![]() DTMF Tones ExplainedĪlternatively, DTMF tones can be digitally removed from the audio RTP stream at the source and separately encoded as digital data payloads known as a Named Telephone Events (NTE), transmitted on the same network path alongside the other RTP packets. If G.729 or G.723 are used, signaling normally fails as a result of the compression. In-band DTMF transmission is only reliable when the uncompressed G.711 codec is used. The standard in-band method is to simply transmit the tones along with the audio, but this can result in unreliable signals due to codec compression, packet loss or audio interference. ![]() Out-of-band VoIP DTMF signaling can be implemented using protocols like SIP and MGCP, whereby special message types are defined for the transmission of digits. But in voice over IP, DTMF signals can be transmitted in-band or out-of-band. Traditional DTMF is an in-band signaling system, meaning the signals are transmitted using the same channel as the voice traffic. ![]() # 941 Hz 1477 Hz In-Band and Out-of-Band Signaling The list below shows the dual tones used to represent each standard telephone key. It is standardized under ITU-T Recommendation Q.23. One tone is high frequency while the other is low frequency. With DTMF, each key generates two simultaneous tones of specific frequencies, designed so that a human voice can’t replicate the tones. ‘Touch-Tone’ remained a registered trademark of Bell Systems, and later AT&T, until 1984. Over the following decades, it gradually replaced the earlier Pulse Dialing or Rotary Dialing technology. DTMF was invented by the Bell Telephone Company in 1963. Dual Tone Multi-Frequency is a signal tone generated when buttons are pushed on a telephone keypad – hence the term ‘Touchtone Phone’.
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