Quality of service
The four measures of a link (throughput, latency, jitter, loss), what each use tolerates, best effort versus integrated and differentiated services, classifying and marking, queues, policing versus shaping and the token bucket.
- 28 cards
- 6 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Questions
- Sort and link
- The numbers
- Scenarios
- Understand
A few cards from the deck
Quality of service (QoS): what is it?
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The set of mechanisms that bound the throughput, delay, jitter or loss of certain flows, so that they are treated better than the others when the link is saturated.
It does not create bandwidth: on an idle link the queues are empty and nothing changes. It only decides who waits or loses packets when demand exceeds capacity.
A VoIP call keeps breaking up. Readings: 40 ms average delay, delays from 5 to 120 ms from one packet to the next, 0% loss, 0.1 Mb/s used on a 100 Mb/s link. Which measure is at fault?
- Jitter
- Latency
- Throughput
- Loss
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Jitter
A 40 ms average delay suits voice, the link is nearly empty and nothing is lost. What chops the voice is the 5 to 120 ms spread between delays: the receiver's buffer runs dry before the next packet arrives.
In DiffServ, a packet's mark is the … code, written in its … header; a configured router reads this mark to pick the packet's queue.
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In DiffServ, a packet's mark is the DSCP code, written in its IP header; a configured router reads this mark to pick the packet's queue.
The mark is set once, close to the source or at the edge of the trusted network; after that each router classifies from it without re-analysing the flow. The usual code for voice is EF (value 46).
Latency: what is it?
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The time a packet takes to travel from its source to its destination, or to make the round trip.
It adds up propagation in the cables, transmission, processing in the routers and waiting in queues. It is mainly the queueing part that QoS can reduce.