Computer Network | Sliding Window protocols Summary With Questions

3

Prerequisites – Stop & Wait, Go Back N, Selective Repeat

Summary of all the protocols –

Before starting with the questions a quick recap for all the protocols.

Stop and wait –

  1. Sender window size (Ws) = 1
  2. Receiver window size (Wr) = 1
  3. Sequence Number >= 1 + 1
  4. Uses independent acknowledgement
  5. Discards out of order packets
  6. Packet Loss → Retransmit packet after time out
  7. Acknowledgement loss → Resends packet after time out
  8. Efficiency = 1/(1+2a) where a = Tp/Tt

Go Back N –

  1. Sender window size Ws = N
  2. Receiver window size Wr = 1
  3. Sequence number >= N + 1
  4. Can use both cumulative or independent acknowledgement depends on acknowledge timer
  5. Discards out of order packets
  6. Packet Loss → Track back N size from the last packet within the window limit to the lost packet and retransmit them
  7. Acknowledgement loss → If not received before timeout the entire window N size is resend
  8. Efficiency = N/(1+2a) where a = Tp/Tt

Selective Repeat –

  1. Sender window size Ws = N
  2. Receiver window size Wr = N
  3. Sequence Number >= N + N
  4. Uses only independent acknowledgement
  5. Can Accept out of order packets
  6. Packet Loss → Resend only the lost packet after timeout
  7. Acknowledgement loss → Resend if not receive before timeout
  8. Efficiency = N/(1+2a) where a = Tp/Tt

Practice Questions –

  • Example-1. In Stop and wait protocol every 4th packet is lost and we need to send total 10 packets so how many transmission it took to send all the packets ?
  • Explanation –
    1 2 3 4 5 6 7 8 9 10 (Initially)
          ^
    1 2 3 4 4 5 6 7 8 9 10 (Packet no. 4 retransmitted) 
                  ^
    1 2 3 4 4 5 6 7 7 8 9 10 (Packet no. 10 retransmitted)
                           ^
    1 2 3 4 4 5 6 7 7 8 9 10 10 (Result)
    

    So, we retransmitted packet noumber 4, 7, 10
    Total count = 13

  • Example-2. In S&W protocol if Error probability is p and no. of packets to send is ‘n’.How many packets we have to send ?
  • Explanation –
    Total retransmissions
    = n+ n*p+ n*p^(2) + n*p^(3) + n*p^(4) + …
    = n(1 + p + p^2 + p^3 + p^4 + …)
    = n*(1/1-p) using infinite GP formula
  • Example-3. In GBN sender Window size = 10 and Tp = 49.5ms & Tt = 1ms. What is the Efficiency of the protocol and Throughput given Bandwidth=1000 bps ?
  • Explanation –
    Efficiency = N/(1+2a), N = 10 (given), a = Tp/Tt = 49.5
    Efficiency = 10/(1+2*49.5) = 10/100 = 0.1 or 10%
    Throughput = Efficiency * Bandwidth
    = 0.1*1000 = 100
  • Example-4. In GB3 if every 5th packet is lost & we need to send 10 packets so how many retransmissions are required ?
  • Explanation –
    1 2 3 4 5 6 7  | 8 9 10 
            ^   $            (packet no. 5 lost)
    1 2 3 4 5 6 7 5 6 7 8 9 | 10
                  *   ^   $       
    1 2 3 4 5 6 7 5 6 7 8 9 7 8 9 10
                            *   ^  $
    1 2 3 4 5 6 7 5 6 7 8 9 7 8 9 10 9 10 (count starts from * till ^)
    (from ^ to $ retranmission is done)
    

    Note – From Last packet is window size to lost pocket we resend the entire window.
    Total no. of transmissions = 18

  • Example-5. In SR Ws= 5 and we are sending 10 packets where every 5th packet is lost find number of retransmissions ?
  • Explanation –
    1 2 3 4 5 6 7 8 9 10
            ^
    1 2 3 4 5 5 6 7 8 9 10
                      ^
    1 2 3 4 5 6 7 8 9 9 10
    

    We see here there is no role of Window size in SR only the lost packet is resent.
    Total transmissions = 12

  • Example-6. If there is K bits sequence no. define require sender window size and receiver window size for S&W, GBN & SR?

    Explanation –
    Given, K bits, For S&W Ws = 1 and Wr= 1
    For GBN, Ws = 2^(K)-1 and Wr = 1
    For SR, Ws = 2^(K-1) and Wr = 2^(K-1)

  • This article is contributed by SHAURYA UPPAL. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

    Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

    GATE CS Corner    Company Wise Coding Practice

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.

Recommended Posts:



3 Average Difficulty : 3/5.0
Based on 1 vote(s)










Writing code in comment? Please use ide.geeksforgeeks.org, generate link and share the link here.