CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application JP2012-269224 filed on Dec. 10, 2012, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a station-side apparatus. 
     The recent prevalence of the Internet is enhancing the demand for higher-speed network communications. To meet this demand for higher-speed communications, ADSL (Asymmetric Digital Subscriber Line) or PON (Passive Optical Network) is growing. The PON includes B-PON (Broadband PON), E-PON (Ethernet PON), and G-PON (Gigabit-capable PON). 
     The PON is a network system which connects an accommodation station (OLT: Optical Line Terminal) installed in a vendor&#39;s station and network units (ONUs: Optical Network Units) installed in user premises; in the PON, the signal in an optical fiber connected to the OLT is split into a plurality of fibers with an optical splitter; the plurality of fibers are connected to the ONUs one by one. If a network is configured with the PON, low cost fiber installation and high-speed communications by employment of optical transmission are attained. For this reason, the PON is spreading all over the world. 
     Among the techniques utilizing the PON, TDM-PON (Time Division Multiplexing Passive Optical Network) is widely employed, which uses optical signals having different wavelengths in downstream transmission from the OLT to ONUs and upstream transmission from the ONUs to the OLT and further applies time-division multiplexing to the signals depending on the ONU. This TDM-PON is employed in B-PON, E-PON, G-PON, 10G-EPON, and XG-PON. 
     In the TDM-PON, the OLT controls the timing of sending optical signals from ONUs to prevent a conflict among the optical signals from the ONUs in upstream transmission. Specifically, the OLT sends each ONU a control frame to specify a permitted transmission period. Each ONU sends an upstream control signal and upstream data during the period specified by the received control frame. 
     Each ONU also sends the OLT a control frame to request a required bandwidth to the OLT, based on the data volume of a frame received from the connected terminal. In typical, the permitted transmission period for an ONU is determined through DBA (Dynamic Bandwidth Allocation) control by the OLT. The DBA control is a control method that dynamically determines the period based on the required bandwidth requested by the ONU. 
     Furthermore, to recover significant optical transmission loss generated in the TDM-PON, it has been generally known to employ FEC (Forward Error Correction), which is a kind of error correction coding. For example, the 10G-EPON (IEEE 802.3av standard) requires employment of Reed-Solomon (255,223) code, which is one of the FEC codes. 
     A system like the 10G-EPON employs the FEC regardless of transmission loss between OLT and ONUs. However, communications with short-distant ONUs generating small transmission loss do not require the FEC, although communications with long-distant ONUs generating great transmission loss do. Accordingly, in a system like the 10G-EPON, transmission efficiency will be degraded as a result of transmission of optical signals after attaching unnecessary FEC redundant codes to the signals for the short-distant ONUs. 
     In view of the above, a method for improving the transmission efficiency has been proposed that does not use the FEC in communications with short-distant ONUs but uses the FEC in communications with long-distant ONUs. 
     As to the DBA control for the TDM-PON, various control methods are known. In order to achieve fair bandwidth allocation to the ONUs, there exists a method of controlling bandwidth allocation which considers data volume information and the degree of FEC redundancy for upstream transmission data to determine upstream data transmission bandwidths to be allocated to individual subscriber&#39;s communications apparatuses (for example, refer to JP 2009-010530 A). 
     In the meanwhile, as a further next generation PON of the 10G-EPON or the XG-PON, there exists WDM/TDM-PON that bundles traditional TDM-PONs with a plurality of wavelengths. This WDM/TDM-PON enables still larger volume of communications by using a plurality of wavelengths. 
     For this WDM/TDM-PON, a technique has been proposed that dynamically changes the communication wavelength using a wavelength-tunable optical transceiver in each ONU (for example, refer to S. Kimura, “10-Gbit/s TDM-PON and over-40-Gbit/s WDM/TDM-PON systems with OPEX-effective burst-mode technologies”, OFC2009, OWH-6, March, 2009) 
     SUMMARY OF THE INVENTION 
     The WDM/TDM-PON system attains high transmission efficiency by applying the FEC depending on the distance and the transmission loss and changing the wavelength dynamically. However, if a WDM (Wavelength-Division Multiplexing) PON system employing selectable FEC code rate for each ONU simply assigns wavelengths based on the number of registered ONUs, communication bandwidths (transmission rates) to actually send and receive data will become unfair among a plurality of wavelengths used in the PON system. 
     It is desired, even in such a case, that the communication bandwidths at the plurality of wavelengths be kept fair in the overall PON system. 
     This invention has been made in view of this problem and a primary object of this invention is to achieve wavelength assignment to ONUs that can reduce the unfairness in the communication bandwidth among the wavelengths in a WDM/TDM-PON system. 
     An aspect of this invention is a network system including subscriber apparatuses and a station-side apparatus for communicating with the subscriber apparatuses. The station-side apparatus communicates with the subscriber apparatuses using wavelengths. The station-side apparatus determines a wavelength to be used by each of at least one subscriber apparatus of the subscriber apparatuses based on effective transmission rates used by the subscriber apparatuses in communications with the station-side apparatus. 
     According to an embodiment of this invention, the unfairness in communication bandwidth among wavelengths can be reduced even if the network system includes ONUs communicating at different effective transmission rates in transmission and receiving. 
     Objects, configurations, and effects of this invention other than those described above will be clarified in the description of the following embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an optical access network employing the WDM/TDM-PON in Embodiment 1; 
         FIG. 2  is a block diagram illustrating a configuration of an OLT in Embodiment 1; 
         FIG. 3  is a block diagram illustrating a configuration of an ONU in Embodiment 1; 
         FIG. 4  is an explanatory diagram illustrating an ONU wavelength management table in Embodiment 1; 
         FIG. 5  is a flowchart illustrating overall processing by a wavelength assignment controller in the OLT in Embodiment 1; 
         FIG. 6  is a sequence diagram illustrating operations in wavelength assignment between the OLT and ONUs at registration of an ONU in Embodiment 1; 
         FIG. 7  is an explanatory diagram illustrating a λSET frame for setting wavelengths to an ONU in Embodiment 1; 
         FIG. 8  is an explanatory diagram illustrating a λSET_ACK frame for responding to the λSET frame in Embodiment 1; 
         FIG. 9  is a block diagram illustrating an optical access network employing the WDM/TDM-PON in Embodiment 2; 
         FIG. 10  is a block diagram illustrating a configuration of an OLT in Embodiment 2; 
         FIG. 11  is a block diagram illustrating a configuration of an ONU in Embodiment 2; and 
         FIG. 12  is an explanatory diagram illustrating an ONU wavelength management table in Embodiment 2. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of this invention will be described in detail with reference to the drawings. The components common to the drawings are assigned the same reference signs. The descriptions about control frames in the PON area are provided based on the control frames defined by the 10G-EPON standard unless specified otherwise. 
     Embodiment 1 
       FIG. 1  is a block diagram illustrating an optical access network employing the WDM/TDM-PON in Embodiment 1. 
     The optical access network in Embodiment 1 includes an OLT  10 , an optical splitter  30 , a plurality of ONUs  20  ( 20 - 1 - 1  to  20 - 4 -N 4 ), and a plurality of terminals  5  ( 5 - 1 - 1  to  5 - 4 -N 4 ). The OLT  10  is an optical line terminal and the ONU  20  is an optical network unit. 
     The OLT  10  is connected to the optical splitter  30  via a trunk line optical fiber  40 - 0 . The optical splitter  30  is connected to the ONUs  20 - 1 - 1  to  20 - 4 -N 4  via branch line optical fibers  40 - 1 - 1  to  40 - 4 -N 4 . The terminals  5 - 1 - 1  to  5 - 4 -N 4  are connected to their respective ONUs  20 - 1 - 1  to  20 - 4 -N 4 . 
     Next, methods of downstream transmission and upstream transmission are described. The ONUs  20 - 1 - 1  to  20 - 1 -N 1  communicate with the OLT  10  using a downstream wavelength λD 1  and an upstream wavelength λU 1 . The ONUs  20 - 2 - 1  to  20 - 2 -N 2  communicate with the OLT  10  using a downstream wavelength λD 2  and an upstream wavelength λU 2 . The ONUs  20 - 3 - 1  to  20 - 3 -N 3  communicate with the OLT  10  using a downstream wavelength λD 3  and an upstream wavelength λU 3 . The ONUs  20 - 4 - 1  to  20 - 4 -N 4  communicate with the OLT  10  using a downstream wavelength λD 4  and an upstream wavelength λU 4 . It should be noted that the wavelengths for upstream communications and downstream communications used by the ONUs  20  are not fixed but dynamically selectable in this WDM/TDM-PON system. 
     The downstream transmission from the OLT  10  to the ONUs  20  in the WDM/TDM-PON is described. The OLT  10  sends signals addressed to the ONUs  20 - 1 - 1  to  20 - 1 -N 1  by way of downstream optical signals at the downstream wavelength λD 1 . The OLT  10  sends signals addressed to the ONUs  20 - 2 - 1  to  20 - 2 -N 2  by way of downstream optical signals at the downstream wavelength λD 2 . The OLT  10  sends signals addressed to the ONUs  20 - 3 - 1  to  20 - 3 -N 3  by way of downstream optical signals at the downstream wavelength λD 3 . The OLT  10  sends signals addressed to the ONUs  20 - 4 - 1  to  20 - 4 -N 4  by way of downstream optical signals at the downstream wavelength λD 4 . 
     Accordingly, the optical signal sent from the OLT  10  is an optical signal obtained by wavelength-division multiplexing signals having downstream wavelengths λD 1 , λD 2 , λD 3 , and λD 4 . The wavelength-division multiplex optical signal is fed to the ONUs  20 - 1 - 1  to  20 - 4 -N 4  through the optical splitter  30  and the optical fibers  40 - 1 - 1  to  40 - 4 -N 4 . Each ONU  20  has a wavelength-tunable optical transceiver that can select wavelengths in transmission and receiving (which will be described later), and can only send and receive signals having specified wavelengths. 
