Abstract:
A synchronization detection method executed by a processor included in a transmission apparatus, the synchronization detection method includes receiving a frame in accordance with a predetermined timing; calculating an unmatched bit number that indicates a number of unmatched bits between a bit stream of the received frame and an expected bit stream that indicates a bit stream that has been expected to be received; acquiring an accumulated number by accumulating the unmatched bit number at the predetermined timing; determining whether the received frame is synchronized with a predetermined signal by comparing the accumulated number and a predetermined threshold; and starting processing for establishing synchronization between the received frame and the predetermined signal when it is determined that the received frame is not synchronized with the predetermined signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-232323, filed on Nov. 27, 2015, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a synchronization detection method and a transmission apparatus. 
       BACKGROUND 
       [0003]    In a synchronized state of frame data (in frame (IF)), when bit errors of frame data continues over a plurality of pieces of frame data and for a predetermined number of times (for example, five times) in succession, or when a synchronization phase of a frame being received is changed, a receiving apparatus determines synchronization loss of the frame data. When synchronization loss (out of frame (OOF)) is determined, the receiving apparatus shifts the status thereof from “IF” to “OOF” and starts up synchronization pull-in processing for establishing synchronization of frame data. Accordingly, when the synchronization phase of the frame of the frame data is displaced, OOF is desirably determined, originally. As related arts, Japanese Laid-open Patent Publication No. 11-122233, Japanese Laid-open Patent Publication No. 9-130741, and Japanese Laid-open Patent Publication No. 2007-267085 are disclosed, for example. 
         [0004]    However, in a receiving apparatus, even in the case of a bit error of only one bit in a bit stream within the frame data, for example, the error is counted as a bit error. With this, when the count value exceeds a predetermined number of times, even if no change of the frame phase actually occurs, the status of the frame data shifts to OOF in some cases. 
         [0005]    The receiving apparatus also has a function of correcting a bit error in frame data using forward error correction (FEC). However, when the status is OOF, the FEC function is unusable. With this, when the state in which the status is OOF frequently occurs, the state in which the FEC function is unusable also frequently occurs, whereby the data transmission efficiency is lowered. 
         [0006]    For this reason, in the receiving apparatus, it is conceivable to increase the predetermined number of times for determining that the status has shifted to OOF in order to avoid a shift to OOF due to a minor bit error. However, this makes it difficult to shift the status to OOF when OOF actually occurs. As a result, even when change of the frame phase actually occurs, a shift to OOF is delayed. 
         [0007]    An aspect is to provide a receiving apparatus and a synchronization detection method with which synchronization loss may be detected with high accuracy. 
       SUMMARY 
       [0008]    According to an aspect of the invention, a synchronization detection method executed by a processor included in a transmission apparatus, the synchronization detection method includes receiving a frame in accordance with a predetermined timing; calculating an unmatched bit number that indicates a number of unmatched bits between a bit stream of the received frame and an expected bit stream that indicates a bit stream that has been expected to be received; acquiring an accumulated number by counting the unmatched bit number at the predetermined timing; determining whether the received frame is synchronized with a predetermined signal by comparing the accumulated number and a predetermined threshold; and starting processing for establishing synchronization between the received frame and the predetermined signal based on a synchronization pattern in the bit stream of the received frame, when it is determined that the received frame is not synchronized with the predetermined signal. 
         [0009]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is an explanatory diagram illustrating an example of a transmission system according to a first embodiment; 
           [0012]      FIG. 2  is a block diagram illustrating an example of a configuration in a transmission apparatus according to the first embodiment; 
           [0013]      FIG. 3  is an explanatory diagram illustrating an example of a format configuration of a frame; 
           [0014]      FIG. 4  is a block diagram illustrating an example of a configuration in a first synchronization determination unit; 
           [0015]      FIG. 5  is an explanatory diagram illustrating an example of a state shift of a state machine; 
           [0016]      FIG. 6  is a flowchart illustrating an example of a processing operation of the first synchronization determination unit related to first synchronization monitoring processing; 
           [0017]      FIG. 7  is a timing diagram illustrating an example of each signal in the first synchronization determination unit at the time of synchronization pull-in according to the first embodiment; 
           [0018]      FIG. 8  is a timing diagram illustrating an example of each signal in the first synchronization determination unit at the time of OOF according to the first embodiment; 
           [0019]      FIG. 9  is a block diagram illustrating an example of a configuration in a second synchronization determination unit according to a second embodiment; 
           [0020]      FIG. 10  is an explanatory diagram illustrating an example of a state shift of a state machine; 
           [0021]      FIG. 11  is a flowchart illustrating an example of a processing operation of the second synchronization determination unit related to second synchronization monitoring processing; 
           [0022]      FIG. 12  is a timing diagram illustrating an example of each signal in the second synchronization determination unit at the time of synchronization pull-in according to the second embodiment; and 
           [0023]      FIG. 13  is an explanatory diagram illustrating an example of a relation between a BER before FEC correction and an OOF occurrence interval. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    Hereinafter, with reference to the drawings, embodiments of a receiving apparatus and a synchronization detection method disclosed by the present application will be described in detail. These embodiments are not intended to limit the disclosed techniques. The embodiments described below may be combined as appropriate in a range in which no contradiction is generated. 
