Circuit switched networks physically connect two endpoints using a communication channel through the network. This utilises a fixed bandwidth for the session whilst the two endpoints remain connected. Packet switched networks offer the ability to split the communication channel into small packets to be sent across the network using multiple routes offering redundancy and increased performance. This utilises dynamic bandwidth management which can be optimised for each connected party and allows for many more services and connections to be active at the same time using the same bandwidth.
In the cellular network GSM (2G) and UMTS (3G) use circuit switching for voice, circuit switched dialup (CSD) and short messaging service (SMS) traffic and use packet switching for GPRS data traffic. In LTE (4G) the entire network uses packet switching and has no capability for circuit switched network support. As a result, the voice and SMS services must be moved over to a packet switched network when using the LTE (4G) network. In addition to this, CSD is no longer supported and GPRS data must be used instead.
Please see ‘Circuit Switched Dialup (CSD) Migration to LTE Application Note’ here.
Voice Overview
Many applications in use today utilise voice as part of the service offering. This may be to connect two parties together to communicate such as in an elevator emergency failure system, to implement an automated payment service using a dial keypad or report a fault with a system using normal spoken language. Whatever the reason for using the voice service in a cellular system, there are some differences with the LTE network which are described in more detail below.
On legacy devices supporting GSM (2G) and UMTS (3G) services, the voice and SMS components of the system use circuit switching as the transmission channel. For GSM the whole system was built on an analogue infrastructure and digital data was added to the system later with the advance of GPRS packet data. The UMTS system is based on the GSM standard and therefore also supports both digital and analogue components. The UMTS framework allows much higher data rates over GSM but is still based on a similar infrastructure.
LTE on the other hand is entirely digital and has no analogue components at all. Therefore, new methods are used to take advantage of the new features offered by the LTE network as analogue voice calling is no longer possible and the packet switched network must be adopted.
VoLTE (Voice over LTE) Overview
The LTE network supports voice and many other additional services in the same way as GSM and UMTS, however, as there is no analogue component the voice must be encoded and sent digitally across the network using packets. The 3GPP body have specified the voice service on LTE which is highly optimised and takes advantage of the additional bandwidth and low latency of the LTE network. This service is called Voice over LTE (VoLTE) and uses the digital packet switched network in a much more efficient way than traditional services. This means that VoLTE can support up to 3 times more voice capacity than previous UMTS networks and up to 6 times more capacity than older GSM networks.
The VoLTE service is provided on LTE over the IP Multimedia System (IMS) network service which is a standardized framework for delivering multimedia services over TCP/IP. It has been developed to allow all content rich media from wireless and wireline applications and allows for advanced services to be provided over the existing infrastructure without the need for individual control functions for each service.
The network provider must support IMS to be able to offer VoLTE as a service to its customers. The IMS framework must be integrated within the network’s core architecture and is fundamental to the operation of the network.
Network Usage with Circuit Switch Fall Back (CSFB)
With existing GSM (2G) and UMTS (3G) networks, the core functions are built into the application and can be supported with any hardware platform that supports voice and any SIM card that supports voice. This allows for a very simple implementation and calls can be established without any special configuration to the end equipment.
With newer modems that support LTE (4G) in addition to GSM (2G) and UMTS (3G) the firmware in the modem performs a subtle trick to continue having seamless voice support. When connected to the network for transmitting digital packet data, the unit connects to and uses the LTE network. When attempting to dial a voice number the unit automatically switches to UMTS (3G) or GSM (2G) to establish the call. Once the call is finished it reverts to the LTE network. This technology is called circuit switch fall back and is a short-term solution to allow all network providers to offer voice services on the LTE network whilst they implement their own full IMS solution.
This process is fine whilst there are UMTS (3G) and GSM (2G) networks available to use but the analogue networks are being gradually sunset to be replaced entirely with the LTE network.
With the LTE (4G) network it is necessary to configure the device to allow calls to be made using the LTE only network over the IMS service.
SMS Overview
The short messaging service (SMS) can be used to send and receive small text messages which can be used in a variety of ways for applications that use the cellular network. The messaging service can be used to configure settings in remote equipment as well as reporting on the current configuration. In addition, the messaging service can inform users directly of a triggered alarm state or indicate when a certain criterion has been met. As with voice, the existing GSM (2G) and UMTS (3G) core functions are built into the system and can be supported with any hardware platform that supports the SMS messaging service and any SIM card that has SMS functionality enabled. This allows for a simple implementation and messages can be sent and received without any special configuration to the end equipment.
