Abstract:
An optical reception apparatus comprises an optical receiver to receive an optical signal from an optical transmission line, an error corrector to correct an error of the received signal and to transmit the error-corrected signal and error rate information before error correction, a judging apparatus to judge transmission quality of the optical transmission line according to the error rate information from the error corrector and a threshold value equal to an error rate lower than an error correction limit of the error corrector, and a selective breaker to transmit the signal whose error is corrected by the error corrector in normal state and to block transmission of the signal whose error is corrected by the error corrector when the judged result by the judging apparatus indicates deterioration of the transmission quality of the optical transmission line.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-199599, filed Jun. 29, 2001, the entire contents of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   This invention relates to an optical reception apparatus and an optical transmission system, and more specifically relates to an optical reception apparatus capable of informing deterioration of transmission quality in an optical communication system to the following apparatuses and an optical transmission system to control line connections using the optical reception apparatus. 
   BACKGROUND OF THE INVENTION 
   In an optical communication system, an optical transmitter error correction code encodes a SONET/SDH flame of OC-48/STM-16 or OC-192/STM-64 and sends it onto an optical fiber transmission line, and an optical receiver corrects transmission errors using an error correction code and outputs the SONET/SDH flame of OC-48/STM-16 or OC-192/STM-64. As an error correcting system, a foward error correction (FEC) system has been known. An FEC system having more advanced error-correcting ability is also well known (see, for instance, T. Mizouchi, et al., “Transparent multiplexer featuring super FEC for optical transport networking”, proceedings of SubOptic 2001, pp. 484–487). 
   Generally, quality of an optical transmission line is unstable, and the unstableness appears more clearly as the transmission line distance becomes longer and the bit rate becomes higher. By introducing error-correcting technology with high correcting ability, it is possible to obtain transmission of practically error-free data even in a transmission line having poor quality. For instance, in an optical transmission line which quality varies in a range from 2×10 −3  to 4×10 −3  with its average quality of 3×10 −3 , an error rate after error correction varies between an error-free state and a frame-loss state. 
   Also, an optical transmission system, specifically an optical transmission system of trunk lines is designed to be able to select from a plurality of lines or routes to prepare for line fault. An electric crossconnector or a photonic crossconnector is well known as a means to switch traffic. 
   A large number of transmission errors can be corrected by the FEC system. Since high correction ability is obtained in the error correcting technology described in the above paper, it is possible not only to realize practically error-free transmission in a wide range but also to use optical transmission lines having inferior quality. 
   In this error correcting technology, however, an error rate after error correction rapidly increases if an error rate before error correction exceeds a certain threshold value (see  FIG. 6  of the above-described paper). For example, when an error rate before error correction deteriorates from 4×10 −3  to 8×10 −3 , an error rate after error correction greatly worsens from 10 −11  to 10 −2 . 
   In an optical transmission line requiring a high error correcting capability, a minor deterioration of transmission characteristics is likely to cause disabled conditions. Accordingly, when a transmission error sufficiently serious to cause such disablement occurs in a line, the line is switched to another line by a crossconnector. 
   However, any means to easily monitor such deterioration in transmission characteristics has not been proposed yet. Conventionally, an abnormal condition in a transmission line is merely detected on a result obtained by restoring a data carried by an optical carrier. Therefore, in a conventional system, alarm signals are output from many parts to inform deterioration of transmission characteristics in an optical fiber transmission line. 
   Conventionally, a crossconnector switches lines manually or automatically when an abnormal condition occurs in a transmission line as mentioned above. Some electric crossconnectors have an error monitoring function. When such an electric crossconnector is employed, it is possible to make the electric crossconnector automatically switch working lines when the error rate reaches a certain value. When such an automatic switching system is used, however, transmission characteristics often become unstable because the line switching causes chattering. 
   In addition, since signals are transmitted in an inferior transmission quality until the line having a high error rate is switched to another line, packet retransmission is repeated. This increases loads in the network. 
   In a photonic crossconnector, a special electric device must be installed to calculate an error rate of input optical signals. This reduces merits to utilize a photonic crossconnector instead of an electric crossconnector. 
