📝 S/MIME secure email protocol (23 MCQs)

📖 Explanation: An I‑frame, also called an intra‑coded frame, is self‑contained; it can be decoded without reference to any other frame. This independence allows a decoder to start playback at any I‑frame. The other options describe P‑ or B‑frames or unrelated concepts, making B the correct definition.

📖 Explanation: A P‑frame (predicted frame) relies on the immediately preceding I‑ or P‑frame and stores only the changes, reducing the amount of data needed. Unlike I‑frames, it cannot be decoded on its own, and unlike B‑frames, it does not use future information. Therefore option D accurately captures its role.

📖 Explanation: Because a B‑frame depends on both a previous and a future reference, the loss of its future I‑frame removes essential prediction data. Without that reference, the decoder cannot reconstruct the B‑frame correctly, leading to a noticeable artifact. The other options assume fallback mechanisms that MPEG does not provide for B‑frames.

📖 Explanation: B‑frames are bidirectional and can reference both past and future frames, allowing them to exploit temporal redundancy more effectively than P‑frames (which reference only past frames) and I‑frames (which contain full image data). Consequently, B‑frames usually achieve the greatest compression, making option C correct.

📖 Explanation: Periodic I‑frames provide complete picture information, enabling a decoder to begin playback at any point in the stream. Without regular I‑frames, a viewer joining late would encounter incomplete data and could not reconstruct the video, which is why option B is the correct justification.

📖 Explanation: The two B‑frames that immediately follow the I‑frame reference that I‑frame (as the past reference) and the subsequent P‑frame (as the future reference). Both share the same I‑frame as their past reference, so exactly two B‑frames depend on that I‑frame, making option B correct.

📖 Explanation: The bitrate is calculated as $1920 \times 1080 \times 30 \times 24 = 1,492,992,000$ bits per second, which is approximately 1.5 Gbps. This matches option C, while the other choices underestimate or overestimate the required bandwidth for the given parameters.

📖 Explanation: Bandwidth is directly proportional to frame rate when all other factors (resolution, colour depth) remain unchanged. Doubling the frame rate from 30 fps to 60 fps therefore doubles the amount of data transmitted each second, so the required bandwidth also doubles, making option A correct.

📖 Explanation: At 30 fps, two seconds contain 60 frames, and one I‑frame is placed in that interval. One minute equals 60 seconds, which contains 30 intervals of two seconds, so 30 I‑frames are transmitted. Therefore option B correctly reflects the count.

📖 Explanation: Adding B‑frames improves compression because they can reference both past and future frames, but each B‑frame requires the decoder to wait for future reference frames, which adds decoding delay. Hence, the increase in B‑frames simultaneously raises compression efficiency and latency, making option D the best description.

📖 Explanation: A pattern with a single I‑frame followed by multiple B‑frames and a single P‑frame (I B B B P B B B I) maximizes temporal compression while limiting the distance between reference frames, which keeps the decoding delay under half a second. This arrangement therefore satisfies both bandwidth and latency requirements, making option D correct.

📖 Explanation: B‑frames can serve as reference frames for subsequent B‑frames. When a B‑frame is missing, any later B‑frames that would have used it as a past or future reference lose that information, potentially causing visual artifacts or decoding errors, while P‑frames remain unaffected. Hence option C is correct.

📖 Explanation: MPEG‑4 was designed to improve compression efficiency and often employs a larger proportion of B‑frames compared to MPEG‑2, leveraging their bidirectional prediction to achieve higher quality at lower bitrates. This distinction makes option B the accurate statement.

📖 Explanation: Sending an I‑frame every half‑second guarantees that a viewer who starts at any moment will encounter a full picture within at most 0.5 seconds, well within the two‑second requirement. The other options either do not provide a complete picture or are unrelated to video synchronization, so option A is correct.

📖 Explanation: Examining the pattern reveals P‑frames at the fourth and seventh positions. No other frames are labeled P, so the total count of P‑frames is two, which corresponds to option A.

📖 Explanation: When audio and video are compressed together, algorithms can identify and remove redundancies that span both streams (e.g., shared timing information), potentially achieving a lower combined bitrate than compressing each independently. This cross‑modal advantage makes option B the correct choice.

📖 Explanation: The raw frame size is $1920 \times 1080 \times 24 = 49,766,400$ bits, or about 6.22 MB. Reducing this by a factor of 50 yields roughly 0.124 MB (≈ 0.12 MB). Therefore option A accurately reflects both the raw and compressed sizes.

📖 Explanation: A missing P‑frame removes a reference that later P‑ and B‑frames depend on, causing a cascade of errors until the next I‑frame. In contrast, a missing B‑frame does not serve as a reference, so only that frame is degraded. Forward error correction or retransmission of critical P‑frames can mitigate the more severe propagation.

📖 Explanation: High‑motion video benefits from predictive coding; mixed I/P/B streams can represent changes with far fewer bits than a sequence of full I‑frames, which must encode each picture independently. The resulting entropy reduction can be on the order of 60 % or more, making option A the accurate assessment.

📖 Explanation: Encapsulating each GOP in a S/MIME container provides end‑to‑end confidentiality and authenticity while keeping the MPEG frame boundaries intact. Session‑based key management allows efficient encryption without re‑encrypting the whole stream, making option C the most practical and secure solution.

📖 Explanation: Each frame requires $1280 \times 720 \times 24 = 22,118,400$ bits. With a 5 Mbps channel, the maximum frame rate is $5,000,000 / 22,118,400 \approx 0.226$ fps, which rounds to about 0.2 fps. Thus option A correctly reflects the theoretical limit.

📖 Explanation: Adding more B‑frames introduces extra bidirectional references, requiring the decoder to store additional future and past frames and perform more motion‑compensation calculations. Consequently, both computational complexity and the size of the reference buffer grow, confirming option B as the correct description.

📖 Explanation: Deep‑learning models can capture complex temporal patterns, offering superior compression ratios. However, they typically require substantial processing power and introduce additional latency due to inference time, posing challenges for real‑time playback. Option A accurately captures both the benefit and the drawback of such an approach.