EMZETT.QoS: Prioritising network traffic
In short: Quality of Service ensures that time-critical data such as telephony and video conferences get priority in the network over less urgent data such as downloads.
In more detail: When the line is full, packets pile up in queues. A download can wait seconds, a voice call cannot: delay (latency), fluctuation (jitter) and loss destroy the conversation. QoS distinguishes kinds of traffic and treats them differently.
In Depth
Metrics
- Bandwidth: amount of data per time.
- Latency: delay of a packet (for voice preferably under 150 ms one way).
- Jitter: fluctuation of the delay.
- Packet loss: lost packets. Real-time applications are sensitive, downloads hardly.
How QoS works
- Classify: traffic is recognised, for example by port, protocol, address or a marking in the packet (DSCP in the IP header).
- Mark: packets receive a class (for example “EF” for voice).
- Queues and schedulers: important classes go first (priority queuing, weighted fair queuing).
- Limit and smooth: policing and shaping slow traffic that exceeds its rate.
Where it is used
- Company networks: prefer VoIP phones and video conferences.
- Home routers: put games or video calls ahead of streaming and downloads, especially with low upload bandwidth.
- Provider networks and mobile for their own services. In the home network the smart queue management (SQM) feature with CAKE or fq_codel often already helps, fighting bufferbloat (overlong queues in the router with high latency under load).
Limits and net neutrality
QoS only helps at bottlenecks: if there is enough bandwidth you do not need it. It only works where you control the devices yourself; across the public internet DSCP markings are often ignored or deleted. Preferential treatment by providers is legally limited by net neutrality. Misconfigured, QoS can also harm if important classes starve everything else.
See also: IP addresses, Layer 3 switch, Router, Ping