Abstract:
An optical line terminal (OLT) channel termination (CT) comprises a receiver configured to receive an upstream message which comprises a correlation tag from an optical network unit (ONU), wherein the correlation tag represents a unique number, a processor coupled to the receiver and configured to process the upstream message, and generate a downstream message based on the upstream message, wherein the downstream message comprises the correlation tag, and a transmitter coupled to the processor and configured to transmit the downstream message to the ONU.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional patent application No. 62/016,852 filed Jun. 25, 2014 by Xuming Wu, et al., and titled “Optical Line Terminal (OLT) Support of Optical Network Unit (ONU) Calibration,” which is incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       REFERENCE TO A MICROFICHE APPENDIX 
       [0003]    Not applicable. 
       BACKGROUND 
       [0004]    A passive optical network (PON) is one system for providing network access over the last mile, which is the final portion of a telecommunications network that delivers communication to customers. A PON is a point-to-multipoint (P2MP) network comprised of an optical line terminal (OLT) at a central office (CO), an optical distribution network (ODN), and optical network units (ONUs) at the user premises. PONs may also comprise remote nodes (RNs) located between the OLTs and the ONUs, for instance at the end of a road where multiple customers reside. 
         [0005]    In recent years, time-division multiplexing (TDM) PONs such as gigabit-capable PONs (GPONs) and Ethernet PONs (EPONs) have been deployed worldwide for multimedia applications. In TDM PONs, the total capacity is shared among multiple users using a time-division multiple access (TDMA) scheme, so the average bandwidth for each user may be limited to below 100 megabits per second (Mb/s). 
         [0006]    Wavelength-division multiplexing (WDM) PONs are considered a very promising solution for future broadband access services. WDM PONs can provide high-speed links with dedicated bandwidth up to 10 gigabits per second (Gb/s). By employing a wavelength-division multiple access (WDMA) scheme, each ONU in a WDM PON is served by a dedicated wavelength channel to communicate with the CO or the OLT. Next-generation PONs (NG-PONs) and NG-PON2s may include point-to-point WDM PONs (P2P-WDM PONs), which may provide data rates higher than 10 Gb/s. 
         [0007]    NG-PONs and NG-PON2s may also include time- and wavelength-division multiplexing (TWDM) PONs, which may also provide data rates higher than 10 Gb/s. TWDM PONs may combine TDMA and WDMA to support higher capacity so that an increased number of users can be served by a single OLT with sufficient bandwidth per user. In a TWDM PON, a WDM PON may be overlaid on top of a TDM PON. In other words, different wavelengths may be multiplexed together to share a single feeder fiber, and each wavelength may be shared by multiple users using TDMA. 
       SUMMARY 
       [0008]    In one embodiment, the disclosure includes an OLT channel termination (CT) comprising a receiver configured to receive an upstream message which comprises a correlation tag from an ONU, wherein the correlation tag represents a unique number, a processor coupled to the receiver and configured to process the upstream message, and generate a downstream message based on the upstream message, wherein the downstream message comprises the correlation tag, and a transmitter coupled to the processor and configured to transmit the downstream message to the ONU. 
         [0009]    In another embodiment, the disclosure includes an ONU comprising a transmitter configured to transmit an upstream message to an OLT CT, wherein the upstream message comprises a correlation tag, wherein the correlation tag represents a unique number, and a receiver coupled to the transmitter and configured to receive a downstream message from the OLT CT, wherein the downstream message comprises the correlation tag. 
         [0010]    In yet another embodiment, the disclosure includes a method for ONU online calibration, comprising receiving an upstream message from an ONU, wherein the upstream message comprises a correlation tag, wherein the correlation tag represents a unique number, generating a downstream message based on the upstream message, wherein the downstream message comprises the correlation tag, and transmitting the downstream message to the ONU. 
         [0011]    In yet another embodiment, the disclosure includes another method for ONU online calibration, comprising transmitting an upstream message to an OLT CT, wherein the upstream message comprises a correlation tag, wherein the correlation tag represents a unique number, and receiving a downstream message from the OLT CT, wherein the downstream message comprises the correlation tag. 
         [0012]    These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
           [0014]      FIG. 1  is a schematic diagram of a TWDM PON. 
           [0015]      FIG. 2  is a message sequence diagram illustrating ONU online calibration. 
           [0016]      FIG. 3  is a flowchart illustrating a method for ONU online calibration according to an embodiment of the disclosure. 
           [0017]      FIG. 4  is a flowchart illustrating another method for ONU online calibration according to an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    It should be understood at the outset that, although illustrative implementations of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
         [0019]    Prior PON technologies are single-wavelength PONs and employ an OLT with a single OLT CT, which is an OLT port communicating with ONUs. Therefore, the prior PON technologies do not support ONU wavelength tuning. NG-PONs and NG-PON2s, including multiple-wavelength PONs such as WDM PONs, P2P-WDM PONs, and TWDM PONs may have multiple OLT CTs in order to support the multiple wavelengths. Each OLT CT may provide data transmission in a pair of upstream and downstream wavelengths. Upstream may refer to the direction of communication from an ONU to the OLT. Downstream, on the other hand, may refer to the direction of communication from the OLT to the ONU. 
