Patent Publication Number: US-2023146851-A1

Title: Communication system and olt system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a communication system and an OLT system. 
     BACKGROUND ART 
     A service that provides optical access can be offered economically by using a passive optical communication network (PON) on which a terminal station device and a plurality of endpoint devices are connected (for example, see NPL 1). A PON is a point-to-multipoint network on which a station provides a service to multiple subscribers. For example, on a PON, a downlink optical signal from a station is split by an optical coupler linked to a single main fiber and distributed to multiple subscribers. An optical network unit (ONU) on the lower side and an optical line terminal (OLT) on the upper side are used on the PON. 
     CITATION LIST 
     Non Patent Literature 
     
         
         NPL 1: “Basic Technical Course GE-OPON Technology”, [online], NTT Technical Journal, August 2005, p. 71 to 74, [retrieved on Feb. 28, 2020], Internet URL: https://www.ntt.cojp/journal/0508/files/jn200508071.pdf. 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, PONs of the related art has difficulty to maintain communication when a failure such as a facility failure occurs. 
     In view of the above circumstance, it is an object of the present disclosure to provide a technology that enables to increase the likelihood of maintaining communication even when a failure occurs in a PON. 
     Means for Solving the Problem 
     An aspect of the present disclosure is a communication system of a passive optical communication network. The communication system includes an optical line terminal (OLT) system including a first OLT, a second OLT, and an OLT control device that controls the first OLT and the second OLT, a plurality of splitters that connect the first OLT and the second OLT with an optical communication path, and an optical network unit (ONU) that is connected to each of the splitters with an optical communication path. The splitter distributes and outputs an optical signal transmitted from the OLT system to the ONU connected to the splitter and a succeeding device that is another splitter or the OLT system, and the OLT control device determines a distribution ratio at the splitter, the distribution ratio indicating a ratio between an intensity of the optical signal distributed to the succeeding device and an intensity of the optical signal distributed to the ONU. 
     An aspect of the present disclosure is an optical line terminal (OLT) system of a passive optical communication network. The OLT system includes a first OLT, a second OLT, and an OLT control device that controls the first OLT and the second OLT. The first OLT and the second OLT are connected with a plurality of splitters and an optical communication path, each of the splitters distributes and outputs an optical signal transmitted from the first OLT or the second OLT to the ONU connected to the splitter and a succeeding device, and the OLT control device determines a distribution ratio at the splitter, the distribution ratio indicating a ratio between an intensity of the optical signal distributed to the succeeding device and an intensity of the optical signal distributed to the ONU. 
     Effects of the Invention 
     The present disclosure makes it possible to increase the likelihood of maintaining communication even when a failure occurs in a PON. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating an exemplary system configuration of a communication system  100  according to the present disclosure. 
         FIG.  2    is a diagram illustrating an exemplary configuration of an OLT system  20 . 
         FIG.  3    is a diagram illustrating an exemplary configuration of a splitter  30  and an ONU  40 . 
         FIG.  4    is a diagram showing a first specific example in a distribution ratio table. 
         FIG.  5    is a diagram showing a second specific example in the distribution ratio table. 
         FIG.  6    is a diagram illustrating a specific exemplary operation of the communication system  100  during normal communication in a first operation example. 
         FIG.  7    is a diagram illustrating a specific exemplary operation of the communication system  100  during failed communication in the first operation example. 
         FIG.  8    is a diagram illustrating a specific exemplary operation of the communication system  100  during normal communication in a second operation example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described in detail with reference to the drawings. 
       FIG.  1    is a diagram illustrating an exemplary system configuration of a communication system  100  according to the present disclosure. The communication system  100  includes an upper device  10 , an OLT system  20 , a plurality of splitters  30 , a plurality of ONUs  40 , a lower device  50 , and a user device  60 . The upper device  10  is connected to an upper network of the communication system  100 . Although three splitters  30  and three ONUs  40  are provided in  FIG.  1   , this number “three” is merely a specific example. That is, at least two splitters  30  and at least two ONUs  40  are required to be provided. 
     The OLT system  20  is provided so as to communicate with the upper device  10 . The OLT system  20  functions as an OLT on a PON. The splitters  30  receive an input of optical signals and output an optical signal to a plurality of paths. The splitters  30  may be formed with, for example, two-input two-output optical splitters. 
