Patent Publication Number: US-2023155674-A1

Title: Optical communication system and master station

Description:
TECHNICAL FIELD 
     The present invention relates to an optical communication system and a master station device in the optical communication system. 
     BACKGROUND ART 
     A PON (Passive Optical Network) system is known as an optical communication system. In recent years, a TWDM-PON (Time and Wavelength Division Multiplexing—PON) system that uses WDM (Wavelength Division Multiplexing) technology has also been proposed. 
     NPL 1 discloses a bus-topology WDM Access System as an example of a TWDM-PON system. The network topology of an OLT (Optical Line Termination, or Optical Line Terminal) and ONUs (Optical Network Units) is a bus topology. Such a bus-topology WDM access system is applied to a MFH (Mobile Fronthaul) when deploying a mobile communication area linearly, for example. 
     NPL 2 discloses a protection method for improving reliability in a bus-topology WDM access system. In the case of a bus topology, if the trunk fiber becomes disconnected, communication may be cut off for many ONUs. In view of this, a loop-shaped communication path is formed by combining the main trunk fiber with a backup trunk fiber. In a normal situation where no disconnection has occurred, the backup path is cut off by an optical switch, and communication between the OLT and ONUs is performed only via the normal path. If a disconnection occurs in a portion of the normal path, the backup path is enabled in addition to the normal path. The OLT then uses the backup path to perform communication with disconnected ONUs that cannot perform communication via the normal path. This makes it possible to resume communication with all disconnected ONUs without waiting for repair of the optical fiber that constitutes the normal path. 
     CITATION LIST 
     Non Patent Literature 
     [NPL 1] Harada et al., “Cyclic Wavelength Allocation Scheme Reducing the Number of Wavelengths in Bus-Topology WDM Access Systems,” IEICE (Institute of Electronics, Information and Communication Engineers) 2020, B-8-12, p. 159, March 2020. 
     [NPL 2] Ujikawa et al., “Protection Architecture for Reliable Bus-topology WDM Access Systems,” IEICE (Institute of Electronics, Information and Communication Engineers) 2020, B-8-14, p. 161, March 2020. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In an optical communication system that includes a master station device and multiple slave station devices, the master station device executes communication control processing for controlling communication with each of the slave station devices. Consider the case where this communication control processing is executed based on the “RTT” (Round Trip Time) between the master station device and each of the slave station devices. In the case of executing communication control processing based on RTT, the master station device needs to be aware of the RTT for all of the slave station devices. 
     For example, in a PON system, uplink communication from multiple slave station devices (ONU) to a master station device (OLT) is performed using TDMA (Time Division Multiple Access). In order to prevent uplink optical signals transmitted from the ONUS from colliding with each other, the OLT controls the transmission timing of the uplink optical signal from each ONU based on the corresponding RTT for that ONU. To achieve this, the OLT needs to be aware of the RTT for all of the communication partner ONUS in advance. Accordingly, when the OLT executes registration processing (discovery) for registering an ONU, the OLT measures the RTT for the ONU and holds the RTT together with other registration information. 
     Here, consider a protection method that uses a backup path as disclosed in NPL 2 described above. As described above, the OLT uses the backup path instead of the normal path to perform communication with disconnected ONUS that cannot perform communication via the normal path. In order to achieve this, it is necessary to execute the above-mentioned registration processing for all of the disconnected ONUs and measure the new RTT in the case of performing communication via the backup path. However, performing registration processing for all of the disconnected ONUs and measuring the new RTTs takes a long time. In other words, the amount of time required to resume communication increases. 
     One object of the present invention is to provide a technique for making it possible to shorten the time required for resuming communication in a protection method that uses a backup path in an optical communication system that includes a master station device and multiple slave station devices. 
     Solution to Problem 
     A first aspect of the present invention relates to an optical communication system.
         The optical communication system includes:   a plurality of slave station devices that are connected to a loop path in parallel; and   a master station device that is connected to the loop path and configured to perform communication with each of the slave station devices.       

     Communication paths between the master station device and each of the slave station devices include:
         a normal path that extends along the loop path from the master station device to each of the slave station devices in a first direction, and   a backup path that extends along the loop path from the master station device to each of the slave station devices in a second direction that is opposite to the first direction.       

     The master station device is configured to execute communication control processing for controlling communication with each of the slave station devices based on an RTT (Round Trip Time) between the master station device and the slave station device. 
     A normal path RTT is the RTT in a case of communication performed via the normal path. 
     A backup path RTT is the RTT in a case of communication performed via the backup path. 
     A first slave station device is one of the slave station devices. 
     A first normal path RTT is the normal path RTT between the master station device and the first slave station device. 
     A first backup path RTT is the backup path RTT between the master station device and the first slave station device. 
