code: def parse_transmission_id(header, reference_id=None):
      
      m = _TID_PATTERN.search(header)
      if not m:
          return None
      return m.group(1)
docs: \
      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.