📝 Admission control in quality of service (10 MCQs)
📖 From Data Communication and Networks • 30. Quality of Service • 10 questions available
What is Admission control in quality of service?
Admission control is the decision-making process at network entry points that determines whether to accept or reject a new flow request based on whether the network currently has enough available resources to satisfy its QoS requirements without degrading existing services.
📝 All Admission control in quality of service MCQs
Q1. A router implements call admission control that reserves exactly 2 Mbps for each voice call. The total available bandwidth is 10 Mbps and three calls are already active. How many additional calls can be admitted without exceeding the bandwidth limit?
📖 Explanation: Each active call consumes 2 Mbps, so three calls use 6 Mbps. The remaining bandwidth is 10 Mbps − 6 Mbps = 4 Mbps. Since each new call also requires 2 Mbps, at most 4 Mbps ÷ 2 Mbps = 2 additional calls can be admitted. Therefore option A is correct.
Q2. In the context of network admission control, what does the acronym CAC stand for?
📖 Explanation: CAC is the standard abbreviation used in networking literature to denote the mechanism that decides whether a new traffic flow can be admitted based on current resource availability. It stands for Call Admission Control, which directly reflects its purpose of managing call or session admission. Hence option A is the accurate definition.
Q3. Which statement best describes why admission control is essential for maintaining QoS in VoIP networks?
📖 Explanation: Admission control deliberately restricts the number of simultaneous voice sessions so that the network never exceeds its capacity. By preventing congestion, packet loss and jitter remain low, which are critical for voice quality. The other options either describe unrelated mechanisms or would degrade QoS, making option B the most accurate.
Q4. When comparing H.323 and SIP for large‑scale video‑conferencing deployments, which factor most strongly favors SIP?
📖 Explanation: SIP is a lightweight, stateless protocol that can operate without maintaining persistent sessions, which simplifies scaling. Its text‑based messages traverse NAT devices more readily than H.323’s binary, stateful exchanges. Consequently, SIP incurs lower signaling overhead and scales better in large deployments, making option B the strongest advantage.
Q5. A network employs a CAC threshold of 80 % of link utilization. Current utilization is 78 % and a new flow would increase usage by 3 %. What action will the CAC module take?
📖 Explanation: The current utilization (78 %) plus the additional 3 % would raise total usage to 81 %, which exceeds the predefined 80 % threshold. CAC is designed to reject any flow that would push utilization beyond the limit, so the correct response is to reject the flow, option C.
Q6. Assume call arrivals follow a Poisson process with rate λ = 4 calls/min, each call consumes 1.5 Mbps, and the link capacity is 12 Mbps. Using the Erlang B formula, which approximate probability correctly represents the chance that an incoming call finds enough free bandwidth to be admitted?
📖 Explanation: First determine the number of equivalent circuits: 12 Mbps ÷ 1.5 Mbps = 8 circuits. Assuming an average holding time of 2 min, the offered traffic A = λ·T = 4·2 = 8 Erlangs. Applying the Erlang B formula for N = 8 and A = 8 yields a blocking probability of about 0.45, so the admission probability is 1 − 0.45 ≈ 0.55, matching option B.
Q7. In a two‑class service network (premium and best‑effort), the CAC algorithm reserves 30 % of total bandwidth exclusively for premium users. If premium traffic demands 20 % of the link and best‑effort traffic demands 70 % of the link, what is the resulting admission decision for a new best‑effort flow requiring 5 % of bandwidth?
📖 Explanation: Premium users are guaranteed 30 % of the bandwidth, leaving a maximum of 70 % for best‑effort traffic. Since best‑effort already occupies the full 70 % share, any additional 5 % request would surpass its allocation, leading to rejection. Hence option D correctly describes the outcome.
Q8. A CAC system uses adaptive thresholds that increase by 2 % after each successive admission and decrease by 3 % after each rejection. Starting from a threshold of 60 %, the system admits three consecutive flows and then rejects one. What is the new threshold value?
📖 Explanation: Begin at 60 %. After three admissions the threshold rises three times: 60 % + 2 % = 62 %, then 64 %, then 66 %. The subsequent rejection reduces it by 3 %: 66 % − 3 % = 63 %. Therefore the new threshold is 63 %, option C.
Q9. Which of the following best explains the impact of adaptive CAC on bursty traffic patterns?
📖 Explanation: Adaptive CAC monitors recent traffic intensity and temporarily reduces the admission limit when a burst is detected, preventing the network from becoming overloaded. By throttling new admissions during peaks, it helps maintain low jitter and packet loss, which are essential for real‑time services. The other options misrepresent the behavior, making option A correct.
Q10. A network operator evaluates two CAC policies: Policy X admits calls up to 80 % link utilization, while Policy Y admits calls up to 70 % but provides a 10 % bandwidth headroom for priority packets. Which policy is more likely to meet strict latency requirements for real‑time video?
📖 Explanation: Policy Y deliberately reserves a 10 % margin, ensuring that priority video packets encounter less competition for bandwidth even when the link is heavily loaded. This extra headroom translates into lower queuing delay and jitter, which are critical for strict latency. Consequently, Policy Y (option B) is more suitable for real‑time video.