CCNA 3 Module 9 Quiz – QoS Concepts (Answers)

1. What is the term used to indicate a variation of delay?

  • speed mismatch
  • serialization delay
  • jitter
  • latency

Explanation: Jitter is a phenomenon caused by a variation in delay. Delay (or latency) is the time it takes for the packet to arrive at its destination. Jitter describes how the voice packets arrive at the destination at varying intervals (not in a steady, consistent stream).

2. A network engineer performs a ping test and receives a value that shows the time it takes for a packet to travel from a source to a destination device and return. Which term describes the value?

  • jitter
  • bandwidth
  • priority
  • latency

Explanation: Latency refers to the time it takes for a packet to travel from a source device to a destination device. Jitter is the variation in the latency values of received packets. Bandwidth is the measurement of how much traffic is transmitted in a single second. Priority is the preferential treatment of specific classes of traffic.

3. What role do network devices play in the IntServ QoS model?

  • Network devices provide a best-effort approach to forwarding traffic.
  • Network devices ensure that resources are available before traffic is allowed to be sent by a host through the network.
  • Network devices use QoS on a hop-by-hop basis to provide excellent scalability.
  • Network devices are configured to service multiple classes of traffic and handle traffic as it may arrive.

Explanation: The IntServ QoS model uses resource reservation to guarantee bandwidth and packet-loss rates from end to end. IntServ uses a connection-oriented approach to ensure that available resources are sufficient in the network for the traffic to have a specific level of QoS.

4. Which device would be classified as a trusted endpoint?

  • firewall
  • switch
  • IP phone
  • router

Explanation: Trusted endpoints are devices that have the capability to mark application traffic at Layer 2 or Layer 3. Trusted endpoints include the following:
– IP phones
– Wireless access points
– Videoconferencing gateways and systems
– IP conferencing stations and more

5. Under which condition does congestion occur on a converged network with voice, video, and data traffic?

  • if video traffic requests more bandwidth than voice traffic requests
  • if the request for bandwidth exceeds the amount of bandwidth available
  • if voice traffic latency begins to decrease across the network
  • if a user downloads a file that exceeds the file limitation that is set on the server

Explanation: Network congestion is caused by devices requesting more bandwidth than is available on a network. Congestion causes traffic to be temporarily queued or ultimately dropped.

6. Which type of traffic does Cisco recommend be placed in the strict priority queue (PQ) when low latency queuing (LLQ) is being used?

  • management
  • video
  • data
  • voice

Explanation: Cisco recommends that only voice traffic be placed in the priority queue (PQ) when using the low latency queuing (LLQ) method. All other traffic types are to be placed in other traffic queues and will be forwarded after voice traffic.

7. Which model is the only QoS model with no mechanism to classify packets?

  • hard QoS
  • DiffServ
  • best-effort
  • IntServ

Explanation: The best-effort model has no mechanism to classify packets and treats all networks packets equally. A voice packet would be treated in the same manner as an email packet.

8. What happens when the memory queue of a device fills up and new network traffic is received?

  • The network device will drop the arriving packets.
  • The network device sends the received traffic immediately.
  • The network device drops all traffic in the queue.
  • The network device queues the received traffic while sending previously received traffic.

Explanation: Packets received by a network device experiencing congestion cannot be transmitted until previous packets have been processed. If the queue is full, the network device will begin to drop packets as new traffic arrives.

9. What are two characteristics of voice traffic? (Choose two.)

  • It is bursty.
  • It consumes few network resources.
  • It is insensitive to packet loss.
  • It can tolerate latency up to 400 ms.
  • It is delay sensitive.

Explanation: Voice traffic does not consume much in the way of network resources, such as bandwidth. However, it is very sensitive to delay and dropped packets. For good voice quality, the amount of latency should be less than 150 milliseconds and packet loss less than 1%.

1o. Which QoS model is very resource intensive and provides the highest guarantee of QoS?

  • IntServ
  • DiffServ
  • soft QoS
  • best-effort

Explanation: The IntServ QoS model uses resource reservation and admission control mechanisms to schedule network resources.

11. What happens when an edge router using IntServ QoS determines that the data pathway cannot support the level of QoS requested?

  • Data is not forwarded along the pathway.
  • Data is forwarded along the pathway using a best-effort approach.
  • Data is forwarded along the pathway using DiffServ.
  • Data is forwarded along the pathway using IntServ but not provided preferential treatment.

Explanation: When the IntServ QoS model is being used, if network devices along a data path can reserve the required bandwidth, the originating application can begin transmitting. If the requested resource reservation cannot be allocated along the path, the originating application does not send any data.

12. In QoS models, which type of traffic is commonly provided the most preferential treatment over all other application traffic?

  • voice traffic
  • web traffic
  • email
  • file transfers

Explanation: Voice traffic from IP phones is commonly provided preferential treatment over all other application traffic such as email, web traffic, and file transfers.

13. Which queuing mechanism supports user-defined traffic classes?

  • WFQ
  • FCFS
  • FIFO

Explanation: Class-based weighted fair queuing (CBWFQ) goes beyond weighted fair queuing (WFC) by providing support for user-defined traffic classes. With CBWFQ, an administrator can use access control lists to define different traffic classes. Once traffic is classified, it receives is own queue for the egress interface.

14. What mechanism compensates for jitter in an audio stream by buffering packets and then replaying them outbound in a steady stream?

  • WFQ
  • voice codec
  • digital signal processor
  • playout delay buffer

Explanation: Jitter on the network can cause audio problems that can be heard and which are distracting in voice conversations. To compensate for the effects of jitter, a mechanism called a playout delay buffer is used. The playout delay buffer receives and buffers voice packets and then retransmits them in a steady stream with the jitter removed.

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