Select Page

A common request we receive is to supply a pair of industrial 3G modems, set up to provide a transparent serial link from a remote location back to an office. This kind of requirement usually falls into the category of “M2M” (machine to machine) communications. 

Industrial 3G Modem Link-Up

Dial-up vs. IP networking

A few years ago this kind of remote serial link-up would have been done using dial-up data connections (CSD – Circuit Switched Data) on the GSM 2G network, or even using a land line phone connection. However these days most telcos are discouraging use of dial-up data connections over the cellular networks, and it is often time consuming and expensive to install a land line connection. And the old GSM 2G networks in Australia have already been shut down in favour of 3G and 4G networks. 

The modern way to achieve a point to point serial link is to use IP (packet switched internet) connections over 3G or 4G networks.

Challenges of using IP over 3G networks

There are a few challenges with using IP over 3G networks for this kind of application. If you just buy a simple 3G serial modem, it may not have the smarts to automatically set up a connection over IP to another modem. And if it does have this functionality, how will it find the other modem, in the vast reaches of the public internet? Also how will it be protected from curious hackers if it’s sitting on the public internet?

If you buy a standard data SIM card from a phone store, you’ll likely find that it does not even have an IP address on the public internet. This prevents hackers from getting access to mobile broadband devices such as smartphones. So it is impossible to request a connection to that device from anywhere on the public internet. The only thing the SIM will allow is for your device to send connection requests to the public internet, but not to receive connection requests from the internet. But what we need to do is have one modem initiate connection to the other modem, and both are on the 3G network.

To get around this problem there are 2 common methods:

1. Use special SIM cards designed for M2M communications, which have been set up with a “virtual private network”. Each SIM can be provided with a local IP address that forms part of a virtual “local network”. The data runs through the public 3G network but as far as the modems are concerned, they only see the few devices on their “local network”, which would typically be other modems that they are supposed to communicate with. The local network would also be “firewalled” from the public internet to prevent hacking.

2. Use a standard telco data SIM and request the telco to provide a public IP address. On Telstra this is done by requesting access to the “telstra.extranet” APN. However this does not guarantee that each device will have a fixed IP address, only that the IP address is visible on the public internet. So to work around the dynamically changing IP address issue you would also need to use a service called Dynamic DNS. This allows the modem to have a permanent URL set up and a cloud service will make sure that this URL always points to the latest IP address of the modem. For this to work, the modem must have built-in support for Dnyamic DNS with the particular provider you want to use. Many industrial 3G modems include this feature. Another potential issue with this method is that the modems will now be on the public internet, so they are exposed to potential hacking attempts. If they do not have strong enough security (such as a good firewall) then your critical serial link could be taken down by hackers.

For point to point applications in Australia with relatively low data requirements, option 1 is usually the best. This simplifies the modem setup, and provides protection against hacking. It can also be more reliable than using Dynamic DNS. 

Which industrial 3G modem to use?

A modem that we commonly use for serial links like this is the Netcomm NTC-3000. This device is very low cost by comparison with most industrial 3G modems and routers, and it is designed for this kind of scenario. It only has a serial port and a USB host connection, but no network port. The NTC-3000 has built-in firmware that supports “PAD Mode” which is designed for this kind of serial link. 

PAD Mode allows you to set up one modem as a TCP Server and the other modem as a TCP Client. The Server modem will wait for a request over the cellular network, and when the request is received, it will set up a direct link from the requested network TCP socket to its serial port. At the other end of the link, the Client modem will automatically try to request this TCP socket from the server, and when this connects through, it will also connect its serial port to the same TCP socket. This produces a direct point to point serial link over TCP/IP via the cellular network.

What else is important in an industrial 3G modem?

Other features to look out for in an industrial 3G modem include reliability features for remote unattended use. Most consumer grade modems are not designed for unattended use. If something goes wrong with the modem or the 3G connection, consumer grade modems will likely hang, requiring the user to physically unplug or power cycle the modem.

 This is not good enough for a remote unattended modem. It needs to have built-in mechanisms for recovering from that kind of situation. Automatic recovery is commonly done using either a watchdog timer, or a fixed reset interval. A watchdog timer will automatically reset the unit if the internal software stops responding. Fixed reset intervals force the unit to reset itself regularly, for example once every 24 hours. Both of these options allow the modem to recover without further human intervention if it ever gets “hung”.

Another feature that can be important is the modem’s environmental ratings. Often modems are installed in outdoor enclosures that can heat up to 70 degrees C in direct sunlight, or cool down below freezing at night. So the modem will need to cope with temperature extremes. The power supply voltage is another consideration – often 12 or 24V DC power is available on industrial sites, but if the modem is used in a vehicle, the power can sometimes spike up above 36V DC. And if it’s installed as part of a solar powered system, low power consumption will also be critically important.

Conclusion

With the advent of industrial modems such as the Netcomm NTC-3000 and dedicated M2M cellular services, it is now quite straightforward and cost effective to set up a point to point serial link over a 3G network. The modems themselves are low cost and fully industrial grade. The setup is just a matter of sending a few commands to each modem over the serial port. And the SIM cards can be set up by an M2M service provider to make it all easy and reliable.

Esis routinely supplies complete systems for this kind of application, including industrial 3G modems, M2M SIM cards, and expert services to set them up to meet individual customer needs. Contact us now to discuss your needs.

Call Now Button