# Overview

The FOTA image’s first sub-image is the FOTA Bluetooth Low Energy Stack (fota.bin) and the second one is the user application (app.bin). The Python utility mkfotaimg.py generates the FOTA image, as illustrated below in the "Image Format" figure:

Figure: Image Format

The .fota file can then be used as input to the updater.py tool to perform a UART firmware update via the bootloader. For more information about the bootloader, see the Bootloader User's Guide.