10 장 . 데이터 링크 제어 (Data Link Control)

1. 10장. 데이터 링크 제어(Data Link Control) 10.1 Line Discipline 10.2 흐름 제어(Flow Control) 10.3 에러 제어(Error Control) 10.4 요약

2. 데이터 링크 제어(Data Link Control) • Data Link Layer

3. 데이터 링크 제어(Data Link Control) • Data Link Layer functions

4. 10.1 Line Discipline • Line discipline answers the question, who should send now? • can be done in two ways ① ENQ/ACK(Enquiry/acknowledgment) ② poll/select

5. Line Discipline(cont’d) • Line discipline categories • ENQ/ACK : peer-to-peer communication • Poll/Select : primary-secondary communication

6. Line Discipline(cont’d) • ENQ/ACK

7. Line Discipline(cont’d) • How it work

8. Line Discipline(cont’d) • Poll/Select

9. Line Discipline(cont’d) • How it works • polling : If the primary want to receive data, it asks the secondaries if they have anything to send. • select : If primary want to send data, it tells the target secondary to get ready to receive.

10. Line Discipline(cont’d) • Address ~ identify each frame as being either to or from a specific device on the link (address field or header on the protocol) • P  S : recipient of the data • S  P : originator of the data

11. Line Discipline(cont’d) • Select ~ is used whenever the primary device has something to send(select(SEL) frame)

12. Line Discipline(cont’d) • Poll ~ is used by the primary device to solicit transmissions from the secondary devices

13. 10.2 Flow Control ~ refers to a set of procedures used to restrict the amount of data the sender can send before waiting for acknowledgment

14. Flow Control(cont’d) • Stop and Wait

15. Flow Control(cont’d) • Stop-and-Wait ~ the sender sends one frame and waits for an acknowledgment before sending the next frame • Advantage : simplicity • Disadvantage : inefficiency

16. Flow Control(cont’d) • Sliding window ~ several frames can be in transit at a time

17. Flow Control(cont’d) • Sender window

18. Flow Control(cont’d) • Receiver window

19. Flow Control(cont’d) • Example

20. 10. 3 Error Control ~ refers primarily to methods of error detection and retransmission • ARQ(Automatic Repeat Request) ~ means retransmission of data in three cases : damaged frame, lost frame, and lost acknowledgment

21. Error Control(cont’d) • Categories of error control

22. Error Control(cont’d) • Stop-and-Wait ARQ ~ For retransmission to work, four features are added to the basic flow control mechanism. • sending device keeps a copy of the lost from transmitted • data frame and ACK frame are numbered alternately 0 and 1 • NAK frame(not numbered) • timer(sending device)

23. Error Control(cont’d) • Damaged Frames

24. Error Control(cont’d) • Lost Data Frame

25. Error Control(cont’d) • Lost Acknowledgment

26. Error Control(cont’d) • Sliding Window ARQ • go-back-n ARQ • selective-reject ARQ

27. Error Control(cont’d) • Three features are added to the basic flow control mechanism(sliding window) • sending device keeps copies of all transmitted frame until they have been acknowledged • ACK(carry the number of the next frame expected) • NAK(carry the number of the damaged frame itself) • equipped with a timer to enable it to handle lost acknowledgments

28. Error Control(cont’d) • Go-Back-n ARQ ~ if one frame is lost or damaged, all frame sent since the lost frame acknowledged are retransmitted

29. Error Control(cont’d) • Damaged Frame

30. Error Control(cont’d) • Lost Data Frame

31. Error Control(cont’d) • Lost Acknowledgment

32. Error Control(cont’d) • Selective-Reject ARQ ~ only the specific damaged or lost frame is retransmitted