Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
1999-04-12
2003-11-25
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C341S094000, C341S100000, C341S101000, C341S102000, C341S103000
Reexamination Certificate
active
06654562
ABSTRACT:
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 transmitter
201
, an optical transmission line
203
and an optical receiver
202
, as schematically shown in
FIG. 1
, and the system is in conformity with SDH (Synchronous Digital Hierarchy) that is a set of international, digital transmission standards. The optical transmitter
201
has, for each of k channels CH
i
(i=1, . . . , k), individually an SOH (Section Over Head) inserting unit
204
for inserting an SOH, an electrical-optical converter (OS)
205
and a wavelength-multiplexer
206
. The optical receiver
202
also has, for each of the k channels, individually a wavelength-demultiplexer
207
, an optical-electrical converter (OR)
208
and an SOH terminating unit
209
.
The SOH inserting unit
204
at the optical transmitter
201
inserts the SOH into an electrical signal for one of the corresponding channels CH
i
. Each electrical signal for the every channel is then provided to the optical-electrical converter
205
and converted to an optical signal with a wavelength &lgr;
i
corresponding to the channel CH
i
. The optical signals having the wavelength of &lgr;
i
are multiplexed by the wavelength-multiplexer
206
and resulting wavelength-multiplexed signals are transmitted to the optical transmission line
203
.
The wavelength demultiplexer
207
at the optical receiver
202
separates the multiplexed signals received from the optical transmitter
201
through the optical transmission line
203
into the signals corresponding to the wavelengths &lgr;
1
to &lgr;
k
, respectively. These optical signals having the wavelength of &lgr;
1
to &lgr;
k
, respectively, are converted to corresponding electrical signals by the optical-electrical converter
208
, and then the SOH of the electrical signals is terminated by the SOH terminating unit
209
. The electrical signals having their SOH terminated are transmitted to a further stage (not shown in
FIG. 1
) on an each (i.e., individual) channel basis. Thus, the data comprising the electrical signals for each of the channels CH
1
to CH
k
can be transmitted from the optical transmitter
201
to the optical receiver
202
over the signal optical transmission line
203
.
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 B
1
and B
2
.
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.
SUMMARY OF THE INVENTION
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 transmitter
201
and the receiver
202
, 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 applications.
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 &lgr;
1
to &lgr;
n
, 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 &lgr;
1
to &lgr;
n
, respectively, by a wavelength demultiplexer, converts the signals having the different wavelengths &lgr;
1
to &lgr;
n
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 &lgr;
1
to &lgr;
n
, 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
Fujitsu Limited
Pascal Leslie
Payne David
Staas & Halsey , LLP
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