Computer Network Questions and Answers

Answer: A

Correct answer: doubleplexer

Multiplexers, concentrators and controllers all let several devices share one line. Doubleplexer is not a device at all, which is what makes it the answer.

Answer: B

Correct answer: 56 kbps

T1 PCM samples at 8000 samples per second and uses seven of the eight bits of each sample for data, the eighth being taken for signalling:

\[ 8000\times 7 = 56{,}000\ \text{bps} \]

Answer: C

Correct answer: 6000 bps

By Nyquist's formula for a noiseless channel,

\[ C = 2B\log_{2}V = 2\times 3000\times \log_{2}2 = 6000\ \text{bps} \]

Binary signalling means two levels, so each symbol carries one bit and the rate is twice the bandwidth.

Answer: B

Correct answer: False

The statement is false. A positive-edge-triggered flip-flop responds on the rising edge of the clock - the transition from low to high. A negative-edge-triggered device responds on the falling edge. On a circuit diagram the two are distinguished by a small triangle at the clock input, with an added inversion bubble marking negative-edge triggering. The choice between them is a design convention rather than a matter of correctness, but mixing the two in one design creates a half-cycle timing difference that must be accounted for, and is a common source of error. Two timing requirements attach to either kind. The data must be stable for the setup time before the active edge and for the hold time after it. Violating them can leave the flip-flop metastable, hovering between levels for an unpredictable interval - the reason asynchronous inputs are passed through two flip-flops in series before use.

Answer: C

Correct answer: synchronization

In asynchronous serial transmission the receiver has no clock from the sender, so each character is framed by a start bit that marks its beginning and a stop bit that returns the line to idle. They carry no data and exist only to keep the receiver in step.

Answer: C

Correct answer: 1950, USA

Modems were developed in the United States in the 1950s, for the SAGE air defence system, which needed to carry radar data over telephone lines.

Answer: A

Correct answer: Unshielded Twisted pair

UTP is Unshielded Twisted Pair - pairs of wires twisted together to cancel interference, with no metal screen around them. Shielding gives STP.

Answer: B

Correct answer: 45

A full-duplex line serves both directions, and no station connects to itself, so the number of connections is the number of unordered pairs:

\[ \binom{10}{2} = \frac{10\times 9}{2} = 45 \]

Answer: B

Correct answer: Maximum

The collision domain is bounded by the greatest distance a frame may travel and still be detected as a collision by the sender before it finishes transmitting. It is therefore a maximum distance, and it shrinks as the data rate rises.

Answer: A

Correct answer: \( \overline{A + B} = \bar{A} \cdot \bar{B} \)

De Morgan’s first theorem states that the complement of a sum is the product of the complements: \[ \overline{A + B} = \bar{A} \cdot \bar{B}. \] In words, NOT (A OR B) is the same as (NOT A) AND (NOT B). The second theorem is its dual: \[ \overline{A \cdot B} = \bar{A} + \bar{B}. \] A truth table confirms the first at once. Taking A and B as 0,0 gives \( \overline{0} = 1 \) on the left and \( 1 \cdot 1 = 1 \) on the right; every other combination gives 0 on both sides. The practical rule for applying either theorem is: break the bar, change the sign. Split the overbar, and swap AND for OR or OR for AND. The theorems matter because they allow any circuit to be rebuilt using only NAND gates or only NOR gates, which is why those two are called universal.