     A second slave station device is a slave station device that cannot perform communication via the normal path, among the plurality of slave station devices. 
     A second normal path RTT is the normal path RTT between the master station device and the second slave station device. 
     A second backup path RTT is the backup path RTT between the master station device and the second slave station device. 
     The master station device holds the normal path RTT for each of the slave station devices and the first backup path RTT. 
     If the second slave station device does not exist, the master station device executes the communication control processing with respect to the slave station devices based on the normal path RTTs. 
     If the second slave station device is detected, the master station device calculates the second backup path RTT based on the first normal path RTT, the first backup path RTT, and the second normal path RTT, and resumes the communication control processing with respect to the second slave station device based on the calculated second backup path RTT. 
     A second aspect of the present invention relates to a master station device that performs communication with each of a plurality of slave station devices in an optical communication system. 
     The slave station devices are connected to a loop path in parallel. 
     Communication paths between the master station device and each of the slave station devices include:
         a normal path that extends along the loop path from the master station device to each of the slave station devices in a first direction, and   a backup path that extends along the loop path from the master station device to each of the slave station devices in a second direction that is opposite to the first direction.       

     The master station device is configured to execute communication control processing for controlling communication with each of the slave station devices based on an RTT (Round Trip Time) between the master station device and the slave station device. 
     A normal path RTT is the RTT in a case of communication performed via the normal path. 
     A backup path RTT is the RTT in a case of communication performed via the backup path. 
     A first slave station device is one of the slave station devices. 
     A first normal path RTT is the normal path RTT between the master station device and the first slave station device. 
     A first backup path RTT is the backup path RTT between the master station device and the first slave station device. 
     A second slave station device is a slave station device that cannot perform communication via the normal path, among the plurality of slave station devices. 
     A second normal path RTT is the normal path RTT between the master station device and the second slave station device. 
     A second backup path RTT is the backup path RTT between the master station device and the second slave station device. 
     The master station device holds the normal path RTT for each of the slave station devices and the first backup path RTT. 
     If the second slave station device does not exist, the master station device executes the communication control processing with respect to the slave station devices based on the normal path RTTs. 
     If the second slave station device is detected, the master station device calculates the second backup path RTT based on the first normal path RTT, the first backup path RTT, and the second normal path RTT, and resumes the communication control processing with respect to the second slave station device based on the calculated second backup path RTT. 
     Effects of the Invention 
     There are two types of communication paths between the master station device and each of the slave station devices, namely a normal path and a backup path. Accordingly, if there are second slave station devices that cannot perform communication via the normal path, communication can be resumed via the backup path. However, in that case, a second backup path RTT is needed for all of the second slave station devices. According to the present invention, the master station device holds a normal path RTT for each of the slave station devices and a first backup path RTT for the first slave station device. If a second slave station device is detected, the master station device calculates the second backup path RTT for each second slave station device based on the normal path RTT and the first backup path RTT for the first slave station device. The master station device then resumes communication control processing for each second slave station device based on the calculated second backup path RTT. It is not necessary to execute registration processing and directly measure the second backup path RTT for all of the second slave station devices. Accordingly, the time required to resume communication is shortened. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a conceptual diagram schematically showing a configuration of an optical communication system according to an embodiment of the present invention. 
         FIG.  2    is a conceptual diagram for describing normal operation of the optical communication system according to the embodiment of the present invention. 
         FIG.  3    is a conceptual diagram for describing a situation in which a fiber has become disconnected in the optical communication system according to the embodiment of the present invention. 
         FIG.  4    is a conceptual diagram for describing operations performed using a normal path and a backup path in the optical communication system according to the embodiment of the present invention. 
         FIG.  5    is a conceptual diagram for describing a method of acquiring a backup path RTT for a disconnected ONU according to an embodiment of the present invention. 
         FIG.  6    is a conceptual diagram showing an example of the configuration of an optical communication system according to an embodiment of the present invention. 
         FIG.  7    is a conceptual diagram showing an example of the configuration of the master station device (OLT) in the optical communication system according to the embodiment of the present invention. 
         FIG.  8    is a flowchart showing a first processing example performed in the optical communication system according to the embodiment of the present invention. 
         FIG.  9    is a flowchart showing a second processing example performed by the optical communication system according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention is described below with reference to the accompanying drawings. 
     1. Overview 
     1-1. Basic Configuration 
       FIG.  1    schematically shows the configuration of an optical communication system  10  according to the present embodiment. The optical communication system  10  includes a master station device  100  and multiple slave station devices  200 . In the example shown in  FIG.  1   , the optical communication system  10  includes m slave station devices  200 - i  (i=0 to m- 1 ). Here, m is an integer greater than or equal to 2. The master station device  100  is connected to the slave station devices  200  via an optical fiber and performs optical communication with each of the slave station devices  200 . 