   SUMMARY OF THE INVENTION 
   An optical reception apparatus according to the invention comprises an optical receiver to receive an optical signal from an optical transmission line, an error corrector to correct an error of the signal received by the optical receiver and to transmit the error-corrected signal and error rate information before error correction, a judging apparatus to judge transmission quality of the optical transmission line according to the error rate information from the error corrector and a threshold value equal to an error rate which is lower than an error correction limit of the error corrector, and a selective breaker to transmit the signal whose error was corrected by the error corrector in normal state and to block transmission of the signal whose error was corrected by the error corrector when the judged result by the judging apparatus indicates deterioration of the transmission quality of the optical transmission line. 
   An optical transmission system according to the invention comprises first and second optical fiber lines, an optical transmission terminal to selectively transmit an optical signal to either of the first and second optical fiber lines, and an optical reception terminal to selectively receive an optical signal from either of the first and second optical fiber lines, wherein the optical transmission terminal comprises first and second optical transmission apparatuses respectively connecting to the first and second optical fiber lines, each optical transmission apparatus having a error correction code encoding circuit to error correction code encode a transmission signal and a first selector to selectively supply the transmission signal to either of the first and second optical transmission apparatuses and to supply the transmission signal to the first optical transmission apparatus in the initial state, and the optical reception terminal comprises first and second optical reception apparatuses respectively connecting to the first and second optical fiber lines, each optical reception apparatus correcting an error of received signal and to transmit error rate information before error correction, a second selector selectable of either signal from the first and second optical reception apparatuses, a judging apparatus to judge transmission quality of the first optical fiber line according to the error rate information before error correction from the first optical reception apparatus, and a selective breaker to break signal transmission between the first optical reception apparatus and the second selector when the judged results of the judging apparatus indicate deterioration of the transmission quality. 
   A line switching method according to the invention to switch working lines in an optical transmission system comprising first and second optical fiber lines, an optical transmission terminal having a first selector to select one of the first and second fiber lines to transmit an optical signal and an optical reception terminal having a second selector to select either signal from the first and second optical fiber lines comprises a reception step to correct an error of signal from the first optical fiber line and to transmit the error corrected signal and error rate information before error correction, a judging step to judge transmission quality of the first optical fiber line by comparing the error rate information before error correction with a predetermined threshold value equal to an error rate lower than an error correction limit of the reception step, a selective breaking step to supply the error corrected signal from the reception step to the second selector at the beginning and to break transmission of the error corrected signal from the reception step to the second selector when the judged result of the judging step indicates deterioration of transmission quality of the first optical fiber line, an input detecting step to detect whether any signal enters a first port to which signals from the first optical fiber line enters in the second selector, a first switching step to switch from the first port of the second selector to a second port to which signals from the second optical fiber line enter when the first port has no input signal and to inform the optical transmission terminal to switch to the second optical fiber line, and a second switching step at the optical transmission terminal to make the first selector select the second optical fiber line to transmit the optical signal according to the switching information from the optical reception terminal. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a schematic block diagram of an embodiment according to the invention; and 
       FIG. 2  illustrates error rate characteristics before and after error correction. 
   

   DETAILED DESCRIPTION 
   Embodiments of the invention are explained below in detail with reference to the drawings. 
     FIG. 1  shows a schematic block diagram of an embodiment according to the invention. A plurality of optical fiber lines (2 lines are shown in the illustration)  14  and  16  are selectable between a transmission terminal  10  and a reception terminal  12 . It is assumed that the optical fiber line  14  is being selected at the beginning. 
   The configuration of the transmission terminal  10  is explained below. An optical router  20  applies data to be transmitted for the reception terminal  12  to a photonic crossconnector  22  out of input data (for example, OC-48/STM-16 form data) carried by a light wave. A controller  24  controls a route in the photonic crossconnector  22 . Optical transmission apparatuses  26  . . .  28  respectively connect to a plurality of output ports of the photonic crossconnectors  22 . At the beginning, the controller  24  controls the photonic crossconnector  22  to supply the input data to the optical transmission apparatus  26 . 
   In the optical transmission apparatus  26 , an optical receiver  30  receives an optical signal from the photonic crossconnector  22 , and an opto-electric converter  32  converts the output from the optical receiver  30  into an electric signal and applies it to an FEC encoder  34 . The FEC encoder  34  adds an error correction code of FEC to the signal from the opto-electric converter  32 . An electro-optical converter  36  converts the error correction code encoded data from the FEC encoder  34  into an optical signal, and an optical transmitter  38  outputs the optical signal from the electro-optical converter  36  onto the optical fiber line  14  in a predetermined form. 
