Abstract:
A system and method for data synchronization in Passive Optical Networks are disclosed. According to an embodiment, the present invention provides a method for providing upstream data synchronization in an optical communication network. The method includes sending data from an Optical Network Unit. The data includes a first data frame, which includes a header sequence, a synchronization segment, and a data segment. The synchronization segment includes 66 bits, which includes a first number of bits having nonzero values and a second number of bits having a value of zero. The first number is different from the second number. The method further includes receiving at least the first data frame by an Optical Line Terminal. The method also includes processing the first data frame. The method additionally includes selecting a first segment of the first data frame, the first segment including 66 bits.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2008/071395, filed on Jun. 20, 2008. This application claims priority to International Patent Application No. PCT/CN2007/071056, filed on Nov. 13, 2007 by Leung, Raymond W. K. et al., and PCT/CN2007/071253 which claimed the priority of the application PCT/CN2007/071056, filed on Dec. 17, 2007 by Leung, Raymond W. K. et al, which are incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to telecommunication techniques, more particularly, to a method and system for providing data synchronization in Passive Optical Networks (PONs). 
     BACKGROUND OF THE INVENTION 
     To improve readability and clarity of this application, acronyms are used. A listed of acronyms is provided below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 PON 
                 Passive Optical Network 
               
               
                   
                 VoIP 
                 Voice over Internet Protocol 
               
               
                   
                 HDTV 
                 High Definition Television 
               
               
                   
                 OLT 
                 Optical Line Terminal 
               
               
                   
                 ONU 
                 Optical Network Unit 
               
               
                   
                 ODN 
                 Optical Distribution Network 
               
               
                   
                 TDM 
                 Time Division Multiplexing 
               
               
                   
                 TDMA 
                 Time Division Multiple Access 
               
               
                   
                 ID 
                 Identification 
               
               
                   
                 SOD 
                 Start of Data 
               
               
                   
                 BD 
                 Burst Delimiter 
               
               
                   
                 HFP 
                 High Frequency Pattern 
               
               
                   
                 AGC 
                 Automatic Gain Control 
               
               
                   
                 CDR 
                 Clock and Data Recovery 
               
               
                   
                 FSC 
                 False Synchronization Candidates 
               
               
                   
                 HD 
                 Hamming Distance 
               
               
                   
                 HDC 
                 Hamming Distance Calculation 
               
               
                   
                 SDM 
                 Synchronization Decision Module 
               
               
                   
                 MSB 
                 Most Significant Bit 
               
               
                   
                 LSB 
                 Least Significant Bit 
               
               
                   
                   
               
             
          
         
       
