What is Quality of Service (QoS) and why is it important?

QoS is the primary mechanism for reliable delivery — it prioritizes time-sensitive traffic (like voice and video) when bandwidth demand exceeds capacity.

QoS is the primary mechanism to ensure reliable delivery of content for all users by managing network congestion and prioritizing traffic. With a QoS policy in place, the router can more easily manage the flow of data and voice traffic.

* When the link is congested, a QoS policy sends time-sensitive traffic first and lets delay-tolerant traffic wait. *

Why needed: When bandwidth demand exceeds available capacity, some traffic must be prioritized — otherwise live video stutters and calls break up.

Priority examples:

High-priority traffic needs smooth, uninterrupted delivery; the lower-priority traffic can tolerate some delay.

Go deeper:

• Quality of service (Wikipedia) — explains prioritizing time-sensitive traffic (VoIP/video) over other queues during congestion.
• The Network as a Platform (Cisco Press) — the ITN framing of QoS as a core architecture characteristic for priority delivery when demand exceeds capacity.