1.2 Key Characteristics of Scalable Internetworks
1.2.3 Making the network responsive
A network's responsiveness is typically measured by its end users as they access the network to perform day-to-day tasks. Today's users expect network resources to respond quickly, as if network applications were running from a local hard drive. You must tailor networks to meet the needs of applications, especially delay sensitive applications such as voice and video. The Cisco IOS offers traffic prioritization features to tune responsiveness in a congested network. Routers can be configured to prioritize certain kinds of traffic based on protocol information, such as TCP port numbers. As shown in the figure, traffic prioritization ensures that packets carrying mission-critical data take precedence over less important traffic.

If the router schedules these packets for transmission on a first-come, first-served basis, users could experience an unacceptable lack of responsiveness. Therefore, an end user sending delay-sensitive voice traffic, may be forced to wait too long while the router empties its buffer of a long train of queued packets.

The Cisco IOS addresses priority and responsiveness issues through queuing. The question of priority is most important on routers that maintain a slow WAN connection and therefore experience frequent congestion. Queuing refers to the process that the router uses to schedule packets for transmission during periods of congestion. By using the queuing feature, you can configure a congested router to reorder packets so that mission-critical and delay-sensitive traffic is sent out first. These higher priority packets are sent first even if other low-priority packets arrive first. The Cisco IOS supports four methods of queuing, as described in the following sections: first-in, first out (FIFO) queuing; priority queuing; custom queuing, and weighted fair queuing (WFQ). Only one of these queuing methods can be applied per interface because each method handles traffic in a unique way.