Patent Publication Number: US-7725031-B2

Title: Passive optical network system and ranging system thereof

Description:
INCORPORATION BY REFERENCE 
     The present application is a Continuation of U.S. application Ser. No. 11/729,974, filed Mar. 30, 2007, now, U.S. Pat. No. 7,489,869, and claims priority from Japanese application JP2006-279446 filed on Oct. 13, 2006, the entire contents of each of which are hereby incorporated by reference into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     The present application pertains to a PON (Passive Optical Network) system in which a plurality of subscriber connection devices share an optical transmission line. 
     As an optical access system, there is known the PON which makes a 1-to-n connection (n being an integer equal to or greater than 2) between an OLT (Optical Line Terminal) arranged on the station side and an ONU (Optical Network Unit) arranged on the subscriber side, by means of a device passively carrying out combining of optical signals, such as an optical splitter. A plurality of ONUs are connected to the terminals (e.g. PCs or the like) of the respective subscribers, the electrical signals from a terminal being converted into optical signals and transmitted toward the OLT. The optical splitter having received optical signals from the plurality of ONUs optically multiplexes (time division) the same optical signals. Inversely, an optical signal from the OLT is split, by means of the optical splitter, into a plurality of optical signals and transmitted toward a plurality of ONUs, each ONU selectively receiving and processing signals, from among the transmitted destined for it. 
     As mentioned above, an uplink signal transmitted from a plurality of ONUs toward an OLT is time division multiplexed by means of an optical splitter. The OLT determines/notifies the transmission timing of the optical signals with respect to the respective ONUs so that the optical signals from the plurality of ONUs do not collide, each ONU transmitting sequentially the optical signals at the notified timing. As specified in Ch. 8 and Ch. 9 of ITU-T Recommendation G.984.1, since the optical fiber length is e.g. set arbitrarily to one range from among the ranges 0-20 km, 20-40 km, and 40-60 km for each ONU, the distances between the OLT and the ONUs, i.e. the optical fiber lengths, are not necessarily equal, also leading to a difference in the transmission delay times of the optical signals transmitted from each ONU toward the OLT. Consequently, there is a need for the OLT to determine the transmission timing of the optical signals by taking into account the optical signal transmission delay times arising from the difference in the distance of each ONU. 
     In order to implement this, the OLT uses so-called ranging which is described in Ch. 10 of ITU-T Recommendation G.984.3, and by means of this, the OLT adjusts the transmission timing of the respective ONUs as if each ONU had been installed at equal distances, and the optical signals from a plurality of ONUs are made not to mutually interfere. In other words, the OLT assumes that all the ONUs are separated by just the identically same distance, determines/notifies the timing at which each ONU transmits an optical signal, and the OLT further notifies each ONU of the optical signal delay time arising from the difference between the concerned assumed distance and the distance at which each ONU is actually installed, and each ONU transmits an optical signal at a timing delayed, from the transmission timing notified from the OLT, by just the notified delay time. 
     In ranging, it is necessary for the OLT to transmit a signal for measuring the distance with respect to the ONU. When the ONU returns the distance measurement frame, the OLT receives the same signal, measures the time from the request for transmission of the signal for distance measurement until the reception of the signal for distance measurement, i.e. the roundtrip delay time, and finds out how much the ONU is separated from the OLT. Next, the OLT, in order to make all the ONUs appear to be at an equal distance, sends instructions for each ONU to delay transmission by just a time called the equalization delay (EqD). E.g., in order to make all the ONUs have a roundtrip delay time of 20 km, it indicates to the ONU an equalization delay equal to (“20 km roundtrip delay time”)−(“measured roundtrip delay time”). The ONU is provided with a circuit that transmits data with a fixed delay of just the indicated equalization delay, and by means of the aforementioned instruction, and uplink data transmission is carried out so that all the ONUs have a 20 km roundtrip delay time. 
     Also, in the Ethernet™ PON system defined in Ch. 64 of the IEEE 802.3 Standard, the aforementioned distance measurement is carried out notwithstanding the fact that no equalization delay instruction is present. Instead, after the distance measurement, in case the OLT sends a grant to the ONU, the Start value of the grant is compensated on the basis of the measured roundtrip delay time. 
