Meshtastic
Meshtastic is a communications system which utilizes an ad hoc wireless mesh network. Operating in ISM (Industrial Scientific, and Medical) radio frequencies, Meshtastic enables anyone to obtain an inexpensive radio, and communicate with other local people, independently of the internet and cellular systems. An amateur radio operator license is not required in most countries, including the United States.
How it works
Each user must obtain some form of radio device, capable of transmitting on the local LoRa band(s), and compatible with Meshtastic firmware. In the 2020s, such devices could typically be purchased for prices between $10 and $100, with many quality options in the $40-$70 range.[1] Once obtained and configured, that device (called a "node") now attempts to communicate with other Meshtastic devices within range. Using its LoRa transmitter, that node announces itself to others, and listens for messages. Some Meshtastic nodes include screens and keyboards, but most connect using Bluetooth to the owner's smartphone. Either though the smartphone application, or the node's own keyboard and screen, the owner can now send out text messages. Messages can be sent as a general broadcast to everyone in range, or to specific people using private channels or direct messages. When one person transmits a message, and other nodes recive it, that message is repeated by those other nodes in the local mesh network, causing the message to "hop" multiple times from one group of users to another. Through this mechanism, messages can be exchanges over significant distances. However, each transmitter is relatively weak, so without neighbors, the sender must be well clear of obstructions.[2][3]
The exact mechanism of message distribution is determined largely based on the "role" of each node. In its default configuration ("client" role), a node will listen for messages and send its own user's messages. When it receives someone else's message, it will listen to see if it hears anyone else locally re-transmit that message. If no one else does, it will rebroadcast that message, to help pass it on to the next collection of nodes. Other roles can be set, for specific purposes such as nodes on radio towers (capable of repeating messages over a large area), or to prevent re-transmission of messages.