# Understanding NRF24L01 specifications

I was reading this tutorial about the NRF24L01 and came across a few terms which I did not clearly understand.

It uses the 2.4 GHz band

1. Does this refer to the frequency of the EM waves that the device uses to communicate?

If used in open space and with lower baud rate its range can reach up to 100 meters.

1. I know baud rate refers to how many "groups of bits" are transmitted per second. I don't quite understand why the range increases for a lower baud rate.

The module can use 125 different channels which gives a possibility to have a network of 125 independently working modems in one place. Each channel can have up to 6 addresses, or each unit can communicate with up to 6 other units at the same time.

1. Here, does a channel mean a single frequency? How is the range of possible channels decided (why 125) and how is the smallest detectable resolution decided (smallest change in frequency which would cause a change in channel)? So does a single unit work on only one frequency or could the same unit communicate at different frequencies at the same time and so be a part of different networks simultaneously? In effect, I am not able to make sense of this paragraph!

It uses the 2.4 GHz band

Does this refer to the frequency of the EM waves that the device uses to communicate?

Yes. It generates (and receives) radio waves in the region of 2.5GHz.

If used in open space and with lower baud rate its range can reach up to 100 meters.

I know baud rate refers to how many "groups of bits" are transmitted per second. I don't quite understand why the range increases for a lower baud rate.

Think of it like talking to someone. If you're stood right next to them you can speak nice and fast and they can understand you. Your speech comes across nice and clear.

However if you're the other side of a quarry you have to shout really loud, and if you talk too fast they can't understand you. You. Have. To. Shout. Slowly. so that they can understand you.

The environment makes a lot of difference to how your speech propagates. In large areas there's lots of echo and other things. It's similar with RF communications. There's more noise and things that can interrupt communication. By having lower baud rates you're giving the receiver more time to understand what it's receiving and make sense of the data.

The module can use 125 different channels which gives a possibility to have a network of 125 independently working modems in one place. Each channel can have up to 6 addresses, or each unit can communicate with up to 6 other units at the same time.

Here, does a channel mean a single frequency? How is the range of possible channels decided (why 125) and how is the smallest detectable resolution decided (smallest change in frequency which would cause a change in channel)? So does a single unit work on only one frequency or could the same unit communicate at different frequencies at the same time and so be a part of different networks simultaneously? In effect, I am not able to make sense of this paragraph!

There's a number of different factors at play here. First there is the "Bandwidth" of the channel. This is the maximum amount of data that can be squeezed into a particular carrier frequency with a particular amount of "space" around the signal. You can think of the channel width as the diameter of a pipe. The wider the pipe the more water can be pushed through the pipe per second.

You want to slice the RF allocation up into enough channels to give enough concurrent access to lots of devices, but at the same time you want to have enough bandwidth per channel that you can transfer reasonable amounts of data.

It's all a balancing act.

Nordic decided 125 channels was the optimal balance for their application.

One nRF24L01+ can work at one frequency at a time. That's not to say you can't instruct it to change frequencies at any time, but it can only be listening on the one frequency at once. Just like a normal FM radio - you can only listen to one radio station at a time.

But the nRF24L01+ protocol uses addresses within that one chosen channel, so lots of devices can all be on the same channel to talk together, and each is distinct by its address.

The protocol also includes the concept of "pipes" which are virtual wireless circuits between two physical devices. Once you have configured a pipe with an address any other device can communicate with that pipe. You can configure up to 6 pipes, so up to 6 devices can send to the same target at the same time and each devices data is kept separate and distinct from the others. It's kind of like having 6 serial ports all connected to different devices at once.

• Not sure where you get the 100 MHz (min? max?) from. Most users of the band use very different bandwidths: 22 MHz (802.11b), 20 MHz (802.11g), 40 MHz (802.11n), 1 MHz (Bluetooth), 2 MHz (BLE), 2 MHz (802.15.4 / Zigbee / Thread)... 100 MHz is the total size of the 2.4 GHz ISM band, but in many regions it's actually slightly smaller than that. – jcaron Oct 4 '20 at 23:13
• Wikipedia on the ism page. But I probably misunderstood it. It's badly written. – Majenko Oct 4 '20 at 23:17
1. Yes. They call it a band, because the nRF24L01 can operate on frequencies from 2.400GHz to 2.525GHz

2. It's like talking slower to someone. It makes it easier to distinguish the signal from the noise.

3. Kind of. Every channel is 1Mhz apart from the next (`2.4Ghz + (125 * 1Mhz) = 2.525GHz` which is the range specified above). This difference is chosen more of less arbitrarily. The 125 channels is because it has to stay inside the ISM band 2.4-2.5Ghz. Note that the different channels can still interfere with each other, unless they are far enough apart from each other. Just like with WiFi where you had 11 channels, but were advised to only use channels 1, 6, or 11. The NFR can only run at one frequency/channel at a time. You can however change channels while it's running. But its better to have all running on the same channel and use the NRFs pipes to create separate "networks". Or even use the same pipe, but have the packet contain the info on what kind of data it contains.

• Note that not all channels may be usable. In the EU (ETSI) region for instance, the 2.4 GHz ISM band stop at 2.4835 GHz, so you can only use channels 1-83 at 1 MHz (up to 1 Mbps) and only half of those at 2 MHz. – jcaron Oct 4 '20 at 23:29
• @jcaron good point. I wanted to include this, but I couldn't find a good source. It looks like north america is even more restrictive, with a max of 2.473GHz. You definitely want to stay within these limits, and not risk fines or jail. – Gerben Oct 5 '20 at 15:18