     SUMMARY OF THE INVENTION 
     In ranging, the signal for distance measurement sent by the OLT is received by a plurality of ONUs, and each ONU having received this signal transmits to the OLT a response signal with respect to the same signal. Since the timing at which the response signals from each ONU at this point in time arrive at the OLT is not adjusted, there is the possibility that the OLT receives a number of response signals within a short time period. In order to prevent this, there is provided in the OLT, for a certain fixed time interval after a signal for distance measurement has been transmitted to the ONU, a non-signal domain (ranging window) devised not to receive any response signal other than the response signal first received. 
     In this way, the OLT transmits a signal for distance measurement, since there is computed for each ONU an equalization delay time for the same ONU, it is not possible, within the ranging window of one ONU, for other ONUs to transmit an optical signal with respect to the OLT. For this reason, in order to carry out ranging of an ONU located within the range of e.g. a fiber length of 0-60 km, a non-signal domain (ranging window) with a length of 600 μs corresponding to a 60 km roundtrip delay time is necessary. As mentioned above, since the distance measurement is carried out with respect to the ONUs within one ranging window, for e.g. the distance measurement of e.g. 128 ONUs, there is needed an extended non-signal domain of 76.8 ms, lengthened 128 times. Further, when the stability of the system is taken into account, it is desirable for the distance measurement to take an average over a plurality of times, and if the ranging is e.g. carried out using an average of four times the distance measurement result, the domains which cannot be utilized by the user are further lengthened by a factor 4, there being necessary an extended non-signal domain of 307.2 ms. 
     In order to suppress the loss of signal domain due to this ranging, it is acceptable to broaden the interval during which ranging is implemented and to make the loss uplink domain sufficiently small. E.g. in the aforementioned example, if the ranging interval is chosen to be 30 seconds, the portion occupied by the 307.2 ms extended non-signal domain becomes on the order of 1%, something which can be thoroughly neglected. However, in this case, there occurs the new problem that 30 seconds are required to activate all the ONUs at once. If the importance of communications service is taken into consideration, it is desirable, in order to suppress to the utmost the service interruption time resulting from a temporary failure, for the time to activate all the ONUs at once have a sufficiently small value, e.g. 1 second. 
     The present invention has for an object to provide an OLT, ONU, and PON system making possible the activation of one hundred or more ONUs while making efficient use of the band, and in a short time. 
     In order to combine a low band loss and a short activation time, it is acceptable to enable distance measurements of a plurality of ONUs inside one ranging window. 
     The aforementioned task is implemented by means of a method wherein the OLT is provided with a plurality of distance measurement circuits, receives a plurality of distance measurement signals transmitted from the plurality of ONUs inside one interval of the ranging window, validates a delimiter detection circuit directly after receiving a distance measurement signal, and carries out a reset of an automatic threshold circuit. 
     Also, as another means for attaining the task, the OLT periodically generates, regardless of the presence of an uplink ranging response in the ranging window, plurality of ATC reset pulses, and when the ONU receives a ranging request, it transmits a plurality of ranging responses at an interval which is different from an integer multiple of the aforementioned period. By transmitting a plurality of ranging responses, at least one signal is able to bring the distance measurement to success without colliding with the ATC reset pulse. 
     According to the present invention, it is possible to provide an optical access system which, while making effective use of the band, enables the activation of one hundred ONUs or more in a short time, within 1 second. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an embodiment of a PON network configuration according to the present invention. 
         FIG. 2  is a diagram showing an embodiment of a downlink PON signal frame. 
         FIG. 3  is a diagram showing an embodiment of an uplink PON signal frame. 
         FIG. 4  is a diagram showing an embodiment of an OLT functional block. 
         FIG. 5  is a diagram showing an embodiment a functional block related to ranging processing. 
         FIG. 6  is a diagram showing an embodiment of an optical signal receiving portion. 
         FIG. 7  is a diagram showing an embodiment of a functional block supplying an ATC reset. 
         FIG. 8  is a diagram showing a time chart of the first embodiment. 
         FIG. 9  is a diagram showing a time chart of the second embodiment. 
         FIG. 10  is a diagram showing a time chart of the third embodiment. 
         FIG. 11  is a diagram showing a time chart of the fourth embodiment. 
         FIG. 12  is a diagram showing the operation of an optical signal receiving portion of an OLT. 
         FIG. 13  is a diagram showing an example of ranging operation in a PON. 
         FIG. 14  is a diagram showing an embodiment of the hardware configuration of an OLT. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Below, embodiments of the present invention will be explained. 
