def prepare_test_bytes(b_field: bytes) -> bytes:
    """
    Returns the bytes of the B-field which are to be CRCed.
    It can be thought of as "2 on 6 off cont.".

    >>> prepare_test_bytes(bytes.fromhex('0b 0b 0c 0d 0e 0f aa bb cc'))
    b'\\x0b\\x0b\\xcc'
    """
    # bytes be CRCed
    test_bytes = bytearray()
    i = 1
    for e in b_field:
        i = i % 8

        if i == 1 or i == 2:
            test_bytes.append(e)
        i += 1
    return bytes(test_bytes)