Dual-Tone Multi-Frequency

DTMF (dual tone multi frequency) is the signal to the phone company that you generate when you press an ordinary telephone's touch keys. In the United States and perhaps elsewhere, it's known as "Touch-Tone®" phone (formerly a registered trademark of AT&T). With DTMF, each key you press on your phone generates two tones of specific frequencies. So that a voice can't imitate the tones, one tone is generated from a high-frequency group of tones and the other from a low frequency group. Here are the signals you send when you press your Touchtone phone keys:












In technical words: Dual Tone Multi-Frequency, or DTMF is a method for instructing a telephone switching system of the telephone number to be dialed, or to issue commands to switching systems or related telephony equipment.

The DTMF keypad is laid out in a 4×4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as '1' ) will send a sinusoidal tone of the two frequencies (697 and 1209 hertz (Hz)). The original keypads had levers inside, so each button activated two contacts. The multiple tones are the reason for calling the system multifrequency. These tones are then decoded by the switching center to determine which key was pressed.

DTMF keypad frequencies table





DTMF event frequencies





The engineers had envisioned phones being used to access computers, and surveyed a number of companies to see what they would need for this role. This led to the addition of the number sign (#) and star (*) keys (also known as humphries), as well as a group of keys for menu selection: A, B, C and D. In the end, the lettered keys were dropped from most phones, and it was many years before the humphries became widely used for vertical service codes such as *67 in the United States and Canada to suppress caller ID.

Present-day uses of the A, B, C and D keys on telephone networks are few, and exclusive to network control. The levels of priority available were Flash Override (A), Flash (B), Immediate (C), and Priority (D), with Flash Override being the highest priority. For example, the A key is used on some networks to cycle through different carriers at will (thereby listening in on calls). Their use is probably prohibited by most carriers. The A, B, C and D tones are used in amateur radio phone patch and repeater operations to allow, among other uses, control of the repeater while connected to an
active phone line.

DTMF tones are also used by some cable television networks and radio networks to signal the local cable company/network station to insert a local advertisement or station identification.

Spectral Analysis of Dual-Tone Multi Frequency (DTMF)

References:
searchnetworking.com/
nemesis.lonestar.org/

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Call Centers have been affected by the damage of Undersea Cables

An earthquake off Taiwan, measuring 6.7-7.1 on the Richter scale, knocked out Internet links to India for 20-25 minutes and affected Reliance Communications’ FLAG and VSNL’s SEA-ME-WE-3 under-sea cable systems, even as telecommunications around Asia was severely disrupted, with Internet services slowing and financial transactions being hindered, particularly in the currency market. However, BPO and IT services across India remained largely unaffected.

Internet services have been disrupted in large parts of the Middle East and India following damage to two undersea cables in the Mediterranean. There was disruption to 70% of the nationwide network in Egypt, and India suffered up to 60% disruption.

UK firms such as British Airways have told the BBC that call centers have been affected by the outage.

Industry experts said it could take up to one week to repair the damaged cables and resume full service.

International telephone calls, which have also been affected, are being rerouted to work around the problem.

Neither of the cable operators have confirmed the cause or location of the outage but some reports suggest it was caused by a ship's anchor near the port of Alexandria in Egypt.

"Information technology companies, software companies and call centres that provide online services to the UK or the US east coast are the worst affected," Rajesh Chharia, president of the Internet Service Providers' Association of India, told Agence France-Presse (AFP).

India is the world leader in offshore outsourcing, with the remote servicing of IT or other business processes worth an estimated £24bn.