QOS - The Need for QOS

Question 1

What are the 3 leading causes of quality issues?

Answer 1

• Lack of bandwidth
• Latency and jitter
• Packet loss

Question 2

What does the available bandwidth on the data path from a source to a destination equal to?

Answer 2

The capacity of the lowest-bandwidth link.

Question 3

What 3 types of traffic should get prioritized forwarding?

Answer 3

• Voice
• Video
• Business-critical traffic

Question 4

What is another name for one-way end-to-end delay?

Answer 4

Network latency

Question 5

What is network latency?

Answer 5

The time it takes for packets to travel across a network from a source to a destination.

Question 6

What does ITU Recommendation G.114 recommend for maximum latency regardless of the application?

Answer 6

That a network latency of 400 ms should not be exceeded

Question 7

What does ITU Recommendation G.114 recommend for maximum latency for real-time traffic?

Answer 7

For real-time traffic network latency should be less than 150 ms

Question 8

At what latency level has Cisco and ITU determined when real-time traffic significantly degrades?

Answer 8

When network latency exceeds 200 ms

Question 9

What are the 2 types of network latency?

Answer 9

• Fixed latency
• Variable latency

Question 10

What are the 3 types of Fixed latency?

Answer 10

• Propagation delay
• Serialization delay
• Processing delay

Question 11

What is the 1 type of variable latency?

Answer 11

Delay variation

Question 12

What is propagation delay?

Answer 12

The time it takes for a packet to travel from the source to a destination at the speed of light over a medium such as fiber-optic cables or copper wires.

Question 13

What is the speed of light?

Answer 13

299,792,458 meters per second in a vacuum.

Question 14

In the case of fiber or copper cabling does the lack of a vacuum slow down the speed of light?

Answer 14

Yes

Question 15

What is the refractive index?

Answer 15

The ratio indicating how much the speed of light is slowed down due to the lack of a vacuum.

Question 16

What is the formula to determine the speed of light through a medium

Answer 16

Speed of light through a medium = Speed of light in a vacuum / Refractive index

Question 17

What is the propagation delay for satellite circuits?

Answer 17

The time it takes a radio wave traveling at the speed of light to go from the earth’s surface to the satellite.

Question 18

What is worse - a high refractive index or a low refractive index?

Answer 18

The larger the refractive index is the slower the speed of light is.

Question 19

What is serialization delay?

Answer 19

The time it takes to place all the bits of a packet onto a link.

Question 20

What is the formula to calculate serialization delay?

Answer 20

serialization delay = packet size in bits / link speed in bits per second

Question 21

What is processing delay?

Answer 21

The fixed amount of time it takes for a networking device to take the packet from an input interface and place the packet onto the output queue of the output interface.

Question 22

What are 5 factors that affect processing delay?

Answer 22

• CPU speed
• CPU utilization (load)
• IP packet switch mode (process switching, CEF)
• Router architecture (centralized or distributed)
• Configured features on input and output interfaces

Question 23

What is another name for Delay Variation?

Answer 23

Jitter

Question 24

What is Delay Variation?

Answer 24

The difference in the latency between packets in a single flow.

Question 25

If one packet takes 50 ms to traverse the network from the source to destination, and the following packet takes 70 ms, what is the jitter (Delay Variation)?

Answer 25

70ms - 50ms = 20ms jitter (delay variation)

Question 26

What causes jitter?

Answer 26

When there is a queueing delay experienced by packets during periods of network congestion.

Question 27

What are 3 things that queuing delay depends on?

Answer 27

* The number and sizes of packets already in the queue * Link speed * Queuing mechanism

Question 28

What can improve the effects of jitter?

Answer 28

De-jitter buffers that can help smooth out changes in packet arrival times due to jitter.

Question 29

What 2 factors help define a de-jitter buffer?

Answer 29

* It is often dynamic * Can adjust for approximately 30 ms changes in arrival times of packets

Question 30

What does a de-jitter buffer do if a packet arrives outside the 30 ms?

Answer 30

It is dropped.

Question 31

How is jitter prevented for high priority real-time traffic?

Answer 31

Use queuing mechanisms such as low-latency queueing (LLQ) that allow matching packets to be forwarded prior to any other low priority traffic during periods of network congestion.

Question 32

What causes packet loss?

Answer 32

Congestion on an interface.

Question 33

What 4 things can be done to prevent packet loss?

Answer 33

* Increase link speed * Implement QoS congestion-avoidance and congestion-management mechanism * Traffic policing * Traffic shaping

Question 34

Why is traffic shaping not recommended for real-time traffic?

Answer 34

Because it relies on queuing and queuing can cause jitter.