     First Embodiment 
       FIG. 1  shows the configuration of an optical network in which the present invention is applied. 
     PON  10  is composed of an optical splitter  100 , an OLT  200  being a device on the station side installed in an office building of a telecommunications operator or the like, a trunk fiber  110  connecting OLT  200  and the optical splitter, a plurality of ONUs  300  being subscriber side devices installed inside the respective subscriber residences or in the vicinity thereof, and a plurality of branch fibers  120  respectively connecting optical splitter  100  and a plurality of ONUs  300 . OLT  200  can be connected, via trunk fiber  110 , optical splitter  100 , and branch fibers  120 , to e.g. 32 ONUs  300 . Also, user terminals such as telephones  400  and Personal Computers  410  are respectively connected to the plurality of ONUs  300 . PON  10  is connected via OLT  200  to a PSTN (Public Switched Telephone Network) or the Internet  20  and transmits and receives data to/from external networks. 
     In  FIG. 1 , five ONUs are illustrated which respectively have differing fiber lengths from OLT  200 . In  FIG. 1 , ONU  300 - 1  has a fiber length from OLT  200  of 1 km, ONU  300 - 2  has a fiber length from OLT  200  of 10 km, ONU  300 - 3  has a fiber length from OLT  200  of 20 km, ONU  300 - 4  has a fiber length from OLT  200  of 10 km, and ONU  300 - n  has a fiber length from OLT  200  of 15 km. In a signal  130  transmitted in the downlink direction from OLT  200  to ONU  300 , the respective signals destined for ONU  300  are time division multiplexed. Each ONU  300  receives signal  130 , determines whether or not the signal is destined for it, and moreover, in case the signal was one destined for it, delivers the signal to a telephone  400  or a Personal Computer  410 , based on the recipient of the signal. 
     Also, in the uplink direction from ONU  300  to OLT  200 , signal  150 - 1  transmitted from ONU  300 - 1 , signal  150 - 2  transmitted from ONU  300 - 2 , signal  150 - 3  transmitted from ONU  300 - 3 , signal  150 - 4  transmitted from ONU  300 - 4 , and signal  150 - n  transmitted from ONU  300 - n , become a signal  140  after having passed optical splitter  100  and being time division multiplexed, and reach OLT  200 . Since OLT  200  understands in advance from which ONU a signal was received at which timing, it carries out processing by identifying the signal from each ONU in response to the received timing. 
     In  FIG. 2 , there is shown an example of a downlink PON signal frame transmitted from OLT  200  to each ONU  300 . The downlink frame is composed of a frame synchronization pattern  201 , a PLOAM domain  202 , a grant instruction domain  203 , and a frame payload  204 . Grant instruction domain  203  corresponds to a so-called “US Bandwidth MAP” shown in Section 8.1.3.6 of the same Recommendation, and the OLT specifies the uplink transmission grant timing of each ONU, using this domain. The “US Bandwidth MAP” domain comprises a “Start” value designating the beginning of the transmission grant and an “End” value designating its completion, with a designation of the respective byte units being carried out. This value, having the meaning of granting transmission, is also called a grant value. 
     Further, in the individual ONUs, a plurality of band allocation units called T-CONT (Trail CONTainer) can be allocated, the designations of uplink and downlink transmission grant timing being carried out for each T-CONT. In grant instruction domain  203 , there are stored, for each T-CONT, a “Start” value expressing the timing at which the optical signal transmission starts and an “End” value expressing the timing at which the optical signal transmission ends. T-CONT is a band allocation unit in DBA and, in case e.g. the ONU has a plurality of buffers, T-CONT IDs which are identification information items concerning T-CONT, are given to the respective buffers, it also being possible to control for each buffer from the OLT. 
     The “ranging time” message in  FIG. 13  which will be subsequently described is stored in PLOAM domain  202  and a “ranging request” signal  310 - 1  and a “grant, request report” signal  320  including information as to the timing at which transmission of optical signals start in each ONU are stored in “Grant” instruction domain  203 . In frame payload  204 , user signals and the like from OLT  200  toward ONU  300  are stored. Details are described in ITU-T Recommendation G.984.3. 