     
     PON is one of the most promising access network technologies. This type of network provides many benefits, including low maintenance cost, high bandwidth, low implementation cost, etc. PON can be an ideal platform for multi-play applications such as VoIP, data transmission, HDTV, etc. 
     Typically, PON is implemented as a point-to-multipoint medium based on a tree topology including an Optical Line Terminal (OLT), some Optical Network Units (ONUs) and an Optical Distribution Network (ODN) with splitters/couplers. One of the most attractive features of a PON is that the PON does not need any active component in the ODN. 
     Usually, PON system employs a point-to-multipoint access protocol so that all subscribed ONUs can share an OLT over an optical fiber. For example, the Time Division Multiplexing (TDM) broadcast for downstream transmission and Time Division Multiple Access (TDMA) for upstream transmission is widely used in current PON systems. 
     As merely an example,  FIG. 1  illustrates a downstream transmission process in a PON system. The OLT broadcasts signals to all subscribed ONUs in the downstream transmission, the destination ONU will extract its belonging packets according to the destination identification (ID) of a packet and discard all other packets as in  FIG. 1 . For example, ONU- 1  extracts the packet with its destination ID and sends to its corresponding end user; ONU- 2  sends packet- 2  to its end user, and so on. 
     As merely an example,  FIG. 2  illustrates an upstream transmission process in a PON system. The ONU transmits its signal in upstream channel of a PON system in a burst mode, which is different from the conventional point to point continuous mode transmission. ONU will first set up a communication link with the OLT, thereafter OLT will allocate different time slots to different ONUs in a TDMA fashion so that their signals will not overlap with each other when they reach the coupler in the ODN. As shown in  FIG. 2 , the ONU- 1  only transmit its signal in its time slot (i.e., No.  1 ) and ONU- 2  transmits its signal in its time slot (i.e., No.  2 ), and so on. 
       FIG. 3  is a simplified diagram illustrating a frame structure for upstream data. A high frequency pattern (HFP) “0x 55 55 . . . ” (0x means hexadecimal numbers and its binary form is 01010101 01010101 . . . ) is a special preamble sequence used by the OLT for Automatic Gain Control (AGC) and Clock and Data Recovery (CDR). The HFP is followed by a 66 bits long Start of Data &lt;SOD&gt; delimiter, or it is called Burst Delimiter (BD). The SOD is used to delineate the boundary of the data frame. The length of SOD delimiter is compatible with the 66-bit data frame structure. The section labeled FEC protected “ . . . ” could be one or several IDLE blocks that are used by OLT for de-scrambler re-synchronization and codeword delineation. Data are appended after this (these) IDLE block(s). 
     The SOD is useful in data synchronization.  FIG. 4  is a simplified diagram illustrating the correlation between SOD and False Synchronization Candidates (FSC). After the OLT detects incoming signal and synchronizes the signal with its clock reference, the OLT will send the received signal to the Boundary Detector. A SOD Correlator is embedded within the Boundary Detector module to test the correlation between the SOD delimiter and the received signal. The SOD Correlator will calculate the Hamming distance (HD) between the SOD delimiter and the received 66 bits data to determine the validation of burst synchronization, which is the same as delineate the boundary of the data frame. A false locking synchronization will output a truncated data frame to its higher layer, this may degrade the system performance. Therefore, the employed SOD delimiter should provide a false locking probability as low as possible. The SOD delimiter should be designed to minimize the correlation between the SOD delimiter and the FSC, in other words, maximize the HD between the SOD delimiter and the FSC. 
     As can be seen from the above description that, various conventional techniques are available for data synchronization in optical networks. Unfortunately, these techniques are often inadequate for various reasons. 
     Therefore, improved system and method for data synchronization are desired. 
     SUMMARY OF THE INVENTION 
     The present invention relates in general to telecommunication techniques. More particularly, the invention provides a method and system for providing data synchronization in PONs. In a specific embodiment, the present invention provides a technique for upstream synchronization using optimized SOD sequences and the hardware implementation thereof. Merely by way of example, the invention is described as it applies to PONs, but it should be recognized that the invention has a broader range of applicability. For example, the invention can be applied to any communication systems uses specified sequences for data synchronization. 
     According to an embodiment, the present invention provides a method for providing upstream data synchronization in an optical communication network. The method includes sending data from an ONU. The data includes a first data frame, which includes a header sequence, a synchronization segment, and a data segment. The synchronization segment includes 66 bits, which includes a first number of bits having nonzero values and a second number of bits having a value of zero. The first number is different from the second number. The method further includes receiving at least the first data frame by an OLT. The method also includes processing the first data frame. The method additionally includes selecting a first segment of the first data frame, the first segment including 66 bits. The method further includes comparing the first segment with a synchronization delimiter. Moreover, the method includes determining a Hamming distance based on the first segment. The method additionally includes determining a boundary of the data frame. 
     According to another embodiment, the present invention provides a PON system includes a number of ONUs. The optical network includes a transmitter. Each ONU is configured to send data using the transmitter in a TDMA fashion. The data include a first data frame, which includes a header sequence, a synchronization segment, and a data segment. The synchronization segment including 66 bits, which includes a first number of bits having nonzero values and a second number of bits having a value of zero. The first number is different from the second number. The system also includes an OLT. The OLT includes a receiver that is configured to receive at least the first data frame. The OLT also includes a shifter register for storing the first data frame. The OLT includes a logic circuit for comparing the first segment with a synchronization delimiter. The OLT additionally includes a Hamming distance module for determining a Hamming distance based on the first segment. The OLT includes a Synchronization Decision Module (SDM) determining a boundary of the first data frame. 
