Patent Publication Number: US-2023140715-A1

Title: Optical transmission device, optical communication system, and optical communication method

Description:
TECHNICAL FIELD 
     The present invention relates to an optical transmission device, an optical communication system, and an optical communication method. 
     BACKGROUND ART 
     As a technology for providing a FTTH (Fiber To The Home) service at high speed and at a low price, there is, for example, a PON (Passive Optical Network) technology. A PON is an optical communication system constructs a network without photoelectric conversion by branching an optical signal using a splitter as a passive element. 
     Some of optical communication systems, such as a PON, establish connection by using TCP (Transmission Control Protocol) and implement highly reliable communication between a terminal device (client) and a server. 
     Examples of a terminal device used in the above-described optical communication system include an information processing device having a wired or wireless communication function, such as a PC (personal computer). For example, the terminal device communicates with a content server or the like which provides content via an ONU (Optical Network Unit), a splitter, and an optical transmission device, such as an OTL (Optical Line Terminal). 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: Takeshita, Takafumi and three others, “masutaringu TCP/IP nyumon hen” (mastering TCP/IP: elementary level), 5th ed., Ohmsha, Ltd., Feb. 25, 2012, p. 230-256 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     For example, in TCP communication, an algorithm called slow start is used at the start of communication in order to prevent a client, such as a terminal device, from transmitting massive data to a server from the beginning to cause an excess over the capacity of a network. 
     Slow start has parameters, such as a congestion window (CW) and a slow start threshold (SST). 
     The amount of data to be transmitted by a terminal device depends on a CW. For the terminal device, an initial value for the CW is set to be small. Upon receipt of a positive acknowledgment (ACK) from a server, the terminal device exponentially increases a value of the CW. 
     In TCP communication, a terminal device can only transmit data, the amount of which is dependent on a CW, until the terminal device receives an ACK. A round trip time (RTT) thus largely affects throughput. 
     An OLT (optical transmission device) allocates communication resources (a band) for an ONU on the basis of a buffer amount notification transmitted by the ONU. For this reason, the terminal device may suffer an increase in delay and a reduction in throughput, depending on a band allocated by the OLT and a time period taken for the OLT to allocate the band. 
     An object of the present invention is to provide an optical transmission device, an optical communication system, and an optical communication method capable of enhancing throughput of a terminal device even when the terminal device establishes connection with a server to perform communication. 
     Means for Solving the Problem 
     An optical transmission device according to one aspect of the present invention is an optical transmission device which performs relaying between a terminal device and a server via an optical network device, the terminal device and the server establishing connection with each other, including a request detection unit which detects a connection establishment request transmitted to the server by the optical network device on the basis of a connection establishment request transmitted to the server by the terminal device, a response detection unit which detects an acknowledgment transmitted to the terminal device by the server on the basis of the connection establishment request detected by the request detection unit, and a band allocation unit which, if the response detection unit detects the acknowledgement, performs allocation so as to expand a band over which the optical network device is capable of transmission to the optical transmission device, regardless of presence or absence of a buffer amount notification from the optical network device. 
     An optical communication system according to one aspect of the present invention is an optical communication system including a server and an optical transmission device which performs relaying between a terminal device and the server via an optical network device, the terminal device and the server establishing connection with each other, wherein the optical transmission device includes a request detection unit which detects a connection establishment request transmitted to the server by the optical network device on the basis of a connection establishment request transmitted to the server by the terminal device, a response detection unit which detects an acknowledgment transmitted to the terminal device by the server on the basis of the connection establishment request detected by the request detection unit, and a band allocation unit which, if the response detection unit detects the acknowledgement, performs allocation so as to expand a band over which the optical network device is capable of transmission to the optical transmission device, regardless of presence or absence of a buffer amount notification from the optical network device. 
     An optical communication method according to one aspect of the present invention is an optical communication method for performing relaying between a terminal device and a server via an optical network device, the terminal device and the server establishing connection with each other, including a request detection step of detecting a connection establishment request transmitted to the server by the optical network device on the basis of a connection establishment request transmitted to the server by the terminal device, a response detection step of detecting an acknowledgment transmitted to the terminal device by the server on the basis of the detected connection establishment request, and a band allocation step of, if the acknowledgement is detected, performing allocation so as to expand a band over which the optical network device is capable of transmission to the optical transmission device, regardless of presence or absence of a buffer amount notification from the optical network device. 
