Cellular is increasingly being adopted as a reliable failover strategy of last resort for network infrastructure. But what if your data center is underground and/or loaded with high-density devices that emit interference that results in poor signal strength or signal interference? It’s vital that you have a way to check your signal strength using a Web or command line interface (CLI) to make sure that in the event of a cellular failover your devices remain connected to a network. Here are some strategies for doing that.
Why Cellular Rocks as a Failover Option
First, a little shout out for cellular failover. A cellular connection is a fraction of the cost of a WAN interface or PSTN connection. (Check out this calculator to see the ROI of using cellular over POTS.) 4G LTE provides speeds up to 1000 times faster than a 56K modem. 5G will be even faster. So it’s no wonder that data center and Internet of Things (IoT) device developers are gravitating to the use of cellular as a second or third level of failover.
But what do you do if your deployment is in a basement, underground or in some other location or region with spotty coverage? How do you ensure a strong wireless signal?
Measure Signal Strength
First, measure the signal strength of your wireless connection and device. Received Signal Strength Indicator (RSSI) is a measurement of the radio frequency (RF) power present in a radio signal that is received by a mobile device. The best cellular throughput is achieved when a device and/or its antennas are positioned to maximize RSSI. The signal your phone receives from a cell tower is measured in decibel-milliwatts (dBm).
| RSSI Readings | |
|---|---|
| 69 dBm or less | Strong signal |
| 89 to 70 dBm | Medium to high signal |
| 99 to 90 dBm | Weak signal |
| 100 dBm or more | Unacceptable signal (check antenna connection) |
Before measuring signal strength, configure your cellular connection to ensure that the readings you get are measuring the appropriate carrier and band. You do this through the Access Point Name (APN) gateway from your carrier’s cellular network to another network (usually the Internet or, by special arrangement with your carrier, to your private corporate network). The configured APN determines the IP address of your device.
Usually, configuring your cellular model only requires configuring the APN and adding a user name and password. (Verizon customers can leave the APN field blank as it’s automatically programmed.)
There are three ways to check signal strength. You can use a tool with a Web interface, a command line interface (CLI) or check it using the chassis LEDs.
Physical LEDs
If your device has signal strength LEDs, you can determine signal strength based on how many LEDs are lit. The LED status is updated every 5 seconds.
| LED Reeadings | |
|---|---|
| 4 LEDs lit | Strong signal |
| 2 LEDs lit | Medium to high signal |
| 1 LED lit | Weak signal |
| No LEDs lit | Unacceptable signal (check antenna connection) |
Using the CLI
Login to your CLI and type the following below to obtain a display cellular information
cellctl -l -s
Using a Web Interface
Using a management product with a Web-based user interface, you can look at the current state of the cellular modem, including RSSI.
Conducting a signal strength site survey before an installation is a very good idea. But you can do one at any time. Specialized equipment is available but a standard cellular phone with a SIM card from your carrier will typically allow you to check signal strength throughout your site.
The Right Antenna and Placement Are Key
If you are consistently receiving a poor or marginal signal using a standard antenna, consider a specialized high gain or directional antenna, an extender or a booster. Extenders or boosters enable you to capture signal away from the device, where poor or marginal signal strength can’t be remedied.
When placing antennas, first ensure that both Main and Aux antennas are properly connected. Make sure to place the antenna outside any metal racks and cabinets to avoid the Faraday cage effect, which blocks electromagnetic fields. Keep the antenna and cable away from AC wiring and other interference from other electrical or radio devices.
The optimum placement for an antenna is close to an exterior window or in a higher point in the building, such as on top of tall cabinets. Another factor to consider in antenna placement is the location of the carrier’s closest cell tower. Position it facing the tower if possible.
If All Else Fails…
If your signal strength still isn’t satisfactory, it might be time to look for another wireless carrier.
At Opengear we believe in giving our customers freedom of choice when it comes to wireless failover. That’s why our out-of-band (OOB) management consoles support pre-provisioning your wireless connection with dual SIMs, allowing you to test each carrier quickly and easily so you can choose the optimal carrier―the carrier that could be the difference between failover and downtime when all other network connections go down.
