Optical transmission system and optical transmission device

An optical transmission system and optical transmission devices in the optical transmission system that can achieve a high quality transmission using considerably simple arrangements are disclosed. At a transmitting-end optical transmission device, encoding means having n outputs, forms k data by aligning phases of data on k channels with each other and for generating (n−k) error correction bits for said k data and adding said (n−k) error correction bits to said k data, and wavelength-multiplexing means connected to the encoding means, converts both said k data and said (n−k) error correction bits to n optical signals having different wavelengths and for wavelength-multiplexing said n optical signals so as to be delivered to the optical transmission line. At a receiving-end optical transmission device, wavelength-demultiplexing means separates the wavelength-multiplexed optical signals from the optical transmission line into n optical signals, each corresponding to one of the different wavelengths, and decoding means connected to the wavelength-multiplexing means, generates k error corrected data by correcting error bits using the (n−k) error correction bits contained in said n separated optical signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an optical transmission system that can be applicable to a long distance, high capacity transmission.

The present invention also relates to optical transmission devices, such as a transmitter and a receiver for the optical transmission system.

Such types of high capacity transmission systems using optical signals have been developed and designed so as to be adapted to multimedia applications. Many TDM (Time Division Multiplexing) transmission systems or WDM (Wavelength Division Multiplexing) transmission systems have been known. Typically, those systems have been intended to efficiently make use of a transmission line. In these high capacity transmission systems, it is particularly demanded that a reliable transmission can be achieved.

Therefore, the present invention relates to, in particular, the optical transmission system that has their transmission reliability been improved and the optical transmission device used in this system.

2. Description of the Related Art

A conventional wavelength-multiplexing transmission system includes an optical transmitter201, an optical transmission line203and an optical receiver202, as schematically shown inFIG. 1, and the system is in conformity with SDH (Synchronous Digital Hierarchy) that is a set of international, digital transmission standards. The optical transmitter201has, for each of k channels CHi(i=1, . . . , k), individually an SOH (Section Over Head) inserting unit204for inserting an SOH, an electrical-optical converter (OS)205and a wavelength-multiplexer206. The optical receiver202also has, for each of the k channels, individually a wavelength-demultiplexer207, an optical-electrical converter (OR)208and an SOH terminating unit209.

The SOH inserting unit204at the optical transmitter201inserts the SOH into an electrical signal for one of the corresponding channels CHi. Each electrical signal for the every channel is then provided to the optical-electrical converter205and converted to an optical signal with a wavelength λicorresponding to the channel CHi. The optical signals having the wavelength of λiare multiplexed by the wavelength-multiplexer206and resulting wavelength-multiplexed signals are transmitted to the optical transmission line203.

The wavelength demultiplexer207at the optical receiver202separates the multiplexed signals received from the optical transmitter201through the optical transmission line203into the signals corresponding to the wavelengths λ1to λk, respectively. These optical signals having the wavelength of λ1to λk, respectively, are converted to corresponding electrical signals by the optical-electrical converter208, and then the SOH of the electrical signals is terminated by the SOH terminating unit209. The electrical signals having their SOH terminated are transmitted to a further stage (not shown inFIG. 1) on an each i.e., (individual) channel basis. Thus, the data comprising the electrical signals for each of the channels CH1to CHkcan be transmitted from the optical transmitter201to the optical receiver202over the signal optical transmission line203.

Several error correction techniques have been also proposed in order to improve a transmission quality by correcting transmission errors involved in the transmitted data. For example, one of the known techniques, also called an “FEC (Forward Error Correction)” method, consists in generating and adding an error correction bit to the data representing one frame or the data of a predetermined length and performing the error correction at a receiver side.

Adding a parity bit to the transmitted data is also a common technique used for determining a presence/absence of the transmission error within the transmitted data. In this case, the SOH may be also provided with error monitoring bits, named B1and B2.

The earlier described error correction techniques consist in, for every frame or every block of the transmission data, generating an error correction bit and adding it to each frame or block. Therefore, in contrast with a transmission system without correcting transmission errors, the conventional transmission system provided with the error correction technique has to increase a transmission rate, because a number of bits to be transmitted are increased. Alternatively, if the transmission rate is set to a predetermined value, the transmission system should reduce an amount of the transmission data so that the error correction bit can be transmitted together with the transmission data within the predetermined transmission rata.

Furthermore, in some of the conventional transmission systems, erroneous bits included in the transmission data cannot be corrected when parity bits are contained in the data. One solution for improving a capability of correcting the erroneous bits in the data is to increase the number of the error correction bits added to the transmission data. However, this solution may be not practical, because a considerably high transmission rate is required for increasing the number of error correcting redundant bits to be added to the transmission data.

Another possible solution is to insert the error correction bits into reserved bits within the SOH. The reserved bits means that those bits are reserved for a variety of future applications. In this case, since a lot of redundant bits are to be inserted into some particular locations in the SOH, a problem may occur that a size of a circuit comprising a transmission device, such as the transmitter201and the receiver202, is enlarged. This solution has a further drawback in that the error correction bits, which have been already assigned to the reserved bits, cannot be made use of, if the reserved bits are decided to be used for one of the future appiications.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an optical transmission system for allowing a high capacity and high quality transmission and which can be easily and simply manufactured or implemented.

Another object of the present invention is to provide an optical transmitter and an optical receiver suitable for used in the optical transmission system according to the present invention.

The object of the present invention is achieved by an optical transmission system which is operable to form a set of k data by aligning each of phases from k channels in phase, generate and add a set of (n−k) error correction bits to the set of k data so as to produce n data in total, convert the n data into different signals having different wavelengths λ1to λn, respectively, by an electrical-optical converter, multiplex these signals by an wavelength multiplexer, and send the multiplexed signals to an optical transmission line.

The inventive optical transmission system further operable to receive the multiplexed signals through the optical transmission line, separate the received multiplexed signals into signals having different wavelengths λ1to λn, respectively, by a wavelength demultiplexer, converts the signals having the different wavelengths λ1to λn, respectively, to electrical signals by an optical-electrical converter, and correct errors within the k data by means of the (n−k) error correction bits contained in the n data.

In the optical transmission system according to the present invention, the k data concurrently transmitted are added to in parallel by the (n−k) error correction bits. Then, the k data being added to by the error correction bits are converted to optical signals having the different wavelengths λ1to λn, respectively, so as to be transmitted as the wavelength-multiplexed optical signals. This allows the optical transmission system to correct the errors at a receiver and transmit data with the high quality without increasing the transmission rate. In addition, since the error correction bits are generated for the k data at the same timing, an error correction decoding process has to be performed at the receiver. To do this, a frame synchronous byte may be added to each of the k data, each data containing the error correction bits.

