver recalculates EDV and checks it against the received EDV
Mathematical
calculations
Mathematical
calculations
DATA
EDV
DATA
EDV
DATA ? =
EDV
EDV 19

20. Error Detection Techniques
Parity checks
Cyclic Redundancy Check (CRC) 20

21. Parity Checking One of the oldest and simplest
A single bit added to each character
Even parity:
Odd parity:
Receiving end recalculates parity bit
If one bit has been transmitted in error the received parity bit will differ from the recalculated one 21

22. Examples of Using Parity
Sending the letter A ( ) using odd parity bit
sender
receiver 22

23. Cyclic Redundancy Check (CRC)
Treats entire message as a number, divides by a preset number and uses the remainder as the check code in the message
Preset number is chosen by sender and receiver to ensure that the remainder is no longer than 16 or 32 bits depending on detection mode.
Detection modes:
CRC-16
CRC-32 23

24. Error Correction Once detected, the error must be corrected
Error correction techniques
Retransmission (or, backward error correction)
Forward Error Correction 24