     When an ONU  20  receives a wavelength-division multiplex downstream optical signal, it only receives a specified wavelength. For example, the ONUs  20 - 1 - 1  to  20 - 1 -N 1  select only signals having the downstream wavelength λD 1  to receive the selected signals. The ONUs  20 - 2 - 1  to  20 - 2 -N 2  select only signals having the downstream wavelength λD 2  to receive the selected signals. The ONUs  20 - 3 - 1  to  20 - 3 -N 3  select only signals having the downstream wavelength λD 3  to receive the selected signals. The ONUs  20 - 4 - 1  to  20 - 4 -N 4  select only signals having the downstream wavelength λD 4  to receive the selected signals. 
     The downstream optical signal at each wavelength includes signals addressed to ONUs  20  that have been time-division multiplexed. For example, a downstream optical signal at the downstream wavelength λD 1  includes signals to the ONU  20 - 1 - 1  to  20 - 1 -N 1  that have been time-division multiplexed; accordingly, each ONU  20  is capable of analyzing a frame received from the OLT  10 , determining whether the frame is addressed to the ONU  20  itself, and selecting only a frame addressed to the ONU  20  itself. 
     Next, the upstream transmission from the ONUs  20  to the OLT  10  in the WDM/TDM-PON is described. Each ONU  20  selects one of the upstream wavelengths λU 1  to λU 4  and sends an upstream optical signal having the selected wavelength during a period specified by the OLT  10 . It should be noted that the upstream optical signal sent from the ONU  20  is a burst optical signal because the ONU  20  sends the upstream optical signal only in the specified period. 
     For example, the ONU  20 - 1 - 1  to  20 - 1 -N 1  send upstream burst optical signals having the upstream wavelength λU 1 ; the ONU  20 - 2 - 1  to  20 - 2 -N 2  send upstream burst optical signals having the upstream wavelength λU 2 ; the ONU  20 - 3 - 1  to  20 - 3 -N 3  send upstream burst optical signals having the upstream wavelength λU 3 ; and the ONU  20 - 4 - 1  to  20 - 4 -N 4  send upstream burst optical signals having the upstream wavelength λU 4 . 
     The upstream optical signals (upstream burst optical signals) sent from the ONUs  20  are multiplexed by the optical splitter  30  and fed to the OLT  10 . Accordingly, the OLT  10  receives a signal obtained by time-division multiplexing and wavelength-division multiplexing upstream optical signals having upstream wavelengths λU 1  to λU 4 . 
     In this way, the WDM/TDM-PON bundles traditional TDM-PONs with a plurality of wavelengths. As a result, one OLT  10  can accommodate a larger number of ONUs  20  and the WDM/TDM-PON can provide a larger capacity for data transmission between the OLT  10  and the ONUs  20 . 
     In the network system shown in  FIG. 1 , the ONUs  20 - 1 - 1  to  20 - 1 -N 1 , the ONUs  20 - 2 - 1  to  20 - 2 -(N 2 - 1 ), and the ONU  20 - 3 - 1  employ the FEC in communications with the OLT  10 . The ONUs  20 - 2 -N 2 , the ONUs  20 - 3 - 2  to  20 - 3 -N 3 , and the ONUs  20 - 4 - 1  to  20 - 4 -N 4  do not employ the FEC in communications with the OLT  10 . 
     All of the ONUs  20  in Embodiment 1 shown in  FIG. 1  are assigned the same transmission rate and are configured in advance to send and receive signals at the assigned transmission rate. However, the rate (effective transmission rate) at which an ONU  20  actually sends or receives data may be different from the transmission rate preset to the ONU  20 . The example described below is a case where the effective transmission rate in actual sending or receiving data is different among wavelengths in communications of the ONUs  20 . 
     In the network system shown in  FIG. 1 , it is assumed that each value for N 1 , N 2 , N 3 , and N 4  is 16. Furthermore, it is assumed that the ONU  20 - 1 - 1  to  20 - 1 - 16 , and the ONU  20 - 2 - 1  to  20 - 2 - 16  are installed at long distant places from the OLT  10  so that they need to apply the FEC to their communications, but the ONU  20 - 3 - 1  to  20 - 3 - 16 , and the ONU  20 - 4 - 1  to  20 - 4 - 16  are installed at short distant places from the OLT  10  so that they do not need to apply the FEC to their communications. 
     The number of ONUs  20  that communicate at each of the aforementioned wavelengths (combinations of a downstream wavelength λD 1 , λD 2 , λD 3 , or λD 4  and an upstream wavelength λU 1 , λU 2 , λU 3 , or λU 4 ) is 16. In these situations, whether to apply the FEC causes difference in the bandwidth available for communications of each ONU  20 . 
     An example is studied in which the transmission rate is 10 Gbps and the Reed-Solomon (255,223) code is employed for the FEC. An average effective bandwidth per each ONU  20  for the ONUs  20 - 1 - 1  to  20 - 1 - 16  and the ONUs  20 - 2 - 1  to  20 - 2 - 16  is 10 Gbps*(223/255)*1/16=547 Mbps. On the other hand, an average effective bandwidth per ONU  20  for the ONUs  20 - 3 - 1  to  20 - 3 - 16  and the ONUs  20 - 4 - 1  to  20 - 4 - 16  is 10 Gbps*1*1/16=625 Mbps. As a result, the average effective bandwidth for the ONUs  20  employing the FEC is lower than the average effective bandwidth for the ONUs  20  without employing the FEC. 
     In this way, if a WDM/TDM-PON system includes ONUs  20  different in FEC code rate, the TDM-PON system therein includes ONUs  20  different in effective transmission rate in transmission and receiving by an ONU  20 . Embodiment 1 can eliminate unfairness in communication bandwidth among the wavelengths caused by the co-existence of ONUs  20  employing the FEC code rate and ONUs  20  without employing it in a network system. 
       FIG. 2  is a block diagram illustrating a configuration of the OLT  10  in Embodiment 1. 
     The OLT  10  includes a WDM coupler (Wavelength Division Multiplexing coupler)  100 , optical transceivers  111  ( 111 - 1  to  111 - 4 ), PHY processors  121  ( 121 - 1  to  121 - 4 ), a MAC processor  131 , an NNI processor  140 , a wavelength assignment controller  151 , an FEC controller  160 , an ONU wavelength management table  171 , and an MPCP controller  180 . For example, the communication interface of the OLT  10  may include the WDM coupler  100 , the optical transceivers  111 , and the PHY processors  121 . The functions of these devices and processors are described below. 
     The WDM coupler  100  multiplexes and demultiplexes upstream optical signals including upstream wavelengths λU 1  to λU 4  and multiplexes and demultiplexes downstream optical signals including downstream wavelengths λD 1  to λD 4 . The WDM coupler  100  demultiplexes a multiplex upstream optical signal including upstream wavelengths of λU 1  to λU 4  fed to the OLT  10 . Then, the WDM coupler  100  feeds the upstream optical signal having the upstream wavelength λU 1  to the optical transceiver  111 - 1 , the upstream optical signal having the upstream wavelength λU 2  to the optical transceiver  111 - 2 , the upstream optical signal having the upstream wavelength λU 3  to the optical transceiver  111 - 3 , and the upstream optical signal having the upstream wavelength λU 4  to the optical transceiver  111 - 4 . 
     The WDM coupler  100  also multiplexes a downstream optical signal having a downstream wavelength λD 1  fed from the optical transceiver  111 - 1 , a downstream optical signal having a downstream wavelength  102  fed from the optical transceiver  111 - 2 , a downstream optical signal having a downstream wavelength λD 3  fed from the optical transceiver  111 - 3 , and a downstream optical signal having a downstream wavelength of λD 4  fed from the optical transceiver  111 - 4 . Then, it feeds the multiplex downstream optical signal to the optical fiber  40 - 0 . 
     The optical transceivers  111 - 1  to  111 - 4  receive upstream optical signals having upstream wavelengths λU 1  to λU 4  fed by the WDM coupler  100  and convert the received upstream optical signals into electric current signals. Furthermore, each optical transceiver  111  converts the obtained current signal into a voltage signal, amplifies it, and feeds it to each PHY processor  121 . 
     The optical transceivers  111 - 1  to  111 - 4  also convert electric signals fed by the PHY processors  121 - 1  to  121 - 4  into optical signals having the downstream wavelengths λD 1  to λD 4  and feed the obtained optical signals to the WDM coupler  100 . 
     The PHY processors  121 - 1  to  121 - 4  extracts clocks from electric signals fed by the optical transceivers  111 - 1  to  111 - 4 , retime the electric signals with the extracted clocks, and convert the electric signals to digital signals. Each PHY processor  121  performs decoding on the digital signal, and further performs FEC decoding on the digital signal as necessary. Each PHY processor  121  extracts a frame from the digital signal and feeds the extracted frame to the MAC processor  131 . 
     Each PHY processor  121  also encodes a frame fed by the MAC processor  131  and performs FEC encoding as necessary. The PHY processor  121  converts a coded frame into an electric signal waveform based on a clock held by the OLT  10  to generate an electric signal. The PHY processor  121  feeds the generated electric signal to one of the optical transceivers  111 - 1  to  111 - 4 . 
     It should be noted that each PHY processor  121  can hold an ON/OFF value indicating whether to perform FEC decoding and FEC encoding for each ONU  20  in connection and switch the value between ON and OFF in accordance with an instruction from the FEC controller  160 . 
