DualCam Communication Protocol

Camera file transfer reconnection

If FMU1YX device has bad reception, server is not reachable or wrong server details are configured, then the device tries to open a link to a camera server few consecutive times. If no connection was possible to be established, then the connection is postponed for 30 minutes and tried again (or tried every configured sending interval if periodic image sending is enabled).

DualCam camera file transfer support

Once camera has at least one file captured, it starts connection to a remote server, which is configured by parameters “Domain” and “Port” found in the “Camera Settings” tab.

Initialization packet

On connection, a device sends an initialization packet.

Header(0x0000)	Protocol ID	IMEI	Settings
2 bytes	2 bytes	8 bytes	4 bytes

Protocol ID – just a reference for the protocol version that is running on a device (for server cross compatibility with older versions). Firmware FMB.Ver.03.27.00.Rev.100 and up have protocol ID 5. Settings flag contains information on what is available for download. Structure is provided below:

Settings, 4 B
Byte 3								Byte 2								Byte 1								Byte 0
		1	2	3	4

1. Video, rear (%videor)
2. Video, front (%videof)
3. Photo, rear (%photor)
4. Photo, front (%photof)

If identifier sent to a server is not valid, device disconnects.

General command structure

General communication packet structure is as in the table bellow. It consist of CMD_ID (2 bytes), Data length of a command and a payload.

Command ID	Data length	Data
2 bytes	2 bytes	[data length] bytes

Communication protocol

Close a session command (CMD ID 0x0000)
In case when a device connects to a server, but the server does not expect it to connect, the server will respond by sending a CLOSE command after which the connection will be terminated. This command is also used when the device connects to a server for a custom file sending and the server finishes to send all custom files to the device.

Command ID	Data length
0x0000	0x0000

Start file transfer command (CMD ID 0x0001)
After device received file request command from the server (0x0008) device sends START command with file data (packet count).

Command ID	Data length	File Packets (4 bytes)	Blank field (2 bytes)
0x0001	0x0006	0x12345678	0x0000 (always same value)

File request command (CMD ID 0x0008)
After the device is connected for a file upload, the server initiates file transfer by sending a FILE REQ command.

Command ID	Data length	File Identifier
0x0008	2 bytes	See the table below

The device should answer with a START command described above indicating the size and CRC of the requested file.

File source, type	Identifier (ASCII chars)
Photo from camera, rear	%photor
Photo from camera, front	%photof
Video from camera, rear	%videor
Video from camera, front	%videof

Resume file transfer command (CMD ID 0x0002)
In a response to the START command a RESUME command must be sent from server.

Command ID	Data length	Packet offset (4 bytes)
0x0002	0x0004	0x00000001

To begin file transfer from the start, offset should be set to 1 (4 bytes value). In case when the file transfer was interrupted, to resume file transfer, offset can be set to the desired value (1 ≤ [offset] ≤ [file packets]).

Synchronize file transfer command (CMD ID 0x0003)
In a response to the RESUME command a SYNC command is sent from device.

Command ID	Data length	File offset (4 bytes)
0x0003	0x0004	0x00000001

By sending the SYNC command it is ensured that the next data command will contain file data starting from the specified offset.

File data transfer command (CMD ID 0x0004)
After sending a SYNC command, a file data transfer is started by sending DATA commands.

Command ID	Data length	File data (up to 1024 bytes)	Data CRC (2 bytes)
0x0004	0x0402	...	...

The file is sent in chunks of 1024 bytes, but the server may receive it in various packet sizes depending on a modem and provider packet forming procedures. .
Note: if a command with a bad CRC is received, RESUME command should be sent with the last valid file offset, after receiving a RESUME command, the server will stop sending DATA commands and continue communication from “Resume file transfer” step.
CRC polynomial expression: 0x8408 The initial value, when calculating CRC, is the previously received packet (CMD ID 0x0004) CRC value.

File transfer status command (CMD ID 0x0005)
After a file transfer is completed and no more files are required from the device, a server should send a COMPLETED command to the device (this command does not work after executing repeat init command0 x0009 – in this case, the server should send a CLOSE SESSION 0x0000 command mentioned before).

Command ID	Data length	Status (4 bytes)
0x0005	0x0004	0x00000000

In case of the server using invalid arguments, commands or not following the file request flow, the device will send this command with a Status field set to one of the few possible error codes. A list of possible ones is provided below.

Status value (hexadecimal)	Description	Notes
0x00000000	File transfer process completed	Sent from server
0x00000002	Failed to close GPRS	Sent from device
0x00000003	Failed to close socket	Sent from device
0x00000005	Invalid response from server to init packet	Sent from device
0x00000011	This error code forces the device to disconnect from server	Sent from device. Possible causes: Camera is not configured The requested file is not available by camera

After a COMPLETED command device should disconnect from the server.

