Patent Publication Number: US-2005129408-A1

Title: Optical transmission system for removing skew between optical channels

Description:
BACKGROUND OF THE INVENTION  
      This application claims the priority of Korean Patent Application No. 2003-91334, filed on Dec. 15, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
      1. Field of the Invention  
      The present invention relates to an optical communication system, and more particularly, to an apparatus for removing skew between channels occurring in an optical communication system.  
      2. Description of the Related Art  
      Problems such as a rise in price due to a high-priced transmitting/receiving module, reduction in an optical transmission distance due to the nonlinearity of fiber optics, and current technical restriction occur when transmitting an optical signal having a high speed of several tens of Gbps. Such problems can be solved by demultiplexing a signal having a high speed of several tens of Gbps into a low-speed multi-channel signal and then by optically transmitting the low-speed signal.  
       FIG. 1  is a block diagram of conventional N-channel (M/N) Gbps optical transmission systems  100  and  200  which perform M Gbps signal transmission with an overhead. Referring to  FIG. 1 , the optical transmission systems  100  and  200  include optical signal transmitters  110  and  210  and optical signal receivers  160  and  260 , respectively.  
      The optical signal transmitters  110  and  210  include demultiplexers  130  and  230  and N (M/N) Gbps E/O converters  151 - 15   n  and  251 - 25   n , respectively. The optical signal transmitters  110  and  210  demultiplex an M Gbps signal with an overhead into N (M/N) Gbps optical signals and perform medium and long distance transmission to the opposite optical transmission systems  200  and  100  at intervals of several tens of kms via fiber optics. The optical signal receivers  160  and  260  include N (M/N) Gbps O/E converters  171 - 17   n  and  271 - 27   n  and multiplexers  190  and  290 , respectively, and multiplex N (M/N) Gbps optical signals transmitted from the opposite optical transmission systems  200  and  100  into original M Gbps signals.  
      Compared to current high-speed transmission technology, a low-speed optical transmitting/receiving module is low-priced, long distance transmission using fiber optics can be executed, and the development of technology and the manufacture and supply of a product have stabilized. Thus, the optical signal transmitters  110  and  210  demultiplex a high-speed signal into a low-speed multi-channel signal and optically transmit the signal, and the optical signal receivers  160  and  260  multiplex the signal received from the optical signal transmitters  160  and  260  and restore the signal to an original signal, causing technical and economical advantages.  
      However, it is very difficult to make the transmission distance between N channels equal, and due to the time delay of a circuit constituting optical signal transmitters and receivers, the actual transmission distance may vary for each channel. Thus, even though the optical signal transmitters  110  and  210  have transmitted N signals having a frame structure at the same position and at the same time, the optical signal receivers  160  and  260  receive N signals at different points of time. Thus, in order to multiplex N signals received by the optical signal receivers  160  and  260  into original M Gbps signals, it is very important to trace the position of a frame between the N received signals and to remove a difference in time between frames, that is, skew.  
     SUMMARY OF THE INVENTION  
      The present invention provides an optical transmission system for removing skew between channels that may occur when in an optical communication system for demultiplexing a signal with an overhead into an N-channel signal, optically transmitting and receiving the signal and multiplexing the signal into an original signal.  
      According to an aspect of the present invention, there is provided an optical transmission system for removing skew between optical channels. The optical transmission system includes an optical signal transmitter inserting serial numbers into an overhead of a frame, delaying the frame by a time corresponding to skew information transmitted from an opposite optical transmission system, and transmitting the frame; and an optical signal receiver receiving the frame from an opposite optical transmission system via fiber optics, removing skew existing in the received frame in units of both a byte and a frame by referring to the serial numbers stored in the overhead of the frame, and transmitting the skew information collected when the skew is removed, to the opposite optical transmission system.  