     Effects of the Invention 
     According to the present invention, it is possible to enhance throughput of a terminal device even when the terminal device establishes connection with a server to perform communication. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram showing an example of a configuration of an optical communication system. 
         FIG.  2    is a functional block diagram showing an outline of functions which an OLT has. 
         FIG.  3    is a sequence chart showing an example of operation of an optical communication system including the OLT. 
         FIG.  4    is a functional block diagram showing an outline of functions which an OLT according to one embodiment has. 
         FIG.  5    is a sequence chart showing an example of operation of an optical communication system including the OLT according to the one embodiment. 
         FIG.  6    is a diagram showing an example of a hardware configuration of the OLT according to the one embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A background against which the present invention has been made will be described first.  FIG.  1    is a diagram showing an example of a configuration of an optical communication system  1 . As shown in  FIG.  1   , the optical communication system  1  has, for example, three terminal devices  10 - 1  to  10 - 3 , ONUs  15 - 1  to  15 - 3 , a splitter  100 , an OLT  20 , and a server  30 . Note that the number of units of each type among units constituting the optical communication system  1  is not limited. 
     The terminal devices  10 - 1  to  10 - 3  are each, for example, a PC (personal computer), and establish, for example, TCP connection with the server  30  via the ONUs  15 - 1  to  15 - 3 , the splitter  100 , and the OLT  20  and performs communication. 
     For example, the terminal devices  10 - 1  to  10 - 3  each include, in an uplink signal, buffer information indicating a capacity of a transmit-receive buffer which the terminal device includes and transmit the respective uplink signals to the ONUs  15 - 1  to  15 - 3 . If any of a plurality of components, such as the terminal devices  10 - 1  to  10 - 3 , is not specified, each terminal device will simply be referred to as a terminal device  10  or the like hereinafter. 
     The ONUs  15 - 1  to  15 - 3  are terminating devices (optical network devices) on a user side which are arranged near the terminal devices  10 - 1  to  10 - 3 , respectively, and are connected to the one OLT  20  via an optical fiber and the splitter  100 . An ONU  15  also transmits a band request requesting a communication resource amount (band) to the OLT  20  on the basis of buffer information included in an uplink signal which is transmitted from the terminal device  10 . 
     The ONUs  15 - 1  to  15 - 3  each convert an optical signal which is transmitted by the OLT  20  into an electrical signal and output the respective electrical signals to the terminal devices  10 - 1  to  10 - 3 . The ONUs  15 - 1  to  15 - 3  convert respective electrical signals which are transmitted by the terminal devices  10 - 1  to  10 - 3  into optical signals and output the optical signals to the OLT  20 . 
     The splitter  100  is an optical splitter which performs multiplexing and demultiplexing of optical signals which are transmitted between the ONUs  15 - 1  to  15 - 3  and the OLT  20  through the optical fiber. 
     The OLT  20  is an optical transmission device which is connected to the server  30  via a wired or wireless network and performs optical communication with each of the terminal devices  10 - 1  to  10 - 3 . For example, the OLT  20  is a terminating device on a telecommunications carrier side. 
     The OLT  20  transfers, as a predetermined signal, optical signals which are transmitted by the ONUs  15 - 1  to  15 - 3  to the server  30  and transfers a signal which is output by the server  30  to each of the ONUs  15 - 1  to  15 - 3 . 
     The server  30  is, for example, a content server. Note that the server  30  only needs to be a device which establishes connection with the terminal device  10  and performs communication and may be, for example, a PC (personal computer). 
       FIG.  2    is a functional block diagram showing an outline of functions which the OLT  20  has. As shown in  FIG.  2   , the OLT  20  has, for example, a PON-I/F (I/F: interface)  200 , an uplink communication control unit  202 , a NW-I/F (NW: network)  204 , a band allocation unit  206 , and a downlink communication control unit  208 . 
     The PON-I/F  200  has a function as an optical communication unit which performs optical communication with the ONUs  15 - 1  to  15 - 3  and switches between uplink communication and downlink communication with the ONUs  15 - 1  to  15 - 3  (the terminal devices  10 - 1  to  10 - 3 ). For example, the PON-I/F  200  receives uplink signals which are transmitted by the ONUs  15 - 1  to  15 - 3  and outputs the uplink signals to the uplink communication control unit  202  and the band allocation unit  206 . The PON-I/F  200  also transmits a downlink signal which is processed by the downlink communication control unit  208  and bands (band amounts) which are allocated by the band allocation unit  206  to the ONUs  15 - 1  to  15 - 3  (the terminal devices  10 - 1  to  10 - 3 ). 
