Data Link Control Surasak Sanguanpong nguan@ku.ac.th - - PDF document

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Applied Network Research Group Department of Computer Engineering, Kasetsart University 1/14

Data Link Control

Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan

Last updated: 10 Feb 2001 Applied Network Research Group Department of Computer Engineering, Kasetsart University 2/14

Frame Synchronization Addressing Error Control Flow Control

Data Link Layer functions

Data Link Control

Link Management

To identify the nodes by ID number To identify the beginning end of data frame Regulation of the transmission rate to avoid overwhelming receiver Detection and correction of errors Procedures for management of link setup, maintenance and terminate

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Flow Control

technique for controlling the data transmission so that receivers have sufficient buffer space to accept before processing

Stop-and-Wait Stop-and-Wait Sliding Windows Sliding Windows

Flow Control Flow Control

Send one frame at a time Send several frames at a time

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X-On and X-OFF

transmit sender receiver dddddd dddddd

X-OFF X-ON

stop transmit

receiver sends X-OFF (DC3) if its buffer is full sender stops sending when receives X-OFF receiver sends X-ON (DC1) when buffer space

becomes available

senders starts sending when receives X-ON

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Stop-and-Wait Flow Control

sends back an acknowledgement to the frame just received transmit a frame wait until it receives ACK before sending the next frame

sender receiver

d a t a ACK data ACK Applied Network Research Group Department of Computer Engineering, Kasetsart University 6/14

Sliding-Window Flow Control

frames already transmitted Windows of frames that may be transmitted lower window pointer upper window pointer frame sequence number

Sender’s buffer

frames already received Windows of frames that may be accepted lower window pointer upper window pointer frame sequence number

Receiver’s buffer

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7

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Sliding-Window example

0,1,2 ACK 3 3,4,5,6 ACK 4

1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Applied Network Research Group Department of Computer Engineering, Kasetsart University 8/14

Error Detection

Checksum Checksum CRC CRC

Error Detection Error Detection

• uses dynamic bit pattern attached with

data

• can detect various errors with

appropriated code

• all single bit error
• all double bit errors
• any odd number of errors
• Summation of data and attached

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Checksum

data data checksum Sum and complement

+ + + + + = + + + + + + =

00..00

complement

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CRC concept

M F T k-bit n-bit

T=2nM+F What should be F ? T =2nM+R let F equals R 2nM P R P Q+ = T P = 2nM+R divides T by R P P R P Q+ = + Q T is divisible by P = R P Q ,R quotient, remainder P is predetermine divisor

Given a k-bit block of bits (M), the transmitter generates an n-bit sequence (F), so that the resulting frame, consisting of k+n bits (T), is exactly divisible by some predetermine divisor (P). The receiver then divides the incoming frame (T) by that number (P), if there is no remainder, assume there was no error. Given a k-bit block of bits (M), the transmitter generates an n-bit sequence (F), so that the resulting frame, consisting of k+n bits (T), is exactly divisible by some predetermine divisor (P). The receiver then divides the incoming frame (T) by that number (P), if there is no remainder, assume there was no error.

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CRC in polynomials

M = 110011 then M(X) = X5 + X4 + X+1 P = 1101 then P(X) = X3+X+1 Standard P(X) CRC-16 = X16+ X15+ X2+1 CRC-CCITT = X16+ X12+ X5+1 CRC-32 = X32+ X26+X23+ X22+ X16+ X12+ X11+ X10+ X8 + X7+ X5 + X4+ + X2+1

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Error Control

What is error control

detect and correct errors occurs in transmission Type of errors loss frame

damaged frame

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Error Control Techniques

Error control techniques error detection positive ACK Retransmission after timeout Negative ACK Standard techniques Stop-and-wait ARQ Go-back-N ARQ Selective Repeat ARQ

known as Automatic Repeat Request (ARQ)

}

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Stop-and-Wait ARQ

f r a m e i A C K i f r a m e I + 1 A C K I + 1 f r a m e I + 2 f r a m e I + 2 A C K I + 2 f r a m e I + 3 A C K I + 3 f r a m e I + 3 A C K I + 3

sender receiver

time out time out frame lost retransmit ACK lost Receiver discards duplicate frame retransmit

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Go-back-N ARQ

f r a m e 1

sender receiver

error 7 and 8 will be discarded f r a m e 2 f r a m e 3 A C K 4 f r a m e 4 f r a m e 5 A C K 6 f r a m e 6 f r a m e 7 f r a m e 8 N A C K 6 A C K 9 f r a m e 7 f r a m e 6 f r a m e 8 6, 7 and 8 retransmitted

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Selective-repeat ARQ

f r a m e 1

sender receiver

error f r a m e 2 f r a m e 3 A C K 4 f r a m e 4 f r a m e 5 A C K 6 f r a m e 6 f r a m e 7 f r a m e 8 N A C K 6 A C K 9 f r a m e 9 f r a m e 6 f r a m e 1

• nly 6 is

retransmitted