      According to another aspect of the present invention, there is provided an optical transmission system for removing skew between optical channels. The optical transmission system includes a serial number inserter inserting serial numbers into an overhead of a frame; a demultiplexer demultiplexing the frame into which the serial numbers are inserted, into N electrical signals; an aligner delaying the N electrical signals for a predetermined amount of time in response to skew information transmitted from an opposite optical transmission system; N E/O converters converting the N electrical signals output from the aligner into N optical signals, so as to transmit the N electrical signals to an opposite optical transmission system; N O/E converters converting the N optical signals received from the opposite optical transmission system via fiber optics, into N electrical signals; a skew remover removing skew existing in the N electrical signals in units of both a byte and a frame by referring to the serial numbers stored in the overhead of the N electrical signals and transmitting the skew information collected when the skew is removed, to the opposite optical transmission system; and a multiplexer multiplexing a signal output from the skew remover into an original signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above aspects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a block diagram of conventional N-channel (M/N) Gbps optical transmission systems which perform M Gbps signal transmission with an overhead;  
       FIG. 2  is a block diagram showing the structure of N-channel (M/N) Gbps optical transmission systems which include an apparatus for removing skew between optical channels according to an embodiment of the present invention and perform M Gbps signal transmission;  
       FIG. 3  is a block diagram showing the structure of 4-channel 10 Gbps optical transmission systems (M=40, N=4) which include an apparatus for removing skew between optical channels according to an embodiment of the present invention and perform 40 Gbps STM-256 signal transmission;  
       FIG. 4  shows the structure of a 40 Gbps STM-256 signal frame according to an embodiment of the present invention;  
       FIG. 5  shows the structure of a signal frame in which the 40 Gbps STM-256 signal frame shown in  FIG. 4  is deinterleaved into four channels;  
       FIG. 6  is a block diagram showing the structure of an aligner of the optical signal transmitter shown in  FIG. 3 ;  
       FIG. 7  is a block diagram showing the structure of a skew remover of the optical signal receiver shown in  FIG. 3 ;  
       FIG. 8  is a block diagram showing the structure of a byte unit skew remover shown in  FIG. 7 ;  
       FIG. 9  shows an input/output signal of the byte unit skew remover shown in  FIG. 8 ;  
       FIG. 10  is a block diagram showing the structure of a frame unit skew remover shown in  FIG. 7 ;  
       FIG. 11  shows an input/output signal of the frame unit skew remover shown in  FIG. 10 ; and  
       FIG. 12  shows an operation of a total skew quantity calculator shown in  FIG. 7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
       FIG. 2  is a block diagram showing the structure of N-channel (M/N) Gbps optical transmission systems  300  and  400  which include an apparatus for removing skew between optical channels according to an embodiment of the present invention and perform M Gbps signal transmission. Referring to  FIG. 2 , the optical transmission systems  300  and  400  include optical signal transmitters  310  and  410  and optical signal receivers  360  and  460 , respectively.  
      The optical signal transmitters  310  and  410  include serial number inserters  320  and  420 , demultiplexers  330  and  430 , aligners  340  and  440 , and N (M/N) Gbps E/O converters  351 - 35   n  and  451 - 45   n , respectively. The serial number inserters  320  and  420  insert serial numbers into part of an overhead of an M Gbps frame in the order of frames. The serial numbers serve as a kind of recognition mark when removing skew in the optical signal receivers  360  and  460 .  
      The demultiplexers  330  and  430  demultiplex an M Gbps frame, into which the serial numbers are inserted, into N (M/N) Gbps electrical signals. The aligners  340  and  440  receive skew information from skew removers  480  and  380  of the opposite optical transmission systems  400  and  300  and delay an electrical signal frame generated by the demultiplexers  330  and  430  for a predetermined amount of time. In this case, the skew information includes byte information of a signal frame received by the opposite optical transmission systems  400  and  300  and delay information of a frame. The N (M/N) Gbps E/O converters  351 - 35   n  and  451 - 45   n  convert the electrical signal output from the aligners  340  and  440  into N (M/N) Gbps optical signals. The optical signals converted by the E/O converters  351 - 35   n  and  451 - 45   n  are transmitted to the opposite optical transmission systems  400  and  300  via fiber optics.  
      The optical signal receivers  360  and  460  include N (M/N) Gbps O/E converters  371 - 37   n  and  471 - 47   n , skew removers  380  and  480 , and multiplexers  390  and  490 .  
      The N (M/N) Gbps O/E converters  371 - 37   n  and  471 - 47   n  convert N (M/N) Gbps optical signals transmitted from the opposite optical transmission systems  400  and  300  to N (M/N) Gbps electrical signals. The skew removers  380  and  480  perform a skew-removing operation in each byte and a skew-removing operation in each frame on each of the N (M/N) Gbps electrical signals generated by the (M/N) Gbps O/E converters  371 - 37   n  and  471 - 47   n . The multiplexers  390  and  490  multiplex signals output from the skew removers  380  and  480  into the original M Gbps signal.  
