Discover the most reliable and secure ways to connect remote devices via the 4G networks (updated in 2024). At Esis we’re often asked to supply a transparent serial link from a remote location back to an office, via the 4G network. This kind of requirement usually falls into the category of “M2M” (machine to machine) communications, and we would typically do it using a pair of industrial 4G modems, with M2M SIM cards, all pre-configured by our engineers to provide a reliable point to point connection. Read on to find out the pitfalls and how best to do this.
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 have stopped supporting dial-up data connections over the cellular networks. And the old GSM 2G networks in Australia have already been shut down in favour of 3G (soon to be shut down) 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 4G networks for remote connectivity
There are a few challenges with using IP over 4G networks for this kind of application. If you just buy a simple 4G serial modem, it may have the smarts to create a connection over IP to another modem, but this needs to be set up carefully by someone who knows what they’re doing. A major sticking point that almost everyone will find, is how will it find the other modem, in the vast reaches of the public internet? Also how will it be protected from malicious hackers if it’s sitting on the public internet?
If you buy a standard data SIM card from a telco or 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 the cellular device from anywhere else on the public internet, as the device is hidden behind a firewall and doesn’t have a public IP address. The only thing the SIM will allow is for the cellular device to send connection requests to the public internet. It can’t receive connection requests from the internet like a server.
What we want to do, to establish a point to point link, is have one modem initiate connection to the other modem, and both are on the 4G network. So we can’t do this just using regular SIM cards, as one of the modems needs to act like a server and receive connections from the other modem.
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 4G 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. This usually means that the carrier will dynamically allocate a different IP address to the device each time it connects or re-connects. To work around the issue of the dynamically changing IP address, 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 4G modems include this feature. Another 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 (including a good firewall, strong password protection, and other protections) then your critical link could be taken down by hackers, or your cellular data charges could go through the roof.
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 4G modem to use?
Some modems that we commonly use for serial links like this are Lantronix M113, M114 and Netcomm NTC-100. The M113 and NTC-100 are very low cost by comparison with most industrial 4G modems and routers, and they are designed for this kind of scenario. These devices only have a serial port and a USB host connection, but no network port. All these devices have built-in firmware that supports sending and receiving serial data over the network using a TCP socket, which is designed for this kind of application.
This mode of operation 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 4G modem?
Other features to look out for in an industrial 4G 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 4G 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 or if the network connection is lost. 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 rating. 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-100 and dedicated M2M cellular services, it is now quite straightforward and cost effective to set up a point to point serial link over a 4G 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 4G modems, M2M SIM cards, and engineering services to set them up to meet individual customer needs. You’ll receive a complete set of pre-configured devices with SIM card installed and accessories, so that you can just plug them in and start using them straight away. Contact us now to discuss your needs.
