6 Support of DTMF across the air interface
23.0143GPPRelease 17Support of Dual Tone Multi-Frequency (DTMF) signallingTS
6.1 General
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.
6.2 Specific
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:
a) START DTMF: Containing the digit value (0-9,A,B,C,D,*,#);
b) 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;
c) STOP DTMF: No further info;
d) 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 [3] (for PCS 1900 for North America this is operator specific);
– Tone sending levels are defined in ETSI ES 201 235-2 [4] (for PCS 1900 for North America this is operator specific);
– Durations as specified below.
6.3 Tone durations
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 [4] is achieved. For PCS 1900 for North America this is operator specific.
NOTE 1: In ETSI ES 201 235-2 [4] the minimum duration of a DTMF tone is 65 ms.
NOTE 2: In ETSI ES 201 235-2 [4] the minimum gap between DTMF tones is 65 ms.
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.
Ta Association time for DTMF Generator in MSC, implementation dependent but low.
Tp Pre-determined maximum tone length, operator option.
T1 Minimum length of tone.
NOTE: If the Network operator implements the time limit option (see subclause 6.2), then the tone ends if the timer expires before the ‘Stop DTMF’ is received.
Figure 1: Single DTMF Transmission
Ta Association time for DTMF Generator in MSC, implementation dependent but low.
Tp Pre-determined maximum tone length, operator option.
T1 Minimum length of tone.
Figure 2: Single DTMF Transmission, Short Key Press
Ta Association time for DTMF Generation in MSC, implementation dependant but low.
Tp Pre-determined maximum tone length, operator option.
T2 Minimum gap between tones.
Figure 3: Two Single DTMF Transmission
T1 Minimum length of tones
T2 Minimum gap between tones.
Figure 4: Two Single DTMF Transmissions, Short Gap Between Key Presses