     The uplink communication control unit  202  buffers an uplink signal which is input from the PON-I/F  200 , restores a PON frame to an original frame, and outputs the frame to the NW-I/F  204 . 
     The NW-I/F  204  has a function as a network interface for transmission and reception of signals to and from the server  30  located upstream of the OLT  20 . That is, the NW-I/F  204  transmits an uplink signal which is input from the uplink communication control unit  202  to the server  30  and outputs a downlink signal which is received from the server  30  to the downlink communication control unit  208 . 
     The band allocation unit  206  allocates respective uplink communication resource amounts (bands) for the ONUs  15 - 1  to  15 - 3  on the basis of band requests (pieces of buffer information) included in an uplink signal which is input from the PON-I/F  200  and outputs pieces of band information indicating the allocated bands to the PON-I/F  200 . 
     The downlink communication control unit  208  buffers a downlink signal which is input from the NW-I/F  204 , converts the downlink signal into a PON frame, and outputs the PON frame to the PON-I/F  200 . 
       FIG.  3    is a sequence chart showing an example of operation of the optical communication system  1  including the OLT  20 . As shown in  FIG.  3   , the terminal device  10  first makes a connection establishment request (SYN packet) to the ONU  15  in order to request establishment of connection with the server  30  (S 10 ). 
     In response to the connection establishment request from the terminal device  10 , the ONU  15  transmits a band request to the OLT  20  (S 100 ). 
     The OLT  20  makes a response of intent to allocate a band for connection to the ONU  15  (S 110 ). 
     The ONU  15  transmits a SYN packet for establishing TCP connection using a band which is allocated by the OLT  20  to the server  30  via the OLT  20  (S 120 ). 
     The server  30  transmits a response (ACK) to the SYN packet transmitted from the ONU  15  to the terminal device  10  via the OLT  20  and the ONU  15  (S 130 ). 
     Note that a time period from when the terminal device  10  transmits a SYN packet to the server  30  to when the terminal device  10  receives a response (ACK) from the server  30  is a round trip time (RTT). 
     The terminal device  10  sets a congestion window (CW) to 1 MSS (Maximum Segment Size) and transmits data to the ONU  15  (S 135 ). Although the ONU  15  receives the data to be transmitted at this time, no band has yet been allocated. For this reason, the ONU  15  temporarily stores the data in a buffer and makes a band request (buffer amount notification) based on an amount by which the buffer is occupied to the OLT  20 . 
     Note that the congestion window (CW) is a value which is passed in a connection establishment request and a response thereto and is expressed as n×MSS. As to an initial value for the congestion window (CW), although n is set at 1 in  FIG.  3   , n is defined as 4 in RFC 2581, and n is defined as 10 in RFC 6928. 
     The OLT  20  requests a band for connection from the OLT  20  on the basis of the band request from the ONU  15  (S 140 ). 
     The OLT  20  makes a response of intent to allocate a band for connection to the ONU  15  (S 150 ). 
     The ONU  15  transmits the data with the CW of 1 MSS stored in the buffer to the server  30  via the OLT  20  (S 160 ). 
     The server  30  transmits a response (ACK) to the data transmitted with the CW of 1 MSS from the ONU  15  to the terminal device  10  via the OLT  20  and the ONU  15  (S 170 ). 
     The terminal device  10  then sets the CW to 2 MSS and transmits data to the ONU  15  (S 175 ). 
     The ONU  15  requests, to the OLT  20 , a band after reception of the data from the terminal device  10  (S 180 ). 
     The OLT  20  makes a response of intent to allocate a band to the ONU  15  (S 190 ). 
     The ONU  15  transmits the data with the CW of 2 MSS stored in the buffer to the server  30  via the OLT  20  (S 200 ). 
     The server  30  transmits a response (ACK) to the data transmitted with the CW of 2 MSS from the ONU  15  to the terminal device  10  via the OLT  20  and the ONU  15  (S 210 ). 
