Cisco QoS FAQ: Quality of Service Overview
Q1. List the four traffic characteristics that QoS tools can affect.
Q2. Describe some of the characteristics of voice traffic when no QoS is applied in a network.
Q3. Describe some of the characteristics of video traffic when no QoS is applied in a network.
Q4. Describe some of the characteristics of data traffic when no QoS is applied in a network.
Q5. Interpret the meaning of the phrase, “QoS is both ‘managed fairness,’ and at the same time ‘managed unfairness’.”
Q6. Define bandwidth. Compare and contrast bandwidth concepts over point-to-point links versus Frame Relay.
Q7. Compare and contrast bandwidth and clock rate in relation to usage for QoS.
Q8. List the QoS tool types that affect bandwidth, and give a brief explanation of why each tool can affect bandwidth.
Q9. Define delay, compare/contrast one-way and round-trip delay, and characterize the types of packets for which one-way delay is important.
Q10. List the categories of delay that could be experienced by all three types of traffic: data, voice, and video.
Q11. Define, compare, and contrast serialization and propagation delay.
Figure: Serialization and Propagation Delay for Selected Packet and Link Lengths
Q12. Define network delay.
Q13. List the QoS tool types that affect delay and give a brief explanation of why each tool can affect delay.
Answer: Queuing, link fragmentation and interleaving, compression, and traffic shaping. Queuing methods use an algorithm to choose from which queue to take the next packet for transmission, which can decrease delay for some packets and increase delay for others. LFI tools break large frames into smaller frames, so that smaller delay-sensitive frames can be sent after the first short fragment, instead of having to wait for the entire large original frame to be sent.
Compression helps delay because it reduces the overall load in the network, reducing congestion, reducing queue lengths, and reducing serialization delays. Finally, traffic shaping actually increases delay, but it can be applied for one type of traffic, allowing other traffic to be sent with less delay.
Q14. Define jitter. Give an example that shows a packet without jitter, followed by a packet with jitter.
Q15. List the QoS tool types that affect jitter and give a brief explanation of why each tool can affect jitter.
Q16. Define packet loss and describe the primary reason for loss for which QoS tools can help.
Q17. List the QoS tool types that affect loss and give a brief explanation of why each tool can affect loss.
Q18. Describe the contents of an IP packet carrying the payload for a G.729 VoIP call.
Q19. Describe the amount of bandwidth required for G.711 and G.729 VoIP calls, ignoring data-link header/trailer overhead.
Q20. List the delay components that voice calls experience, but which data-only flows do not experience.
Q21. Define the meaning of the term “packetization delay” in relation to a voice call.
delay refers to the (default) 20 ms of delay, waiting for the speaker to speak long enough to fill the packet with the correctly sized payload.
Q22. List the different one-way delay budgets as suggested by Cisco and the ITU.
Q23. Define the term “codec delay” and discuss the two components when using a G.729 codec.
Q24. Describe the affects of a single lost packet versus two consecutive lost packets, for a G.729 voice call.
Q25. Describe a typical video payload flow in terms of packet sizes and packet rates.
Q26. Discuss the delay requirements of video traffic.
Q27. List the basic differences between TCP and UDP traffic.
28 Contrast the QoS characteristics needed by interactive data applications, as compared to the QoS needs of voice payload flows.
Delay for interactive data can be relatively longer than for voice, but the key measurement for data is application response time, which includes round-trip packet delays. Finally, data applications are much more tolerant of packet loss, because either the application will resend the data, or rely on TCP to resend the data, or just not care whether some data is lost.