     Each of the ONUs  40  is communicatively connected to the lower device  50 . Each of the lower devices  50  is communicatively connected to one or a plurality of user devices  60 . Each of the devices will be described in detail below. However, the upper device  10 , the lower devices  50 , and the user devices  60  will be described prior to describing the OLT system  20 , the splitters  30 , and the ONUs  40  for the sake of convenience in explanation. 
     The upper device  10  is communicatively connected to a plurality of lower devices  50  via the OLT system  20 , the splitters  30 , and the ONUs  40 . The upper device  10  is a device that realizes a predetermined function by communicating with the plurality of lower devices  50 . The upper device  10  is, for example, a base station apparatus (a base band unit or BBU) on a mobile network. The upper device  10  may be, for example, communication equipment constituting a relay network. 
     The lower devices  50  are devices that realize predetermined functions by communicating with the upper device  10 . The lower devices  50  are devices that are provided at positions closer to the users than to the upper device  10 . In a case in which the upper device  10  is a BBU, for example, the lower devices  50  are radio devices (remote radio heads:RRHs) on the mobile network. In this case, the communication path between the lower device  50  and the user device  60  is an access section on the mobile network. On the other hand, in a case in which the upper device  10  is communication equipment constituting a relay network, the lower devices  50  may be devices such as a set-top box. In this case, the communication path between the lower device  50  and the user device  60  may be a network such as a home network. Each lower device  50  accommodates one or a plurality of user devices  60 , for example. Further, each user device  60  may be directly connected to the ONU  40  without the lower device  50 . 
     The user devices  60  are devices connected to the lower devices  50  via communication paths to be communicatively connected to other devices. Each of the user devices  60  is an information processing device, for example, a smartphone, a tablet, or a personal computer. The user device  60  may be, for example, a sensor for the Internet of Things (IoT). The user device  60  may be a device for business applications, for example, an automatic teller machine (ATM), a vending machine, or a point-of-sale (POS) terminal. 
     Next, the OLT system  20  will be described.  FIG.  2    is a diagram illustrating an exemplary configuration of the OLT system  20 . The OLT system  20  includes a first OLT  21 , a second OLT  22 , and an OLT control device  23 . The first OLT  21  and the second OLT  22  are devices that each provide the OLT functions on the PON of the related art. That is, the OLT system  20  has at least two devices that provide the OLT functions. The first OLT  21  and the second OLT  22  may each be formed as a separate device having an individual housing or may be formed as a board or a chip. In any case, the first OLT  21  and the second OLT  22  are formed as software. Because the basic configurations of the first OLT  21  and the second OLT  22  are the same, the first OLT  21  will be described below as a representative. 
     The first OLT  21  includes a transmission unit  211 , a reception unit  212 , a light meter  213 , a signal processing unit  214 , and a control unit  215 . The transmission unit  211  outputs an optical signal generated by the signal processing unit  214  to a communication path. The transmission unit  211  transmits an optical signal to the ONUs  40  via the splitters  30 . Optical signals addressed to the plurality of ONUs  40  may be superimposed on the optical signal transmitted by the transmission unit  211 . 
     The reception unit  212  receives an optical signal from a communication path and outputs the received optical signal to the light meter  213 . The reception unit  212  receives optical signals from the ONUs  40  via the splitters  30 . Optical signals transmitted from the plurality of ONUs  40  may be superimposed on the optical signals received by the reception unit  212 . 
     The light meter  213  outputs information regarding the light intensity of the optical signal received by the reception unit  212 . The light meter  213  may output a value representing, for example, the light intensity of the optical signal. The light meter  213  may output a binary value indicating whether the light intensity of the optical signal exceeds a predetermined threshold, for example. The light meter  213  may output any information as long as the information enables the control unit  215  to determine whether the reception unit  212  has received optical signals normally. 
     The signal processing unit  214  functions as an OLT of the related art. Exemplary processing of the signal processing unit  214  will be described below. The signal processing unit  214  converts an electrical signal transmitted from the upper device  10  to the lower devices  50  into an optical signal and outputs the converted signal to the transmission unit  211 . The signal processing unit  214  may superimpose (multiplex) optical signals addressed to the plurality of lower devices  50 . The signal processing unit  214  converts the optical signal received by the reception unit  212  into an electrical signal and outputs the electrical signal to the upper device  10  to be transmitted via the OLT control device  23 . 
