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The most important responsibility :Flow Control and Error Control

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•

amount of data that the sender can send beforewaiting for acknowledgment.

• Error Correction is both error detection and error

correction. (Simply : Anytime an error is detectedin an exchange, specified frames areretransmitted. This process called Automatic

.

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• Stop-and-Wait ARQ• Go-Back-N-ARQ

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Stop-and-Wait ARQ• This is the simplest flow and error control mechanism. It has

the following mechanism : – The sending device keep the copy of the last frame transmitted

un rece ves an ac now e gmen or a rame. – For identification purpose, both Data Frame and

Acknowledgment (ACK) frame are numbered alternately 0 and.

duplicate transmission.

A data 0 frame ACK 1 - A damaged and lost frame is treated in the same manner by the

receiver. ( Its simply discards ).- ,

variable R.- The sender start a timer when it send a frame.-

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received safe and sound.

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There are four situation in a frame transmission : Normal situation,, ,

acknowledgment is delayed.

Normal si tuation

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Lost or Damaged Frame

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Lost ACK Frame

The receiverdiscards the 2 nd

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copy of Frame 1

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the acknowledgment is delayed

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Numbered ACKs are needed of an acknowledgment is delayed

and the frame is lost.

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If station A and B both have data to send. Piggybacking is amethod to combine a data frame and an acknowledgment.

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Go-Back N ARQ

• Go-Back-N ARQ improves the efficiency of Stop andWait ARQ in term of using transmission medium.

• It lets more than one frame be outstanding; It can sendup to W frames before worrying about acknowledgment.

with a range from 0 to 2 m -1.

• The window slides to includes new unsent frames whenthe correct acknowledgments are received (SlidingWindow).

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Example : Sender Sliding Window

Receiver SlidingWindow

The receiver is alwayslooking for the specific

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rame o arr ve n a spec corder.

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Control Variables

S = Variable holds the sequence number of the recently

sent frame.S F= Holds the sequence number of the first frame.

S L= Holds the sequence number of the last frame.

The size of the window :

= –

The receiver only has onevariable “ R” that only hold

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e sequence o e rame

expect to receive

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• Acknowledgment – The receiver send positive ACK if a frame has arrived and

sound and in order. – If frame is dama ed or is received out of order the

receiver is silent and will discard all subsequent framesuntil receives the one it is expecting. –

unacknowledged frame to expire.

– It causes the sender to go back and resend all frame,eg nn ng w e one w exp re mer.• For Example, suppose the sender has already sent

frame 6, but the timer for frame 3 expires. This meansthat frame 3 has not been acknowledged, so thesender goes back and send frame 3, 4, 5, 6 again. ( Go-Back-N ARQ .

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Go-Back-N ARQ Normal Operation

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Go-Back-N ARQ; Damaged or Lost Frame

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Damaged or Lost Acknowledgment …..?e aye c now e gmen …

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Sender Window Size

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- - ,than 2 m; the size of the receiver window is always 1

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Selective Repeat ARQ

• The Go-Back-N protocol is very inefficient.

• The Selective Repeat ARQ only resendone rame s amage . ore e c ent or

noisy link but more complex process att e rece ver .• The window size at the sender and the

receiver is the same size:• W = one half of the value 2 m

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,

RF = Receive First

L = ece ve as

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Selective Repeat ARQ; Lost Frame

e ec ve epea ARQ also definesa negativeacknowledgment(NAK) that reportsthe sequencenumber of adamaged framebefore the timer

expires

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Selective Repeat ARQ; Lost andDela ed ACKs and

NAKs……….???

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Selective Repeat ARQ, Sender and Window Size

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