Page 370 - From GMS to LTE

356 From GSM to LTE-Advanced Pro and 5G

Figure 5.10 An RTP frame with an embedded DTMF signaling message. The message was produced
after the user pressed the digit ‘7’ on the dial pad. Source: Gerald Combs / Wireshark.

media gateway is then responsible for converting the RTP DTMF messages into an
audible tone and injecting this into the speech path data stream.
From an implementation point of view it makes sense to transport DTMF signaling
messages as part of the data bearer stream instead of generating the tone at the originat-
ing device. Generating the tone as far down the transmission chain as possible ensures
that transcoding from one speech codec to another or from digital to analog has as little
impact as possible on the DTMF tone. Furthermore, sending the message as part of the
speech path and not as individual signaling messages over the IMS infrastructure means
that the DTMF tones are delivered without delay and no resources are required in the
IMS signaling network to transport them.

5.3.12 SMS over IMS
Non‐VoLTE‐capable LTE devices send and receive traditional SMS messages via an LTE
signaling channel and the SGs interface as described in Chapter 4. VoLTE‐capable LTE
devices can transmit and receive SMS messages over SIP. As shown in Figure 5.11 this
is done by embedding SMS messages in SIP ‘Message’ requests in a container that is
transparent to the IMS. That means that the SMS message is encoded in exactly the
same way as if it was transmitted over GSM, UMTS or via LTE over the SGs interface.
On the network side the IP‐Short‐Message‐Gateway (IP‐SM‐GW) bridges the old cir-
cuit‐switched signaling world in which the SMS Service Center is located and the IMS IP
world as described in 3GPP TS 23.204 [18]. As the format of the SMS message is not
changed, all SMS‐related services, such as, for example, the delivery notification of the
terminating device to the originating device or the transmission of binary data inside SMS
messages to update information on the SIM card, are implicitly supported as well without
any additional functionality required in the IMS network or the end user’s device.