6 Support of DTMF across the air interface

03.143GPPSupport of Dual Tone Multi-Frequency Signalling (DTMF) via the GSM SystemTS

6.1 General

A message based signalling system is used across the GSM 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 a been received.

The tones that are to be generated by the MSC are specified as follows:

– Frequencies are defined in CEPT Recommendation T/CS 34-08 (for PCS 1900 for North America this is operator specific);

– Tone sending levels are defined nationally (for PCS 1900 for North America this is operator specific);

– Durations as specified below.

6.3 Tone durations

The minimum length of tone generated by the switch should be according to CEPT recommendation T/CS 46-02. For PCS 1900 for North America this is operator specific.

The minimum gap between two subsequent tones should be according to CEPT recommendation T/CS 46-02. 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 6.2.

Figures 1 to 3 show an overview of how the DTMF should operate.

NOTE: The transmission time of the messages over the air interface on FACCH/F or FACCH/H, see GSM 05.02, ensures that the minimum length of tones and minimum gap between tones according to T/CS 46-02 are fulfilled. This does not apply to PCS 1900 for North America.

MMI MS BSS MSC

key X depressed
──────────────> START DTMF (X)
──────────────> START DTMF (X)
───────────────>─┬─
│Ta

START DTMF │
START DTMF ACKNOWLEDGE (X) │ Tone X ON
ACKNOWLEDGE (X) <─────────────────├─────────>
ACKNOWLEDGE <───────────── │/ / /
<────────────── │ / /
│ / / /
│ / / /
Key X Released │Tp / /
──────────────> STOP DTMF │ / /
─────────────> STOP DTMF │ Tone OFF
────────────────>├─────────>
STOP DTMF │
STOP DTMF ACKNOWLEDGE │
ACKNOWLEDGE ────────────────│
ACKNOWLEDGE <───────────── │
<————– │

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.

NOTE: If the Network operator implements the time limit option (see section 6.2), then the tone ends if the timer expires before the ‘Stop DTMF’ is received.

MMI MS BSS MSC

key X depressed
──────────────> START DTMF (X)
──────────────> START DTMF (X)
key X released ───────────────>─┬─
──────────────> │Ta

START DTMF │
START DTMF ACKNOWLEDGE (X) │ Tone X ON
ACKNOWLEDGE (X) <─────────────────├─────────>
ACKNOWLEDGE <───────────── │ / / /
<————– │/ / /
│ Tp / /
│ / / /
│ / / /
STOP DTMF │/ / /
─────────────> STOP DTMF │ Tone OFF
────────────────>├─────────>
STOP DTMF │
STOP DTMF ACKNOWLEDGE │
ACKNOWLEDGE ────────────────│
ACKNOWLEDGE <───────────── │
<————– │

Figure 2: Single DTMF Transmission, short key depression

Ta Association time for DTMF Generator in MSC, implementation dependent but low.

Tp Pre-determined maximum tone length, operator option.

MMI MS BSS MSC

key X depressed
──────────────> START DTMF (X)
key X released ──────────────> START DTMF (X)
──────────────> ───────────────>─┬─
key Y depressed │Ta
──────────────> │
key Y released │
──────────────> START DTMF │
START DTMF ACKNOWLEDGE (X) │ Tone X ON
ACKNOWLEDGE (X) <─────────────────├─────────>
ACKNOWLEDGE <───────────── │ / / /
<————– │/ / /
│ Tp / /
│ / / /
│ / / /
STOP DTMF │/ / /
─────────────> STOP DTMF │ Tone OFF
────────────────>├─────────>
STOP DTMF │
STOP DTMF ACKNOWLEDGE │
ACKNOWLEDGE ────────────────│
ACKNOWLEDGE <───────────── │
<————– │


START DTMF (Y)
──────────────> START DTMF (Y)
───────────────>─┬─
│Ta
START DTMF │
START DTMF ACKNOWLEDGE (Y) │ Tone Y ON
ACKNOWLEDGE (Y) <─────────────────├─────────>
ACKNOWLEDGE <───────────── │ / / /
<————– │/ / /
│ Tp / /
│ / / /
│ / / /
STOP DTMF │/ / /
─────────────> STOP DTMF │ Tone OFF
────────────────>├─────────>
STOP DTMF │
STOP DTMF ACKNOWLEDGE │
ACKNOWLEDGE ────────────────│
ACKNOWLEDGE <───────────── │
<————–

Figure 3: Two single DTMF Transmissions, short key depressions

Ta Association time for DTMF Generator in MSC, implementation dependent but low.

Tp Pre-determined maximum tone length, operator option.