     In the following description, the optical communication system  10  is a PON (Passive Optical Network) system. The master station device  100  is hereinafter referred to as an “OLT (Optical Line Termination, or Optical Line Terminal)  100 ”. The slave station devices  200  are hereinafter referred to as “ONUs (Optical Network Units)  200 ”. 
     In the present embodiment, the network topology of the OLT  100  and the ONUs  200  is a bus topology. More specifically, as shown in  FIG.  1   , the optical communication system  10  includes a loop path  300  that is formed by a trunk fiber arranged in a loop. The OLT  100  is connected to the loop path  300 . The ONUs  200  are connected to the loop path  300  in parallel. 
     Due to the inclusion of the loop path  300 , two types of communication paths exist between the OLT  100  and each of the ONUs  200 , namely a normal path PN and a backup path PS. For example, when considering the downlink direction from the OLT  100  to each of the ONUs  200 , the normal path PN is a communication path that extends in a first direction D 1  along the loop path  300 . On the other hand, the backup path PS is a communication path that extends in a second direction D 2 , which is opposite to the first direction D 1 , along the loop path  300 .  FIG.  1    shows the normal path PN and the backup path PS for the ONU  200 - 1  as one example. 
     1-2. Normal Operation 
       FIG.  2    is a conceptual diagram for describing normal operation of the optical communication system  10 . In normal operation, the OLT  100  performs communication with each of the ONUs  200  via the normal path PN. To achieve this, the OLT  100  enables the normal path PN and disables the backup path PS. 
     First, the OLT  100  executes “registration processing (discovery)” for registering the ONUs  200  in order to establish communication links with the ONUs  200  that are connected to the PON network. In the registration processing, the OLT  100  detects the ONUs  200  that are connected to the PON network and assigns an identifier to each detected ONU  200 . The OLT  100  notifies the ONUs  200  of the assigned identifiers, and the ONUs  200  each hold the notified identifier. Also, in the registration processing, the OLT  100  executes “ranging processing” for measuring the round-trip propagation time (hereinafter referred to as RTT (Round Trip Time)) between the OLT  100  and each of the ONUs  200 . The OLT  100  holds an identifier and an RTT in association with each of the ONUs  200 . After the registration processing is complete, the OLT  100  begins performing communication with the ONUs  200 . 
     The OLT  100  executes “communication control processing” for controlling communication with each of the ONUs  200 . Uplink communication from the ONUs  200  to the OLT  100  is performed using the same wavelength through TDMA (Time Division Multiple Access). In order to prevent uplink optical signals transmitted from the ONUs  200  from colliding with each other, the OLT  100  determines a transmission timing and a transmission amount for the uplink optical signals from each of the ONUs  200  based on the corresponding RTTs of the ONUs  200 . The OLT  100  then generates transmission permission information that includes an identifier, a transmission timing, and a transmission amount for each of the ONUs  200 . 
     The OLT  100  transmits the generated transmission permission information to each of the ONUs  200 . The transmission permission information reaches the ONUs  200  through the normal path PN. The ONUs  200  reference the identifiers included in the transmission permission information and identify a corresponding portion of the transmission permission information. The ONUs  200  each then transmit an uplink optical signal in accordance with the transmission timing and the transmission amount indicated by the transmission permission information. The uplink optical signals transmitted from the ONUs  200  reach the OLT  100  through the normal path PN. 
     In this way, the OLT  100  holds an RTT for each of the ONUs  200  and executes communication control processing for controlling communication with the ONUs  200  based on the RTTs. For convenience, the RTT for the case of communication via the normal path PN is hereinafter referred to as “normal path RTT”. During normal operation shown in  FIG.  2   , the OLT  100  holds a normal path RTT for each of the ONUs  200 , and executes communication control processing for the ONUs  200  based on the normal path RTTs. 
     1-3. Protection 
     Next, a protection technique used in the optical communication system  10  according to the present embodiment will be described. 
       FIG.  3    is a conceptual diagram for describing a situation in which a fiber has become disconnected in the normal path PN. In a bus topology, if a fiber becomes disconnected, communication with many ONUs  200  may be cut off. In the case of the example shown in  FIG.  3   , a fiber disconnection occurs in the section between the ON  200 - 0  and the ONU  200 - 1 . As a result, communication with the ONUs  200 - 1  to  200 -( m - 1 ) is cut off. The ONUs  200  that cannot perform communication via the normal path PN are each hereinafter referred to as a “disconnected ONU  200 - j ”. In the example shown in  FIG.  3   , the ONUs  200 - 1  to  200 -( m - 1 ) are respectively disconnected ONUs  200 - j  (j=1 to m- 1 ). According to the present embodiment, the above-described backup path PS is used in order to quickly resume (recover) communication with the disconnected ONUs  200 - j.    
