Adaptive or dynamic directory used in packet routing changes
Answer: A
Correct answer: within each user session
An adaptive routing table is rebuilt as conditions change - as links fail or queues lengthen - so it may well change several times during one session. A table fixed at system generation is static routing.
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Maximum data rate of a channel of 3000 Hz bandwidth and SNR of 30 dB is
Answer: C
Correct answer: 30,000 bps
Shannon's formula applies, with 30 dB meaning a power ratio of 1000:
\[ C = B\log_{2}\!\left(1+\frac{S}{N}\right) = 3000\log_{2}(1001) \approx 3000\times 9.97 \approx 30{,}000\ \text{bps} \]
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Minimum number of bits required to represent B is
Answer: C
Correct answer: 3
Three bits distinguish eight values, which is the smallest power of two large enough for the symbol set in question: \( 2^{2}=4 \) is too few and \( 2^{3}=8 \) suffices.
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A 3000 Hz bandwidth noisy channel transmits bits with a signal to thermal noise ratio of 30 dB. What is the maximum data rate of the channel ?
Answer: A
Correct answer: 30 kbps
This is Shannon's limit again, with a signal-to-noise ratio of 30 dB, that is 1000:
\[ C = 3000\log_{2}(1001) \approx 30{,}000\ \text{bps} \]
Note that the answer does not depend on how many levels the signalling uses; Shannon's bound holds whatever the modulation.
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In a broad sense, a railway track is an example of
Answer: B
Correct answer: half-duplex
A single railway track carries traffic both ways but only one way at a time, which is the definition of half duplex.
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Which of the following access control methods is probabilstic ?
Answer: D
Correct answer: Contention
In a contention scheme such as CSMA/CD a station transmits when it judges the medium free, and whether it succeeds depends on what other stations happen to do - so access is probabilistic. Polling and token passing grant access deterministically, by turn.
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In stop-and-wait ARQ, if data 1 has an error, then receiver sends which frame ?
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.
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In which communication, poll/select method used to determine control of the line ?
Answer: D
Correct answer: primary-to-secondary
Poll and select belong to a configuration with one primary station controlling several secondaries: the primary polls a secondary to invite it to send, and selects it to warn it that data is coming. Peers by definition need no such controller.
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The secondary device in a multipoint configuration sends data in response to which of the following event ?
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.
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Which of the following decides the role (sender or receiver) of a device on a network ?
Answer: C
Correct answer: Line discipline
Line discipline is the part of the data link layer that settles who may transmit and who must listen, by enquiry and acknowledgement or by poll and select.
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