     In  FIG. 3 , there is shown an example of an uplink PON frame transmitted from an ONU to the OLT. Uplink signal  150 - 1  coming from ONU  300 - 1  is composed of a preamble domain  301 , a delimiter domain  302 , a PLOAM domain  303 , a queue length domain  304 , and a frame payload  305 . The aforementioned “Start” value indicates the start position of PLOAM domain  303  and “End” value  313  indicates the end position of frame payload  305 . Immediately before each uplink signal, there is set a guard time for preventing a collision with the previous burst signal. The difference between the aforementioned “End” value and the subsequent “Start” value corresponds to the guard time which is a domain of no uplink signal. In other words, the time from the end position of frame payload  305  of the uplink signal until preamble domain  301  of the subsequent uplink signal corresponds to the guard time. Further, in the present embodiment, by detecting the signal of the delimiter domain, it is identified that the data from the delimiter domain onward are new data. In other words, the delimiter domain is used as information for identifying breaks between signals. 
     In  FIG. 4 , a configuration example of an OLT  200  according to the present invention is shown. An ONU transmission and reception part  401  is a part transmitting and receiving optical signals to and from ONU  300  which carries out processing such as converting the optical signal received from the ONU into an electrical signal by means of a optical signal reception processing part  403 , transmitting as optical signals the electrical signal inside the device by means of an optical signal transmission processing part  404 , and transmitting the signal to the ONU. Network transmission and reception part  402  carries out transmission and reception of PSTN or Internet  20  signals to and from higher-level networks. A control part  409  carries processing and the like according to the PON Protocol with respect to input and output signals. A received signal processing part  405  carries out processing such as cutting and dividing electrical signals received from optical signal reception processing part  403  into PON frames. A ranging processing part  406  carries out ranging processing to be subsequently described. Transmission granting part  407  sets the “Start” value and the “End” value of each ONU, from the values for the communication bands allocated to each ONU by means of DBA processing, and notifies each ONU of these values. A transmitted signal processing part  408  generates PON frames transmitted to each ONU. 
     In  FIG. 14 , there is shown an example of a hardware configuration of OLT  200 . OLT  200  has a control board  1400  managing the operation of the whole device and a plurality of network interface boards  1440 ,  1450 , and  1460  being respectively connected to the network and carrying out signal transmission and reception. Control board  1400  has a memory  1410  and a CPU  1420  and controls each network interface board through a hub  1430 . Each network interface board has an ONU transmission and reception part  401  and a network transmission and reception part  402  as well as a CPU  1470 , carrying out processing required for the transmission and reception of signals occurring between the ONU and the Internet or a PSTN, and a memory  1480 . A wide variety of processing types occurring in the present embodiment function e.g. by CPU  1470  executing programs stored in memory  1480 . Otherwise, as the need arises, dedicated hardware (LSI and the like) specialized in each type of processing is available, it being acceptable to execute processing by means of this. Further, the configuration of the OLT hardware is not limited hereto, it being acceptable for various devices to carry out the processing in response to an appropriate need. 
     In  FIG. 13 , there are shown the ranging signals occurring in the optical access network of the present embodiment. OLT  200  transmits a “ranging request” signal  310 - 1  toward ONU  300 - 1 . ONU  300 - 1 , after receiving “ranging request” signal  310 - 1 , transmits a “ranging response” signal  311 - 1  after a determined fixed time. OLT  200  determines, from the difference in the transmission timing of “ranging request” signal  310 - 1  and the reception timing of “ranging response” signal  311 - 1 , the distance to ONU  300 - 1 . Next, OLT  200  transmits a “ranging time” message  312 - 1  and sets an equalization delay  330 - 1  with respect to ONU  300 - 1 . By the functioning of this equalization delay  330 - 1 , regardless of the physical installation position of ONU  300 - 1 , the distance from OLT  200  is regulated as if it were 20 km. Below, the distance measurement of ONU  300 - 2  and ONU  300 - 3  is carried out in the same way. 
