Abstract:
The present invention provides an optical parallel transmission system which can achieve reduction of the processing time without adding many processing functions. A pattern data adder adds, to transmission data, pattern data determined from the transmission data so that the transmission data per one bit becomes data having a form close to a repetition of “1”, “0”, and optical parallel transmission by AC coupling is realized thereby.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical transmission system, and more particularly to an optical parallel transmission system which converts data of a plurality of bits into an optical signal to transmit the same parallelly. 
     2. Description of the Related Art 
     Conventionally, in optical transmission, serial transmission is usually employed so that no skew may occur between bits in long distance transmission, and is used in various fields relating to communications. Since AC coupling which involves determination of presence or absence of data from a variation of data is used in serial transmission, data of successive “0”s cannot be handled. Therefore, in serial transmission, data are transmitted after they are converted into different data having a form close to a repetition of “1” and “0”. For this conversion, coding called 8 B/10 B conversion which converts 8-bit data into 10-bit data is used very frequently. 
     FIG. 1 is a diagrammatic view showing a conventional serial optical transmission system. 
     On the transmission side, 8-bit data to be transmitted are first converted into 10-bit data. 
     Then, the transmission data converted into 10-bit data are converted into serial data. 
     Thereafter, the transmission data converted into serial data are converted into an optical signal and transmitted over an optical fiber. 
     On the reception side, when the optical signal transmitted from the transmission side is received over the optical fiber, the received optical signal is first converted into an electric signal. 
     Then, a clock signal is extracted from the data converted into an electric signal and is converted into parallel data of 10 bits. 
     Thereafter, the 10-bit data converted into parallel data are converted into 8-bit data. 
     Serial optical transmission of data is realized by the sequence of processes described above. 
     However, when a large amount of data is transmitted, such serial transmission as described above increases the transmission amount per transmission line to such a level that transmission of the data cannot be realized. Particularly, transmission between clusters of computers is likely to suffer from the problem described above because a very large amount of data is transmitted. 
     Further, when serial transmission is carried out, parallel data must be converted into serial data, and this gives rise to another problem that a long processing time is required. 
     As a countermeasure for solving such problems as described above, realization of optical parallel transmission is demanded, and optical parallel transmission is being put into practical use to a certain extent. 
     FIG. 2 is a diagrammatic view illustrating a data transmission method in a conventional optical parallel transmission system. 
     On the transmission side, data of 8 bits to be transmitted and a clock signal are converted into an optical signal and transmitted over nine optical fibers provided in parallel. 
     On the reception side, when the optical signal transmitted from the transmission side is received over the optical fibers, the received optical signal is first converted into an electric signal. 
     Thereafter, skew adjustment is performed between the 8-bit data converted into the electric signal. 
     In such a conventional optical parallel transmission system as described above, when parallel data are transmitted as they are, it is often the case that data which include successive data of “0” cannot be transmitted in an AC fashion, and DC coupling by which a signal of successive “0”s or “1”s can be transmitted is demanded. 
     However, when it is tried to realize DC coupling, the module becomes expensive and is not practical. Besides, since various restrictions are applied, parallel transmission cannot be made taken advantage of. 
     Further, since parallel transmission of data produces a skew between data which is a difference in transmission time between transmission lines, there is a problem that a function of adjusting the skew must be newly added. 
     Furthermore, the optical parallel transmission system has many problems in regard to manufacture of parallel optical modules such as a yield and further has many problems also in regard to implementation of the same. However, in recent years, devices with which augmentation in yield can be achieved such as surface emitting lasers have exhibited remarkable augmentation in performance, and expectation for optical parallel transmission is increasing also due to perfection of modules. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical parallel transmission system which can achieve reduction of the processing time without adding many processing functions. 
     In the present invention, pattern data addition means adds, to transmission data, pattern data determined from the transmission data so that the transmission data per one bit becomes data having a form close to a repetition of “1”, “0”. Consequently, optical parallel transmission by AC coupling can be realized. 
