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docs/about/overview/mesh-alg.mdx
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@ -19,29 +19,30 @@ need to consider the MeshPacket and Sub-packet message types.
All data is converted into LoRa symbols which are sent to the radio for transmission. The details are described elsewhere, but it is worth noting that in addition to the converted packet bytes described below, there is also a preamble sent at the start of any data packet.
This preamble allows receiving radios to synchronize clocks and start framing. We use a preamble length of 32, which is longer than the minimum preamble length of 8, to maximize the amount of time the LoRa receivers can stay asleep, which dramatically lowers power consumption.
This preamble allows receiving radios to synchronize clocks and start framing. We use a preamble length of 16, which is longer than the minimum preamble length of 8, to let SX126x LoRa receivers sleep for a while, which lowers power consumption.
After the preamble comes the LoRa Physical Header, which contains information about the packet length as well as a sync word to distinguish networks. For Meshtastic, it is set to `0x2B`.
### Layer 1: Unreliable Zero Hop Messaging
This layer is conventional non-reliable LoRa packet transmission. The transmitted packet has the following representation before encoding for transmission:
This layer is conventional non-reliable LoRa packet transmission. A packet generated by a Meshtastic device has the following representation before encoding for transmission:
| Offset | Length | Type | Usage |
| :----------: | :---------------------------: | :-----: | :--------------------------------------------------------------------------------------- |
| 0x00 | 1 byte | Integer | syncWord, always `0x2B`. |
| 0x01 | 4 bytes | Integer | Packet header: Destination. The destination's unique NodeID. `0xFFFFFFFF` for broadcast. |
| 0x05 | 4 bytes | Integer | Packet Header: Sender. The sender's unique NodeID. |
| 0x09 | 4 bytes | Integer | Packet Header: The sending node's unique packet ID for this packet. |
| 0x0D | 32 bits | Bits | Packet Header: Flags. See the [header flags](#packet-header-flags) for usage. |
| 0x11 .. 0xFD | Varies, maximum of 237 bytes. | Bytes | Actual packet data. Unused bytes are not transmitted. |
| 0xFE .. 0xFF | 2 Bytes | Bytes | Unused. |
| Offset | Length | Type | Usage |
| :----: | :--------------------------------------: | :-----: | :--------------------------------------------------------------------------------------- |
| 0x00 | 4 bytes | Integer | Packet Header: Destination. The destination's unique NodeID. `0xFFFFFFFF` for broadcast. |
| 0x04 | 4 bytes | Integer | Packet Header: Sender. The sender's unique NodeID. |
| 0x08 | 4 bytes | Integer | Packet Header: The sending node's unique packet ID for this packet. |
| 0x0C | 1 byte | Bits | Packet Header: Flags. See the [header flags](#packet-header-flags) for usage. |
| 0x0D | 1 byte | Bits | Packet Header: Channel hash. Used as hint for decryption for the receiver. |
| 0x0E | Max. 237 bytes (excl. protobuf overhead) | Bytes | Actual packet data. Unused bytes are not transmitted. |
#### Packet Header Flags
| Index | # of Bits | Usage |
| :-----: | :-------: | :----------------------------- |
| 0 | 3 | HopLimit (see note in Layer 3) |
| 3 | 1 | WantAck |
| 4 .. 32 | 28 | Currently unused |
| Index | # of Bits | Usage |
| :----: | :-------: | :----------------------------- |
| 0 | 3 | HopLimit (see note in Layer 3) |
| 3 | 1 | WantAck |
| 4 .. 7 | 4 | Currently unused |
#### Usage Details