     Next, an OLT  20   a  according to one embodiment will be described.  FIG.  4    is a functional block diagram showing an outline of functions which the OLT  20   a  according to the one embodiment has. The OLT  20   a  is replaced by the OLT  20  shown in  FIG.  1    to constitute an optical communication system  1 . The OLT  20   a  performs relaying between a terminal device  10  and a server  30  which establish connection with each other by TCP. 
     As shown in  FIG.  4   , the OLT  20   a  has a PON-I/F  200   a , an uplink communication control unit  202   a , a request detection unit  210 , a NW-I/F  204 , a response detection unit  212 , a band allocation unit  206   a , and a downlink communication control unit  208   a.    
     The PON-I/F  200   a  has a function as an optical communication unit which performs optical communication with ONUS  15 - 1  to  15 - 3  and switches between uplink communication and downlink communication with the ONUS  15 - 1  to  15 - 3  (the terminal devices  10 - 1  to  10 - 3 ). For example, the PON-I/F  200   a  receives uplink signals which are transmitted by the ONUS  15 - 1  to  15 - 3  and outputs the uplink signals to the uplink communication control unit  202   a  and the band allocation unit  206   a . The PON-I/F  200   a  also transmits a downlink signal which is processed by the downlink communication control unit  208   a  and bands (band amounts) which are allocated by the band allocation unit  206   a  to the ONUS  15 - 1  to  15 - 3  (the terminal devices  10 - 1  to  10 - 3 ). 
     The uplink communication control unit  202   a  buffers an uplink signal which is input from the PON-I/F  200   a , restores a PON frame to an original frame, and outputs the frame to the NW-I/F  204   a  and the request detection unit  210 . 
     The request detection unit  210  detects a connection establishment request which is transmitted to the server  30  by the ONU  15  on the basis of, for example, a processed uplink signal which is input from the uplink communication control unit  202   a  and outputs information indicating the transmission of the connection establishment request to the server  30  by the ONU  15  and a destination address to the band allocation unit  206   a  and the response detection unit  212 . 
     For example, the request detection unit  210  detects a SYN packet for establishing TCP connection which is transmitted to the server  30  by the ONU  15  and outputs information indicating the detection and a communication partner (transmission information and a destination address) to the band allocation unit  206   a  and the response detection unit  212 . 
     That is, the request detection unit  210  detects a connection establishment request which is transmitted to the server  30  by the ONU  15  on the basis of a connection establishment request which is transmitted to the server  30  by the terminal device  10 . 
     The NW-I/F  204  has a function as a network interface for transmission and reception of signals to and from the server  30  located upstream of the OLT  20 . That is, the NW-I/F  204  transmits an uplink signal which is input from the uplink communication control unit  202  to the server  30  and outputs a downlink signal which is received from the server  30  to the downlink communication control unit  208 . 
     The downlink communication control unit  208   a  buffers a downlink signal which is input from the NW-I/F  204 , converts the downlink signal into a PON frame, and outputs the PON frame to the response detection unit  212  and the PON-I/F  200   a.    
     The response detection unit  212  detects an acknowledgment (ACK) which is transmitted to the terminal device  10  by the server  30  on the basis of, for example, information which is input from the request detection unit  210  and a downlink signal processed by the downlink communication control unit  208   a . The response detection unit  212  outputs information indicating the transmission of the acknowledgment (ACK) to the terminal device  10  by the server  30 , a predetermined instruction (to be described later), and the like to the band allocation unit  206   a.    
     That is, the response detection unit  212  detects an acknowledgement which is transmitted to the terminal device  10  by the server  30  on the basis of a connection establishment request which is detected by the request detection unit  210 . 
     Note that, after information is input from the request detection unit  210 , the response detection unit  212  waits for a fixed time period for an acknowledgement (ACK) which is a communication opposite in transmission information and destination address to an uplink signal. The fixed time period is set to a length equal to or more than a time period (a retransmission time-out time period) taken for a transmitting side (the terminal device  10 ) to retransmit data. 
     For example, if the response detection unit  212  detects an acknowledgement (ACK) within the fixed time period, since a CW is expected to be increased in later TCP communication, the response detection unit  212  instructs the band allocation unit  206   a  to allocate a band wider than last time. 
