3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 1 QoS Overview3Com Corporation1-2Those new applications have one thing in common, i.e. high requirements forbandwidth, delay, and jitter. For instance, videoconference and VOD need theassurance of wide bandwidth, low delay and jitter. As for mission-critical applications,such as transaction and Telnet, they may not require wide bandwidth but do requirelower delay and be handled by priority during congestion.The new emerging applications demand higher requirements for service performanceof IP network. Better network services during packets transmission are required otherthan simply delivering the packets to their destination, such as providing user-specificbandwidth, reducing packet loss ratio, avoiding congestion, regulating network traffic,setting priority of the packets. To meet those requirements, the network should beprovided with better service capability.1.4 Congestion: Causes, Impact, and CountermeasuresNetwork congestion is a key factor to degrade the service quality of the traditionalnetwork. Congestion refers to such a fact that the service rates are decreased due torelative deficiency of the resources supply (leading to extra delay).1.4.1 CausesCongestion will easily occur in complex packet switching circumstances in the Internet,with two cases illustrated in the following Figure:100M 10M 100M 100M100M100MTraffic congestion at interfaceswith different speedTraffic congestion at interfaceswith the same speedFigure 1-1 Diagram for traffic congestion1) The packet streams enter a router from a high speed link and are forwarded via alow speed link;2) Packet streams enter to a router from several interfaces with a same speed andare forwarded through an interface with the same speed as well;When traffic arrives at wire speed, congestion may occur for network resourcebottleneck.Besides the bottleneck of link bandwidth, congestion will also be caused by resourcesdeficiency in normal packet forwarding, such as the deficiency of assignable processortime, buffer and memory. In addition, congestion may occur if the arrival traffic is notmanaged efficiently and the assignable network resources are inadequate.
