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Configuring VoLTE with Your Siretta Modem

November 20, 2024 January 8th, 2025

Applicable Products

Part number Description
ZETA-GEP-LTE4 (EU) Low Power LTE Cat 4 European Modem with GPIO and GNSS
ZETA-NEP-LTE4 (EU) Low Power LTE Cat 4 European Modem with GPIO
ZETA-NLP-LTE1 (EU) Ultra Low Power European LTE Cat 1 Modem
ZETA-NSP-LTE1 (EU) Low Power European LTE Cat 1 Modem

VoLTE or Voice over LTE, uses 4G (LTE) technology to transmit voice audio from one point to another using digital packets as opposed to 3G (UMTS) and 2G (GSM) networks which transfer voice audio over a dedicated analogue channel.

VoLTE boasts several advantages over traditional CS (circuit switching) voice calls. VoLTE delivers superior voice quality, achieved through higher data rates and an expanded frequency range for voice transmission, resulting in a substantial enhancement in voice quality. The integration of EVS (Enhanced Voice Services) codec further extends this range. VoLTE has more data carrying capacity in comparison to 3G and 2G technologies and as such can connect more calls simultaneously.

Solution

This application note provides users with guidance on transferring voice audio over LTE technology using a Siretta ZETA modem.
The VoLTE implementation allows users to initiate voice calls from a Siretta Modem to a mobile phone or landline. Upon call connection, the PC linked to the Siretta Modem can seamlessly transmit voice audio, music audio, or DTMF tones to the mobile or landline that responds, facilitated through the USB interface.

Demonstration of Solution

Resources used

Note: The device must be running Firmware version: M0F.220010 or later. Other versions are not compatible.

Modem Configuration

  1. Refer to the ZETA-xxP Quick Start Guide, which provides step-by-step instructions for initializing the modem and establishing a connection with Tera Term.

Note: It is crucial to use a 4G/LTE-enabled SIM card that supports VoLTE (IMS).

  1. Verify the network registration state by executing the AT command: AT+CREG?
  • The modem will respond with one of the following when registered:
    1. +CREG: 0,1 – Registered to a home network
    2. +CREG: 0,2 – Not registered, but is currently searching for an operator to register to
    3. +CREG: 0,3 – Registration denied
    4. +CREG: 0,5 – Registered to a roaming network
COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT+CREG?

+CREG: 0,1

 

OK

3. Set modem to 4G (LTE) by issuing the AT command: AT+WS46=28

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT

OK

AT+WS46=28

OK

 

4. Set the default value for AT#DIALMODE=0 (Setting for voice call only).

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT#DIALMODE?

#DIALMODE: 0

 

OK

5. Set service class as voice by issuing the AT command: AT+FCLASS=8

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT+FCLASS=8

OK

6. Check if device is registered with IMS using AT command: AT+CIREG?

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT+CIREG?

+CIREG:0,1

OK

Note: IMS is required for VoLTE and is also known as the IP Multimedia Subsystem.

7. When successfully registered to IMS, the terminal should display: +CIREG: 0,1.
If +CIREG: 0,0 is displayed, it indicates that the SIM is not registered with IMS, potentially suggesting a lack of VoLTE support. Verify all parameters to ensure no details were overlooked. Should this persist, check whether the IMS service is enabled or not by issuing the AT command: AT#IMSPDPSET?

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT#IMSPDPSET?

#IMSPDPSET: IMS

OK

8. Alternatively, check IMS using the AT command: AT+CGDCONT?

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT+CGDCONT?

+CGDCONT: 1,” IP”,” internet”,””,0,0,0,0

+CGDCONT: 2,” IPV4V6”,” ims”,””,0,0,0,0

+CGDCONT: 3,” IPV4V6”,” sos”,””,0,0,0,1

OK

Note: The ZETA-NLP-LTE1 (EU) modem uses profile 2 for the IMS service as seen above.

9. Issue the AT command: AT$QCPDPIMSCFGE? to confirm profile 2 is enabled.

10. 1 Indicates enabled, 0 indicates disabled.

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT$QCPDPIMSCFGE?

$QCPDPIMSCFGE: 1,0,0,0

$QCPDPIMSCFGE: 2,1,0,0

$QCPDPIMSCFGE: 3,1,0,0

OK

Voice call configurations for the ZETA-NLP-LTE1 (EU) are now complete.