         [0020]    In a typical TWDM PON, there may be up to eight wavelengths in both the downstream direction and the upstream direction. Calibration is a process of adjusting the wavelength of upstream optical signals that an ONU transmits within a specific error margin. Calibration may be necessary for an un-calibrated ONU or an already-calibrated or pre-calibrated ONU that is experiencing performance degradation. Offline calibration may refer to calibration when an ONU is pre-calibrated before registering with a PON. The ONU may know the wavelengths of all working upstream wavelength channels and know how to calibrate itself to transmit in a specific channel. Online calibration may refer to calibration when the ONU must register with the PON in order to determine from an OLT which upstream wavelength channel to use. The ONU may not know the wavelengths of any working upstream wavelength channels or know how to calibrate itself to transmit in a specific upstream wavelength channel. 
         [0021]    Existing standard draft, International Telecommunication Union Telecommunication (ITU-T) Standardization Sector G.989.3 (draft ITU-T G.989.3) discloses a process for ONU online calibration. After powering up, the ONU conducts downstream scanning and calibration. The ONU chooses a downstream channel for upstream channel calibration. The ONU learns the system and channel profiles from the chosen downstream channel. As part of the calibration, the ONU sends an upstream message to the OLT. 
         [0022]    However, a direct response to the upstream message from the OLT to the ONU is not provided in the draft ITU-T G.989.3. Therefore, the ONU doesn&#39;t have enough information for online calibration. For example, the ONU doesn&#39;t know if the upstream wavelength of the ONU is correct, if the ONU needs to calibrate more upstream wavelength channels, and if the ONU is ready to activate the upstream wavelength channel for operation. 
         [0023]    Disclosed herein are embodiments for online calibration of an ONU. The ONU transmits an upstream message to an OLT CT, which comprises a unique index number. The OLT CT receives the upstream message and transmits a downstream message based on the upstream message to the ONU, which comprises the unique index number. Both the upstream message and the downstream message may be physical layer operation, administration and maintenance (PLOAM) messages. The disclosed embodiments are respect to a TWDM PON, but may also apply to any PON supporting the ONU online calibration. 
         [0024]      FIG. 1  is a schematic diagram of a TWDM PON  100 . The TWDM PON  100  may be suitable for implementing the disclosed embodiments. The TWDM PON  100  may be a communications network that does not require any active components to distribute data between an OLT  105  and ONUs 1-n    150   1-n . Instead, the TWDM PON  100  may use passive optical components to distribute data between the OLT  105  and the ONUs 1-n    150   1-n . The TWDM PON  100  may adhere to any standard related to multiple-wavelength PONs. The TWDM PON  100  comprises the OLT  105 , a splitter  140 , and the ONUs 1-n    150   1-n , where n may be any positive integer. 
         [0025]    The OLT  105  may be any device configured to communicate with the ONUs 1-n    150   1-n  and another network. Specifically, the OLT  105  may act as an intermediary between the other network and the ONUs 1-n    150   1-n . For instance, the OLT  105  may forward data received from the network to the ONUs 1-n    150   1-n , and may forward data received from the ONUs 1-n    150   1-n  to the other network. When the other network uses a network protocol that differs from the PON protocol used in the TWDM PON  100 , the OLT  105  may comprise a converter that converts the network protocol to the PON protocol. The OLT  105  converter may also convert the PON protocol into the network protocol. 
         [0026]    The OLT  105  may comprise a media access control (MAC) module  110 , OLT CT 1-m    115   1-m , a wavelength multiplexer (WM)  120 , a wavelength demultiplexer (WDEM)  125 , a local oscillator (LO)  130 , and a bi-directional optical amplifier (OA)  135 . The MAC module  110  may be any module suitable for processing signals for use at a physical layer in a protocol stack. The WM  120  may be any suitable wavelength multiplexer, such as an arrayed waveguide grating (AWG). The WM  120  may multiplex the wavelength channels and thus combine the signals into a combined transmitted signal, then forward the combined transmitted signal to the LO  130 . The LO  130  may add characteristics to the combined transmitted signal in order for the ONUs 1-n    150   1-n  to properly extract the signals. The LO  130  may then forward the combined transmitted signal to the OA  135 , which may amplify the combined transmitted signal as needed in order to forward the combined transmitted signal to a splitter  140  via an optical fiber  137 . The OA  135  may also receive a combined received signal from the splitter  140  via the optical fiber  137  and amplify the combined received signal as needed in order to forward the combined received signal to the WDEM  125 . The WDEM  125  may be similar to the WM  120  and may demultiplex the combined received signal into multiple optical signals, then forward the multiple optical signals to the OLT CT 1-m    115   1-m . 
