In the Telephony Community

Telephony specialists approach communications technology from a background shaped by
the traditional telephone network, the Public Switched Telephone Network (PSTN). The
telephone service provided by the PSTN is called “plain old telephone service” (POTS).
This “plain” type of telephone network we all take for granted uses “circuit-switched”
connections, which means that when you make a call, you receive a dedicated circuit, from
one telephone to the other, through everything that’s in the middle. The typical dedicated
circuit through the PSTN has evolved from a physical connection to a logical connection
that involves many switches. When you speak into a phone, a microphone creates an analog
transmission that’s passed on the circuit through the network.
Decades of knowledge, experience, and innovation have allowed the public telephone network
to achieve the quality and reliability that it has today. When you pick up a phone, you
get a dial tone almost instantly. And when you dial a number, the destination phone starts
ringing, usually within a few seconds. Can you even recall the last time your traditional
telephone call was dropped by the network? Research shows that because the PSTN is so
reliable, people are rarely willing to tolerate reduced-quality or dropped calls, and their
tolerance usually comes only with additional convenience, such as the convenience provided
by mobile phones.
The level of quality that’s expected from the PSTN is sometimes referred to as “five-nines.”
This term means that the entire network must be available and functional for 99.999% of the
time. If you apply this principle over the period of one year:
365 days * 24 hours/day * 60 minutes/hour = 525,600 minutes
“Five-nines” means that the network can be down for a grand total of less than 6 minutes
during a year!