What Is QoS (Quality of Service)?
QoS (Quality of Service) — a set of techniques that prioritise certain network traffic over others — ensuring critical applications (voice, video) get bandwidth and low latency even when the network is busy.
How it works
Without QoS, all traffic competes equally — a large file download can starve a voice call of bandwidth, causing choppy audio. QoS classifies traffic (e.g. by marking with DSCP values), then applies policies: priority queuing for real-time traffic, guaranteed minimum bandwidth for business apps, and policing/shaping to control excess.
Why it matters
QoS is essential wherever voice, video and data share the same links — virtually every modern network. It appears throughout the CCNA and CCNP blueprints (classification, marking, queuing, shaping vs policing) and is a common interview topic for enterprise network roles.
Frequently asked questions
What does QoS do?
It prioritises certain network traffic (like voice and video) over less time-sensitive traffic (like file downloads), ensuring critical applications perform well even under congestion.
What is the difference between QoS shaping and policing?
Shaping buffers and delays excess traffic to smooth it to a rate; policing drops or remarks traffic that exceeds a rate immediately, without buffering.
Why does voice traffic need QoS the most?
Voice is extremely sensitive to latency, jitter and packet loss — QoS gives it priority queuing so calls stay clear even when the network is busy with other traffic.
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