         [0027]    The splitter  140  may be any device suitable for splitting the combined optical signals and forwarding the split signals to the ONUs 1-n    150   1-n . The splitter  140  may also be any device suitable for receiving signals from the ONUs 1-n    150   1-n , combining those signals into a combined received signal, and forwarding the combined received signal to the OA  135 . For example, the splitter  140  splits a downstream optical signal into n split downstream optical signals in the downstream direction (e.g. from the OLT  105  to the ONUs 1-n    150   1-n ), and combines n upstream optical signals into one combined upstream optical signal in the upstream direction (e.g. from the ONUs 1-n    150   1-n  to the OLT  105 ). 
         [0028]    The ONUs 1-n    150   1-n  may be any devices suitable for communicating with the OLT  105 . The ONUs 1-n    150   1-n  may comprise wavelength tunable components  155   1-n  and MAC modules 1-n    160   1-n . The wavelength tunable components  155   1-n  comprise wavelength tunable transmitters and wavelength tunable filters (not shown). The MAC modules 1-n    160   1-n  are similar to the MAC module  110 . 
         [0029]    The TWDM PON  100  may provide WDM capability by associating a downstream wavelength (e.g. λ 1   d,  λ 2   d,  . . . , and λnd) and an upstream wavelength (e.g. λ 1   u,  λ 2   u,  . . . , and λnu) with each OLT CT 1-m,    115   1-m  so that a plurality of wavelengths are present. The TWDM PON  100  may then combine those wavelengths into a single optical fiber  137  and distribute the wavelengths to the ONUs 1-n    150   1-n  through the splitter  140 . The TWDM PON  100  may provide TDM as well. 
         [0030]      FIG. 2  is a message sequence diagram  200  illustrating ONU online calibration. The TWDM PON  100  of  FIG. 1  may implement the ONU online calibration. The diagram  200  illustrates messages exchanged between an OLT CT  210  and an ONU  220 . The OLT CT  210  and the ONU  220  may be any OLT CT 1-m    115   1-m  and any ONU 1-n    150   1-n . 
         [0031]    The ONU  220  initializes and calibrates its receiver and transmitter (not shown). Initialization may comprise self-configuration and ranging to the OLT CT  210 . Self-calibration of the transmitter and the receiver may comprise calibrating the wavelengths of upstream optical signals that the transmitter transmits and downstream optical signals the receiver receives by adjusting control parameters including temperature, current, and voltage on the ONU. After the self-calibration, the wavelength of the upstream optical signals that the transmitter of the ONU  220  transmits should match the wavelength of the upstream optical signals that the receiver of the OLT CT  210  receives. Similarly, the wavelength of the downstream optical signals that the receiver of the ONU  220  receives should match the downstream wavelength of the optical signals that the transmitter of the OLT CT  210  transmits. 
         [0032]    The ONU  220  transmits the OLT CT  210  an upstream message  230 . The upstream message  230  is a PLOAM message and comprises a correlation tag that differentiates the ONU  220  from other ONUs. The correlation tag may represent a unique non-zero number in various forms. For example, the correlation tag may comprise a unique non-zero number in a 16-bit field. The unique non-zero number changes to a different unique non-zero number when the ONU  220  tunes the wavelength, power, or both of the upstream optical signals. 
         [0033]    The OLT CT  210  receives the upstream message  230 , evaluates the upstream message  230 , and transmits a downstream message  240  to the ONU  220 . The evaluation of the upstream message  230  includes determining if the ONU  220  transmits the upstream signals with right wavelength and power, if the ONU  220  needs to calibrate more upstream wavelength channels, and if the ONU  220  is ready to be activated for operation under the existing control parameters. The downstream message  240  is also a PLOAM message. 
         [0034]    In one embodiment, the downstream message  240  may be a new calibration downstream message created by the OLT CT  210 . Table 1 describes parameters of the downstream message  240  according to an embodiment of the disclosure. The downstream message  240  comprises an unassigned ONU identification (ID), a message type ID, a sequence number (SeqNo), a vendor-specific serial number, a message index, a calibration ID, an upstream wavelength channel ID, a received signal strength indicator (RSSI), a padding, and a message integrity check. The message index is the correlation tag copied from the upstream message  230 . 
         [0035]    The calibration ID comprises “ 0 ” bits and a calibration status bit. For example, table 1 shows that the calibration ID comprises 8 bits, 7 “0” bits followed by a calibration status bit. The calibration status bit is decided based on the evaluation of the upstream message  230  of the OLT CT  210 . The calibration status bit has a first binary value indicating the ONU  220  should continue calibration, and a second binary value indicating the ONU  220  should be activated for operation, where the first binary value and the second binary value are different. In one embodiment, table 1 shows the first binary value is 0 and the second binary value is 1. In another embodiment, the first binary value may be 1 and the second binary value may be 0. 
         [0036]    The upstream wavelength channel ID is either the upstream wavelength channel in which the upstream wavelength is being calibrated when the calibration status bit has the first binary value (e.g. 0 as shown in table 1), or the upstream wavelength channel which is ready for activation when the calibration status bit has the second binary value (e.g.  1  as shown in table 1). The RSSI indicates optical power received from the ONU  220  at the OLT CT  210 . The RSSI may be used by the ONU  220  as a reference for online calibration. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The downstream message according to an embodiment of the disclosure. 
               
