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JSON/CBOR acquisition format

To send data from any device or any system to Edge Impulse you will need to deliver the data in the Data acquisition format. This is a small specification that describes the type of data, the sensor data itself, and information about the device that generated the data. You can optionally sign this data so you have cryptographic proof on the origin and integrity of the data. There are end-to-end examples in C++, Python and Node.js here.

If you have your data already in CSV, JPG, PNG or WAV format, you can also just upload the data either using the uploader feature in the Studio, or using the CLI, see CLI Uploader for more information. For CSV files, either use CSV data acquisition specification or use the CSV Wizard feature for more flexibility.

If you are using an Edge AI hardware with an Edge Impulse firmware it will already send data in this format, and if you want to collect data from other devices the data forwarder will automatically convert and sign your data.

Data acquisition format specification

The Edge Impulse Data Acquisition format is an optimized binary format for time-series data. It allows cryptographic signing of the data when sampling is complete to prove the authenticity of the data. Data is encoded using CBOR or JSON and signed according to the JSON Web Signature specification.

Data can easily be generated by any language that has a CBOR or JSON library available, but there are examples available for C, C++, Node.js and Python.

Example in JSON:

{
    "protected": {
        "ver": "v1",
        "alg": "HS256",
        "iat": 1564128599
    },
    "signature": "b0ee0572a1984b93b6bc56e6576e2cbbd6bccd65d0c356e26b31bbc9a48210c6",
    "payload": {
        "device_name": "ac:87:a3:0a:2d:1b",
        "device_type": "DISCO-L475VG-IOT01A",
        "interval_ms": 10,
        "sensors": [
            { "name": "accX", "units": "m/s2" },
            { "name": "accY", "units": "m/s2" },
            { "name": "accZ", "units": "m/s2" }
        ],
        "values": [
            [ -9.81, 0.03, 1.21 ],
            [ -9.83, 0.04, 1.27 ],
            [ -9.12, 0.03, 1.23 ],
            [ -9.14, 0.01, 1.25 ]
        ]
    }
}

Fields

  • protected - information about the signature format.

    • ver - always v1 (required).

    • alg - the algorithm used to sign this file. Either HS256 (HMAC-SHA256) or none (required).

    • iat - date when the file was created in seconds since epoch. Only set this when the device creating the file has an accurate clock (optional).

  • signature - cryptographic signature for this file (see signing data) (required).

  • payload - sensor data.

    • device_name - unique identifier for this device. Only set this when the device has a globally unique identifier (e.g. MAC address). If this field is set the device shows up on the 'Devices' page in the studio (optional).

    • device_type - device type, for example the exact model of the device. Should be the same for all similar devices (required).

    • interval_ms - the frequency of the data in this file (in milliseconds). E.g. for 100Hz fill in 10 (new data every 10 ms.). You can use a float here if you need the precision (required). If you have sensors with different sampling frequencies, you should upscale to the highest frequency to keep the finest granularity. Example: sensor A is 100 Hz, sensor B is 5 Hz. Upscale sensor B to 100 Hz by duplicating each value 20 times (100/5). You could also smooth values over between samples.

    • sensors - array with sensor axes.

      • name - name of the axis.

      • units - type of data on this axis. Needs to comply to SenML units (required).

    • values - array of sensor values. One array item per interval, and as many items in this array as there are sensor axes. If you have a single sensor, you are allowed to flatten this array to save space. An array of string values can be used to refer to binary attachments sent to the ingestion service using multipart requests (see Binary payloads and Binary attachments).

Signing data

Files can be signed to establish a trust chain between device and Edge Impulse. Because the iat and device_name fields are included in the signed file you can validate authenticity of the data, which device created the data, and when the data was captured. Currently data can be signed with a symmetric HMAC key. You can create HMAC keys under 'Dashboard' in the studio.

