Multi-frequency signaling
In
Multi-frequency signaling defines electronic signals that consist of a combination of two audible frequencies, usually selected from a set of six frequencies. Over several decades, various types of MF signaling were developed, including national and international varieties. The
Because of the in-band transmission characteristic of MF signaling, the systems proved vulnerable to misuse and fraud by
Multifrequency signaling is a technological precursor of
Operation
Digits are represented by two simultaneous tones selected from a set of five (MF 2/5), six (MF 2/6), or eight (MF 2/8) frequencies. The frequency combinations are played, one at a time for each digit, to the remote multi-frequency receiver in a distant telephone exchange. MF is used for signaling in trunking applications.
Using MF signaling, the originating telephone switch sends a start signal to seize the line, taking the circuit off-hook. The terminating office acknowledges the seizure with a ready state by responding with a wink start signal, which is a momentary off-hook condition. The originating office then sends address information to the terminating switch. In R1 MF signaling this address information normally is a KP tone, the numeric digits of the destination number, and an ST tone to indicate the end of the address. Other information may also be added, such as the caller's number, using KP2 as a delimiter.
MF is a type of in-band signaling. Depending on the type and configuration of switching equipment, it may or may not be audible to the telephone user, but the technology was vulnerable to abuse with a method called phreaking with a blue box which generates the tones required to control remote telephone switches.
Multi-frequency signals
Code | 700 Hz | 900 Hz | 1100 Hz | 1300 Hz | 1500 Hz | 1700 Hz |
---|---|---|---|---|---|---|
1 | X | X | ||||
2 | X | X | ||||
3 | X | X | ||||
4 | X | X | ||||
5 | X | X | ||||
6 | X | X | ||||
7 | X | X | ||||
8 | X | X | ||||
9 | X | X | ||||
0/10 | X | X | ||||
11/ST3 | X | X | ||||
12/ST2 | X | X | ||||
KP | X | X | ||||
KP2 | X | X | ||||
ST | X | X |
The Bell System published the following standards for MF tone timing:[citation needed]
- KP (110 ms)
- Digits 0-9, ST: 55 ms
The interval between digits is the same as the digit duration (55 ms)
These standards are, for the most part, still in place where MF signaling is in use in legacy exchanges. MF signaling is still used in North America for inter-office signaling, although it is increasingly rare.[citation needed]
Demise
In-band signaling fell into disfavor in the public switched telephone network (PSTN) as electronic switching systems displaced electro-mechanical switching systems, but legacy offices may still exist in some countries, that are still using some electromechanical and other legacy switching equipment .[2]
Out-of-band
Most 911
See also
References
- ^ Pearce, J. Gordon (2013). Telecommunications Switching. Springer. p. 243.
- ^ "In-Band Signaling in the former Soviet Union". Binary Revolution Forums. Archived from the original on 2021-05-16. Retrieved 2019-08-08.
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: CS1 maint: bot: original URL status unknown (link) - ^ "NENA 03-002 v2: Enhanced MF Signaling, E9-1-1 Tandem to PSAP".
External links
- "Speeding Speech", a 1950s Bell System film, depicts a 2-1-1 long-distance operator manually entering a number on an MF keypad just prior to the introduction of direct distance dialing. The keypad, visible at 0:01:41 and 0:05:20, has two columns of five digits plus KP (key pulse) and ST (start).
- [1] Details of MF signaling from Bell Laboratories Record. Pages 221-225