9.2.8 Handling of RTP telephone events

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

DTMF digits, telephony tones and signals (telephone events) can be transferred using different mechanisms. For the IM CN Subsystem, 3GPP TS 24.229 [9] defines the usage of the RTP payload format defined for DTMF Digits, Telephony Tones and Telephony Signals in RFC 4733 [105]. When BICC signalling is used in the CS network, telephony signals may be sent either inband or out-of-band as defined in ITU-T Recommendation Q.1902.4 [30] and in ITU-T Recommendation Q.765.5 [35]. If ISUP signalling is used the DTMF tones are sent inband. The following paragraphs describe the Mn interface procedures to transfer DTMF between RTP format defined in RFC 4733 [105] and the CS CN.

Before the actual usage of the telephony signals can occur the sending/receiving of telephone events need to be agreed with the SDP offer-answer mechanism defined in RFC 3264 [36]. The outcome of the negotiation can be e.g. that no telephone events are sent in RTP payload, telephone events are sent only in one direction or in both directions. If the outcome of the negotiation is that RTP payload telephone-events are sent in both directions, the IM-MGW may nevertheless be configured to interwork only mobile originated telephone-events.

When the offer-answer mechanism based session parameters negotiation results in an agreement that telephone events are sent in the RTP payload and the needed preconditions are fulfilled, telephone events can be sent in RTP payload. This negotiation can be done at call control signalling phase or during an ongoing call.

If the MGCF and IM-MGW support the reception and/or transmission of the RTP MIME type "telephone event" (as defined in RFC 4733 [105]) with the IMS, the following applies:

– For CS Network Originating Sessions, the MGCF shall include the MIME type "telephone events" with default events in the first SDP offer. After the usage of telephone events is agreed in the subsequent offer-answer parameter exchanges and the needed preconditions defined in RFC 3312 [37] are fulfilled, telephone events can be sent as RTP payload.

– In case of IM CN Subsystem Originating Sessions, the MGCF shall accept the MIME type "telephone events" with default events in any SDP answer when it received such an offer.

9.2.8.1 Sending DTMF digits out-of-band to CS CN (BICC)

Furthermore, the MGCF shall use the "Detect IMS RTP Tel Signal" procedure to request the MGW to detect incoming telephone events from the IMS and notify the MGCF about the detected events. The MGW shall use the "Notify IMS RTP Tel Event" procedure for this notification. The termination used to receive DTMF shall be placed in the same context used for the speech of the same call. The MGCF shall request to be notified when the MGW detects the end of a digit and may also request to be notified when the MGW detects the start of a digit. An IM-MGW not supporting the notification about the detection of the start of a digit may ignore the request to provide this notification. If the IM-MGW received a "Detect IMS RTP Tel Event" procedure for a termination, the IM-MGW shall not forward inband to the CS network any DTMF received at this termination.

Figure 48 shows an example message sequence chart when a DTMF digit is received from the IM CN subsystem in the RTP payload and the MGCF has requested to be notified only about the detection of the end of a digit. Figure 48a shows an example message sequence chart when a DTMF digit is received from the IM CN subsystem in the RTP payload and the MGCF has requested to be notified about the detection of the start and the end of a digit.

Figure 48: Activation of notification of DTMF digits received in RTP and examples of sending the digits out-of-band to CS CN, a whole digit received by IM-MGW before sending further (message sequence chart)

Figure 48a: Activation of notification of DTMF digits received in RTP and examples of sending the digits out-of-band to CS CN, IM-MGW starts sending the digit further when the start of the digit is recognized (message sequence chart)

9.2.8.2 Sending and receiving DTMF digits inband to/from CS CN (ISUP or BICC)

For the IM CN subsystem terminated session, the MGCF shall use the "Configure IMS Resources" procedure as described in clause 9.2.3. For the IM CN subsystem originating session, the MGCF shall use the "Reserve IMS Connection Point and Configure Remote Resources" procedure as described in clause 9.2.2. If DTMF is supported and the MGCF wants to configure the IM_MGW to send and receive DTMF to/from the CS network side, the MGCF shall include "telephone event" along with the selected speech codecs within the "local IMS resources" parameter of these procedures to request the MGW to detect incoming telephone events and transform them into speech signals on the CS side and shall not apply the "Detect IMS RTP Tel Event" procedure. When receiving this configuration, an MGW supporting DTMF shall detect DTMF encoded according as RTP Tel Event and transform this into DTMF tones encoded within the speech codec used at the CS CN network and may in addition optionally detect incoming telephone events received inband from the CS CN network and transform them into telephone events on the IMS side. The same termination shall be used to receive and transmit DTMF and speech of the same call.

Figure 49 shows the message sequence chart to configure the IM-MGW to receive DTMF detection on the IMS side and transfer the DTMF inband on the CS side. When receiving this configuration, the IM-MGW may in addition optionally detect DTMF inband on the CS side and transmit DTMF on the IMS side.

Figure 49: Activation of processing of DTMF digits received in RTP for sending the digits inband to CS CN (message sequence chart)

9.2.8.3 Receiving DTMF digits out-of-band from CS CN (BICC)

Furthermore, the MGCF shall use the "Send IMS RTP Tel Event" and may use the "Stop IMS RTP Tel Event" procedures to request the MGW to play out DTMF to the IM CN subsystem whenever it receives out-of-band DTMF indications from the BICC network.

Figure 49a shows an example message sequence chart when DTMF digits are transmitted to the IM CN subsystem in the RTP payload. For the first digit, the received APM message contains all information including the duration and only a single notification is received. For the second digit, the start and the end of the DTMF digit are notified separately.

Figure 49a: Examples of receiving DTMF digits out-of-band from the CS CN
and transmitting them in RTP (message sequence chart)