     The control unit  215  controls operations of the first OLT  21  following instructions from the OLT control device  23 . When there is an instruction from a control unit  233  to perform an operation in a communication state, the control unit  215  controls the first OLT  21  such that the first OLT functions as an OLT on a normal PON. In addition, the control unit  215  outputs an output from the light meter  213  to the control unit  233 . When the control unit  215  is instructed by the control unit  233  to operate in a standby state, the control unit  215  controls each of the functional units such that the first OLT  21  does not function as an OLT on the normal PON. In this case, the control unit  215  transmits the output from the light meter  213  to the control unit  233 . 
     Some or all of operations of the control unit  215  may be implemented by software executed by, for example, a processor such as a CPU and a memory. Some or all of the control unit  215  may be implemented by hardware including an electronic circuit (or circuitry) in which, for example, a large scale integration circuit (LSI), an application specific integrated circuit (ASIC), a programmable logic device (PLD), a field programmable gate array (FPGA), or the like is used. 
     The transmission unit  211 , the reception unit  212 , the light meter  213 , the signal processing unit  214 , and the control unit  215  of the first OLT  21  each correspond to a transmission unit  221 , a reception unit  222 , a light meter  223 , a signal processing unit  224 , and a control unit  225  of the second OLT  22 . 
     The OLT control device  23  includes a signal splitting unit  231 , a signal concentration unit  232 , the control unit  233 , a storage unit  234  and a splitter control unit  235 . The signal splitting unit  231  outputs a downlink signal output from the upper device  10  (a signal to be transmitted to the ONUs  40 ) to at least one of the first OLT  21  or the second OLT  22  following control of the control unit  233 . 
     The signal concentration unit  232  outputs an uplink signal output from either the first OLT  21  or the second OLT  22  (a signal received from the ONUs  40 ) to the upper device  10 . 
     The control unit  233  manages information indicating the OLT assigned to each ONU  40 . The control unit  233  controls operations of the signal splitting unit  231  and operations of the first OLT  21  and the second OLT  22  based on a predetermined criterion. The predetermined criterion is a criterion for maintaining communication even if an equipment failure occurs in the communication system  100 . Examples of operations of the control unit  233  include a first operation example and a second operation example. 
     In the first operation example, the control unit  233  controls each device differently in a normal communication state and in a failed communication state. The normal communication state is a state in which no particular communication problem has been detected in the communication system  100 . In this case, the control unit  233  exerts control such that one of the first OLT  21  and the second OLT  22  is in a communication state and the other is in a standby state. In the normal communication state, the control unit  233  controls the signal splitting unit  231  so as to transmit an uplink signal output from an OLT in the normal state to the upper device  10 . In addition, in the normal communication state, the control unit  233  controls the signal splitting unit  231  so as to output a signal transmitted from the upper device  10  to the OLT in the normal state. 
     The control unit  233  receives information regarding an output of a light meter from the OLT in the standby state to detect a communication problem in the communication system  100 . For example, it is determined that a communication problem has occurred when the light intensity of an optical signal received by the OLT in the standby state is lower than a predetermined threshold. In this case, the control unit  233  operates in the failed communication state. In other words, the control unit  233  shifts the OLT, which has been in the standby state, to a communication state. Thereafter, both the first OLT  21  and the second OLT  22  of the OLT system  20  function as OLTs. During the failed communication, the signal processing unit  214  transfers uplink signals output from the OLTs (both OLTs) in the normal state to the signal concentration unit  232  to transmit to the upper device  10 . In addition, during the failed communication, the control unit  233  controls the signal splitting unit  231  such that a signal transmitted from the upper device  10  is output to the OLTs (both OLTs) in the normal state. 
     In the second operation example, the control unit  233  controls both the first OLT  21  and the second OLT  22  in the communication state, regardless of whether the communication system  100  has a failure. For this reason, the signal processing unit  214  transfers uplink signals output from the OLTs (both OLTs) in the normal state to the signal concentration unit  232  to transmit to the upper device  10 . In addition, the control unit  233  controls the signal splitting unit  231  to output a signal transmitted from the upper device  10  to the OLTs (both OLTs) in the normal state. The storage unit  234  and the splitter control unit  235  will be described after the description of the splitter  30  and the ONU  40  for convenience of description. 
