Computer Network Questions and Answers

Answer: B

Correct answer: Reliability

Whether the network keeps working through failures - equipment faults, power cuts, natural disasters - is a question of reliability. Security concerns deliberate attack and performance concerns speed.

Answer: C

Correct answer: presentation layer

The presentation layer deals with the representation of data - character encoding, compression and encryption - so that the two ends interpret it the same way. Encryption may also be applied elsewhere in practice, but in the OSI model it belongs here.

Answer: C

Correct answer: Four

Binary Coded Decimal stores each decimal digit in its own group of four bits, so 27 becomes 0010 0111. The six combinations from 1010 to 1111 are unused, which makes BCD less compact than pure binary.

Answer: A

Correct answer: simplify Boolean expressions

A Karnaugh map arranges a truth table so that adjacent cells differ in one variable, letting groups of ones be combined by inspection. It is a visual substitute for repeated algebraic manipulation.

Answer: B

Correct answer: the value of D just before the clock edge

A D flip-flop simply captures whatever is on its D input at the active clock edge and holds it until the next one. Its characteristic equation is \( Q_{next} = D \). The D stands for data or delay, and both readings are apt: the device stores one bit of data, and it delays a signal by exactly one clock period. Its virtue is that it has no forbidden or ambiguous input combination, unlike the SR flip-flop, and needs no input decoding, unlike the JK. That simplicity is why it is the workhorse of synchronous design and why registers, shift registers and pipeline stages are built from it. For the capture to be reliable the D input must be stable for the setup time before the edge and the hold time after it. Violating either can leave the output metastable - hovering between levels for an unpredictable period - which is why an asynchronous input is passed through two D flip-flops in series before being used.

Answer: C

Correct answer: move stored bits one position at each clock pulse

A shift register is a chain of flip-flops in which the output of each feeds the input of the next, so that on every clock pulse the whole pattern of bits moves one position along. The four configurations are named by how data enters and leaves. Serial-in serial-out is used for delay. Serial-in parallel-out converts a bit stream into a word, which is what a receiver does. Parallel-in serial-out does the reverse and is what a transmitter does. Parallel-in parallel-out acts as a simple storage register. Serial-to-parallel conversion is the reason shift registers are fundamental to communications, since data travels serially over a single line but is processed in parallel words. Shifting also performs arithmetic: shifting left multiplies a binary number by two and shifting right divides it by two, which is far cheaper in hardware than a multiplier. Feeding the output back to the input gives a ring or Johnson counter.

Answer: B

Correct answer: 8

A byte is 8 bits. The convention was settled by the IBM System/360 in the 1960s and has been universal since; a byte of eight bits can represent 256 distinct values, which was enough for a character set with room to spare. A group of four bits is a nibble, and it corresponds exactly to one hexadecimal digit - which is why a byte is written as two hexadecimal characters, from 00 to FF. Above the byte the units run kilobyte, megabyte, gigabyte and terabyte. There is a real ambiguity here worth knowing: in the decimal convention a kilobyte is 1,000 bytes, while in the binary convention it is \( 2^{10} = 1{,}024 \). The standard prefixes for the binary values are kibibyte, mebibyte and gibibyte, though they are rarely used outside standards documents. Storage manufacturers quote decimal values and operating systems often report binary ones, which is why a drive sold as 1 TB shows as roughly 931 GB.

Answer: A

Correct answer: CMOS

CMOS - complementary metal-oxide-semiconductor - consumes very little power, which is why it displaced every earlier family and is used in essentially all modern integrated circuits. The reason lies in its structure. Each gate pairs a p-channel and an n-channel transistor so that in either stable state one of the two is off, and almost no current flows from supply to ground. Power is drawn chiefly when the output changes, charging and discharging the capacitance of the next stage - which is why CMOS power consumption rises with clock frequency. TTL, transistor-transistor logic, is faster than early CMOS but draws current continuously. ECL, emitter-coupled logic, is the fastest of the three because its transistors never saturate, but it consumes the most power of all. RTL was among the earliest families and is obsolete. CMOS also tolerates a wide supply range and has high noise immunity, though it is sensitive to static discharge.

Answer: C

Correct answer: NOT

The NOT gate is the inverter. It has a single input and a single output, and the output is always the complement of the input: 0 becomes 1 and 1 becomes 0. Its Boolean expression is \( Y = \bar{A} \). On a circuit diagram it is drawn as a triangle with a small circle, or bubble, at its output. That bubble is the general symbol for inversion, which is why a NAND gate is drawn as an AND gate with a bubble and a NOR gate as an OR gate with one. The NOT gate is the only basic gate with one input; AND, OR, NAND, NOR and XOR all take two or more. It is easily built from a universal gate: tying both inputs of a NAND or a NOR together produces an inverter, which is part of what makes those gates universal. Two inverters in series cancel, giving \( \bar{\bar{A}} = A \) - the involution law of Boolean algebra.

Answer: C

Correct answer: 8

A counter with \( n \) bits passes through \( 2^{n} \) distinct states, so with three bits \[ 2^{3} = 8, \] running from 000 to 111 - decimal 0 to 7 - before returning to 000. That return is why such a device is called a modulus-8 counter, and why its highest output is the clock frequency divided by eight. Counters are therefore used as frequency dividers as often as they are used to count. The same relation works in reverse: a counter of modulus \( N \) needs the smallest \( n \) for which \( 2^{n} \ge N \). A decade counter of modulus 10 needs four flip-flops, and the six surplus states are removed by feedback that clears the counter when it reaches ten. It is the identical arithmetic that fixes the number of select lines on a multiplexer, the outputs of a decoder and the address lines of a memory - one relation worth being fluent in.