       FIG. 2  shows data flow in transmission and reception directions of the two optical transmission systems  300  and  400 . The skew-removing operations are the same in the transmission and reception directions, and a skew-removing operation in each of the transmission and reception directions is performed independently.  
       FIG. 3  is a block diagram showing the structure of 4-channel 10 Gbps optical transmission systems  500  and  600  (M=40, N=4) which include an apparatus for removing skew between optical channels according to an embodiment of the present invention and perform 40 Gbps STM-256 signal transmission, and  FIG. 4  shows the structure of a 40 Gbps STM-256 signal frame according to an embodiment of the present invention.  
      The basic configuration of the four-channel 10 Gbps optical transmission systems  500  and  600  shown in  FIG. 3  is the same as that of the N-channel (M/N) Gbps optical transmission systems  300  and  400  shown in  FIG. 2 , except that the number of processed channels is different from each other. Thus, repeated descriptions will be omitted.  
       FIG. 3  shows data flow in transmission and reception directions of the two optical transmission systems  500  and  600 . The skew-removing operations are the same in the transmission and reception directions, and a skew-removing operation in each of the transmission and reception directions is performed independently. Thus, for explanatory convenience, the optical transmission system  500  which performs optical transmission of a 40 Gbps STM-256 signal using four channels (M=40 and N=4) will now be described. First, the structure and operation of an optical signal transmitter  510  of the optical transmission system  500  will be described with reference to  FIG. 3 .  
      A serial number inserter  520  inserts serial numbers into part of an overhead of a STM-256 frame in the order of frames.  
      Referring to  FIG. 4 , the STM-256 frame is divided into an overhead and a payload, and reserved bytes are contained in the overhead. The serial number inserter  520  inserts serial numbers (for example, 01, 02, and 03) into the reserved 4 bytes arranged in series in an overhead of a frame in the order of the frames, because the STM-256 signal is demultiplexed into four signals.  
      For example, if the STM-256 signal is demultiplexed into N STM-K signals (where N×K=256), N bytes in total are needed. Inserting serial numbers in each byte intends to deinterleave the STM-256 signal in each byte when the STM-256 signal is demultiplexed into the four-channel STM-64 signal. If a demultiplexing technique is used, serial numbers corresponding thereto are inserted.  
       FIG. 5  shows the structure of a signal frame in which the 40 Gbps STM-256 signal frame shown in  FIG. 4  is deinterleaved into four channels.  FIG. 5  shows only one signal frame, but the other three signal frames have the same frame structure.  
      Referring to  FIGS. 3 and 5 , the serial number inserter  520  sets an initial number and a maximum number when inserting serial numbers and starts from the initial number and stops when the serial number reaches the maximum number. The serial numbers and a boundary between A1 and A2 bytes serves as kinds of recognition marks when removing skew in an optical signal receiver  660 . In a synchronous digital hierarchy (SDH) frame structure, the A1 byte generally has a value of F6 (hexa), and the A2 byte has a value of 28 (hexa).  
      When inserting the serial numbers is performed, a demultiplexer  530  demultiplexes a 40 Gbps frame, into which the serial numbers are inserted, into four 10 Gbps electrical signals and outputs the demultiplexed electrical signals to an aligner  540 .  
       FIG. 6  is a block diagram showing the structure of the aligner  540  of the optical signal transmitter  510  shown in  FIG. 3 . Referring to  FIGS. 3 and 6 , the aligner  540  includes a buffer unit  541  comprising four variable buffers  5411 ,  5412 ,  5413 , and  5414 , and a skew information interpreter  545 .  
      The skew information interpreter  545  interprets the quantity of delay at each channel in response to skew information for each channel transmitted from a skew remover  680  of the opposite optical transmission system  600  and transmits the interpreted delay-quantity information for each channel into the variable buffers  5411 ,  5412 ,  5413 , and  5414  allocated to each channel. In this case, the skew information supplied from the skew remover  680  includes byte delay information of a signal frame for each of four channels received from the optical signal receiver  660  of the opposite optical transmission system  600  and delay information of the frame. Each of the variable buffers  5411 ,  5412 ,  5413 , and  5414  delays an electrical signal frame of a corresponding channel by the quantity of delay for each channel transmitted from the skew information interpreter  545  and outputs the delayed signal frame to each of E/O converters  551 ,  552 ,  553 , and  554 . The E/O converters  551 ,  552 ,  553 , and  554  convert the electrical signals output from the aligner  540  into four 10 Gbps optical signals. The optical signal converted by the E/O converters  551 ,  552 ,  553 , and  554  is transmitted to the optical transmission system  600  via fiber optics.  
