1-11 QoS OverviewThis chapter covers the following topics:z Introduction to QoSz Introduction to QoS Service Modelsz QoS Techniques OverviewIntroduction to QoSFor network traffic, the Quality of Service (QoS) involves bandwidth, delay, and packet loss rate duringtraffic forwarding process. In a network, you can improve the QoS by guaranteeing the bandwidth, andreducing the delay, jitter, and packet loss rate.The network resources are always scarce. QoS requirements exist on any occasion where traffic flowscontend for network resources. QoS is a relative concept for traffic flows, that is, guaranteeing QoS fora certain traffic flow may damage QoS of other traffic flows. For example, in the case of fixed bandwidth,if a traffic flow gets more bandwidth, the other traffic flows will get less bandwidth and may be affected.Therefore, the network administrator should reasonably plan and allocate network resources based onthe characteristics of various traffic flows, thus utilizing the network resources effectively.The following part introduces the QoS service models, and some mature QoS techniques used mostwidely. Using these techniques reasonably in the specific environments, you can improve the QoSeffectively.Introduction to QoS Service ModelsThis section covers three typical QoS service models:z Best-effort servicez Integrated service (IntServ)z Differentiated service (DiffServ)Best-Effort Service ModelBest effort is a single service model and also the simplest service model. In the best effort service model,the network delivers the packets at its best effort but does not guarantee delay or reliability.The best-effort service model is the default model in the Internet and is applicable to most networkapplications. It is implemented through FIFO queuing.IntServ Service ModelIntServ is a multiple services model that can accommodate multiple QoS requirements. In this model,an application must request a specific kind of service from the network before it can send data. Therequest is made by RSVP signaling. RSVP runs on each device from the source end to the destinationend, and monitors each data flow to prevent each data flow from consuming more resources than therequested, reserved, and pre-purchased resources. The Inter-Serv model can definitely identify andguarantee QoS for each data flow, and provides the most granularly differentiated QoS.