       FIG.  4    is a conceptual diagram for describing operations performed using the normal path PN and the backup path PS in the optical communication system  10 . The OLT  100  enables the backup path PS in addition to the normal path PN. In the case of the ONU  200 - 0  at a location before the fiber disconnection point, the OLT  100  performs communication with that ONU via the normal path PN. On the other hand, in the case of the disconnected ONUs  200 - j  (j=1 to m- 1 ), the OLT  100  performs communication with those ONUs via the backup path PS instead of the normal path PN. This makes it possible to resume communication with all of the disconnected ONUs  200 - j  without waiting for repair of the optical fiber that constitutes the normal path PN. 
     However, the RTT for communication via the backup path PS is required in order to execute communication control processing for the disconnected ONUs  200 - j . The RTT for communication via the backup path PS is hereinafter referred to as the “backup path RTT”. Before the start of communication performed via the backup path PS, the OLT  100  needs to be aware of the backup path RTT for all of the disconnected ONUs  200 - j . In other words, it is necessary to switch the RTT for all of the disconnected ONUs  200 - j  from the normal path RTT to the backup path RTT. 
     However, it takes a long time to perform the above-mentioned registration processing for all of the disconnected ONUS  200 - j  and measure the new backup path RTT. In other words, the amount of time required to resume communication increases. As the number of disconnected ONUS  200 - j  increases, the number of times that registration processing needs to be performed to acquire the backup path RTT for all of the disconnected ONUS  200 - j  increases, and the time required to resume communication increases. Also, uplink communication from the registered ONUS  200  needs to be temporarily suspended during the QuietWindow period set in the registration processing (ranging processing). Accordingly, the execution of registration processing for the disconnected ONUS  200 - j  is accompanied by temporary suspension of uplink communication on the normal path PN and uplink delay. 
     In view of this, the present embodiment provides a technique capable of quickly acquiring the backup path RTT for all disconnected ONUS  200 - j.    
       FIG.  5    is a conceptual diagram for describing a method of acquiring a backup path RTT for a disconnected ONU  200 - j . The normal path RTT and the backup path RTT for each ONU  200 - i  (i=0 to m- 1 ) are represented by Ti_normal and Ti_protect, respectively. 
     A reference ONU  200 - k  (first slave station device) is any one of the ONUS  200 - i . The normal path RTT (Tk_normal) between the OLT  100  and the reference ONU  200 - k  is hereinafter referred to as the “first normal path RTT”. The backup path RTT (Tk_protect) between the OLT  100  and the reference ONU  200 - k  is hereinafter referred to as the “first backup path RTT”. 
     The disconnected ONU  200 - j  (second slave station device) is an ONU  200  that cannot perform communication via the normal path PN as described above. The normal path RTT (Tj_normal) between the OLT  100  and the disconnected ONU  200 - j  is hereinafter referred to as the “second normal path RTT”. The backup path RTT (Tj_protect) between the OLT  100  and the disconnected ONU  200 - j  is hereinafter referred to as the “second backup path RTT”. 
     As shown in  FIG.  5   , the sum of the second normal path RTT (Tj_normal) and the second backup path RTT (Tj_protect) for the disconnected ONU  200 - j  corresponds to the sum of the first normal path RTT (Tk_normal) and the first backup path RTT (Tk_protect) for the reference ONU  200 - k . Accordingly, the OLT  100  can calculate the second backup path RTT (Tj_protect) based on the first normal path RTT (Tk_normal), the first backup path RTT (Tk_protect), and the second normal path RTT (Tj_normal). 
     In particular, if the connection distances between the loop path  300  and the ONUS  200 - i  and are substantially the same, the second backup path RTT (Tj_protect) is expressed by Expression 1 shown below. Here, “the connection distances . . . are substantially the same” means that the connection distances are the same, or that the variation in connection distance is small enough to be ignored. 
       Tj_protect=Tk_normal+Tk_protect−Tj_normal  Expression 1:
 
     The first normal path RTT (Tk_normal) and the second normal path RTT (Tj_normal) have been measured in the above-mentioned registration processing and are known. Even after a disconnected ONU  200 - j  is detected, the OLT  100  holds the registration information (identifier, second normal path RTT) for that disconnected ONU  200 - j  instead of deleting it. Thereafter, if the first backup path RTT (Tk_protect) for one reference ONU  200 - k  is known, the second backup path RTT (Tj_protect) can be calculated for all disconnected ONUS  200 - j.    