     For example, if the network system employs the FEC code type of Reed-Solomon (255,223) and a PHY processor  121  holds a value indicating FEC ON for the ONU  20 - 1 - 1 , the PHY processor  121  attaches a parity bit string to a frame to be sent to the ONU  20 - 1 - 1 . If the PHY processor  121  holds a value indicating FEC OFF for the ONU  20 - 1 - 1 , it does not attach a parity bit to a frame to be sent to the ONU  20 - 1 - 1 . Through these operations, the PHY processor  121  can switch whether to perform FEC encoding. 
     If a PHY processor  121  holds a value indicating FEC ON for the ONU  20 - 1 - 1 , it performs decoding using a parity bit string in a frame sent from the ONU  20 - 1 - 1 . If the PHY processor  121  holds a value indicating FEC OFF for the ONU  20 - 1 - 1 , it performs decoding without using a parity bit string in a frame sent from the ONU  20 - 1 - 1 . Through these operations, the PHY processor  121  can switch whether to perform FEC decoding. 
     The MAC processor  131  analyzes header information in frames received from the PHY processors  121 - 1  to  121 - 4  to identify whether each frame is a user data frame or a control frame. The MAC processor  131  aggregates user data frames sent from the PHY processors  121  to feed the aggregated user data frames to the NNI processor  140 . 
     The MAC processor  131  further identifies the type of a control frame and feeds it to the wavelength assignment controller  151  or the MPCP controller  180  depending on the type of the control frame. The types of the control frames include wavelength assignment control frame such as the later-described λSET_ACK frame and MPCP control frame. If the received frame is of a type relating to wavelength assignment, the MAC processor  131  feeds the frame to the wavelength assignment controller  151 . 
     The MAC processor  131  also distributes user data frames received from the NNI processor  140 , wavelength assignment control frames (λSET frames) received from the wavelength assignment controller  151 , and MPCP control frames received from the MPCP controller  180  to the PHY processors  121 - 1  to  121 - 4  in accordance with the destination address included in the frame. The MAC processor  131  multiplexes the user data frames and control frames by destination address and feeds the obtained multiplex frames to the PHY processors  121 - 1  to  121 - 4 . 
     The NNI processor  140  converts user data frames received from the MAC processor  131  into signals conformable to the NNI (Network Node Interface) and feeds the converted user data frames to the network  6 . The NNI processor  140  also transfers user data frames fed from the network  6  to the MAC processor  131 . 
     The wavelength assignment controller  151  determines the wavelength to be used in transmission or receiving by ONUs  20  based on the ONU wavelength management table  171 . Furthermore, the wavelength assignment controller  151  creates a wavelength assignment control frame (λSET frame) for setting the determined wavelength to an ONU  20  or terminates a wavelength assignment control frame (λSET_ACK frame). The format of the wavelength assignment control frame will be described later. 
     The FEC controller  160  creates and terminates a control frame to specify the FEC ON/OFF for each ONU  20  in each PHY processor  121 . 
     The ONU wavelength management table  171  holds ONU identifiers, FEC code rates for the ONUs  20 , and identifiers of wavelengths in transmission and receiving. A specific example of this table will be described later. 
     The MPCP controller  180  creates a GATE frame for specifying the period for an ONU  20  to send upstream optical signals and terminates a REPORT frame for requesting a required bandwidth by an ONU  20 . The MPCP controller  180  also creates and terminates an MPCP control frame such as a DiscoveryGATE or a REGISTER for registering a newly connected ONU  20 . 
     In the configuration of the OLT  10  shown in  FIG. 2 , one MPCP controller  180  performs MPCP control for four wavelengths; however, the OLT  10  may have a plurality of MPCP controllers which individually perform MPCP control for a specific wavelength. 
     The OLT  10  as configured in Embodiment 1 can send and receive optical signals having different FEC code rates depending on the ONU  20 , calculate transmission and receiving wavelengths for individual ONUs  20  based on the different FEC code rates of the ONUs  20 . The OLT  10  can further send and receive control frames for setting the wavelengths to the individual ONUs  20 . 
       FIG. 3  is a block diagram illustrating a configuration of an ONU  20  in Embodiment 1. 
     The ONU  20  includes a wavelength-tunable optical transceiver  210 , a PHY processor  220 , a MAC processor  230 , a UNI processor  240 , a wavelength assignment controller  250 , an FEC controller  260 , and an MPCP controller  280 . The functions of these devices and processors are described below. 
     The wavelength-tunable optical transceiver  210  is an optical transceiver that can tune the transmission wavelength and the receiving wavelength. The wavelength-tunable optical transceiver  210  receives an instruction to set a transmission wavelength and a receiving wavelength from the wavelength assignment controller  250 . The wavelength-tunable optical transceiver  210  sets the transmission wavelength for received upstream optical signals at one of the upstream wavelengths λU 1  to λU 4  in accordance with the instruction and sends upstream signals toward the OLT  10 . The wavelength-tunable optical transceiver  210  receives downstream optical signals to which the receiving wavelength has been set at one of the downstream wavelengths λD 1  to λD 4 . 
     Now, operations of the wavelength-tunable optical transceiver  210  are described, assuming that the transmission wavelength is set at the upstream wavelength λU 1  and the receiving wavelength is set at the downstream wavelength λD 1 . When the wavelength-tunable optical transceiver  210  receives a downstream optical signal which is obtained by wavelength-division multiplexing downstream wavelengths λD 1  to λD 4  and sent from the OLT  10 , it cuts off the wavelengths other than the downstream wavelength λD 1 . Through this operation, the wavelength-tunable optical transceiver  210  selects only the downstream optical signal having the downstream wavelength λD 1  to receive the selected downstream optical signal. The wavelength-tunable optical transceiver  210  may include an optical filter that can tune the wavelength to pass through to perform such an operation. 
     The wavelength-tunable optical transceiver  210  converts the downstream optical signal having the downstream wavelength λD 1  into an electric current signal, converts the electric current signal into an voltage signal, and then amplifies the voltage signal to generate an electric signal. The wavelength-tunable optical transceiver  210  feeds the generated electric signal to the PHY processor  220 . The wavelength-tunable optical transceiver  210  also converts an electric signal fed from the PHY processor  220  to an upstream optical signal having the upstream wavelength λU 1  and outputs the converted upstream optical signal toward the OLT  10 . 
     The PHY processor  220  extracts a clock from an electric signal fed from the wavelength-tunable optical transceiver  210 , retimes the electric signal with the extracted clock, and converts the electric signal into a digital signal. Furthermore, the PHY processor  220  performs decoding on the digital signal and performs FEC decoding as necessary to extract a frame from the digital signal. Then, the PHY processor  220  feeds the extracted frame to the MAC processor  230 . 
     The PHY processor  220  also performs encoding on a frame fed from the MAC processor  230  and performs FEC encoding as necessary. The PHY processor  220  converts the received frame into an electric signal waveform based on a clock held in the ONU  20  to generate an electric signal, and feeds the generated electric signal to the wavelength-tunable optical transceiver  210 . 
     It should be noted that the PHY processor  220  has a value ON/OFF indicating whether to perform the FEC decoding and FEC encoding and can switch the value ON/OFF in accordance with an instruction from the FEC controller  260 . 
     For example, if the PHY processor  220  employs the FEC code type of Reed-Solomon (255,223) and holds a value indicating FEC ON, the PHY processor  220  attaches a parity bit string to an upstream optical signal in FEC encoding. If the PHY processor  220  holds a value indicating FEC OFF, it processes the signal without attaching a parity bit. Through these operations, the PHY processor  220  can switch whether to perform FEC encoding. 
     If the PHY processor  220  holds a value indicating FEC ON, it performs decoding using a parity bit string. If the PHY processor  220  holds a value indicating FEC OFF, it performs decoding without using a parity bit string. Through these operations, the FEC ON/OFF can be switched. 
     The MAC processor  230  analyzes header information in frames received from the PHY processor  220  to identify whether the frame is a user data frame or a control frame. The MAC processor  230  feeds user data frames to the UNI processor  240  and feeds control frames to the wavelength assignment controller  250  or the MPCP controller  280  after identifying the type of the control frame. 
     The MAC processor  230  also multiplexes user data frames received from the UNI processor  240 , wavelength assignment control frames received from the wavelength assignment controller  250 , and MPCP control frames received from the MPCP controller  280  and feeds the obtained multiplex frames to the PHY processor  220 . 
     The UNI processor  240  converts user data frames received from the MAC processor  230  into signals conformable to the UNI (User Network Interface) and sends the obtained frames to the terminal  5 . The UNI processor  240  also transfers user data frames sent from the terminal  5  to the MAC processor  230 . 
     The wavelength assignment controller  250  performs control of changing the transmission wavelength and the receiving wavelength in the wavelength-tunable optical transceiver  210  based on the wavelength information included in a wavelength assignment control frame (λSET frame) and information on the time to change the wavelength received from the OLT  10 . It also creates a wavelength assignment control frame (λSET_ACK frame) upon completion of the wavelength change in the wavelength-tunable optical transceiver  210 . The wavelength assignment controller  250  feeds the created wavelength assignment control frame (λSET_ACK frame) to the MAC processor  230 . 
     The FEC controller  260  creates and terminates control frames for specifying the FEC ON/OFF in the PHY processor  220 . The FEC controller  260  also controls switching of the FEC ON/OFF in the PHY processor  220  based on the received control frame. 
     The MPCP controller  280  terminates GATE frames for specifying the period to send upstream optical signals and creates REPORT frames for requesting a required bandwidth. The MPCP controller  280  also creates and terminates MPCP control frames such as DiscoveryGATE and REGISTER for registering a newly connected ONU  20 . 
     The ONU  20  as configured in this embodiment can send and receive optical signals at a selected FEC code rate, and also change the wavelength of optical signals to be sent and received by the ONU  20  based on wavelength assignment control frames received from the OLT  10 . 