       First Embodiment 
       [0025]      FIG. 1  is an explanatory diagram illustrating an example of a transmission system  1  according to a first embodiment. The transmission system  1  illustrated in  FIG. 1  includes a plurality of transmission apparatuses  2 . Each of the transmission apparatuses  2  is connected to an optical network  4 A using an optical fiber  3  or other as medium or a user network  4 B using an electric line or other as medium, for example. The optical network  4 A is a network such as optical transport network (OTN) and synchronous optical network (SONET)/synchronous digital hierarchy (SDH), for example. The user network  4 B is a network such as Ethernet®, for example. Each transmission apparatus  2  is connected to a transmission apparatus  2  on an opposite side thereof with the optical fiber  3  and transmits an OTN frame, for example. 
         [0026]      FIG. 2  is a block diagram illustrating an example of a configuration in a transmission apparatus  2  according to the first embodiment. For the convenience of explanation, the transmission apparatus  2  will be described. However, the transmission apparatus  2  on the opposite side thereof has substantially the same configuration and thus is denoted with the same reference numeral, and the overlapped explanation for the configuration and operation thereof will be omitted. 
         [0027]    The transmission apparatus  2  includes a client interface (CLIF)  11 , a network interface (NWIF)  12 , and a switch (SW)  13 . The CLIF  11  is, for example, a communication IF that administers communication with an electric line connected to a terminal  5  in the user network  4 B. The NWIF  12  is a communication IF that administers communication with the optical fiber  3  in the optical network  4 A. The SW  13  is a switch that switchably connects communication between the CLIF  11  and the NWIF  12 , or between each of the CLIFs  11 . 
         [0028]    The NWIF  12  includes a frame generation unit  21 , an FEC encode unit  22 , a scramble unit  23 , a first synchronization determination unit  24 , a descramble unit  25 , an FEC decode unit  26 , and a frame processing unit  27 . The frame generation unit  21  is, for example, a generation unit that assembles frames such as OTN frames. The FEC encode unit  22  adds FEC into a frame. The scramble unit  23  is a processing unit that scrambles the entire frame. 
         [0029]    The first synchronization determination unit  24  is a determination unit that determines a synchronization state of a synchronization pattern in a reception frame, for example, IF or OOF. Cases where the synchronization determination is shifted to OOF includes a first case where a reception frame has disappeared, a second case where the phase of a reception frame is displaced, and a third case where a bit error rate (BER) is high on a transmission path such as the optical fiber  3 . The first case is, for example, a case where a reception frame itself is hard to receive due to causes such as disconnection of a transmission path or failure of the transmission apparatus  2  on an opposite side, whereby an OOF state continues. The second case is a case where although reception frames are received, due to causes such as switching of frame data of the transmission apparatus  2  on an opposite side or the influence of a jitter component of a transmission path, the frame is received at a timing deviated from a timing of an IF state that has already been established. In the second case, extractions of overhead information and client data from reception frames are disabled. The third case is a case where although no timing deviation is present in receiving reception frames and an IF state is kept, the transmission path quality (line quality) is deteriorated and a bit error is thus generated. 
         [0030]    The descramble unit  25  is a processing unit that cancels scrambling of reception frames that have been scrambled. The FEC decode unit  26  is a processing unit that corrects a bit error with respect to a bit stream in a reception frame using FEC in the reception frame. The frame processing unit  27  is a processing unit that disassembles the reception frame corrected by the FEC decode unit  26 . 
         [0031]      FIG. 3  is an explanatory diagram illustrating an example of a format configuration of a frame  100 . The frame  100  illustrated in  FIG. 3  corresponds to an OTN frame. The frame  100  includes an overhead region  101 , a payload region  102 , and an FEC region  103 . The frame  100  is composed of 4 rows×4080 bytes. Transmission is performed in the order of 1st to 4080th bytes in the 1st row, 1st to 4080th bytes in the 2nd row, 1st to 4080th bytes in the 3rd row, and 1st to 4080th bytes in the 4th row. An overhead region  101  is, for example, a region for storing therein an administering function or a monitoring function, in particular, a synchronization pattern among the transmission apparatuses  2 . The synchronization pattern is a pattern specified by a predetermined bit stream for the transmission apparatus  2  at the reception side to acknowledge the head of the frame. The payload region  102  is, for example, a region for storing therein client data. The FEC region  103  is a region for storing FEC of the frame. 
         [0032]    The frame generation unit  21  in the NWIF  12  causes the overhead region  101  in the frame  100  to store therein the synchronization pattern or the administering function, causes the payload region  102  in the frame  100  to store therein the client data, and causes the FEC region  103  in the frame  100  to store therein FEC. The frame generation unit  21  generates frame data. The NWIF  12  continuously transmits the frame  100  to the optical fiber  3  with a predetermined frame cycle specified by the OTN standard. The NWIF  12  acknowledges the synchronization pattern in the reception frame and thereby determines the synchronization state of the frame data, that is, whether the state is IF or  00 F. The NWIF  12  then outputs a frame timing in the state of IF to the descramble unit  25 , the FEC decode unit  26 , and the frame processing unit  27 . The descramble unit  25 , based on the frame timing in the state of IF, performs descrambling processing of the reception frame. Furthermore, the FEC decode unit  26 , based on the frame timing in the state of IF, performs error correction of a bit stream in the reception frame. Furthermore, the frame processing unit  27 , based on the frame timing in the state of IF, extracts the overhead information from the overhead region  101  in the frame  100  and extracts the client data from the payload region  102  in the frame  100 . 
