Therefore, some vendors have developed offload strategies in which local switch equipment senses ISP calls at dialup and routes them around the circuit-switched infrastructure. The transition scenario for the disappearance or coexistence of circuit switching and packet switching is uncertain, but we do know for certain that local telecommunications switches will be present for years to come. Therefore, information about the distribution of local switch outages over the 24-hour day is important because of the shift in peak traffic from daytime to nighttime. Service providers schedule outages during these hours to allow new hardware and software installations. In addition, all outages aren't equal.
Some outages last longer than others, and some switches have more access lines than others.
For example, if a 10,000-line switch is out for 12 minutes, this is equivalent to 2,000 lines lost for one complete hour. Said another way, an average of 910 of the 158,000,000 lines were out for a complete hour each hour of the day throughout the year. Of course, more than 910 lines actually experienced outages, as most switch outages last less than an hour. However, we can use this equivalent metric to make other calculations, such as availability. The overall PSTN access line availability was 99.9994 percent, and the average line outage was 3.0 minutes per line per year.