Computer Network Questions and Answers

Answer: A

Correct answer: False

The statement is false. With both S and R at 1 the SR flip-flop enters its forbidden or invalid condition. The problem is twofold. While both inputs are held high, Q and its complement are driven to the same level, so the outputs are no longer complementary and the device is not storing a meaningful bit. Worse, when the inputs are released together the circuit settles into whichever state wins a race between its two gates, which depends on minute differences in propagation delay - so the resulting state is genuinely unpredictable. The JK flip-flop was designed to remove this defect. By feeding the outputs back into the input gating, it converts the J = K = 1 case into a useful toggle, and that toggle is what makes binary counters possible. The D flip-flop avoids the problem differently, by deriving both internal inputs from a single data line so the forbidden combination can never arise.

Answer: A

Correct answer: the D input just before the edge

The D flip-flop simply stores whatever is on its D input at the active clock edge, holding it until the next edge. This makes it the natural building block for registers and memory cells.

Answer: B

Correct answer: AND gate

The rules describe an output that is HIGH only when both inputs are HIGH and LOW in every other case, which is the AND function. An OR gate would be HIGH whenever either input was HIGH.

Answer: C

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

The XOR function is 1 when the two inputs differ, which happens in exactly two cases: A is 1 and B is 0, or A is 0 and B is 1. Written as a sum of products that is \[ A \oplus B = A\bar{B} + \bar{A}B. \] The second option, \( AB + \bar{A}\bar{B} \), is 1 when the inputs are the same, and is therefore the complement - the XNOR function, also called the equivalence or coincidence gate. XOR has properties that make it unusually useful. \( A \oplus 0 = A \) and \( A \oplus 1 = \bar{A} \), so one input acts as a controlled inverter. \( A \oplus A = 0 \), which is why XOR finds the unpaired element in a list and why it underlies simple ciphers and the swap-without-a-temporary trick. In arithmetic it produces the SUM output of a half adder, and cascaded it generates and checks parity bits.

Answer: D

Correct answer: American Standard Code for Information Interchange

ASCII is the American Standard Code for Information Interchange, a character encoding published in 1963 that assigns a numeric value to each letter, digit and punctuation mark. It is a 7-bit code, giving 128 values: 0 to 31 are control characters such as carriage return and line feed, 32 is the space, 48 to 57 are the digits 0 to 9, 65 to 90 are the capital letters and 97 to 122 the small ones. Two consequences of that layout are worth knowing. The gap of exactly 32 between a capital and its small counterpart means the case of a letter is changed by flipping a single bit. And because the digits begin at 48, a numeric character is converted to its value by subtracting 48. Extended ASCII uses the eighth bit for a further 128 characters. Unicode superseded it for international text, but keeps ASCII as its first 128 code points, which is why the two remain compatible.

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: D

Correct answer: Binary

Digital computers work internally in binary, base 2, using only the digits 0 and 1. The reason is physical rather than mathematical. A circuit can distinguish two voltage levels reliably and cheaply - on or off, high or low - and that reliability degrades quickly if more levels must be told apart. Two states also map directly onto the switching behaviour of a transistor and onto the true and false of Boolean algebra, which is what makes logic gates possible. Octal and hexadecimal are used by people rather than by machines, as compact ways of writing binary: each octal digit stands for exactly three bits and each hexadecimal digit for exactly four, so conversion is done by grouping rather than by division. A byte such as 11010110 becomes D6 in hexadecimal at a glance. Decimal is what humans use, and converting between it and binary requires actual arithmetic, which is why it is not used internally.

Answer: C

Correct answer: To detect errors in transmitted data

A parity bit is appended so that the total number of 1s is always even or always odd, letting the receiver spot a single flipped bit. It detects an odd number of errors but cannot say which bit changed, nor correct it.

Answer: A

Correct answer: computer bus

A computer's internal bus runs at rates no external line approaches: it is short, parallel and driven at the processor's own clock. Telephone and leased lines are limited by the distance they must cover and by the medium available.

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.