N10-009 Question 319
Single answerCongestion/contention: Bottlenecking, Bandwidth, Throughput capacity, Latency, Packet loss, JitterA company recently moved its customer support team to a cloud-based VoIP platform. During the afternoon, agents report that calls sound choppy and words are occasionally cut off. File transfers and web browsing still work, but users notice a slight delay before hearing the other party speak. A network administrator reviews interface statistics on the WAN edge router and sees the link is consistently near 100% utilization during those periods, with increasing output drops on the egress interface. Which issue is the MOST likely cause of the poor call quality?
- A
High jitter and packet loss caused by WAN congestion at a bandwidth bottleneck
- B
Low latency caused by underutilization of the WAN link
- C
A DNS resolution failure affecting the VoIP server
- D
Duplex mismatch on user workstations causing increased local CPU usage
Show answer and explanation
Correct answer: A
Explanation
This question tests the ability to connect performance metrics to user symptoms. VoIP is highly sensitive to latency, jitter, and packet loss because voice traffic is real-time and cannot tolerate significant delay or retransmissions the way many data applications can. When the WAN edge interface is near full utilization and shows output drops, that indicates the egress queue is filling and packets are being discarded. As congestion increases, packets experience variable delay (jitter), longer queueing delay (latency), and eventual loss. These are classic signs of a bandwidth bottleneck limiting throughput capacity.
In practice, best-practice troubleshooting would include confirming utilization trends, queue drops, QoS policy behavior, and whether voice traffic is being prioritized. Industry guidance from vendors such as Cisco and general voice-network design best practices consistently note that VoIP quality is degraded by excessive one-way delay, jitter, and packet loss, especially during periods of congestion. The key applied takeaway is that bandwidth alone is not the only concern; usable throughput under load and proper traffic prioritization are critical to avoid contention and bottlenecking for latency-sensitive applications.
- A. Correct.
Correct. The scenario points to congestion/contention on the WAN link: utilization is near 100%, output drops are increasing, and the symptoms include choppy audio, clipped words, and conversational delay. In real-time traffic such as VoIP, a bandwidth bottleneck reduces available throughput capacity and leads to queueing delay, jitter, and packet loss. These effects directly degrade call quality much more noticeably than they affect ordinary file transfers or web browsing.
- B. Incorrect.
Incorrect. Low latency would not explain delayed audio, choppy calls, or output drops. Underutilization generally reduces contention rather than creating it. The symptoms and interface statistics indicate the opposite condition: the WAN link is saturated, increasing delay variation and drops.
- C. Incorrect.
Incorrect. DNS issues typically prevent endpoints from locating or connecting to services, especially during call setup or registration. They do not usually cause ongoing choppy audio only during peak utilization while other traffic continues to pass. The presence of high interface utilization and output drops strongly indicates a transport problem related to congestion, not name resolution.
- D. Incorrect.
Incorrect. A duplex mismatch can cause collisions, frame errors, and poor performance on a specific Ethernet segment, but this scenario identifies output drops on the WAN router's egress interface and a time-based pattern tied to peak usage. That is more consistent with a throughput bottleneck on the WAN circuit than with workstation NIC settings. Also, local CPU usage on workstations is not a defining symptom of duplex mismatch.