     Further, since the pattern data addition means adds, to one or more positions of the transmission data, information for detecting synchronism among bits can be detected by synchronization processing means, so that there is no necessity of newly providing a function for adjusting a skew between bits of the transmission data. 
     Further, since it is discriminated by data discrimination means whether the transmission data is data which includes “1” and “0” or successive “0” data and a control signal for controlling processing on the reception side is produced based on the result of this discrimination and outputted, data transmission is performed smoothly. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view showing a conventional serial optical transmission system; 
     FIG. 2 is a diagrammatic view illustrating a data transmission method in an optical parallel transmission system; 
     FIG. 3 is a diagrammatic view showing a first embodiment of an optical parallel transmission system of the present invention; 
     FIG. 4 a  is a diagrammatic view illustrating data to which pattern data are added in the optical parallel transmission system shown in FIG. 3; 
     FIG. 4 b  is a view illustrating motion per one bit in the optical parallel transmission system shown in FIG. 3; 
     FIG. 5 is a diagrammatic view illustrating information added for detecting synchronism among bits by a pattern data adder shown in FIG. 3; and 
     FIG. 6 is a block diagram showing a construction of the transmission side in a second embodiment of the optical parallel transmission system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     The present embodiment includes, as shown in FIG. 3, pattern data adder  10  for adding to transmission data of 8 bits pattern data determined from the transmission data and information for detecting synchronism between the bits, electrical/optical converter  20  which serves as first conversion means for converting the transmission data to which the pattern data and the information have been added by pattern data adder  10  into an optical signal of 8 bits and transmitting the optical signal of 8 bits, optical/electrical converter  30  which serves as second conversion means for converting the optical signal transmitted thereto into an electric signal, clock extractor  40  for extracting a clock signal from the data converted into an electric signal by optical/electrical converter  30 , and synchronization processor  50  for synchronizing the data from which the clock signal has been extracted by clock extractor  40  based on the information added by pattern data adder  10 . 
     In the following, a data transmission method of the optical parallel transmission system having the construction described above will be described. 
     When data of 8 bits is to be transmitted, pattern data adder  10  first adds pattern data as shown in FIG. 4 a . Here, the pattern data added to the transmission data is determined based on preceding transmission data so that, as a result of addition of the pattern data, transmission data per bit (transmitted by one optical fiber) may infinitely approach a repetition of “1”, “0”. In the present embodiment, as shown in FIG. 4 b,  to transmission data of “0”, “0”, pattern data of “1”, “1” is added; to transmission data of “0”, “1”, pattern data “0”, “1” is added; to transmission data of “1”, “0”, pattern data of “1”, “0” is added; and to transmission data of “1”, “1”, pattern data of “0”, “0” is added. 
     This allows transmission by AC coupling. 
     However, as described above, if transmission data is transmitted with pattern data added thereto, then the data amount increases to twice in the time base direction. However, in the present condition of the optical transmission technology and the LSI technology, the foregoing is a reasonable construction. For example, in parallel transmission of 400 Mbit/second, while the optical part is required to have the performance of 800 Mbit/second, a rate of this order can be realized without any trouble by the present technology. Further, it is possible to realize electric transmission of the order of 400 Mbit/sec although there is some difficulty with regard to mounting. 
     On the other hand, if it is tried to realize this rate by serial transmission, a performance of 4 Gbit/sec is required, and very high technology is required and the cost is very high. 
     Further, information for detecting synchronism between the bits is added as shown in FIG. 5 by pattern data adder  10 . 
     More particularly, information that can make variation between “0” and “1” definite such as 00001111 is added to the position at the top of the transmission data. This information is composed of a unit of approximately 4 to 6 bits at the maximum. If the information is composed of more than that unit, it may be impossible for the reception side to detect normal data because an excessively large number of data of “0” or “1” successively appear. Meanwhile, if processing is performed bit by bit, fine control can be achieved and a repetition of “1”, “0” can be realized infinitely. In this instance, however, high speed processing is required and high LSI technology is required. Therefore, an optimum technique must be selected with the foregoing taken into consideration. 