       FIG.  3    is a diagram illustrating an exemplary configuration of the splitter  30  and the ONU  40 . The splitter  30  is configured with a two-input two-output splitter for optical signals. The splitter  30  includes a distribution ratio control unit  31 . The splitter  30  distributes and outputs optical signals input from a communication path connecting the OLT system  20  formed in a ring shape and each splitter  30  (hereinafter referred to as a “main communication path”) to a succeeding device and the ONU  40  connected to the splitter  30 . A distribution ratio at this time is controlled by the distribution ratio control unit  31 . The distribution ratio control unit  31  controls the distribution ratio in accordance with an instruction output from the splitter control unit  235  of the OLT control device  23 . For example, when the splitter control unit  235  outputs an instruction for a distribution ratio “31:1”, the ratio of the intensity of the optical signal output to the succeeding device to the optical signal output to the ONU  40  is controlled to be 31:1. 
     The succeeding device is a device different from a device from which the optical signal input to a reference device is output, out of two devices connected to the reference device via the main communication path. For example, if a splitter  30 - 1  is set as a reference splitter in  FIG.  1   , the succeeding device is a splitter  30 - 2  when an optical signal input from the OLT system  20  is distributed. For example, if a splitter  30 - 2  is set as a reference splitter in  FIG.  1   , the succeeding device is a splitter  30 - 3  when an optical signal input from the splitter  30 - 1  is distributed. For example, if a splitter  30 - 2  is set as a reference splitter in  FIG.  1   , the succeeding device is a splitter  30 - 1  when an optical signal input from the splitter  30 - 3  is distributed. Note that, the succeeding device is a device from which an optical signal input to the reference device is output, out of two devices connected to the reference device via the main communication path. 
     In addition, the splitter  30  outputs an optical signal to be output to the ONU  40  that is connected to the splitter  30  to the ONU  40  on a different communication path according to an input port. For example, the splitter  30  outputs the optical signal coming from the upper left of  FIG.  3    to the upper right and lower right. Thus, taking the splitter  30 - 1  of  FIG.  1    as an example, an optical signal coming from the first OLT  21  is output to a path  91 - 2  and the splitter  30 - 2 . The optical signal output to the path  91 - 2  is input to the ONU  40 - 1 . In addition, for example, the splitter  30  outputs an optical signal coming from the upper right of  FIG.  3    to the upper left and lower left. Thus, taking the splitter  30 - 1  of  FIG.  1    as an example, an optical signal coming from the splitter  30 - 2  is output to a path  91 - 1  and the first OLT  21 . The optical signal output to the path  91 - 1  is input to the ONU  40 - 1 . Further, the path  91 - 1  and the path  91 - 2  are each connected to different light meters. For example, the path  91 - 1  is connected to a first light meter  41  and the path  91 - 2  is connected to a second light meter  42 . 
     The ONU  40  includes the first light meter  41 , the second light meter  42 , an optical switch  43 , a control unit  44 , a signal processing unit  45 , and a communication unit  46 . The first light meter  41  receives an optical signal output from the splitter  30 . The first light meter  41  outputs information indicating the light intensity of the received optical signal to the control unit  44 . The first light meter  41  outputs the received optical signal to the optical switch  43 . The second light meter  42  receives an optical signal output from the splitter  30 . The second light meter  42  outputs information indicating the light intensity of the received optical signal to the control unit  44 . The second light meter  42  outputs the received optical signal to the optical switch  43 . 
     The optical switch  43  outputs either an optical signal output from the first light meter  41  or an optical signal output from the second light meter  42  to the signal processing unit  45  according to control of the control unit  44 . The optical switch  43  outputs an optical signal output from the signal processing unit  45  to the splitter  30  via either the first light meter  41  or the second light meter  42  according to control of the control unit  44 . 
     The control unit  44  receives information regarding light intensity from each of the first light meter  41  and the second light meter  42 , and selects the optical signal of the information from either the first light meter  41  or the second light meter  42  according to a predetermined criterion. The control unit  44  controls the optical switch  43  so that the selected optical signal is output to the signal processing unit  45 . Highly reliable optical signal is selected in accordance with the predetermined criterion. The level of reliability may be determined based on the light intensity of the optical signal. For example, the control unit  44  may select an optical signal having a higher light intensity. 
