Computer Network Questions and Answers

Answer: D

Correct answer: transponders

A communication satellite carries a set of transponders, each of which receives on one frequency, amplifies, shifts the signal to another frequency and retransmits it. Each transponder is in effect one repeater in the sky.

Answer: D

Correct answer: Control unit

In a time-division switch the samples sit in RAM and the control unit decides the order in which they are read out, which is what determines where each one goes. The memory holds the data; the control unit does the switching.

Answer: A

Correct answer: binary digit

Bit is a contraction of binary digit. The word was coined by John Tukey in 1947 and popularised by Claude Shannon in his 1948 paper founding information theory. A bit is the smallest unit of information: it distinguishes between exactly two possibilities, and in a circuit it is a single high or low voltage. The units built on it are worth keeping straight. Four bits make a nibble, which corresponds to one hexadecimal digit. Eight bits make a byte, giving 256 values. A word is whatever width the processor handles naturally - 32 or 64 bits on modern machines - so it is architecture-dependent rather than fixed. A distinction that causes constant confusion in practice: network speeds are quoted in bits per second and file sizes in bytes, and the abbreviations differ only in case - Mb for megabit, MB for megabyte. A 100 Mbps link therefore transfers about 12.5 MB per second.

Answer: A

Correct answer: Does not require global time synchronization

Pure ALOHA lets a station transmit the moment it has data, so no station needs to know where any slot boundary lies. Slotted ALOHA gains throughput precisely by imposing that synchronisation.

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.