1-11 QoS OverviewThis chapter covers the following topics:z Introduction to QoSz Networks Without QoS Guaranteez QoS Requirements of New Applicationsz Congestion: Causes, Impacts, and Countermeasuresz QoS Technology ImplementationsIntroduction to QoSQuality of Service (QoS) reflects the ability of a network to meet customer needs. In an internet, QoSevaluates the ability of the network to forward packets of different services.The evaluation can be based on different criteria because the network may provide various services.Generally, QoS performance is measured with respect to bandwidth, delay, jitter, and packet loss ratioduring packet forwarding process.Networks Without QoS GuaranteeOn traditional IP networks without QoS guarantee, devices treat all packets equally and handle themusing the first in first out (FIFO) policy. All packets share the resources of the network and devices. Howmany resources the packets can obtain completely depends on the time they arrive. This service iscalled best-effort. It delivers packets to their destinations as possibly as it can, without any guarantee fordelay, jitter, packet loss ratio, and so on.This service policy is only suitable for applications insensitive to bandwidth and delay, such as WordWide Web (WWW) and E-Mail.QoS Requirements of New ApplicationsThe Internet has been growing along with the fast development of networking technologies.Besides traditional applications such as WWW, E-Mail and FTP, network users are experiencing newservices, such as tele-education, telemedicine, video telephone, videoconference andVideo-on-Demand (VoD). Enterprise users expect to connect their regional branches together with VPNtechnologies to carry out operational applications, for instance, to access the database of the companyor to monitor remote devices through Telnet.These new applications have one thing in common, that is, they all have special requirements forbandwidth, delay, and jitter. For example, videoconference and VoD require high bandwidth, low delayand jitter. As for mission-critical applications, such as transactions and Telnet, they may not require highbandwidth but do require low delay and preferential service during congestion.