         [0033]      FIG. 4  is a block diagram illustrating an example of a configuration in the first synchronization determination unit  24 . The first synchronization determination unit  24  illustrated in  FIG. 4  includes a shift register  31 , a first comparison unit  32 , a synchronous counter  33 , a frame counter  34 , a state machine  35 , a distance calculation unit  36 , an accumulation unit  37 , and a second comparison unit  38 . The distance calculation unit  36  and the accumulation unit  37  are calculation units. The second comparison unit  38  is a determination unit. The state machine  35  is, for example, a determination unit and a control unit. 
         [0034]    The shift register  31  shifts a bit stream in the reception frame bit by bit and converts the shifted bit stream into a bit stream with the same width as that of the synchronization pattern. The shift register  31  then outputs that bit stream to the descramble unit  25 , the first comparison unit  32 , and the distance calculation unit  36 . The first comparison unit  32  compares a bit stream of the synchronization pattern in the reception frame and an expected value of a predetermined synchronization pattern. The first comparison unit  32  has a region for storing therein the predetermined synchronization pattern. When the bit stream and the expected value perfectly match each other, the first comparison unit  32  outputs a pulse of a match signal to the state machine  35 , the synchronous counter  33 , and the accumulation unit  37 . 
         [0035]    The synchronous counter  33  is a counter that counts detection of a synchronization pattern in accordance with the match signal. The frame counter  34  is a counter that starts a count operation of the frame timing of the reception frame in accordance with the match signal from the first comparison unit  32 . The frame timing is, for example, a head timing of the reception frame in the predetermined cycle specified by the OTN standard. The frame counter  34  outputs the frame timing to the synchronous counter  33 , the state machine  35 , and the distance calculation unit  36 . For example, at the time of synchronization pull-in, when the first match signal is input and then the next match signal is input at a timing the next frame timing is input, the synchronous counter  33  outputs a synchronization signal for shifting the synchronization determination to IF to the state machine  35 . The next match signal becomes a match signal delayed by one clock from the timing at which a match signal is detected. 
         [0036]    The distance calculation unit  36  compares a bit stream of the synchronization pattern in the reception frame and an expected value of a predetermined synchronization pattern from the shift register  31 . The distance calculation unit  36  then calculates the unmatched bit number between the bit stream and the expected value, that is, a hamming distance HD. The accumulation unit  37  accumulates the hamming distance HD acquired at the distance calculation unit  36  for each frame timing and calculates an accumulated hamming distance AHD obtained by the accumulation. The accumulation unit  37  has a region for storing therein the accumulated hamming distance AHD. The accumulation unit  37  resets the accumulated hamming distance AHD being stored in accordance with a load signal from the state machine  35 . 
         [0037]    The second comparison unit  38  determines whether the accumulated hamming distance AHD has exceeded a threshold THD. The threshold THD is the hamming distance HD with which the synchronization state of the reception frame is determined as  00 F, for example, 7 times. The second comparison unit  38  has a region for storing therein the threshold THD. The second comparison unit  38  outputs a comparison result when the accumulated hamming distance AHD has exceeded the threshold THD to the state machine  35 . 
         [0038]      FIG. 5  is an explanatory diagram illustrating an example of a state shift of the state machine  35 . When the state machine  35  illustrated in  FIG. 5  detects a synchronization signal from the synchronous counter  33 , the state machine  35  shifts the synchronization determination of the reception frame to IF and outputs a load signal to the frame counter  34  and the accumulation unit  37 . When the state machine  35  detects a comparison result in a case where the accumulated hamming distance AHD has exceeded the threshold THD, the state machine  35  shifts the synchronization determination of the reception frame to OOF and outputs an OOF notification thereof. When the state machine  35  detects a comparison result in a case where the accumulated hamming distance AHD has not exceeded the threshold THD, the state machine  35  continues IF as the synchronization determination of the reception frame. 
         [0039]    When the synchronization determination is shifted to  00 F, the first synchronization determination unit  24  starts the synchronization pull-in operation again. Furthermore, when the first synchronization determination unit  24  detects a synchronization signal from the synchronous counter  33 , the first synchronization determination unit  24  shifts the synchronization determination of the reception frame to IF. When the synchronization determination is IF, as long as the bit stream of the synchronization pattern in the reception frame and the expected value perfectly match each other at the first comparison unit  32 , the first synchronization determination unit  24  continues IF as the synchronization determination. 
         [0040]    Next, an operation of the transmission system  1  according to the first embodiment will be described.  FIG. 6  is a flowchart illustrating an example of a processing operation of the first synchronization determination unit  24  related to first synchronization monitoring processing. The first synchronization monitoring processing is processing for detecting the synchronization pattern, accumulating the hamming distance HD of the synchronization pattern, and when the accumulated hamming distance AHD has exceeded a predetermined threshold THD, shifting the synchronization determination to OOF. The state machine  35  of the first synchronization determination unit  24  illustrated in  FIG. 4  is at the time of synchronization pull-in in the initial state thereof. With this, the synchronization determination is to be OOF (S 11 ), and whether the synchronization pattern has been detected is determined (S 12 ). The processing for determining whether the synchronization pattern has been detected is performed with the first comparison unit  32  determining whether a bit stream of the synchronization pattern in the reception frame and an expected value of the synchronization pattern perfectly match each other. When the perfect match is determined, it is determined that the synchronization pattern has been detected. 
         [0041]    When the synchronization pattern has been detected (Yes at S 12 ), the state machine  35  sets the synchronization determination to OOF. The state machine  35  then starts a count operation of the frame timing at the frame counter  34  (S 13 ). The state machine  35  determines whether the synchronization pattern has been detected at the frame timing at the frame counter  34  (S 14 ). 
