TUCoPS :: Truly Miscellaneous :: histtone.txt

Tone signaling history - MF-KP vs DTMF

 From : Mark J. Cuccia                                    26 Jul 97  17:06:34
 Subj : MF-KP vs. DTMF (was Telephone / Gilligan)                               

Leonid A. Broukhis wrote:

> Mark J. Cuccia <mcuccia@mailhost.tcs.tulane.edu> writes:

>> As for signaling the called number, most inter-office trunks don't
>> use dialpulses. And international or overseas trunks most certainly
>> don't use dialpulse. Most likely, there was SF 2400/2600-Hz
>> supervisory tones, and CCITT #5 address signaling, which was an
>> extension of existing Bell-System 'domestic' signaling, using all
>> fifteen possible MF tone-pair combinations.

> Aren't there 16 MF tone-pair combinations? 0-9, #, *, A-D?
> DTW, I had a modem that would produce A-D tones (as in ATDTABCD), but
> the they were ignored by the telco.

There are indeed sixteen DT-MF (Dual-Tone Multi-Frequency) pairs:

1 2 3 A  (697 Hz)
4 5 6 B  (770 Hz)
7 8 9 C  (852 Hz)
* 0 # D  (941 Hz)

Column-1 (1,4,7,*) is 1209 Hz
Column-2 (2,5,8,0) is 1336 Hz
Column-3 (3,6,9,#) is 1477 Hz
Column-4 (A,B,C,D) is 1633 Hz

But that is 'touchtone', a 4x4 arrangement of four 'high' frequncies
(columns) and four 'low' frequencies (rows). A 'digit' is a pair of
one of the 'low' and one of the 'high'.

Touchtone/DTMF is sometimes used on interoffice trunks, but mostly is
for customer loop signaling, and customer-produced 'end-to-end'
signaling, such as entering your calling-card number, credit-card
number, etc. ad-nauseum.

In the original article, what I was referring to by "MF" was
"Multi-frequency Keypulsing", which was developed by the Bell System
as early as 1939 or 1940 (one of the first locations for its use was
between #1XB offices in Baltimore), and eventually used for most all
interoffice/toll signaling in the later 1950's all the way through the
1980's. There are six distinct frequencies (700 Hz, 900 Hz, 1100 Hz,
1300 Hz, 1500 Hz, 1700 Hz) for MF. Each tone-pair is two of these six
frequencies in combination, and you get fifteen tone-pair

For most NANP/DDD/Bell-System 'domestic' toll addressing applications,
only '1' thru '9', '0', 'Kp', and 'St' were used. But on
international/overseas trunks (CCITT #5), there was also 'Kp-1' (which
was NANP/DDD 'Kp'), 'Kp-2', 'Code-11', and 'Code-12', in addition to
the twelve NANP/DDD domestic MF-tones.

In the later 1970's, some of the other frequency pairs used on CCITT #5
were also used domestically, for various TSPS signaling applications.

One of the earliest (1948) experimental pushbutton tone-dialing phones
is pictured in Bell's magazines and the book "History of Engineering &
Science in the Bell System, Switching Technology, 1925-75" (authored by
Amos E. Joel, Jr, of Bell Labs). There was an experiment with such
pushbutton tone-dialing in Pennsylvania (the town of "Media", IIRC), at
an early #5XB central office. The phones were standard WECO model 302
phones, but instead of a rotary dial, there were two horizontal rows of
buttons. The first row was numbered '1' thru '5', from left to right,
and the second row was numbered '6' thru '9', '0', from left to right.

The interesting thing about this phone was that "MF-KP" frequncies
were used (numerical digits only - there was no 'Kp' nor 'St'), and
_NOT_ the frequencies of "Touchtone" (DTMF) which was being developed
in the late 1950's and early 1960's. Also, the frequencies were
produced by 'plucking metal reeds', and not electronically. While the
transistor was just being invented at that time (by Bell Labs),
_vacuum-tubes_ were still the order of the day for electronics. And
having residential and commercial telephone dialsets using vacuum
tubes and external continuous power (to heat the tubes) was considered
too costly and cumbersome! Of course, people had been using radios,
phonograph players, etc. (and soon televisions) home-entertainment
devices with tube amplifiers for years, but Bell didn't seem to want
vacuum-tubes in telephones used by the general public, thus the
'plucked reed' method of tone-generation!

Widespread use of MF-KP was being replaced with CCIS#6 beginning in
the mid-1970's, and by SS7 in the late-1980's and onward. CCIS#6 and
SS7 are 'out-of-voiceband' signaling, while MF-KP was in the voicepath
(similar to touchtone/DTMF).


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