3G Failover using the Huawei B310

Huawei B310 3G router

I first implemented a 3G failover using the TP-Link MR3420, but that had a few problems. It got a little erratic with less than 20 IoT devices connected. When in backup 3G mode the uplink often failed when run for too long and the 3G USB modem then needed to be power cycled. In a thunderstorm 3G reception often got a lot worse. But worst of all I could not alter the failover criteria.

This is because my benighted ADSL service provider Talikom Malaysia plumbed news depths in quality of service. The ADSL connection would get erratic, going on and off within a few minutes. Sometimes there would be an actual ADSL connection but it would not be able to reach the TM gateway and from there the Internet. Sometimes the link simply got slow.

This would sometimes cause the MR3420 to remain in ADSL mode and not fail over to 3G. Worse this sometimes caused it to randomly drop connections to some client WiFi devices. Power-cycling it usually helps, but the random dropouts are nevertheless annoying.

Huawei B310 with external antennae

The Huawei B310 has a simple failover criteria: it needed its WAN input cable disconnected before it goes into 3G mode. I could do that with an electromechanical relay; this allowed me to make a custom failover program to handle TM's myriad failure modes. Also the B310 has two connectors for 3G external antennae which are separately bought. And it does not disconnect clients in either 3G or ADSL mode.

The setup is much the same as before:

The Huawei B310's WAN uplink is taken from the Archer D50 via the DES-100S LAN switch. This allows me to disconnect the WAN link by turning off the DC power to the LAN switch. I do this using a Raspberry Pi Model B and a Piface hat.

Because of frequent electrical storms, the ADSL line can also be disconnected by an AS3935 lightning detector relay module.

The Raspberry Pi runs Raspbian. The Piface is controlled by the pifacedigitalio library and the failover program for now is a simple bash script. Here is a first-pass version of the failover program, it did not take more than an afternoon to code and test.

Because of the AS3935 lightning detector, the failover program subscribes to and publishes to a local MQTT server. The piface digitalio library is not one of you best supported ones, but there are a few hints if you run into trouble.

As always the choice of components is not necessarily optimal, if not downright obsolescent. Most of the time they are used simply because they are available on hand. Indeed you can probably get an all-singing all-dancing 3G failover modem router for a very reasonable price somewhere. But it goes against grain not to use working parts simply because they are a little too old.

Much better to go out with your boots on. Happy Trails.