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Dual Tone Multi Frequency (DTMF) is an inband one out of four plus one out of four signalling system, primarily used from terminal instruments in telecommunication networks. The international recommendations which apply are ETSI ES 201 235 , ,  as detailed in subclause 6.2 and subclause 6.3. For PCS 1900 for North America the Standards which apply are operator specific.
In the 3GPP system the MSC must support DTMF in the mobile to land direction.
The support of this facility in the land to mobile direction is for further study.
The use of DTMF is only permitted:
when the speech teleservice is being used or during the speech phase of alternate speech/data and alternate speech/facsimile teleservices; the DTMF is transmitted across the radio interface as specified in subclause 6 of this specification; and
during a multimedia call; the DTMF is transmitted across the radio interface using the H.245 UserInputIndication message (see ITU-T H.245 ). This is transparent for the MSC.
The responsibility for checking this lies in the MS.
A user may cause a DTMF tone to be generated by depression of a key in the Mobile Station (MS). Optionally (on a MS basis) manufacturers of mobile equipment may choose to allow DTMF to be controlled from a remote terminal.
The man-machine interface questions associated with this facility are not discussed further in the present document.
A message based signalling system is used across the 3GPP system air interface.
This requires that the relevant user action (e.g. a key depression) is interpreted by the MS as a requirement for a DTMF digit to be sent, this is converted by the MS into a message, the message is transmitted across the air interface, and is converted by the MSC into a DTMF tone which is applied towards the network, which should then respond with an acknowledgement. When the user completes the key depression, an message that the DTMF sending should cease is also passed to the MSC, which again will respond with an acknowledgement.
The messages to be sent across the air interface will use the frame stealing mode of transmission.
The messages when sent across the air interface should contain the following information:
START DTMF: Containing the digit value (0-9, A, B, C, D, *, #);
START DTMF ACKNOWLEDGE: Containing the digit value (0-9, A, B, C, D, *, #) corresponding to the DTMF tone that the network applies towards the remote user;
STOP DTMF: No further info;
STOP DTMF ACKNOWLEDGE: No further info.
Only a single digit will be passed in each START DTMF and START DTMF ACKNOWLEDGE message.
The messages will be passed transparently through the base station and interpreted at the MSC.
On receipt of a START DTMF message, the MSC will connect the correct dual-tone to line. This tone will remain connected until either the call is cleared or a STOP DTMF message is received.
As an operator option, the tone may be ceased after a pre-determined time whether or not a STOP DTMF message has been received.
The tones that are to be generated by the MSC are specified as follows:
Frequencies are defined in ETSI ES 201 235-1  (for PCS 1900 for North America this is operator specific);
Tone sending levels are defined in ETSI ES 201 235-2  (for PCS 1900 for North America this is operator specific);
The network shall ensure that the minimum length of tone and the minimum gap between two subsequent tones according to ETSI ES 201 235-2  is achieved. For PCS 1900 for North America this is operator specific.
There is no defined maximum length to the tone, which will normally cease when a STOP DTMF message is received from the MS. However, the operator may choose to put a pre-defined time limit on the duration of tones sent to line as mentioned in clause 6.2.
Figures 1 to 3 show an overview of how the DTMF should operate.
Depending on the exact moment when handover occurs, there may be a slight possibility of cutting short a DTMF tone.
For protocol reasons, in the case of an MSC receiving a STOP DTMF message when no tone is being sent, it should respond with an acknowledgement as usual.
No other impact is seen due to external handover.