The need for quality of service technology has increased as enterprise networks with different characteristics and priorities such as SNA-based transaction traffic, realtime multimedia, and Web browsing are combined to create a single enterprisewide multiservice utility network. In the future, voice will also ride the data network infrastructure, providing an even greater need for quality of service that's adequate for realtime voice traffic. Before we dive into a discussion of quality of service alternatives, let's define our terminology. Originally, quality of service had a distinct meaning associated only with ATM technology.
Some standards work makes a distinction between QoS and class of service .
QoS traffic categories can be fine grained, sometimes specifying specific network performance parameters for individual applications of individual users. In contrast, CoS organizes services into classes that are treated separately, often only by prioritization. Strictly speaking, CoS is a subset of QoS. CoS is simpler to implement, particularly on multihop networks, and it scales more easily to large networks. Recent IETF QoS work has concentrated on Differentiated Class of Services to provide a scalable method for grouping traffic flows for different levels of service quality. In common usage, QoS encompasses CoS. To develop a comprehensive technical approach to QoS, you must evaluate and select alternatives differently for LANs, which usually offer abundant free bandwidth, and WANs, where bandwidth is more constrained or more costly.