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Stop-and-Wait ARQ Protocol

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This presentation shows how the Stop-and-wait ARQ protocol works

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Stop-and-Wait ARQ Protocol

  1. 1. Team: 1 5th A BVBCSE 2012-2016
  2. 2. Introduction o Stop-and-wait ARQ is a method used in telecommunications to send information between two connected devices. o It ensures that information is not lost due to dropped packets and that packets are received in the correct order. o It is the simplest kind of automatic repeat-request (ARQ) method.
  3. 3. Description  A stop-and-wait ARQ sender sends one frame at a time.  It is a special case of the general sliding window protocol with both transmit and receive window sizes equal to 1.  After sending each frame, the sender doesn't send any further frames until it receives an acknowledgement (ACK) signal.  After receiving a good frame, the receiver sends an ACK.  If the ACK does not reach the sender before a certain time, known as the timeout, the sender sends the same frame again.
  4. 4.  Typically the transmitter adds a redundancy check number to the end of each frame.  The receiver uses the redundancy check number to check for possible damage. If the receiver sees that the frame is good, it sends an ACK.  If the receiver sees that the frame is damaged, the receiver discards it and does not send an ACK—pretending that the frame was completely lost, not merely damaged.
  5. 5. Need for sequence numbers o 1 bit sequence number in the header of the frame, alternating (from 0 to 1) in subsequent frames. o When the receiver sends an ACK, it includes the sequence number of the next packet it expects. This way, the receiver can detect duplicated frames by checking if the frame sequence numbers alternate. o If two subsequent frames have the same sequence number, they are duplicates, and the second frame is discarded. Similarly, if two subsequent ACKs reference the same sequence number, they are acknowledging the same frame.
  6. 6. Case 1  Sender sends the frame 0 to the destination and simultaneously a timer is started.  The destination accepts the frame since it was expecting frame 0.  Now the receiver window slides one position ahead.  Receiver sends an acknowledgement ACK 1 informing the sender that it has successfully received frame 0 and is expecting for frame 1.  This acknowledgement reaches sender before the timer of frame 0 expires and Sending window slides to the next frame and sends frame1.  See the animation for better understanding.
  7. 7. Request Sn 0 1 0 1 0 1 0 1 0 1 0 1 Arrived Sn 0 1 0 1 0 1 Rn Sender Receiver Start Stop Arrived
  8. 8. Case 2  Sender sends frame 0 to the destination and simultaneously timer is started.  Frame 0 is lost before reaching the destination.  Since destination didn’t receive frame 0 no acknowledgement is sent.  Eventually timer for frame 0 expires and sender resends frame 0.  This process continues until it reaches acknowledgement for frame 1.  See the animation in the next slide for better understanding.
  9. 9. Request 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 timeout Arrived Sn Sn Rn Arrived Stop
  10. 10. Case 3  Sender sends frame 0 to the destination and simultaneously timer is started.  The destination accepts the frame since it was expecting frame 0.  Now the receiver window slides one position ahead.  Receiver sends an acknowledgement ACK 1 informing the sender that it has successfully received frame 0 and is expecting for frame 1.  The acknowledgement gets lost and it doesn’t reach the sender.  Eventually timer for frame 0 expires and sender resends frame 0.  Now since the receiver is expecting for frame 1, it will safely discard the frame 0 and once again sends acknowledgement ACK 1.
  11. 11. Sn Request 010101 Start Rn 0 1 0 1 0 1 Arrived ACK 1 Sn 010101 Lost 0 1 0 1 0 1 Arrived Sn 0 1 0 1 0 1 Rn Discard, Duplicate timeout Stop
  12. 12. Advantages of stop-and-wait ARQ over simple stop and wait protocols  It can be used for noisy channels.  It has both flow and error control mechanism.  It has timer implementation.
  13. 13. Disadvantages of Stop and wait ARQ  Efficiency is very less.  Only one frame is sent at a time.  Timer should be set for each individual frame.  No pipelining.  Sender window size is one(disadvantage over go back and ARQ).  Receiver window size is one(disadvantage over selective repeat ARQ).
  14. 14.  By-  Pushkar D  Praneeta Y  Amith A  Komal S  Pallavi D  Chaitra H  Ambaresh P  Ankit R Thank you
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This presentation shows how the Stop-and-wait ARQ protocol works

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