(1) Field of the Invention
The present invention relates to a transmission system having a plurality of terminals linked by a working channel line and a protection channel line in a redundant structure wherein a connection is switched from the working channel line to the protection channel line while avoiding instantaneous cutoff when the working channel line has failed.
It is desired to provide reliable data communication in a transmission system having a plurality of terminals linked by a working channel line and a protection channel line in a redundant structure. In order to attain this objective, it is necessary that the transmission system reliably switch a connection from the working channel line to the protection channel line while avoiding instantaneous cutoff when one of the two channel lines has failed.
(2) Description of the Related Art
FIG. 22 shows a conventional transmission system having a plurality of terminals linked by a working channel line and a protection channel line in a redundant structure. In the conventional transmission system, a connection is switched from the working channel line to the protection channel line while avoiding instantaneous cutoff when a switching command to switch one of the channel lines to the other is received.
As shown in FIG. 22, the conventional transmission system includes a transmission terminal 101 (which is called a terminal A), a reception terminal 103 (which is called a terminal B), and a transport terminal 102 (which is called a terminal C). The terminal A and the terminal B are linked by a working channel line, and the terminal A, the terminal C and the terminal B are linked by a protection channel line. In the conventional transmission system of FIG. 22, transmission of a digital signal in the existing synchronous digital hierarchy (SDH) frame format is assumed.
In the transmission terminal 101 (the terminal A), a frame pulse insertion unit (FP INS) 104 and a distribution unit (DIS) 105 are provided. The FP INS 104 inputs the SDH frame and inserts a frame pulse (FP) in the SDH frame at a given location of the SDH frame. The frame pulse (FP) inserted by the FP INS 104 indicates a reference position in the SDH frame on the related channel line. When a switching command is externally supplied to the reception terminal 103, the FP in the SDH frame is used by the reception terminal 103 to synchronize the SDH frame on the working channel line with the SDH frame on the protection channel line.
The DIS 105 supplies the SDH frame (with the FP inserted) from the FP INS 104 to both the terminal B through the working channel line and the terminal C through the protection channel line.
In the transport terminal 102 (the terminal C), a data delay unit 106 is provided. The data delay unit 106 provides a delay for the SDH frame on the protection channel line from the terminal A.
In the reception terminal 103 (the terminal B), a fixed delay unit 107, a frame pulse detection unit (FP DET) 108, and a control unit 109 are provided. The fixed delay unit 107 provides a fixed delay for the SDH frame on the working channel line from the terminal A. The fixed delay is provided by the fixed delay unit 107 such that a total delay for the SDH frame on the working channel line due to the transmission between the terminal A and the terminal B and due to the transmission through the fixed delay unit 107 within the terminal B is always greater than a delay for the SDH frame on the protection channel line due to the transmission between the terminals A, C and B and due to the transmission through the data delay unit 106 within the terminal C.
The frame pulse detection unit (FP DET) 108 detects the frame pulse (FP) in the SDH frame on the working channel line, and separates the frame pulse (FP) from the SDH frame on the working channel line. The FP DET 108 supplies the frame pulse to the control unit 109 and outputs the reconstructed SDH frame (with no frame pulse) which is the same as the SDH frame originally sent on the working channel line from the terminal A.
Further, in the reception terminal 103, a data delay unit 110, a frame pulse detection unit (FP DET) 111, and a selector 112 are provided. The data delay unit 110 provides a variable delay for the SDH frame on the protection channel line from the terminal C. The delay provided for the SDH frame on the protection channel line by the data delay unit 110 is controlled by the control unit 109 such that a position of the FP in the SDH frame on the protection channel line matches with a position of the FP in the SDH frame on the working channel line.
The frame pulse detection unit (FP DET) 111 detects the frame pulse (FP) in the SDH frame on the protection channel line, and separates the frame pulse (FP) from the SDH frame on the protection channel line. The FP DET 111 supplies the frame pulse (FP) to the control unit 109 and outputs the reconstructed SDH frame (with no frame pulse) which is the same as the SDH frame originally sent on the protection channel line from the terminal A.
The control unit 109 receives the frame pulse (FP) from the FP DET 108 and the frame pulse (FP) from the FP DET 111, and controls the variable delay of the data delay unit 110 based on the FP from the FP DET 108 such that a position of the FP in the SDH frame on the protection channel line matches with a position of the FP in the SDH frame on the working channel line. Further, the control unit 109 controls the selector 112 in response to an externally supplied switching command, so that the selector 112 outputs a selected one of the SDH frame on the working channel line from the FP DET 108 and the SDH frame on the protection channel line from the FP DET 111.
The selector 112 outputs the selected one of the SDH frame on the working channel line from the FP DET 108 and the SDH frame on the protection channel line from the FP DET 111, under the control of the control unit 109, as the output data from the terminal B.
FIG. 23 shows an operation of the conventional transmission system of FIG. 22.
In the above-described conventional system, when a digital signal in the SDH frame format is input to the transmission terminal 101, the frame pulse insertion unit 104 inserts a frame pulse (FP) in the SDH frame. The SDH frame with the FP inserted is produced at the output of the FP INS 104.
The distribution unit 105 outputs the SDH frame (with the FP inserted) to both the reception terminal B through the working channel line and the transport terminal C through the protection channel line ("S1" in FIG. 23).
A first delay due to the transmission of the digital signal on the working channel line is provided for the SDH frame having the FP on the working channel line. The delayed SDH frame is input to the fixed delay unit 107 in the terminal B ("S2" in FIG. 23).
A second delay due to the transmission of the digital signal on the protection channel line and through the data delay unit 106 of the terminal C is provided for the SDH frame having the FP on the protection channel line. The delayed SDH frame is input to the data delay unit 110 in the terminal B ("S3" in FIG. 23).
A third delay is further provided for the SDH frame having the FP on the working channel line by the fixed delay unit 107 in the terminal B. The delayed SDH frame is output from the fixed delay unit 107 ("S4" in FIG. 23).
The third delay (or the fixed delay) is provided by the fixed delay unit 107 such that a total delay (the first delay plus the third delay) for the SDH frame on the working channel line input to the FP DET 108 is always greater than the second delay for the SDH frame on the protection channel line input to the FP DET 111.
The control unit 109 controls the variable delay of the data delay unit 110 based on the FP from the FP DET 108 such that a position of the FP in the SDH frame on the protection channel line matches with a position of the FP in the SDH frame on the working channel line. Further, the control unit 109 controls the selector 112 in response to an externally supplied switching command, so that the selector 112 outputs a selected one of the SDH frame on the working channel line from the FP DET 108 and the SDH frame on the protection channel line from the FP DET 111. The selector 112 normally outputs the SDH frame on the working channel line from the FP DET 108. When the switching command to switch the working channel line to the protection channel line is externally supplied to the control unit 109, the selector 112 outputs the SDH frame on the protection channel line from the FP DET 111.
Accordingly, when both the working channel line and the protection channel line normally function and a switching command to switch one of the channel lines to the other is externally supplied, the conventional transmission system of FIG. 22 can switch a connection from the working channel line to the protection channel line without causing instantaneous cutoff in response to the command. However, when the working channel line has failed, the switching command cannot be supplied to the control unit 109 in the reception terminal 103. Therefore, when the working channel line has failed, it is impossible for the conventional transmission system of FIG. 22 to switch a connection from the working channel line to the protection channel line while avoiding instantaneous cutoff. It is difficult for the conventional transmission system of FIG. 22 to provide reliability for data communication if the working channel line has failed.