The object of the present invention can be achieved by a further optical transmission system which is operable to form a set of k data by aligning data from k channel CH1to CHkwith each other, add an SOH containing error monitoring bytes B1and B2to the set of k data, generate and add a parity bit to the set of the k data so as to form a sequence of (k+1) data, convert the sequence of the (k+1) data to optical signals having different wavelengths, wavelength-multiplex the optical signals and send the multiplexed signals to an optical transmission line. At the receiver, after receiving the multiplexed signals through the optical transmission line, the received multiplexed signals are separated into signals corresponding to wavelengths, respectively, and then the separated signals are converted to electrical signals. Subsequently, a parity check is performed on the basis of a sequence of (k+1) data from the electrical signals, and another parity check corresponding to channels CH1to CHkis carried out by means of an error check byte within the SOH. In this case, depending on results of the parity checks, a position of the error bit is located and the error bit may be corrected. Thus, the error correction can be achieved solely by additionally assigning the parity bit corresponding to a vertical parity to the data.

The object of the present invention can be achieved by a still further optical transmission system for serially transmitting data such as TDM (Time-Division Multiplexing) transmission data. The optical transmission system is operable to generate and add a set of (n−k) error correction bits to k bits of the transmission data, convert the k bits of the transmission data and the (n−k) error correction bits to different optical signals having different wavelengths, and the optical signals are multiplexed so as to be transmitted as wavelength-multiplexed signals through an optical transmission line. The optical transmission system further receives the wavelength-multiplexed signals through the optical transmission line, separates the received multiplexed signals into different signals having corresponding wavelengths, and perform error correction decoding process for the n bits corresponding to the transmission data by means of the (n−k) error correction bits. That is to say, in this case, for a serial data of the k bits, the (n−k) error correction bits are converted into the signals having the different wavelengths such that the serial data of k bits can be transmitted in parallel.

The object of the present invention can be achieved by a still further optical transmission system for transmitting data through k channels. The optical transmission system, at a transmitter, generates and adds (n−k) error correction bits to k data present at the same timing so as to form a sequence of n data, multiplexes the sequence of the n data, converts the multiplexed data to optical signals and sends the optical signals to an optical line. The optical transmission system, at a receiver, receives the optical signals through the optical line, converts the received optical signals to electrical signals, separates the electrical signals into a sequence of n data and performs an error correction decoding on the k data from the sequence of the n data by means of the (n−k) error correction bits. In this case, if a plurality of reserved channels exist in the channels available in a TDM (Time Division Multiplexing) transmission, the reserved channels can be assigned to the error correction bits. Alternatively, if a number of the error correction bits assigned to busy channels is more than that of the reserved channels, an error correction coding is performed only on some significant channels from the busy channels. It leads to that the number of the error correction bits within an error correction code is limited by the number of the reserved channels and the data transmitted through the significant channels can be transmitted using the error correction coding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, principles and embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 2shows a first embodiment of an optical transmission system according to the present invention. As shown inFIG. 2, the optical transmission system comprises an optical transmission device1provided at a transmitting side, an optical transmission device2provided at a receiving side and an optical transmission line connecting the optical transmission device1and the optical transmission device2. Hereinafter, the optical transmission device1provided at the transmitting side is assumed to be a transmitting-end station and the optical transmission device2provided at the receiving side is a receiving-end station.

The optical transmission system further comprises an SOH inserting unit4, an encoder5, a phase alignment unit6, an electrical-optical converter (OS)7, a wavelength-multiplexing unit8, a wavelength-demultiplexing unit11, an optical-electrical converter (OR)12, a decoder13and an SOH terminating unit. These elements are included in either the transmitting-end station1or the receiving-end station2, as described hereinafter. In this embodiment, transmission data are transmitted using both of SDH (Synchronous Data Hierarchy) and WDM (Wavelength Division Multiplexing) methods.

The transmitting-end station1includes the SOH inserting unit4that adds an individual SOH (Section Over Head) to each transmission data coming from k channels CH1to CHkand the encoder5for error correction coding. For example, the encoder5may be such that the encoder5performs an operation of (n, k) Hamming coding. In this embodiment, the encoder5generates (n−k) error correction bits for k bits corresponding to k data present at the same timing from the channels CH1to CHk.

The transmitting-end station1further includes the phase alignment unit6that is connected to the encoder5and which together performs a data generating function. The encoder5provides a signal of all n bits comprising the k data from the channels and the generated (n−k) error correction bits to the phase alignment unit6. The phase alignment unit6compensates for a delay due to the error correction coding so as to phase all the n bits. The phase alignment unit6may be, for example, a delay circuit capable of aligning a delay time appropriately. The signals comprising the n bits in phase are then passed to the electrical-optical converter7, which is also included in the transmitting-end station1. The electrical-optical converter7converts the electrical signals of the n bit into optical signals having wavelengths λ1, to λn, respectively.

The transmitting-end station1has the wavelength-multiplexing unit8, which is provided by the optical signals from the electrical-optical converter7, for wavelength-multiplexing the optical signals and delivering the multiplexed signals to the optical line3.

The wavelength-multiplexing unit8may be constructed by a wavelength-combination unit for multiplexing the optical signals having the wavelengths λ1to λntogether. It should be noted that the error correction bits are transmitted with the wavelengths different from those of the signals corresponding to the data from the channels CH1to CHk. This leads to the fact that the transmission rate for the data coming from the channels CH1to CHkis not adversely affected by the addition of the error correction bits. It should be also noted that reserved bits within the SOH are not used to transmit the error correction bits so as to overcome the problem caused by the conventional error correction method. Therefore, according to the first embodiment of the present invention, it is advantageous that an error correcting coding scheme can be easily added to various existing WDM transmission systems.

The receiving-end station2comprises the wavelength-demultiplexing unit11for receiving the wavelength-multiplexed signals from the transmitting-end station1through the optical line3and separating the received wavelength-multiplexed signals into n optical signals having wavelengths λ1to μn, respectively. The receiving-end station2further comprises the optical-electrical converter12being connected to the wavelength-demultiplexing unit11and for converting the n optical signals input from the wavelength-demultiplexing unit11to corresponding electrical signals. The receiving-end station2has the decoder13, which is connected to the optical-electrical converter11and receives and decodes the electrical signals.

The electrical signals received by the decoder13are formed by k bits, each corresponding to one of the channels CH1to CHk, and (n−k) error correction bits. Then the decoder13performs a data regenerating function, which includes error correction decoding by means of the k bits representing the data from the channels CH1to CHkand the (n−k) error correction bits and sends the decoded signals to the SOH termination unit14, which is also included in the receiving-end station2. The SOH termination unit14terminates the SOHs and delivers the signals with the SOHs to a succeeding device (not shown inFIG. 2) as data representing the data coming from the channels OH1to CHk.