     After this, OLT  200  requests, by transmitting “grant and request report” signal  320  with respect to ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3 , that the requested transmission volume be notified together with giving an uplink transmission grant. Corresponding to this signal, ONU  300 - 1  transmits user data and a report  321 - 1 . In the report, the volume of uplink signals waiting for transmission inside ONU  300 - 1  is displayed with the number of bytes and notified to OLT  200 . The transmission of user data and report  321 - 1  is carried out after receiving the user data and report  321 - 1  with a timing delayed, from the instructed timing  331 - 1  based on the grant, by just equalization delay  330 - 1 . The transmission control of ONU  300 - 2  and ONU  300 - 3  is also similar, so by means of this operation, when OLT  200  receives an uplink signal, the user data from ONU  300 - 1  and report  321 - 1 , the user data from ONU  300 - 2  and report  321 - 2 , and the user data from ONU  300 - 3  and report  321 - 3 , are lined up efficiently without mutually colliding or being greatly separated and are received by OLT  200 . In this way, on the basis of the respective transmission requests of ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3 , Dynamic Bandwidth Allocation (DBA) is implemented by means of changing the volume of uplink transmission requests. 
     In case a plurality of ONU distance measurements are implemented within one ranging window, in order that the length of the preamble signal for synchronization permitted in the uplink burst signal defined in ITU-T Recommendation G.984.3 does not exceed several bytes, a manipulation applying a reset to the receiver by means of a timing known in advance is indispensable for carrying out the pullback of the identification threshold value of the uplink signal and the clock with a short preamble like this. In practice, in a steady state after ONU activation, since the arrival time of the uplink burst signal is controlled with the OLT instruction, applying a reset to the uplink receiver is simple. However, in the ranging process, since the arrival times of the distance measurement signals differ depending on the distance between the OLT and the ONU, it is not possible to apply a reset to the receiver by means of a timing known in advance. If it is an Ethernet™ PON defined in the IEEE 802.3ah standard in which a preamble with a length of several hundred bytes is permitted, signal reception is possible using fast-tracking AGC (Automatic Gain Control) and not using a reset, but in the short preamble defined in Recommendation G.984.3, no method is proposed for carrying out a distance measurement of a burst signal of several bytes within one ranging window. Since this is implemented in the present embodiment, an improvement is added to OLT ranging processing. 
     Using  FIG. 5 , an explanation will be given of the details of the functional block related to the ranging processing of the OLT  200  in the present embodiment. An optical signal received from trunk fiber  110  is converted into an electrical signal by an O/E conversion part  501  and there is carried out an identification of a value “0” or a value “1” on the basis of an appropriate threshold value in an ATC (Automatic Threshold Control) unit  503 . Subsequently, clock extraction and retiming are carried out, a delimiter detection part  504  detecting delimiter domain  302  shown in  FIG. 3  and identifying a break in the uplink signal. PON frame decomposition part  505  decomposes the uplink PON frame explained in  FIG. 3  and sends the queue length report stored in queue length domain  304  to a grant generation part  509 . Also, a distance measurement part  507  implements the distance measurement occurring in the ranging operation explained in  FIG. 13  and computes the equalization delay for each ONU. Grant generation part  509  carries out DBA processing using the queue length report from the PON frame decomposition part, determines the communication band allocated to each ONU, and generates a “Start” value and an “End” value. Moreover, this “Start” value and this “End” value are handed over to a reset timing part  506  and are also used for the reset of ATC  208 . A PON frame generation part  510  stores the signal from grand generation part  509 , based on the downlink PON frame signal format explained in  FIG. 2 , in grant instruction domain  203  and transmits it. Also, the equalization delay computed by distance measurement part  507  is also stored in the “Ranging time” message format by PON frame generation part  510  and is transmitted toward each ONU. A driver  511  converts the electrical signal from PON frame generation part  510  from a voltage to a current signal and E/O conversion part  502  converts the current signal into an optical signal and transmits it to trunk fiber  110 . 
       FIG. 6  shows a configuration example of the optical signal reception portion of the OLT in the present invention. Inside O/E conversion part  501 , an APD (Avalanche Photo Diode) connected to a high-voltage bias source  601  is given a reverse bias at a high voltage and the received optical signal is amplified and converted into a current signal by means of the avalanche effect. The converted current is converted into voltage with a TIA (Trans Impedance Amplifier)  244  composed of a resistance  604  and an amplifier  605 . Together with the voltage of the received signal being output in digital form with an A/D converter, in ATC  503 , a threshold value is set at half amplitude and a signal identified as having a value “0” or a value “1” is output. The output of an amplifier  606  has a peak detection carried out on it using the diode function from the base to the emitter of a transistor  607 , is held in a capacitor  608 , and is provided as the threshold value of an amplifier  609 . Immediately before the reception of signals from each ONU, a reset signal is provided to transistor  609  and the threshold value held in capacitor  608  is discharged and reset to the “0” level. 