     The transmission data to which the pattern data and the information have been added by pattern data adder  10  is converted into an optical signal of 8 bits by electrical/optical converter  20  and transmitted over the optical fibers. 
     When the signal transmitted over the optical fibers is received, the optical signal is first converted into an electric signal by optical/electrical converter  30 . 
     Then, clock extractor  40  extracts a clock signal from the data converted into an electric signal by optical/electrical converter  30 . Since the transmission data exhibits at any successive bits thereof a combination proximate to a repetition of “1”, “0”, it consequently allows clock extraction from any bit by clock extractor  40 , so that there is no need of performing clock extraction for each bit. 
     Thereafter, synchronization processing of the data from which the clock signal has been extracted by clock extractor  40  is performed by synchronization processor  50  based on the information added to the transmission data by pattern data adder  10 . 
     It is to be noted that a cause of a skew appearing between bits originates in the optical cable serving as a transmission line and driving and driven LSIs and optical parts, and since a skew detected once is varied little by conditions of use such as temperature, once detection of a skew is established, use can be continued by changing the mode to the usual data transmission mode. However, since some variation may possibly occur in use over a long period of time, the skew can be maintained if the information is added after each fixed interval of time. 
     Second Embodiment 
     In parallel transmission of data, transmission data is not always generated, and frequently no data is generated. When no data is present, “0” as blank data is always generated and transmitted so that data transmission can be performed immediately when transmission data is generated. However, since data of “0” appears repetitively when no data is present, this is very undesirable for data transmission. Therefore, in the present embodiment, information in a state when no data is present is transmitted with a control signal. 
     The transmission side in the present embodiment includes, as shown in FIG. 6, data discriminator  60  which receives data of 8 bits to be transmitted and a clock signal as inputs thereto and discriminates the data of 8 bits to be transmitted, pattern data adder  70  for adding pattern data determined in advance to the data of 8 bits to be transmitted, control signal production unit  80  for producing a control signal based on a result of the discrimination of data discriminator  60 , and electrical/optical converter  90  for converting the data to which the pattern data has been added by pattern data adder  70  and the control signal produced by control signal production unit  80  into an optical signal. 
     Next, a data transmission method in the optical parallel transmission system having the construction described above will be described. 
     First, it is discriminated by data discriminator  60  whether data to be transmitted is data including “1” and “0” or data of successive “0”s. 
     If successive “0” data are detected by data discriminator  60 , control signal production unit  80  produces and outputs a control signal representing that the data is a repetition of “0” data. 
     On the other hand, if it is discriminated by data discriminator  60  that the data to be transmitted is data including “1” and “0”, then control signal production unit  80  produces and outputs a control signal representing that the data is data including “1” and “0”. 
     Further, pattern data is added by pattern data adder  70 , and consequently, the transmission data becomes repetitive data of “1”, “0”. The resulting transmission data is converted into an optical signal by electrical/optical converter  90  and transmitted over optical fibers. 
     On the reception side, if a control signal that the data is successive “0” data is received, then it is discriminated that the data transmitted is “0”s, and “0” data is outputted successively. 
     On the other hand, if the reception side receives a control signal representing that the data includes “1” and “0”, then it outputs the data transmitted. 
     It is to be noted that, since also the control signal produced by control signal production unit  80  is an optical signal, it is not preferable that the signal has a fixed value, and the two kinds of control signals can be distinguished from each other if, for example, a control signal of “100100 . . . ” is produced for “0” data, but another control signal of “101010 . . . ” is produced for data including “1” and “0”. 
     In transmission of general data, the second embodiment operates similarly to the first embodiment described above. 
     Further, since an independent control signal is used, also a cable for transmission of a clock signal is added, and the necessity for a circuit to extract a clock signal on the reception side is eliminated, 
     Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.