Radio modem or cellular service for telemetry: comparing cost and reliability

The task is one, the solutions are fundamentally different

Telemetry is the regular transmission of small portions of data from objects to one point: readings of meters, pump status, levels, alarms. There are few data, there are almost no speed requirements, there are many reliability requirements.

Cellular channel solves this problem with foreign infrastructure. A modem with a SIM card is put, the operator works further. Quick, familiar, without permission.

The radio channel decides its own infrastructure. A couple of modems and antennas are installed, the channel is your entire, there are no monthly payments, but you need frequency, permission and installation work.

The choice between them is not a debate about technology. It is a choice between rent and property, with all the usual consequences: renting is cheaper at the start and more expensive at the distance.

Where Cellular Communication Loses Immediately

There is no sustainable coverage. Object in the field, in the quarry, on the river, over the hill. The modem shows one stick, the data goes away after a time, and there is no one to deal with it.

Coverage is there, but unstable. This is worse than total absence: the system is considered operational, and gaps in the data are detected when the technical guide of the accident.

Many objects with frequent surveys. The subscription fee is multiplied by the number of points, and in a few years the cost of its own channel runs over.

The demand for independence. In a mass event, the cellular network is overloaded first, and that is when telemetry is most needed.

Long delays in establishing the connection. For a polling once a minute it is critical, for a polling once an hour - no.

Where the radio channel doesn't fit

Objects are far apart and without a line of sight to a common point. A radio channel requires geometry, and it is not always possible to build one.

One or two objects. Building your own infrastructure for the sake of two points is unprofitable, it is easier to put a cellular modem.

We need large amounts of data: videos, photos, program updates over the channel. A narrowband radio channel is not designed for this.

A tight deadline. Registration of frequencies takes months, cellular card works on the same day.

What to count in the estimate of the radio channel

A pair of modems for each direction and one on the central point. Sometimes the central point serves many objects, and then the count is different.

Antennas and masts. On objects, this is usually a pin antenna on the bracket, at the central point - directed or circular on the mast. The mast is often the most expensive line.

Feeder, lightning, ground. Not an option: a modem on a mast without protection lives until the first thunderstorm.

Power supply with reserve. A telemetry object usually has no on-duty personnel, and any interruption turns into a data pass.

Registration of frequency and registration of means of communication. It's money and time, but once.

Commissioning: power selection, level check at each facility, setting up the polling period and repeats.

The settings on which everything depends

Exchange rate. The lower the speed, the further and more stable the connection. For telemetry, a slow and reliable channel is almost always better than a fast and breaking channel.

Interview period. Every extra poll is a busy airwave. If there are many objects, their schedules are blown away, otherwise they clog the channel to each other.

Repeats. The modem should try again if it fails, but not indefinitely: the object stuck on the replays blocks the others.

Address. Each object has its own address, and it must be recorded in the passport of the object, not only in the memory of the modem.

Communications control. The system must distinguish between “the object is silent because everything is normal” and “the object is silent because it has refused.” Without a periodic signal of presence, this is indistinguishable.

How to decide in practice

Mixed circuit works best: objects with reliable coverage - on the cellular channel, objects without coverage and critical - on the radio channel.

The second working option is the radio channel as the main and cellular as a backup at the central point. Then independence is maintained, and when its infrastructure fails, the data keeps going.

The benefits should be calculated for the life of the system, not for a year. The radio channel is more expensive at the start and cheaper further, the cellular – on the contrary.

And the last thing you forget is that you have to maintain your own channel. These are antenna inspections, level checks, battery replacements. If there's no one to do it, renting someone else's network is fairer.