Loop around
Encyclopedia
A loop line or loop around is a telephone company
test circuit. The circuit has two associated phone numbers. When one side of the loop is called (side A), the caller receives a test tone of approximately 1000 Hz
(milliwatt test
). When the second number (side B) is called, it produces dead silence, but the party on side A hears the milliwatt test tone drop, and is connected to the person on side B. The purpose of the loop around test is to allow circuit testing to a distant central office without needing a person at the far end. The technician can send a tone down either line and measure the response tone on the second line to determine the path loss
parameters.
When a line is connected to side A, multiple telephone lines, within limits, may connect to side B and thus be connected into a conference with the person on side 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 test facility could be used as party lines
or chat room
s and would dial up the primary number and wait for someone at random to call its mate. Phreaks would use the loop around in a similar manner, to exchange information that they had learned about the phone company.http://www.lospadres.info/thorg/lbb.html
Loop lines are far less common today than they were in the 1960s
, however they reportedly continued to exist past the turn of the century. One such loop line is certain to exist: it is a TPI-560P located at (416) 981-0001 owned by Telus. It also comes with an automatic number announcement circuit
.
Because of the potential for abuse, however, telephone companies seek to protect them. The most common protection techniques are:
Eventually, telephone companies designed devices to prevent this misuse of the loop around. Some devices blocked the audio if frequencies other than the milliwatt tone were present, which thwarted the average abuser, but skilled violators discovered that playing a 1000 Hz tone while speaking softly, the suppression unit could be subverted, and a notch filter could be used while listening to remove the tone at the other end.
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
test circuit. The circuit has two associated phone numbers. When one side of the loop is called (side A), the caller receives a test tone of approximately 1000 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(milliwatt test
Milliwatt test
A Milliwatt test is a test method or test facility used in telecommunications to measure line quality and transmission loss between stations or points in an analog telephone system....
). When the second number (side B) is called, it produces dead silence, but the party on side A hears the milliwatt test tone drop, and is connected to the person on side B. The purpose of the loop around test is to allow circuit testing to a distant central office without needing a person at the far end. The technician can send a tone down either line and measure the response tone on the second line to determine the path loss
Path loss
Path loss is the reduction in power density of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system....
parameters.
When a line is connected to side A, multiple telephone lines, within limits, may connect to side B and thus be connected into a conference with the person on side 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 test facility could be used as party lines
Party line (telephony)
In twentieth-century telephone systems, a party line is an arrangement in which two or more customers are connected directly to the same local loop. Prior to World War II in the United States, party lines were the primary way residential subscribers acquired local telephone service...
or chat room
Chat room
The term chat room, or chatroom, is primarily used by mass media to describe any form of synchronous conferencing, occasionally even asynchronous conferencing...
s and would dial up the primary number and wait for someone at random to call its mate. Phreaks would use the loop around in a similar manner, to exchange information that they had learned about the phone company.http://www.lospadres.info/thorg/lbb.html
Loop lines are far less common today than they were in the 1960s
1960s
The 1960s was the decade that started on January 1, 1960, and ended on December 31, 1969. It was the seventh decade of the 20th century.The 1960s term also refers to an era more often called The Sixties, denoting the complex of inter-related cultural and political trends across the globe...
, however they reportedly continued to exist past the turn of the century. One such loop line is certain to exist: it is a TPI-560P located at (416) 981-0001 owned by Telus. It also comes with an automatic number announcement circuit
Automatic number announcement circuit
An automatic number announcement circuit is a special telephone number that is meant to be used by phone company technicians and other telecommunications technicians to determine the phone number of a particular line....
.
Because of the potential for abuse, however, telephone companies seek to protect them. The most common protection techniques are:
- Filters that bandstopBand-stop filterIn signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter...
voice (these can be switched on or off) - Recordings which make the line appear out of service
- Selective loop activation by operators
- Restrictions on which lines may call
- Using uncommon DTMF tones (A through D) in the telephone number
Eventually, telephone companies designed devices to prevent this misuse of the loop around. Some devices blocked the audio if frequencies other than the milliwatt tone were present, which thwarted the average abuser, but skilled violators discovered that playing a 1000 Hz tone while speaking softly, the suppression unit could be subverted, and a notch filter could be used while listening to remove the tone at the other end.