         [0042]    When the synchronization pattern has been detected at the frame timing (Yes at S 14 ), the state machine  35  determines that the synchronization pattern has been detected twice continuously, and sets the synchronization determination to IF and sets the accumulated hamming distance AHD to 0 C(S 15 ). Furthermore, the state machine  35  determines whether the synchronization pattern has been detected at the frame timing at the frame counter  34  (S 16 ). 
         [0043]    When the synchronization pattern has not been detected at the frame timing (No at S 16 ), the state machine  35  calculates the hamming distance HD of the synchronization pattern of the frame timing (S 17 ). The distance calculation unit  36  calculates the hamming distance HD between the bit stream of the synchronization pattern of the frame timing in the reception frame and the expected value of the predetermined synchronization pattern. The state machine  35  sets the synchronization determination to IF and accumulates the hamming distance HD of the synchronization pattern on the accumulated hamming distance AHD (S 18 ). The state machine  35  then determines whether the accumulated hamming distance AHD is equal to or higher than 1 and has exceeded the threshold THD (S 19 ). 
         [0044]    When the accumulated hamming distance AHD is equal to or higher than 1 and has exceeded the threshold THD (Yes at S 19 ), the state machine  35  moves to S 11  in order to shift the synchronization determination to OOF. As a result, the state machine  35  shifts synchronization determination to OOF because the accumulated hamming distance AHD has exceeded the threshold THD. When the accumulated hamming distance AHD is equal to or higher than 1 and has not exceeded the threshold THD (No at S 19 ), the state machine  35  moves to S 16  in order to determine whether the synchronization pattern has been detected at the frame timing. As a result, the state machine  35  continues IF as the synchronization determination even in the third case where a minor bit error occurs because the accumulated hamming distance AHD has not exceeded the threshold THD. 
         [0045]    When the synchronization pattern has not been detected (No at S 12 ), the state machine  35  moves to S 11  in order to shift the synchronization determination to OOF. When the synchronization pattern has not been detected at the frame timing (No at S 14 ), the state machine  35  moves to S 11  in order to shift the synchronization determination to OOF. 
         [0046]    When the synchronization pattern has been detected at the frame timing (Yes at S 16 ), the state machine  35  moves to S 15  in order to set the synchronization determination to IF and set the accumulated hamming distance AHD to 0. 
         [0047]    The state machine  35  that performs the first synchronization monitoring processing shifts the synchronization determination to IF when the synchronization pattern has been detected continuously at the time of synchronization pull-in. As a result, the transmission apparatus  2  may achieve pull-in from OOF to IF. 
         [0048]    When the synchronization pattern has not been detected at the frame timing, the state machine  35  calculates the hamming distance HD of the synchronization pattern of the frame timing and accumulates the hamming distance HD to calculate the accumulated hamming distance AHD. When the accumulated hamming distance AHD has exceeded the threshold THD, the state machine  35  shifts the synchronization determination to OOF. As a result, because there is a high risk of the second case where OOF actually occurs when the accumulated hamming distance AHD has exceeded the threshold THD, the transmission apparatus  2  may quickly detect OOF. 
         [0049]    When the accumulated hamming distance AHD has not exceeded the threshold THD, the state machine  35  continues IF as the synchronization determination. As a result, because the case merely is the third case where a minor bit error occurs when the accumulated hamming distance AHD has not exceeded the threshold THD, the transmission apparatus  2  may suppress erroneous detection of OOF by continuing IF as the synchronization determination without rashly shifting to OOF. 
         [0050]      FIG. 7  is a timing diagram illustrating an example of each signal in the first synchronization determination unit  24  at the time of synchronization pull-in according to the first embodiment. When a bit stream of the synchronization pattern in the reception frame and the expected value perfectly match each other, the first comparison unit  32  outputs a first match signal M 1 . At this time, the state machine  35  shifts the state of the synchronization determination to OOF in accordance with the match signal M 1  and outputs a load signal L 1  in accordance with the match signal M 1  to the synchronous counter  33  and the frame counter  34 . 
         [0051]    The frame counter  34  starts a count operation of the frame timing in accordance with the load signal L 1 . Furthermore, the synchronous counter  33  counts the synchronization pattern 0x1 in accordance with the match signal M 1 . 
         [0052]    The first comparison unit  32  outputs a next match signal M 2 . At this time, the state machine  35  outputs a load signal L 2  in accordance with the match signal M 2 . The synchronous counter  33  counts the synchronization pattern 0x2 in accordance with the match signal M 2 , delays the match signal M 2  by one clock, and when the match signal M 2  and a frame timing F 1  at the frame counter  34  match each other, outputs a synchronization signal S 1  to the state machine  35 . As a result, the state machine  35  shifts the synchronization determination from OOF to IF in accordance with the synchronization signal S 1 . With this, the synchronization pull-in operation is completed. The first comparison unit  32  then outputs a next match signal M 3 . At this time, the state machine  35  outputs a load signal L 3  in accordance with a match signal M 3 . Furthermore, the synchronous counter  33  counts the synchronization pattern 0x3 in accordance with the match signal M 3 . 
         [0053]      FIG. 8  is a timing diagram illustrating an example of each signal in the first synchronization determination unit  24  at the time of OOF according to the first embodiment. The example in  FIG. 8  is an example of an operation until shifting to OOF during continuation of IF as the synchronization determination. For the convenience of explanation, description is made on the assumption that the predetermined threshold is “4”. 
