Computer Network Questions and Answers

Answer: D

Correct answer: NAK

Stop-and-wait ARQ has only one frame outstanding at a time, so a negative acknowledgement needs no sequence number: there is only one frame it could refer to. Numbered NAKs belong to the sliding-window protocols.

Answer: D

Correct answer: half duplex

Half duplex allows both directions but only one at a time, so the line must be turned round between transmissions. Full duplex allows both at once and simplex only one direction ever.

Answer: D

Correct answer: Three

A full adder takes three inputs - the two bits being added and a carry-in from the previous stage - and produces a sum and a carry-out. That carry-in is exactly what a half adder lacks, which is why half adders cannot be chained.

Answer: A

Correct answer: First three

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

Answer: A

Correct answer: E-mail

A mail message is small and its delivery is not time-critical, so it travels perfectly well over a slow link. Bulk file transfer and the traffic between nodes of a LAN both need the bandwidth.

Answer: A

Correct answer: less power than a TTL IC

CMOS draws significant current only while switching, so its static dissipation is minute compared with TTL, which draws current continuously through its output stage. Lowering the supply to 3 V reduces it further still, since dynamic power varies with the square of the supply voltage.

Answer: A

Correct answer: 2 (m - 2)

In a binary tree of \( 2^{m}-1 \) nodes the path from a leaf to the root has \( m-1 \) hops, so a message going up and back down again traverses about \( 2(m-2) \) links on average once the shared part of the two paths is discounted.

Answer: A

Correct answer: successive values differ in exactly one bit

Gray code is an ordering of binary values in which each differs from the next in exactly one bit. For three bits it runs 000, 001, 011, 010, 110, 111, 101, 100. That single-bit property solves a real problem. When several bits change at once in ordinary binary - 0111 to 1000, for instance - they will not switch at precisely the same moment, and a reading taken during the transition can be wildly wrong. With Gray code only one bit ever changes, so the worst error is one count. It is therefore used in shaft encoders and position sensors, where a mechanical brush or optical reader may sample mid-transition. It is also why the axes of a Karnaugh map are labelled in Gray code: adjacent cells then differ in one variable, which is what makes grouping valid. Conversion from binary is simple: the first bit is copied, and each later bit is the XOR of the corresponding binary bit with the one before it.

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: True

The statement is true. XNOR - exclusive NOR - gives 1 when its inputs are equal and 0 when they differ, which is the complement of XOR. Its truth table runs: 0,0 gives 1; 0,1 gives 0; 1,0 gives 0; 1,1 gives 1. For that reason it is also called the equivalence or coincidence gate, and its Boolean expression is \( AB + \bar{A}\bar{B} \). Its natural application is comparison. A chain of XNOR gates, one per bit pair, with their outputs ANDed together, forms an equality comparator: the result is 1 only if every bit matches. XOR does the opposite job and is used for parity generation and checking, since its output tells whether the number of 1s is odd. A useful identity: \( A \odot B = \overline{A \oplus B} \), so an XNOR is simply an XOR followed by an inverter, and the bubble on the symbol shows exactly that.