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Sliding Window protocols Summary With Questions

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  • Difficulty Level : Medium
  • Last Updated : 03 May, 2022

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 number 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*p0+ n*p1+ n*p2 + n*p3 + n*p4 + … = n(1 + p + p2 + p3 + p4 + …) = n*(1 / (1-p)) using infinite GP sum 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 $ retransmission is done)
  • Note – From the 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 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 = 2K-1 and Wr = 1 For SR, Ws = 2K-1 and Wr = 2(K-1)

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