**1.2 Quality of Service (QoS) requirements**

*Quality of Service (QoS)* generally refers to a broad collection of networking technologies and techniques. QoS refers to the ability of a network to deliver predictable results. Traditional QoS characteristics include availability, bandwidth, delay or delay jitter (Huston, 2000). However, QoS demands that no task be late (Krings & Azadmanesh, 1999) in hard real-time systems. QoS represents the quantified ratio of tasks that may not be executed – i.e. the total amount of work not contributing to the value of the system – in firm real-time systems. Delay and delay jitter are eliminated from consideration in both cases. Perfect QoS characterization proves to be difficult at the application level. It would be desirable that the scheduling policy adapts to changes in user QoS requirements. Such policy should strive to achieve the desired QoS in an environment with variable resources as well as complex and variable application demands. Consequently, this chapter adresses consideration of the control of the deadline miss ratios as a QoS concern.
