It’s the first of the new year, which means we ought to do something momentous
to mark the occasion, like a short piece about telephones. Why so much on
telephones lately? I think I’m just a little burned out on software at the
moment and I need a vacation before I’m excited to write about failed Microsoft
ventures again, but the time will surely come. Actually I just thought of a
good one I haven’t mentioned before, so maybe that’ll be next time.

Anyway, let’s talk a little bit about phones, but not quite about long distance
carriers this time. Something you may or may not have noticed about the
carriers we’ve discussed, perhaps depending on how interesting you find data
communications, is that we have covered only the physical layer. So far, there
has been no consideration of how switches communicated in order to set up and
tear down connections across multiple switches (i.e. long distance calls).
Don’t worry, we will definitely get to this topic eventually and there’s plenty
to be said about it. For the moment, though, I want to take a look at just one
little corner of the topic, and that’s multifrequency tone systems.

Most of us are at least peripherally familiar with the term “dual-tone
multifrequency” or “DTMF.” AT&T intended to promote Touch-Tone as the consumer
friendly name for this technology, but for various reasons (mainly AT&T’s
trademark) most independent manufacturers and service providers have stuck to
the term DTMF. DTMF is the most easily recognizable signaling method in the
telephone system: it is used to communicate digital data over phone lines, but
generally only for “meta” purposes such as connection setup (i.e. dialed
digits). An interesting thing about DTMF that makes it rather recognizable is
that it is in-band, meaning that the signals are sent over the same audio
link as the phone call itself… and if your telephone does not mute during
DTMF (some do but most do not), you can just hear those tones.

Or, really, I should say: if your phone just makes the beep boop noises for fun
pretend purposes, like cellphones, which often emit DTMF tones during dialing
even though the cellular network uses entirely on-hook dialing and DTMF is not
actually used as part of call setup. But that’s a topic for another day.

DTMF is not the first multi-frequency signaling scheme. It is directly based on
an earlier system called, confusingly, multifrequency or MF. While DTMF and MF
have very similar names, they are not compatible, and were designed for
separate purposes.

MF signaling was designed for call setup between switches, mostly for
long-distance calling. Whenever a call requires a tandem switch, so say you
call another city, your telephone switch needs to connect you to a trunk on a
tandem switch but also inform the tandem switch of where you intend to call.
Historically this was achieved by operators just talking to each other over the
trunk before connecting it to your local loop, but in the era of direct dialing
an automated method was needed. Several different techniques were developed,
but MF was the most common for long-distance calling in the early direct dial
era.

An interesting thing about MF, though, is that it was put into place in a time
period in which some cities had direct long distance dialing but others did
not. As a result, someone might be talking to an operator in order to set up a
call to a city with direct dial. This problem actually wasn’t a new one, the
very earliest direct dialing implementations routinely ran into this issue, and
so it became common for operators switchboards to include a telephone dial
mounted at each operator position. The telephone dial allowed the operator to
dial for a customer, and was especially important when connecting someone into
a direct dial service area.

MF took the same approach, and so one could say that there were two distinct
modes for MF: