Communication Protocol

Originally, we just needed to transmit the door open / closed status; that is, one bit of information. But pretty quickly one realizes that there is a need to pass on some timing information, some means to track message integrity, undelivered messages, etc. So, it ends up with some sort of a mini-protocol, defined in MailBoxMessage.h header.

Each communication node (transmitter or receiver) is assigned an address Communication Channel.Receiver ID.Mailbox ID:

• Communication Channel (1–127) corresponds to AT+Cnnn setting of HC-12 (see its user manual);
• Receiver ID (1–15) identifies receiver working on a given channel. 0 means message addressed to any receiver listening;
• Mailbox ID (1–15) identifies mailbox reporting to a given receiver. 0 is reserved for the receiver itself.

Examples:

• 07.01.02 — mailbox #2 reporting to receiver #1 on communication channel #7 (435.8 MHz);
• 15.03.00 — receiver #3 working on communication channel #15.

All nodes who wish to communicate must use the same communication channel. Even though HC-12 allows for reconfiguration at run-time, it is not being used. All HC-12 are pre-programmed to a given channel (see Getting Started); the rest of the software makes no assumption on the channel being used.

Receiver must accept all messages carrying its ID, as well as messages with receiver ID 0; other messages must be discarded (but may be logged). Mailbox (transmitter) must identify itself with a non-zero ID.

Each message carries a version of a protocol and a checksum; messages with mismatch in protocol or checksum must be discarded (but may be logged). Protocol version 0 is reserved for development purposes; applications may accept it at their own risk. Systems in production must use a non-zero version of the protocol. Protocol version must be incremented every time an incompatible change is made. The current version of the protocol is 2.