   An inside configuration of the optical transmission apparatus  28  is the same with that of the optical transmission apparatus  26 . The optical transmission apparatus  28  processes an optical signal from the photonic crossconnector  22  in the same way with that of the optical transmission apparatus  26  and transmits onto an optical fiber line  16 . 
   The configuration of the reception terminal  12  is explained below. Optical reception apparatuses  40  and  42  receive an optical signal from the optical fiber lines  14  and  16 , respectively. The configurations of the optical reception apparatuses  40  and  42  are the same and the details are described later. The optical signals received by the optical reception apparatuses  40  and  42  enter a photonic crossconnector  52  through normally closed optical switches  44  and  46 , respectively. Judging circuits  48  and  50  evaluate transmission quality of the optical fibers  14  and  16  according to error rate information before error correction of the optical fibers  14  and  16  by the optical reception apparatuses  40  and  42  and a judging threshold value Ref, and open or close the optical switches  44  and  46  according to the evaluated result. A controller  54  controls optical routes in the photonic crossconnector  52 . The optical signal selected by the photonic crossconnector  52  is applied to an optical router  56 . The optical router  56  then transmits the optical signal from the photonic crossconnector  52  to an apparatus (which is not illustrated) designated by the optical signal. 
   In the optical reception apparatus  40 , an optical receiver  60  receives the optical signal from the optical fiber line  14 , and an opto-electric converter  62  converts the output from the optical receiver  60  into an electric signal and applies it to an FEC decoder  64 . The FEC decoder  64  corrects errors of the transmission data using the error correction code added by the FEC encoder  34  and applies the corrected data to an electro-optical converter  66  and an error rate before error correction to a judging circuit  48 . The electro-optical converter  66  converts the error-corrected data from the FEC decoder  64  into an optical signal and applies it to an optical transmitter  68 . The optical transmitter  68  applies the output optical signal from the electro-optical converter  66  to the photonic crossconnector  52  through the optical switch  44  in a predetermined form. 
   The judging circuit  48  compares the error rate before error correction by the FEC decoder  64  with a predetermined threshold value Ref to judge transmission quality of the optical fiber line  14 . The threshold value Ref is set to the error rate before error correction which is smaller than a correction limit of the FEC. When the transmission quality is poor, the judging circuit  48  opens (turns off) the optical switch  44 . That is, the connection of the optical signals between the optical receiver  40  and the photonic crossconnector  52  is blocked. Judging conditions of the judging circuit  48  are explained later. 
   The characteristic operation of this embodiment is explained below. The data to be transmitted from the terminal  10  to the terminal  12  passes through the router  20 , the photonic crossconnector  22 , the optical transmission apparatus  26 , the optical fiber line  14 , the optical reception apparatus  40 , the optical switch  44 , the photonic crossconnector  52 , and the router  56 . The FEC encoder  34  in the optical transmission apparatus  26  encodes the transmission data, and the FEC decoder  64  in the optical reception apparatus  40  corrects the transmission error in the optical fiber line  14  using an error correction code added by the FEC encoder  34 . The FEC decoder  64  transmits the error corrected data to the electro-optical converter  66  and applies the error rate information before error correction to the judging circuit  48 . 
   The judging circuit  48  judges the transmission quality of the optical fiber line  14  according to the error rate from the FEC decoder  64  and the predetermined threshold value Ref and turns off (opens) the optical switch  44  when the transmission quality is poor and turns on (closes) the optical switch  44  when the transmission quality is satisfactory. When the optical switch  44  is turned off, the optical signal transmission from the optical reception apparatus  40  to the photonic crossconnector  52  is shut off. 
   When the transmission of the optical signal from the optical reception apparatus  40  to the photonic crossconnector  52  is shut off, the data of the terminal  10  emitted from the optical router  56  becomes practically non-signal. An operator or a controller, which is not illustrated, can switch from the optical fiber  14  to the optical fiber line  16  before the transmission quality of the optical fiber  14  deteriorates exceeding the error correcting ability of the FEC system. 