     It is to be appreciated that embodiments of the present invention provides various advantages over conventional techniques. Among other things, by using optimized synchronization delimiter sequences and the hardware thereof, the synchronization process is optimized both for speed and reliability. In addition, embodiments of the present invention can be easily implemented using and/or in conjunction with conventional systems with minimal modification. There are other benefits as well, which are described below. 
     Depending upon embodiment, one or more of these benefits may be achieved. These benefits and various additional objects, features and advantages of the present invention can be fully appreciated with reference to the detailed description and accompanying drawings that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a downstream transmission process in a PON system. 
         FIG. 2  illustrates an upstream transmission process in a PON system. 
         FIG. 3  is a simplified diagram illustrating a frame structure for upstream data. 
         FIG. 4  is a simplified diagram illustrating the correlation between SOD delimiter and false synchronization candidates. 
         FIG. 5  is a simplified diagram illustrating upstream transmission from an ONU to an OLT in a PON system. 
         FIG. 6  is a simplified diagram illustrating SOD delimiter being used in data frame. 
         FIG. 7  is a simplified diagram illustrating an upstream data frame structure of a 10 G EPON system according to an embodiment of the present invention. 
         FIG. 8A  is a simplified diagram illustrating a synchronization circuit of the SOD Correlator according to an embodiment of the present invention. 
         FIG. 8B  is a simplified diagram illustrating hard logic for providing SOD delimiter logic according to an embodiment of the present invention. 
         FIG. 9  is a graph illustrating the comparison of false locking probability between existing techniques and an embodiment of the present invention. 
         FIG. 10  is a simplified diagram illustrating an upstream data frame structure of a 10 G EPON system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention relates in general to telecommunication techniques. More particularly, the invention provides a method and system for providing data synchronization in PONs. In a specific embodiment, the present invention provides a technique for upstream synchronization using optimized SOD sequences and the hardware implementation thereof. Merely by way of example, the invention is described as it applies to PONs, but it should be recognized that the invention has a broader range of applicability. For example, the invention can be applied to any communication systems uses specified sequences for data synchronization. 
     As discuss above, conventional data synchronization techniques are often inadequate for various reasons, which are explained in detail below. 
       FIG. 5  is a simplified diagram illustrating upstream transmission from an ONU to an OLT in a PON system. The FEC codewords are composed to form an upstream data frame in the Frame Formator. 
     The HFP and the SOD delimiter sequence will then be appended to the beginning of the data frame as shown in  FIG. 3 , respectively. The currently employed SOD is a 66 bits binary sequence. For example, the binary sequence is the following, {00 01010100 10101110 11111001 11011010 01111000 00111101 11000010 01000110} and its hexadecimal representation is {0x 0 54 AE F9 DA 78 3D C2 46}. It should be noted that every hexadecimal number represent 4 binary bits, except the first hexadecimal number or the leading number, which represents 2 binary bits. The minimum distance of the current employed SOD delimiter and the FSC is 31. As an example,  FIG. 6  is a simplified diagram illustrating SOD delimiter being used in data frame. 
     In a PON system, the distances between subscribed ONUs and the OLT are different, and hence the optical signal power loss and channel penalties vary with different ONUs. For example, power levels for signals are different when they arrive at the OLT. Therefore, it is usually a requirement for the OLT to automatically adjust the received power level and synchronize the received signal correctly with its clock reference. Usually, these functions are performed by the AGC and CDR module in the OLT. 
     At the OLT side, the Boundary Detector includes a SOD Correlator. Among other things, the SOD Correlator is used for delineating the data frame boundary of the upstream signal from the ONU. For example, as shown in  FIG. 3 , the SOD delimiter is not protected by the FEC code, and the bit error probability is relatively high. The SOD Correlator is required to tolerate bit errors since the transmitted SOD delimiter at the receiver side is often very likely be corrupted by bit errors. For example, the number of bit errors can be tolerated by the SOD Correlator is defined by a predetermined synchronization threshold. Often the synchronization threshold in the SOD Correlator is decided according to the operating bit error level. 
     By setting up a suitable synchronization threshold, the SOD Correlator of OLT can effectively delineate the received signal quickly and minimize the mean time of false lock occurrence. Typically, conventional systems set the synchronization threshold of the SOD Correlator at 12. If the computed HD between the SOD delimiter and the 66 bits received data is less than 12, then the OLT declares a successful synchronization with the received signal. On the other hand, if the HD is equal or larger than 12 then the received signal is shifted by one bit and the SOD Correlator re-calculates the HD between the SOD delimiter and the new 66 bits data until a successful synchronization is declared. 
     There are various problems with the conventional approach discussed above. Among other things, since the SOD delimiter is not protected by the FEC code, the bit error probability of the SOD delimiter over the transmission channel is often high. As a result, it requires the SOD delimiter to have a large HD between the SOD delimiter and the FSC. For example, conventional SOD delimiter and the FSC have a minimum HD equal to 31. 
     However, the theoretical suggested minimum HD between a SOD delimiter and FSC can be calculated using Equation 1 below: 
     
