The 'Technical Journal'

There are actually several articles in various Bell System publications that included the in-band trunk signaling tones that provided the technical information needed to build Blue Boxes.

One article was "Signaling Systems...An International Concern", by G. H. Johannessen, Bell Laboratories Record, volume 48, no. 1 (January, 1970), pp 13-18. This article focused on signaling problems for international calling. One table shows CCITT's (International Telegraph and Telephone Consultative Committee) Number 5 Signaling System, including all the trunk MF signaling tones in use at the time. This table is reproduced at the bottom of the page. The Bell Laboratories Record was a glossy monthly with a wide audience.

The ITU-T (International Telecommunication Union-Telecommunication Standardization Sector, which replaced the CCITT in March, 1993) has published this information as Recommendation Q.140 (1988) - Definition and function of signals (Signaling System No. 5).

The most detailed article I am aware of is "Signaling Systems for Control of Telephone Switching", by C. Breen and C. A. Dahlbom, Bell System Technical Journal, volume 39, no. 6 (November, 1960), pp 1381-1444. This journal was directed to the scientific community within the telecommunications industry.

"Signaling Systems..." reviews the various methods available for inter-office trunk signaling. Since this was the critical decision that ultimately cost Ma Bell untold dollars, its discourse on the advantages and disadvantages of particular methods of signaling is intriguing. Starting on page 1399:

The obvious advantages of out-of-band signaling are, of course, freedom from the effects of speech currents, compandors and echo suppressors. In general, out-of-band signaling also allows relatively simple terminal equipment and has the additional advantage of permitting signaling to take place during the talking interval. The necessity of providing additional bandwidth, filters to provide the signaling slot, and the possibility of switching to a trunk over which it is possible to signal but impossible to talk due to a trouble in the separate speech path, are disadvantages.

In-band signaling systems have the advantage of not requiring additional bandwidth. In addition, since the signaling currents utilize the same path as the speech currents, the amplification provided by voice amplifiers also renews the strength of the signaling currents. With in-band signaling it is possible to quickly substitute another voice channel in case of trouble, since the signaling is carried along with the speech circuit.

In-band signaling systems are advantageous when telephone facilities within a trunk are connected in tandem. Under these conditions signaling equipment need only be furnished at the terminals of the trunk and not at the intermediate point within the trunk. This is economically significant in the Bell System toll plant, where an average of 1.4 transmission links comprise a trunk. In addition to the economic advantage, there is no additional signal distortion introduced through equipment at intermediate points.

In-band signaling systems tend to be more complex because of the need for protection against speech-generated signals, but the Bell System type of in-band signaling provides protection adequately and economically by signal-to-guard arrangements in the signaling receiver. The guard uses the energy present in the frequency spectra outside of that assigned to the signaling frequency itself. Such guard energy, when combined with the energy present at the signaling frequency, determines whether the receiver is to operate or not. The ratio of signal-to-guard voltages can be adjusted at the time of design of the receiver, and this, together with timing, provides protection against speech operation.

Well, not really. All the above only protects against accidental operation of the signaling system. Nothing was ever discussed about un-authorized operation. The authors concluded:

[After World War II] it was possible to raise the signaling frequency to 2600 cycles. At this higher frequency the problem of protection against speech operation was made considerably easier and permitted the use of smaller components as well as less complex circuitry. Present standard voice-frequency signaling systems employ tones of 1600 to 2600 cycles and use all frequencies, except the signaling frequency, to guard against false operation of the signaling receiver on speech currents.

Considering the explosive growth of both our society and technology after World War II, it was probably the most economic decision they could make. Before the implementation of this method, there were several types of different signaling systems being employed. This modern method allowed for implementation while leaving in place existing plant resources. Since in-band signaling had to be repeated at every trunk termination, it allowed for integration with the existing out-of-band and pulsing in-band systems, by using translators, with a minimum of expense.

And as we have found out, with the maturing of technology and the realization of the fraud occurring on their system, Ma Bell has moved to digital out-of-band signaling. I have not researched the details of this new system, but maybe I should....

So, here's the holy grail:

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Last updated April 8, 2004
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