Data and Computer Communications

1. Data and Computer Communications Chapter 7 – Data Link Control Protocols Ninth Edition by William Stallings

2. Data Link Control Protocols • when sending data, to achieve control, a layer of logic is added above the Physical layer • data link control or a data link control protocol • to manage exchange of data over a link: • frame synchronization • flow control • error control • addressing • control and data • link management

3. Types of Error Single-Bit Error Burst Error

4. Note: In a single-bit error, only one bit in the data unit has changed.

5. 10.1Single-bit error

6. Note: A burst error means that 2 or more bits in the data unit have changed.

7. Burst error of length 5

8. Detection methods

9. Even-parity concept

10. Note: In parity check, a parity bit is added to every data unit so that the total number of 1s is even (or odd for odd-parity).

11. Example 1 Suppose the sender wants to send the word world. In ASCII the five characters are coded as 1110111 1101111 1110010 1101100 1100100 The following shows the actual bits sent 11101110 11011110 11100100 11011000 11001001

12. Example 2 Now suppose the word world in Example 1 is received by the receiver without being corrupted in transmission. 11101110 11011110 11100100 11011000 11001001 The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). The data are accepted.

13. Example 3 Now suppose the word world in Example 1 is corrupted during transmission. 11111110 11011110 11101100 11011000 11001001 The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). The receiver knows that the data are corrupted, discards them, and asks for retransmission.

14. Two-dimensional parity

15. Example 4 Suppose the following block is sent: 10101001 00111001 11011101 11100111 10101010 However, it is hit by a burst noise of length 8, and some bits are corrupted. 1010001110001001 11011101 11100111 10101010 When the receiver checks the parity bits, some of the bits do not follow the even-parity rule and the whole block is discarded. 10100011 10001001 11011101 11100111 10101010

16. Note: In two-dimensional parity check, a block of bits is divided into rows and a redundant row of bits is added to the whole block.

17. CRC generator and checker

18. Binary division in a CRC generator

19. Binary division in CRC checker

20. A polynomial representing a divisor

21. A polynomial representing a divisor

22. Standard polynomials

23. Example 6 The CRC-12 x12 + x11 + x3 + x + 1 which has a degree of 12, will detect all burst errors affecting an odd number of bits, will detect all burst errors with a length less than or equal to 12, and will detect, 99.97 percent of the time, burst errors with a length of 12 or more.

24. Checksum : • Checksum • The sender follows these steps: • The unit is divided into k sections, each of n bits. • All sections are added using one’s complement to get the sum. • The sum is complemented and becomes the checksum. • The checksum is sent with the data.

25. Note: • The receiver follows these steps: • The unit is divided into k sections, each of n bits. • All sections are added using one’s complement to get the sum. • The sum is complemented. • If the result is zero, the data are accepted: otherwise, rejected.

26. Example 7 Suppose the following block of 16 bits is to be sent using a checksum of 8 bits. 10101001 00111001 The numbers are added using one’s complement 10101001 00111001 ------------Sum 11100010 Checksum 00011101 The pattern sent is 10101001 00111001 00011101

27. Example 8 Now suppose the receiver receives the pattern sent in Example 7 and there is no error. 10101001 00111001 00011101 When the receiver adds the three sections, it will get all 1s, which, after complementing, is all 0s and shows that there is no error. 10101001 00111001 00011101 Sum 11111111 Complement 00000000means that the pattern is OK.

28. Example 9 Now suppose there is a burst error of length 5 that affects 4 bits. 1010111111111001 00011101 When the receiver adds the three sections, it gets 10101111 11111001 00011101 Partial Sum 1 11000101 Carry 1 Sum 11000110 Complement 00111001the pattern is corrupted.

29. Flow Control • ensure sending entity does not overwhelm receiving entity • prevent buffer overflow • influenced by: • transmission time • time taken to emit all bits into medium • propagation time • time for a bit to traverse the link • assumption is all frames are successfully received with no frames lost or arriving with errors

30. Model of Frame Transmission

31. Stop and Wait • simplest form of flow control • works well for a message sent in a few large frames • stop and wait becomes inadequate if large block of data is split into small frames by source

32. Note: Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment.

33. Normal operation

34. Stop-and-Wait ARQ, lost frame

35. Stop-and-Wait ARQ, lost ACK frame

36. Note: In Stop-and-Wait ARQ, numbering frames prevents the holding of duplicate frames.

37. 11.4Stop-and-Wait ARQ, delayed ACK

38. Sliding Windows Flow Control • allows multiple numbered frames to be in transit • receiver has buffer W long • transmitter sends up to W frames without ACK • ACK includes number of next frame expected • sequence number is bounded by size of field (k) • frames are numbered modulo 2k • giving max window size of up to 2k – 1 • receiver can ACK frames without permitting further transmission (Receive Not Ready) • must send a normal acknowledge to resume • if have full-duplex link, can piggyback ACKs

39. Sliding Window Diagram

40. Sliding Window Example

41. Error Control Techniques • detection and correction of errors such as: • lost frames • -a frame fails to arrive at the other side • damaged frames • -frame arrives but some of the bits are in error

42. Automatic Repeat Request (ARQ) • collective name for error control mechanisms • effect of ARQ is to turn an unreliable data link into a reliable one • versions of ARQ are: • stop-and-wait • go-back-N • selective-reject

43. Stop and Wait ARQ • source transmits single frame • waits for ACK • no other data can be sent until destination’s reply arrives • if frame received is damaged, discard it • transmitter has timeout • if no ACK within timeout, retransmit • if ACK is damaged, transmitter will not recognize • transmitter will retransmit • receiver gets two copies of frame • use alternate numbering and ACK0 / ACK1

44. Stop and Wait ARQ

45. Go-Back-N ARQ • most commonly used error control • based on sliding-window • use window size to control number of outstanding frames • if no error, ACK as usual • if error, reply with rejection • destination will discard that frame and all future frames until frame in error is received correctly • transmitter must go back and retransmit that frame and all subsequent frames

46. Go Back N - Handling • Damaged frame • error in frame i so receiver rejects frame i • transmitter retransmits frames from i • Lost frame • frame i lost and either • transmitter sends i+1 andreceiver gets frame i+1 out of sequence and rejects frame i • or transmitter times out and sends ACK with P bit set which receiver responds to with ACK i • transmitter then retransmits frames from i

47. Go Back N - Handling

48. Go-Back-N ARQ, normal operation

49. Go-Back-N ARQ, lost frame

50. Note: In Go-Back-N ARQ, the size of the sender window must be less than 2^m; the size of the receiver window is always 1.