Computer Network Questions and Answers

Answer: A

Correct answer: NAND

NAND is a universal gate, meaning that any logic function whatever can be built using NAND gates alone. NOR is the other universal gate. The proof is constructive. An inverter is a NAND with both inputs tied together. An AND is a NAND followed by that inverter. An OR is a NAND fed by two inverters, which is De Morgan’s theorem realised in hardware: \( \overline{\bar{A} \cdot \bar{B}} = A + B \). Since AND, OR and NOT together are sufficient for every Boolean function, and all three can be made from NAND, NAND alone is sufficient. The property matters commercially. A manufacturer can fabricate a whole logic family from one repeated cell, which simplifies design, testing and inventory, and the regularity suits automated layout. AND, OR and XOR are not universal, because none of them can produce an inversion: with only non-inverting gates, an input of all 1s can never yield a 0.

Answer: A

Correct answer: NAND

By De Morgan's law \( \overline{A}+\overline{B} = \overline{A\cdot B} \), which is the NAND function. It is the expression for a NAND written with the inversion pushed inwards onto the variables.

Answer: A

Correct answer: True

The statement is true. This is the distributive law, and it is the same identity as in ordinary algebra, where multiplication distributes over addition. What has no ordinary counterpart is its dual, which also holds in Boolean algebra: \[ A + B\cdot C = (A + B)(A + C). \] Addition distributes over multiplication, which is false for numbers and true here - a good illustration that Boolean algebra is a different system despite the familiar notation. Every Boolean identity has such a dual, obtained by exchanging AND with OR and 0 with 1. The pairing runs through the whole set: \( A + 0 = A \) with \( A \cdot 1 = A \), \( A + 1 = 1 \) with \( A \cdot 0 = 0 \), and De Morgan’s two theorems with each other. The distributive law is what allows an expression to be converted between sum-of-products and product-of-sums form, which is the first step in most simplification.

Answer: A

Correct answer: True

The statement is true, and it is the standard distinction between the two. A latch is transparent while its enable input is at the active level: as long as the enable is high, changes at the data input pass straight through to the output. A flip-flop responds only at the instant the clock changes - on the rising or falling edge - and ignores its input at every other moment. That difference matters greatly in synchronous design. Because a flip-flop samples at a single instant, its output is stable for a whole clock period, which makes timing analysis tractable: one need only check that data arrives before the setup time and holds past the hold time. A latch’s transparency window makes the same analysis far harder and can allow a signal to race through two stages in one cycle. Latches are still used where their transparency is wanted, and internally a flip-flop is often built as two latches in a master-slave arrangement.

Answer: C

Correct answer: UUCP

UUCP - Unix to Unix Copy - copies files between machines over a single link, and was the basis of early mail and news distribution. TFTP is a trivial transfer protocol and UART is a hardware device.

Answer: D

Correct answer: 2000

A single-stage crossbar needs one crosspoint for every input-output pair:

\[ 40\times 50 = 2000 \]

Answer: C

Correct answer: 8-12

Digits 8 to 12 identify the terminal, following the four-digit DNIC and the three-digit area code.

Answer: B

Correct answer: 12 cells

The burst is limited by the interval L and the peak cell rate, and the excess over what the line itself would deliver in that time works out at 12 cells for the figures given.

Answer: C

Correct answer: Poll

A secondary never transmits unbidden. It sends data only in response to a poll - the primary's invitation to do so. Select is the opposite direction, the primary announcing that it has data to send.

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.