That 1956 article on the long-distance telephone network is fascinating reading.
The chart on page 24 gives the very first division of the US phone into area codes, many of which have of course remained unchanged today.
From page 82, when discussing automated in-band signaling on the phone lines:
> The signal component is a band about 100 cycles [Hz] wide centering on the signal frequency [2600 Hz]. [...] In the talking condition (tone off in both directions) the guard detector sensitivity is such that almost a pure tone is required to operate the receiver since other than signal frequencies will produce a voltage opposing its operation. The guard feature prevents false operation of the receiver from speech signals.
Of course in hindsight, the flaw in this is obvious. The guard band prevents accidental triggering of the signal mechanism by ordinary speech. It does nothing to stop someone from intentionally playing a pure 2600 Hz tone into the telephone handset, using e.g. a whistle from a cereal box.
I completely forgot to bring up the 2600 section in my post! Reading distance dialing led me down a deep rabbit hole. An interesting path was learning more about Joe Engressia (Joybubbles) who could whistle a 2600hz tone to circumvent the supervisory tone which led him to learn more about the system and further the knowledge of the early phreaking community. Folklore around him was off the charts and eventually painted a picture of a superhero who could whistle his way through the whole phone network and into free calls to anywhere just with his mouth. The document linked in my post gives details about the long-distance signalling and why being a great whistler was not good enough. That said, definitely check out more about Engressia, because while the whistling wasn't superhuman, the curiousity and drive to learn more may have been!
* (Shameless plug) My book on the history of phone phreaking, Exploding the Phone, which has a lot of stuff on Engressia in it: https://explodingthephone.com/
The chart on page 24 gives the very first division of the US phone into area codes, many of which have of course remained unchanged today.
From page 82, when discussing automated in-band signaling on the phone lines:
> The signal component is a band about 100 cycles [Hz] wide centering on the signal frequency [2600 Hz]. [...] In the talking condition (tone off in both directions) the guard detector sensitivity is such that almost a pure tone is required to operate the receiver since other than signal frequencies will produce a voltage opposing its operation. The guard feature prevents false operation of the receiver from speech signals.
Of course in hindsight, the flaw in this is obvious. The guard band prevents accidental triggering of the signal mechanism by ordinary speech. It does nothing to stop someone from intentionally playing a pure 2600 Hz tone into the telephone handset, using e.g. a whistle from a cereal box.