QoS networks offer services to application traffic. Such services may be of high or low quality. High-quality services generally offer specific quantifiable service parameters and are of high integrity. Low-quality services offer little in the way of quantifiable parameters and/or are of low integrity. Different applications require different qualities of service.
Efficiency is a measure of the amount of bandwidth required in a network to support the demands of the applications making use of the network. When a network can be operated with less bandwidth, it operates more efficiently. When more bandwidth is required, the network operates less efficiently.
In general, the network manager is faced with a trade-off between the QoS that the network can provide and the efficiency with which the network can be operated. The more efficiently a network is operated, the lower the quality of services that can be offered. The less efficiently a network is operated (the more bandwidth is made available), the higher the quality of services that can be offered. Thus, the product of the average QoS that can be offered by the network and the average efficiency with which it is operated is a constant. This constant is referred to as the quality/efficiency (QE) product of the network.
The QE product of a network can be raised by enabling more sophisticated QoS mechanisms in the network. These mechanisms carry overhead, which must be weighed against the expected improvement in QE product. Network managers face this trade-off in each part of the network. Depending on the characteristics of different parts of the network, and depending on the demands on different parts of the network, different QoS mechanisms may be appropriate for that part of the network.
Because any subnetwork is likely to be required to support multiple service qualities, it is helpful to partition a physical subnetwork into four logical networks. These logical networks may each employ different QoS mechanisms to control a subset of the underlying physical resources. Traffic is assigned to a logical network based on the QoS that it requires.