      The structure and operation of the optical signal receiver  660  of the optical transmission system  500  will now be described below.  
      When the four 10 Gbps optical signals are received from the opposite optical transmission systems  400  and  300 , four O/E converters  571 ,  572 ,  573 , and  574  convert the received optical signals into 10 Gbps electrical signals, and a skew remover  580  removes skew in each of the four 10 Gbps electrical signals generated in the O/E converters  571 ,  572 ,  573 , and  574 .  
       FIG. 7  is a block diagram showing the structure of the skew remover  580  of the optical signal receiver  560  shown in  FIG. 3 . Referring to  FIG. 7 , the skew remover  580  includes a byte unit skew remover  581 , a frame unit skew remover  587 , and the total skew quantity calculator  589 .  
      The byte unit skew remover  581  removes skew in units of a byte due to a difference in lengths of optical transmission paths to this end, the byte unit skew remover  581  makes a boundary between A1 and A2 bytes contained in the four frames received from the O/E converters  571 ,  572 ,  573 , and  574  be the same, thereby removing skew in each frame.  
      The frame unit skew remover  587  removes skew in units of a frame that may occur during the transmission/reception of an optical signal. A time delay that may occur during the transmission/reception of an optical signal may exceed the period of one frame. In a synchronous digital hierarchy (SDH) frame structure, the period of one frame is 125 ms. Skew caused by the time delay in units of a frame is removed using serial numbers inserted into the frame.  
      The total skew quantity calculator  589  calculates the total skew quantity by receiving the skew quantity in units of a byte and the skew quantity in units of a frame for each channel from the byte unit skew remover  581  and the frame unit skew remover  587 , respectively, and transmits skew information for each channel to an aligner  640  of the opposite optical signal transmission system  600  which transmits current data. In this case, the skew information for each channel is transmitted to the aligner  640  by using a control line path separated from four optical channels or by transmitting information calculated in an overhead of the signal frame to be transmitted via an optical channel and analyzing the information using the opposite optical signal transmission system  600 . In this way, the optical signal transmission system according to the present invention removes skew in received signals using a skew remover in units of a byte and in units of a frame and feeds back skew information analyzed during skew removal to an aligner of an opposite optical signal transmission system. The opposite optical signal transmission system delays a frame to be transmitted by a predetermined amount of time based on input skew information so that skew does not occur during signal transmission. As a result, skew that occurs during transmission/reception of an optical signal can be effectively removed.  
       FIG. 8  is a block diagram showing the structure of the byte unit skew remover  581  shown in  FIG. 7 . Referring to  FIG. 8 , the byte unit skew remover  581  includes four demultiplexers  5811  connected to each channel, four FIFOs  5813 , four frame pulse generators  5814 , four variable buffers  5819 , a byte unit skew quantity calculating unit  5816  commonly connected between the four frame pulse generators  5814  and the four variable buffers  5819 , and a reference clock source  5812  connected to the four FIFOs  5813 . The byte unit skew quantity calculating unit  5816  includes a byte unit skew quantity calculator  5817  and a 4:1 selector  5818 .  
      The four-channel signal received from the O/E converters  571 ,  572 ,  573 , and  574  is demultiplexed using the demultiplxer  5811  for each channel, is converted into a parallel signal, and is input into the FIFOs  5813 , so as to perform processing on an electronic circuit. Each parallel signal stored in the FIFOs  5813  is synchronized with a reference clock signal generated in the reference clock source  5812  and is output to the frame pulse generators  5814  and the variable buffers  5819  for each channel.  
      The frame pulse generators  5814  find a boundary between A1 and A2 bytes by analyzing the structure of a frame for each channel and generates a pulse signal according to the period of the frame. In this case, it is assumed that there is no skew between parallel signals of one channel because the parallel signals are separated in parallel on the same electronic circuit and transmitted and this assumption is reasonable.  
