FreeDATA/documentation/FreeDATA-protocols.md

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# FreeDATA - Protocols
## ARQ Sessions
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)
```
### Frame details
#### SESSION_OPEN_REQ
ISS sends this first
DATAC13 Mode (12 bytes)
|field|bytes|
|-|-|
|session id|1|
|origin|6|
|destination_crc|3|
#### SESSION_OPEN_ACK
Sent by the IRS in response to a SESSION_OPEN_REQ
DATAC13 Mode (12 bytes)
|field|bytes|
|-|-|
|session id|1|
|origin|6|
|destination_crc|3|
|protocol version|1|
|snr|1|
#### SESSION_INFO
ISS sends this in response to a SESSION_OPEN_ACK
DATAC13 Mode (12 bytes)
|field|bytes|
|-|-|
|session id|1|
|total bytes|4|
|total crc|4|
|snr|1|
#### SESSION_INFO_ACK
IRS sends this in response to a SESSION_INFO
DATAC13 Mode (12 bytes)
|field|bytes|
|-|-|
|session id|1|
|total crc|4|
|snr|1|
|speed level|1|
|frames per burst|1|
#### Data Burst
ISS sends this to send data to IRS
Mode according to handshake speed level
Frames per burst according to handshake
##### Modulation
Each burst is composed of frames_per_burst frames:
|preamble|f1|f2|f3|...|postamble|
##### Each data frame
|field|bytes|
|-|-|
|session id|1|
|offset|4|
|payload|(the remaining payload length)|
#### DATA_BURST_ACK
Sent by the IRS following successful decoding of burst.
|field|bytes|
|-|-|
|session id|1|
|next offset|4|
|next speed level|1|
|next frames per burst|1|
|snr|1|
#### DATA_BURST_NACK
Sent by the IRS following unsuccessful decoding of burst or timeout.
|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.
| field |bytes|
|------------|-|
| session id |1|
| state |1|
| snr |1|