That means that the testing lab that certifies the product will measure the field strength of the transmission (including the transmitter’s antenna) at 3 meters using a calibrated measurement antenna. In the event mode application, 15.231.b applies and the transmit power is limited to about 10,000uV/meter at 3 meters. In the case of 433 MHz, there are two separate limitations. The FCC regulations also limit transmitter power output. You’re not allowed to transmit that often. For example, you might want to log temperature every second to monitor the operation of your air conditioner. However, for our trend logging IoT application, this 10-second limitation can be a big problem. Their display will update once every 10 seconds, but the user will never notice. There are, in fact, inexpensive indoor/outdoor wireless temperature sensors with displays that currently operate under this rule, but they only tell us the current temperature. Under this rule, we can only transmit one time in 10 seconds. In this case, we will be regularly sending data to the gateway from the temperature sensor so the 15.231.e rule will apply. Say we want to collect temperature readings to monitor trends. If activities like turning wall sockets on and off are not the only thing we want to do in our IoT application, this is where the second mode comes in. While that might be fine if you simply want to turn a couple of lights on and off, it is wholly inadequate if you want to do more. Consider a transmission period of only 2mSec you could transmit 1,000 of these messages in an hour. Because you can only transmit for a maximum of two seconds an hour you will be limited in how many events you can handle even if you have very short transmission periods. This would be used for things like activating a wall socket when some event, such as throwing a switch, happens. The first is what I will call an event mode. Therefore, there are two modes, if you will, that you can use in this band. This restriction to one transmission in 10 seconds is the important thing to glean here. Second, 15.231.e makes a provision for more frequent periodic polling transmissions as long as “the duration of each transmission shall not be greater than one second and the silent period between transmissions shall be at least 30 times the duration of the transmission but in no case less than 10 seconds.”Īlthough this ruling contemplates a maximum of one second of transmission time, it really says that if you want to transmit at the maximum repetition rate, you can only transmit for 300mSec every 10 seconds. First, 15.231.a.3 prohibits scheduled periodic data transmissions, but it does allow polling transmissions (with data) that cannot exceed two seconds of transmission time per hour. There are two main restrictions for data applications to consider.
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