In this embodiment, the transmission data on the channels CH1to CHkmay be formed in accordance with various transmission schemes such as a frame transmission scheme or an ATM (Asynchronous Transfer Mode) cell scheme. Since the phase alignment unit6is provided in the transmitting-end station1, it is sufficient that the SOH inserting unit4adds the SOH to each of the data corresponding to the channels CH1to CHk.

FIG. 3is a schematic diagram of an encoder for (n, k) Hamming coding according to the first embodiment of the present invention. Since Hamming coding is a well-known coding scheme, for a detail explanation thereof, it is recommended that a general textbook concerning “communications” should be referred to. Briefly speaking, for any positive integer m, there is a Hamming code with parameters k=2m−m−1 and n=2m−1. For example, we consider a k=4, n=7 Hamming code. This means that the number of the channels is k=4 and the number of the error correction bits is (n−k)=3. The encoder, illustrated inFIG. 3, performs (7, 4) Hamming coding. The encoder includes exclusive-OR logic circuits15-1,15-2and15-3. The data corresponding to the channels CH1to CH4are denoted as D1to D4, respectively, and the error correction bits generated by the encoder are denoted as D5to D7, as shown inFIG. 3. Thus, the encoder generates three error correction bits D5to D7for the four data D1to D4. In this case, a generator matrix G for this coding scheme is represented as:

G=(1000101010011100101100001011)
Then this coding scheme can perform one bit error correction with a minimum intersymbol distance of 3.

FIG. 4is a schematic diagram of a decoder according to the first embodiment of the present invention. The decoder is constructed to be a (7, 4) Hamming code decoder such that its decoding scheme is adapted to the encoder shown inFIG. 3. The decoder includes exclusive-OR logic circuits16-1,16-2and16-3, inverters17-1,17-2and17-3, and AND logic circuits18-1,18-2,18-3and18-4. In this case, a check matrix for the decoder is represented as:

Referring again toFIG. 3, an operation of the encoder5of the transmitting-end station1is explained. It is assumed that all the data D1to D4have value “1”. After the (7, 4) Hamming coding, since the exclusive-OR circuits15-1,15-2and15-3have the same behavior as that of a modulo-2 adder, the error correction bits D5to D7become “1” at the same timing. Thus, the data D1to D7output by the encoder5are represented as “1111111.”

With reference toFIG. 4, the decoder13of the receiving-end station2is operable to make output signals from the exclusive-OR circuits16-1,16-2and16-3to have a value of “0” in response to the received signals D1to D7representative of “1111111.” In this case, the AND circuits18-1to18-4are operable to produce an output signal having a value of “0”, and the exclusive-OR circuits19-1to19-4are operable to allow the data D1to D4, respectively, to pass through the corresponding exclusive-OR circuits. Therefore, the data D1to D4provided to the decoder13are directly output from the decoder13and include no errors.

Now, we consider that an error has occurred with one bit such that a true data D2=“1” is replaced with an erroneous data D2=“0” for the channel CH2. In this case, each of the exclusive-OR circuits16-1,16-2and16-3outputs a signal having a value of “1”, and each of the AND circuits18-1,18-3and18-4generates a signal having a value of “0.” However, the AND circuit18-2will output a signal “1.” Then, the exclusive-OR circuit19-2receives a value of “0” on a line D2and a value of “1” from the AND circuit18-2and generate “1” as a result of an exclusive-OR operation between “0” and “1”. Thus, the data on the line D2is inverted in the decoder13and the erroneous data D2can be corrected. Other data D1, D3and D4having the true value and input to the decoder13are directly output from the decoder13.

A second embodiment of an optical transmission system according to the present invention is shown inFIG. 5. As shown inFIG. 5, the optical transmission system comprises a transmitting-end station21, a receiving-end station22, an optical transmission line connecting the transmitting-end station21and the receiving-end station22. The transmitting-end station21includes an SOH inserting unit24, a parity generator25, a phase alignment unit26, an electrical-optical converter (OS)27and a wavelength-multiplexing unit28. The receiving-end station22includes a wavelength-demultiplexing unit31, an optical-electrical converter (OR)32, a parity detector33, an SOH terminating unit34and an error correction unit. InFIG. 5, the encoding unit and its function, as inFIG. 2, are incorporated in the parity generator25and the decodinci unit and its function, as inFIG. 2, are incorporated in the parity detector33.

At the transmitting-end station24, the SOH inserting unit24adds an individual SOH (Section Over Head) to each transmission data coming from k channels CH1to CHkand supplies the k transmission data with the individual SOH to the parity generator25. The parity generator25calculates a parity bit for the supplied k transmission data and outputs the calculated parity bit together with the k transmission data, and thus, passing (k+1) data to the phase alignment unit26. The phase alignment unit26compensates for a delay caused by the parity generator25and sends resulting in-phase (k+1) data to the electrical-optical converter27. The electrical-optical converter27converts the in-phase (k+1) data to (k+1) optical signals having different wavelengths λ1to λk+1and passes the optical signals to the wavelength-multiplexing unit28. The wavelength-multiplexing unit28multiplexes the (k+1) optical signals and sends the multiplexed signals to the optical transmission line23. In this case, the parity bit calculated for the k transmission data on the channels CH1to CHkcorresponds to the vertical parity.

At the receiving-end station22, the wavelength-demultiplexing unit31receives the wavelength-multiplexed signals through the optical transmission line23and separates them into the optical signals having the different wavelengths λ1to λk+1. The optical signals corresponding to the wavelengths λ1to λk+1are passed to the optical-electrical converter32and then converted to (k+1) electrical signals in the optical-electrical converter32. Then, the (k+1) electrical signals are transferred from the optical-electrical converter32to the parity detector33.

The parity detector33performs not only one parity check on the data conveyed on the channels CH1to CHkby detecting the parity bit added to the transmission data at the parity generator25, but also the other parity check on a frame corresponding to the channels CH1to CHkby detecting error monitoring bytes B1and B2within the SOH. As a result of the parity checks, the parity detector33determines an error position signal identifying a position of an erroneous bit and sends the error position signal to the error correction unit35.

The SOH terminating unit34terminates the SOHs of the k data conveyed on the k channels CH1to CHkand sends the k transmission data to the error correction unit35. Then the error correction unit35performs the error correction on the transmission data and delivers the error corrected transmission data to a further device (not shown inFIG. 5).

FIG. 6is a diagram for explaining a configuration of an STM-1 (Synchronous Transfer Module Level One) frame. This STM-1 frame is used for the SDH and its bit rate is 155.52 Mbps. The STM-1 signals are formed by multiplexing three STM-0 signals having the bit rate of 51.84 Mbps. For example, STM-4 signals formed by multiplexing the four STM-1 signals have a bit rate of 622.08 Mbps. The SOH within the STM-1 signal is represented as a 9 row by 9 column structure as depicted inFIG. 6, and a payload is represented as a 9 row by 261 column structure.