         [0054]    The state machine  35  outputs a load signal L 4  in accordance with the match signal M 4  from the first comparison unit  32 . The distance calculation unit  36  calculates “0” as the hamming distance HD of the synchronization pattern of the timing of the match signal M 4 . The accumulation unit  37  accumulates the hamming distance HD of “0” on the accumulated hamming distance AHD. Furthermore, the accumulation unit  37  resets the accumulated hamming distance AHD in accordance with the load signal L 4  from the state machine  35 . 
         [0055]    Next, the state machine  35  outputs a load signal L 5  in accordance with a match signal M 5  from the first comparison unit  32 . The distance calculation unit  36  calculates “0” as the hamming distance HD of the synchronization pattern of the timing of the match signal M 5 . The accumulation unit  37  accumulates the hamming distance HD of “0” on the accumulated hamming distance AHD. Furthermore, the accumulation unit  37  resets the accumulated hamming distance AHD in accordance with the load signal L 5  from the state machine  35 . 
         [0056]    The first comparison unit  32  does not output a match signal M 6  because the bit stream of the synchronization pattern and the expected value do not perfectly match each other. The distance calculation unit  36  calculates “1” as the hamming distance HD between the bit stream and the expected value. The accumulation unit  37  accumulates the hamming distance HD of “1” on the accumulated hamming distance AHD. Because the accumulated hamming distance AHD has not exceeded the predetermined threshold THD, the second comparison unit  38  does not output a comparison result. 
         [0057]    The state machine  35  outputs a load signal L 7  in accordance with a match signal M 7  from the first comparison unit  32 . The distance calculation unit  36  calculates “0” as the hamming distance HD of the synchronization pattern of the timing of the match signal M 7 . The accumulation unit  37  accumulates the hamming distance HD of “0” on the accumulated hamming distance AHD. Furthermore, the accumulation unit  37  resets the accumulated hamming distance AHD in accordance with the load signal L 7  from the state machine  35 . 
         [0058]    The first comparison unit  32  does not output a match signal M 8  because the bit stream of the synchronization pattern and the expected value do not perfectly match each other. The distance calculation unit  36  calculates “2” as the hamming distance HD between the bit stream and the expected value. The accumulation unit  37  accumulates the hamming distance HD of “2” on the accumulated hamming distance AHD “0”. Because the accumulated hamming distance AHD “2” has not exceeded the predetermined threshold THD “4”, the second comparison unit  38  does not output a comparison result. 
         [0059]    Next, the first comparison unit  32  does not output a match signal M 9  because the bit stream of the synchronization pattern and the expected value do not perfectly match each other. The distance calculation unit  36  calculates “1” as the hamming distance HD between the bit stream and the expected value. The accumulation unit  37  accumulates the hamming distance HD of “1” on the accumulated hamming distance AHD “2”. Because the accumulated hamming distance AHD “3” has not exceeded the predetermined threshold THD “4”, the second comparison unit  38  does not output a comparison result. 
         [0060]    Next, the first comparison unit  32  does not output a match signal M 10  because the bit stream of the synchronization pattern and the expected value do not perfectly match each other. The distance calculation unit  36  calculates “2” as the hamming distance HD between the bit stream and the expected value. The accumulation unit  37  accumulates the hamming distance HD of “2” on the accumulated hamming distance AHD “3”. Because the accumulated hamming distance AHD “5” has exceeded the predetermined threshold THD “4”, the second comparison unit  38  outputs a comparison result C 1  to the state machine  35 . As a result, the state machine  35  shifts the synchronization determination from IF to OOF in accordance with the comparison result C 1  and starts the synchronization pull-in operation. 
         [0061]    In the second case, the bit stream of the synchronization pattern in the reception frame and the expected value do not match each other with respect to a half of the bits stochastically. The first synchronization determination unit  24  thus accumulates the hamming distance HD on the accumulated hamming distance AHD. For example, because the number of bits in the synchronization pattern is 24 bits by the OTN standard, the first synchronization determination unit  24  accumulates the hamming distance HD corresponding to 12 bits, which is half of the above-described number, on the accumulated hamming distance AHD. The accumulated hamming distance AHD then exceeds the threshold THD (for example, 7 times). As a result, in the second case, the first synchronization determination unit  24  may immediately shift the synchronization determination to OOF and start the synchronization pull-in operation. 
         [0062]    In the third case, in the synchronization pattern in the reception frame, a minor bit error, for example, an error of one bit is dominant stochastically. The first synchronization determination unit  24  thus accumulates the hamming distance HD “1” on the accumulated hamming distance AHD. Because the accumulated hamming distance AHD does not easily exceed the threshold THD, the first synchronization determination unit  24  does not rashly shift the synchronization determination to OOF and continues IF. As a result, in the third case, because the hamming distance HD accumulated for each frame is around 1 and does not easily exceed the threshold THD, erroneous detection of OOF may be suppressed. 
         [0063]    The transmission apparatus  2  according to the first embodiment calculates the hamming distance HD of the synchronization pattern of the frame timing, sequentially accumulates the hamming distance HD on the accumulated hamming distance AHD, and when the accumulated hamming distance AHD has exceeded the threshold THD, shifts the synchronization determination to OOF. As a result, because there is a high risk that OOF actually occurs, the transmission apparatus  2  may quickly detect OOF. 