             
          
           
               
                 Field 
                 Content 
                 Description 
               
               
                   
               
             
          
           
               
                 #1 
                 Unassigned ONU ID 
                 10-bit unassigned ONU-ID. 
               
               
                 #2 
                 Message Type ID 
                 Message type ID “Calibration”. 
               
               
                 #3 
                 SeqNo 
                 Broadcast PLOAM sequence number. 
               
               
                 #4 
                 Vendor-ID 
                 32-bit ONU Vendor-ID code, a four-character combination discovered 
               
               
                   
                   
                 at SN acquisition. 
               
               
                 #5 
                 VSSN 
                 32-bit vendor-specific serial number, a four-byte unsigned integer 
               
               
                   
                   
                 discovered at SN acquisition. 
               
               
                 #6 
                 Message index 
                 Index of the corresponding ONU PLOAM message. 
               
               
                 #7 
                 Calibration ID 
                 0000 000A 
               
               
                   
                   
                 A - Calibration indication, one bit field with the following significance: 
               
               
                   
                   
                 A = 0: the ONU should keep on scanning the upstream wavelength. 
               
               
                   
                   
                 A = 1: the ONU should stop the calibration process, and get ready for 
               
               
                   
                   
                 the activation process. 
               
               
                   
                   
                 Other values reserved. 
               
               
                 #8 
                 Upstream wavelength 
                 0000 UUUU 
               
               
                   
                 channel ID 
                 UUUU - Four-bit upstream wavelength Channel ID, 
               
               
                   
                   
                 When A = 0, this four-bit Channel ID indicates which upstream 
               
               
                   
                   
                 wavelength Channel the upstream wavelength is in. 
               