The SOH contains frame synchronous bytes A1and A2, a STM identifier byte C1, a monitoring byte B1, also called BIP-8 (Bit Interleaved Parity 8), an orderwire byte E1for voice communications, a user byte F1for specifying a failure, data link bytes D1, D2and D3for DCC (Data Communication Channel), an AU pointer bytes H1, H2and H3, a monitoring byte B2, also called a BIP-Nx24 (Bit Interleaved Parity Nx24) byte, control bytes K1and K2for APS (Automatic Protection Switch), data link bytes D4to D12for data communication channels, reserved bytes Z1and Z2, and an orderwire byte E2for voice communications. Blank bytes and bytes marked with X are reserved or undefined bytes.

The error monitoring byte B1for a current frame is derived from the previous frame by calculating parity bits for each bit position over the entire bytes. The parity detector33in the receiving-end station22, as shown inFIG. 5, can perform parity check for each bit position by calculating the parity bits for every bit position over the entire bytes in the current frame and referring to the B1byte of the SOH in the next frame.

The error monitoring byte B2represents a bit interleaved parity-Nx24, where N is a level number for the STM-N. The byte B2is derived from the previous frame by grouping BIP-Nx24s from the previous frame into a block and calculating parity bits for each bit position over the entire bytes in the block. Assuming N is equal to 1, as shown inFIG. 6, the parity bits for the 24 bits are calculated and added to the frame in accordance with a bit order, and thus resulting in 3 bytes as the parity bits. Any suitable bit generation and addition schemes, as well as correspondingly suitable bit detection schemes can be applied to the scheme for generating, adding and detecting the error monitoring bits B1and B2.

The parity detector33in the receiving-end station22is operable to perform parity check on each bit position within the received frame by means of either or both of the error monitoring bits B1and B2. In other words, the parity detector33can perform horizontal parity check. At the same time, the parity detector33performs another parity check, i.e. vertical parity check, by means of the (k+1) data comprising the k data corresponding to the channels CH1to CHkfollowed by the parity bits.

As the parity detector33detects a vertical parity error in the current frames, it is appreciated that some of the currently received frames corresponding to the channels CH1to CHkcontain one bit error. The parity detector33also detects a horizontal parity error in order to identify the channel and the bit position where the parity error occurs. Thus, the parity detector33can determine the position of the error bit on the basis of the channel and the bit position of the parity error.

The position of the error bit is determined as follows. For example, a bit sequence comprising bits1to8contained in the payload, as depicted inFIG. 6, appears repeatedly at times t1, t2, t3and so on. Then, one parity check using the error monitoring bit B1is performed so as to detect the parity error at a position of the bit3in the bit sequence and the other parity check is also performed using the bit (k+1) representing the parity bit following the transmission data applied to the parity detector33at the time t11in order to detect the parity error. As a result of the parity checks, it is determined that a transmission error has occurred at a bit position, as illustrated as a shaded box inFIG. 6. Subsequently, a signal representing the bit error occurring position is sent to the error correction unit35for correcting the bit with a transmission error. And thus, the error bit can be corrected. As described above, the bit error occurring position can be determined by a combination of the vertical parity check and the horizontal parity check, the erroneous bit can be easily corrected by holding the k data conveyed on the k channels CH1to CHkuntil the bit error occurring position is detected in the SOH terminating unit34or the error correction unit35.

In the above-mentioned first embodiment of the present invention, when the (7, 4) Hamming coding is employed, redundant bits comprising three bits have to be added to the transmission data by performing the error correction coding. On the contrary, in the second embodiment of the present invention, as shown inFIG. 4, it is sufficient that only a single additional bit is added, as a parity bit, to the transmission data. This means that a combination of the parity generator25and the parity detector33, as shown inFIG. 4, may be much simply constructed than that of the error correction encoder5and the error correction decoder13, as shown inFIG. 2. Thus, a high quality transmission can be achieved by the optical transmission system according to the second embodiment of the present invention without making structures of the transmitting-end station21and the receiving-end station22. Advantageously, according to the second embodiment of the present invention, data rates for primary data on the channels CH1to CHkare not adversely affected by the parity bit. This is because the parity bit is transmitted over the optical transmission line23after being converted to a wavelength other than those of the primary data.

Furthermore, in one variation of the second embodiment of the present invention, a further parity check can be implemented in addition to the parity check using the monitoring bits B1and B2. In this case, at the transmitting-end, one frame is divided into a plurality of blocks, a parity bit is calculated for each block, and the calculated parity bits are transmitted in reserved bits within the SOH. Then, at the receiving-end, the parity bits corresponding to the blocks are received, and the further parity check is performed based on the received parity bits for the blocks. In other words, a small number of parity bits, this number being equal to a number of the blocks, are added to the SOH at the reserved bits thereof, so that the number of the bits capable of being corrected can be increased.

FIG. 7is a schematic diagram of an optical transmission system according to a third embodiment of the present invention. The optical transmission system comprises a transmitting-end station41, a receiving-end station42and an optical transmission line43for connecting the transmitting-end station41and the receiving-end station42.

The transmitting-end station41includes a frame generating and SOH inserting unit44, an encoder45, an electrical-optical converter (OS)46and a wavelength-multiplexing unit47. The receiving-end station42includes a wavelength-demultiplexing unit51, an optical-electrical converter (OR)52, a memory unit53, a decoder54, an SOH terminating unit55and a top-of-frame (“TOF”) detector56.

At the transmitting-end station41, the frame generating and SOH inserting unit44generates a frame for each of channels CH1to CHk, such that tops from every frames are synchronous with others, inserts a frame number for each of the channels CH1to CHkinto a reserved byte within an SOH, and sends the frames to the encoder45. The encoder45, for example, performing a (n, k) Hamming coding, may accomplish an error correction coding on every bits other than k frame synchronization bytes within the SOH over the frame and add the frame synchronization byte for a sequence of (n−k) error correction bits.

In other words, the frame generating and SOH inserting unit44adds the frame synchronization byte for the n channels and sends control information including the frame synchronization byte to the encoder45. Then, the encoder45generates the (n−k) error correction bits for the k data on the k channels, respectively, without taking the frame synchronization bytes into account in response to the control information received from the unit44and adds the generated (n−k) error correction bits to the k data. Thus, the n data, n is calculated from an equation:
n=(n−k)+k
are generated by the encoder45. This means that the encoder45generates k sequences of the transmission data for the k channels and (n−k) sequences of the error correction bits. The encoder45also adds the frame synchronization bit to the (n−k) sequences of the error correction bits. Therefore, all of the n sequences are individually added to by the frame synchronization byte.

The electrical-optical converter46converts the n sequences received from the encoder45into n optical signals having different wavelengths λ1to λn, and then sends the optical signals to the wavelength-multiplexing unit47. The wavelength-multiplexing unit47multiplexes the n optical signals and sends the multiplexed signals to the receiving-end station42over the optical transmission line43.

