What is Quality of Service on a router?
A set of techniques for deciding which traffic gets priority when there is not enough capacity for everything — because the only time prioritisation matters is when a link is full.
The problem. A network link is a queue. When more traffic arrives than can be sent, packets wait. Without any policy, they are served first-come-first-served, so a large file upload can fill the queue and add hundreds of milliseconds of delay to a video call sharing the same connection.
What QoS does. Classifies traffic and treats classes differently:
Prioritisation — latency-sensitive traffic such as voice and video is sent ahead of bulk traffic such as downloads and backups.
Bandwidth allocation — guaranteeing a minimum or capping a maximum for a class or device.
Queue management — controlling how deep queues are allowed to grow.
Why it only works on the upload, generally. Your router controls what it sends. It has little control over what arrives, because the congestion point for incoming traffic is at your ISP, upstream of your equipment. Some routers approximate download control by deliberately slowing acknowledgements, which is indirect and imperfect.
Bufferbloat — the problem QoS most usefully solves. Equipment manufacturers fitted large buffers assuming that never dropping a packet was good. The result is the opposite: when a link saturates, packets sit in enormous queues, adding latency of hundreds of milliseconds or more. Interactive traffic becomes unusable while a single large transfer runs, which is why a video call falls apart the moment someone starts an upload.
The fix is not prioritisation but queue discipline. Modern algorithms — CoDel, FQ-CoDel and CAKE — keep queues short by dropping or marking packets early, and fairly share capacity between flows. This is frequently labelled Smart Queue Management (SQM) and it is far more effective for household use than manually prioritising applications.
Practical advice: enable SQM if your router supports it; set the bandwidth slightly below your actual line speed, so the queue forms in your router where you control it rather than in the ISP's equipment where you do not; and prefer automatic classification over hand-written rules.