       FIG. 4  is an explanatory diagram illustrating the ONU wavelength management table  171  in Example 1. 
     The ONU wavelength management table  171  held in the OLT  10  in Example 1 is described. The ONU wavelength management table  171  includes ONU identifiers  1711 , FEC code rates  1712 , and wavelength identifiers  1713 . 
     The ONU identifier  1711  is an identifier uniquely representing an ONU  20  and the FEC code rate  1712  is an FEC code rate used in communications with the ONU  20  represented by an ONU identifier  1711 . The FEC code rate  1712  is a value obtained based on communications with the ONU  20  in later-described discovery process. 
     The wavelength identifier  1713  is an identifier uniquely representing a wavelength (a combination of a transmission wavelength and a receiving wavelength) assigned to the ONU  20  represented by an ONU identifier  1711 . This embodiment employs a limited number of wavelengths to be used in communications between the OLT  10  and the ONUs  20 ; the wavelength identifier  1713  is also one of a predetermined limited number of kinds of identifiers. 
     The following description explains that the OLT  10  and the ONUs  20  use the FEC code type of Reed-Solomon (255,233) to switch application of the FEC by holding a value ON or OFF. 
     It is assumed that, in an ONU  20  having an ONU identifier  1711  of “1”, the FEC application is OFF; the transmission wavelength is an upstream wavelength λU 1 ; and the receiving wavelength is a downstream wavelength  201 . In the entry having the ONU identifier  1711  of “1” in the ONU wavelength management table  171 , the FEC code rate  1712  is “1” (1=255/255) and the wavelength identifier  1713  is “1”. 
     It is also assumed that, in an ONU  20  having an ONU identifier  1711  of “2”, the application of the FEC is ON; the transmission wavelength is an upstream wavelength λU 4 ; and the receiving wavelength is a downstream wavelength λD 4 . In the entry having the ONU identifier  1711  of “2” in the ONU wavelength management table  171 , the FEC code rate  1712  is “0.875” (0.875=223/255) and the wavelength identifier  1713  is “4”. 
     The ONU identifier  1711  in  FIG. 4  is one of the values of 1 to 256; however, it may be the MAC address or the serial number of the ONU  20 , as far as the value can identify the ONU  20 . 
       FIG. 5  is a flowchart illustrating overall processing by the wavelength assignment controller  151  in the OLT  10  in Embodiment 1. 
     After start of the wavelength assignment control, the wavelength assignment controller  151  detects a change in the ONUs  20  connected to the OLT  10  by detecting registration of a new ONU  20  or cancellation of registration of an ONU  20  (S 302 ). 
     Specifically, the wavelength assignment controller  151  determines that it has detected registration of a new ONU  20  at S 302  when registration of the new ONU  20  has been detected and completed in a later-described discovery process. The wavelength assignment controller  151  also determines that registration of an ONU  20  has been canceled at S 302  when an ONU  20  that had not sent a REPORT frame to the OLT  10  for a predetermined period has been detected and the registration of the ONU  20  has been canceled. 
     After S 302 , the wavelength assignment controller  151  calculates an average effective bandwidth per ONU  20  Beff(λ) at each wavelength (S 303 ). The average effective bandwidth per ONU  20  Beff(λ) means an average bandwidth for one ONU  20  in the case where the ONUs  20  assigned the same wavelength fairly share the bandwidth. For example, the wavelength assignment controller  151  may calculate the average effective bandwidth Beff(λ) with the following Formula 1: 
                     Beff   ⁡     (   λ   )       =       {       ∑     i   =   1       N   ⁡     (   λ   )         ⁢           ⁢     1     R   ⁡     (     λ   ⁢           ⁢   i     )           }       -   1               (   1   )               
where, N(λ) represents the number of ONUs  20  that communicate at a wavelength λ and are registered in the OLT  10 ; the effective transmission rate R(λ,i) represents an effective transmission rate of an ONU  20  that communicates at the wavelength λ and has an ONU identifier  1711  of “i”.
 
     For example, if the ONU  20 - 1 - 1  has a transmission rate of 10 Gbps in the PON area and an FEC code rate  1712  of 0.875, the effective transmission rate R(λ,i) of the ONU  20 - 1 - 1  is 8.75 Gbps=10 Gbps*0.875. And, if the ONU  20 - 1 - 2  has a transmission rate of 10 Gbps in the PON area and an FEC code rate  1712  of 1, the effective transmission rate R(λ,i) of the ONU  20 - 1 - 2  is 10 Gbps=10 Gbps*1. 
     When these two ONUs  20  sends and receives signals at a wavelength λ 1  (a combination of an upstream wavelength λU 1  and a downstream wavelength λD 1 ), the wavelength assignment controller  151  calculates an average effective bandwidth per ONU  20  Beff(λ 1 ) at the wavelength λ 1  by Formula 1 as follows:
 
Beff(λ1)=1/(1/8.75 Gbps+1/10 Gbps)=4.67 Gbps
 
     The wavelength assignment controller  151  obtains FEC code rates  1712  for the ONUs  20  which communicate at each wavelength from the ONU wavelength management table  171  and calculates the average effective bandwidth per ONU  20  Beff(λ) at each wavelength based on the Formula 1. 
     It should be noted that the transmission rates of all the ONUs  20  are the same in Embodiment 1; accordingly, the wavelength assignment controller  151  does not need to use the transmission rates of the ONUs  20  to calculate the aforementioned effective transmission rate R(λ,i). Specifically, the wavelength assignment controller  151  may obtain the value of the FEC code rate  1712  for the effective transmission rate R(λ,i) to calculate the average effective bandwidth Beff(λ). If the wavelength assignment controller  151  has been instructed that the transmission rate does not need to be considered, it does not need to use the transmission rates of the ONUs  20  to calculate the effective transmission rate R(λ,i), either. 
     After S 303 , the wavelength assignment controller  151  extracts the greatest value Beff_max and the smallest value Beff_min from all of the average effective bandwidths Beff(λ) at individual wavelengths λ calculated at S 303  (S 304 ). 
     After S 304 , the wavelength assignment controller  151  determines whether the average effective bandwidth Beff(λ) is unfair among the wavelengths λ, based on the greatest value Beff_max and the smallest value Beff_min (S 305 ). Specifically, the wavelength assignment controller  151  obtains an evaluation value (Beff_max/Beff_min), which is a result of dividing the greatest value Beff_max by the smallest value Beff_min, and compares the evaluation value (Beff_max/Beff_min) with a predetermined certain range. 
     As a result of the comparison, if the evaluation value (Beff_max/Beff_min) is out of the predetermined range, the wavelength assignment controller  151  determines that the bandwidth is unfair among the wavelengths and performs  5306 . If the evaluation value (Beff_max/Beff_min) is within the predetermined range, the wavelength assignment controller  151  determines that the bandwidth is fair among the wavelengths and performs S 307 . 
     In the above description, the evaluation value (Beff_max/Beff_min) is a result of a division, but the wavelength assignment controller  151  may obtain an evaluation value (Beff_max−Beff_min) by subtracting the smallest value Beff_min from the greatest value Beff_max. And at S 305 , the wavelength assignment controller  151  may determine that the bandwidth is unfair among the wavelengths λ, if the evaluation value (Beff_max−Beff_min) is greater than a predetermined threshold. 
     At S 306 , the wavelength assignment controller  151  determines assignment of wavelengths used in transmission and receiving by the ONUs  20  to eliminate the unfairness in bandwidth among the wavelengths based on the FEC code rates  1712  of the individual ONUs  20 . After determining the wavelength assignment, the wavelength assignment controller  151  returns to S 303 . 
     Then, the wavelength assignment controller  151  performs  5303  and S 304  with the wavelengths in accordance with the determined assignment. If, at S 305 , the wavelength assignment controller  151  determines that the unfairness in bandwidth among the wavelengths has been eliminated, it performs S 307 . 
     At S 307 , the wavelength assignment controller  151  sends a wavelength assignment control frame (λSET frame) for setting a wavelength to each ONU  20  based on the wavelength determined at  5306 . Thereafter, when each ONU  20  completes the wavelength change and the wavelength assignment controller  151  receives a wavelength assignment control frame (λSET_ACK frame) sent from each ONU  20 , the wavelength assignment controller  151  registers a wavelength identifier  1713  indicating the wavelength after change in the wavelength identifier  1713  in the ONU wavelength management table  171  for each ONU  20  (S 307 ). 
     Through the processing shown in  FIG. 5 , the wavelength assignment controller  151  determines assignment of wavelengths for the ONUs  20  to use in transmission and receiving based on the FEC code rates  1712  of the ONUs  20  so as to eliminate the unfairness in the average effective bandwidth per ONU  20  Beff(λ) and sets wavelengths to the ONUs  20  based on the determined wavelengths at which the unfairness can be eliminated. 
     Now, some specific methods of assigning wavelengths at  5306  are provided. 
     The first method of assigning wavelengths is that the wavelength assignment controller  151  calculates an evaluation value (Beff_max/Beff_min) indicating the unfairness for every potential combination of wavelengths which might be assigned to the ONUs  20  based on the FEC code rate  1712 . The wavelength assignment controller  151  extracts a combination of wavelengths with which the evaluation value (Beff_max/Beff_min) is smallest from all of the potential combinations and determines the extracted combination of wavelengths to be assigned to the ONUs  20 . 
     When employing the first method of assigning wavelengths, the wavelength assignment controller  151  can determine the assignment of wavelengths that results in the smallest unfairness evaluation value. 
     The second method of assigning wavelengths is that the wavelength assignment controller  151  determines the wavelengths to be assigned to the ONUs  20  so that the number of ONUs  20  having the same FEC code rate will be as fair as possible among the wavelengths. 
