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 • Providing well-defined 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. Services • Unacknowledged connectionless service • Appropriate for • Low error rate  Let higher layer(s) recover errors • Real-time traffic, e.g., speech, video • Most LANs, such as Ethernet • Acknowledged connectionless service • Useful over unreliable channels • Each frame sent individually acknowledged • e.g., wireless systems, e.g. 802.11 (WiFi)

8. 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

9. 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

10. Framing • Approaches • Byte count • Flag bytes with byte stuffing • Flag bits with bit stuffing • Physical layer coding violations

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

12. Flag Bytes with Byte Stuffing • A frame delimited by flag bytes • Four examples of byte sequences before and after stuffing

13. Flag Bits with Bit Stuffing • Each frame begins and ends with special bit pattern (flag byte): 01111110 • Problem: 6 consecutive 1s in data • Solution: Bit Stuffing: inserting a 0 after 5 consecutive 1s • Used in USB Original Data After Stuffing After received and destuffed

14. 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 classic Ethernet • Combination of approaches is possible • E.g., Ethernet and 802.11 use preamble (well-defined pattern) + byte count

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

16. 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?

17. 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

18. 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

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

20. 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

21. Implementation of Physical, Data Link, and Network Layers

22. Data Structures and Primitives

23. Data Structures and Primitives

24. Data Structures and Primitives

25. 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

26. Unrestricted Simplex Protocol - Sender

27. Unrestricted Simplex Protocol - Receiver

28. 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

29. Simplex Stop-and-Wait Protocol - Sender

30. Simplex Stop-and-Wait Protocol - Receiver

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

32. 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?

33. PAR Protocol – Sender

34. PAR Protocol – Sender (Cont’d)

35. PAR Protocol – Receiver