       
         
           
             
               
                 
                   
                     
                       Minimum 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Hamming 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Distance 
                     
                     = 
                     
                       
                         ⌊ 
                         
                           
                             N 
                             - 
                             1 
                           
                           2 
                         
                         ⌋ 
                       
                       ⁢ 
                       
                         
 
                       
                       ⁢ 
                       
                           
                       
                       = 
                       
                         
                           ⌊ 
                           
                             
                               66 
                               - 
                               1 
                             
                             2 
                           
                           ⌋ 
                         
                         ⁢ 
                         
                           
 
                         
                         ⁢ 
                         
                             
                         
                         = 
                         32 
                       
                     
                   
                   , 
                   
                     
 
                   
                   ⁢ 
                   
                     N 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     is 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     length 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     of 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     SOD 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     The theoretical suggested value of the maximum synchronization threshold, so that the performance of SOD Correlator can be calculated using Equation 2 below: 
     
       
         
           
             
               
                 
                   
                     Delineation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Threshold 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     T 
                   
                   = 
                   
                     
                       
                         ⌊ 
                         
                           N 
                           4 
                         
                         ⌋ 
                       
                       - 
                       1 
                     
                     = 
                     
                       
                         16 
                         - 
                         1 
                       
                       = 
                       15 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     It is to be appreciated that an objective of the present invention is to provide a set of Start of Data &lt;SOD&gt; delimiters in which they comply with the theoretical suggested value. For example, the minimum HD between the SOD delimiter and the FSC is 32. 
     In a specific embodiment, the present invention provides a SOD correlation circuit based on the using of one or more SOD delimiters. Among other things, the embodiment provides a fast synchronization algorithm for various synchronization thresholds. 
     Depending on the application, various SOD delimiters may be used. As an example, Table 1 below illustrates five exemplary SOD delimiters: 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 SOD 
                   
                 Minimum HD 
                 Allowable Max 
               
               
                 length 
                 SOD Delimiter (hexadecimal 
                 between 
                 Synchroniza- 
               
               
                 (bits) 
                 numbers) 
                 SOD and FSC 
                 tion Threshold 
               
               
                   
               
             
             
               
                 66 
                 1 16 A2 DC 69 F0 CD EE 40 
                 32 
                 15 
               
               
                 66 
                 1 5A E3 94 B6 66 C7 E0 03 
                 32 
                 15 
               
               
                 66 
                 1 7F A0 96 0E 14 A7 33 66 
                 32 
                 15 
               
               
                 66 
                 1 70 3A 08 6D ED 4E 99 66 
                 32 
                 15 
               
               
                 66 
                 0 41 BD B2 B3 D5 A7 C8 F0 
                 32 
                 15 
               
               
                   
               
             
          
         
       
     
