## Sliding-Window-Protocol

 Question 1

Host A is sending data to host B over a full duplex link. A and B are using the sliding window protocol for flow control. The send and receive window sizes are 5 packets each. Data packets (sent only from A to B) are all 1000 bytes long and the transmission time for such a packet is 50 µs. Acknowledgement packets (sent only from B to A) are very small and require negligible transmission time. The propagation delay over the link is 200 µs. What is the maximum achievable throughput in this communication?

 A 7.69 × 106 bps B 11.11 × 106 bps C 12.33 × 106 bps D 15.00 × 106 bps
Computer-Networks       Sliding-Window-Protocol       GATE 2003
Question 1 Explanation:
Given, Tt = 50 μs, Tp = 200 μs, L = 1000 bytes, N = 5,
Transmission rate , Tt = L / B.W
Therefore, B.W. = L / Tt = 1000 bytes/ 50 μs = 8000 bits / 50 μs=160 Mbps
Efficiency = N / 1 + 2a, where a = Tp / Tt
Efficiency = 5 * 50 / (50+400) = 250/450 = 5/9
Maximum achievable throughput = Efficiency * B.W = (5/9)*160 Mbps = 88.88 Mbps = = 11.11 x 106 bytes per second
*Actual option should be in bytes per second.
 Question 2

The maximum window size for data transmission using the selective reject protocol with n-bit frame sequence numbers is:

 A 2n B 2n-1 C 2n - 1 D 2n-2
Computer-Networks       Sliding-Window-Protocol       GATE 2005
Question 2 Explanation:
In Selective Reject (or Selective Repeat), maximum size of window must be half of the maximum sequence number = 2n/ 2 = 2n-1
For Go-back N, the maximum window size can be 2n - 1.
 Question 3

Station A uses 32 byte packets to transmit messages to Station B using a sliding window protocol. The round trip delay between A and B is 80 milliseconds and the bottleneck bandwidth on the path between A and B is 128 kbps. What is the optimal window size that A should use?

 A 20 B 40 C 160 D 320
Computer-Networks       Sliding-Window-Protocol       GATE 2006
Question 3 Explanation:
Tt = L / B = 32 bytes/ 128 kbps = 32*8/128 ms = 2 ms
Round trip delay = 2 * Tp = 80 ms (given)
Optimal window size is = (Tt + 2*Tp) / Tt = 82 / 2 = 41
Option is not given, closest option is 40.
 Question 4

Station A needs to send a message consisting of 9 packets to Station B using a sliding window (window size 3) and go-back-n error control strategy. All packets are ready and immediately available for transmission. If every 5th packet that A transmits gets lost (but no acks from B ever get lost), then what is the number of packets that A will transmit for sending the message to B?

 A 12 B 14 C 16 D 18
Computer-Networks       Sliding-Window-Protocol       GATE 2006
Question 4 Explanation:
Window size is 3, so maximum 3 packets can be remained unacknowledged. In go back ‘n’ if acknowledge for a packet is not received then packets after that packet is also retransmitted.
Frame sequence for 9 frame is shown below. Frame with bold sequence number gets lost.
1 2 3 4 [5 6 7] 5 6 [7 8 9] 7 8 9 9 = 16
 Question 5
Consider the three-way handshake mechanism followed during TCP connection established between hosts P and Q. Let X and Y be two random 32-bit starting sequence numbers chosen by P and Q respectively. Suppose P sends a TCP connection request message to Q with a TCP segment having SYN bit = 1, SEQ number = X, and ACK bit = 0. Suppose Q accepts the connection request. Which one of the following choices represents the information present in the TCP segment header that is sent by Q to P?
 A SYN bit = 1, SEQ number = X+1, ACK bit = 0, ACK number = Y, FIN bit = 0 B SYN bit = 0, SEQ number = X+1, ACK bit = 0, ACK number = Y, FIN bit = 1 C SYN bit = 1, SEQ number = Y, ACK bit = 1, ACK number = X+1, FIN bit = 0 D SYN bit = 1, SEQ number = Y, ACK bit = 1, ACK number = X, FIN bit = 0
Computer-Networks       Sliding-Window-Protocol       GATE 2021 CS-Set-2       Video-Explanation
Question 5 Explanation:

Q will send the SYN bit = 1 to the connection establishment.

Q Seq number will be Y different from X

ACK bit = 1 because sending the ACK

ACK number = X+1 (Next seq number id)

FIN bit = 0 (Because establishing the connection)

There are 5 questions to complete.

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