out-of-order signal

In a communications system, such as a communications network or a computer network, a signal that is sent in the backward direction indicating that a call cannot be completed or a message cannot be received because the destination user terminal, such as a computer, or the destination user terminal access line (a) is in the outage state, (b) has been taken out of service, or (c) has a fault that renders it unserviceable. See also access line, backward direction, call, communications system, communications network, completed call, computer, computer network, destination user, fault, message, outage, outage state, receive, service, signal, terminal.