     The SOD delimiters as shown in Table 1 comply with the theoretical suggested minimum HD between a SOD delimiter and FSC. Among other things, the SOD delimiters have sixteen “0”s and seventeen “1”s on the 33 even positions and on the 33 odd positions, or vice versa. As a consequence, the SOD have a number of “0” not equal to the number of “1”, (e.g., either 32 “0”s plus 34 “1”s or 34 “0” s plus 32 “1”s). As an example, all the SOD delimiters, in their binary forms, all conform to the described requirement. For example, the following SOD delimiters are shown in hexadecimal numbers and their binary forms have 34 “0”s and 32 “1”s, 
     0x1 16 A2 DC 69 F0 CD EE40 
     01 00010110 10100010 11011100 01101001 11110000 11001101 11101110 01000000 
     0x1 5A E3 94 B6 66 C7 E0 03 
     01 01011010 11100011 10010100 10110110 01100110 11000111 11100000 00000011 
     0x1 7F A0 96 0E 14 A7 33 66 
     01 01111111 10100000 10010110 00001110 00010100 10100111 00110011 01100110 
     0x1 70 3A 08 6D ED 4E 99 66 
     01 01110000 00111010 00001000 01101101 11101101 01001110 10011001 01100110 
     The following delimiter has 32 “0”s and 34 “1”s, 
     0x 0 41 BD B2 B3 D5 A7 C8 F0 
     00 01000001 10111101 10110010 10110011 11010101 10100111 11001000 11110000 
     The SOD delimiters shown in Table 1 comply with the theoretical suggested minimum HD between the SOD delimiter and the FSC. For example, these SOD delimiters, as well as other SOD delimiters contemplated by the present invention, can be used to replace conventional SOD delimiter so that the minimum HD can be increased from 31 to 32 without additional complexity or modifying the existing data frame structure. In other words, the present invention can decrease the false locking probability without any extra cost. 
     In an embodiment, a SOD delimiter complies with the theoretical suggested value of the minimum HD between a SOD delimiter and FSC. The SOD delimiter must have sixteen “0”s and seventeen “1”s on the 33 even positions and on the 33 odd positions, or vice versa. Consequently, the number of “0”s must not be equal to the number of “1”s, (e.g., 32 “0”s plus 34 “1”s, or 34 “0”s plus 32 “1”s). 
     To implement the embodiment, the binary form of the Hexadecimal sequence is a 66 bits long sequence. For example, the binary form of {0x 1 16 A2 DC 69 F0 CD EE 40} is {01 00010110 10100010 11011100 01101001 11110000 11001101 11101110 01000000}. It should be noted that every hexadecimal number represent 4 binary bits, except the first hexadecimal number or the leading number, which represents 2 binary bits. 
       FIG. 7  is a simplified diagram illustrating an upstream data frame structure of a 10 G EPON system according to an embodiment of the present invention. This diagram is merely an example, which should not limit the scope of the claims in any way. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 7 , the SOD delimiters as listed in Table 1 are used in the data frames. 
     It is to be appreciated that embodiments of the present invention have a wide range of applicability and can be used in any systems that use SOD delimiters for synchronization with the received signal or for delineating the boundary of data frame. In a specific embodiment, the invention is to be used in a 10 G EPON system based on IEEE 802.3 standards. 
     At the ONU transmitter side, the SOD delimiter is appended to the beginning of the FEC coded data frame as well as the HFP. For example, the HFP is used as a preamble for the transmitted upstream signal. 
     At the OLT receiver side, the SOD Correlator calculates the HD between the received signal and the SOD delimiter to test if the HD is less than the system&#39;s synchronization threshold. In contrast to conventional systems, the synchronization threshold is adjustable according to the requirement of the system. For example, the false locking probability can be minimized if T is set to be 0. 
       FIG. 8A  is a simplified diagram illustrating a synchronization circuit of the SOD Correlator according to an embodiment of the present invention. This diagram is merely an example, which should not limit the scope of the claims in any way. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 8A , a shifter register is used to process the receive ONU packets and provide the processed packets to the Hamming distance calculation (HDC) module. The Hamming distance calculation module determines the delineation based on the Hamming distance calculation. 
       FIG. 8B  is a simplified diagram illustrating hard logic for providing SOD delimiter logic according to an embodiment, of the present invention. This diagram is merely an example, which should not limit the scope of the claims in any way. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown, the SOD delimiter for this particular situation is “0x 1 16 A2 DC 69 F0 CD EE 40”. 
     As long as the OLT detects an upstream signal from the ONU, it synchronizes its clock reference with the upstream signal. The OLT then sends the received data into the Shift Register of the SOD Correlator. Once the Shift Register has been filled with 66 bits received data, it passes these 66 bits through an electrical circuit that is based on the SOD delimiter. The electrical circuit is defined as the following: every bit of the SOD delimiter responds to a direct electric logic form the Shift Register to HDC module. If the bit of the corresponding bit of the SOD delimiter is “0”, then the originally received data bit is sent to the HDC module unchanged. On the other hand, if the corresponding bit of the SOD is “1”, then it passes the binary complement value of the received data bit to the HDC module (i.e., “0” changed to “1” or “1” changed to “0”). The HDC module calculates the corresponding HD and passes the output to the Synchronization Decision Module (SDM). Finally, the SDM determines if it is a valid synchronization or not. If a successful synchronization is declared, then the OLT knows the beginning of the data frame and starts to receive data. 
     In an embodiment, the present invention provides a fast synchronization algorithm on the binary format of the HD from the SDM. The algorithm is implemented with the 66-bits SOD delimiter. The minimum HD between the SOD delimiter and all possible 66 bits binary sequence is 0. The maximum HD between the SOD delimiter and all possible 66 bits binary sequence is 66. Since 2 6 &lt;66&lt;2 7 , it requires at least 7 binary bits to represent the resulted HD in a binary format. 
     According to Table 2, the SDM can count on the number of consecutive “0” bits from the Most Significant Bit (MSB) to Least Significant Bit (LSB) to determine whether it is a valid synchronization or not, if the synchronization threshold T is to be set as T=8=2 3  or T=16=2 4 . For example, if T=8, then the SDM just need to check if the first 4 consecutive bits are 0 or not. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Hamming 
                 HD binary format 
               
               
                   
