The Data Rate Story
When it comes to determining the data rate at which to send packets, there are three variables you can adjust:
- Transmission Power
- Bandwidth
- Spreading Factor
When dealing with transmission (TX) power, you’re dealing with a trade-off: a lower TX power results in a lower battery drain, at the cost of a shorter signal range. Conversely, if you use more TX power, you can send a signal farther.
The data rate itself is a combination of both bandwidth (BW) and spreading factor (SF). It determines how quickly bytes are transmitted. If you increase the data rate (make the bandwidth wider or reduce the spreading factor) you can transmit the same number of bytes in a shorter time. Here’s a way to calculate a good approximation of your data rate:
- Doubling the bandwidth, for example, from BW125 to BW250, allows you to send twice as much data in the same amount of time.
- Reducing the spreading factor by one step, for instance, from SF10 to SF9, allows you to send twice as many bytes in the same amount of time.
A caveat about spreading factors: Lowering the spreading factor makes it more difficult for the gateway to receive a transmission because doing so makes the gateway more sensitive to noise. Consider this comparison: two people are talking in a noisy place, such as a restaurant or bar. If they are far away from each other, they need to speak slowly (SF10) to be heard. However, if they are near one another, they can speak more quickly (SF7) and still be heard.