Glossary · Networking

QoS (Quality of Service)

QoS is a set of techniques that prioritize certain network traffic, such as voice or video, so it gets consistent bandwidth, low delay and low loss.

Updated October 6, 2026

QoS (Quality of Service) refers to mechanisms that classify traffic and treat classes differently, using queues, priorities and rate limits so important flows are delivered ahead of less important ones during congestion.

Why it matters

When a link is saturated, a large download can delay a video call. QoS marks and queues packets, often using DSCP values in the IP header, so voice gets priority while bulk transfers wait. It only matters when there is contention; on an idle link it changes nothing. Markings are respected only within networks that agree to honor them, so QoS seldom works across the public Internet.

Example

Priority 1  voice (DSCP EF)     max 1 Mbps, low delay
Priority 2  video calls
Priority 3  web browsing
Priority 4  backups, updates

During a backup, call audio is still sent first, so it stays smooth.

Common mistakes

  • Prioritizing everything, which makes the priorities meaningless.
  • Setting QoS bandwidth limits above the real line rate, so the router never queues.
  • Expecting DSCP markings to survive the trip across the Internet.

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