Off-network Messages

Off-network messages use family 02, command 04, and subcommand 01. The message ID is a 4-byte uint32-be value. The sender field is 16-byte UTF-8 padded on the right with spaces.

Types

type	Content	Frame
`01`	Text	Start frame
`02`	Text	Data frame
`03`	Voice	Start frame
`04`	Voice	Data frame
`05`	Image	Start frame
`06`	Image	Data frame

The acknowledgment payload for each business frame is 00 82 04 01 00.

Short Text

UTF-8 text up to 180 bytes uses one frame.

Field	Length	Encoding
Fixed header	5	`00 02 04 01 01`
Message ID	4	uint32-be
Reserved	1	`00`
Sender	16	UTF-8 padded on the right with `20`
Text length	2	uint16-le
Fragment count and sequence	2	`01 00`
Text	N	UTF-8

Fragmented Text

Split text into 131-byte chunks. The start frame contains no text. Send the data frames in sequence.

Frame	payload
Start frame	`00 02 04 01 01 [msgId be32] 00 [sender 16] [streamLength le16] [partCount] 00`
Data frame	`00 02 04 01 02 [msgId be32] [seq be16] 00 [up to 131-byte text]`

streamLength is the sum of all fragment lengths plus one byte for each fragment.

Voice Message

Voice uses AMR-NB MR475. Each frame covers 20 ms. BLE data retains the 12-byte MR475 payload. Split voice data into 132-byte chunks.

Frame	payload
Start frame	`00 02 04 01 03 [msgId be32] 00 [sender 16] [dataLength le16] [partCount le16] [durationSeconds le16]`
Data frame	`00 02 04 01 04 [msgId be32] [seq be16] 00 [up to 132-byte voice]`

Image Message

Images use JPEG with a 300 px long edge and quality 75. Split image data into 132-byte chunks.

Frame	payload
Start frame	`00 02 04 01 05 [msgId be32] 00 [sender 16] [dataLength le16] [partCount] 00 [width le16] [height le16]`
Data frame	`00 02 04 01 06 [msgId be32] [seq be16] 00 [up to 132-byte jpeg]`

Session Control

Operation	payload
Enable off-network session	`00 02 04 07 01`
Disable off-network session	`00 02 04 07 00`