Chi-Cheng Lin, Winona State University CS 313 Introduction to Computer Networking & Telecommunication Data Link Layer Part I – Designing Issues and Elementary Protocols

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

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

4 Introduction l 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 l Relationship between packets and frames.

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

7 Services l 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 l Acknowledged connectionless service  Useful over unreliable channels  Each frame sent individually acknowledged  e.g., wireless systems, e.g (WiFi)

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

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

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

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

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

14 Physical Layer Coding Violations l Encoding on physical medium l Framing by invalid physical code  Use some code that does not represent 0 nor 1 l Example  Manchester encoding in classic Ethernet l Combination of approaches is possible  E.g., Ethernet and use preamble (well-defined pattern) + byte count

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

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

17 Reliability and Acknowledgement l Case 1: no error SenderReceiver TimeData Ack. l Case 2: data lost Sender Receiver TimeData Ack. X Timeout Timeout and retransmission

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

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

20 Elementary Data Link Protocols l 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

Implementation of Physical, Data Link, and Network Layers 21

22 Data Structures and Primitives

23 Data Structures and Primitives

24 Data Structures and Primitives

25 Unrestricted Simplex Protocol l Utopia protocol l 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 l Assumptions  Unidirectional data transmission  Transmitting/receiving network layers are always ready  Finite processing speed  Finite buffer capacity  No errors l Problem: Sender sends too fast l 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 l For noisy channel l Positive acknowledgement w/ retransmission (PAR) l Sender waits for a positive acknowledgement before advancing to the next data item l A. k. a. ARQ (Automatic Repeat reQuest)

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

33 PAR Protocol – Sender

34 PAR Protocol – Sender (Cont’d)

35 PAR Protocol – Receiver