Quick answer: QoS (Quality of Service) prioritises important traffic (VoIP, video calls, business apps) to reduce congestion, latency, jitter and packet loss. Basic configuration: (1) classify and mark traffic (DSCP/CoS) — e.g. voice EF, video AF41; (2) set queueing/priority policies on switches and routers; (3) on the edge router, enable QoS/traffic shaping to reserve and limit bandwidth by class; (4) many business and Wi-Fi routers offer per-app or per-device QoS — enable it and set priorities; (5) verify by measuring latency/jitter under load. Common models: DiffServ (DSCP marking) and IntServ (RSVP, rare). QoS only works when applied consistently along the whole path.
What is QoS and why it matters
QoS manages network traffic so critical, latency-sensitive services (voice, video, ERP) get the bandwidth and low latency they need, even when the link is busy. Without it, a large download can starve a VoIP call.
QoS models
- DiffServ: marks packets with a DSCP value; devices treat each class per policy. The common enterprise choice.
- IntServ: reserves bandwidth per flow with RSVP; powerful but not scalable, rarely used.
How to configure QoS
- Identify priorities: protect VoIP, video and business apps; let backups/downloads be best-effort.
- Classify & mark: set DSCP/CoS at the access layer (phones mark voice EF/DSCP 46).
- Trust & queue on switches: trust markings and map classes to priority/weighted queues.
- Shape at the edge: on the WAN router, apply policing/shaping to guarantee and cap bandwidth per class.
- Wi-Fi/WMM: enable WMM and per-SSID or per-app QoS on access points.
- Verify: generate load and measure latency, jitter and loss for priority traffic.
Troubleshooting
- Markings reset mid-path: ensure every device trusts/propagates DSCP.
- VoIP still choppy: confirm the voice queue is priority and the WAN shaper matches the real circuit speed.
- Asymmetric links: configure QoS in both directions and ask the ISP about upstream QoS.
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