     The band allocation unit  206   a  allocates respective communication resource amounts (bands) for the ONUs  15  (the terminal devices  10 ) on the basis of information which is input from the PON-I/F  200   a , information which is input from the request detection unit  210 , information which is input from the response detection unit  212 , and the like and outputs pieces of band information indicating the allocated bands to the PON-I/F  200   a.    
     For example, if the response detection unit  212  detects an acknowledgment (ACK), the band allocation unit  206   a  performs allocation so as to expand a band over which the terminal device  10  is capable of transmission to the server  30 , regardless of presence or absence of a buffer amount notification from the ONU  15  (the terminal device  10 ). 
     In the presence of a buffer amount notification from the ONU  15  (the terminal device  10 ), the band allocation unit  206   a  may allocate a band over which the terminal device  10  is capable of transmission to the server  30 , regardless of the magnitude of a buffer amount in question. That is, the band allocation unit  206   a  may be configured to ignore a buffer amount in question if the ONU  15  transmits a buffer amount notification. 
     In normal TCP communication, a congestion window (CW) increases exponentially. In contrast, the band allocation unit  206   a  performs band allocation, for example, on the assumption that a congestion window (CW) is twice wider than last time. Note that a band to be allocated by the band allocation unit  206   a  is not limited to an amount twice a width of a most recently allocated band if the response detection unit  212  detects an acknowledgment. 
     For example, the band allocation unit  206   a  may allocate a band on the assumption that the congestion window (CW) increases to a maximum value for a standard. Alternatively, the band allocation unit  206   a  may perform band allocation for the congestion window (CW) having an initial value after a SYN packet for establishing TCP connection is detected, and a first ACK packet is received. 
     The band allocation unit  206   a  may determine the initial value on the basis of the fact that n=4 in RFC 2581, the fact that n=10 in RFC 6928, or the like. Alternatively, the band allocation unit  206   a  may monitor uplink traffic thus far, learn how wide a band is needed as an initial value, and allocate a band. For example, if the response detection unit  212  detects an acknowledgment, the band allocation unit  206   a  may perform allocation so as to expand a band on the basis of statistics obtained by learning traffic amounts after past acknowledgment detection by the response detection unit  212 . 
     An example of operation of the optical communication system  1  including the OLT  20   a  will be described.  FIG.  5    is a sequence chart showing the example of the operation of the optical communication system  1  including the OLT  20   a  according to the one embodiment. As shown in  FIG.  5   , the terminal device  10  first transmits a connection establishment request (SYN packet) to the ONU  15  in order to request establishment of connection with the server  30  (S 30 ). 
     The ONU  15  transmits a band request to the OLT  20  after reception of the connection establishment request from the terminal device  10  (S 300 ). 
     The OLT  20  makes a response of intent to allocate a band to the ONU  15  (S 310 ). 
     The ONU  15  transmits a SYN packet for establishing TCP connection using a band which is allocated by the OLT  20  to the server  30  via the OLT  20  (S 320 ). 
     At this time, in the OLT  20   a , the request detection unit  210  detects a connection establishment request which is transmitted to the server  30  by the ONU  15  (S 325 ). 
     The server  30  transmits a response (ACK) to the SYN packet transmitted from the ONU  15  to the terminal device  10  via the OLT  20  and the ONU  15  (S 330 ). 
     At this time, in the OLT  20   a , the response detection unit  212  detects the acknowledgment (ACK) transmitted to the terminal device  10  by the server  30  on the basis of the connection establishment request detected by the request detection unit  210  (S 335 ). 
     Note that a time period from when the terminal device  10  transmits a SYN packet to the server  30  to when the terminal device  10  receives a response (ACK) from the server  30  is a round trip time (RTT). 
     If the response detection unit  212  detects the acknowledgment (ACK), the OLT  20   a  performs allocation so as to expand a band over which the ONU  15  is capable of transmission to the OLT  20   a , regardless of presence or absence of a buffer amount notification from the ONU  15  (the terminal device  10 ) (S 345 ). 
     Even without the ONU  15  requesting band allocation by the OLT  20   a , the OLT  20   a  expands and allocates the band in the process in S 345 . Thus, once the ONU  15  receives data which is transmitted with the CW of, for example, 1 MSS from the terminal device  10  to the server  30  (S 355 ), the ONU  15  transmits the received data to the server  30  via the OLT  20  (S 360 ). 