     The operation of ATC at the time of implementing a distance measurement of a plurality of ONUs within one ranging window is explained by means of  FIG. 12 . The length of the preamble signal for synchronization permitted in the uplink burst signal defined in ITU-T Recommendation G.984.3 does not exceed several bytes. To carry out the pullback of the uplink signal identification threshold value and the clock pullback with a short preamble such as this, a circuit called ATC (Automatic Threshold Control) shown in  FIG. 6  is made necessary. ATC  503  detects the amplitude of the received signal for each input burst at high speed, and by inputting the same threshold value in the capacitor and holding it there, it is stable even for data with consecutive 0&#39;s and can receive. The reverse face thereof is that, as shown in  FIG. 12 , a manipulation of applying a reset to ATC  503  by means of a timing known in advance is indispensable after the burst signal has ended. If there is no reset, the threshold value is left held at the value of the previous signal, so if subsequently a smaller signal is received, the threshold value is too big and correct signal identification is not carried out. 
     Especially at the time of ranging, since the signal is returned at an earlier time and with larger amplitude the closer the ONU is, it is normal for the subsequently received signal to have an amplitude which gradually gets smaller. Since, in the steady state after ONU activation, the arrival time of the uplink burst signal is controlled by OLT assignment, it is easy to apply a reset to the aforementioned receiver. Since, however, in the ranging process, the arrival times differ for the distance measurement signals depending on the distance between the OLT and the ONU, it is not possible to apply a reset to the receiver by means of a timing known in advance and there is a need for determining a timing at which OLT  200  applies a reset to ATC  503 . 
     In  FIG. 7 , there is shown a block diagram of a reset timing generation part  506  supplying a reset signal to ATC  503 . A start edge detector  701  receives a “Start” value/“End” value form grant generation part  509  and when it is time to start reception of optical signals from the ONU, a start edge signal is generated. The present signal is used for delimiter detection validation in a normal operating state after the ONU has been activated and ATC reset. 
     On the other hand, the start and end timing of the ranging window, which is output by grant generation part  509 , is notified to a periodic timing generation part  702 , or during the interval of the ranging window, by making the signal be put in the ON state, or the like, a ranging window signal indicating the interval of the ranging window is input. Further, there is also input, into periodic timing generation part  702 , a delimiter detection notification signal indicating that a delimiter which is output by delimiter detection part  504  and included in the signal from the ONU has been detected. Periodic timing generation part  702  generates a delimiter detection validation signal which validates the processing of delimiter detection part  504  and hastens the carrying out of new delimiter detection processing and an ATC reset signal for resetting ATC  503 , if a timing detection notification signal is input while it is being indicated by means of a ranging window signal that there is currently a ranging window interval. 
     The fact of validating delimiter detection part  504  together with the resetting of ATC  503  means there is a possibility, if delimiter detection is carried out continuously, of erroneously recognizing as the delimiter a signal inside the payload, being random data, in a distance measurement signal. In order to prevent an erroneous recognition such as this, once delimiter detection part  504  detects a delimiter, the following delimiter detection operation is temporarily halted. When delimiter detection part  504  receives a delimiter detection validation signal, delimiter detection starts for the second time. In this way, even if there is currently a ranging window interval, by resetting ATC  503  whenever a delimiter signal from a different ONU is detected, it becomes possible to receive and process “ranging request” signals from a plurality of ONUs even within one ranging window. Further, after the ATC reset, reception of the following distance measurement signal becomes precisely possible. Since the distance measurement signal has as far as possible a length of several tens of seconds, the probability that a distance measurement signal from a different distance collides is small and several ONU distance measurements can be carried out within one ranging window. 
     Logical summing part  703  merges and outputs the delimiter detection validation and ATC reset signal in the aforementioned normal operating state coming from start edge detector  701 , the aforementioned delimiter detection validation signal within the aforementioned ranging window and the ATC reset signal coming from periodic timing generation part  702 , and validates delimiter detection part  504  together with resetting ATC  503 . 
