\
    Returns the transmission ID of the cable. If no transmission identifier was
    found, ``None`` is returned.

    `header`
        The cable's header
    `reference_id`
        The cable's reference ID.

def parse_transmission_id(header, reference_id=None):
    """\
    Returns the transmission ID of the cable. If no transmission identifier was
    found, ``None`` is returned.

    `header`
        The cable's header
    `reference_id`
        The cable's reference ID.
    """
    m = _TID_PATTERN.search(header)
    if not m:
        return None
    return m.group(1)