1. CS412 Introduction to Computer Networking & Telecommunication
Data Link Layer Part I – Designing Issues and Elementary Protocols
Chi-Cheng Lin, Winona State University

2. Topics Introduction Framing Error Control Flow Control
Elementary Data Link Protocols

3. Introduction
Algorithms to achieve reliable, efficient communication between two adjacent machines at the layer
Adjacent:
Physically connected by a communication channel

4. Introduction Issues Services to network layer Framing Error control
Determining how bits are grouped into frames
Error control
Flow control

5. Functions of the Data Link Layer
Relationship between packets and frames.

6. Services to Network Layer
Transferring data between network layers of machines

7. Placement of Data Link Protocol

8. Services Unacknowledged connectionless service
Appropriate for
Low error rate, higher layers recover errors
Real-time traffic
e.g., speech, video
Most LANs
Acknowledged connectionless service
Useful over unreliable channels
e.g., wireless systems

9. Services Acknowledged connection-oriented service Guarantees
Each frame sent is received without error
All frames sent are received in right order
 Network layer always receives correct packets in the order in which the packets were sent
Three phases:
Connection establishment
Variables and counters initialization
Frame transmission
Connection release
Variables, buffers, resources freed up

10. Framing Fact Data link layer detects/corrects errors
Raw bit stream delivered by physical layer is not error free
Data link layer detects/corrects errors
Framing
Computing checksum
Handling error if any

11. Framing How about inserting time gaps? Approaches Trust no time!!
Character count
Starting/ending character w/ character stuffing
Starting/ending flags w/ bit stuffing
Physical layer coding violations

12. Character Count
A field in header specifies number of characters in a frame.
Problem?

13. Starting/Ending Characters w/ Character Stuffing
A frame delimited by flag bytes.
Four examples of byte sequences before and after stuffing.

14. Starting/Ending Flags w/ bit Stuffing
Flag byte (special bit pattern):
Problem: 6 consecutive 1s in data
Solution: Bit Stuffing: Inserting a 0 after 5 consecutive 1s
Original
Data
After Stuffing
After received
and destuffing

15. Physical Layer Coding Violations
Encoding on physical medium
Framing by invalid physical code
Use some code that does not represent 0 nor 1
Example
Manchester encoding in 802 LAN standard (Ch. 4)
Combination of approaches is possible
E.g., character count + one other method

16. Error Control Using acknowledgement
Positive
Negative
Problem: In some cases, sender waits for acknowledgement forever
Solution: Timer
Problem: Duplicate transmission
Solution: Sequence number

17. Positive Acknowledgement
Sender sends a message, waits for acknowledgement from receiver, and then sends next message
There’s no free lunch, though
 overhead, delay
How does it work?

18. Reliability and Acknowledgement
Case 1: no error
Sender Receiver
Case 2: data lost
Sender Receiver
Time
Data
Time
Data X
Timeout
Ack.
Data
Ack.
Timeout and retransmission

19. Reliability and Acknowledgement
Case 3: data error
Sender Receiver
Case 4: ack. lost
Sender Receiver
Time
Data
Time
Data
Error
Timeout
Timeout X
Data
Data
Ack.
Ack.
Timeout and retransmission
New problem? Duplicate
Solution: Sequence number

20. Flow Control Needed Problem Solution
When frames are transmitted faster than receiver can accept, frames will be lost
Solution
Flow control by feedback mechanism

21. Elementary Data Link Protocols
Key Assumptions
Network, data link, and physical layers are independent processes communicating by sending messages
Machine A wants to send a long stream of data to machine B over a reliable, connection-oriented service

22. Data Structures and Primitives
Just think of it as a class containing data only…

24. Unrestricted Simplex Protocol
Utopia protocol
Assumptions
Unidirectional data transmission
Transmitting/receiving network layers are always ready
Processing time is ignored
Infinite buffer space
No errors

25. Unrestricted Simplex Protocol - Sender

26. Unrestricted Simplex Protocol - Receiver

27. Simplex Stop-and-Wait Protocol
Assumptions
Unidirectional data transmission
Transmitting/receiving network layers are always ready
Finite processing speed
Finite buffer capacity
No errors
Problem: Sender sends too fast
Stop-and-wait
Senders sends one frame and then waits for an acknowledgement before processing

28. Simplex Stop-and-Wait Protocol - Sender

29. Simplex Stop-and-Wait Protocol - Receiver

30. Simplex PAR Protocol For noisy channel
Positive acknowledgement w/ retransmission
Sender waits for a positive acknowledgement before advancing to the next data item
A. k. a. ARQ (Automatic Repeat reQuest)

31. PAR Protocol Assumptions Timer + sequence number
Unidirectional data transmission
Transmitting/receiving network layers are always ready
Finite processing speed
Finite buffer capacity
Errors, can be detected
Timer + sequence number
Size (i.e., # bits) of sequence number?

32. PAR - Sender

33. PAR - Receiver