The analog telephone was a mature product more than 100 years ago. The POTS residential line of today is what every line was in those days. Amazingly those old phones will work on today’s analog service. Many central office switches still accept rotary (pulse) dialing as well as dual-tone multifrequency (DTMF or TouchTone) signals. For longevity and preserved backward compatibility, it’s hard to find anything as durable as POTS.

Telephones are so ingrained in our lives that many users are not aware of how the traditional service sets our expectations in many ways. There’s a story, allegedly true, about a call to a computer support desk from a person whose computer wouldn’t turn on. The support agent asked the caller to look at the back of the computer for loose cables. The caller said that was impossible because it was too dark—the power was out.

The expectation for phone service (powered from the CO) was transferred to a computer (on local power). It’s hard to see how old telephone expectations won’t continue to apply in some significant ways to telephones and telephone service after the packet revolution.


As described in Chapter 1, analog phones draw operating power over the local loop from the telco. To ensure continuity of operation, COs contain large banks of batteries, typically enough for over 8 hours of operation, powering the switch as well as the phones. Many COs have ...

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