Mabaloo - An introduction to telephone tone dialling

An introduction to telephone tone dialling

Written by Atul Sunsunwal

Today every person either has his personal mobile or has access to telephone in some way or the other. The telephones have been upgraded from the old pulse dialing to new hi speed tone dialing. But has anyone ever thought what does all this means?

The one thing that irritates most us about the tone dialing is the tones itself. We often hear when dialing/redialing a number through a land line or mobile, or while typing a message on the mobile. Generally mobile users do not come across these tones very often. Thanks to the silent option for keypad tones. We usually ignore these tones, have you ever thought that when these tones are so irritating why do we have them?

Pulse Dialing

Before tone dialing, pulse dialing was the only way to dial a number. You must have seen those antique phones with a circular thing on top having numbers 0 to 9 (actually 1 to 0). That phone dialed a digit when the circular thing is rotated by certain angle allotted to that digit as per the construction of rotor. The pulse dialing used to dial by rapidly disconnecting the phone line particular number of times. For example, if i dial 2, the phone will get disconnected twice (or by any other number as set by the company).

Different countries use different standards,

U.S.

standard is one disconnect per digit and dialing 0 means disconnecting ten times. These phones made dialing very slow, so new phones were introduced which had ICs and buttons to dial the number. With this, one didn’t wait for the rotor to return to original position before dialing another digit. ICs served as short memory and helped dialing the digit with proper pulse rate. Since this method contains rapid connection and disconnection, you can dial a number by stroking the hook-key rapidly. Interesting?? Try it. It works. Apart from being slower pulse dialing also has the limitation that its audio band signal can travel only as far as local phone exchange. To overcome these limitations, tone dialing was introduced which is faster and its audio band signal can travel down the phone lines further than pulse.

Keypad frequency

Tone Dialing

When 1 is dialed the waves with frequencies 1209Hz and 697Hz are added and the resultant signal is sent. The addition is not arithmetic as the waves are added, not the frequencies. The frequencies used are same all over the world. The interesting thing here is: A, B, C, and D, these are the 4 buttons not present in our telephone. Then why are they present in DTMF? They might not be useful to us but they are used within the telephone networks and in military purposes where more frequencies are required using less number of combinations of keys. If u want to hear the desired DTMF tone u can download software for generating such tones or hear it online. Another interesting thing is that you can use these tones to dial from your telephone. It can serve you as auto dialer (this is what software claims). In other words you can dial a number from your phone without touching it. The transmitter and receiver of the telephone are restricted to various conditions like: Maximum and minimum strength of the signal. The minimum strength varies for upper and lower band frequencies, Leakage of tones, Inter digit interval etc.

The signal strength for telephones is usually expressed in dB-microvolts per metre (dBµV/m) or in decibels above a reference level of one milliwatt (dBm). Sometimes our phone is unable to detect signals, it is due to destructive interference or some obstacle which weakens the signal (like construction). The signals having higher frequency are more prone to get affected but they also have the advantage of using reflection and diffraction phenomenon to cross the obstacle. This is the reason why we have low or no signal in some areas like basement and lifts.

Today most of the phones come with an internal amplifier which amplifies the signal so that phone can work in places like basement, lifts, hospitals etc. The earlier phone which were DTMF enabled used small value of inter digit interval like 40-50ms. It was then found that shorter tones were more likely to cause dialing troubles. Most of the phones today use longer DTMF length as default setting. There are some security issues connected with the tone dialing. Since the code for a particular digit is unique and same all over the world, it can be easily decoded if one has the decoder. That means you can know what your friend is dialing by just standing near him and carrying a decoder. Make sure it remains a prank…..don’t cross the limit….

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Comments (1)

1 Written by This e-mail address is being protected from spam bots, you need JavaScript enabled to view it , on 29-04-2008 09:21
excellent...this solved the basic doubt of pulse and tone dailling..8