CS 313 Introduction to Computer Networking & Telecommunication

1. CS 313 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 • 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 • 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? • Trust no time!! • Approaches • 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): 01111110 • 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 2: data lost Sender Receiver • Case 1: no error 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 • 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 • 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