Sliding window protocol(ARQ technique)

Sliding window protocol(ARQ technique)

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  Prepared By JESADIYA SHILPA
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  Introduction  If framegets lost,damage,or corrupted during transmission, then retransmission of that frame is called ERROR CONTROL....  The error control technique is also known as Automatic Repeat Request(ARQ)..
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  Automatic Repeat Request(ARQ).. The ARQ is error control technique..  In this technique, if the data transmitted by the sender arrives at the receiver without any problem, then the receiver reverts by sending an acknowledgment(ACK).  On the contrary to this, if the data to be transmitted gets lost or corrupted in between then the receiver sends a negative acknowledgment(NAK).  Consequently, the sender retransmit the same data again.  There is also provision of the timer with the sender so that if ACK gets lost on the way, then the sender can again retransmit the same data again.
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  Types of ARQ... ARQtechnique Sliding Window Protocol Stop and Wait ARQ Go-Back-N Selective Reject
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   The slidingwindow ARQ technique has two two categories, namely, 1. Go-Back –N 2. Selective Repeat
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  • Concept – Introducea window of size n – Can inject n packets into net before hearing an ACK • Sliding window – Label each packet with a sequence number – A window is a collection of adjacent sequence numbers – The size of the collection is the sender’s window size
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  Go-Back-N ARQ • Wecan send up to W frames before worrying about ACKs. • We keep a copy of these frames until the ACKs arrive. • This procedure requires additional features to be added to Stop-and-Wait ARQ.
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  Go-Back-N ARQ, normaloperation • The sender keeps track of the outstanding frames and updates the variables and windows as the ACKs arrive.
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   In above,example  It can be seen in figure that error occurs in Frame 5.Hence the receiver sends negative acknowledgment of Frame 5 to the sender.  In such a case, the sender needs to retransmit Frame 5 and all the succeeding frames (Frame 6 and Frame 7)
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  Go-Back-N ARQ, lostframe • Frame 2 is lost • When the receiver receives frame 3, it discards frame 3 as it is expecting frame 2 (according to window). • After the timer for frame 2 expires at the sender site, the sender sends frame 2 and 3. (go back to 2)
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  Go-Back-N ARQ, damaged/lost/delayedACK • If an ACK is damaged/lost, we can have two situations: • If the next ACK arrives before the expiration of any timer, there is no need for retransmission of frames because ACKs are cumulative in this protocol. • If ACK1, ACK2, and ACk3 are lost, ACK4 covers them if it arrives before the timer expires. • If ACK4 arrives after time-out, the last frame and all the frames after that are resent. • Receiver never resends an ACK. • A delayed ACK also triggers the resending of frames
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  Go-Back-N ARQ, senderwindow size • Size of the sender window must be less than 2 m. Size of the receiver is always 1. If m = 2, window size = 2 m – 1 = 3. • Fig compares a window size of 3 and 4. Accepts as the 1st frame in the next cycle-an error
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  Selective Repeat ARQ,sender and receiver windows • Go-Back-N ARQ simplifies the process at the receiver site. Receiver only keeps track of only one variable, and there is no need to buffer out-of-order frames, they are simply discarded. • However, Go-Back-N ARQ protocol is inefficient for noisy link. It bandwidth inefficient and slows down the transmission. • In Selective Repeat ARQ, only the damaged frame is resent. More bandwidth efficient but more complex processing at receiver. • It defines a negative ACK (NAK) to report the sequence number of a damaged frame before the timer expires.
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  Selective Repeat ARQ,lost frame • Frames 0 and 1 are accepted when received because they are in the range specified by the receiver window. Same for frame 3. • Receiver sends a NAK2 to show that frame 2 has not been received and then sender resends only frame 2 and it is accepted as it is in the range of the window.
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  Selective Repeat ARQ,sender window size • Size of the sender and receiver windows must be at most one-half of 2 m. If m = 2, window size should be 2 m /2 = 2. Fig compares a window size of 2 with a window size of 3. Window size is 3 and all ACKs are lost, sender sends duplicate of frame 0, window of the receiver expect to receive frame 0 (part of the window), so accepts frame 0, as the 1st frame of the next cycle – an error.