Information to be sent is structured by the signaling-control unit (level 2) into a Signal Unit (SU). The SU is based on the HDLC protocol. There are three types of signaling units: Message Signal Unit (MSU) which transfers information between two signaling nodes; Link-status Signal Unit (LSSU) which is used to set up or close down the signaling link or indicate and adjust its status; Fill-in Signal Unit (FISU). When there are no status or information messages to send, fill-in messages are transmitted to establish that the link is activated and to provide a bit-error check on the link.