Editing Blue box (section)

==In other countries==
Another signaling system widely used on international circuits (except those terminating in North America) was [[CCITT]] Signaling System No. 4 (friendly named 'SS4').

Technical definitions are specified in formerly CCITT (now [[ITU-T]]) Recommendations Q.120 to Q.139.<ref>CCITT SS4 / ITU-T Q.120–139 https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-Q.120-Q.139-198811-I!!PDF-E&type=items</ref>

This was also an in-band system but, instead of using multifrequency signals for digits, it used four 35 ms pulses of tone, separated by 35 ms of silence, to represent digits in four-bit binary code, with 2400{{nbs}}Hz as a "0" and 2040{{nbs}}Hz as a "1". The supervisory signals used the same two frequencies, but each supervisory signal started with both tones together (for 150 ms) followed, without a gap, by a long (350 ms) or short (100 ms) period of a single tone of 2400{{nbs}}Hz or 2040{{nbs}}Hz. Phreaks in Europe built System 4 blue boxes that generated these signals. Because System 4 was used only on international circuits, the use of these blue boxes was more specialized.

Typically, a phreak would gain access to international dialing at low or zero cost by some other means, make a dialed call to a country that was available via direct dialing, and then use the System 4 blue box to clear down the international connection and make a call to a destination that was available only via operator service. Thus, the System 4 blue box was used primarily as a way of setting up calls to hard-to-reach operator-only destinations.{{Citation needed|date=August 2019}}

A typical System 4 blue box had a keypad (for sending four-bit digit signals) plus four buttons for the four supervisory signals (clear-forward, seize-terminal, seize-transit, and transfer-to-operator). After some experimentation, nimble-fingered phreaks found that all they needed was two buttons, one for each frequency. With practice, it was possible to manually generate all the signals with sufficient timing precision, including the digit signals. This made it possible to make the blue box quite small.

A refinement added to some System 4 blue boxes was an anti-acknowledgment-echo guard tone. Because the connection between the telephone and the telephone network is two-wire, but the signaling on the international circuit operates on a four-wire basis (totally separate send and receive paths), signal-acknowledgment tones (single pulses of one of the two frequencies from the far end of the circuit after receipt of each digit) tended to be reflected at the four-wire/two-wire conversion point. Although these reflected signals were relatively faint, they were sometimes loud enough for the digit-receiving circuits at the far end to treat them as the first bit of the next digit, messing up the phreak's transmitted digits.

What the improved blue box did was to continuously transmit a tone of some other frequency (e.g., 600{{nbs}}Hz) as a guard tone whenever it was not sending a System 4 signal. This guard tone drowned out the echoed acknowledgment signals so that only the blue box-transmitted digits were heard by the digit-receiving circuits at the far end.