14.4 DTMF

23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS

DTMF information can be transported either inband or out of band. In order to minimise the interworking between out of band and in band DTMF signalling, the general principle is to use the DTMF signalling method of the preceding node whenever possible. A node supporting OoB DTMF shall also be able to receive inband DTMF digits, but no DTMF digits shall be duplicated, i.e. any detected digit shall either be sent forward by inband or out-of-band, but never by both methods. Transitions between inband and out-of-band may occur due to changes to an ongoing call (Explicit Call Transfer for example) but digits shall not be sent both inband and OoB for the same link.

If out-of-band transcoder control is applied for a speech call, it shall be performed in accordance with 3GPP TS 23.153 [3]

14.4.1 DTMF Tone Generation

14.4.1.1 Inband DTMF Tone Generation

This option uses inband signalling to transport DTMF digits in the core network.

The DTMF tone generation shall be performed in accordance with 3GPP TS 23.108 [18]. The following clauses describe the additional requirements for the bearer independent CS core network.

14.4.1.1.1 Start DTMF

When the MSC server receives the Start DTMF message from the UE, it uses the Send DTMF procedure to request the MGW to modify the bearer termination to play a tone for the pressed digit. The result of the tone sending by the bearer termination will be received by the MSC server and sent to the UE (bullet 1 in figure 14.3).

14.4.1.1.2 Stop DTMF

When the MSC server receives the Stop DTMF message from the UE, it uses the Stop DTMF procedure to request the MGW to modify the bearer termination to stop digit playing. When the response is received from the MGW, the MSC server will acknowledge the Stop DTMF (bullet 2 in figure 14.3).

The MGW shall check the minimum duration and minimum interval in accordance with the DTMF timing defined in TS 23.014 [33].

14.4.1.1.3 Example

Figure 14.3 shows an example where out-of-band signalling of DTMF information is not supported by the call control protocol. When the UE sends Start DTMF and Stop DTMF messages , the MSC server uses resources in the MGW to generate tones by modifying the bearer termination.

Figure 14.3: Inband DTMF generation (message sequence chart)

14.4.1.2 Out-of-Band DTMF Tone Generation

This option uses out-of-band network signalling to transport DTMF digits in the core network, where the information is sent on a call control layer.

The DTMF Tone Generation shall be performed in accordance with 3GPP TS 23.108 [18]. The following clauses describe the additional requirements for the bearer independent CS core network.

14.4.1.2.1 Start DTMF

When the MSC server receives a Start DTMF message from the UE, it indicates digit playing using out-of-band signalling. The corresponding result received from the preceding/succeeding node will be sent to the UE (bullet 1 in figure 14.4).

14.4.1.2.2 Stop DTMF

When the MSC server receives a Stop DTMF message from the UE, it indicates stop digit playing using out-of-band signalling. The succeeding node will indicate that digit playing is stopped. The MSC server will send the result back to the UE (bullet 2 in figure 14.4).

14.4.1.2.3 Example

Figure 14.4 shows the message sequence example for the out-of-band DTMF during a call. When the MSC server receives the Start DTMF and Stop DTMF messages from the UE, it shall send the information using signalling on call control layer. The MSC server will not use any dedicated resources of the MGW.

Figure 14.4: Out-of-Band DTMF generation (message sequence chart)

14.4.2 DTMF Detection

14.4.2.1 Inband DTMF Detection

The (G)MSC server/gsmSSF/gsmSRF requests the MGW to detect DTMF tones using Detect DTMF procedure (bullet 1 in figure 14.5).

At detection of the DTMF tone the MGW reports the digit to the (G)MSC server/gsmSSF/gsmSRF using the Report DTMF procedure (bullet 2 in figure 14.5). At reception of the DTMF tone report the (G)MSC server/gsmSSF/gsmSRF either expects the MGW to detect other DTMF tones (in which case no new Detect DTMF request needs to be sent) or requests the MGW to stop the detection of DTMF tone (bullet 3 in figure 14.5) using the Stop DTMF Detection procedure.

Figure 14.5: Inband DTMF detection (message sequence chart)

14.4.2.2 Out-of-Band DTMF Detection

The (G)MSC server/gsmSSF/gsmSRF starts collecting out-of-band DTMF tones. One DTMF tone consists of Start DTMF (bullet 1 in figure 14.6) and Stop DTMF messages (bullet 2 in figure 14.6).

Figure 14.6: Out-of-Band DTMF detection
(message sequence chart)