With newer modems that support LTE (4G) in addition to GSM (2G) and UMTS (3G) the firmware in the modem supports circuit switched SMS (using the analogue network) and packet switched SMS (using the digital network) offering seamless SMS support. The unit can be configured to send packet switched SMS when connected to the LTE network for transmitting digital packet data by selecting appropriate configuration option. Alternatively, the unit can be configured to send SMS via the circuit switched network. When attempting to send an SMS via the circuit switched network, the unit automatically switches to UMTS (3G) or GSM (2G) from LTE (4G) to send the SMS. These options are described in more detail later.
This process works perfectly whilst there are UMTS (3G) and GSM (2G) networks available to use but the analogue networks are being gradually sunset to be replaced entirely with the LTE (4G) network.
With an LTE (4G) only network it is necessary to configure the device to send SMS using the packet switched network as the circuit switched networks will no longer be available to use.
SIM Configuration
To support VoLTE and packet switched SMS it is necessary to use a SIM card on a network provider that supports the IMS service. Your SIM card provider offers IMS services in similar to the existing SMS, circuit switched voice, circuit switched SMS and packet switched GPRS data services. Most SIMs come pre-configured allowing analogue voice SMS and GPRS, but you can switch these services on and off as required. IMS is usually enabled as a service with large global suppliers and is switched on by default with providers who offer the service.
You will need to change to a SIM from a network provider that offers this service if you are currently unable to use the IMS service from your existing supplier.
The following table shows the typical services available for a SIM card from most large global suppliers and what services are generally turned on by default.
| Name | Description | Default | Notes |
| Analogue Voice | Make and receive calls | x | Available on 2G/3G only |
| SMS | Short messaging service | x | Available on 2G/3G only |
| MMS | Multimedia message service | Available on 2G/3G only | |
| CSD | Circuit switched dialling | Available on 2G/3G only | |
| GPRS | Send and receive digital data | x | Available on all technologies |
| Fixed Private IP | Allocated IP fixed on private network | Available on all technologies | |
| Fixed Public IP | Allocated IP fixed on public network | Available to all | |
| IMS VoIP | Voice over LTE (VoLTE) | x | Available on LTE only |
| IMS SMS | SMS over LTE | x | Available on LTE only |
Circuit Switched Voice and SMS Sunset
As GSM (2G) and UMTS (3G) become less used around the world they will become increasingly obsolete and as such there will be more systems which will not be able to rely entirely on the GSM (2G) and UMTS (3G) networks. There have already been several SIM providers. who have announced sunset on the UMTS (3G) networks around the world which only leaves GSM (2G) to support circuit switched voice and SMS services in the short term where 3G is no longer available. Over time GSM (2G) will also be turned off and there will be no legacy networks available to support circuit switched network operation.
A few of the larger global network providers have committed to supporting GSM (2G) until late 2025 but none have committed past this date at the currently. GSM (2G) and UMTS (3G) have been superseded by LTE (4G) and most internet capable devices have already been moved over to the LTE (4G) network. However, in many existing installations there is a pressing requirement for LTE (4G) due to the increased voice and SMS capacity offered by the LTE (4G) network. As a result of this developers and customers will be required to make changes to their applications to facilitate the move over to an entirely LTE (4G) only network infrastructure.
Industrial ZETA Modem IMS Support
The Siretta ZETA family of LTE enabled industrial modems is shown below:
| ZETA Model | LTE Category | VoLTE Support | IMS SMS Support |
| ZETA-N-LTE (EU) | 4 | No
|
No
|
| ZETA-NLP-LTEM (GL) | M / NBIoT | No
|
No |
| ZETA-NLP-LTE1 (EU) | 1 | Yes
|
Yes
|
| ZETA-NSP-LTE1 (EU) | 1 | Yes
|
Yes
|
| ZETA-NEP-LTEM (GL) | M/NB
|
No
|
No
|
| ZETA-NEP-LTE4 (EU) | 4
|
Yes
|
Yes
|
| ZETA-GEP-LTE4 (EU) | 4
|
Yes
|
Yes
|
Circuit Switched Voice Migration to VoLTE
When using analogue voice and SMS services to communicate with remote parties, equipment and devices, the process to connect local devices to remote systems is shown below.
Voice Procedure:
- Local unit dials remote party
- Remote unit answers incoming call
- Conversation takes place
- Either local or remote unit can terminate the call
SMS Procedure:
- Local unit sends message to remote party
- Remote unit receives incoming message
- Either party can initiate sending a message to the other
When using VoLTE or packet switched SMS the initiation procedure is identical but there are a few configuration requirements for either process which must be met first.
