One of the popular optical transmission systems that compensates for deterioration of the optical signal-to-noise ratio based on forward error correction (FEC) is the one recommended by ITU-T G.975. FIG. 22 is a block diagram of an FEC multiplexer at a transmitting end that employs the FEC method recommended by ITU-T G.975. FIG. 23 is a block diagram of an FEC demultiplexer at a receiving end that employs the FEC method.
As shown in FIG. 22, a first demultiplexer 211 receives and demultiplexes an STM-16 data of 2.5 Gbps into 16 parallel data stream of 156 Mbps and outputs the 16 parallel data stream to a second demultiplexer 212. The second demultiplexer 212 receives and further demultiplexes the 16 parallel data stream into 128 parallel data stream of 19 Mbps, and outputs the 128 parallel data stream to a first rate converter 213.
The first rate converter 213 receives the 128 parallel data stream of 19 Mbps and adds a redundant data area to the data stream. The first rate converter 213 then rate-converts the 128 parallel data stream of 19 Mbps into 128 parallel data stream of 21 Mbps and outputs it to an overhead inserting circuit 214. The overhead inserting circuit 214 inserts an overhead (OH) bit such as a frame synchronization data that is necessary for the maintenance and the operation of the optical system, and outputs the overhead (OH) data to an encoder 215.
The encoder 215 carries out error correction encoding by means of Reed Solomon (255,239) codes and outputs the encoded data to a first multiplexer 216. The first multiplexer 216 multiplexes the 128 parallel data stream into 16 parallel data stream of 167 Mbps and outputs the 16 parallel data stream to a second multiplexer 217. The second multiplexer 217 receives the 16 parallel data stream and further multiplexes it to a FEC frame of 2.66 Gbps. The second multiplexer 217 then converts the FEC frame of 2.66 Gbps into an optical signal via an optical transmitter and outputs the optical signal to an optical transmission channel.
The optical signal from the optical transmission channel is converted into an electrical signal by an optical receiver and output the electrical signal to a third demultiplexer 221. The third demultiplexer 221 receives and demultiplexes the FEC frame of 2.66 Gbps into 16 parallel data stream of 167 Mbps, and outputs the 16 parallel data stream of 167 Mbps to a fourth demultiplexer 222. The fourth demultiplexing 222 receives the 16 parallel data stream, further demultiplexes it into the 128 parallel data stream of 21 Mbps, and outputs the 128 parallel data stream of 21 Mbps to a frame synchronization circuit 223.
The frame synchronization circuit 223 detects the header position of the FEC frame based on the frame synchronization data included in an overhead data area and outputs frame-synchronized data to a decoder 224. The decoder 224 detects and corrects bit errors by means of the Reed Solomon (255,239) codes and outputs the corrected data to an overhead separating circuit 225.
The overhead separating circuit 225 separates the overhead data area. A second rate converter 226 removes the redundant data area in which the error correction codes are stored, converts the remaining data into the 128 parallel data stream of 19 Mbps, and outputs it to a third multiplexer 227. The third multiplexer 227 receives and multiplexes the 128 parallel data stream of 19 Mbps to the 16 parallel data stream of 156 Mbps, and outputs the 16 parallel data stream of 156 Mbps to a fourth multiplexer 228. The fourth multiplexer 228 receives and multiplexes the 16 parallel data stream of 156 Mbps and outputs it as the STM-16 data stream of 2.5 Gbps.
The FEC frame is composed of subframes 1 through 128 that include one column of the overhead bit, 238 columns of the STM-16 data, and 16 columns of a Reed Solomon (255,239) redundant data. The error correction encoding is carried out after every eighth subframe.
For instance, in the subframes 1 through 8, the error correction encoding is carried out for the overhead bit and the STM-16 data, and the Reed Solomon (255,239) redundant data is stored in columns R0-0 through R0-15. As shown in FIG. 24B, the FEC frame is generated sequentially multiplexing the subframes 1 through 128. If we assume that the Reed Solomon (255,239) codes 0 through 15 are multiplexed for ‘f’ number of times, then FIG. 24 shows an example when f=16.
The transmission rate in the FEC frame increases 15/14 times with respect to the original rate of the STM-16. Hence, the transmission rate of the FEC frame becomes 2.66 Gbps. In this way, the bit errors can be corrected by adding the FEC frame and a high quality service can be offered in an optical transmission system. Hence, an optical transmission system that facilitates transmission of high capacity of signals over long distances can be built.
When a Reed Solomon (127,111) encoding that has reduced error correction codes is carried out on the Reed Solomon (255,239) codes by, for instance, reducing the STM-16 data of the subframes from 238 columns to 110 columns, the percentage of the redundant data corresponding to the data increases and the error correction capability can be improved.
However, in the optical transmission system described above, when the transmission distance is increased or the number of wavelengths is increased using a wavelength multiplexing system, deterioration of the signal-to noise ratio is considerable. A conventional way to prevent the deterioration of the signal-to noise ratio is to reduce the error correction codes to improve the constant error correction capability. However, when the error correction codes are reduced, the percentage of the redundant data with respect to the data that is supposed to be transmitted increases, resulting in an increase in the transmission rate. Thus, an optical transmission system that maintains a certain quality and facilitates transmission of high capacity of signals over a long distance cannot be built.
For instance, in the Reed Solomon (127,111) encoding, the transmission rate of the FEC frame becomes 127/111 times to 2.89 Gbps with respect to the rate of 2.5 Gbps of the STM-16 data, resulting in more deterioration of the optical transmission characteristics. As a result, even if the code length is reduced, an optical transmission system that maintains a certain quality and facilitates transmission of high capacity of signals over a long distance cannot be built.