In your daily life, you probably enjoy uninterrupted access to mobile networks, allowing you to place calls, send messages, and browse the web from your phone. We all tend to take this luxury for granted, but you will face times when this connectivity is unavailable. Perhaps you're embarking on a camping expedition in a remote area lacking mobile network coverage. Or you might be with your friends at a crowded festival where the mobile network becomes unreliable. In worst-case scenarios, you might find yourself in a disaster where the mobile network is completely down.

What if you could set up your own mobile network in such scenarios? It turns out you can. Meshtastic [1] lets you to establish your own local wireless mesh network using affordable, low-power devices. The mesh network allows you to stay connected with friends or family and send text messages to each other in areas without an existing or reliable communications infrastructure.

Meshtastic is not a complete replacement for a mobile network. You can't place phone calls or download massive amounts of data, and the range depends on your environment and the location of other devices in the mesh network. However, Meshtastic offers connectivity where other options fail. In most situations, the direct range is up to a few kilometers, but Meshtastic devices have the capability to relay messages from others, thereby extending the range. To better understand these bandwidth and range limitations, it is important to start with a look at the underlying technology.

Unlicensed Spectrum

A large portion of the radio spectrum is licensed by national regulators to specific users. The licensing system is why mobile operators or TV broadcasters have to pay hefty fees for the exclusive right to broadcast signals on certain frequencies. Likewise, this is why you need a ham radio license to transmit on specific frequencies.

In contrast, you can operate without a license on some frequency bands. However, because the devices on these free bands don't have exclusive rights, interference is much more likely. Therefore, these devices must comply with some regulations, including limits on the permitted transmit power and the duty cycle (the maximal ratio of transmission time to total time). Meshtastic relies on LoRa [2], a radio protocol developed by Semtech, operating in this unlicensed radio spectrum. Anyone can buy a LoRa chip and start using it (see the box entitled "Open Source on Proprietary Hardware").

The frequency spectrum and associated restrictions for LoRa depend on your geographical region, following radio regulations from the International Telecommunication Union [5]. Within Europe, the most popular LoRa frequency band for Meshtastic is 868MHz, offering a maximum power of +27dBm with a 10 percent duty cycle. Alternatively, the 433MHz band is also used in Europe, allowing up to +12dBm with the same duty cycle. In North America, the 915MHz frequency band permits up to +30dBm without duty cycle restrictions. It's important to buy LoRa hardware compatible with your region's frequency band and configure the Meshtastic software correspondingly.

Range and Data Rate

LoRa, which stands for "Long Range," enables long-range transmissions with low power consumption at slow data rates. Depending on the settings, the data rate can range from a few hundred bits per second to tens of kilobits per second. Although this bandwidth might appear minimal, it is sufficient for sending text messages, location information, or sensor data to nearby users, which is precisely what Meshtastic does.

Several parameters directly influence the range and data rate of your LoRa transmissions. Range and data rate are inversely proportional. Increasing the data rate will reduce range. Conversely, if you change the settings to improve range, the data rate decreases.

Meshtastic uses three LoRa settings to change this range and data rate: the spreading factor, bandwidth, and coding rate. The spreading factor, a number ranging from 7 to 12, determines how much data is spread over time during transmission. Each step up in spreading factor doubles the air time to transmit. The higher the spreading factor, the longer the range, but the slower the data rate. Bandwidth represents how big of a slice of the radio spectrum is used. Each doubling of the bandwidth doubles the data rate but cuts the range by half. And coding rate determines how much redundancy is built into the transmissions to resist radio noise. Increasing the coding rate increases reliability but decreases the net data rate because of the overhead.

Which settings should you use? To prevent analysis paralysis, Meshtastic has defined eight LoRa radio presets, which have been proven to work well. You can find these presets in Table 1. The default preset is Long Range/Fast, or in short, Long Fast. This setting uses a bandwidth of 250kHz, a coding rate of 4/5 (1 bit overhead for every 4 bits), and a spreading factor of 11, leading to a data rate of 1.07kbps. This overall data rate includes protocol overhead, which means the effective net data transfer rate is even lower.

Mesh Network

Meshtastic implements a mesh network architecture layered upon these direct point-to-point LoRa connections. Devices can send messages either directly or through intermediary devices that rebroadcast messages to further extend the network's reach. By default, Meshtastic devices send their messages with a maximum hop count of three. With each retransmission, this hop count is decreased by one. When a device receives a message with a hop count of zero, it will not rebroadcast it further.

However, if every device would rebroadcast all messages it receives, this would result in a broadcast storm, wasting bandwidth and energy, and ultimately diminishing the overall efficiency of the mesh network. For this reason, the Meshtastic protocol was carefully designed with various device roles you can choose depending on the use and location.

The default device role is Client, which works well in various circumstances. Upon receiving a message, a Client listens briefly to check if another device has already rebroadcast the message. If so, the device refrains from rebroadcasting it again. If not, it does rebroadcast it. This approach is called "managed flooding" and attempts to reduce the network load. There's also a Client Mute role. Client Mute devices don't rebroadcast messages from other devices. This role is useful if you have a couple of devices always in each other's vicinity. It doesn't make sense to have them all listen and rebroadcast; one of them suffices.

Another important role is Router: this device always rebroadcasts messages once. You might think that it makes sense to configure every device as a Router to improve coverage. However, it's not that simple. The presence of a Router in a suboptimal location could leave a Client in a more advantageous location ignoring messages due to previous rebroadcasts. So, adding a Router in that location could inadvertently reduce range. This phenomenon is called "eating a hop."

Most of us will never deploy a Meshtastic router, as they're only useful in exposed positions, such as on a mountain top, a radio tower, or a tall building that stretches at least 20 stories tall. In such strategic positions, they work extremely well, with the potential to expand your mesh network's range with tens or even hundreds of kilometers. However, a device on a typical house rooftop should remain configured as a Client: This will allow it to rebroadcast messages, but only if a better-placed Router has not yet done so.