[This is preliminary documentation and subject to change.]
Quality of Service (QOS) provides applications (or network administrators) with a means by which network resources, such as available bandwidth and latency performance, can be predicted and managed on both local machines and devices throughout the network.
With such an all-network encompassing definition, QOS functionality requires cooperation among and between end nodes, switches, routers, and WAN links through which data must pass. Without some level of cooperation among those network devices, the quality of data transmission services would potentially break down. In other words, if each of the above network devices were left to make its own decisions about transmitting data, it would likely treat all data equally, and thus provide simple first-come first-served service. Although such service may be satisfactory in network devices or transmission mediums that are not heavily loaded, when congestion occurs, such equal treatment could mean all data passing through the device would get delayed. With this information, we can extend the definition of QOS, by adding that quality of service allows preferential treatment for certain subsets of data as it traverses any QOS-enabled part of (or device in) the network.
Windows NTŪ 5.0 implements quality of service by including a number of components, each of which potentially cooperates with (or invokes) one another. These components are found in the incarnation of DLLs, protocols, NT services and individual network device functionality.