H.245 is line transmission of nontelephone signals. It includes receiving and transmitting capabilities as well as mode preference from the receiving end, logical channel signalling, and control and indication. Acknowledged signalling procedures are specified to ensure reliable audiovisual and data communication. The ITU-T H.245 recommendation specifies the H.323 call control messages that govern how the endpoints operate across the half-duplex logical connections. H.245 messages control endpoint capabilities exchange, opening and closing of logical channels, flow control, and so forth.