Initialization packet repeat command (CMD ID 0x0009)
When sent, the initialization packet is repeated. This is used, when all of the files are downloaded and an additional check is carried out for any additional files, that may have been captured during the download operation.

Query file metadata request (CMD ID 0x000A)
This command is used for retrieving extra information about a selected file. A file identifier is used to determine which file’s information will be received. File identifiers are identical and used in the same way as in the File Request Command (0x0008).

Command ID (2 bytes)	Data length (2 bytes)	File Identifier (7 bytes)
0x000A	0x0007	0xFFFFFFFFFFFFFF

Can be sent after the initialization packet or after the file transfer is complete. It is not a mandatory command and doesn’t have to be used. The response for this command is 0x000B (more details below).

Note: 0x000A command was added since 03.27.04.Rev.104 firmware version and would not work with older versions.

File metadata response (CMD ID 0x000B)
0x000B is a response command to the request command 0x000A.

Response command returns information about a specified file: file type, timestamp, trigger, and video length. Complete command’s structure:

Command ID (2 bytes)	Data length (2 bytes)	Command version (1 byte)	File type (1 byte)	Timestamp (8 bytes)	Trigger (1 byte)	Video length (2 bytes)
0x000B	0x000D	0xFF	0xFF	0xFFFFFFFFFFFFFF	0xFF	0xFFFF

Note: 0x000B command was added since 03.27.04.Rev.104 firmware version and would not work with older versions.

Response command meta data:

Data	Description
Command version	Iteration of the command itself to determine to parse of the data
File type	Media type for confirming which file’s metadata is received. Byte structure is the same as byte 3 in the initialization packet settings data
Timestamp	Timestamp at the time of the trigger. Same used in a corresponding SOS record
Trigger	Trigger values. Same values as video sending trigger in SOS record.
Video length	Duration of the video in seconds. If the file type is a photo, this value is 0x0000.

CRC-16 calculation

CRC is calculated with using 3 input variables:

1. init_value - last packet CRC (or 0 if it's the first packet)
2. poly - binary polynomial expression (0x8408 in our case)
3. data - raw data of the packet (not including the first 4 bytes (cmd ID and packet size) and 2 last bytes (the CRC)

An example of structure can be found below:

function crc16_generic(init_value, poly, data) {
    let RetVal = init_value;
    let offset;

    for (offset = 0; offset < data.length; offset++) {
        let bit;
        RetVal ^= data[offset];
        for (bit = 0; bit < 8; bit++) {
            let carry = RetVal & 0x01;
            RetVal >>= 0x01;
            if (carry) {
                RetVal ^= poly;
            }
        }
    }
    return RetVal;
}

CRC (Cyclic Redundancy Check) is an error - detecting code using for detect accidental changes to RAW data. The algorithm how to calculate CRC - 16 (also known as CRC - 16 / IBM) you will find below.

var crc = {
   extractCRC: function (message) {
       crcData = message.slice(message.length - 2, message.length);
       return crcData[0] * 256 + crcData[1];
   },
   calculateCRC: function(message, previous_crc) {
       let crc_poly = 0x8408;
       let idx = 0;
       let crc = 0x0000;

       /* the payload length is calculated with removing size of header and CRC (4 + 2 bytes) */
       let payload_length = message.length - 6; 

       /* the payload is separated from the message header and CRC (4 bytes ofset + length)*/
       let payload = message.slice(4, 4 + payload_length);

       /* Init value for the first message is zero and for the upcoming messages it is the CRC value of the previous message */
       crc = previous_crc;

       for (idx = 0; idx < payload_length; idx++) {
           let bit;
           /* Use XOR operation for initial value and payload */
           crc ^= payload[idx];

           for (bit = 0; bit < 8; bit++) {
               let carry_bit = crc & 0x01;
               crc >>= 0x01;

               if (carry_bit != 0) {
                   crc ^= crc_poly;
               }
           }
       }       
       return crc;
   }
};
exports.crc = crc;

File transfer visual flow

Video conversion example and settings

For converting DualCam h265 videos to a more popular mp4 video format, a free open-source converter FFmpeg is used in the example. Although any other converter from h265 would work as well.

To convert DualCam video properly, the converter has to know the frame rate of the video. The frame rate is configured as Config Id: 66003 and could be either 20, 25, or 30 FPS. That same value could be obtained by using the metadata command (see more details at File metadata response (CMD ID 0x000B)).

Recommended parameters using the FFmpeg for the conversion:

ffmpeg -r <fps> -i <input>.h265 -ss 00:00:0.9 -c:a copy -c:v libx264 <output>.mp4

<fps> - configured frame rate of the video;

<input> - file name of the video file from the camera;

<output> - file name of the video that will be converted to mp4.

Video files and conversion

Videos downloaded from the camera are in the raw h265 format. By default, they could be viewed using the ZMVideoPlayer program in the h265 format. If the videos are needed in a more common format, they can be converted.