QOS - The Need for QOS Flashcards
What are the 3 leading causes of quality issues?
- Lack of bandwidth
- Latency and jitter
- Packet loss
What does the available bandwidth on the data path from a source to a destination equal to?
The capacity of the lowest-bandwidth link.
What 3 types of traffic should get prioritized forwarding?
- Voice
- Video
- Business-critical traffic
What is another name for one-way end-to-end delay?
Network latency
What is network latency?
The time it takes for packets to travel across a network from a source to a destination.
What does ITU Recommendation G.114 recommend for maximum latency regardless of the application?
That a network latency of 400 ms should not be exceeded
What does ITU Recommendation G.114 recommend for maximum latency for real-time traffic?
For real-time traffic network latency should be less than 150 ms
At what latency level has Cisco and ITU determined when real-time traffic significantly degrades?
When network latency exceeds 200 ms
What are the 2 types of network latency?
- Fixed latency
- Variable latency
What are the 3 types of Fixed latency?
- Propagation delay
- Serialization delay
- Processing delay
What is the 1 type of variable latency?
Delay variation
What is propagation delay?
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.
What is the speed of light?
299,792,458 meters per second in a vacuum.
In the case of fiber or copper cabling does the lack of a vacuum slow down the speed of light?
Yes
What is the refractive index?
The ratio indicating how much the speed of light is slowed down due to the lack of a vacuum.
What is the formula to determine the speed of light through a medium
Speed of light through a medium = Speed of light in a vacuum / Refractive index
What is the propagation delay for satellite circuits?
The time it takes a radio wave traveling at the speed of light to go from the earth’s surface to the satellite.
What is worse - a high refractive index or a low refractive index?
The larger the refractive index is the slower the speed of light is.
What is serialization delay?
The time it takes to place all the bits of a packet onto a link.
What is the formula to calculate serialization delay?
serialization delay = packet size in bits / link speed in bits per second
What is processing delay?
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.
What are 5 factors that affect processing delay?
- CPU speed
- CPU utilization (load)
- IP packet switch mode (process switching, CEF)
- Router architecture (centralized or distributed)
- Configured features on input and output interfaces
What is another name for Delay Variation?
Jitter
What is Delay Variation?
The difference in the latency between packets in a single flow.
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)?
70ms - 50ms = 20ms jitter (delay variation)
What causes jitter?
When there is a queueing delay experienced by packets during periods of network congestion.
What are 3 things that queuing delay depends on?
- The number and sizes of packets already in the queue
- Link speed
- Queuing mechanism
What can improve the effects of jitter?
De-jitter buffers that can help smooth out changes in packet arrival times due to jitter.
What 2 factors help define a de-jitter buffer?
- It is often dynamic
- Can adjust for approximately 30 ms changes in arrival times of packets
What does a de-jitter buffer do if a packet arrives outside the 30 ms?
It is dropped.
How is jitter prevented for high priority real-time traffic?
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.
What causes packet loss?
Congestion on an interface.
What 4 things can be done to prevent packet loss?
- Increase link speed
- Implement QoS congestion-avoidance and congestion-management mechanism
- Traffic policing
- Traffic shaping
Why is traffic shaping not recommended for real-time traffic?
Because it relies on queuing and queuing can cause jitter.