     The following description provides an example of a four-wavelength multiplex WDM/TDM-PON system which includes only two groups of ONUs  20 , one having an FEC code rate  1712  of “1” and the other having an FEC code rate  1712  of “0.875”. It is assumed that the group of the FEC code rate  1712  of “1” consists of 32 ONUs  20  and the group of the FEC code rate  1712  of “0.875” consists of 16 ONUs  20 , namely, a total of 48 ONUs  20  are registered in an OLT  10 . 
     In this case, the wavelength assignment controller  151  divides the ONUs  20  having the FEC code rate  1712  of “1” into four groups of eight ONUs (because there are four wavelengths) by the second method and assigns the four groups to different wavelengths. The wavelength assignment controller  151  divides the ONUs  20  having the FEC code rate  1712  of “0.875” into four groups of four ONUs and assigns the four groups to different wavelengths. 
     Through these operations, the number of ONUs  20  having the FEC code rate  1712  of “1” becomes fair among the wavelengths and the number of ONUs  20  having the FEC code rate  1712  of “0.875” becomes fair among the wavelengths. As a result, the average effective bandwidths per ONU  20  will be fair in all the wavelengths; accordingly, the wavelength assignment controller  151  attains the fairness among the wavelengths. 
     The third method of assigning wavelengths is that the wavelength assignment controller  151  assigns an ONU  20  newly registered in the OLT  10  the wavelength at which the average effective bandwidth has been broadest until the registration of the new ONU  20 . Since the average effective bandwidth at the wavelength assigned to the new ONU  20  is narrowed by the registration of the new ONU  20 , this method can reduce the unfairness in bandwidth. 
     The second and the third methods of assigning wavelengths can speed up the processing of the wavelength assignment controller  151  particularly in a system including a greater number of ONUs  20 . 
     The wavelength assignment controller  151  in Embodiment 1 may omit S 302  to S 305  in  FIG. 5  to perform only S 306  and S 307 . For example, in the case of receipt of an instruction from the administrator or operator, the wavelength assignment controller  151  may perform only S 306  and S 307 . 
     In the case of employing the second or the third method and performing only S 306  and S 307 , the wavelength assignment controller  151  can determine the wavelength assignment based on the FEC code rate  1712  only. 
     Wavelength assignment at new registration of an ONU  20  in Embodiment 1 is described. 
       FIG. 6  is a sequence diagram illustrating operations in wavelength assignment between the OLT  10  and the ONUs  20  at registration of an ONU  20 - 4  in Embodiment 1. 
     For simpler explanation, the following example is described assuming that three ONUs, an ONU  20 - 1 , an ONU  20 - 2 , and an ONU  20 - 3 , have already been registered in an OLT  10  and an ONU  20 - 4  is newly registered. Although the ONUs  20  actually connected to the OLT  10  include other ONUs than the ONUs  20 - 1 ,  20 - 2 ,  20 - 3 , and  20 - 4 ,  FIG. 6  explains operations between the OLT  10  and the ONUs  20 - 1 ,  20 - 2 ,  20 - 3 , and  20 - 4 , which change the wavelengths as a result of determination of wavelengths by the OLT  10 . 
     First, the ONU  20 - 4  is newly connected, turned on, and activated. In this embodiment, an ONU  20  communicates with the OLT  1  at a wavelength λ 1  (a combination of an upstream wavelength λU 1  and a downstream wavelength λD 1 , namely the wavelength identifier  1713  of “1”) until receipt of a λSET frame. At the start of the sequence of  FIG. 6 , the ONU  20 - 1  is assigned a wavelength λ 1  having a wavelength identifier  1713  of “1”; the ONU  20 - 2  is assigned a wavelength λ 2  having a wavelength identifier  1713  of “2” (an upstream wavelength λU 2  and a downstream wavelength λD 2 ); and the ONU  20 - 3  is assigned a wavelength λ 3  having a wavelength identifier  1713  of “3” (an upstream wavelength λU 3  and a downstream wavelength λD 3 ). 
     The OLT  10  periodically performs a discovery process at each wavelength. For this reason, in a discovery process  310  at the wavelength λ 1  after the startup of the ONU  20 - 4 , the OLT  10  exchanges MPCP control frames and FEC control frames with the ONU  20 - 4 . Through this operation, the OLT  10  measures a round trip time to the ONU  20 - 4  or sets initial parameters of the ONU  20 - 4  to the OLT  10 . 
     In the discovery process  310 , the wavelength assignment controller  151  in the OLT  10  acquires an FEC code rate  1712  to be applied to the ONU  20 - 4 . 
     Specifically in the discovery process  310 , the wavelength assignment controller  151  in the OLT  10  measures communications conditions between the ONU  20 - 4  and the OLT  10  and acquires an FEC code rate for the ONU  20 - 4  determined based on the result of the measurement as the FEC code rate  1712 . 
     For example, if a value of the round trip time between the ONU  20 - 4  and the OLT  10  calculated by the MPCP controller  180  is longer than a predetermined threshold, the MPCP controller  180  determines the FEC code rate at the value ON. The wavelength assignment controller  151  acquires the determined FEC code rate as the FEC code rate  1712 . 
     Upon acquisition of the FEC code rate  1712  for the ONU  20 - 4 , the wavelength assignment controller  151  adds a new entry to the ONU wavelength management table  171 . The wavelength assignment controller  151  stores an identifier indicating the ONU  20 - 4  to the ONU identifier  1711  in the new entry and stores the FEC code rate  1712  for the ONU  20 - 4  to the FEC code rate  1712  in the new entry in the ONU wavelength management table  171 . The wavelength assignment controller  151  also stores an identifier indicating the wavelength λ 1 , which is a default for the transmission and receiving wavelengths, to the wavelength identifier  1713  in the new entry. 
     After the foregoing operations, the OLT  10  terminates the registration of the ONU  20 - 4  by the discovery process  310 . 
     After the registration of the ONU  20 - 4 , the wavelength assignment controller  151  in the OLT  10  determines the wavelengths to be assigned to the ONUs  20  so that the average effective bandwidth Beff(λ) will be fair among the wavelengths with reference to the ONU wavelength management table  171  ( 311 ). The operations at the step  311  correspond to S 302  to S 306  shown in  FIG. 5 . 
     In  FIG. 6 , the wavelengths for four ONUs  20 , the ONU  20 - 1 , ONU  20 - 2 , ONU  20 - 3 , and ONU  20 - 4 , are changed. At the step  311  in  FIG. 6 , the wavelength assignment controller  151  determines to assign the ONU  20 - 1  the wavelength λ 4  (the upstream wavelength λU 4  and the downstream wavelength  104 ) having the wavelength identifier  1713  of “4”, the ONU  20 - 2  the wavelength λ 3  having the wavelength identifier  1713  of “3”, the ONU  20 - 3  the wavelength λ 2  having the wavelength identifier  1713  of “2”, and the ONU  20 - 4  the wavelength λ 2  having the wavelength identifier  1713  of “2”. 
     After the determination of the wavelength assignment, the OLT  10  sets the wavelengths to the ONUs  20  which are determined to change the wavelength ( 312  to  315 ). The steps  312  to  315  correspond to  5307  in  FIG. 5 . 
     In the example shown in  FIG. 6 , the OLT  10  sets the wavelengths to the ONU  20 - 4 , ONU  20 - 3 , ONU  20 - 2 , and ONU  20 - 1  in this order. 
     At the step  312 , the OLT  10  sends a λSET (wavelength λ 2 ) frame, which is a control frame for setting the assigned wavelength λ 2  to an ONU  20 , to the ONU  20 - 4 . The wavelength-tunable optical transceiver  210  in the ONU  20 - 4  sets the transmission wavelength at an upstream wavelength λU 2  and the receiving wavelength at a downstream wavelength λD 2 . 
     After completion of the setting, the ONU  20 - 4  sends a λSET_ACK (wavelength λ 2 ) frame to the OLT  10  during the period specified by the λSET frame. The λSET_ACK frame is a control frame to notify the OLT  10  that the wavelength has properly been set. 
     At the step  313 , the OLT  10  sends a λSET (wavelength λ 2 ) frame to set the wavelength λ 2  to the ONU  20 - 3 . The wavelength-tunable optical transceiver  210  in the ONU  20 - 3  sets the transmission wavelength at the upstream wavelength λU 2  and the receiving wavelength at the downstream wavelength λD 2 . After completion of the setting, the ONU  20 - 3  sends a λSET_ACK (wavelength λ 2 ) frame to the OLT  10  during the period specified by the λSET frame. 
     At the step  314 , the OLT  10  sends a λSET (wavelength λ 3 ) frame to set the wavelength λ 3  to the ONU  20 - 2 . The wavelength-tunable optical transceiver  210  in the ONU  20 - 2  sets the transmission wavelength at an upstream wavelength λU 3  and the receiving wavelength at a downstream wavelength λD 3 . After completion of the setting, the ONU  20 - 2  sends a λSET_ACK (wavelength λ 3 ) frame to the OLT  10  during the period specified by the λSET frame. 
     At the step  315 , the OLT  10  sends a λSET (wavelength λ 4 ) frame to set the wavelength λ 1  to the ONU  20 - 1 . The wavelength-tunable optical transceiver  210  in the ONU  20 - 1  sets the transmission wavelength at an upstream wavelength λU 4  and the receiving wavelength at a downstream wavelength λD 4 . After completion of the setting, the ONU  20 - 1  sends a λSET_ACK (wavelength λ 4 ) frame to the OLT  10  during the period specified by the λSET frame. 
     Through the foregoing operations, upon registration of a new ONU  20 , the OLT  10  can determine wavelength assignment to achieve fair bandwidths among the wavelengths and set transmission and receiving wavelengths to the ONUs  20 . Although this description provides a procedure of setting transmission and receiving wavelengths to each ONU  20 , the transmission and receiving wavelengths may be set to a plurality of ONUs  20  collectively. 
