1-11 QoS OverviewThis chapter covers the following topics:z Introduction to QoSz QoS Service Modelsz QoS Techniques OverviewIntroduction to QoSIn data communications, Quality of Service (QoS) is the ability of a network to provide differentiatedservice guarantees for diverse traffic in terms of bandwidth, delay, jitter, and drop rate.Network resources are always scarce. The contention for resources demands that QoS prioritizeimportant traffic flows over trivial traffic flows. When making a QoS scheme, a network administratormust consider the characteristics of various applications to balance the interests of diversified usersand fully utilize network resources.The subsequent section describes some typical QoS service models and widely-used mature QoStechniques. By appropriately using these techniques, you can improve QoS effectively.QoS Service ModelsThis section covers three typical QoS service models:z Best-Effort Service Modelz IntServ Modelz DiffServ ModelBest-Effort Service ModelBest effort is a single service model and also the simplest service model. In the best effort service model,the network does its best to deliver packets but does not guarantee delay or reliability.The best-effort service model is the default model in the Internet and applies to most networkapplications. It uses the first in first out (FIFO) queuing mechanism.IntServ ModelThe integrated service (IntServ) model is a multiple-service model that can accommodate multiple QoSrequirements. It provides the most granularly differentiated QoS by definitely identifying andguaranteeing QoS for each data flow.In the IntServ model, an application must request a specific kind of service from the network before itsends data. IntServ signals the service request with the Resource Reservation Protocol (RSVP). Allnodes that receive the request reserve resources as requested and maintain state information for theapplication flow.The IntServ model demands high storage and processing capabilities, because it requires that all nodesalong the transmission path maintain resource state information for each flow. The model is suitable for