      The pulse signal generated in each of the frame pulse generators  5814  is input into the byte unit skew quantity calculator  5817  of the byte unit skew quantity calculating unit  5816 , and the quantity of skew between channels is calculated in units of a byte. In this case, a relative skew quantity is calculated in units of a byte based on a pulse signal that reaches the byte unit skew quantity calculator  5817  the latest during a period corresponding to one frame. In other words, the pulse signal that reaches the byte unit skew quantity calculator  5817  the latest, is selected as a reference pulse using the 4:1 selector  5818  and is transmitted to each of the variable buffers  5819 , and the variable buffers  5819  output the signal based on the reference pulse. In this case, the byte unit skew quantity calculator  5817  transmits the quantity of skew calculated in units of a byte for each channel to each of the variable buffers  5819 , each of which correspond to one channel. The variable buffers  5819  adjust the size of a buffer so that the position of each frame output from the variable buffers  5819  is the same.  
      In a procedure of removing skew in units of a byte, skew between four channels in units of a byte is removed, and the position of the frame is made the same, but skew in units of a frame is not removed. Removing skew in units of a frame is performed using the frame unit skew remover  587  connected to an output terminal of the byte unit skew remover  581 .  
      The quantity of skew calculated in units of a byte for each channel using the byte unit skew quantity calculator  5817  is transmitted to the total skew quantity calculator  589  of the skew remover  580 . In this case, information on a channel that reaches the byte unit skew quantity calculator  5817  the latest during the period corresponding to one frame is together transmitted to the total skew quantity calculator  589 .  
       FIG. 9  shows an input/output signal of the byte unit skew remover  581  shown in  FIG. 8 . Referring to  FIGS. 8 and 9 , with respect to data input for each channel during time T of one frame, a boundary between A1 and A2 bytes of three channels reaches the byte unit skew remover  581  the latest. In this case, the byte unit skew remover  581  makes the A1 and A2 bytes of each channel be the same by delaying a frame of different channels (that is, a first channel, a second channel, and a fourth channel) based on the information on the boundary between the A1 and A2 bytes of the three channels that reaches the byte unit skew remover  581  the latest. An output of the byte unit skew remover  581  is input into the frame unit skew remover  587 .  
      The byte unit skew remover  581  can make the position of bytes in the frame equal by introducing a delay in units of a byte in each frame of each channel using information on the boundary between the A1 and A2 bytes of the overhead of the frame. However, when a difference in time between a frame reaching the byte unit skew remover  581  the earliest and a frame reaching the byte unit skew remover  581  the latest is greater than a half-period of the frame, a delay in units of a frame occurs. Thus, it is difficult to restore the position of a frame for each channel during signal transmission of the optical signal transmitter using only the byte unit skew remover  581 . Thus, a skew removing operation is performed in units of both a byte and a frame so that skew can be more effectively removed.  
       FIG. 10  is a block diagram showing the structure of the frame unit skew remover  587  shown in  FIG. 7 .  
      Referring to  FIGS. 7 and 10 , the frame unit skew remover  587  includes four serial number extractors  5875  connected for each channel, four frame unit delay units  5879 , and a frame unit skew quantity calculating unit  5876  commonly connected between the four serial number extractors  5875  and the four frame unit delay units  5879 . The frame unit skew quantity calculating unit  5876  includes a frame unit skew quantity calculator  5877  and a 4:1 selector  5878 .  
      The four-channel signal received from the byte unit skew remover  581  is input into the serial number extractor  5875  and the frame unit delay unit  5879 . The serial number extractor  5875  extracts serial numbers by retrieving the overhead of the received signal. The serial numbers extracted by each of the serial number extractors  5875  for each of the four channels are input into the frame unit skew quantity calculator  5877 , and the quantity of skew is calculated in units of a frame for each of the four channels. The calculated skew quantity is input into each of the frame unit delay units  5879 , and a frame signal of each channel is delayed in units of a frame. Each of the frame unit delay units  5879  uses a reference pulse signal output from the frame unit skew quantity calculator  5877  as a reference signal. The frame unit skew quantity calculator  5877  outputs a frame pulse signal that reaches the frame unit skew quantity calculator  5877  the latest during a period of one frame, as the reference pulse signal. A calculation result (that is, a skew quantity in units of a frame) of the skew quantity of each channel using the frame unit skew quantity calculator  5877  is transmitted to the total skew quantity calculator  589  of the skew remover  580 . In this case, information on a channel that reaches the frame unit skew quantity calculator  5877  the latest during a period corresponding to one frame, is together transmitted to the total skew quantity calculator  589 .  