       FIG. 6  shows a procedure in which the wavelengths are set after the discovery process  310 ; however, the steps  311  to  315  are performed after cancellation of registration of an ONU  20  is detected. 
     Now, frame formats of a λSET frame and a λSET_ACK are described. The λSET frame is for the OLT  10  to instruct an ONU  20  to set an assigned wavelength and the λSET_ACK frame is for an ONU  20  to notify the OLT  10  of completion of the wavelength setting. 
     The λSET frame and λSET_ACK frame in Embodiment 1 are control frames similar to the REPORT frame and the GATE frame, which are MPCP control frames. The λSET frame employs configurations specified in IEEE 802.3av Clause 77.3.6 for its fields. 
       FIG. 7  is an explanatory diagram illustrating a λSET frame in Embodiment 1 for setting a wavelength to an ONU  20 . 
     First, the λSET frame is described. The λSET frame includes fields of Destination Address (F 321 ), Source Address (F 322 ), Length/Type (F 323 ), Opcode (F 324 ), Timestamp (F 325 ), Assigned US wavelength ID (F 326 ), Assigned DS wavelength ID (F 327 ), Grant # 1  Start time (F 328 ), Grant # 1  Length (F 329 ), Pad/Reserved (F 330 ), and FCS (F 331 ). 
     The Destination Address (F 321 ) indicates the destination address. The Source Address (F 322 ) indicates the source address. The Length/Type (F 323 ) indicates the length and the type of the frame. The Opcode (F 324 ) indicates the type of the control frame. The Timestamp (F 325 ) indicates the time of sending the control frame. The Assigned US wavelength ID (F 326 ) indicates the identifier of the assigned upstream wavelength. The Assigned DS wavelength ID (F 327 ) indicates the identifier of the assigned downstream wavelength. The Grant # 1  Start time (F 328 ) indicates the start time of the permitted period for upstream transmission. The Grant # 1  Length (F 329 ) indicates the length of the permitted period for upstream transmission. The Pad/Reserved (F 330 ) is a field used as a padding or reserve. The FCS (F 331 ) is a bit string to check whether the received frame contains an error. Hereinbelow, each field is described in detail. 
     In the Length/Type (F 323 ) and the Opcode (F 324 ), values for identifying the frame as a λSET frame, an MPCP control frame, or a user data frame are stored and they are used for the OLT  10  and the ONU  20  to transfer the frame depending on its type. For example, when sending a λSET frame, the OLT  10  stores “0x8808” in the Length/Type (F 323 ) and “0x0007” in the Opcode (F 324 ). 
     In the Assigned US wavelength ID (F 326 ), the identifier of an upstream wavelength λU for the ONU  20  determined by the wavelength assignment controller  151  in the OLT  10  is stored. For example, if an upstream wavelength λU 1  has been assigned, a value “0x01” indicating the upstream wavelength λU 1  is stored in the Assigned US wavelength ID (F 326 ) in the λSET frame to be sent to the ONU  20 . 
     In the Assigned DS wavelength ID (F 327 ), the identifier of a downstream wavelength λD for the ONU  20  determined by the wavelength assignment controller  151  in the OLT  10  is stored. For example, if a downstream wavelength λD 1  has been assigned, a value “0x01” indicating the downstream wavelength λD 1  is stored in the Assigned DS wavelength ID (F 327 ) in the λSET frame to be sent to the ONU  20 . 
     In the Grant # 1  Start time (F 328 ) and the Grant # 1  Length (F 329 ), information indicating a transmission period for the ONU  20  to send a λSET_ACK frame in response to the λSET frame is stored. Values to be stored in the Grant # 1  Start time (F 328 ) and the Grant # 1  Length (F 329 ) are determined by the OLT  10 , in consideration of the time required for receiving an instruction by the λSET frame and setting the wavelength to the wavelength-tunable optical transceiver  210  and the periods allocated to the upstream bandwidths for the other ONUs  20  assigned the same wavelength as the ONU  20  that has received the λSET frame. 
     It should be noted that the frame format shown in  FIG. 7  is a format that allows separate settings of an upstream wavelength and a downstream wavelength; however, if the same value is always set to the upstream wavelength and the downstream wavelength, the λSET frame in this embodiment may have a field for storing a single wavelength identifier. 
       FIG. 8  is an explanatory diagram illustrating a λSET_ACK frame in Embodiment 1 for responding to a λSET frame. 
     The λSET_ACK frame is described. The λSET_ACK frame includes fields of Destination Address (F 341 ), Source Address (F 342 ), Length/Type (F 343 ), Opcode (F 344 ), Timestamp (F 345 ), Flags (F 346 ), Echoed US wavelength ID (F 347 ), Echoed DS wavelength ID (F 348 ), Pad/Reserved (F 349 ), and FCS (F 350 ). 
     The Destination Address (F 341 ) indicates the destination address. The Source Address (F 342 ) indicates the source address. The Length/Type (F 343 ) indicates the length and the type of the frame. The Opcode (F 344 ) indicates the type of the control frame. The Timestamp (F 325 ) indicates the time of sending the control frame. The Flags (F 346 ) indicates the response to the λSET frame. The Echoed US wavelength ID (F 347 ) indicates the identifier of the assigned upstream wavelength. The Echoed DS wavelength ID (F 348 ) indicates the identifier of the assigned downstream wavelength. The Pad/Reserved (F 349 ) is a field used as a padding or reserve. The FCS (F 350 ) is a bit string to check whether the received frame contains an error. 
     Hereinbelow, each field is described in detail. As to the fields the same descriptions in the λSET frame are applicable, descriptions are omitted. 
     In the Length/Type (F 343 ) and the Opcode (F 344 ), values for identifying the frame as a λSET_ACK frame, an MPCP control frame, or a user data frame are stored and they are used for the OLT  10  and the ONU  20  to transfer the frame depending on its type. For example, when sending a λSET_ACK frame, the OLT  10  stores “0x8808” in the Length/Type (F 343 ) and “0x0008” in the Opcode (F 344 ). 
     In the Flags (F 346 ), information indicating whether the ONU  20  has completed the wavelength setting is stored. For example, if the wavelength setting has properly been completed in the ONU  20 , the ONU  20  stores “0x01” in the Flags (F 346 ); if the wavelength setting is not properly completed, it stores “0x00” in the Flags (F 346 ). 
     In the Echoed US wavelength ID (F 347 ), the identifier of an upstream wavelength λU for the ONU  20  determined by the wavelength assignment controller  151  in the OLT  10  is stored. Specifically, in the Echoed US wavelength ID (F 347 ), the same identifier as the Assigned US wavelength ID (F 326 ) in the λSET frame is stored. For example, if the upstream wavelength λU 1  has been assigned, the ONU  20  stores a value “0x01” in the Echoed US wavelength ID (F 347 ). 
     In the Echoed DS wavelength ID (F 348 ), the identifier of a downstream wavelength λD for the ONU  20  determined by the wavelength assignment controller  151  in the OLT  10  is stored. Specifically, in the Echoed DS wavelength ID (F 348 ), the same identifier as the Assigned DS wavelength ID (F 327 ) in the λSET frame is stored. For example, if the downstream wavelength λD  1  has been assigned, the ONU  20  stores a value “0x01” in the Echoed DS wavelength ID (F 348 ). 
     In a WDM/TDM-PON system, Embodiment 1 calculates an average effective bandwidth per ONU  20  Beff(λ) at each wavelength based on the FEC code rates  1712  for the ONUs  20  and determines the wavelength assignment to the ONUs  20  so that the difference in the calculated average effective bandwidth Beff(λ) will be minimum. This approach can eliminate the unfairness in communication bandwidth among the wavelengths. In turn, the elimination of the unfairness among the wavelengths enables the FEC code rates  1712  for individual ONUs  20  to be set at values that allow transmission with reliability and at high transmission efficiency. Accordingly, Embodiment 1 can improve the throughput in the WDM/TDM-PON system. 
     Embodiment 2 
     The foregoing Embodiment 1 is an embodiment to achieve wavelength assignment so that the average effective bandwidth per ONU  20  will be fair among the wavelengths in a WDM/TDM-PON system including ONUs  20  having different FEC code rates  1712 . 
     Embodiment 2 is an embodiment to achieve wavelength assignment so that the average effective bandwidth per ONU  20  will be fair among the wavelengths in a WDM/TDM-PON system including ONUs  20  having different transmission rates. 
     A PON system supporting a plurality of transmission rates is defined by existing standards; for example, the 10G-EPON and the 1G-EPON may be implemented in the same optical network. Such an optical network includes ONUs  20  having a transmission rate of 10 Gbps and ONUs  20  having a transmission rate of 1 Gbps together. Hereinafter, differences from Embodiment 1 are mainly described. 
       FIG. 9  is a block diagram illustrating an optical access network employing the WDM/TDM-PON in Embodiment 2. 
     Before wavelength assignment in Embodiment 2, ONUs  20 - 1 - 1  to ONU  20 - 1 -N 1  communicate using an upstream wavelength λU 1  and a downstream wavelength λD 1 ; ONUs  20 - 2 - 1  to ONU  20 - 2 -N 2  communicate using an upstream wavelength λU 2  and a downstream wavelength λD 2 ; ONUs  20 - 3 - 1  to ONU  20 - 3 -N 3  communicate using an upstream wavelength λU 3  and a downstream wavelength λD 3 ; and ONUs  20 - 4 - 1  to ONU  20 - 4 -N 4  communicate using an upstream wavelength λU 4  and a downstream wavelength λD 4 , like in Embodiment 1. 
