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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

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데이터 링크 제어 (Data Link Control)

Data Link Layer

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데이터 링크 제어 (Data Link Control)

Data Link Layer functions

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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

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Line Discipline(cont’d)

Line discipline categories

ENQ/ACK : peer-to-peer communication Poll/Select : primary-secondary communication

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Line Discipline(cont’d)

ENQ/ACK

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Line Discipline(cont’d)How it work

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Line Discipline(cont’d)

Poll/Select

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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.

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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

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Line Discipline(cont’d)

Select

~ is used whenever the primary device has something to send(select(SEL) frame)

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Line Discipline(cont’d)Poll

~ is used by the primary device to solicit transmissions from the secondary devices

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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

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Flow Control(cont’d)Stop and Wait

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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

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Flow Control(cont’d)

Sliding window

~ several frames can be in transit at a time

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Flow Control(cont’d)

Sender window

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Flow Control(cont’d)

Receiver window

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Flow Control(cont’d)

Example

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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

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Error Control(cont’d)

Categories of error control

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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)

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Error Control(cont’d)

Damaged Frames

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Error Control(cont’d)

Lost Data Frame

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Error Control(cont’d)

Lost Acknowledgment

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Error Control(cont’d)

Sliding Window ARQ

go-back-n ARQ

selective-reject ARQ

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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

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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

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Error Control(cont’d)Damaged Frame

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Error Control(cont’d)Lost Data Frame

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Error Control(cont’d)

Lost Acknowledgment

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Error Control(cont’d)Selective-Reject ARQ

~ only the specific damaged or lost frame is retransmitted