         [0064]    When the accumulated hamming distance AHD has not exceeded the threshold THD, the transmission apparatus  2  shifts the synchronization determination to IF. As a result, because only a minor bit error occurs, the transmission apparatus  2  continues IF as the synchronization determination without rashly shifting to OOF, whereby erroneous detection of OOF may be suppressed. 
         [0065]    When the accumulated hamming distance AHD is small, the transmission apparatus  2  increases the number of frames before OOF is determined. By contrast, when the accumulated hamming distance AHD is large, the transmission apparatus  2  decreases the number of frames before OOF is determined. Accordingly, in the second case, OOF may be detected with high accuracy. In the third case, erroneous detection of OOF may be suppressed. 
         [0066]    The first comparison unit  32  according to the first embodiment compares the bit stream of the synchronization pattern in the reception frame and the expected value, and when the bit stream and the expected value perfectly match each other, outputs a match signal. However, instead of a match signal, the hamming distance HD between the bit stream and the expected value may be output. An embodiment for this case will be described below as a second embodiment. 
       Second Embodiment 
       [0067]      FIG. 9  is a block diagram illustrating an example of a configuration in a second synchronization determination unit  24 A according to the second embodiment. A component having the same structure as that in the transmission apparatus  2  according to the first embodiment is denoted with the same reference numeral, and any overlapped explanation of the structure and operation thereof will be omitted. The difference between the first synchronization determination unit  24  and the second synchronization determination unit  24 A is that instead of the first comparison unit  32  and the distance calculation unit  36 , a first distance calculation unit  36 A is arranged in the second synchronization determination unit  24 A. 
         [0068]    The first distance calculation unit  36 A compares a bit stream of the synchronization pattern in the reception frame from the shift register  31  and an expected value of a predetermined synchronization pattern. The first distance calculation unit  36 A compares the bit stream and the expected value and outputs the hamming distance HD which is a result of the comparison to a state machine  35 A, the synchronous counter  33 , and the accumulation unit  37 . When the hamming distance HD is 0, the synchronous counter  33  counts detection of a synchronization pattern. Furthermore, when the synchronization patterns are continuously detected, the synchronous counter  33  outputs a synchronization signal to the state machine  35 A. 
         [0069]    The accumulation unit  37  accumulates the hamming distance HD on the accumulated hamming distance AHD and outputs the accumulated hamming distance AHD to the second comparison unit  38 . The second comparison unit  38  compares the accumulated hamming distance AHD from the accumulation unit  37  and the threshold THD and outputs a comparison result to the state machine  35 A. 
         [0070]      FIG. 10  is an explanatory diagram illustrating an example of a state shift of the state machine  35 A. When the state machine  35 A illustrated in  FIG. 10  detects a comparison result indicating that the accumulated hamming distance AHD has exceeded the threshold THD, the state machine  35 A shifts the synchronization determination to  00 F. When the state machine  35 A does not detect the comparison result indicating that the accumulated hamming distance AHD has exceeded the threshold THD, the state machine  35 A continues IF as the synchronization determination. 
         [0071]    When the hamming distance HD is 0, the state machine  35 A outputs a load signal to the frame counter  34 , the synchronous counter  33 , and the accumulation unit  37 . The frame counter  34  and the synchronous counter  33  perform reset in accordance with the load signal. The accumulation unit  37  sets the accumulated hamming distance AHD to 0 in accordance with the load signal. 
         [0072]    When the synchronization determination is shifted to  00 F, the second synchronization determination unit  24 A starts the synchronization pull-in operation again. Furthermore, when the second synchronization determination unit  24 A detects a synchronization signal from the synchronous counter  33 , the second synchronization determination unit  24 A shifts the synchronization determination of the reception frame to IF. When the synchronization determination is IF, as long as the hamming distance HD between the bit stream of the synchronization pattern in the reception frame and the expected value is 0 at the first distance calculation unit  36 A, the second synchronization determination unit  24 A continues IF as the synchronization determination. 
         [0073]    Next, an operation of the transmission apparatus  2  according to the second embodiment will be described.  FIG. 11  is a flowchart illustrating an example of a processing operation of the second synchronization determination unit  24 A related to second synchronization monitoring processing. The second synchronization monitoring processing is processing for detecting the synchronization pattern using the hamming distance HD, accumulating the hamming distance HD of the synchronization pattern, and when the accumulated hamming distance AHD equals to a predetermined threshold THD, shifting the synchronization determination to OOF. 
         [0074]    The state machine  35 A of the second synchronization determination unit  24 A, as the synchronization pull-in operation, shifts the synchronization determination to OOF (S 31 ). The first distance calculation unit  36 A then calculates the hamming distance HD (S 32 ). The state machine  35 A determines whether the hamming distance HD is 0 (S 33 ). When the hamming distance HD is 0 (Yes at S 33 ), the state machine  35 A sets the synchronization determination to OOF and starts a count operation at the frame counter  34  (S 34 ). 
         [0075]    The state machine  35 A determines whether the hamming distance HD of the bit stream of the frame timing at the frame counter  34  is 0 (S 35 ). When the hamming distance HD is 0 (Yes at S 35 ), the state machine  35 A sets the synchronization determination to IF and sets the accumulated hamming distance AHD to 0 (S 36 ). Furthermore, the state machine  35 A determines whether the hamming distance HD of the bit stream of the frame timing at the frame counter  34  is 0 (S 37 ). When the hamming distance HD of the bit stream of the frame timing at the frame counter  34  is 0 (Yes at S 37 ), the state machine  35 A moves to S 36  in order to set the synchronization determination to IF and set the accumulated hamming distance AHD to 0. 