At the receiving-end station42, the wavelength-demultiplexing unit51receives the multiplexed signals through the optical transmission line43, and then separates the multiplexed optical signals into optical signals having the different wavelengths λ1to λn. The separated optical signals are applied to the optical-electrical converter52, where the separated optical signals are converted to electrical signals. The optical-electrical converter52stores the electrical signals to the memory unit53and supplies the electrical signals to the top-of-frame detector56. Then, the top-of-frame detector56determines a top of every frame of the n sequences by detecting each frame synchronization byte for the n sequences. Then the top-of-frame detector56reads the n sequences from the memory unit53by controlling a read-out timing so as to align the top of the frame for each of the n sequences with the others, and sends the read out n sequences to the decoder54.

However, in a long distance transmission environment, even if the wavelength-multiplexed signals are sent from the transmitting-end station41via the optical transmission line43, the optical signals having the different wavelengths may be usually received by the receiving-end station42at different timings. This is because the fiber may have different transmission rates for the n sequences having the different wavelengths. According to the third embodiment of the present invention, advantageously, this problem is overcome by controlling the read out timing for the n sequences stored in the memory unit53in order to allow the top of each of the n sequences to be synchronous with the others, and thus keeping the n sequences in phase. It is noted that the memory unit53may be formed by an FIFO buffer suitable for controlling the read out timing.

The decoder54is designed so as to be reversibly operable with the encoder45such that the decoder54performs an error correction decoding which is adapted to the error correction coding implemented by the encoder45. The decoder54can correct erroneous data present at the channels CH1to CHkand send the error corrected k data to the SOH terminating unit55. The SOH terminating unit55terminates the SOHs of the k data and sends the k data to a further processing stage.

In the third embodiment of the present invention, the frame generating and SOH inserting unit44in the transmitting-end station41is followed by the encoder45. Alternatively, the frame generating and SOH inserting unit44may be divided into a frame generating unit and an SOH inserting unit such that the frame generating unit is followed by the encoder45which is followed by the SOH inserting unit. In this case, at the receiving-end station42, the top-of-frame detector can detect the tops of the frames and keep the tops of the frames in phase, and then, the decoder54can perform the error correction decoding.

As earlier described, the optical transmission line43has different transmission rates of the optical signals depending on the wavelengths of the optical signal. For example, in a high speed, long distance transmission with 100 Gbps, the phases of the optical signals that are demultiplexed by the wavelength-demultiplexing unit51may deviate from the others. In order to avoid this deviation of the phases, it is ensured that the encoder54can perform the error correction decoding on the high speed transmission data by aligning the frames in phase. In this case, the in-phase frames can be achieved from the memory unit53by detecting the tops of the frames on the basis of frame synchronization bytes added to the frames in order to keep the frames in phase at the transmitting-end station41.

FIG. 8is a schematic diagram of an optical transmission system according to a fourth embodiment of the present invention. The optical transmission system comprises a transmitting-end station61and a receiving-end station62and is provided with optical relay transmission devices63and64arranged between the transmitting-end station61and the receiving-end station62. The optical transmission system further includes a first optical transmission line65for connecting the transmitting-end station61and the optical relay transmission device63, a second optical transmission line66for connecting the optical relay transmission device63and the optical relay transmission device66, and a third optical transmission line67for connecting the optical relay transmission device64and the receiving-end station62. In the optical transmission system, a high quality transmission of wavelength-multiplexed signals can be achieved between the optical relay transmission devices63and64via the second optical transmission line66. As shown inFIG. 8, the optical relay transmission device63includes a wavelength-demultiplexing unit71, an encoder72, a wavelength converter73, a phase alignment unit74and a wavelength-multiplexing unit75. The optical relay transmission device64includes a wavelength-demultiplexing unit76, a decoder77and a wavelength-multiplexing unit78.

The optical relay transmission device63may, for example, be operable to perform an error correction coding and a wavelength multiplexing in order to function in the same manner as the transmitting-end station1shown inFIG. 2. The optical relay transmission device64may also be operable to perform a wavelength demultiplexing and an error correction decoding in order to function in the same manner as the receiving-end station2inFIG. 2.

The wavelength-demultiplexing unit71separates the received signals into k optical signals having wavelengths λ1to λkand supplies the k optical signals to the encoder72for performing the error correction coding.

The encoder72may be formed by an optical logic circuit and perform the error correction coding with respect to the optical signals without converting the optical signals to electrical signals. Assuming that a (n, k) Hamilton code is applied to the encoder72, then the encoder72generates (n−k) error correction bits. The wavelength converter73converts the wavelengths for the (n−k) error correction bits generated and received from the encoder72to wavelengths λk+1to λnother than the wavelengths λ1to λkof the k optical signals. Then, the phase alignment unit74receives the k optical signals having the wavelengths λ1to λkand corresponding to the channels CH1to CHkfrom the encoder72and (n−k) error correction bits having the wavelengths λk+1to λn. Thus, the phase alignment unit74receives n optical signals in total.

The phase alignment unit74may be also formed by an optical logic circuit. The phase alignment unit74compensates for a delay due to a processing of the encoder72, matches the phase of each of the n optical signals with the others, and sends the n optical signals in phase to the wavelength-multiplexing unit75. The wavelength-multiplexing unit75multiplexes the n optical signals and delivers the multiplexed optical signals to the optical transmission line66.

Alternatively, the optical relay transmission device63may be arranged so as to convert the k optical signals separated by the wavelength-demultiplexing unit71to electrical signals. In this case, the optical relay transmission device63performs an error correction coding in the same manner as described in the first embodiment, as shown inFIG. 2, and converts the k coded electrical signals along the (n−k) generated error correction bits to n optical signals by an electrical-optical converter. In this configuration, the wavelength converter73can be dispensed with and the electrical-optical converter are provided at a succeeding stage to the phase alignment unit74, such that the n electrical signals are converted to the n optical signals having the wavelengths λ1to λnand supplies the n optical signals to the wavelength-multiplexing unit75.

The decoder77in the optical relay transmission device64may be formed by an optical logic circuit. In this case, the n optical signals, having the wavelengths λ1to λn, separated by the wavelength-demultiplexing unit76are directly applied to the decoder77, where the error correction decoding is performed on the n optical signals. Then, the k decoded optical signals having the wavelengths λ1to λkare provided to the wavelength-multiplexing unit78for multiplexing the k optical signals so as to be delivered to the receiving-end station62via the optical transmission line67. As a result, the optical signals input to the optical relay transmission device64can be processed in the optical relay transmission device64and then be delivered to the optical transmission line67without being converted to the electrical signals. Alternatively, the optical relay transmission device64may be provided with an optical fiber amplifier following the wavelength-multiplexing unit78for intermediately amplifying the multiplexed signals.

