FreeDATA/documentation/FreeDATA-protocols.md

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# FreeDATA - Protocols
## ARQ Sessions
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An ARQ Session represents a reliable data transmission session from a sending station (A) to a receiving station (B). It uses automatic repeat request on top of different codec2 modes according to the transmission channel conditions.
So lets say A wants to send some data to B. A typical scenario would be like this:
```
ISS->(1)IRS:<datac13> OPEN_REQ(session id, origin, dest)
IRS->(1)ISS:<datac13> OPEN_ACK (session id, proto version, speed level, frames, snr)
ISS->(1)IRS:<datac13> INFO(id, total_bytes, total_crc)
IRS->(1)ISS:<datac13> INFO_ACK(id, total_crc)
ISS->(1)IRS:BURST (ID, offset, payload),(ID, offset, payload),(ID, offset, payload)
IRS->(1)ISS:BURST_ACK (ID, next_offset, speed level, frames, snr)
ISS-->(1)IRS:Lost BURST (total or part)
IRS->(1)ISS:BURST_NACK (ID, next_offset, speed level, frames, snr)
ISS->(1)IRS:BURST (ID, offset, payload),(ID, offset, payload),(ID, offset, payload)
IRS->(1)ISS:DATA ACK NACK (ID, next_offset, speed level, frames, snr)
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```
### Frame details
#### SESSION_OPEN_REQ
ISS sends this first
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DATAC13 Mode (12 bytes)
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| field | bytes |
| --------------- | ----- |
| session id | 1 |
| origin | 6 |
| destination_crc | 3 |
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#### SESSION_OPEN_ACK
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Sent by the IRS in response to a SESSION_OPEN_REQ
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DATAC13 Mode (12 bytes)
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| field | bytes |
| ---------------- | ----- |
| session id | 1 |
| origin | 6 |
| destination_crc | 3 |
| protocol version | 1 |
| snr | 1 |
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#### SESSION_INFO
ISS sends this in response to a SESSION_OPEN_ACK
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DATAC13 Mode (12 bytes)
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| field | bytes |
| ----------- | ----- |
| session id | 1 |
| total bytes | 4 |
| total crc | 4 |
| snr | 1 |
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#### SESSION_INFO_ACK
IRS sends this in response to a SESSION_INFO
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DATAC13 Mode (12 bytes)
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| field | bytes |
| ---------------- | ----- |
| session id | 1 |
| total crc | 4 |
| snr | 1 |
| speed level | 1 |
| frames per burst | 1 |
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#### Data Burst
ISS sends this to send data to IRS
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Mode according to handshake speed level
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Frames per burst according to handshake
##### Modulation
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Each burst is composed of frames_per_burst frames:
|preamble|f1|f2|f3|...|postamble|
##### Each data frame
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| field | bytes |
| ---------- | ------------------------------ |
| session id | 1 |
| offset | 4 |
| payload | (the remaining payload length) |
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#### DATA_BURST_ACK
Sent by the IRS following successful decoding of burst.
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| field | bytes |
| --------------------- | ----- |
| session id | 1 |
| next offset | 4 |
| next speed level | 1 |
| next frames per burst | 1 |
| snr | 1 |
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#### DATA_BURST_NACK
Sent by the IRS following unsuccessful decoding of burst or timeout.
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| field | bytes |
| --------------------- | ----- |
| session id | 1 |
| next offset | 4 |
| next speed level | 1 |
| next frames per burst | 1 |
| snr | 1 |
#### DATA ACK NACK
Sent by the IRS after receiving data with a state information.
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| field | bytes |
| ---------- | ----- |
| session id | 1 |
| state | 1 |
| snr | 1 |