1 10 장. 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) zData Link Layer

3 데이터 링크 제어 (Data Link Control) zData Link Layer functions

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

5 Line Discipline(cont’d) zLine discipline categories yENQ/ACK : peer-to-peer communication yPoll/Select : primary-secondary communication

6 Line Discipline(cont’d) zENQ/ACK

7 Line Discipline(cont’d) zHow it work

8 Line Discipline(cont’d) zPoll/Select

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

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

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

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

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) zStop and Wait

15 Flow Control(cont’d) zStop-and-Wait ~ the sender sends one frame and waits for an acknowledgment before sending the next frame yAdvantage : simplicity yDisadvantage : inefficiency

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

17 Flow Control(cont’d) zSender window

18 Flow Control(cont’d) zReceiver window

19 Flow Control(cont’d) zExample

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

21 Error Control(cont’d) zCategories of error control

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

23 Error Control(cont’d) zDamaged Frames

24 Error Control(cont’d) zLost Data Frame

25 Error Control(cont’d) zLost Acknowledgment

26 Error Control(cont’d) zSliding Window ARQ ygo-back-n ARQ yselective-reject ARQ

27 Error Control(cont’d) zThree features are added to the basic flow control mechanism(sliding window) ysending device keeps copies of all transmitted frame until they have been acknowledged yACK(carry the number of the next frame expected) yNAK(carry the number of the damaged frame itself) yequipped with a timer to enable it to handle lost acknowledgments

28 Error Control(cont’d) zGo-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) zDamaged Frame

30 Error Control(cont’d) zLost Data Frame

31 Error Control(cont’d) zLost Acknowledgment

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