Alternatively, the optical relay transmission device64may be arranged so as to convert the n optical signals separated by the wavelength-demultiplexing unit76to n electrical signals. In this case, the optical relay transmission device64performs an error correction decoding in the same manner as described in the first embodiment and shown inFIG. 2, and converts the k decoded electrical signals to n optical signals corresponding to the k wavelengths for the k channels, respectively, by an electrical-optical converter. Then, the k decoded optical signals having the wavelengths λ1to λkcan be provided to the wavelength-multiplexing unit78for multiplexing the k optical signals so as to be delivered to the receiving-end station62via the optical transmission line67.

It can be easily appreciated that the transmission implemented between the optical relay transmission device63and the optical relay transmission device64can be applied to the transmission between the transmitting-end station61and the optical relay transmission device63or the optical relay transmission device64and the receiving-end station62. This means that the transmission is implemented using wavelength-multiplexed optical signals with error correction bits which are generated at the transmitting-end station61or the optical relay transmission device64, respectively. In this case, the optical relay transmission device63should be modified to include a decoder and the optical relay transmission device64should be modified to include an encoder. The optical relay transmission devices63and64are operable so as to receive the multiplexed optical signals, perform the error correction decoding on the received multiplexed optical signals, determine whether the error has been detected or not. If the error has been detected, the optical relay transmission devices63and64perform the error correction coding in order to allow the error to be corrected and multiplex the error corrected optical signals. Otherwise, the optical relay transmission devices63and64directly pass the received multiplexed optical signals to the following optical line.

FIG. 9is a schematic diagram of an optical transmission system according to a fifth embodiment of the present invention. The optical transmission system implements a TDM (Time Division Multiplex) transmission. The optical transmission system comprises a transmitting-end station81, a receiving-end station82and an optical transmission line for connecting the transmitting-end station81and the receiving-end station82, as shown inFIG. 9.

We consider that a number of bits assigned to a time slot used for TDM equals to k. In this case, the encoder84implements a (n, k) Hamming coding and generates (n−k) error correction bits. It is noted that the time slot or a frame is a non-limiting example of the data to be error correction coded. For example, the error correction coding may be applied to a series of data by dividing the series of the data into any blocks containing k bits and performing the (n, k) Hamming coding.

The phase alignment unit85is operable so as to match a top of the (n−k) error correction bits with the top of k bits representing primary signals. To do this, the phase alignment unit85aligns the phases of the (n−k) error bits. The primary signals having the k bits are converted to an optical signal having a wavelength λ1by the electrical-optical converter86and the phase aligned (n−k) error bits are converted to an optical signal with a wavelength λ2by the electrical-optical converter87. Then the wavelength-multiplexing unit88multiplexes the optical signal with the wavelength λ1and another optical signal with wavelength λ2and delivers the multiplexed signals to the receiving-end station82via the optical transmission line83. Thus, the (n−k) redundant bits representing the error correction bits can be transferred with out affecting a transmission rate for the k bits representing the primary signal.

Assuming that the encoder84performs the (7, 4) Hamming coding, as shown inFIG. 3, a series of four bits, D1to D4, are input to a shift register. When the four bits D1to D4are stored in the shift register at the same time, redundant three bits D5to D7are concurrently calculated by means of a parallel output comprising the four bits D1to D4. Then the four bits D1to D4are applied to the electrical-optical converter86in series and the redundant three bits D5to D7are applied to the electrical-optical converter87in series. Alternatively, the phase alignment unit85may be dispensed with by appropriately controlling a timing for outputting the four serial bits D1to D4and the redundant three serial bits D5to D7.

Though the phase alignment unit85, as shown inFIG. 9, is provided between the encoder84and the electrical-optical converter87, the phase alignment unit85may be provided between the encoder84and the electrical-optical converter86such that a difference between processing times required for the primary signal having the k bits and the (n−k) redundant bits. Typically, the above-mentioned numeric numbers k and n are selected such that the relation between k and (n−k) is written as:
k>(n−k).
Preferably, in order to facilitate a frequency division of clock signals, these two numerical numbers are selected such that k equals to a multiple of (n−k).

At the receiving-end station82, the wavelength-demultiplexing unit91separates the multiplexed optical signals into optical signals having wavelengths λ1and λ2and passes the separated two optical signals to the optical-electrical converters92and93, where the optical signals are converted to k-bits and (n−k)-bits electrical signals. The decoder94receives the k bits representing the primary signal from the optical-electrical converter92as well as the (n−k) bits representing the redundant bits, and performs an error correction decoding. Therefore, this optical transmission system can automatically correct error bits due to transmission errors. Assuming that this decoder94employs the (7, 4) Hamming code, a decoding scheme, as shown inFIG. 4, can be applied to the decoder94. This means that the decoder94receives in parallel the data D1to D7that are serially input to the receiving-end station82, correct any errors included the data D1to D4, convert the error corrected data D1to D4into a parallel format, and outputs the data D1to D4in parallel.

It should be noted that, according to the fifth embodiment of the present invention, a transmission rate for the primary signal could be maintained at the transmission rate achieved when no error correction bits are added. Therefore, advantageously, the existing optical transmission system can be improved with respect to a transmission quality by providing the encoder84and the wavelength-multiplexing unit88at the transmitting-end station as well as the wavelength-demultiplexing unit91and the decoder94for error correction decoding at the receiving-end station. Furthermore, the primary signal may not be limited to TDM signals, but may be extended to, for example, STM-N signals for one channel, as earlier described, by wavelength-multiplexing and transferring error correction bits with different wavelengths. Also, in this case, since the error correction bits do not occupy reserved bytes within an SOH, a high quality transmission can be achieved using this error correction-coding scheme.

FIG. 10is a schematic diagram of an optical transmission system according to a sixth embodiment of the present invention. The optical transmission system comprises a transmitting-end station101, a receiving-end station102and an optical transmission line connecting the transmitting-end station101and the receiving-end station102.

The transmitting-end station101includes an encoder104, a multiplexing and frame generating unit105and an electrical-optical converter (OS)106.

The receiving-end station102includes an optical-electrical converter (OR)107, an SOH terminating and separating unit108and a decoder109.

We consider that an n-multiplexing TDM transmission apparatus is operable so as to perform a time-division multiplexing with data for k channels CH1to CHk. When the (n−k) channels CHk+1to CHnare reserved, the encoder104at the transmitting-end station101performs a coding such as a (n, k) Hamming coding so as to generate (n−k) error correction bits for the data conveyed on the k channels CH1to CHk. Then, the multiplexing and frame generating unit105time-division multiplexes the complete data for the n channels while the multiplexing and frame generating unit105adds frame synchronization patterns to the multiplexed data in order to produce frames. The multiplexed data formed by the frames are transferred to the electrical-optical converter106where the multiplexed data with the frame synchronization patterns are converted to optical signals so as to be transmitted to the receiving-end station102through the optical transmission line103.