     The control unit  44  determines the OLT to which the ONU  40  is assigned. The control unit  44  may determine, for example, the OLT (the first OLT  21  or the second OLT  22 ) to which the ONU  40  is assigned according to the selected light meter (the first light meter  41  or the second light meter  42 ). For example, when the first light meter  41  is selected, it may be determined that the ONU  40  is assigned to the first OLT  21 . When the received optical signal contains transmission source information (information indicating the OLT serving as the transmission source), the control unit  44  may determine the OLT to which the ONU  40  is assigned in accordance with the transmission source information. The control unit  44  notifies the OLT system  20  of the information indicating the OLT to which the ONU  40  is assigned. Some or all of the operations of the control unit  44  may be implemented by hardware including an electronic circuit using an LSI, an ASIC, a PLD, or an FPGA, for example. 
     The signal processing unit  45  functions as an ONU on the PON of the related art. Hereinafter, exemplary processing of the signal processing unit  45  will be described. The signal processing unit  45  converts an optical signal indicating a signal transmitted from the upper device  10  to the lower device  50  into an electrical signal and outputs the converted signal to the communication unit  46 . At this time, if optical signals addressed to a plurality of lower devices  50  are superimposed (multiplexed), an optical signal addressed to the lower device  50  connected to the ONU  40  is taken out from the optical signals and then converted into an electrical signal. The signal processing unit  45  converts an electrical signal received by the communication unit  46  into an optical signal and outputs the converted signal to the optical switch  43 . 
     The communication unit  46  is a communication interface that enables communication with the lower device  50 . 
     Next, the storage unit  234  and the splitter control unit  235  will be described with reference to  FIG.  2   . The storage unit  234  is provided with a storage device such as a magnetic hard disk device or a semiconductor storage device. The storage unit  234  stores data to be used by the control unit  235  to execute control processing. For example, the storage unit  234  stores a distribution ratio table for each splitter  30  to be controlled. 
       FIGS.  4  and  5    are diagrams illustrating exemplary specific distribution ratio tables. The distribution ratio tables include assigned OLTs and values of distribution ratios. Each assigned OLT denotes an OLT (the first OLT  21  or the second OLT  22 ) that is assigned to the ONU  40  connected to the splitter  30  to be controlled. The distribution ratio indicates a distribution ratio used when the associated assigned OLT is assigned to the ONU  40 . The distribution ratio indicates a ratio between the intensity of an optical signal to be distributed to the succeeding device and the intensity of an optical signal to be distributed to the ONU  40  at the splitter  30 . For example, in a case in which the distribution ratio is 14:18, the ratio of the intensity of an optical signal output to the succeeding device to the intensity of an optical signal output to the ONU  40  is controlled to be 14:18. 
     In the following description, the ONU  40  connected to the splitter  30  for which the distribution ratio table is used is referred to as a target ONU. Contents of the distribution ratio table may be determined according to the distance between the OLT corresponding to the assigned OLT and the target ONU. The distance to the assigned OLT may be represented, for example, using a physical distance value, the number of hops (the number of splitters  30 ), or other values. 
     For example, as the distance between the assigned OLT and the target ONU increases, in the distribution ratio, the ratio of the intensity of the optical signals distributed to the succeeding device decreases, and as the distance between the assigned OLT and the target ONU decreases, in the distribution ratio, the ratio of the intensity of the optical signal distributed to the succeeding device increases. As a result of this configuration, the splitter  30  where the distance between the assigned OLT and the target ONU is short distributes a larger amount of optical signals to the succeeding device. A plurality of splitters  30  and ONUs  40  are connected to a subsequent stage of the splitter  30  where the distance between the assigned OLT and the target ONU is short, and thus the distance to each splitter  30  and ONU  40  is long. Because a larger amount of optical signals is distributed to the succeeding devices, it is possible to make the optical signals reliably reach these devices with high accuracy. An optical signal with a strong light intensity reaches the splitter  30  where the distance between the assigned OLT and the target ONU is short. As a result, even if the distribution ratio with respect to the ONU  40  is low, it is possible to bring an optical signal with a sufficient amount of light to the target ONU. 
