A policy for pressure
QoS chooses which trade-off matters first.
Voice and interactive video are sensitive to delay and jitter. A bulk backup can usually tolerate waiting, but it benefits from throughput. QoS classifies traffic, marks its intended treatment, and applies a scheduling or rate policy when multiple flows compete for the same resource.
QoS is local to the devices and links that enforce it. A priority queue can protect a voice flow on one router, but it cannot guarantee an end-to-end result across networks that do not honor the same markings.
Build the path
A flow becomes a treatment.
Classify
Identify the application, address, port, or policy class.
Mark
Attach a value such as a DSCP codepoint to the packet.
Queue
Schedule classes with priority, weighted service, or fairness.
Shape
Delay excess traffic so its rate fits a defined profile.
Four QoS tools
Different controls solve different problems.
Classification
Groups packets by observable attributes and policy.
Marking
Communicates a class so later devices can recognize it.
Scheduling
Chooses which queued packet is served next.
Shaping
Smooths a sender's rate by buffering excess traffic.
Mini investigation
Choose a treatment for each flow
Select a traffic type and identify the most defensible first policy.
Voice needs bounded delay and low jitter more than maximum throughput.
Classify it as real-time traffic and give it a bounded, carefully monitored priority treatment.
Knowledge check
Test your QoS model
Answer eight questions. The answer order changes each time.