         [0076]    When the hamming distance HD of the bit stream of the frame timing is not 0 (No at S 37 ), the state machine  35 A calculates the hamming distance HD of the bit stream of the frame timing (S 38 ). Furthermore, the state machine  35 A sets the synchronization determination to IF and accumulates the hamming distance HD on the accumulated hamming distance AHD (S 39 ). The state machine  35 A then determines whether the accumulated hamming distance AHD is equal to or higher than 1 and has exceeded the threshold THD (S 40 ). 
         [0077]    When the accumulated hamming distance AHD is equal to or higher than 1 and has exceeded the threshold THD (Yes at S 40 ), the state machine  35 A moves to S 31  in order to shift the synchronization determination to OOF. When the accumulated hamming distance AHD is equal to or higher than 1 and has not exceeded the threshold THD (No at S 40 ), the state machine  35 A moves to S 37  in order to determine whether the hamming distance HD of the bit stream of the frame timing is 0. 
         [0078]    When the hamming distance HD is not 0 (No at S 33 ) or the hamming distance HD of the bit stream of the frame timing is not 0 (No at S 35 ), the state machine  35 A moves to S 31  in order to shift the synchronization determination to OOF. 
         [0079]    When the hamming distance HD of the bit stream of the frame timing at the time of the synchronization pull-in is 0, the state machine  35 A that performs the second synchronization monitoring processing determines that the synchronization pattern is detected, and when the synchronization patterns are continuously detected, the state machine  35 A shifts the synchronization determination to IF. As a result, the transmission apparatus  2  may achieve pull-in from OOF to IF as the synchronization determination. 
         [0080]    When the hamming distance HD of the bit stream of the frame timing is not 0, the state machine  35 A sequentially accumulates the hamming distance HD on the accumulated hamming distance AHD. The state machine  35 A then determines whether that accumulated hamming distance AHD has exceeded the threshold THD. When the accumulated hamming distance AHD has exceeded the threshold THD, the state machine  35 A shifts the synchronization state to OOF. As a result, because there is a high risk of the second case where OOF actually occurs when the accumulated hamming distance AHD has exceeded the threshold THD, the transmission apparatus  2  may quickly detect OOF. 
         [0081]    When the accumulated hamming distance AHD has not exceeded the threshold THD, the state machine  35 A continues IF as the synchronization determination. As a result, because the case merely is the third case where a minor bit error occurs when the accumulated hamming distance AHD has not exceeded the threshold THD, the transmission apparatus  2  may suppress erroneous detection of OOF by continuing IF as the synchronization determination without rashly shifting to OOF. 
         [0082]    In the second case, the bit stream of the synchronization pattern in the reception frame and the expected value do not match each other with respect to a half of the bits stochastically. The second synchronization determination unit  24 A thus accumulates the hamming distance HD on the accumulated hamming distance AHD. For example, because the number of bits in the synchronization pattern is 24 bits by the OTN standard, the second synchronization determination unit  24 A accumulates the hamming distance HD corresponding to 12 bits, which is half of the above-described number, on the accumulated hamming distance AHD. The accumulated hamming distance AHD then exceeds the threshold THD (for example, 7 times). As a result, the second synchronization determination unit  24 A may immediately shift the synchronization determination to OOF and start the synchronization pull-in operation. 
         [0083]    In the third case, in the synchronization pattern in the reception frame, a minor bit error, for example, an error of one bit is dominant stochastically. The second synchronization determination unit  24 A thus accumulates the hamming distance HD “1” on the accumulated hamming distance AHD. Because the accumulated hamming distance AHD does not easily exceed the threshold THD, the second synchronization determination unit  24 A does not rashly shift the synchronization determination to OOF and continues IF. As a result, in the third case, because the hamming distance HD accumulated for each frame is around 1 and does not easily exceed the threshold THD, erroneous detection of OOF may be suppressed. 
         [0084]      FIG. 12  is a timing diagram illustrating an example of each signal in the second synchronization determination unit  24 A at the time of synchronization pull-in according to the second embodiment. The first distance calculation unit  36 A calculates the hamming distance HD between the bit stream of the synchronization pattern in the reception frame and the expected value. The first distance calculation unit  36 A then outputs the hamming distance HD to the synchronous counter  33 , the accumulation unit  37 , and the state machine  35 A. At this time, when the hamming distance HD is “0”, the state machine  35 A shifts the state of the synchronization determination to OOF. The state machine  35 A then outputs a load signal L 21  to the synchronous counter  33  and the frame counter  34 . 
         [0085]    The frame counter  34  starts a count operation of the frame timing in accordance with the load signal L 21 . Furthermore, when the hamming distance HD is “0”, the synchronous counter  33  counts the synchronization pattern 0x1. 
         [0086]    When the next hamming distance HD is “0”, the state machine  35 A outputs a load signal L 22 . Furthermore, when the hamming distance HD is “0”, the synchronous counter  33  counts the synchronization pattern 0x2 and delays the timing at which the hamming distance HD is “0” by one clock. When that timing and a frame timing F 21  at the frame counter  34  match each other, the synchronous counter  33  further outputs a synchronization signal S 21  to the state machine  35 A. 