                 Distance 
                 (MSB) (LSB) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 0 0 0 0 0 0 0 
               
               
                   
                 1 
                 0 0 0 0 0 0 1 
               
               
                   
                 2 
                 0 0 0 0 0 1 0 
               
               
                   
                 3 
                 0 0 0 0 0 1 1 
               
               
                   
                 4 
                 0 0 0 0 1 0 0 
               
               
                   
                 5 
                 0 0 0 0 1 0 1 
               
               
                   
                 6 
                 0 0 0 0 1 1 0 
               
               
                   
                 7 
                 0 0 0 0 1 1 1 
               
               
                   
                 8 
                 0 0 0 1 0 0 0 
               
               
                   
                 9 
                 0 0 0 1 0 0 1 
               
               
                   
                 10 
                 0 0 0 1 0 1 0 
               
               
                   
                 11 
                 0 0 0 1 0 1 1 
               
               
                   
                 12 
                 0 0 0 1 1 0 0 
               
               
                   
                 13 
                 0 0 0 1 1 0 1 
               
               
                   
                 14 
                 0 0 0 1 1 1 0 
               
               
                   
                 15 
                 0 0 0 1 1 1 1 
               
               
                   
                 16 
                 0 0 1 0 0 0 0 
               
               
                   
                   
               
             
          
         
       
     
     Assuming the number of bits required to represent the binary form of HD is n, and the synchronization threshold is set to be T=2 m , where 0≦m≦n. Then the SDM can decide if it is a valid synchronization by checking whether the first n-m consecutive bits are 0&#39;s or not. If they are all 0, then SDM can declare a successful synchronization. Otherwise the Shift Register will shift one bit to obtain a “new” 66 bits data to be tested with the SOD delimiter. 
     It is to be appreciated that various embodiments of the present invention provide numerous advantages over conventional techniques. Among other thing, the data frame implemented using an SOD delimiter according to the present invention can decrease the false locking probability. At the same time, this implementation does not introduce any complexity overhead. 
       FIG. 9  is a graph illustrating the comparison of false locking probability between existing techniques and an embodiment of the present invention. As an example, the length of the HFP is 4000 bits. From the graph in  FIG. 9 , it can be seen that the false locking probability is much lower in the embodiment of the present invention. The dotted lines represent the Threshold T=12 and the dash lines represent the Threshold T=15. 
     In addition to better performance, embodiments of the present invention also provide more flexibility when compared to conventional systems. For example, synchronization Threshold T is adjustable according to the system requirement. Fast synchronization algorithms can be adopted for different threshold values. 
     Among other things, embodiments of the present invention provide five Start of Data &lt;SOD&gt; delimiters. Each of the SOD delimiter is ideally suited for Ethernet PON upstream transmission. 
     In embodiments, the HFP can have binary form as “10101010 10101010 . . . ”, which ends with a “0.” The HFP can still provide the AGC and CDR functions. In the embodiments, the hexadecimal number of the SOD is {0x C D5 8A 60 A4 E1 43 BC 9D} and its binary sequence is {11 10101011 01010001 00000110 00100101 10000111 11000010 00111101 10111001}. The minimum distance of the current employed SOD delimiter and the FSC is 31. 
     If the binary sequence “10101010 10101010 . . . ”, which ends with a “0” is used as the HFP in the system, the SOD delimiters, which comply with the HD between the SOD delimiters and FSC, are in the complement form of SOD delimiters provided in Table 1. The corresponding SOD delimiters are shown in Table 3. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 SOD 
                   
                 Minimum HD 
                 Allowable Max 
               
               
                 length 
                 SOD Delimiter (hexadecimal 
                 between 
                 Synchroniza- 
               
               
                 (bits) 
                 numbers) 
                 SOD and FSC 
                 tion Threshold 
               
               
                   
               
             
             
               
                 66 
                 4 97 BA C4 69 F0 4C 88 FD 
                 32 
                 15 
               
               
                 66 
                 4 A5 38 D6 92 99 1C F8 3F 
                 32 
                 15 
               
               
                 66 
                 4 01 FA 96 8F D7 1A 33 99 
                 32 
                 15 
               
               
                 66 
                 4 F1 A3 EF 49 48 8D 66 99 
                 32 
                 15 
               
               
                 66 
                 C 7D 42 B2 32 54 1A EC F0 
                 32 
                 15 
               
               
                   
               
             
          
         
       
     