HMAC SHA256

To sign data using HMAC SHA256:

  1. Set the signature field to 0000000000000000000000000000000000000000000000000000000000000000 (64 times ASCII 0).

  2. Create the full CBOR or JSON object.

  3. Use your language's crypto module to sign the object with HMAC-SHA256. This should give you a 32 byte signature.

  4. Replace the empty signature with the HEX encoded signature (64 characters ASCII).

The C, C++, Node.js and Python SDKs all support signing data using HMAC SHA256. See the examples section.

Binary notation (CBOR)

This is the same example as above, but in binary. Paste it into the CBOR playground to decode:

a3 69 70 72 6f 74 65 63 74 65 64 a3 63 76 65 72 62 76 31 63 61 6c 67 65 48 53 32 35 36 63 69 61 74 1a 5d 3a dd 3d 69 73 69 67 6e 61 74 75 72 65 78 40 62 30 65 65 30 35 37 32 61 31 39 38 34 62 39 33 62 36 62 63 35 36 65 36 35 37 36 65 32 63 62 62 64 36 62 63 63 64 36 35 64 30 63 33 35 36 65 32 36 62 33 31 62 62 63 39 61 34 38 32 31 30 63 36 67 70 61 79 6c 6f 61 64 a5 6b 64 65 76 69 63 65 5f 6e 61 6d 65 71 61 63 3a 38 37 3a 61 33 3a 30 61 3a 32 64 3a 31 62 6b 64 65 76 69 63 65 5f 74 79 70 65 73 44 49 53 43 4f 2d 4c 34 37 35 56 47 2d 49 4f 54 30 31 41 6b 69 6e 74 65 72 76 61 6c 5f 6d 73 0a 67 73 65 6e 73 6f 72 73 83 a2 64 6e 61 6d 65 64 61 63 63 58 65 75 6e 69 74 73 64 6d 2f 73 32 a2 64 6e 61 6d 65 64 61 63 63 59 65 75 6e 69 74 73 64 6d 2f 73 32 a2 64 6e 61 6d 65 64 61 63 63 5a 65 75 6e 69 74 73 64 6d 2f 73 32 66 76 61 6c 75 65 73 9f 83 fa c1 1c f5 c3 fa 3c f5 c2 8f fa 3f 9a e1 48 83 fa c1 1d 47 ae fa 3d 23 d7 0a fa 3f a3 d7 0a 83 fa c1 11 eb 85 fa 3c f5 c2 8f fa 3f 9d 70 a4 83 fa c1 12 3d 71 fa 3c 23 d7 0a f9 3d 00 ff

Examples

// First, add the Edge Impulse C Ingestion SDK to your project.
//     https://github.com/edgeimpulse/ingestion-sdk-c
// See 'C SDK Usage Guide' for more information and porting information

#include "sensor_aq.h"
#include "sensor_aq_mbedtls_hs256.h"

const char *hmac_key = "fed53116f20684c067774ebf9e7bcbdc";

int main() {
    // The sensor format supports signing the data, set up a signing context
    sensor_aq_signing_ctx_t signing_ctx;

    // We'll use HMAC SHA256 signatures, which can be created through Mbed TLS
    // If you use a different crypto library you can implement your own context
    sensor_aq_mbedtls_hs256_ctx_t hs_ctx;
    // Set up the context, the last argument is the HMAC key
    sensor_aq_init_mbedtls_hs256_context(&signing_ctx, &hs_ctx, hmac_key);

    // Set up the sensor acquisition basic context
    sensor_aq_ctx ctx = {
        // We need a single buffer. The library does not require any dynamic allocation (but your TLS library might)
        { (unsigned char*)malloc(1024), 1024 },

        // Pass in the signing context
        &signing_ctx,

        // And pointers to fwrite and fseek - note that these are pluggable so you can work with them on
        // non-POSIX systems too. Just override the EI_SENSOR_AQ_STREAM macro to your custom file type.
        &fwrite,
        &fseek,
        // if you set the time function this will add 'iat' (issued at) field to the header with the current time
        // if you don't include it, this will be omitted
        &time
    };

