| 摘要 |
A protected passive optical communications system (10), having a plurality of terminals (14), a main head end (22) for communicating with the terminals (14) in an operational state, and, a back up head (37) end arranged to allow communication with the terminals (14) in the event of a fault, the main head end (22) and the back up head end (37) each being operable to transmit a sequence of frames for communicating with the terminals (14), each frame having a header portion and a payload portion, the main head end (22) being arranged, in normal operation, to transmit control instructions in the header portion and user data in the payload portion of each frame, the back up head end (37) being arranged, in response to a fault, to transmit control instructions in the payload portion of a frame, and a control message in the header portion of that frame, which control message is indicative of the presence of control instructions in the payload portion, the terminals being responsive to the control message, such that: in the presence of a control message in the header of a frame, each terminal (14) interprets the payload of that frame as containing control instructions for changing one or more operational attributes of a terminal (14); and, in the absence of a control message in the header portion of a frame, each terminal (14) is arranged to interpret the payload of that frame as containing user data and to forward the user data to one or more users external to the terminal (14), in dependence on an address associated with the user data.
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