Efficiency is a nebulous term. In general, it measures how thoroughly one manages to achieve some desired result as a function of the required resources. The biggest problems in implementing efficiency in a computer network are essentially matters of definition. What is the desired result for a network, and what resources are actually required?
With a relatively narrow view of the desired results in network design, it essentially comes down to the factors that I mentioned earlier when talking about network reliability. The network must deliver data to the destination. It must do so within the required application constraints. In most networks, the desired result is effectively quantified with just four parameters: latency, jitter, throughput, and dropped packets. I will define these terms when I come to talk about Quality of Service later in this chapter.
The hard part of describing efficiency is actually in defining the resources. Which resources should the definition include? Some resources are obvious. Everybody would agree that it's necessary to worry about CPU and memory utilization in their routers. The same is true for the bandwidth utilization on the various links that connect network devices. But some resources are harder to quantify or harder to see. How do you compare the relative importance of these resources? Do you want to save bandwidth, for example, at the expense of CPU load?
The ultimate resource for any organization comes down to money, and efficiency has to be defined for the entire organization. Suppose, for example, that you can save money by running a particular application on a particular type of server. The money you save has to be balanced against the extra money it costs to upgrade parts of the network. Perhaps the new implementation will turn out to be more expensive overall than the old way. But perhaps it will also allow the organization to win new business that will more than pay for the difference in cost. Doing such an upgrade is worthwhile for the organization. You just have to understand where you are drawing the line around what resources to include. Conversely, the network engineer may look at a Core router and see abnormally high CPU and memory utilization. If fixing this problem means spending hundreds of thousands of dollars, the organization may not feel that the expense is justified.
Ultimately, efficiency is a matter of the global economics of the organization. This subject, however, is far beyond the scope of this book. The resources that I can reasonably talk about here are the ones that are specific to the network. I can discuss how to make the best use of the resources already in the network, so I look at the four parameters—latency, jitter, throughput, and dropped packets—that describe how well the network works. I also look at network resources such as bandwidth, CPU, and memory utilization. It is never possible to obtain perfect efficiency, though. Therefore, a network designer's most difficult job is to decide what tradeoffs will give the best design possible under the circumstances and will fit in best with larger corporate goals.