    // Payload header
    sensor_aq_payload_info payload = {
        // Unique device ID (optional), set this to e.g. MAC address or device EUI **if** your device has one
        "ac:87:a3:0a:2d:1b",
        // Device type (required), use the same device type for similar devices
        "DISCO-L475VG-IOT01A",
        // How often new data is sampled in ms. (100Hz = every 10 ms.)
        10,
        // The axes which you'll use. The units field needs to comply to SenML units (see https://www.iana.org/assignments/senml/senml.xhtml)
        { { "accX", "m/s2" }, { "accY", "m/s2" }, { "accZ", "m/s2" } }
    };

    // Place to write our data.
    // The library streams data, and does not cache everything in buffers
    FILE *file = fopen("test/encoded.cbor", "w+");

    // Initialize the context, this verifies that all requirements are present
    // it also writes the initial CBOR structure
    int res;
    res = sensor_aq_init(&ctx, &payload, file, false);
    if (res != AQ_OK) {
        printf("sensor_aq_init failed (%d)\n", res);
        return 1;
    }

    // Periodically call `sensor_aq_add_data` (every 10 ms. in this example) to append data
    float values[][3] = {
        { -9.81, 0.03, 1.21 },
        { -9.83, 0.04, 1.28 },
        { -9.12, 0.03, 1.23 },
        { -9.14, 0.01, 1.25 }
    };
    for (size_t ix = 0; ix < sizeof(values) / sizeof(values[0]); ix++) {
        res = sensor_aq_add_data(&ctx, values[ix], 3);
        if (res != AQ_OK) {
            printf("sensor_aq_add_data failed (%d)\n", res);
            return 1;
        }
    }

    // When you're done call sensor_aq_finish - this will calculate the finalized signature and close the CBOR file
    res = sensor_aq_finish(&ctx);
    if (res != AQ_OK) {
        printf("sensor_aq_finish failed (%d)\n", res);
        return 1;
    }

    // Use the HTTP libraries available for your platform to upload
    //      test/encoded.cbor
    // to
    //      https://ingestion.edgeimpulse.com/api/training/data
}

What about applying labels to your samples? You can use the optional x-label header parameter when creating your request. See the header parameters available in the ingestion API here.

Binary payloads (e.g. for audio)

For some payloads, like audio data, it's not feasible to convert the raw sensor data into CBOR values while capturing data on the device. For these cases you can send a binary payload. The same header structure as above applies, but instead of writing the values in the values CBOR field you can append a binary array at the end of the file (in uint8, uint16, uint32, int8, int16, int32 or float32 format). To do so:

  1. Limit your data to a single sensor.

  2. Set the value of the values field to: [ "Ref-Binary-i16" ] (so a string within an array, where i16 indicates int16, alternatively you can use u16 for uint16 or f32 for float32).

  3. Write the CBOR header. Note that this needs to be a valid CBOR structure, and that it requires termination with 0xFF. The easiest to achieve this is by writing the values field as an indefinite length array (which needs to be terminated by 0xFF.

  4. Write your payload (little endian).

    • If you need to align your writes the easiest is to pad the Ref-Binary-i16 string with spaces on the right.

  5. Update the signature with the signature for the full file, including header and payload.

  6. Upload the data to the ingestion service with the Content-Type: application/octet-stream header.

Here is an example of a valid audio sample that includes a binary payload:

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

The header stops at byte ff (you can copy the beginning of the data up until the ff into cbor.me to validate) and the payload starts at f918fa.

Binary attachments (e.g. for images)

For non-time-series data - currently only implemented for images - you can send binary attachments to the ingestion service. Here the same header structure is used as above, but instead of writing the values in the values CBOR field, you reference the content type, the size, and the signed hash (with the same HMAC key) of your attachment. You then need to send both the header and the actual file to the Ingestion service via a multipart/form request (see 'Multipart requests' section).

Note: Including more than 1 binary attachment is currently not supported by the Edge Impulse Studio, such messages won't be rejected by the ingestion service though, these attachments will merely be 'invisible' from the studio.

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