   Most of the photonic crossconnectors  52  comprise a function to monitor the existence of input light in each input port. Using this function, the controller  54  can detect that no optical signal inputs from the optical reception apparatus  40 . When no optical signal inputs although the input port is in use, it means that a fault occurred in a signal transmission system connecting to the input port. In this case, the controller  54  controls the photonic crossconnector  52  to select another line, the optical fiber line  16  in the embodiment, and informs the controller  24  in the photonic crossconnector  22  of the transmission terminal  10  of the line switching (including the information of newly employed lines) through any transmission medium. The controller  24  controls the photonic crossconnector  22  to switch from the optical fiber line  14  to the optical fiber line  16  according to the information from the controller  54 . 
   The judging method of the judging circuit  48  is explained below.  FIG. 2  shows the relation between error rates before and after error correction using the FEC and a threshold value Ref.  FIG. 2  corresponds to  FIG. 6  of the above-mentioned paper. The horizontal axis expresses an error rate before error correction, and the vertical axis expresses an error rate after error correction. Reference numeral  70  denotes a case without a FEC system, reference numeral  72  denotes a case when an initial FEC system is applied, and reference numeral  74  denotes when an improved FEC (super FEC) system is applied, respectively. A threshold value Ref is set to a value equal to an error rate before error correction which is smaller than an error correction ability limit of the improved FEC (super FEC) system. 
   As a second judging method, the judging circuit  48  turns off the optical switch  44  when the error rate from the FEC decoder  64  exceeds the threshold value Ref and continues for a certain period T 1 . The judging circuit  48  turns on the optical switch  44  when the error rate is less than the threshold value Ref for the certain period T 1  or a certain period T 2 . In this method, instantaneous deterioration of transmission quality can be neglected. Because there is a possibility to be able to process such instantaneous deterioration of the transmission quality within the error correction ability since the threshold value Ref is set to be smaller than the error correction ability limit. 
   The transmission quality of an optical fiber line often shows a variation in which gradually changing deterioration and temporary instantaneous deterioration are put together. In a third method, the judging circuit  48  turns off the optical switch  44  when the error rate from the FEC decoder  64  exceeds the threshold value Ref as many times as a predetermined number of times K within a certain period T 3 , and turns on the optical switch  44  when the error rate is continuously smaller than the threshold value for a certain period T 4  which is longer than T 3 . The reason is that, it is preferable to restart the operation after the transmission quality is recovered continuously and satisfactorily. 
   In the above examples, although the judging threshold value to turn on the optical switch  44  is set equal to the judging threshold value Ref to turn off the optical switch  44 , it is preferable that the judging threshold value to turn on the optical switch  44  is smaller than the judging threshold value Ref to turn off the optical switch  44  in consideration of the satisfactory recovery of the transmission quality. Also, this helps to avoid chattering of the optical switch  44 . 
   In the embodiment shown in  FIG. 1 , although the optical switch  44  and the judging circuit  48  are disposed outside the optical reception apparatus  40  to make it understandable, it is obvious that the optical switch  44  and the judging circuit  48  can be disposed in the optical reception apparatus  40 . Also, the optical switch  44  can be disposed in the photonic crossconnector  52 . 
   In a case to employ an electric crossconnector instead of the photonic crossconnector  52 , it is preferable to dispose an electric switch corresponding to the optical switch  44  on an output side of an opto-electric converter disposed between the optical reception apparatus  40  and the electric crossconnector. If the electric switch corresponding to the optical switch  44  is disposed on an input side of the opto-electric converter, it is likely that noises etc. enter the electric crossconnector and make the electric crossconnector misjudge whether any input signal exists or not. 
   In the embodiment, since signals are blocked before the transmission quality of a transmission fiber line becomes a condition impossible to correct, the following apparatuses can detect the transmission quality by monitoring whether any signal exists or not and therefore can afford to switch over lines earlier. By this operation, severe transmission errors can be avoided in advance with a simple configuration. 
   In a method in which alarms are emitted from a number of places, it takes a long time to specify a fault location and a fault source. However, in a method to block signals like the one used in the invention, a number of alarms become relatively small making it easier to find out a fault. 
   As readily understandable from the aforementioned explanation, according to the invention, transmission quality of an optical transmission line can be monitored to switch lines before the transmission quality becomes a condition impossible to correct with a simple configuration using existing elements. That is, severe errors can be avoided in advance using a simple configuration. 
   While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.