# 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:BURST_ACK (ID, next_offset, speed level, frames, snr)
```


### Frame details


#### SESSION_OPEN_REQ

ISS sends this first

DATAC13 Mode (14 bytes)

|field|bytes|
|-|-|
|session id|1|
|origin|6|
|destination|6|
|crc|1|


#### SESSION_OPEN_ACK

IRS sends this in response to a SESSION_OPEN_REQ

DATAC13 Mode (14 bytes)

|field|bytes|
|-|-|
|session id|1|
|origin|6|
|protocol version|1|
|snr|1|
|crc|3|


#### SESSION_INFO

ISS sends this in response to a SESSION_OPEN_ACK

DATAC13 Mode (14 bytes)

|field|bytes|
|-|-|
|session id|1|
|total bytes|4|
|total crc|4|
|snr|1|
|crc|4|


#### SESSION_INFO_ACK

IRS sends this in response to a SESSION_INFO

DATAC13 Mode (14 bytes)

|field|bytes|
|-|-|
|session id|1|
|total crc|4|
|snr|1|
|speed level|1|
|frames per burst|1|
|crc|4|


#### 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|