A recurring objection to VoIP, softswitch, and 802.11 solutions is the perception that these solutions do not match the "five 9s" of reliability of the PSTN. This chapter posits that a Vo802.11 solution is just as reliable (actually "available") as legacy TDM solutions and is potentially more reliable. This chapter will explore what is meant by the "five 9s" and why engineering a network to deliver that level of reliability is only a matter of good engineering not limited to Bell Labs designs. "Five 9s" applies to data networks as well as to telephone switches. Data networks have long been engineered to achieve "five 9s." VoIP is voice over a data network. Ergo, a Vo802.11 network can be engineered to deliver "five 9s" of reliability. A wireless VoIP network can be just as, if not more, reliable than the PSTN (Figure 11.1).

Figure 11.1: Properly engineered, wireless VoIP networks can be more reliable than the PSTN.

Understanding Reliability

Availability is often expressed numerically as a percentage of uninterrupted productive time containing from one to five nines. For instance, 99% availability, or "two 9s," equates to a certain amount of availability versus downtime, as does 99.9% (three 9s), and so on. The downtime calculations shown in Table 11.1 for each of the five 9s are based on 24-hour, year-round operation.