WHEN we were already quite far into the automated era of the modern telephone network, a "magic performance figure" emerged as a design target for each telephone exchange, or switch. It was based on the increasing dependence of society on long-distance communication.

This is the much feted "five nines" figure often quoted in the industry, which says that a switch has to have an availability, or an "up time" in modern parlance, of 99.999pc - a probability of 0.99999. That is, in any one year, the totalised unavailability, or down time, had to be less than 0.001pc, or a probability of 0.00001, which is 0.00365 of a day or 5.3 minutes/year.

Such a figure is not easily achieved and dictates the use of substantial battery power supplies, often backed up by diesel generators, with many items of the control and switchgear at least duplicated by hot standby circuits.

There are not many items of our technology that can boast such a performance, or indeed need such a high figure, but when you consider the concatenation of three to 10 such switches for a single countrywide connection, or 15 or more for international calls, it becomes obvious.

The down time for three concatenated switches falls to about 16 minutes/year, while for 15 it is about 80 minutes/year (still pretty impressive, but barely adequate for modern business). However, these figures are compounded by the number of customers served by each switch, which could be as high as 100,000, and more typically, less than 10,000. So 10,000 x 5.3 minutes is lot of customer down time.

The computer industry looks at five nines with some envy and often struggles to approach two nines. Is your PC up and running for 99pc of the time or more? I doubt it. And how about your internet service provider? Bad news, I suspect. In my experience, ISPs are struggling to give a single-nine performance, that is 90pc of up time.

It is not that it cannot be achieved. It is just very tough to engineer. In my experience, it is not unusual to hear the engaged signal, or to get a modem that does not respond correctly, a synchronisation failure, not to mention the disconnections from the Net or protocol mangles.

The opportunity for an ISP failure with the huge amounts of new, poorly engineered software, and varied hardware is therefore far higher than the well-established telephone network.

To combat this shortfall in a business environment, where I always have to be able to get online somehow from any location on the planet while on the move or not, I have had to sign up with five ISPs. Roughly speaking, this gives me a down time of (0.1)EXP5 = 0.00001, or an up time of five nines.

Do I actually achieve this? Well, not exactly. Although my laptop is extremely robust, and I carry an additional full back-up hard drive, there are times when I run out of battery power, or I have some software glitch, or cannot get an adequate radio connection. I am probably achieving around four nines or a 99.99pc up time. In any one year, my enforced down time is only about one hour.

I suspect that this figure will remain a distant dream for the individual ISP and many people in business, but it is probably a reasonable target for our new mode of operation. To be offline for a full day is clearly unacceptable for any business, but most of us can live with an hour.