     In  FIG. 8 , a time chart of the ranging processing of the present embodiment is shown. A signal generated by periodic timing generation part  702  explained in  FIG. 7  is indicated as ATC reset  802 . OLT  200  transmits toward ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3  distance measurement requests (“ranging requests”)  804 , ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3  respectively independently generating random delays  805 ,  806 , and  807  and transmitting distance measurement signals (“ranging responses”). In case ONUs with nearly equal distances are present within one PON interval, by giving random delays defined in Ch. 10 of Recommendation G.984.3 and making send distance measurement signals, the arrival times at the OLT are randomized, so the collision of distance measurement signals are stochastically avoided, making it possible to bring to success a plurality of ONU distance measurements within one ranging window. 
     OLT  200  carries out ATC reset  802 - 1  at the start position of a ranging window  808  by means of periodic timing generation part  702  and resets the threshold value of a preceding burst signal  803 . Next, OLT  200  receives a first distance measurement signal  809 , immediately thereafter carries out an ATC reset  802 - 2 , and resets the threshold value of first distance measurement signal  809 . Further, OLT  200  receives a second distance measurement signal  810 , immediately thereafter carries out an ATC reset  802 - 3 , and resets the threshold value of second distance measurement signal  810 . Next, OLT  200  receives a third distance measurement signal  811 , immediately thereafter carries out an ATC reset  802 - 4 , and resets the threshold value of third distance measurement signal  811 . 
     In this way, by receiving a distance measurement signal and immediately thereafter carrying out an ATC reset and the validation of a delimiter detection circuit, it is possible to bring a plurality of distance measurements to success within one ranging window. 
     Second Embodiment 
     As another embodiment, there may be considered one in which periodic timing generation part  702  periodically generates and outputs, during the ranging window interval, ATC resets and delimiter detection validation signals. Even in this case, it is possible for periodic timing generation part  702 , by means of a ranging window signal received from grant generation part  509 , to output the aforementioned reset signals and the like periodically just during the ranging window interval. 
     In  FIG. 9 , the time chart of the present embodiment is shown. The signals generated by periodic timing generation part  702  are indicated as ATC resets  902 . OLT  200  transmits distance measurement requests (ranging requests)  905  toward ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3 , and ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3  respectively transmit distance measurement signals (ranging responses). Specifically, ONU  300 - 1  transmits a distance measurement signal  910 , ONU  300 - 2  transmits a distance measurement signal  911 , and ONU  300 - 3  transmits a distance measurement signal  912 . 
     OLT  200  carries out an ATC reset  902 - 1  at the start position of the ranging window and resets the threshold value of a preceding burst signal  904 . Next, OLT  200  periodically carries out ATC resets  902 - 2 ,  902 - 3 ,  902 - 4 ,  902 - 5 ,  902 - 6 , and  902 - 7  at equal intervals  903 . Similarly to the example shown in  FIG. 9 , if distance measurement signals from each ONU can be received in the ATC reset interval, ranging processing with respect to a plurality of ONUs within one ranging window becomes possible. 
     Third Embodiment 
     In the method of the second embodiment, there is the possibility that an ATC reset and a distance measurement signal from an ONU collide, so at this point, the ranging processing with respect to the ONU would fail. As yet another embodiment, there may be considered a method devised so that each ONU having received a ranging request from OLT  200  replies with a plurality of ranging requests, leaving intervals in between. 
     In  FIG. 10 , the time chart of the present embodiment is shown. Regarding portions which are the same, as in the time chart of  FIG. 9 , the same reference numerals are attached. A signal generated by periodic timing generation part  702  is shown as ATC reset  902 . OLT  200  transmits distance measurement requests (ranging requests)  905  toward ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3 . ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3  respectively transmit a plurality of distance measurement signals (ranging responses). Specifically, ONU  300 - 1  transmits a distance measurement signal  910 - 2  leaving an interval  906 - 1 , after having transmitted distance measurement signal  910 - 1 , and further transmits a distance measurement signal  910 - 3  after an interval  906 - 2 . In the same way, ONU  300 - 2  transmits a distance measurement signal  911 - 1  and, after leaving an interval  907 - 1 , transmits distance measurement signal  911 - 2  and, further, after interval  907 - 2 , transmits distance measurement signal  911 - 3 . ONU  300 - 3  also transmits distance measurement signals  912 - 1 ,  912 - 2 , and  912 - 3  with intervals  908 - 1  and  908 - 2  sandwiched in between. 
