Polling sites over a radio link: how many points can one channel serve?

What makes up one polling

The cycle is simple: the center sends a request, the object responds, the center moves to the next.

But to useful data is added the service part: preamble, address, checksum.

There's a pause between the request and the response to switch the receive and transmit, and it's not zero.

At low speeds, the service part eats up more time than the testimony itself.

Therefore, a short package on a slow channel consists almost entirely of overhead.

In the 12.5 kilohertz band, the transmission rate is limited, and this is a physical limitation, not a tuning one.

Doubling the speed requires either a wider bandwidth or worse interference resistance.

Sustainability is more important than speed: Request due to error is more expensive than slow but correct transmission.

The practical approach is to choose the lowest speed at which the cycle still fits into the norm.

The speed reserve is useless if the signal level is at the far point.

Take the time of one complete exchange with one point and multiply by the number of points.

To the result add a margin for requests: in the real network, they are usually from five to fifteen percent.

The time obtained is compared with the required polling period set by technologists.

If it does not fit, it is not the number of points that are reduced, but the amount of data in one answer.

It often turns out that half of the transferred values are not needed by anyone and are surveyed out of habit.

The first solution is to divide the objects into groups with different polling frequency.

Critical parameters are often polled, slowly changing - once in several cycles.

The second is to move from polling to event transfer: the object itself is connected when the change occurs.

The third is the second frequency and the second channel, but this is a new application for frequency assignment.

The fourth, the cheapest, is to cut down the package by removing everything that is not read.

Too long a preamble left from default settings.

Long time-out waiting for an answer: at each silent point, the system waits for nothing.

Repeats tuned to three tries where one is enough.

The slow port between the modem and the controller, which negates all winnings on the air.

And the most insulting is the polling of points that have long been decommissioned, but remained on the list.

List of points with addresses and composition of transferred values for each.

The required polling period agreed with the technologists, not chosen at random.

Calculation of the cycle time with a margin for requests and indicating the accepted speed.

The order of action when adding a new point so that it does not fit into the network in a manifest order.

And a magazine that shows how many requests there were per day: this is the first sign of the degradation of the channel.