Computer Network Questions and Answers

Answer: A

Correct answer: source routing

In IBM's Token Ring the sending station discovers and carries the whole route in the frame, so the bridges simply follow the directions given rather than deciding for themselves. That is source routing, and it is what distinguishes Token Ring bridging from Ethernet's transparent bridging.

Answer: B

Correct answer: time-division

A time-division switch works by writing incoming samples into memory and reading them out in a different order, so each sample must be stored and fetched again. That store-and-fetch is the source of the delay; a space-division crossbar makes a direct path and adds none.

Answer: B

Correct answer: 20

A simplex line carries traffic in one direction only, so each pair of routers needs two of them - one each way. With 5 routers there are \( 5\times 4 = 20 \) ordered pairs, and so 20 lines. Full-duplex lines would need only \( \binom{5}{2}=10 \).

Answer: D

Correct answer: Either (b) or (c)

Both RS-232-C and RS-449 are physical-layer standards for the interface between a terminal and a modem; RS-449 was introduced to carry higher rates over longer distances, but either will serve.

Answer: C

Correct answer: 772

First the propagation delay:

\[ t = \frac{100\times 10^{3}}{2\times 10^{8}} = 500\ \mu\text{s} \]

Then the bits in flight at the T1 rate of 1.544 Mbps:

\[ 1.544\times 10^{6}\times 500\times 10^{-6} \approx 772\ \text{bits} \]

Answer: C

Correct answer: 30,000 GHz

Differentiating \( f = c/\lambda \) gives \( \Delta f = c\,\Delta\lambda/\lambda^{2} \):

\[ \Delta f = \frac{3\times 10^{8}\times 0.1\times 10^{-6}}{(1\times 10^{-6})^{2}} = 3\times 10^{13}\ \text{Hz} = 30{,}000\ \text{GHz} \]

That enormous figure is why a single fibre has so much more capacity than any copper medium.

Answer: C

Correct answer: 1.5 × 10⁻⁶ microns

The screen needs

\[ 480\times 640\times 24\times 60 \approx 4.42\times 10^{8}\ \text{bps} \]

and the spectral width for that bandwidth at \( \lambda = 1\ \mu\text{m} \) is

\[ \Delta\lambda = \frac{\lambda^{2}\Delta f}{c} = \frac{(10^{-6})^{2}\times 4.42\times 10^{8}}{3\times 10^{8}} \approx 1.5\times 10^{-12}\ \text{m} \]

which is \( 1.5\times 10^{-6} \) microns - a minute slice of the fibre's available spectrum.

Answer: C

Correct answer: placing a telephone call prior to a conversion

A virtual circuit is set up before any data flows and torn down afterwards, so the route is fixed for the duration - exactly as dialling establishes a path before the conversation begins. Nothing is physically switched into place, which is what makes the circuit virtual.

Answer: B

Correct answer: First four

The DNIC is the first four digits of an X.121 network user address: three for the country and one to identify the network within it.

Answer: A

Correct answer: First three

The first three digits of the DNIC are the country code; the fourth distinguishes networks within that country.