         [0087]    As a result, the state machine  35 A shifts the synchronization determination from OOF to IF in accordance with the synchronization signal S 21 . With this, the synchronization pull-in operation is completed. 
         [0088]    The transmission apparatus  2  according to the second embodiment calculates the hamming distance HD of the bit stream of the frame timing, sequentially accumulates the hamming distance HD on the accumulated hamming distance AHD, and when the accumulated hamming distance AHD has exceeded the threshold THD, shifts the synchronization determination to OOF. As a result, because there is a high risk that OOF actually occurs, the transmission apparatus  2  may quickly detect OOF. 
         [0089]    When the accumulated hamming distance AHD has not exceeded the threshold THD, the transmission apparatus  2  shifts the synchronization determination to IF. As a result, because only a minor bit error occurs, the transmission apparatus  2  continues IF as the synchronization determination without rashly shifting to OOF, whereby erroneous detection of OOF may be suppressed. 
         [0090]    In the transmission apparatus  2  according to the second embodiment, the first comparison unit  32  and the distance calculation unit  36  according to the first embodiment may be substituted by one unit of the first distance calculation unit  36 A, whereby the circuit scale thereof may be downsized. 
         [0091]      FIG. 13  is an explanatory diagram illustrating an example of a relation between a BER before FEC correction and an OOF occurrence interval. For example, with respect to a FEC code specified by ITU-T G.975.1 I7, a relation between a bit error rate before FEC correction (input BER) and a bit error rate after FEC correction (output BER) is input BER=1.30×10 −3  and output BER=1×10 −15 . When transmission with an OTU2 frame (about 10 Gbps) is assumed, the average time interval at which bit errors occur is 1×10 5  [seconds]. By contrast, by the conventional OTN synchronization determination specified by ITU-T G.798, under the same input BER environment and in the third case, the average occurrence interval of erroneous detection of OOF is 4.4×10 2  [seconds]. In this case, under an environment under which BER is high, erroneous detection of OOF in the third case occurs at a higher rate than that of a bit error after FEC correction. For example, with input BER=1.30×10 −3  and when the threshold THD is set to “7”, the average occurrence interval of OOF in the third case is 5×10 5  [seconds]. This may be longer than the occurrence interval of an error after FEC correction (the average occurrence interval: 1×10 5  [seconds]). In other words, the average occurrence interval of OOF in the third case according to the present application is longer than those of OOF in the third case and OOF after BER correction with a conventional technique, whereby the frequency of shifting to OOF in the third case may be reduced. 
         [0092]    The NWIF  12  in the above-described embodiments is communicably connected with an OTN, and thus includes the frame generation unit  21 , the FEC encode unit  22 , the scramble unit  23 , the first synchronization determination unit  24 , the descramble unit  25 , the FEC decode unit  26 , and the frame processing unit  27  embedded therein. However, the CLIF  11  also includes the frame generation unit  21 , the FEC encode unit  22 , the scramble unit  23 , the first synchronization determination unit  24 , the descramble unit  25 , the FEC decode unit  26 , and the frame processing unit  27  embedded therein when the CLIF  11  is communicably connected with an OTN. 
         [0093]    At S 18  in the first synchronization monitoring processing illustrated in  FIG. 6 , the synchronization determination is set to IF and the hamming distance HD is accumulated on the accumulated hamming distance AHD. However, the hamming distance HD may be accumulated on the accumulated hamming distance AHD at S 18 , and at S 19 , when the accumulated hamming distance AHD is equal to or higher than 1 and has not exceeded the threshold THD, the synchronization determination may be set to IF, and then the processing may move to S 16 . 
         [0094]    At S 39  in the second synchronization monitoring processing illustrated in  FIG. 11 , the synchronization determination is set to IF and the hamming distance HD is accumulated on the accumulated hamming distance AHD. However, the hamming distance HD may be accumulated on the accumulated hamming distance AHD at S 39 , and at S 40 , when the accumulated hamming distance AHD is equal to or higher than 1 and has not exceeded the threshold THD, the synchronization determination may be set to IF, and then the processing may move to S 37 . 
         [0095]    In the above-described embodiments, an OTN frame and a SONET/SDH frame are assumed. However the embodiments are not limited to these communication networks. The above-described embodiments may be applied to a transmission apparatus  2  with a system that uses a synchronization pattern of a reception frame to detect synchronization loss. 
         [0096]    In the above-described embodiments, when the accumulated hamming distance AHD has not exceeded the threshold THD at the second comparison unit  38 , a comparison result is not output. However, the comparison result may be output to the state machine  35 . 
         [0097]    Each component in each unit illustrated does not necessarily have to be configured as illustrated physically. More specifically, the specific mode of distribution and integration of the unit is not limited to the illustrated one, and all or a part thereof may be configured in a functionally or physically distributed or integrated manner in an arbitrary unit, in accordance with various loads, use conditions, and the like. 
         [0098]    Furthermore, with respect to each of various processing functions executed in each apparatus, all or an arbitrary part thereof may be executed on a central processing unit (CPU), a digital signal processor (DSP), a field programmable gate array (FPGA), and the like. With respect to each of the processing functions, all or an arbitrary part thereof may be executed on a program analyzed and executed on a CPU or the like or on hardware using a wired logic. 
         [0099]    A region for storing therein various information may be composed of a read only memory (ROM) or a random access memory (RAM) such as synchronous dynamic random access memory (SDRAM), a magnetoresistive random access memory (MRAM), and a non-volatile random access memory (NVRAM). 
         [0100]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.