     More specifically, a PON system that can split into 32 will be described as an example. Further, the attenuation of an optical signal according to a distance is not considered for the sake of simplified description. The distribution ratio 31:1 is set for a splitter  30  (a first splitter  30 ) that is the first splitter from an OLT. The distribution ratio 30:1 is set for the next splitter  30  (second splitter  30 ). The distribution ratio 29:1 is set for the next splitter  30  (third splitter  30 ). Then, the distribution ratio 1:1 is set for the 31st splitter  30 . In other words, in this specific example, a distribution ratio (32-n): 1  (n is an integer equal to or greater than 1) is set for the splitter  30  that is in the n-th order from the OLT. At the time of failure in the first operation example, the 31st splitter  30  becomes the first splitter  30  when the OLT assigned to the splitter is changed. Thus, the distribution ratio of the splitter  30  is changed from 1:1 to 31:1. 
     As illustrated in  FIG.  4   , in the distribution ratio table, the distribution ratio of the first OLT and the distribution ratio of the second OLT may be a ratio in which the value of the previous term and the value of the latter term are switched. As illustrated in  FIG.  5   , in the distribution ratio table, the distribution ratio of the first OLT and the distribution ratio of the second OLT may not be a ratio in which the value of the previous term and the value of the latter term are switched. 
       FIG.  6    is a diagram illustrating a specific exemplary operation of the communication system  100  during normal communication in the first operation example. The arrows shown in  FIG.  6    represent the flow of a downlink signal (a signal flowing from the upper device  10  to the lower device  50 ). In the example of  FIG.  6   , the first OLT  21  is controlled to be in a communication state, and the second OLT  22  is controlled to be in a standby state. With the control, the downlink signal is transmitted in a counterclockwise direction in the communication system  100  in  FIG.  6   . The control is performed from when the communication system  100  is activated or when the OLT system  20  is activated. When receiving an instruction to operate in a normal state, the first OLT  21  activates equipment needed to function as an OLT, such as the signal processing unit  214 , and operates as an OLT on the normal PON. The first OLT  21  is assigned to each ONU  40 . Thus, the splitter control unit  235  controls the distribution ratio of each splitter as the distribution ratio when the assigned OLT is the first OLT  21 . 
     In  FIG.  6   , the communication paths indicated by the dashed lines of the two communication paths extending from the splitters  30  to the ONUs  40  indicate communication paths through which no optical signals transmitted from the OLT system  20  passes. The solid-line arrows indicate communication paths through which optical signals transmitted from the OLT system  20  pass. 
     In the example of  FIG.  6   , no particular failure has occurred in the communication system  100 . Thus, the second OLT  22  receives an optical signal output from the splitter  30 - 3 . As a result, the control unit  233  of the OLT control device  23  of the OLT system  20  determines that no failure has occurred. 
       FIG.  7    is a diagram illustrating a specific exemplary operation of the communication system  100  during failed communication in the first operation example. The arrows shown in  FIG.  7    represent the flow of a downlink signal (a signal flowing from the upper device  10  to the lower device  50 ). In the example of  FIG.  7   , the communication path between the splitter  30 - 2  and the splitter  30 - 3  has a failure. For this reason, the downlink signal output from the splitter  30 - 2  fails to reach the splitter  30 - 3 . The second OLT  22  does not receive the optical signal output from the splitter  30 - 3 . As a result, the control unit  233  of the OLT control device  23  of the OLT system  20  determines that a failure has occurred. 
     The control unit  233  performs control such that the second OLT  22  transitions to the normal state based on the determination. As a result, the second OLT  22  also outputs an optical signal equivalent to the optical signal output by the first OLT  21  to the communication path. Although the splitter  30 - 3  is not able to receive the optical signal output from the first OLT  21  due to the occurrence of the failure, the splitter can receive the optical signal output from the second OLT  22 . The ONU  40 - 3  receives the optical signal transmitted from the second OLT  22  via the splitter  30 - 3 . As a result, the ONU  40 - 3  changes the assigned OLT from the first OLT  21  to the second OLT  22 . The splitter control unit  235  performs control such that the distribution ratio of the splitter  30 - 3  is changed to the distribution ratio when the assigned OLT is the second OLT  22 . 