               
                   
                   
                 When A = 1, this four-bit Channel ID indicates which upstream 
               
               
                   
                   
                 wavelength Channel the ONU should use for activation. 
               
               
                 #9 
                 RSSI 
                 These two octets are used to indicate the received optical power at the 
               
               
                   
                   
                 OLT side from the ONU. 
               
               
                 #10 
                 Padding 
                 Set to 0x00 by the transmitter; treated as “don&#39;t care” by the receiver. 
               
               
                 #11 
                 MIC 
                 Message integrity check. 
               
               
                   
               
             
          
         
       
     
         [0037]    In another embodiment, the downstream message  240  may be a revised PLOAM message, for example a revised upstream wavelength channel information (US_WLCH_INFO) PLOAM message or a revised assign_ONU_ID PLOAM message. Table 2 describes parameters of the downstream message  240  according to another embodiment of the disclosure. The downstream message  240  is generated by adding the message index, the calibration ID, the upstream wavelength channel ID, and the RSSI to the existing US_WLCH_INFO PLOAM message. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 The downstream message according to another embodiment of the disclosure. 
               
             
          
           
               
                 Field 
                 Content 
                 Description 
               
               
                   
               
             
          
           
               
                 #1 
                 Unassigned ONU ID 
                 Unassigned ONU ID. 
               
               
                 #2 
                 Message Type ID 
                 Message type ID “US_WLCH_INFO”. 
               
               
                 #3 
                 0x00 
                 Sequence number. 
               
               
                 #4 
                 Vendor ID 
                 The code set for the Vendor ID is specified in [ATIS-0300220]. The four 
               
               
                   
                   
                 characters are mapped into the 4-byte field by taking each ASCII/ANSI 
               
               
                   
                   
                 character code and concatenating them. 
               
               
                   
                   
                 Example: Vendor_ID = ABCD → Byte 5 = 0x41, Byte 6 = 0x42, Byte 7 = 
               
               
                   
                   
                 0x43, Byte 8 = 0x44. 
               
               
                 #5 
                 VSSN 
                 Vendor-specific serial number. 
               
               
                 #6 
                 UUU0 000O 
                 UUU - Three-bit upstream wavelength Channel ID. 
               
               
                   
                   
                 O - receiving path indication 
               
               
                   
                   
                 O = 0, in-band, 
               
               
                   
                   
                 O = 1, out-of-band. 
               
               
                 #7 
                 Message index 
                 Index of the corresponding ONU PLOAM message. 
               
               
                 #8 
                 Calibration ID 
                 0000 000A 
               
               
                   
                   
                 A - Calibration indication, one bit field with the following significance: 
               
               
                   
                   
                 A = 0: the ONU should keep on scanning the upstream wavelength. 
               
               
                   
                   
                 A = 1: the ONU should stop the calibration process, and get ready for the 
               
               
                   
                   
                 activation process. 
               
               
                   
                   
                 Other values reserved. 
               
               
                 #9 
                 Upstream wavelength 
                 0000 UUUU 
               
               
                   
                 channel ID 
                 UUUU - Four-bit upstream wavelength Channel ID, 
               
               
                   
                   
                 When Octet 15 = 0000 0000, this four-bit Channel ID indicates which 
               
               
                   
                   
                 upstream wavelength Channel the upstream wavelength is in. 
               
               
                   
                   
                 When Octet 15 = 0000 0001, this four-bit Channel ID indicates which 
               
               
                   
                   
                 upstream wavelength Channel the ONU should turn to. 
               
               
                 #10 
                 RSSI 
                 These two octets are used to indicate the received optical power at the OLT 
               
               
                   
                   
                 side from the ONU. 
               
               
                 #11 
                 Padding 
               
               
                 #12 
                 MIC 
                 Set to 0x00 by transmitter; treated as “don&#39;t care” by receiver. 
               
               
                   
               
             
          
         
       
     
         [0038]    Table 3 describes parameters of the downstream message  240  according to yet another embodiment of the disclosure. The downstream message  240  is generated by adding the message index, the calibration ID, the upstream wavelength channel ID, and the RSSI to the existing assign_ONU_ID PLOAM message. When the calibration ID is 00000000, the ONU ID should be ignored by the ONU. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 The downstream message according to yet another embodiment of the disclosure. 
               