     Unlike in Embodiment 1, however, each ONU  20  in Embodiment 2 sends and receives optical signals at a transmission rate of 10 Gbps or 1 Gbps. For example, the transmission rate of the ONU  20 - 1 - 1  is 1 Gbps and the transmission rate of the ONU  20 - 4 -N 4  is 10 Gbps. The FEC code rates of the ONUs  20  in Embodiment 2 are the same, unlike the FEC code rates in Embodiment 1. 
     Except for the above-described differences, the WDM/TDM-PON system in Embodiment 1 and the WDM/TDM-PON system in Embodiment 2 are the same. 
       FIG. 10  is a block diagram illustrating a configuration of an OLT  10  in Embodiment 2. 
     The OLT  10  in Embodiment 2 includes a WDM coupler  100 , multi-rate optical transceivers  112  ( 112 - 1  to  112 - 4 ), multi-rate PHY processors  122  ( 122 - 1  to  122 - 4 ), a multi-rate MAC processor  132 , an NNI processor  140 , a wavelength assignment controller  152 , an ONU wavelength management table  172 , and an MPCP controller  180 . The WDM coupler  100 , the NNI processor  140 , and the MPCP controller  180  have the same functions in Embodiment 1 and Embodiment 2. 
     Major differences in configuration between the OLT  10  in Embodiment 1 and the OLT  10  in Embodiment 2 are described as follows. The first difference is that the OLT  10  in Embodiment 2 does not have an FEC controller  160 . 
     The second difference is that the OLT  10  in Embodiment 2 supports the rates of 10 Gbps and 1 Gbps. The multi-rate optical transceivers  112 , the multi-rate PHY processors  122 , and the multi-rate MAC processor  132  in Embodiment 2 respectively correspond to the optical transceivers  111 , the PHY processors  121 , and the MAC processor  131  in Embodiment 1; they support the rates of 10 Gbps and 1 Gbps. 
     The third difference is that the OLT  10  in Embodiment 2 uses different transmission rates depending on the ONU  20  instead of using different FEC code rates depending on the ONU  20 . Accordingly, the wavelength assignment controller  152  and the ONU wavelength management table  172  have functions different from those of the wavelength assignment controller  151  and the ONU wavelength management table  171  in Embodiment 1. Hereinbelow, the functions different from those in Embodiment 1 are described. 
     The multi-rate optical transceivers  112 - 1  to  112 - 4  receive upstream optical signals having upstream wavelengths λU 1  to λU 4  fed by the WDM coupler  100 . The upstream optical signal to be received is a burst optical signal obtained by time-division multiplexing a 10-Gbps optical signal and a 1-Gbps optical signal. 
     Each multi-rate optical transceiver  112  converts the received upstream optical signal into an electric current signal and further converts the electric current signal into a voltage signal. Then, it splits the obtained voltage signal into a 10-Gbps signal and a 1-Gps signal, amplifies them, and feeds the amplified 10-Gbps electric signal and 1-Gbps electric signal to one of the multi-rate PHY processors  122 - 1  to  122 - 4 . 
     Each multi-rate optical transceiver  112  also converts electric signals at  10  Gbps and 1 Gbps fed by one of the multi-rate PHY processors  122 - 1  to  122 - 4  into optical signals having one of the downstream wavelengths λD 1  to λD 4  and feeds the obtained optical signal to the WDM coupler  100 . 
     Each of the multi-rate PHY processors  122 - 1  to  122 - 4  extracts a clock from the electric signal obtained by time-division multiplexing a 10-Gbps signal and a 1-Gbps signal fed by each of the multi-rate optical transceivers  112 - 1  to  112 - 4  and retimes the electric signal with the extracted clock. Then, each multi-rate PHY processor  122  converts the received electric signal into a digital signal. 
     Furthermore, each multi-rate PHY processor  122  performs decoding on the digital signal, performs FEC decoding on the digital signal as necessary, and further extracts a frame from the digital signal. Then, each multi-rate PHY processor  122  feeds the extracted frame to the multi-rate MAC processor  132 . 
     Each multi-rate PHY processor  122  also encodes a frame fed by the multi-rate MAC processor  132 , performs FEC encoding as necessary, and converts a coded frame into an electric signal waveform based on a clock held by the OLT  10  to generate an electric signal. Each multi-rate PHY processor  122  feeds the generated electric signal to one of the multi-rate optical transceivers  112 - 1  to  112 - 4 . 
     The multi-rate MAC processor  132  analyzes header information in frames received from the multi-rate PHY processors  122 - 1  to  122 - 4  to identify whether each frame is a user data frame or a control frame. If the received frame is a user data frame, the multi-rate MAC processor  132  aggregates user data frames sent from the multi-rate PHY processors  122  to feed the aggregated user data frames to the NNI processor  140 . 
     If the received frame is a control frame, the multi-rate MAC processor  132  further identifies the type of the control frame and feeds it to the wavelength assignment controller  151  or the MPCP controller  180  depending on the type of the control frame, like the MAC processor  131  in Embodiment 1. 
     The multi-rate MAC processor  132  also distributes user data frames received from the NNI processor  140 , wavelength assignment control frames received from the wavelength assignment controller  152 , and MPCP control frames received from the MPCP controller  180  to the multi-rate PHY processors  122  in accordance with the destination address included in the frame. The multi-rate MAC processor  132  multiplexes the user data frames and control frames by destination address and feeds the obtained multiplex frames to the multi-rate PHY processors  122 - 1  to  122 - 4 . 
     The wavelength assignment controller  152  extracts a wavelength to be used in transmission or receiving by an ONU  20  with reference to the ONU wavelength management table  172 . Furthermore, the wavelength assignment controller  152  creates and terminates a wavelength assignment control frame for setting the extracted wavelength to the ONU  20 . The format of the wavelength assignment control frame in Embodiment 2 is the same as the format of the λSET frame or λSET_ACK frame in Embodiment 1. 
     The ONU wavelength management table  172  in Embodiment 2 differs from the ONU wavelength management table  171  in Embodiment 1. The ONU wavelength management table  172  holds relations among a ONU identifier, a transmission rate of the ONU  20 , and an identifier of the transmission and receiving wavelengths. A specific example of the ONU wavelength management table  172  will be described later. 
     The OLT  10  as configured in Embodiment 2 can send and receive optical signals at transmission rates different depending on the ONU  20 , calculate transmission and receiving wavelengths for the individual ONUs  20  based on the transmission rates of the ONUs  20 . The OLT  10  can further send and receive control frames for setting the wavelengths to the ONUs  20 . 
       FIG. 11  is a block diagram illustrating a configuration of an ONU  20  in Embodiment 2. 
     The ONU  20  includes a wavelength-tunable optical transceiver  210 , a PHY processor  220 , a MAC processor  230 , a UNI processor  240 , a wavelength assignment controller  250 , and an MPCP controller  280 . 
     A major difference between the configuration of the ONU  20  in Embodiment 1 and the configuration of the ONU  20  in Embodiment 2 is that the ONU  20  in Embodiment 2 does not have an FEC controller  260 . The functional components in the ONU  20  in Embodiment 2 are the same as those in Embodiment 1, except for the FEC controller  260 . However, depending on the transmission rate of the ONU  20 , the bit rate supported by the wavelength-tunable optical transceiver  210 , the PHY processor  220 , and the MAC processor  230  is different. 
     If the ONU  20  supports a transmission rate of 10 Gbps, the wavelength-tunable optical transceiver  210 , the PHY processor  220 , and the MAC processor  230  all support 10 Gbps. If the ONU  20  supports a transmission rate of 1 Gbps, the wavelength-tunable optical transceiver  210 , the PHY processor  220 , and the MAC processor  230  all support 1 Gbps. 
     Next, the ONU wavelength management table  172  held by the OLT  10  in Embodiment 2 of this invention. 
       FIG. 12  is an explanatory diagram illustrating the ONU wavelength management table  172  in Example 2. 
     The ONU wavelength management table  172  holds relations among an ONU identifier  1721 , a transmission rate  1722 , and a wavelength identifier  1723 . The ONU identifier  1721  and the wavelength identifier  1723  are the same as the ONU identifier  1711  and the wavelength identifier  1713  in Embodiment 1. 
     The transmission rate  1722  is a value measured or detected in communications with the ONU  20  in the discovery process  310 . In this embodiment, the OLT  10  and the ONUs  20  communicate using two kinds of transmission rates: 10 Gbps and 1 Gbps. 
     For example, it is assumed that an ONU  20  having an ONU identifier  1721  of “1” communicates at a transmission rate of 10 Gbps, an upstream wavelength λU 1 , and a downstream wavelength λD 1 . In the row of  FIG. 12  including the ONU identifier  1721  of “1”, the transmission rate  1722  is “10G” and the wavelength identifier  1723  is “1”. 
     It is also assumed that an ONU  20  having an ONU identifier  1721  of “2” communicates at a transmission rate of 1 Gbps, an upstream wavelength of λU 4 , and a downstream wavelength of λD 4 . In the row of  FIG. 12  having the ONU identifier  1721  of “2”, the transmission rate  1722  is “1G” and the wavelength identifier  1723  is “4”. 
     The ONU identifier  1721  in  FIG. 12  is one of the values of 1 to 256; however, it may be the MAC address or the serial number of the ONU  20 , as far as the value can identify the ONU  20 . 
     Next, processing by the wavelength assignment controller  152  in Embodiment 2 of this invention is described. The processing illustrated in  FIG. 5  applies to the processing of the wavelength assignment controller  152  in Embodiment 2, but the transmission rate  1722  can be a plurality of values in calculating average effective bandwidths per ONU  20  at S 303  and assigning wavelengths at S 306 . 
     At S 303 , the wavelength assignment controller  152  calculates average effective bandwidths per ONU  20  Beff(λ) using Formula 1. 