     There are various conceivable examples of methods for acquiring the first backup path RTT (Tk_protect). For example, before communication is started in normal operation, the OLT  100  selects one reference ONU  200 - k  from among the ONUS  200 - i , uses the backup path PS to preliminarily measure the first backup path RTT, and holds the measured value. As another example, after at least one disconnected ONU  200 - j  is detected, the OLT  100  may select one of the detected disconnected ONUS  200 - j  as the reference ONU  200 - k , use the backup path PS to preliminarily measure the first backup path RTT, and hold the measured value. In either case, it is sufficient to simply measure the first backup path RTT for one reference ONU  200 - k.    
     In this way, the OLT  100  calculates the second backup path RTT for each of the disconnected ONUs  200 - j  based on the normal path RTT and the first backup path RTT for one reference ONU  200 - k . It is not necessary to perform registration processing and directly measure the second backup path RTT for all of the disconnected ONUs  200 - j . After the second backup path RTTs are calculated, the OLT  100  resumes communication control processing for the disconnected ONUs  200 - j  based on the calculated second backup path RTTs. Note that given that the identifier of the disconnected ONU  200 - j  does not change, the identifier does not need to be updated. 
     The OLT  100  may calculate the second backup path RTT for each disconnected ONU  200 - j  using Expression 1 shown above. However, if the connection distances between the loop path  300  and each of the ONUs  200 - i  are different, error may arise in the second backup path RTT calculated using Expression  1 . In that case, the OLT  100  may set a slightly longer Grant period for the disconnected ONUs  200 - j.    
     Also, after communication is resumed, the OLT  100  may periodically measure and update the second backup path RTT. Accordingly, the accuracy of the second backup path RTT is further improved. 
     1-4. Effects 
     As described above, according to the present embodiment, the plurality of ONUs  200  are connected to the loop path  300  in parallel. Accordingly, there are two types of communication paths between the OLT  100  and the ONUS  200 , namely the normal path PN and the backup path PS. If there is a disconnected ONU  200 - j  that cannot perform communication via the normal path PN, communication can be resumed using the backup path PS instead of the normal path PN. This makes it possible to resume communication with all of the disconnected ONUS  200 - j  without waiting for repair of the optical fiber that constitutes the normal path PN. 
     In order to execute communication control processing for the disconnected ONU  200 - j , it is necessary to use the second backup path RTT for the case of communication via the backup path PS. According to the present embodiment, the OLT  100  holds the normal path RTT for each of the ONUS  200  and the first backup path RTT for the reference ONU  200 - k . If a disconnected ONU  200 - j  is detected, the OLT  100  calculates the second backup path RTT for each disconnected ONU  200 - j  based on the normal path RTT and the first backup path RTT for the reference ONU  200 - k . The OLT  100  the resumes communication control processing for each disconnected ONU  200 - j  based on the calculated second backup path RTT. It is not necessary to execute registration processing and directly measure the second backup path RTT for all of the disconnected ONUS  200 - j . Accordingly, the time required to resume communication is further shortened. As the number of disconnected ONU  200 - j  increases, the time saving effect becomes more remarkable. 
     Also, when registration processing is performed, it is necessary to temporarily suspend uplink communication on the normal path PN during the above-mentioned QuietWindow period. According to the present embodiment, it is not necessary to execute registration processing for all of the disconnected ONUS  200 - j , thus suppressing suspension and uplink delay in uplink communication caused by registration processing. In other words, the RTT switching method according to the present embodiment does not interfere with communication on the normal path PN. 
     As a comparative example, a method of measuring the backup path RTT for all of the ONUS  200  in advance is also conceivable. However, in the case of this comparative example, each time a new ONU  200  is connected to the PON network, it is necessary to measure the backup path RTT for the new ONU  200 . For this reason, it is necessary to suspend normal operation, disable the normal path PN, and enable the backup path PS. This is not realistic. According to the present embodiment, even if a new ONU  200  is connected to the PON network, it is not necessary to stop normal operation. 
     Hereinafter, the optical communication system  10  according to the present embodiment is described in more detail. 
     2. Example of Configuration of Optical Communication System 
       FIG.  6    is a conceptual diagram showing an example of the configuration of the optical communication system  10  according to the present embodiment. In this example, the optical communication system  10  is a TWDM-PON (Time and Wavelength Division Multiplexing—PON) system that employs wavelength division multiplexing (WDM) technology. 
     The optical communication system  10  includes the OLT  100 , the ONUs  200 - i  (i=0 to m- 1 ), optical fibers ( 300 N,  300 S,  310 , and  320 - i ), optical branching/coupling units  410  and  420 - i , and an optical switch  430 . The optical branching/coupling units  410  and  420 - i  are optical splitters (power splitters), for example. 
