Quality of Service (QoS) refers to both the capability of a network to deliver data with minimum delay, and the networking methods used to provide bandwidth for real-time multimedia applications. It is particularly useful when using VoIP or other situations where you need to reserve bandwidth for a particular application (or protocol).
Draytek QoS function will allow you classify traffic through router into four transmission classes. Each class has its own priority and reserved bandwidth ratio.
Consider the situation where you are having a VoIP (Voice over Internet Protocol) conversation with a friend when a workmate sends an email with all his holiday photos attached. Suddenly there is more data than the modem can send immediately, and a queue forms as the modem tries to send all that data. This is not a problem for emails, but VoIP is sensitive to fluctuations in data flow and requires the VoIP packets to arrive in a fairly steady stream. This will result in your friend’s VoIP device being unable to compensate for the delays – resulting in your friend hearing your voice chopped up with gaps between.
In this example, with QoS enabled your router would give VoIP packets high priority, and send them almost immediately; while the email/file transfer packets would be fitted between the VoIP packets. Result is that the VoIP conversations remain high quality with your friend noticing no disruption at all – but the email takes a bit longer to send.