11. Initiate a voice call from the modem to a mobile number by issuing the command: ATD”number”.

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
ATD07400245346;

OK

12. The mobile phone will start to ring, indicating an incoming call from the ZETA-NLP-LTE1 (EU).

13. Answer the incoming call.

14. Once a voice call is established issue the AT command: AT+VTS=123456789 to the modem.

COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT+VTS=123456789

OK
  •  DTMF tones corresponding to the numbers 123456789 will be audible through the mobile phone speaker.
  • By this point a voice call has been established successfully, and DTMF tones were transferred over LTE.
    Note: This DTMF feature can be used to automate responses to a third-party system.

Application Example

This example shows how the ZETA-NLP-LTE1 (EU) modem can be integrated with Audacity.
Audacity is a free and open-source digital audio editor and recording application which can be downloaded from the following link https://www.audacityteam.org/download/

  • Download and install Audacity.
  • Connect the ZETA-NLP-LTE1 (EU) modem to a PC using a USB-A to USB-Mini B cable.
  • Go to the PC device manager and check that the Telit USB appears under the modems listing.

Note: If the Telit modem does not appear in device manager, download the USB drivers from https://www.siretta.com/software-library/.

  • Once the modem is connected, issue the AT command: AT#USBCFG=11 (Enables voice over USB).
COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT#USBCFG=11

OK
  • The modem will instantly reboot.
  • The Terminal (Tera Term) will disconnect from the COM port until reboot cycle is completed and connection is re-established.
  • Issue the AT command: AT#ADSPC=6 (Change the Audio path to USB).
COM43 – Tera Term VT
File    Edit     Setup     Control     Window     Help
AT#ADSPC=6

OK

Other supported paths are 0 to 6, where:
• 0 – Automatic (Default)
• 1 – Hands Free.
• 2 – Headset.
• 3 – Handset.
• 4 – Speaker Phone Bluetooth
• 5 – TTY (Teletypewriter)
• USB Universal Serial Bus

  • Go to the PC device manager under Sound, Video and Game controllers to check for AC interface.

 

  • Select AC interface, right click to explore menu.
  • Select Enable device from the menu if not already enabled.
  • Select AC interface again and right click to explore menu.
  • Select Properties then select the details tab.
  • Choose Hardware Ids from the Property drop down menu (see below).

  • Click OK.
  • Disable all other devices under Sound, Video and Game controllers except for AC Interface.

  • A downward arrow indicates a disabled device.
  • Open Hardware and Sound within control panel.

  • Click Sound Control panel.
  • Navigate to the playback tab

  • Right click the speakers icon and enable AC interface.
  • Click OK.

  • Navigate to the Recording tab (See below).

Note: Ensure the AC Interface is set as a default device.

  • Click OK.
  • Open the Audacity application.

  • Set the Microphone to AC interface.
  • Set Channel count to 1 (Mono).
  • Set the Speaker to AC interface.

Note: The ZETA-NLP-LTE1 (EU) modem supports:

  • PCM audio format
  • Mono
  • 16-bits depth
  • 16KHz Sample rate
  • Setup is now complete.
  • Repeat Steps 3 to 14 above to initiate a voice call.
  • Once voice calls are established between the ZETA-NLP-LTE1 (EU) modem and a mobile phone generate Chirp / DTMF Tones / Noise / Tone.

  • Click Play.

  • DTMF Tones/Noise/Tone will be heard on the Mobile phone.
  • Voice from the Mobile Phone to the ZETA-NLP-LTE1 (EU) modem can be captured by clicking the record button within Audacity (Red button).
  • Voice recordings can be played back and heard on the Mobile phone/PC by clicking the Play button in Audacity. The music file can be imported to the application, played and transmitted using Voice over LTE.
  • The example above shows how voice audio can be transmitted/received using VoLTE that is available on the Siretta Modems listed in the “Introduction”.

More information about Siretta modems can be found at: https://www.siretta.com/products/industrial-modems/

Additional Reading

Description Author
Initial Modem Setup Guide Application Note Siretta
ZETA-NxP Quick Start Guide Siretta
AT command manual Telit
Tera Term download
Audacity download Audacity
Download PDF
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