- The SIM and network provider must support IMS
- The device hardware must support IMS
- The device must be configured to allow IMS services
Once these parameters have been met, the standard voice dial command or SMS send commands can be used to establish a voice call or send an SMS message over the IMS network using the VoLTE and packet-based SMS services.
Command Syntax
IMS should be set correctly in the APN which can be checked with the following:
| COM3 – Tera Term VT |
| File Edit Setup Control Window Help |
|
AT+CGDCONT?
+CGDCONT: 1,”IPV4V6″,”internet”,””,0,0,0,0
+CGDCONT: 2,”IPV4V6″,”ims”,””,0,0,0,0
+CGDCONT: 3,”IPV4V6″,”sos”,””,0,0,0,1 |
IMS PDP profile should be set correctly and can be checked with the following:
| COM3 – Tera Term VT |
| File Edit Setup Control Window Help |
|
AT#IMSPDPSET?
#IMSPDPSET: ims |
IMS profile should be enabled which can be checked with the following:
| COM3 – 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 |
For the ZETA-xxP family of modems IMS is shown as active in profile 2 (which is shown in row 2).
IMS registration state should be enabled which can be checked with the following:
| COM3 – Tera Term VT |
| File Edit Setup Control Window Help |
|
AT+CIREG?
+CIREG: 0,1 |
Voice Procedure:
For voice calling, similar to a non-VoLTE call, the standard dial command can be used to establish a VoLTE call with the receiving party once the IMS settings have been established.
ATD
SMS Procedure:
To configure the device to send circuit or packet switched SMS the following settings can be used:
To send circuit switched SMS.
| COM3 – Tera Term VT |
| File Edit Setup Control Window Help |
|
AT+CGSMS=1 |
To send packet switched SMS.
| COM3 – Tera Term VT |
| File Edit Setup Control Window Help |
|
AT+CGSMS=0 |
For packet switched SMS sending, similar to circuit switched SMS, the standard command can be used to send an SMS to the receiving party once the IMS settings have been established.
| COM3 – Tera Term VT |
| File Edit Setup Control Window Help |
|
AT+CMGS= |
Overall System Advantages
Once your system is setup to use IP traffic to initiate VoLTE calls and packet switched SMS messages instead of circuit switched voice and SMS there will be an immediate benefit in the overall operation of the system. This is because using IMS as the method of communication offers several advantages over circuit switched networks. Some of these key points are described below:
| Subject | Description |
| Cost | The cost for voice calls is lower for IP data than for traditional analogue
calls and it is charged for by the amount of data rather than the time. Analogue calls are charged per minute and IP traffic is charged per Mb. In addition, the cost of data has reduced massively over the last decade which has been driven by consumer data usage increasing with mobile devices. SMS messages are charged for per message rather than data used. |
| Capacity | VoLTE traffic uses much less network bandwidth than traditional
analogue voice and as such is much more efficient. This means that less traditional analogue voice calls can take place at the same time as VoLTE calls using the same bandwidth. Therefore, using VoLTE can improve the performance of the network by up to 6 times over traditional GSM voice calls. SMS messages can be transmitted as data rather than dedicated message payload. |
| Flexibility | IP based traffic is supported by all systems connected to the internet
and is native for PC’s and embedded devices. IP systems can connect to multiple locations at the same time and data can be re-routed automatically for redundancy. |
| Scalability | IP based systems are more efficient and can accommodate more
connected devices over a larger service area. This allows for larger networks and improved network coverage using IP connectivity which offers more connections per square meter with a better link budget for all connected devices over circuit switched network connections. |
| Evolution | Circuit switched services are restricted for use only on the GSM (2G)
and UMTS (3G) networks which are now nearing EOL (End of Life). All future LTE networks are IP based which means that moving forwards it will only be possible to communicate using an IP based connection scheme for equipment and devices in the field. |
| Power Consumption | Circuit switched voice calls use a large amount of power and have no
capability to reduce or conserve power. Modern devices are becoming much more power efficient and new legislation is being brought in for devices to be energy efficient which is perfectly suited to IP based traffic. VoLTE improves power consumption as it uses IP data and can make use of advanced power saving schemes. |
Additional Reading
| Description | Author |
| ZETA Hardware user’s manual | Siretta |
| AT command manual | Telit |
| Circuit Switched Dialup (CSD) Migration to LTE Application Note | Siretta |
