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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. 데이터 링크 제어 (Data Link Control). Data Link Layer. 데이터 링크 제어 (Data Link Control). Data Link Layer functions. 10.1 Line Discipline.
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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
데이터 링크 제어(Data Link Control) • Data Link Layer
데이터 링크 제어(Data Link Control) • Data Link Layer functions
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
Line Discipline(cont’d) • Line discipline categories • ENQ/ACK : peer-to-peer communication • Poll/Select : primary-secondary communication
Line Discipline(cont’d) • ENQ/ACK
Line Discipline(cont’d) • How it work
Line Discipline(cont’d) • Poll/Select
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.
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
Line Discipline(cont’d) • Select ~ is used whenever the primary device has something to send(select(SEL) frame)
Line Discipline(cont’d) • Poll ~ is used by the primary device to solicit transmissions from the secondary devices
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
Flow Control(cont’d) • Stop and Wait
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
Flow Control(cont’d) • Sliding window ~ several frames can be in transit at a time
Flow Control(cont’d) • Sender window
Flow Control(cont’d) • Receiver window
Flow Control(cont’d) • Example
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
Error Control(cont’d) • Categories of error control
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)
Error Control(cont’d) • Damaged Frames
Error Control(cont’d) • Lost Data Frame
Error Control(cont’d) • Lost Acknowledgment
Error Control(cont’d) • Sliding Window ARQ • go-back-n ARQ • selective-reject ARQ
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
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
Error Control(cont’d) • Damaged Frame
Error Control(cont’d) • Lost Data Frame
Error Control(cont’d) • Lost Acknowledgment
Error Control(cont’d) • Selective-Reject ARQ ~ only the specific damaged or lost frame is retransmitted