One of your kids cleans her room and has good manners, but she’s always late. The other one doesn’t mow the lawn and his laundry is a heap on the floor, but he’s getting straight As. They both look a little like you and your spouse, so you can’t blame anyone else. It seems wrong; why do two kids who are so similar behave so differently? Your cell phone indicates four bars on the AT&T network, but your mobile router will not connect on the same network. It seems wrong; it’s the same network, why can’t it connect? While we will never understand our own children, there are reason why cell connections behave differently.
At the most basic level, you should not put much faith in the signal strength meter in the top right corner of your cell phone display. While those bars are not completely arbitrary, there is no industry standard for what those bars indicate. The Wilson Amplifiers blog from August of 2016 states it concisely, “In plain English, it’s up to each individual carrier to decide what’s 1, 2, 3, 4 or full bars on their own service. So what could be 1 bar on Verizon, might be 2 bars on AT&T, and 3 bars on Sprint.” In addition, two phones next to each other may show completely different bars, even when receiving the same signal from the same carrier. The bars on a phone are for information, but there is also an incentive for both phone makers and carriers to show a strong signal as often as possible. The “bars” are inaccurate, so for genuine diagnostic purposes every phone has an actual signal meter built-in, mostly hidden from the consumer. It’s pretty easy to find on Andriod, under ABOUT DEVICE: STATUS. You can find it on iPhones too, but it’s not so easy (search Field Test Mode). The meter will show the signal in –dBm, the correct unit of measure.
Cellular signals are radio transmissions, so they are measured in decibel-milliwatts, not bars. The usable range is -50 dBm to -120 dBm. Minus fifty is a strong signal (5 bars, if you must), and -120 dBm is a dead zone. Anything under -110 dBm will make a connection more or less impossible. In the middle of our steel reinforced building, my Samsung S5 shows 1 bar on AT&T, but the signal reads between -114 dBm and -109 dBm, which explains why calls don’t go through.
The arbitrary nature of “bars” is just the tip of the iceberg. The strength of a signal itself can fluctuate, even when the receiving device is stationary. Normal fluctuation can be around ±5 dBm, primarily caused by load on the cell tower and weather. The more people connected to one tower, the weaker the signal. Peak usage times (rush hour, lunch hour, etc.) will result in lower power for all users. Cell broadcasts are VHF, just like over the air TV. Dense clouds, thunder, lightning, temperature, and humidity can all degrade a signal. Five decibels does not seem like much, and at the good end, it isn’t. However if you have a marginal signal, like -95 dBm, your calls will probably hold, but if it falls to -100 dBm it’s likely connections will drop, or not be established at all.
To further cloud the issue, there are also fundamental differences in the way humans use cellular connections compared to how devices use them. Cellular networks have been built primarily for people. They are designed to handle relatively few connections to users who each transfer large amounts of data. As the Internet of Things grows, M2M (Machine-to-Machine) communications are based on a large number of devices connecting often, but moving little data. Human communications are challenged by data flow, but M2M connections challenge the system with signaling traffic.
Some M2M devices are delay tolerant; every smart meter in a town may try to connect at same time, but they will be content to wait in line for their turn. However, there are M2M devices with critical connection requirements, like health monitoring and warning sensors. Cellular carriers must balance between data intensive tasks and connection loads, and the critical requirements of the connected devices. As a result, carriers treat different types of devices differently. If you are interested in what the carriers are up against you can read about it here (Warning! Math).
Carriers know what they are connecting. Before any device can be connected to a network, its MAC address needs to be registered with the carrier. The MAC address has nothing to do with fruit-flavored computers. A MAC address, acronym for Media Access Control, is a unique identifier electronically imbedded in every single device by the manufacturer. Any phone, modem, router, or sensor on a network can be identified by its MAC address. Based on its MAC address, cellular carriers decide how to treat each device as it tries to connect. It is not difficult to guess the priorities that carriers apply.
By federal law, carriers must offer first connection to those enrolled in the WPS (Wireless Priority Service). The FCC lists who is eligible: police, fire departments, 9-1-1 call centers, EMS, essential healthcare providers, or any other “organization that uses telecommunication services necessary for the public health, safety, and maintenance of law and order.” Next are critical health related M2M devices. Both of these high priority users represent a small fraction of connections. No one is admitting where their priorities go from there, but it is a fair guess that individual cellular phone users are high on the list. As anyone can tell from the number of commercials touting connectability, consumer level cellular clients are a major priority for carriers. As relative newcomers to the party, non-critical devices with cellular modems, like most M2M devices, may not be at the top of the food chain.
Let’s go back to the original question. Why is my phone showing four bars when cellular modem in my mobile router can’t connect? First, it is because those four bars are not an accurate indication of current cellular signal. Environmental factors can degrade signal quality, and the variation caused by those conditions may affect one device differently than another. Lastly, the two devices are different, and your carrier knows it. The devices have different connection needs, and your carrier may be giving priority to one class of device over another. You’ll be happiest if you treat your cellular devices like your children. They share some of the same DNA, but they are each unique, with their own quirks and foibles.
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