     For example, in the case where the transmission rate between an ONU  20 - 1 - 1  and the OLT  10  is 10 Gbps, the wavelength assignment controller  152  acquires a value 10 Gbps for the effective transmission rate. In the case where the transmission rate between a different ONU  20 - 1 - 2  and the OLT  10  is 1 Gbps, the wavelength assignment controller  152  acquires a value 1 Gbps for the effective transmission rate. 
     If these two ONUs  20  are assigned the same wavelength λ 1 , the wavelength assignment controller  152  calculates an average effective bandwidth per ONU  20  Beff(λ 1 ) at the wavelength λ 1  by the foregoing Formula 1 as follows:
 
Beff(λ1)=1/[1/10 Gbps+1/1 Gbps]=0.909 Gbps
 
     The transmission rates of the ONUs  20  at each wavelength are acquired in the discovery process  310  and the acquired transmission rates are stored in the ONU wavelength management table  172 ; accordingly, the wavelength assignment controller  152  in Embodiment 2 can calculate the average effective bandwidth per ONU  20  Beff(λ) at each wavelength using Formula 1. 
     At S 306 , the wavelength assignment controller  152  calculates the assignment of transmission and receiving wavelengths to the ONUs  20  to eliminate the unfairness in bandwidth among the wavelengths based on the transmission rates of the ONUs  20 . 
     Now, some specific methods of assigning wavelengths at  5306  in Embodiment 2 are provided. 
     The first method of assigning wavelengths in Embodiment 2 is that the wavelength assignment controller  151  calculates an evaluation value (Beff_max/Beff_min) indicating the unfairness for every potential combination of wavelengths which might be assigned to the ONUs  20  based on the transmission rate. The wavelength assignment controller  152  extracts a combination of wavelengths with which the evaluation value (Beff_max/Beff_min) is smallest from all of the potential combinations and determines the extracted combination of wavelengths to be assigned to the ONUs  20 . 
     The first method of assigning wavelengths in Embodiment 2 is the same as the first method of assigning wavelengths in Embodiment 1. However, it is different from the first method in Embodiment 1 in the point that the evaluation value (Beff_max/Beff_min) is obtained based on the transmission rate only. 
     The second method of assigning wavelengths is that the wavelength assignment controller  152  determines the wavelengths to be assigned to the ONUs  20  so that the number of ONUs  20  at each transmission rate will be as fair as possible among the wavelengths. 
     The following description provides an example of a four-wavelength multiplex WDM/TDM-PON system including only two groups of ONUs  20 , one having the transmission rate  1722  of 10 Gbps and the other having the transmission rate  1722  of 1 Gbps. It is assumed that the group of the transmission rate  1722  of 10 Gbps consists of 32 ONUs  20  and the group of the transmission rate  1722  of 1 Gbps consists of 16 ONUs  20 , namely, a total of 48 ONUs  20  are registered in the OLT  10 . 
     In this case, the wavelength assignment controller  152  divides the ONUs  20  having the transmission rate of 10 Gbps into four groups of eight ONUs and assigns the four groups to different wavelengths. The wavelength assignment controller  152  divides the ONUs  20  having the transmission rate of 1 Gbps into four groups of four ONUs and assigns the four groups to different wavelengths. 
     Through these operations, the number of ONUs  20  having the transmission rate  1722  of 10 Gbps are fair among the wavelengths and the number of ONUs  20  having the transmission rate  1722  of 1 Gbps are fair among the wavelengths. As a result, the average effective bandwidths per ONU  20  Beff(λ) will be fair in all the wavelengths; accordingly, the wavelength assignment controller  152  attains the fairness among the wavelengths. 
     The third method of assigning wavelengths in Embodiment 2 is that the wavelength assignment controller  152  assigns an ONU  20  newly registered in the OLT  10  the wavelength at which the average effective bandwidth Beff(λ) has been broadest until the registration of the new ONU  20 . The third method of assigning wavelengths in Embodiment 2 is the same as the third method in Embodiment 1. 
     The wavelength assignment when an ONU  20  is newly registered in Embodiment 2 is described. The basic processing is the same as that in Embodiment 1 shown in  FIG. 6 ; however, processing in the discovery process  310  is partially different between Embodiment 1 and Embodiment 2. 
     The difference at the discovery process  310  is that the FEC code rate  1712  for the newly registered ONU  20  is determined in the discovery process  310  in Embodiment 1 but the transmission rate  1722  of the newly registered ONU  20  is detected in the discovery process  310  in Embodiment 2. 
     The determining the wavelength assignment (step  311 ) after the discovery process  310  in Embodiment 2 and the setting of wavelengths to the ONUs  20  (steps  312  to  315 ) are the same in Embodiment 1 and Embodiment 2; accordingly, explanation thereof is omitted. 
     Through the above-described wavelength assignment, Embodiment 2 can also register a new ONU  20 , determine wavelengths to be assigned to the ONUs  20  so that the bandwidth will be fair among the wavelengths, and set transmission and receiving wavelengths to the ONUs  20 . 
     Effects of Embodiment 2 of this invention are described. In a WDM/TDM-PON system, Embodiment 2 can configure the wavelengths for the ONUs  20  with the wavelength assignment controller  152  so that the differences in average effective bandwidth per ONU  20  among the wavelengths will be minimum based on the transmission rates of the ONUs  20 . This approach can eliminate the unfairness in communication bandwidth among the wavelengths. In turn, the elimination of the unfairness enables the transmission rates for individual ONUs  20  to be set at values that allow transmission with reliability and at high transmission efficiency. Accordingly, Embodiment 2 can improve the throughput in the WDM/TDM-PON system. 
     In other cases than those of Embodiment 1 and Embodiment 2, different effective transmission rates may coexist. For example, if the bandwidth specified by a communication service contract is different among subscribers, the maximum bandwidth for an ONU  20  may be limited in accordance with the contract. As a result, different effective transmission rates coexist in a network system. 
     In such a case, the OLT  10  configures the assignment of transmission and receiving wavelengths for the ONUs  20  based on the bandwidths contracted for the ONUs  20  to likewise reduce the unfairness among the wavelengths. Specifically, the ONU wavelength management table  172  stores a value of the maximum bandwidth in the contracted bandwidth is stored in the transmission rate  1722 , so that the wavelength assignment controller  152  can assign the ONUs  20  wavelengths by the method as described in Embodiment 2. 
     Determining the wavelength to be assigned depending on the contracted bandwidth allows the OLT  1  in this embodiment to calculate an effective transmission rate R(λ,i) based on the bandwidth set by the operator or administrator. 
     The foregoing has been described assuming that four kinds of wavelengths are multiplexed between the OLT  10  and the ONUs  20 ; however, the number of wavelengths to be multiplexed may be any value not less than 2. 
     Embodiment 1 has been described assuming that two kinds of FEC code rate are provided; however, the number of the kinds of FEC code rate may be three or more. 
     Embodiment 2 has been described assuming that two kinds of transmission rate are provided; however, three or more kinds of transmission rate are applicable. 
     Embodiment 1 has described a case including different FEC code rates only and Embodiment 2 has described a case including different transmission rates only; however, this invention is applicable to a case where different FEC code rates and different transmission rates are both included. In such a case, the wavelength assignment controller  151  or  152  calculates the effective transmission rate R(λ,i) in Formula 1 by multiplication of an FEC code rate by a transmission rate. 
     In the discovery process  310  in this case, the OLT  10  (for example, the MPCP controller  180 ) determines the FEC code rate for a newly registered ONU  20  and further detects the transmission rate of the ONU  20 . The wavelength assignment controller  151  or the MPCP controller  180  stores the determined FEC code rate and the detected transmission rate to the ONU wavelength management table  171  or  172 . 
     In the above-described embodiments, the upstream wavelength λU and the downstream wavelength λD are assigned concurrently, but this is not a requirement. For example, in a system allowing different FEC code rates in upstream transmission and downstream transmission, an ONU  20  can be assigned an upstream wavelength λU based on the FEC code rate for upstream communications and a downstream wavelength λD based on the FEC code rate for downstream communications, so that this invention is applicable. 
     The foregoing has been described based on the frames defined by the 10G-EPON; however, the OLT  10  and the ONUs  20  in this invention may use frames defined by other TDM-PON, such as E-PON, G-PON, or XG-PON. 
     The foregoing embodiments have been provided for the WDM/TDM-PON system; however, this invention is equivalently applicable to any wavelength-division multiplexing type of PON system, for example, a WDM/OFDM-PON system. 
     This invention is not limited to the above-described embodiments but includes various modifications. The above-described embodiments are explained in details for better understanding of this invention and are not limited to those including all the configurations described above. A part of the configuration of one embodiment may be replaced with that of another embodiment; the configuration of one embodiment may be incorporated to the configuration of another embodiment. A part of the configuration of each embodiment may be added, deleted, or replaced by that of a different configuration. 
     The above-described configurations, functions, and processors, for all or a part of them, may be implemented by hardware: for example, by designing an integrated circuit. The above-described configurations and functions may be implemented by software, which means that a processor interprets and executes programs providing the functions. The information of programs, tables, and files to implement the functions may be stored in a storage device such as a memory, a hard disk drive, or an SSD (Solid State Drive), or a storage medium such as an IC card, or an SD card. 
     The drawings shows control lines and information lines as considered necessary for explanations but do not show all control lines or information lines in the products. It can be considered that almost of all components are actually interconnected.
An aspect of this invention is a network system including subscriber apparatuses and a station-side apparatus for communicating with the subscriber apparatuses. The station-side apparatus communicates with the subscriber apparatuses using wavelengths. The station-side apparatus determines a wavelength to be used by each of at least one subscriber apparatus of the subscriber apparatuses based on effective transmission rates used by the subscriber apparatuses in communications with the station-side apparatus.