1. Chapter 4
Data Link Layer

2. Announcements and Outline
Review Assignment Due Weds., 2/1 (Before Class!)
Assessment #1 – Mon., 2/6 MC
Short Answer
Outline
Data Link Layer
Media Access Control
Error Control
Data Link Protocols 2

3. Network Layers
Computer 1
Computer 2

4. 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 4

5. Recap – Media Access Control and Error Detection
Contention
Error Control
Source of errors
Error Detection
Parity Checks
CRC-16 & CRC-32 5

6. Recap – Media Access Control and Error Detection6

7. Recap – Media Access Control and Error Detection7

8. Error Correction Once detected, the error must be corrected
Error correction techniques
Retransmission (or, backward error correction)
Forward Error Correction 8

9. Automatic Repeat reQuest (ARQ)
Process of requesting a data transmission be resent
Main ARQ protocols
Stop and Wait ARQ (A half duplex technique)
Continuous ARQ (A full duplex technique) 9

10. Stop and Wait ARQ
Sender
Receiver 10

11. Stop and Wait ARQ – Timeouts
Sender
Receiver 11

12. Continuous ARQ 12

13. Flow Control with ARQ
Ensuring that sender is not transmitting too quickly for the receiver
Stop-and-wait ARQ
Continuous ARQ 13

14. Flow Control Example 14

15. Data Link Protocols Classification Differ by Asynchronous transmission
Message delineation
Frame length
Frame field structure
frame k-1
frame k
frame k+1 15

16. Synchronous Transmission
Data sent in a large block
Includes addressing information
Includes a series of synchronization (SYN) characters
Used to help the receiver recognize incoming data
Synchronous transmission protocols categories
Bit-oriented protocols: SDLC, HDLC
Byte-count protocols: Ethernet
Byte-oriented protocols: PPP 16

17. Ethernet (IEEE 802.3ac & II – Ethernet 2)
Most widely used LAN protocol, developed jointly by Digital, Intel, and Xerox, now an IEEE standard
Uses contention based media access control
Byte-count data link layer protocol
No transparency problem
uses a field containing the number of bytes (not flags) to delineate frames
Error correction: optional 17

18. Ethernet II Frame 18

19. Transmission Efficiency
An objective of the network:
Move as many bits as possible with minimum errors
 higher efficiency and lower cost
Factors affecting network efficiency:
Characteristics of circuit (error rate, speed)
Speed of equipment, Error control techniques
Protocol used
Information bits (carrying user information)
Overhead bits ( used for error checking, frame delimiting, etc.) 19

20. Transmission Efficiency
Total number of info bits to be transmitted
Total number of bits transmitted = 20

21. Transmission Efficiency of Protocols
Ethernet II Transmission
However, large packets likely to have more errors and are more likely to require retransmission  wasted capacity 21

22. Optimum Packet Size Trade-off between packet size and throughput
Acceptable range
(more costly in terms of circuit capacity to retransmit if there is an error)
(less likely to contain errors) 22

23. Throughput A more accurate definition of efficiency
Total number of information bits received per second; takes into account:
Overhead bits (as in transmission efficiency)
Need to retransmit packets containing errors
Complex to calculate; depends on:
Transmission efficiency
Error rate
Number of retransmission
Transmission Rate of Information Bits (TRIB)
Used as a measurement of throughput 23

24. TRIB = Number of info bits accepted
Total time required to get the bits
Average number of non-info characters per block
Ex:
K=7 bits/character
M = 400 char/block
R= 4.8 Kb/s
C = 10 char/block
P = 1%
T = 25 ms
Probability that a block will require retransmission
Info bits per character
K (M – C) (1 – P)
(M / R) + T
TRIB =
Time between blocks (in seconds) (propagation time + turnaround time) (a.k.a., reclocking time)
7(400-10)(1-0.01)
(400/600)+0.025)
= Kb/s
Packet length in characters
TRIB =
Data xmission rate in char per second 24

25. Implications for Management
Provide a few, widely used data link layer protocols for all networks
Minimize costly customization
Minimize costly translation among many protocols
Less training, simpler network management
Bigger pool of available experts
Less expensive, off-the-shelf equipment 25

26. Next Week:
Monday – Assessment #1 Start Chapter 5 – Network and Transport Layers 26