How to Replace CSD using SirettaLINK

Applicable Products Part number Description

SL500-LTE1 (EU) SL500-LTE1 (EU) Low Power LTE Cat 1 Serial Gateway

SL500-LTEM (GL) SL500-LTEM (GL) Low Power LTE Cat M / NB IoT Serial Gateway

CSD Overview Many aging applications in use today utilise the circuit switched dialing (CSD) service available on older style modems and communication devices. The CSD service allows a device to connect to a remote location using a dial up modem and establish a direct low latency connection to the host.

The benefit of using CSD connections is the simplicity with which the connection can be established and many applications still use dial up services on their equipment to establish an internet connection to their devices. However, this is slowly changing, and much effort is being put in to modernizing the internet connectivity available in remote locations.

On a more modern device supporting 2.5G and EDGE IP data services, the CSD service is being replaced with a PPP dial up adapter on a host PC using a special call id *99***1#' which establishes an IP packet data connection over a dial up adapter.

The CSD service also allows for connections to be made to the device from multiple locations, where the dialling party simply needs to know the devices' data phone number to be able to dial the units directly and establish a connection to the connected equipment.

CSD Connection Speed The downside of CSD data connection services is that the maximum bandwidth available on a 2G connection is limited to 57.6 kbps and in most applications is limited to 9.6 kbps. In today's fast connected world these are very slow speeds and it would take a long time to download even basic information presented with modern systems. The other issue is the cost of downloading large amounts of data. CSD is charged for by the minute and the time taken to download a 1M file is shown in the following table.

Modem Speed Time Taken (min:sec)

9.6 kbps 15:16

14.4 kbps 10:11

28.8 kbps 5:05

33.6 kbps 4:21

56.7 kbps 2:37

So even at the fastest possible speed it would still take nearly 3 minutes to download the file and as this is charged for by the minute this would cost in the region of £0.15-£0.60 depending on the cost of your call charges.

CSD Supported Cellular Technology CSD data services are available on 2G/GSM and 3G/UMTS cellular data carriers. CSD is not supported at all on 4G/LTE due to the digital IP packet based technology used. As a result, any LTE modem which claims support for CSD will automatically fallback to 2G or 3G when establishing a data call. The same is true for analogue voice calls which are also not supported at all on 4G networks. To perform a voice call exclusively on the 4G network it is necessary to use the VoLTE service offered by the IP Multimedia Core Network Subsystem (IMS).

Not all modern modems support IMS and therefore they only support CSD and Voice calls over the 2G and 3G core networks. This means that when the 2G and 3G networks are finally sunset then these modems will stop working for data calls completely and voice calls unless VoLTE is used.

The latest range of Siretta LTE Category 1 and LTE Category 4 modems support IMS over LTE in addition to CSD and Voice over 2G/GSM and 3G/UMTS network technologies. As a result you can be sure when using a Siretta modem you will be able to support all technologies straight out of the box and the services available to you to use will be dictated entirely by your SIM card provider. The details of the SIM services available on 2G (GSM) / 3G (UMTS) / 4G (LTE) and LTE Cat M and LTE Cat NB IoT are described in the following section.

ZETA Model LTE Category

LTE Region UMTS Region GSM Region IMS Support

GPRS Support

CSD Support

SMS Support

ZETA-NLP-LTEM (GL) M/NB GL

GL

Yes

Yes

No

Yes

ZETA-NLP-LTE1 (EU) 1 EU

EU

EU

Yes

Yes

Yes

Yes

ZETA-NSP-LTE1 (EU) 1

EU

EU

EU

Yes

Yes

Yes

Yes

ZETA-NEP-LTEM (GL) M/NB

GL

GL

Yes

Yes

No

Yes

ZETA-NEP-LTE4 (EU)

4

EU

EU

EU

Yes

Yes

Yes

Yes

ZETA-GEP-LTE4 (EU)

4

EU

EU

EU

Yes

Yes

Yes

Yes

ZETA-N-LTE (EU)

4

EU

EU

EU

No

Yes

Yes

Yes

(GL) = Global Region | (EU) = European Region

SIM Configuration To support CSD it is necessary to enable the service with your SIM card provider in a similar way to how you enable SMS, Voice and GPRS data services. Most SIMs come pre-configured allowing Voice SMS and GPRS but you can switch these services on and off as required. CSD is more usually disabled as a service and is usually switched off by default. You will have to request that the CSD service is turned on before starting CSD on your device.

In addition, some SIM providers offer a dedicated data number for CSD and a dedicated voice number for voice but it is also quite normal to have a single number to provide both services. When using a SIM which supports both services over a single number then the type of call being established can be controlled directly by AT commands on the modem and you can specify a voice or data call for the modem.

The following table shows the typical services available for a modern SIM card and what services are generally turned on by default.

Name Description Default Notes

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

CSD Dial

LINK:	https://www.siretta.com/2025/03/how-to-replace-csd-using-sirettalink/...
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