     With the operation described above, all of the ONUs (the ONU  40 - 1 , the ONU  40 - 2 , and the ONU  40 - 3 ) can receive the downlink signal from the OLT system  20  and maintain communication regardless of the occurrence of a failure. 
     Further, although the flow of the downlink signal in the first operation example has been described, an uplink signal is transmitted from each ONU  40  to the OLT system  20  when the signal flows in the opposite direction of the arrows through the paths indicated by the solid lines in each drawing. 
       FIG.  8    is a diagram illustrating a specific exemplary operation of the communication system  100  during normal communication in the second operation example. The arrows shown in  6  represent the flow of a downlink signal (a signal flowing from the upper device  10  to the lower device  50 ). In the example of  FIG.  8   , both the first OLT  21  and the second OLT are controlled to be in a communication state. 
     With the control, the downlink signal is transmitted on both paths in the clockwise and counterclockwise directions in the communication system  100  in  FIG.  8   . The control is performed from when the communication system  100  is activated or when the OLT system  20  is activated. When receiving an instruction to operate in a normal state, the first OLT  21  and the second OLT  22  activate equipment needed to function as OLTs, such as the signal processing unit  214  and the signal processing unit  224 , and operates as OLTs on the normal PON. Either the first OLT  21  or the second OLT  22  is assigned to each ONU  40 . Each ONU  40  notifies the OLT system  20  of information indicating the OLT assigned to the ONU  40 . The splitter control unit  235  exerts control such that the distribution ratio of each splitter  30  is changed to the distribution ratio in accordance with the assigned OLT of the ONU  40  connected to the splitter. 
     In the example of  FIG.  8   , optical signals are transmitted from both the first OLT  21  and the second OLT  22 , and any one of the optical signals is selected and used for processing in accordance with a predetermined criterion (for example, the signal with a higher light intensity) at each ONU  40 . Thus, even if a failure occurs in the middle of a path as illustrated in  FIG.  7   , for example, all of the ONUs (ONU  40 - 1 , ONU  40 - 2 , and ONU  40 - 3 ) can receive the downlink signal from the OLT system  20  and maintain communication without particularly controlling the OLT system  20  for the failure. 
     Further, although the flow of the downlink signals is described in the second operation example, uplink signals are also transmitted to the OLT system  20  on the same path. Further, in  FIG.  8   , although the arrows between the splitters  30  and the ONUs  40  are directed in one direction, this arrow indicates the direction of the downlink signal. In the case of the uplink signal, the signal is transmitted using a path selected by the optical switch  43 . 
     In the communication system  100  configured as described above, the OLT system  20  has functions of a plurality (for example, two) of OLTs. Even if a failure occurs in a communication path or the like, the likelihood that each ONU  40  receives an optical signal from at least one OLT increases. For this reason, the likelihood that communication can be maintained even when a failure occurs in the PON can increase. 
     In addition, in the communication system  100  configured as described above, the distribution ratio of each splitter  30  is controlled according to the assigned OLT of the ONU  40  connected to the splitter  30 . As a result, it is possible to reliably and accurately reach optical signals to ONUs  40 . 
     MODIFIED EXAMPLE 
     In the OLT system  20 , the OLT control device may be configured as a functional unit in a device of either or both of the first OLT  21  and the second OLT  22 . 
     Although the embodiments of the present invention have been described in detail with reference to the drawings, a specific configuration is not limited to the embodiments, and a design or the like in a range that does not depart from the gist of the present invention is included. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure can be applied to a communication system using a PON. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100  Communication system 
               10  Upper device 
               20  OLT system 
               21  First OLT 
               22  Second OLT 
               23  OLT control device 
               30  Splitter 
               40  ONU 
               50  Lower device 
               60  User device 
               211  Transmission unit 
               212  Reception unit 
               213  Light meter 
               214  Signal processing unit 
               215  Control unit 
               221  Transmission unit 
               222  Reception unit 
               223  Light meter 
               224  Signal processing unit 
               225  Control unit 
               231  Signal splitting unit 
               232  Signal concentration unit 
               233  Control unit 
               234  Storage unit 
               235  Splitter control unit 
               41  First light meter 
               42  Second light meter 
               43  Optical switch 
               44  Control unit 
               45  Signal processing unit 
               46  Communication unit