     Here, a time period taken for the ONU  15  to request band allocation by the OLT  20   a  and a later time period taken for the OLT  20   a  to allocate a band are unnecessary. 
     The server  30  transmits a response (ACK) to the data transmitted with the CW of 1 MSS from the ONU  15  to the terminal device  10  via the OLT  20  and the ONU  15  (S 370 ). 
     At this time, in the OLT  20   a , the response detection unit  212  detects the acknowledgment (ACK) transmitted to the terminal device  10  by the server  30  (S 375 ). 
     If the response detection unit  212  detects the acknowledgment (ACK), the OLT  20   a  performs allocation so as to expand the band, over which the ONU  15  is capable of transmission to the OLT  20   a , regardless of presence or absence of a buffer amount notification from the ONU  15  (the terminal device  10 ) (S 385 ). 
     Even without the ONU  15  requesting band allocation by the OLT  20   a , the OLT  20   a  expands and allocates the band in the process in S 385 . For this reason, once the ONU  15  receives data which is transmitted with the CW of, for example, 2 MSS from the terminal device  10  to the server  30  (S 395 ), the ONU  15  transmits the received data to the server  30  via the OLT  20  (S 400 ). 
     Here, a time period taken for the ONU  15  to request band allocation by the OLT  20   a  and a time period taken for the OLT  20   a  to allocate a band to the ONU  15  are unnecessary. 
     The server  30  transmits a response (ACK) to the data transmitted with the CW of 2 MSS from the ONU  15  to the terminal device  10  via the OLT  20  and the ONU  15  (S 410 ). 
     At this time, in the OLT  20   a , the response detection unit  212  detects the acknowledgment (ACK) transmitted to the terminal device  10  by the server  30  (S 415 ) and continues predetermined communication. 
     As described above, if the OLT  20   a  detects a connection establishment request which is transmitted by the ONU  15  (the terminal device  10 ) and detects an acknowledgment which is transmitted by the server  30 , the OLT  20   a  performs allocation so as to expand a band over which the terminal device  10  is capable of transmission, regardless of presence or absence of a buffer amount notification to be transmitted by the ONU  15  (the terminal device  10 ). For this reason, the OLT  20   a  can reduce a time period required for band allocation and enhance throughput of the terminal device  10 . 
     Note that a part or the whole of each of functions of the terminal device  10 , the ONU  15 , the OLT  20 , the OLT  20   a , and the server  30  may be implemented by hardware or may be implemented as a program to be executed by a processor, such as a CPU. 
     That is, the optical communication system  1  according to the present invention can be implemented by using a computer and a program, and the program can be recorded on a storage medium or can be provided through a network. 
       FIG.  6    is a diagram showing an example of a hardware configuration of the OLT  20   a  according to the one embodiment. As shown in  FIG.  6   , for example, an input unit  500 , an output unit  510 , a communication unit  520 , a CPU  530 , a memory  540 , and an HDD  550  are connected via a bus  560 , and the OLT  20   a  has functions as a computer. The OLT  20   a  is configured to be capable of inputting and outputting data from and to a storage medium  570 . 
     The input unit  500  is, for example, a keyboard or a mouse. The output unit  510  is a display device, such as a display. The communication unit  520  is, for example, a wireless or wired network interface. 
     The CPU  530  controls the units constituting the OLT  20   a  and performs the above-described processing. The memory  540  and the HDD  550  store data. The storage medium  570  is configured to be capable of storing, e.g., an optical communication program which causes execution of the functions that the OLT  20   a  has. Note that architecture of the OLT  20   a  is not limited to the example shown in  FIG.  6   . The terminal device  10 , the ONU  15 , the OLT  20 , and the server  30  may have the same configurations as the OLT  20   a.    
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Optical communication system 
               10 - 1  to  10 - 3  Terminal device 
               15 - 1  to  15 - 3  ONU 
               20 ,  20   a  OLT 
               30  Server 
               100  Splitter 
               200 ,  200   a  PON-I/F 
               202 ,  202   a  Uplink communication control unit 
               204  NW-I/F 
               206 ,  206   a  Band allocation unit 
               208 ,  208   a  Downlink communication control unit 
               210  Request detection unit 
               212  Response detection unit 
               500  Input unit 
               510  Output unit 
               520  Communication unit 
               530  CPU 
               540  Memory 
               550  HDD 
               560  Bus 
               570  Storage medium