At the receiving-end station107, the optical-electrical converter107converts the received multiplexed optical signals to electrical signals and sends the electrical signals to the SOH terminating and separating unit108. The SOH terminating and separating unit108detects the frame synchronization patterns, makes the frame to be synchronous with the others and separates the time-division multiplexed signals. The n separated signals corresponding to the n channels CH1to CHnare supplied to the decoder109. The decoder109performs an error correction decoding based on the k bits representing the primary signal and the (n−k) bits representing the error correction bits, and then delivers the decoded signals to a further stage as the data on the available k channels CH1to CHk. In this case, since the frame synchronization is performed at both of the transmitting-end station101and the receiving-end station102, a phase alignment unit can be dispensed with.

A number of the reserved channels capable of being used as the channels for transmitting the error correction bits may be changed due to a modification of a configuration of the optical transmission system. Advantageously, according to the sixth embodiment of the present invention, the encoder104may be adapted so as to perform the appropriate coding depending on numbers of the available channels and the reserved channels. Alternatively, if the number of the reserved channels decreases to one, then the reserved channel may be used for a parity bit. In this case, the optical transmission system can be reconstructed so as to perform an error correction by specifying a bit position where a transmission error occurs by means of monitoring bytes B1and B2within an SOH of an STM-N, as earlier described.

FIG. 11is a schematic diagram of an optical transmission system according to a seventh embodiment of the present invention. The optical transmission system comprises a transmitting-end station111, a receiving-end station112and an optical transmission line113connecting the transmitting-end station111and the receiving-end station112.

We consider a similar environment to that ofFIG. 10where there are n channels CH1to CHncomprising k channels CHito CHkused for transmitting data and other (n−k) channels CHk+1to CHnreserved for future. However, in the seventh embodiment, a coding is performed using any m channels selected from the effective k channels. For example, the encoder114may perform a (n+m−k, m) Hamming coding. An example of a set of these variables is n=9, k=6 and m=4. Then, the encoder114performs the (7, 4) Hamming coding.

With respect to other effective channels rather than the channels used for the Hamming coding, the data on these channels are passed through the encoder114and the identification signal-inserting unit115without being affected.

The data on the m channels to be encoded are marked with an identification signal, inserted by the identification signal-inserting unit115, which specifies an order of coding. The identification signal may be inserted into a control field located at a top of a frame from the channel to be error correction coded or a J1byte within a line over head contained in a payload for an STM-N frame.

The multiplexing unit116time-division multiplexes the k data on the k channels CH1to CHkand the generated (n−k) error correction bits. In this case, the bits1to k and the bits (k+1) to n are multiplexed as the channels1to n, as shown inFIG. 11. The time-division-multiplexed signals are then provided to the electrical-optical converter117and converted to optical signals so as to be delivered to the optical transmission line113.

At the receiving-end station112, the optical-electrical converter118receives the multiplexed optical signals from the optical transmission line113and converts the received optical signals to electrical signals. The electrical signals are passed to the separator119, where the electrical signals that have been time-division multiplexed are separated into n signals corresponding to the n channels, respectively. The separated n signals are supplied to the identification signal detector120. The identification signal detector120detects the identification signals added by the identification signal-inserting unit115at the transmitting-end station111from the separated n signals in order to determine the signals to be decoded. The signals, which do not include the identification signal, are directly passed through the decoder121to a further stage. The other signals containing the identification signal are transferred to the decoder121. The decoder121receives the signals to be decoded in accordance with the order specified by the identification signal and performs the error correction decoding.

Therefore, according to the seventh embodiment of the present invention, advantageously, the error correction coding can be applied to very significant channels selected from the effective channels, even if a number of the effective channels is considerably high.

FIG. 12is a schematic diagram of an optical transmission system according to an eighth embodiment of the present invention. The optical transmission system comprises a transmitting-end station131, a receiving-end station132and an optical transmission line133connecting the transmitting-end station131and the receiving-end station132.

The transmitting-end station131includes an encoder134, an identification signal-inserting unit135, a multiplexing unit136and an electrical-optical converter (OS)137. The receiving-end station132includes an optical-electrical converter (OR)138, a separator139, an identification signal detector140and a decoder141.

At the transmitting-end station131the multiplexing unit136is operable to time-division multiplex n channels and the separator139at the receiving-end station132is operable to separate the time-division-multiplexed channels into n channels. We consider that any m (<k) channels selected from the n channels are subject to an error correction coding, and that (k−m) channels are used for fixed data to be error correction coded by the encoder134. For example, (k−m) data on the channels CH1to CH(k−m)are input to the multiplexing unit136, data on the channels CH(k−m+1)to CHkare input to the encoder134and the fixed data1′ to (k−m)′ corresponding to the channels CH1to CH(k−m)are input to the encoder134. In this manner, the encoder134performs, for example, a (n, k) Hamming coding on the basis of the k data comprising m data on the channels and the (k−m) fixed data.

As a result, the encoder134outputs the (k−m) fixed data1′ to (k−m)′, the m data on the channels CH(k−m+1)to CHkand (n−k) error correction bits. Among the output data from the encoder134, the m data corresponding to the channels CH(k−m+1)to CHkand the (n−k) error correction bits, (k+1) to n bits, are used in the identification signal-inserting unit135, such that the identification signal is inserted into each data. The m data corresponding to the channels CH(k−m+1)to CHkand the (n−k) error correction bits are sent to the multiplexing unit136together with their identification signal. Then, the multiplexing unit136receives and time-division multiplexes the n data consisting of the (k−m) data on the channels CH1to CH(k−m), m data corresponding to the channels CH(k−m+1)to CHkand the (n−k) error correction bits. The time-division-multiplexed signals are transferred to the electrical-optical converter137so as to be converted to optical signals and delivered to the receiving-end station132via the optical transmission line133.

At the receiving-end station132, the optical-electrical converter138receives and converts the multiplexed optical signals to electrical signals. The electrical signals are sent to the separator139by which the electrical signals are separated into signals corresponding to channels CH1to CHnand, thereafter, transferred to the identification signal detector140. The identification signal detector140detects the identification signals attached to the data at the transmitting-end station131and provides the data corresponding to the detected identification signals to the decoder141according to the order specified by the identification signal. The other data without specified by the identification signal are directly passed to a further stage without being decoded, because they have not been subject to the error correction coding. The decoder141receives the data from the identification signal detector140, that is to say, the data on the channels CH(k−m+1)to CHnbeing subject to the error correction coding at the transmitting-end station131as well as the same fixed data1′ to (k−m)′ as input to the encoder134. Subsequently, the decoder141performs an error correction decoding and outputs the error corrected data on the channels CH(k−m+1)to CHn.

