GATE 2015 SET-1 CS Computer Science and information technology

[Q1 – Q25 carry ONE mark each ]

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Explanation:
Statement(I): If the sequence number of a segment is m, then the sequence number of the subsequent segment is always m+1.(False)
1 Byte of Data consumes 1 Sequence number.If Sequence number of segment is m, then the Sequence number of subsequent segment is m+1,only if 1 Byte of Data is transferred. But it is already given that Host transfer karge file. So, the statement is False.

Statement(II): If the estimated round trip time at any given point of time is t sec, the value of the retransmission timeout is always set to greater than or equal to t sec.(True)
TCP sets a timeout when it sends Data and if Data is not acknowledged before timeout expires it retransmit data. Timeout is based on RTT. Timeout is always set to greater than or equal to RTT.

Statement(III): The size of the advertised window never changes during the course of the TCP connection.(False)
Receiver’s window buffer is never changed.But depending upon the available window size, Advertised window is keep changing.
Let’s undestand it with example,
Suppose receiver window size is 1000B, so the size of advertised window is 1000B. Again suppose sender have 400B of data to sent, it sends to receiver. Now receiver window have (1000-400) 600B of available space. Now receiver will sent acknowlege to sender with advertised window of 600B. So, The size of the advertised window keep change during the course of the TCP connection.

Statement(IV): The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.(True)
Because sender will never sends number of Byte greater than the advertised window.

So, Option(II) is correct.

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[Q26 – Q55 carry TWO mark each ]

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Explanation:
Given,
Bit rate (or Bandwidth) = 64 kilobits =64 x 103 bps
Processing Delay (or Tp) = 20ms
Link utilization (or Efficiency) = 50% = 1/2

Frame size = ?

Formula,
For stop-wait protocol,
Efficiency (η) = where, Tp = Propogation Delay
Tt = Transmission Delay = Calculation,
η = => => 1+2a = 2
=> Tt =2*Tp
=> => L = 2 * Tp * Bw
=> L = 2* 20 ms * 64 x 103 bps
=> L = 2* 20* 10-3 s * 64 x 103 bps
=> L = 40 * 64 / 8 Byte = 320 Byte

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