Quality of Service (QoS), in traffic engineering, is a mechanism which prioritizes resource control over service quality in the field of computer networks as well as other packet-switched networks. Service quality is the capacity to assign different priorities to various applications, data flow, or users. It also guarantees data flow’s performance.

Service quality guarantees are useful if the network capacity for some applications may be insufficient, e.g. real-time streaming video, online games, etc. These services frequently need a fixed bit rate and are sensitive to delay, especially in networks of limited capacity. In networks where congestion is not a concern, QoS is not required.

A QoS-compatible protocol or network may have a traffic contract with its application software and resource capacity within the network nodes. During traffic management, QoS may observe the performance level (through data transmission rates and delay) and actively control schedule priority in the networks.

An alternate choice to complex QoS control systems would be the provision of high-quality communication over a network by ‘overestimating’ the network’s approximated capacity so that it will be more than sufficient for predicted peak traffic.

In telephony, Quality of Service is defined as a group of requisites based on the overall performance of one or more devices. QoS consists of requirements pertaining to all connection features, such as cross-talk, echo, frequency response, interruption, loudness levels, loss, signal-to-noise ratio, among other aspects. Under telephony QoS is Grade of Service (GoS), which consists of requirements for network capacity and coverage.

QoS may be utilized as a measure of quality rather than the ability for resource management. High QoS levels are often confused with high performance levels or the achieved quality of service (e.g. low bit error probability, low latency, and high bit rate).