     The OLT  100  is connected to the optical branching/coupling unit  410  via the trunk fiber  310 . In the optical branching/coupling unit  410 , the trunk fiber  310  is branched into a main trunk fiber  300 N and a backup trunk fiber  300 S. The loop path  300  is formed by connecting the main trunk fiber  300 N and the backup trunk fiber  300 S in a loop. The communication path constituted by the main trunk fiber  300 N corresponds to the above-described normal path PN. On the other hand, the communication path constituted by the backup trunk fiber  300 S corresponds to the above-described backup path PS. 
     The ONUs  200 - i  are connected to the loop path  300  (main trunk fiber  300 N) in parallel. More specifically, the optical branching/coupling units  420 - i  are arranged in order on the main trunk fiber  300 N. The ONUs  200 - i  are respectively connected to the optical branching/coupling units  420 - i  via branch fibers  320 - i . Preferably, the branch fibers  320 - i  have substantially the same length. Here, “substantially the same length” means that the lengths are the same, or that the variation in length is small enough to be ignored. 
     The optical branching/coupling unit  410  distributes downlink optical signals from the trunk fiber  310  to the main trunk fiber  300 N and the backup trunk fiber  300 S. The optical branching/coupling unit  410  also outputs uplink optical signals from the main trunk fiber  300 N and the standby trunk fiber  300 S to the trunk fiber  310 . 
     The optical branching/coupling units  420 - i  each distribute optical signals from the trunk fiber on one side to the trunk fiber and the branch fibers  320 - i  on the other side. The optical branching/coupling units  420 - i  each also distribute optical signals from the branch fibers  320 - i  to the trunk fibers on both sides. 
     The optical switch  430  is provided on the main trunk fiber  300 N and the standby trunk fiber  300 S. The optical switch  430  enables/disables the normal path PN by allowing/blocking the passage of optical signals on the main trunk fiber  300 N. Similarly, the optical switch  430  enables/disables the backup path PS by allowing/blocking the passage of optical signals on the backup trunk fiber  300 S. The setting of this optical switch  430  is switched by the OLT  100 . 
       FIG.  7    is a conceptual diagram showing an example of the configuration of the OLT  100 . The OLT  100  includes a plurality of channel termination devices  110 - x  (x=0 to n- 1 ) and a wavelength multiplexer/demultiplexer  120 . Here, n is an integer greater than or equal to 2. The channel termination devices  110  are also called an OLT-CT (Channel Termination) or an OSU (Optical Subscriber Unit). 
     Different wavelengths are assigned to the channel termination devices  110 - x . In other words, the channel termination devices  110 - x  perform communication using optical signals that have different wavelengths. More specifically, the channel termination devices  110 - x  each perform downlink communication using a downlink optical signal that has a wavelength λ Dx , and perform uplink communication using an uplink optical signal that has a wavelength λ Ux . The wavelength λ Dx  and the wavelength λ Ux  are different wavelengths. 
     The wavelength multiplexer/demultiplexer  120  is connected to each of the channel termination devices  110 - x . The wavelength multiplexer/demultiplexer  120  combines the wavelength λ Dx  downlink optical signals output from the channel termination devices  110 - x  to generate a downlink WDM signal, and outputs the downlink WDM signal to the trunk fiber  310 . The downlink WDM signal is distributed to each of the ONUs  200 - i.    
     The ONUs  200 - i  each have a variable wavelength and are assigned to one of the channel termination devices  110 - x  (wavelength λ Dx , λ Ux ). The ONUs  200 - i  each extract the downlink optical signal that has the corresponding assigned wavelength λ Dx  from the downlink WDM signal. Also, the ONUs  200 - i  each transmit an uplink optical signal that has the corresponding assigned wavelength λ Ux . The wavelength multiplexer/demultiplexer  120  demultiplexes uplink optical signals having various wavelengths from the trunk fiber  310 , and outputs the uplink optical signals having the wavelengths λ Ux  to the channel termination devices  110 - x . In this way, communication is performed between the channel termination devices  110 - x  and the ONUs  200 - i.    
     Note that it is assumed that the connection distances between the wavelength multiplexer/demultiplexer  120  and each of the channel termination devices  110 - x  are substantially the same connection distance. Here, “substantially the same connection distance” means that the connection distances are the same, or that the variation in the connection distance is small enough to be ignored. 
     The channel termination device  110  includes a control unit  111 . The control unit  111  performs registration processing (discovery) and registers one or more ONUs  200  that are to be communication partners. The control unit  111  includes a storage unit  113 . The storage unit  113  stores the identifiers of the communication partner ONUs  200  in association with RTTs. 
