Consider water flowing through a series of pipes with varying diameters. The water's flow rate through those pipes is limited to the water's flow rate through the pipe with the smallest diameter. Similarly, as a packet travels from its source to its destination, the packet's effective bandwidth is the bandwidth of the slowest link along that path, as shown in Figure 6-1.
Figure 6-1. Effective Bandwidth
Because a lack of bandwidth represents the primary quality challenge in a network, the logical question is, "How can I increase available bandwidth?" A knee-jerk response to that question is often, "Add more bandwidth!" While there is no substitute for more bandwidth, extra bandwidth comes at a relatively high cost.
Compare your network to a highway system in a large city. During rush hour, the lanes of the highway are congested, but the lanes might be underutilized during other periods of the day. Instead of just building more lanes to accommodate peak traffic rates, highway engineers convert one or more existing lanes to car pool lanes. Cars with two or more riders can use the reserved car pool lane. The riders in these cars enjoy higher priority on the highway. Similarly, you can use QoS features to give your mission-critical applications higher-priority treatment in times of network congestion.