Computer Network Questions and Answers

Answer: B

Correct answer: 47

Each hexadecimal digit carries a place value that is a power of 16, read from the right: \( 16^{0} = 1 \) and \( 16^{1} = 16 \). In 2F the digit F stands for 15, so \[ 2 \times 16 + 15 = 32 + 15 = 47. \] The letters run A = 10, B = 11, C = 12, D = 13, E = 14, F = 15, and it is worth memorising them, since almost every hexadecimal question depends on the conversion. A quicker route for small values goes through binary. Each hexadecimal digit is four bits: 2 is 0010 and F is 1111, giving 00101111, which is \( 32 + 8 + 4 + 2 + 1 = 47 \). Conversion by grouping is faster than arithmetic once the sixteen four-bit patterns are familiar. The largest two-digit hexadecimal number is FF, which is 255 - the maximum value of one byte, and the reason colour codes and byte values are so often written this way.

Answer: C

Correct answer: 8

The 300 bps terminals may use whatever the 1200 bps terminals leave:

\[ 9600 - 6\times 1200 = 2400\ \text{bps} \qquad n = \frac{2400}{300} = 8 \]

Answer: A

Correct answer: 10⁶ to 10⁸ Hz

Land coaxial cable is used from about 1 MHz to 100 MHz. Below that the cable's advantage over twisted pair is slight; above it attenuation rises sharply and waveguide or fibre is preferred.

Answer: A

Correct answer: 0010 0101

In binary coded decimal each decimal digit is encoded separately in four bits. So 25 becomes 2 and 5 encoded individually: 2 is 0010 and 5 is 0101, giving 0010 0101. This is not the same as the pure binary value of 25, which is 11001 - the second option, and the usual trap. BCD deliberately sacrifices compactness for ease of conversion. Because only ten of the sixteen four-bit patterns are used, the six combinations from 1010 to 1111 are invalid in BCD, and arithmetic must correct for them: when a BCD addition produces a result above 9 or generates a carry, 6 is added to bring it back into range. The code is used wherever numbers are displayed or entered digit by digit - calculators, digital clocks, instrument panels - because converting each digit to a seven-segment display needs no division. The waste is about 20 per cent of the available codes.

Answer: C

Correct answer: full duplex lines

Interactive work means a rapid exchange in both directions, and a full duplex line carries both at once so neither side waits for the line to turn round. A simplex line carries one direction only.

Answer: A

Correct answer: 500 m

Thick coaxial cable, the 10Base5 of the original Ethernet, allows a 500 m segment - the 5 in the name records it. Thin coax, 10Base2, manages about 185 m.

Answer: B

Correct answer: 13

Each binary digit carries a place value that is a power of two, read from the right: \( 2^{0}, 2^{1}, 2^{2}, 2^{3} \), that is 1, 2, 4 and 8. For 1101 the set bits are in the 8, 4 and 1 positions: \[ 1\times8 + 1\times4 + 0\times2 + 1\times1 = 8 + 4 + 1 = 13. \] The reverse conversion divides repeatedly by two and reads the remainders upward: 13 gives remainder 1, then 6 gives 0, then 3 gives 1, then 1 gives 1 - reading up, 1101. It is worth memorising the first few powers of two, since almost every question in this area rests on them: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024. Note also that 1101 is D in hexadecimal, since one hexadecimal digit covers exactly four bits - which is why hexadecimal is used as shorthand for binary.

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: C

Correct answer: detect single-bit errors

A parity bit is an extra bit appended to a group of data bits so that the total number of 1s is even, in even parity, or odd, in odd parity. The receiver recomputes it and compares. It detects a single-bit error but cannot correct it, because it shows that something is wrong without indicating which bit. And it fails entirely on an even number of errors, since two flipped bits leave the parity unchanged - which is its principal limitation. It is generated in hardware by a cascade of XOR gates, whose output is 1 exactly when the number of 1s is odd. Stronger schemes exist for more demanding work. Hamming codes use several parity bits over overlapping groups and can locate and correct a single-bit error. Cyclic redundancy checks detect burst errors far more reliably and are what network frames and storage media actually use. Parity survives in memory modules and serial links, where it is cheap and usually sufficient.

Answer: B

Correct answer: message stream network will receive 1024 bytes only at one time

A message stream preserves the boundaries the sender wrote, so two writes of 1024 bytes arrive as two separate messages. A byte stream keeps no boundaries at all, so the same two writes may be read as one block of 2048 bytes, or in any other division.