Loop around

A Loop line or Loop Around is a telephone company test circuit. The circuit did not connect to a specific place, and required two numbers dedicated to it. When one side of the loop was called (Side A), the caller got an approximately 1000 Hz tone (Milliwatt test). When the second number (Side B) was called, it would give the caller dead silence, but the party on the first number would hear the 1000 Hz tone drop and be connected to the person on the second number. The purpose of the loop around test numbers was to allow circuit testing to a distant central office without needing a person at the far end. The technician could send a tone down either line and measure the tone coming back on the second line and calculate the path loss parameters.

When a line is connected to side A, a number of lines may connect to side B (there is usually a limit on this) and thus be connected into a conference with the person on A. The function of the tone on side A was to alert those already connected, when somebody called the B side and connected.

Teenagers discovered that the numbers could be used as "party lines" or "chat rooms" and would dial up the primary number and wait for someone at random to call its mate. Phreaks would use the loop arounds in a similar manner, to exchange information that they had learned about the phone company.

Loop lines are far less common now than they were around 1960, however they reportedly continued to exist past the turn of the century. Because of their nature, however, telephone companies work to protect them (in order to make people use conference calling). The most common methods are:

*Filters which bandstop voice (these can be switched on or off)
*Recordings which make the line appear out of service
*The loop being switched on or off by operators
*Restrictions on which lines may call
*Using tones in the 1633 column of DTMF, A-D, in the number

The phone company eventually designed a device to prevent this misuse of the loop around. The device would block the audio if frequencies other than the 1000 Hz tone used for testing were present, which thwarted the average party line user, but the hardcore phreaks discovered that if one played a 1000 Hz tone and spoke softly, the suppression unit could be fooled, and a notch filter could be used while listening to get rid of the tone at the other end.

ee also

*Loopback
*Milliwatt test