     The control unit  111  also performs communication control processing for controlling communication with the ONUs  200 . Specifically, the control unit  111  determines a transmission timing and a transmission amount for an uplink optical signal from each of the ONUS  200  based on the corresponding RTTs of the ONUS  200 . The control unit  111  then generates transmission permission information (Grant) that includes an identifier, a transmission timing, and a transmission amount for each of the ONUS  200 . The control unit  111  transmits the generated transmission permission information to each of the ONUS  200 . The ONUS  200  each transmit an uplink optical signal in accordance with the transmission timing and the transmission amount indicated by the transmission permission information. 
     The control unit  111  further includes an abnormality detection unit  115 . The abnormality detection unit  115  detects fiber disconnection in the normal path PN (main trunk fiber  300 N). In other words, the abnormality detection unit  115  detects a disconnected ONU  200 - j  that cannot perform communication via the normal path PN. 
     For example, after the above-described transmission permission information is transmitted, the abnormality detection unit  115  monitors the reception status of uplink optical signals from the transmission partner ONUS  200 . If an uplink optical signal is not received from an ONU  200  within a certain period of time after the transmission permission information is transmitted, the abnormality detection unit  115  determines that the corresponding ONU  200  has become a disconnected ONU  200 - j.    
     As another example, the abnormality detection unit  115  may determine that fiber disconnection has occurred if uplink optical signals from multiple ONUs  200  are interrupted at substantially the same time (within a certain period of time) and a DyingGasp signal has not been received. As yet another example, the abnormality detection unit  115  may determine that fiber disconnection has occurred if uplink optical signals from multiple ONUs  200  farther than a certain distance (RTT) are interrupted at substantially the same time (within a certain period of time). As still another example, the abnormality detection unit  115  may detect fiber disconnection by performing a test using an OTDR (Optical Time Domain Reflectometer). 
     The functionality of each of the channel termination devices  110  (control units  111 ) is realized by an optical transceiver that transmits and receives optical signals, a controller that controls the optical transceiver and performs various types of information processing, and the like. Typically, the controller includes a processor and memory. The functionality of the channel termination device  110  is realized by the processor executing a communication control program stored in the memory. The communication program may be recorded on a computer-readable recording medium. The controller may be realized with use of hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). 
     3. Processing Examples 
     Hereinafter, examples of processing performed by the optical communication system  10  shown in  FIGS.  6  and  7    are described. 
     3-1. First Processing Example 
       FIG.  8    is a flowchart showing a first processing example. 
     In step S 1 , the OLT  100  enables the normal path PN and disables the backup path PS by operating the optical switch  430 . Each channel termination device  110 - x  registers one or more ONUS  200  that are communication partners. At this time, the channel termination device  110 - x  measures the normal path RTT for each ONU  200  and assigns an identifier to each ONU  200 . The normal path RTTs and the identifiers of the ONUS  200  are stored in the storage unit  113 . In other words, each channel termination device  110 - x  holds the normal path RTTs and identifiers of the registered ONUS  200  in the storage unit  113 . 
     In step S 2 , the OLT  100  disables the normal path PN and enables the backup path PS by operating the optical switch  430 . One channel termination device  110 - r  selects one reference ONU  200 - k  from among the ONUS  200  that were registered in step S 1 . That channel termination device  110 - r  then uses the backup path PS to measure the first backup path RTT (Tk_protect) for the reference ONU  200 - k . The channel termination device  110 - r  holds the acquired first backup path RTT in the storage unit  113 . The OLT  100  then enables the normal path PN and disables the backup path PS by operating the optical switch  430 . 
     In step S 3 , the channel termination device  110 - r  notifies another channel termination device  110 - s  (s≠r) of the first normal path RTT (Tk_normal) and the first backup path RTT (Tk_protect) for the reference ONU  200 - k . The other channel termination device  110 - s  holds the first normal path RTT and the first backup path RTT that were notified in the storage unit  113 . In this way, the first normal path RTT and the first backup path RTT for the reference ONU  200 - k  are shared by all of the channel termination devices  110 - x  (x=0 to n- 1 ). 
     Preparation for normal operation is thus complete. Thereafter, main signal communication starts. 
     In step S 10 , the channel termination devices  110 - x  each perform communication with the registered ONUS  200  via the normal path PN. At this time, the channel termination device  110 - x  executes communication control processing for the ONUS  20  based on the held normal path RTTs. Note that the channel termination devices  110 - x  may periodically measure and update the normal path RTTs. 
     In step S 20 , the channel termination devices  110 - x  (abnormality detection units  115 ) determine whether or not fiber disconnection has occurred in the normal path PN. In other words, the channel termination devices  110 - x  determine whether or not a disconnected ONU  200 - j  has appeared. If a disconnected ONU  200 - j  does not exist (No in step S 20 ), the processing returns to step S 10 . On the other hand, if at least one disconnected ONU  200 - j  is detected (Yes in step S 20 ), the processing proceeds to step S 21 . Note that even after a disconnected ONU  200 - j  is detected, the corresponding channel termination device  110 - x  holds the registration information (identifier and second normal path RTT) for the disconnected ONU  200 - j  instead of deleting it. 
