QoS

QoS (Quality of Service) is the set of techniques used to prioritize network traffic so that latency-sensitive traffic — VoIP calls, video conferencing — isn't degraded when the network is congested.

Networks don't inherently know the difference between a VoIP packet and a file download. Under load, both get treated equally — which means a large file transfer can cause call quality to deteriorate. QoS fixes this by marking traffic and giving higher-priority traffic preferential treatment in queues.

The most common framework is DSCP (Differentiated Services Code Point) — a 6-bit field in the IP header that marks each packet with a priority class. Voice traffic gets Expedited Forwarding (EF). Video gets Assured Forwarding (AF). Bulk transfers get Best Effort (BE). Switches and routers inspect these markings and serve higher-priority queues first.

QoS matters most at constrained points: the uplink port between your access switch and core, WAN links where available bandwidth is genuinely limited, and Wi-Fi, which has its own QoS mechanisms (WMM — Wi-Fi Multimedia).

A few realities:

QoS doesn't create bandwidth; it manages scarcity. If your WAN link is consistently saturated, QoS helps but more bandwidth ultimately solves the problem.

End-to-end QoS only works if every device in the path honors the markings. If your ISP doesn't preserve DSCP markings — many don't — QoS is only effective within your LAN.

For VoIP deployments, proper QoS configuration on your switches and at the WAN edge is essential. Without it, any sustained traffic spike will produce audible call quality issues.