       FIG. 11  shows an input/output signal of the frame unit skew remover  587  shown in  FIG. 10 .  
      Referring to  FIGS. 9 through 11 , an output of the byte unit skew remover  581  shown in  FIG. 7  is used as an input of the frame unit skew remover  587 . A serial number byte indicates that information on a fourth channel reaches the frame unit skew quantity calculator  5877  by a time corresponding to a period of one frame later than first and second channels and by a time corresponding to a period of two fames later than a third channel. In this case, the frame unit delay unit  5879  shown in  FIG. 10  delays the first and second channels by one frame and delays the third channel by two frames so that the position of frames of the first through third channels is the same as the position of frames of the fourth channel.  
      In this way, the skew remover  580  performs an operation of removing a byte/frame skew between input channels using a combined operation of the byte unit skew remover  581  and the frame unit skew remover  587 . Each of the skew quantity and reference pulse information for each channel output from the skew quantity calculator  5817  of the byte unit skew remover  581  and the skew quantity calculator  5877  of the frame unit skew remover  587  is input into the total skew quantity calculator  589 . The total skew quantity calculator  589  calculate a skew quantity for each channel by analyzing the input information and feeds back the calculated skew information to an aligner of an opposite optical signal transmitter. In this way, the skew is removed and simultaneously, is corrected so that the skew that varies in a real-time can be more easily removed.  
       FIG. 12  shows an operation of the total skew quantity calculator  589  shown in  FIG. 7 .  
      Referring to  FIG. 12 , the total skew quantity calculator  589  calculates a skew quantity for each channel by performing the following operation. If a third channel of the byte unit skew remover  581  generates a reference pulse signal, there are skew quantities such as delta — 1_byte, delta — 2_byte, and delta — 4_byte in the first, second, and fourth channels based on the third channel. And, if the fourth channel of the frame unit skew remover  587  generates the reference pulse signal, there are skew quantities such as delta — 1_frame, delta — 2_frame, and delta — 3_frame in the first, second, and third channels based on the fourth channel. As a result, (delta — 1_byte+delta — 1_frame) is loaded on the first channel of the skew information, (delta — 2_byte+delta — 2_frame) is loaded on the second channel thereof, (0+delta — 3_frame) is loaded on the third channel thereof, and (delta — 4 — +0) is loaded on the fourth channel thereof.  
      The total skew quantity calculator  589  calculates a total skew quantity in this manner and feeds back the calculated skew information to the aligner  640  of the opposite optical signal transmitter  600 . The aligner  640  of the opposite optical signal transmitter  600  interprets the skew information using the skew information interpreter  545  and adjusts the buffering quantity of each of the variable buffers of the aligner  640  using the interpreted skew information so that a final skew quantity of an optical signal receiver is 0.  
      When the buffering quantity of each variable buffer is adjusted, due to rapid adjustment of a skew quantity, the aligner  640  prevents a frame from disappearing at one channel of four channels during a period of one frame, because the skew information interpreter of the aligner  640  adjusts the buffering quantity of each variable buffer gradually by a small quantity according to time. An increase in the skew quantity according to time can be prevented by gradually adjusting the skew quantity using the aligner  640 .  
      An apparatus for removing skew between optical channels or an optical transmission system for removing skew between the optical channels according to the present invention are implemented with a single chip shape or a field programmable gate array (FPGA) and can be applied to actual applications. In the present invention, a byte similar to a sequential indicator H 4  byte of a payload overhead (POH) used in virtual connection is used. However, this is only a part of the invention, and the present invention is not limited to the structure of a frame suggested in an ITU-T G.707 standard document and instead can be applied to a frame with an overhead, such as STM-256, STM-4, STM-16, and STM-64.  
      As described above, in the optical transmission system for removing skew between the optical channels according to the present invention, skew occurring in a received signal is removed in units of both a byte and a frame, and skew information analyzed during skew removal is fed back to an opposite optical signal transmitter, and a frame to be transmitted is delayed for a predetermined amount of time so that skew does not occur during signal transmission. As such, skew, which is a delay or time difference between optical channels that may occur due to a difference in lengths of optical transmission paths and time delay caused by an electronic circuit composing a transceiver, can be effectively removed.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims.