I’m currently on a high speed train between France and Switzerland and wondering how my cell phone is able to keep up with my very rapidly changing position and generally keep data connectivity?
In: Technology
The cell radio on your phone will automatically attach to the tower radio with the strongest signal. In some cases, that means gathering signal from both until one is clearly stronger than another. The phone doesn’t know, or care, what or where the radio is so we use SIM cards to identify the traffic for when there are multiple providers (that’s why 911/119/emergency) works on all providers in most countries). On top of it all, most countries have spectrum regulators to prevent too many radios in any given site in any given radio frequency. These are known as Spectrum Licences.
Among towers of the same provider, we call this process “handoff”. Among towers of a different provider, we call this “roaming”. The system works well because all radios (both on the user handset and the tower) follow the same global standard. Each telecommunications service provider must support the global handoff protocol as if it were a law (i.e as a condition of licence). These are technical updates, like any other software updates, that must be applied on a regular basis. It also requires TSPs to continually upgrade their equipment to remain competitive. I could get into the historical reasons why telecoms (and other common carriage infrastructure like trains &ships ) usually work this way, but it’s ELI5 so I’ll say “not playing nice in the playground makes all the expensive toys your mom bought you useless”.
It seems easy to do, but it gets really challenging to do it with a 99% uptime at a national scale. Like driving a car, it’s dangerous if used improperly because it could cause harmful radiation, drop critical communications, or interfere with other devices like radio altimeters. It requires a great deal of trust among everyone to make sure they are operating safely. It also relies on the global telecoms vendors (of which there’s 3: Huawei, Nokia and Ericsson) to engineer, licence, and build in a spirit of interconnection through global standards making bodies such as 3gpp and the ITU.
Right now, the way that we handle this is fairly primitive because it’s based on the power level of the phone; which is usually taken to mean how close it is to the tower. But next generation technologies could be able to handle this function in a more directed manner depending on which device it is serving. We call the basic form of this idea “Network Shaping” within 5G, but in the future it could use predictive AI to sense people, objects, and vehicles in real time. This could enable you to touch (or stand near, or look at, or shout at) objects and “log in” seamlessly just the same as when a cell phone switches towers.
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