ITPEC FE Morning April 2021 Question 30

Topic: Networks

Which of the following is the response time when a client and server at different locations communicate under the conditions below? Here, the response time is in seconds and starts from the moment the client sends a message until a response is received from the server. 1 MB is 106 bytes. The propagation delay between the client and server can be ignored.

[Conditions]

Line speed between client and server8 Mbit/s
Transmission efficiency60%
Message size1 MB for upstreaming and 2 MB for downstreaming
Processing time at client sideTotal of 0.4 s for sending and receiving
Processing time at server sideTotal of 0.4 s for sending and receiving

Explanation

5.8 seconds — two transfers at the effective line speed plus the processing time on both sides.

  • •Effective speed: 8 × 0.6 = 4.8 Mbit/s, and 4.8 ÷ 8 = 0.6 MB/s
  • •Upstream: 1 ÷ 0.6 ≈ 1.67 s; downstream: 2 ÷ 0.6 ≈ 3.33 s; together 3 ÷ 0.6 = 5.0 s
  • •Processing: 0.4 + 0.4 = 0.8 s
  • •Response time: 5.0 + 0.8 = 5.8 s

Answer (d)

Why not others:

  • •(a) 1.4 — reads the line speed as 8 MB/s instead of 8 Mbit/s: 3 ÷ 4.8 ≈ 0.6 s, plus 0.8 s
  • •(b) 3.8 — ignores the 60% efficiency: 3 MB is 24 Mbit, 24 ÷ 8 = 3.0 s, plus 0.8 s
  • •(c) 5.0 — the transmission time alone, leaving out the 0.8 s of client and server processing

Key rule: Apply the efficiency and convert bits to bytes before dividing, then add every step the message passes through in sequence: both transfers and the processing at each end.

Networks · AI-written, may contain errors

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