Overview

In general, the design of any network involves striking a balance between three requirements: meeting or exceeding the capacity needed to handle the projected demand, minimizing the capital and operational cost of the network, and ensuring high network reliability and availability. In short, we can refer to these three issues as cost, capacity, and quality. Meeting one or more of the requirements often means making sacrifices elsewhere, such that it is impossible to divorce one network design consideration from any of the others. For example, a lower cost might well mean a lower network capacity or a lower network quality. Thus, we will never get a network that is remarkably cheap to implement and operate while still offering high capacity and high quality. Instead, we must aim to establish some happy medium where we satisfy at least the most important criteria to an acceptable degree. Of course, the question is, "What is an acceptable degree?"

If we want to design a carrier-grade network, one design criteria to include is an overall availability of at least 99.999 percent (five-nines), which corresponds to a down time of no more than about 5 minutes per year. Other design criteria will be based upon projected usage of the network and business considerations. For example, network capacity will be driven by expected traffic demand. Network cost will usually be the result of a negotiation between the carrier's engineering organization and the financial/commercial department. At the end of the day, the cost of the network...