def generate_device_letter(index):
    """Returns device letter by index (starts by zero)
       i.e.
       index   = 0, 1,..., 18277
       results = a, b,..., zzz
    """
    base = ord('z') - ord('a') + 1
    unit_dev_name = ''
    while index >= 0:
        letter = chr(ord('a') + index % base)
        unit_dev_name = letter + unit_dev_name
        index = int(index / base) - 1
    return unit_dev_name
