Using Cellular Failover For Network Resilience?

Here’s how to measure signal strength.

Cellular failover has become an increasingly popular method for out-of-band access or WAN failover to remote network infrastructure whenever primary connections experience outages — and for good reason. Devices being used for out-of-band access connecting through mobile networks comes at a fraction of the cost of a secondary WAN interface (cable, T1, etc) or PSTN connection and provides 4G LTE speeds that are 1000 times that of a 56k modem (soon ramping up to 5G).

However, not every data center or remote site location offers an environment that naturally supports cellular failover. Network sites are often in basements, underground, or simply in regions with patchy cellular coverage where signal strength is a persistent issue. The infrastructure itself may also prove problematic by producing interference and limiting (or downright preventing) cellular signals from getting through.

Just as the “no service” signal reading on your cell phone indicates when you’re unreachable, the lack of a strong signal at a network site means you cannot rely on cellular failover to maintain uptime when you need it the most. For this reason, it’s absolutely critical to continually test the cellular signal at any site that will utilize cellular failover (and ideally before installation), and to bolster that signal to the greatest degree possible. The stakes of maintaining uptime are inherently high — don’t let largely avoidable cellular connection problems be a reason for slow resilience and remediation.

Some technical background: Cellular signal strength is quantified as RSSI (received signal strength indicator), with the power of these signals measured in decibel-milliwatts (dBm). An RSSI reading above -69 dBm is considered a strong signal, while readings under -90 dBm convey a weak signal. Anything below -100 is too low to function. When preparing to measure signal strength, it’s necessary to first configure your cellular connection to ensure that only signals from the correct carrier and band are included in the measurement — an all-too-common error during the testing phase. This configuration can be done using the Access Point Name (APN) gateway from your carrier’s cellular network.

Once you’re prepared to make accurate measurements, there are three methods for checking your device’s signal strength:

  • Check the physical LEDs on the device. If you are on-site and these indicators are available on your device, this is a straightforward way for determining your signal strength — usually displayed just as how you’d view the signal indicator bars on a cell phone. But since this isn’t always feasible or available.
  • Use your command line interface. Many devices include commands that can be issued via CLI to the device to display information on the cellular signal.
  • Use a web interface. Network device managers with web-based user interfaces can be used to access RSSI and other cellular modem status information.

Also, a simple but usually effective strategy of checking signal strength within a network infrastructure location is to bring a cell phone with a SIM card from the same carrier to the location, and to check its signal throughout the premises.

So, let’s say you’ve now gone through the tests but discover that the cellular signal at a network location isn’t satisfactory? There are a few methods that might work for improving it, starting with ensuring you’re using the right antenna. If a standard antenna isn’t delivering enough signal, try swapping in a specialized high-gain or directional antenna. Similarly, if the signal strength where the device resides can’t be improved enough, extenders or boosters can also be used, in many cases, to capture a signal at a more optimal location.

You also might find that sometimes it’s not the antenna itself needing to change, but how and where you place it. It’s vital to make sure that both main and aux antennas are connected correctly, and to position them outside of metal racks and cabinets that may block the radio waves of the signal. Likewise, find a place for your antenna that avoids A/C wiring and other electrical or radio devices. Ideal placements include an outside window or, if available, a location high up within your facility. Also advisable is learning where your carrier’s nearest cell tower is, and then aiming the antenna towards it.

If these strategies aren’t enough, it may be time to check the signal strengths available from other carriers. By knowing how to measure your signal — and following these tips for getting the best signal possible for your location — you can give your data centers and remote network sites their best chance of being compatible with cellular failover as a key part of your IT infrastructure management planning.

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