Flow Control

Applied Network Research Group Department of Computer Engineering, Kasetsart University

Data Link Control

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

Last updated: 11 July 2000

2/14

Flow Control

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

z common flow control schemes :

z

X-ON and X-OFF

z

Stop-and-Wait

z

Sliding Windows

Applied Network Research Group Department of Computer Engineering, Kasetsart University

X-On and X-OFF

transmit

sender receiver

dddddd dddddd

X-OFF X-ON

stop

transmit

z

receiver sends X-OFF (DC3) if its buffer is full

z

sender stops sending when receives X-OFF

z

receiver sends X-ON (DC1) when buffer space becomes available

z

senders starts sending when receives X-ON

4/14

Stop-and-Wait Flow Control

data i

ACK

data i+1

sends back an acknowledgement to the frame just received transmits

a frame

wait until it receives ACK before sending the next frame

sender receiver

Applied Network Research Group Department of Computer Engineering, Kasetsart University

Sliding-Window Flow Control

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

frames already transmitted

frames that may be transmitted

lower window pointer upper window pointer frame sequence number

Sender’s buffer

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

frames already received

frames that may be accepted

lower window pointer upper window pointer frame sequence number

receiver’s buffer

6/14

Sliding-Window example

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0,1,2

ACK3

3,4,5,6

ACK4

Applied Network Research Group Department of Computer Engineering, Kasetsart University

Error Detection

z Parity check

z

odd or even parity

z

undetected error, it two or any even number of bits are inverted

z CRC

z

uses dynamic bit pattern attached with data

z

can detect various errors with appropriated code

z

all single bit error

z

all double bit errors

z

any odd number of errors

8/14

CRC concepts

M F

T

k-bit n-bit

T=2

M+F What should be F ?

T =2

M+R let F equals R 2

M

P

R Q+ P

=

T

P = 2

M+R divides T by R P P

R Q+ P

= + R = Q T is divisible by P 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.

Applied Network Research Group Department of Computer Engineering, Kasetsart University

CRC in polynomials

M = 110011 then M(X) = X

+ X

+ X+1 P = 1101 then P(X) = X

+X+1

Standard P(X)

CRC-16 = X

+ X

+ X

+1 CRC-CCITT = X

+ X

+ X

+1

CRC-32 = X

+ X

+X

+ X

+ X

+ X

+ X

+ X

+ X

+ X

+ X

+ X

+ + X

+1

10/14

Error Control

z What is error control

z

detect and correct errors occurs in transmission

z

Type of errors

z

loss frame

z damaged frame

Applied Network Research Group Department of Computer Engineering, Kasetsart University

Error Control Techniques

z

Error control techniques

z

error detection

z

positive ACK

z

Retransmission after timeout

z

Negative ACK

z

Standard techniques

z

Stop-and-wait ARQ

z

Go-back-N ARQ

z

Selective Repeat ARQ

known as

Automatic Repeat Request (ARQ)

}

12/14

Stop-and-Wait ARQ

fra m e i

ACK i

fra m e I+

1

I+1

fra m

e I+

2

fra m e I+

2

I+2

fra m

e I+

3

I+3

fra m

e I+

3

I+3

sender

receiver

time out time out

frame lost

retransmit

retransmit &

duplicate discard

Applied Network Research Group Department of Computer Engineering, Kasetsart University

Go-back-N ARQ

fr am e 1

sender

receiver

error

7 and 8 will be discarded fr am

e 2 fr am

e 3

AC K4

fr am

e 4 fr am

e 5

AC K6

fr am

e 6 fr am

e 7 fr am

e 8

NAC K6

AC K9

fr am

e 7 fr am

e 6

fr am e 8 6, 7 and 8 retransmitted

14/14

Selective-repeat ARQ

fr am e 1

sender

receiver

error

fr am

e 2 fr am

e 3

AC K4

fr am

e 4 fr am

e 5

AC K6

fr am

e 6 fr am

e 7 fr am

e 8

NAC K6

AC K9

fr am

e 9 fr am

e 6

fr am

e 1 0

only 6 is

retransmitted