             
          
           
               
                 Field 
                 Content 
                 Description 
               
               
                   
               
             
          
           
               
                 #1 
                 0x03FF 
                 10-bit broadcast ONU-ID. 
               
               
                 #2 
                 0x03 
                 Message type ID “Assign_ONU-ID”. 
               
               
                 #3 
                 SeqNo 
                 Broadcast PLOAM sequence number. 
               
               
                 #4 
                 ONU ID 
                 LSB-justified 10-bit assigned ONU-ID value padded with 6 MSB 
               
               
                   
                   
                 zeros; range 0-1022 (0x0000-0x03FE). When field #8 is 0, ONU- 
               
               
                   
                   
                 ID should be ignored by the ONU. 
               
               
                 #5 
                 Vendor ID 
                 32-bit ONU Vendor-ID code, a four-character combination 
               
               
                   
                   
                 discovered at SN acquisition. 
               
               
                 #6 
                 VSSN 
                 32-bit vendor-specific serial number, a four-byte unsigned integer 
               
               
                   
                   
                 discovered at SN acquisition. 
               
               
                 #7 
                 Message index 
                 Index of the corresponding ONU PLOAM message. 
               
               
                 #8 
                 Calibration ID 
                 0000 000A 
               
               
                   
                   
                 A - Calibration indication, one bit field with the following 
               
               
                   
                   
                 significance: 
               
               
                   
                   
                 A = 0: the ONU should keep on scanning the upstream 
               
               
                   
                   
                 wavelength. 
               
               
                   
                   
                 A = 1: the ONU should stop the calibration process, and get ready 
               
               
                   
                   
                 for the activation process. 
               
               
                   
                   
                 Other values reserved. 
               
               
                 #9 
                 Upstream wavelength 
                 UUUU - Four-bit upstream wavelength Channel ID, 
               
               
                   
                 channel ID 
                 When Octet 15 = 0000 0000, this four-bit Channel ID indicates 
               
               
                   
                   
                 which upstream wavelength Channel the upstream wavelength is 
               
               
                   
                   
                 in. 
               
               
                   
                   
                 When Octet 15 = 0000 0001, this four-bit Channel ID indicates 
               
               
                   
                   
                 which upstream wavelength Channel the ONU should turn to. 
               
               
                 #10 
                 RSSI 
                 These two octets are used to indicate the received optical power at 
               
               
                   
                   
                 the OLT side from the ONU. 
               
               
                 #11 
                 Padding 
                 Set to 0x00 by the transmitter; treated as “don&#39;t care” by the 
               
               
                   
                   
                 receiver. 
               
               
                 #12 
                 MIC 
                 Message integrity check. 
               
               
                   
               
             
          
         
       
     
         [0039]      FIG. 3  is a flowchart illustrating a method  300  for ONU online calibration according to an embodiment of the disclosure. The method  300  is implemented in an OLT CT in order to activate the ONU. For example, the OLT CT and the ONU may be the OLT CT 1    115   1  and the ONU 1    150   1 . In block  310 , an upstream message which comprises a correlation tag, is received from an ONU. The correlation tag represents a unique number, which differentiates the ONU from other ONUs. The unique number changes to a different number when the ONU tunes the wavelength, power, or both of the upstream optical signals. In block  320 , a downstream message is generated based on the upstream message. The downstream message comprises the correlation tag. In block  330 , the downstream message is transmitted to the ONU. 
         [0040]      FIG. 4  is a flowchart illustrating another method  400  for ONU online calibration according to an embodiment of the disclosure. The method  400  is implemented in an OLT CT in order to activate the ONU. For example, the OLT CT and the ONU may be the OLT CT 1    115   1  and the ONU 1    150   1 . In block  410 , an upstream message which comprises a correlation tag, is transmit to an OLT CT. The correlation tag represents a unique number, which differentiates the ONU from other ONUs. The unique number changes to a different number when the ONU tunes the wavelength, power, or both of the upstream optical signals. In block  420 , a downstream message which comprises the correlation tag is received from the OLT CT. 
         [0041]    While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
         [0042]    In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and may be made without departing from the spirit and scope disclosed herein.