     The SOD delimiters as shown in Table 3 comply with the theoretical suggested minimum HD between a SOD delimiter and FSC. Among other things, the SOD delimiters have sixteen “1”s and seventeen “0”s on the 33 even positions and on the 33 odd positions, or vice versa. As a consequence, the SOD have a number of “0” not equal to the number of “1”, (e.g., either 32 “0”s plus 34 “1”s or 34 “0” s plus 32 “1”s). As an example, all the SOD delimiters, in their binary forms, all conform to the described requirement. For example, the following SOD delimiters are shown in hexadecimal numbers and their binary forms have 34 “0”s and 32 “1”s, 
     0x 4 97 BA C4 69 F0 4C 88 FD 
     10 11101001 01011101 00100011 10010110 00001111 00110010 00010001 10111111 
     0x 4 A5 38 D6 92 99 1C F8 3F 
     10 10100101 00011100 01101011 01001001 10011001 00111000 00011111 11111100 
     0x 4 01 FA 96 8F D7 1A 33 99 
     10 10000000 01011111 01101001 11110001 11101011 01011000 11001100 10011001 
     0x 4 F1 A3 EF 49 48 8D 66 99 
     10 10001111 11000101 11110111 10010010 00010010 10110001 01100110 10011001 
     The following delimiter has 32 “1”s and 34 “0”, 
     0x C 7D 42 B2 32 54 1A EC F0 
     11 10111110 01000010 01001101 01001100 00101010 01011000 00110111 00001111 
     The LSB of binary bits and field (8 bits per field) positions is on the left. Hexadecimal numbers are shown in a normal hexadecimal form and two hexadecimal numbers represent one corresponding field. For example, the field “0x BA” (shown in Table 3) is sent as 01011101, representing 11th to 18th bits of the 66 bits SOD delimiter  1 . The LSB for each field is placed in the lowest number position of the field and is the first transmitted bit of the field. It is noted that a hexadecimal number represents 4 binary bits, except the first hexadecimal number or the leading number, which represents 2 MSBs of corresponding four binary bits representation. For example, the binary representation of “0x 4” is “0010” and the first hexadecimal number “0x 4” represents 10. 
       FIG. 10  is a simplified diagram illustrating an upstream data frame structure of a 10 G EPON system according to an embodiment of the present invention. This diagram is merely an example, which should not limit the scope of the claims in any way. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 7 , the SOD delimiters as listed in Table 3 are used in the data frames. 
     In embodiments, the HFP can be a sequence of n consecutive specific 66-bit blocks, in which the 66-bit block has a binary form as “10 1111 1101 0000 0010 0001 1000 1010 0111 1010 0011 1001 0010 1101 1101 1001 1010”, where the hexadecimal representation is {0x 4 BF 40 18 E5 C5 49 BB 59}. The HFP is suitable for high speed PON systems (e.g. 10 G-EPON system) not only good for the AGC and CDR functions but also suitable for applying Peak Detector or Equalizer at the receiver side of an OLT. In the embodiments, the hexadecimal number of the SOD is {0x 8 6B F8 D8 12 D8 58 E4 AB} and its binary sequence is {01 1101 0110 0001 1111 0001 1011 0100 1000 0001 1011 0001 1010 0010 0111 1101 0101}. The minimum Hamming distance of the current employed SOD delimiter and the FSC is 30. 
     If the above HFP is used in the system, the SOD delimiters that have a large HD between the SOD delimiters and FSC are shown in Table 4. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 SOD 
                   
                 Minimum HD 
                 Allowable Max 
               
               
                 length 
                 SOD Delimiter (hexadecimal 
                 between 
                 Synchroniza- 
               
               
                 (bits) 
                 numbers) 
                 SOD and FSC 
                 tion Threshold 
               
               
                   
               
             
             
               
                 66 
                 8 6B F8 D8 12 D8 58 E4 AB 
                 30 
                 14 
               
               
                   
               
             
          
         
       
     
