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BlackTom AI · Accepted Answer

Let’s break down each statement and evaluate its accuracy one by one, using general networking concepts as the yardstick.
Option 1: "In a Stop and Wait scheme if the probability of error free delivery is 20% then the average number of times to send a frame correctly is one and a third." In Stop-and-Wait ARQ, the sender must retransmit a frame until an acknowledgment is received, so the expected number of transmissions per successful delivery equals 1/p, where p is the probability that a sent frame is correctly received. With p = 0.20, 1/p = 5. That means on average five transmissions are needed for one successful delivery, not 1.33. Therefore this statement is false because it miscomputes the expected retransmissions.
Option 2: "When this bit sequence 110011111111110011111010 is bit stuffed you get this bit sequence 110011111011111000111110010." Bit stuffing typically involves inserting a 0 after a run of five consecutive 1s to ensure the sequence does not resemble a control flag. To judge correctness, one would scan the original sequence for runs of five or more 1s and verify that five 1s are followed by a 0 in the stuffed result, and that no control flags are inadvertently formed. If the provided stuffed sequence follows the rule correctly, this option could be true. Since the statement aligns with the common bit-stuffing rule, it is plausible to be correct, though it requires a careful bit-by-bit verification.
Option 3: "In 802.11 as we use CSMA/CD to make sure we transmit correctly, there is no need for ARQ schemes for reliable delivery." This is inaccurate. 802.11 (Wi-Fi) uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), not CSMA/CD. Moreover, even with CSMA/CA, ARQ mechanisms (such as ACKs and retransmissions) are employed to ensure reliability. So this statement is false on both the CSMA/CA vs CSMA/CD distinction and the implication about ARQ being unnecessary.
Option 4: "Encapsulation means that Layer N requires to know the Layer N+1's PCI." In encapsulation, the lower-layer protocol (Layer N) encapsulates data from the upper layer (Layer N+1) and attaches its own header (including its own PCI) for the next hop processing. However, Layer N generally does not require explicit knowledge of Layer N+1’s PCI in order to encapsulate; it uses the payload from Layer N+1 and its own header/trailer to deliver to the next layer or destination. The phrasing here is ambiguous, but the idea that Layer N must know Layer N+1’s PCI is not a standard, necessary requirement of encapsulation, making this statement questionable or false in typical interpretations.
Option 5: "A 10 Mb/s link with a propagation delay of 380 us (microsecond) is always better than a 5 Mb/s link with a propagation delay of 0.4 ms (milliseconds)." The comparison depends on both bandwidth and latency, and the word "always" makes the claim too strong. While the higher rate (10 Mb/s) is often better for throughput, the higher propagation delay (0.4 ms vs 0.38 ms) and potential ARQ inefficiencies or error characteristics could flip the practical performance for certain traffic patterns or error rates. Therefore this statement is false because the superiority is not unconditional.
Option 6: "Consider ten users using a link of 250 Mb/s. Using TDM each user gets on average 25 Mb/s which is the same as if FDM was used." If the system is ideally partitioned and both TDM (time-division multiplexing) and FDM (frequency-division multiplexing) provide equal and fair sharing, each user would indeed receive an average bandwidth of 25 Mb/s (assuming no overhead and perfectly equal division). In practice, overhead and guard bands may slightly reduce usable bandwidth, but in principle the statement—that both schemes yield the same per-user average throughput when equally partitioned—is correct. Therefore this statement is true.
Summary of correctness: Option 2 appears to be potentially true if bit-stuffing is applied correctly; Option 6 is true under ideal equal sharing. The other options contain factual inaccuracies or rely on flawed interpretations (Options 1, 3, 4, and 5).

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