A 2000-character text file must be sent through a 1,200-baud modem over an asynchronous serial link. Assuming standard 8-N-1 framing (1 start, 8 data, 1 stop = 10 bits/char), how long will the transfer take (ignoring protocol overhead)?

Difficulty: Medium

Correct Answer: None of the above

Explanation:


Introduction / Context:
Serial modem throughput depends on baud rate (symbols/s) and framing. In typical asynchronous links, each character is wrapped with start/stop bits. Calculating transfer time requires converting characters to bits, dividing by the effective bit rate, and comparing against the choices.



Given Data / Assumptions:

  • File length: 2000 characters.
  • Line rate: 1,200 baud (assume 1 bit/symbol as with simple FSK for baseline).
  • Framing: 8-N-1 → 10 bits per character (1 start + 8 data + 1 stop).
  • Ignore higher-layer overhead (no parity, no flow-control gaps, no retransmissions).


Concept / Approach:
Total bits = characters * bits/character. Time = total bits / bit rate. With 8-N-1, each character consumes 10 line bits. Therefore, 2000 chars require 20,000 bits. At 1,200 bit/s, time ≈ 16.67 s. None of the listed numeric options equals 16.67 exactly, so the correct choice among those presented is “None of the above.”



Step-by-Step Solution:
bits_per_char = 10 (8-N-1).total_bits = 2000 * 10 = 20,000 bits.time_seconds = 20,000 / 1,200 ≈ 16.67 s.Compare to options: 2, 12, 20, 120 → none matches ≈16.67 s.


Verification / Alternative check:
If parity were used (8-E-1 → 11 bits/char), time would be ≈ 18.33 s; if one incorrectly ignores start/stop (8 bits only), time ≈ 13.33 s. These also do not match the provided options, reinforcing “None of the above.”



Why Other Options Are Wrong:
12 s: Implies only 8 bits/char at 1,333 bit/s or similar incorrect assumptions.


20 s: Would require 24,000 bits transmitted or a lower effective rate.


2 s and 120 s: Orders of magnitude off for the stated parameters.



Common Pitfalls:
Forgetting framing overhead in asynchronous links and equating baud to characters per second. Always multiply by bits per framed character.



Final Answer:
None of the above

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