     The SOD delimiters have fourteen “1”s and nineteen “0”s on the 33 even positions and on the 33 odd positions, or vice versa. As a consequence, the SOD have a number of “0” equal to the number of “1”, (e.g., either 33 “0”s plus 33 “1”s). As an example, all the SOD delimiters, in their binary forms, all conform to the described requirement. For example, the following SOD delimiters are shown in hexadecimal numbers and their binary forms have 33 “0”s and 33 “1”s, 
     0x 8 6B F8 D8 12 D8 58 E4 AB 
     01 11010110 00011111 00011011 01001000 00011011 00011010 00100111 11010101. 
     The LSB of binary bits and field (8 bits per field) positions is on the left. Hexadecimal numbers are shown in a normal hexadecimal form and two hexadecimal numbers represent one corresponding field. For example, the field “0x BA” (shown in Table 3) is sent as 01011101, representing 11th to 18th bits of the 66 bits SOD delimiter  1 . The LSB for each field is placed in the lowest number position of the field and is the first transmitted bit of the field. It is noted that a hexadecimal number represents 4 binary bits, except the first hexadecimal number or the leading number, which represents 2 MSBs of corresponding four binary bits representation. For example, the binary representation of “0x 4” is “0010” and the first hexadecimal number “0x 4” represents 10. 
     Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims. 
     In embodiments, the HFP can be a sequence of n consecutive specific 66-bit blocks, in which the 66-bit block has a binary form as “10 0111 1101 0110 0000 1010 1001 1111 0101 1000 0010 1010 0111 1101 0110 0000 1010”, where the hexadecimal representation is {0x 4 BE 06 95 AF 41 E5 6B 50}. The HFP is suitable for high speed PON systems (e.g. 10 G-EPON system) not only good for the AGC and CDR functions but also suitable for applying Peak Detector or Equalizer at the receiver side of an OLT. In the embodiments, the hexadecimal number of the SOD is {0x 4 BE E4 B1 DA AA 13 18 B1} and its binary sequence is {10 0111 1101 0010 0111 1000 1101 0101 1011 0101 0101 1100 1000 0001 1000 1000 1101}. The minimum distance of the current employed SOD delimiter and the FSC is 31. 
     If the above HFP is used in the system, the SOD delimiters that have a large HD between the SOD delimiters and FSC are shown in Table 5. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 SOD 
                   
                 Minimum HD 
                 Allowable Max 
               
               
                 length 
                 SOD Delimiter (hexadecimal 
                 between 
                 Synchroniza- 
               
               
                 (bits) 
                 numbers) 
                 SOD and FSC 
                 tion Threshold 
               
               
                   
               
             
             
               
                 66 
                 4 BE E4 B1 DA AA 13 18 B1 
                 31 
                 15 
               
               
                 66 
                 4 BE A4 03 50 32 BF 3A E3 
                 31 
                 15 
               
               
                 66 
                 C AE 85 47 BE 2B 06 24 A7 
                 31 
                 15 
               
               
                 66 
                 4 FE 82 16 48 79 BA 98 B3 
                 31 
                 15 
               
               
                   
               
             
          
         
       
     
     The SOD delimiters have fourteen “1”s and nineteen “0”s on the 33 even positions and on the 33 odd positions, or vice versa. And the number of “0” and “1” on the odd bit and even bit constitutes a cross combination; if on the odd bit, there are 14 “0”, and 19 “1”, then on the even bit, there would be 19 “0” and 14 “1”. As a consequence, the SOD have a number of “0” equal to the number of “1”, (e.g., either 33 “0”s plus 33 “1”s). As an example, all the SOD delimiters, in their binary forms, all conform to the described requirement. For example, the following SOD delimiters are shown in hexadecimal numbers and their binary forms have 33 “0”s and 33 “1”s, 
     0x 4 BE E4 B1 DA AA 13 18 B1 
     10 01111101 00100111 10001101 01011011 01010101 11001000 00011000 10001101 
     0x4BE A4 03 50 32 BF 3A E3 
     10 01111101 00100101 11000000 00001010 01001100 11111101 01011100 11000111 
     0x C AE 85 47 BE 2B 06 24 A7 
     11 01110101 10100001 11100010 01111101 11010100 01100000 00100100 11100101 
     0x 4 FE 82 16 48 79 BA 98 B3 
     10 01111111 01000001 01101000 00010010 10011110 01011101 00011001 11001101 
     The LSB of binary bits and field (8 bits per field) positions is on the left. Hexadecimal numbers are shown in a normal hexadecimal form and two hexadecimal numbers represent one corresponding field. For example, the field “0x BA” (shown in Table 3) is sent as 01011101, representing 11th to 18th bits of the 66 bits SOD delimiter  1 . The LSB for each field is placed in the lowest number position of the field and is the first transmitted bit of the field. It is noted that a hexadecimal number represents 4 binary bits, except the first hexadecimal number or the leading number, which represents 2 MSBs of corresponding four binary bits representation. For example, the binary representation of “0x 4” is “0010” and the first hexadecimal number “0x 4” represents 10. 
     Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.