     In step S 21 , the OLT  100  enables the backup path PS in addition to the normal path PN by operating the optical switch  430 . 
     In step S 24 , the channel termination device  110 - z  that is performing communication with the disconnected ONU  200 - j  switches the RTT for the disconnected ONU  200 - j . Specifically, the channel termination device  110 - z  calculates the second backup path RTT (Tj_protect) based on the first normal path RTT (Tk_normal), the first backup path RTT (Tk_protect), and the second normal path RTT (Tj_normal) that are held. For example, the channel termination device  110 - z  calculates the second backup path RTT (Tj_protect) using Expression 1 shown above. The channel termination device  110 - z  then switches the RTT for the disconnected ONU  200 - j  from the second normal path RTT (Tj_normal) to the second backup path RTT (Tj_protect). Note that given that the identifier of the disconnected ONU  200 - j  does not change, the identifier does not need to be updated. 
     Note that the order of steps S 21  and S 24  may be reversed. 
     In step S 30 , the channel termination device  110 -z resumes communication by performing communication with the disconnected ONU  200 - j  via the backup path PS. More specifically, the channel termination device  110 -z resumes communication control processing for the disconnected ONU  200 - j  based on the second backup path RTT that was calculated in step S 24 . 
     Note that if the branch fibers  320 - i  (see  FIG.  6   ) have different lengths, error may arise in the second backup path RTT calculated using Expression 1 shown above. In this case, the channel termination device  110 -z may set a slightly longer Grant period for the disconnected ONU  200 - j.    
     After communication is resumed, the channel termination device  110 - z  may periodically measure and update the second backup path RTT. Accordingly, the accuracy of the second backup path RTT is further improved. 
     3-2. Second Processing Example 
       FIG.  9    is a flowchart showing a second processing example. Redundant descriptions will not be given for portions that are the same as in the first processing example described above. 
     After step S 1 , main signal communication starts without executing steps S 2  and S 3 . Steps S 10  and S 20  are the same as in the case of the first processing example. If at least one disconnected ONU  200 - j  is detected (Yes in step S 20 ), the processing proceeds to step S 21 . The backup path PS is enabled in step S 21 , and then the processing proceeds to step S 22 . 
     In step S 22 , one channel termination device  110 - r  that was performing communication with the one or more disconnected ONUS  200 - j  selects one reference ONU  200 - k  from among the one or more disconnected ONUS  200 - j . That channel termination device  110 - r  then uses the backup path PS to measure the first backup path RTT (Tk_protect) for the reference ONU  200 - k . The channel termination device  110 - r  holds the acquired first backup path RTT in the storage unit  113 . 
     In step S 23 , the channel termination device  110 - r  notifies another channel termination device  110 - s  (s≠r) of the first normal path RTT (Tk_normal) and the first backup path RTT (Tk_protect) for the reference ONU  200 - k . The other channel termination device  110 - s  holds the first normal path RTT and the first backup path RTT that were notified in the storage unit  113 . In this way, the first normal path RTT and the first backup path RTT for the reference ONU  200 - k  are shared by all of the channel termination devices  110 - x  (x=0 to n- 1 ). 
     The subsequent steps S 24  and S 30  are the same as in the case of the first processing example. 
     According to the second processing example, the setting of the optical switch  430  is switched fewer times than in the first processing example. 
     4. Other Remarks 
     The optical communication system  10  according to the present embodiment is not limited to being a PON system. The technique according to the present embodiment can be applied to any optical communication system that has a loop path  300  and executes communication control processing based on RTT. 
     The optical communication system  10  according to the present embodiment is applicable to, for example, a mobile fronthaul (MFH) in the case where a mobile communication area is deployed linearly or in a plane. 
     REFERENCE SIGNS LIST 
     
         
           10  Optical communication system 
           100  OLT (master station device) 
           110  Channel termination device 
           111  Control unit 
           113  Storage unit 
           115  Abnormality detection unit 
           120  Wavelength multiplexer/demultiplexer 
           200  ONU (slave station device) 
           200 - j  Disconnected ONU 
           200 - k  Reference ONU 
           300  Loop path 
           300 N Main trunk fiber 
           300 S Backup trunk fiber 
           310  Trunk fiber 
           320  Branch fiber 
           410  Optical branching/coupling unit 
           420  Optical branching/coupling unit 
           430  Optical switch 
         PN Normal path 
         PS Backup path