According to the eighth embodiment of the present invention, it is advantageous that a high quality transmission can be achieved for any channels within the transmission system by effectively making use of reserved channels. Obviously, the coding scheme employed in the encoder134should not be limited to the above-mentioned Hamming coding, but any suitable error correction schemes can be applied to the transmission system. Of course, the decoder141at the receiving-end station132should be constructed so as to be adapted to the coding scheme implemented by the encoder134.

FIG. 13is a schematic diagram of an optical transmission system according to a ninth embodiment of the present invention. The optical transmission system comprises a transmitting-end station151, a receiving-end station152and optical transmission lines153-1to153-nconnecting the transmitting-end station151and the receiving-end station152.

The transmitting-end station151includes a frame-generating and SOH-inserting unit154, an encoder155and an electrical-optical converter (OS)156. The receiving-end station152includes an optical-electrical converter (OR)157, a memory unit158, a decoder159, an SOH terminating unit160and a frame number and top detector161.

The optical transmission system according to the ninth embodiment is similar to that ofFIG. 7, except for optical signals not being multiplexed. As shown inFIG. 13, the optical signals are transmitted through the plurality of the optical transmission lines153-1to153-n, which correspond to channels CH1to CHnand transmit the data on the channels CH1to CHnwith error correction bits. The frame-generating and SOH-inserting unit154is operable to add an SOH to each of the channels CH1to CHnconcurrently at the same timing and insert each frame number to a reserved byte within each SOH. Alternatively, each frame number may be inserted into a J1byte within a line overhead in a virtual container. In any cases, the data on the channels CH1to CHk, i.e. primary signals, are input to the encoder155in phase.

As an example, it is assumed that the encoder155performs a (n, k) Hamming coding, that a frame synchronization byte within each SOH is not encoded, and that the frame synchronization byte is added to each sequence of error correction bits. In this case, the frame synchronization byte is attached to every one of the sequences 1 to n and the sequences are applied to the electrical-optical converter156. The electrical-optical converter156converts the n sequences to n optical signals and transmits the n optical signals to the receiving-end station152through the optical transmission lines153-1to153-n, respectively.

At the receiving-end station152, the optical-electrical converter157converts the received n optical signals to electrical signals and provides the converted electrical signals to the frame number and top detector161and the memory unit158.

The frame number and top detector161detects the number and top of each frame formed by the optical signals delivered from the optical transmission lines153-1to153-nregardless of differences of wavelengths or characteristics between the optical signals. Then, the frame number and top detector161can matches a phase of any bits directed to the decoder159with each other by controlling a read out timing of the bits from the memory unit158. Consequently, the decoder159can receive all the n bits in phase and perform an error correction process in conformity with the error correction coding implemented at the transmitting-end station151. Thereafter, the SOH terminating unit160receives the frames and terminates the SOH within each frame so as to transfer the frames, i.e., the data on the channels CH1to CHkto a further stage, not shown inFIG. 13.

Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. For example, any suitable coding scheme, such as error correction codes, cyclic codes, BCH (Bose-Chaudhuri-Hocquenghem) codes, Fire code, and so on, can be applied to the present invention as the coding scheme. In addition, an encoder and a decoder as well as a parity generator and a parity detector, as described in the embodiments and illustrated in the drawings, may be constructed using an optical logic circuit such that an error correction coding and decoding, as well as, a parity generating and detecting can be directly applied to the optical signals. In this case, since those logical operations applied to the optical signals are implemented based on an amplitude modulation for the optical signals, the error correction and parity operations can be applied to the optical signals with different wavelengths by simply introducing a wavelength conversion.

It can be summarized that according to the present invention, a transmitting-end optical transmission device or station is operable to form k data by aligning phases of data on k channels with each other and for generating (n−k) error correction bits for the k data and adding said (n−k) error correction bits to the k data, convert both the k data and the (n−k) error correction bits to n optical signals having different wavelengths and for wavelength-multiplex the n optical signals so as to be delivered to the optical transmission line. A receiving-end optical transmission device or station is operable to separate the wavelength-multiplexed optical signals from the optical transmission line into n optical signals, each corresponding to one of the different wavelengths, and generates k error corrected data by correcting error bits using the (n−k) error correction bits contained in the n separated optical signals. It is advantageous that a high quality transmission can be achieved without increasing a transmission rate for the k transmission data forming primary signals. Also, the present invention provides an advantage that the high quality transmission can be achieved without occupying reserved bytes within an SOH when it is applied to a SDH transmission.

According to another aspect of the present invention, an optical transmission system is characterized in that a transmitting-end optical transmission device or a station is operable to form data by adding an SOH (Section Over Head) including at least one error monitoring byte to data on k channels and aligning phases of the data with each other and generate a parity bit for the k data and adding the parity bit to said k data. The transmitting-end optical transmission device is also operable to convert the k data and the parity bit to (k+1) optical signals having different wavelengths and wavelength-multiplex the (k+1) optical signals so as to be delivered to the optical transmission line. The receiving-end optical transmission device or station is operable to separate the wavelength-multiplexed optical signals from the optical transmission line into (k+1) optical signals, each corresponding to one of the different wavelengths, and, to correct error bits based on one result of a parity check for the separated (k+1) optical signals and the other result of a parity check using the at least one error monitoring byte within the SOH. In this case, a bit position where an erroneous bit locates is determined by the parity checks and the erroneous bit is subject to an error correction. Therefore, it is advantageous that the high quality transmission can be achieved very economically and simply by utilizing the error monitoring bytes in the SOH used for SDH and adding few parity bits.

It should be noted that the present invention could be applied to TDM transmission. Assuming that a transmission data corresponding to one channel is t be transmitted. In this case, a transmitting-end optical transmission device or station comprises an encoder having k input and n outputs, for generating (n−k) error correction bits for every transmission data having k bits, and, a wavelength-multiplexing unit connected to the encoding means, for converting the transmission data and the (n−k) error correction bits to n optical signals having different wavelengths and for wavelength-multiplexing the n optical signals so as to be delivered to the optical transmission line. A receiving-end optical transmission device or station comprises a wavelength-demultiplexing unit for separating the wavelength-multiplexed optical signals from the optical transmission line into n optical signals, each corresponding to one of the different wavelengths and decoder connected to the wavelength-multiplexing means, for correcting error bits of data having k bits contained in the n separated optical signals by using the (n−k) error correction bits contained in the n separated optical signals. Therefore, advantageously, a high quality transmission can be achieved by transmitting error correction bits without adversely affecting a transmission rate for primary signals at a transmitting-end and by performing error correction at a receiving-end.

Furthermore, according to the present invention, when data conveyed on n channels are transmitted after time-division multiplexed, error correction bits may be transmitted by means of reserved channels and a number of the channels used for error correcting coding may not be constrained. Therefore, the present invention provides an advantage that an error correction coding on data for particularly significant channels can be always implemented even if the number of the reserved channels are much reduced.

The present application is based on Japanese priority application No. 10-138556 filed on May 20, 1998, the entire contents of which are hereby incorporated by reference.