A loop line or loop around is a
telephone company
A telephone company, also known as a telco, telephone service provider, or telecommunications operator, is a kind of communications service provider (CSP), more precisely a telecommunications service provider (TSP), that provides telecommunicat ...
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 A Milliwatt test (Milliwatt line) 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.
The test consists of transmitting an analog sinus ...
). 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, or path attenuation, is the reduction in power density (attenuation) 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,
chat room
The term chat room, or chatroom (and sometimes group chat; abbreviated as GC), is primarily used to describe any form of synchronous conferencing, occasionally even asynchronous conferencing. The term can thus mean any technology, ranging from ...
s or alternate phone number 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.
[Ron Rosenbaum]
Secrets of the Little Blue Box
Retrieved December 24, 2012.
Loop lines are far less common today than they were in the 1960s, however they reportedly continued to exist past the start of the 21st 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
An automatic number announcement circuit (ANAC) is a component of a central office of a telephone company that provides a service to installation and service technicians to determine the telephone number of a telephone line. The facility has a tel ...
.
Because of the potential for abuse, however, telephone companies seek to protect them. The most common protection techniques are:
*Filters that
bandstop 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
Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed ...
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
In 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. A notch filter is a ...
could be used while listening to remove the tone at the other end.
See also
*
Loopback
Loopback (also written loop-back) is the routing of electronic signals or digital data streams back to their source without intentional processing or modification. It is primarily a means of testing the communications infrastructure.
There are m ...
*
Conference call
A conference call is a telephone call in which someone talks to several people at the same time. The conference call may be designed to allow the called party to participate during the call or set up so that the called party merely listens into ...
References
{{DEFAULTSORT:Loop Around
Telephone numbers
Communication circuits