     From among the distance measurement signals  910 - 1 ,  910 - 2 , and  910 - 3  transmitted from ONU  300 - 1 , distance measurement signal  910 - 1  is normally received after ATC reset  902 - 1 . Distance measurement signal  910 - 2  collides with ATC reset  902 - 2 , so reception fails. Distance measurement signal  910 - 3 , since distance measurement signal  902 - 2  and ATC reset  910 - 2  have collided, has a reset which is insufficient, but since the immediately preceding signal has been transmitted from the same ONU  300 - 1  and is a signal with the same amplitude, there is a possibility that it is received normally, even if the threshold value is insufficient. In this way, from among the three times the distance measurement signal was sent, it is received normally at least once. 
     In the same way, from among the distance measurement signals  911 - 1 ,  911 - 2 , and  911 - 3  transmitted from ONU  300 - 2 , distance measurement signal  911 - 1  is received normally after ATC reset  902 - 3 , but distance measurement signal  911 - 3  collides with distance measurement signal  912 - 2  from another ONU  300 - 3  and reception fails. Even here, from among the three times the distance measurement signal is sent, it is received normally at least once. Further, from among the distance measurement signals  912 - 1 ,  912 - 2 , and  912 - 3  transmitted from ONU  300 - 3 , distance measurement signal  912 - 1  collides with ATC reset  902 - 5  and reception fails and as for distance measurement signal  912 - 2 , it collides with distance measurement signal  911 - 3  from another ONU  300 - 2 , so reception fails, but distance measurement signal  912 - 3  is received normally after ATC reset  902 - 7 . Either way, from among the three times the distance measurement signal was sent, the signal is received normally at least once. 
     By having the respective ONUs transmit a plurality of distance measurement signals, it is avoided, although the total number of distance measurement signals rises and the probability increases that there will somewhere occur a collision, that the same ONUs are combined and collisions occur repeatedly, by changing the number of intervals of the plurality of distance measurement signals transmitted by the respective ONUs, so there is a high probability that at least once the distance measurement will succeed. In other words, taking interval  906  between transmissions of distance measurement signals by ONU  300 - 1 , interval  907  of ONU  300 - 2 , and interval  908  of ONU  300 - 3  to be different, and further, even if ONU  300 - 1  is the same, it is possible to reduce the probability of collision by taking intervals  906 - 1  and  906 - 2  to be different. 
     The intervals with which ONU  300 - 1  transmits distance measurement signals may be set autonomously by the ONU by using a serial number or an ONU-ID indicated from the OLT, or the OLT may indicate a distance measurement signal interval with respect to the ONU. Alternatively, it is also possible to use dynamically changing random values as intervals of the plurality of distance measurement signals and in this case also, the ONU may be provided with a random counter and autonomously set the intervals, or the OLT may be provided with the random counter and indicate the intervals of the distance measurement signals with respect to the ONUs. 
     Fourth Embodiment 
     In the methods of the aforementioned second and third embodiments, in case ONUs with nearly equal distances are present in one PON interval, it is possible, by conferring random delays defined in e.g. Ch. 10 of Recommendation G.984.3 and making transmit distance measurement signals, to randomize arrival times to the OLT and to stochastically avoid collision of distance measurement signals and so to bring to success a plurality of ONU distance measurements within one ranging window. However, since collision prevention based on random delays is a stochastic avoidance measure, it is good in the case of a small number of ONUs, but in case several hundred multiple ONUs are simultaneously carrying out distance measurements, there is a possibility that there will somewhere occur a collision when attempting distance measurements. An explanation will be given of an embodiment which, in order to avoid this situation, restricts the number of ONUs returning distance measurement signals by means of SN mask generation part  508  in the post stage of delimiter detection part  507  shown in  FIG. 5 . 
     In ITU-T Recommendation G.983.1, there are defined a method wherein the OLT carries out a distance measurement after indicating one eight byte long serial number being an individual ONU identification number; and a method wherein the OLT, without indicating a serial number, after having requested a signal for distance measurement, and when detecting a collision of signals from a plurality of ONUs, requests a signal for distance measurement for a second time while indicating a part of the serial number, and gradually makes adjustments so that only one signal for distance measurement is transmitted. Also, in a GPON, there is in addition to these methods defined a mechanism called random delay wherein the ONU adds a random time delay and transmits a signal for distance measurement; and a method wherein first an ONU serial number is obtained from the signal for distance measurement received at first with random delay, and subsequently, using the obtained serial number, a distance measurement is carried out after indicating one ONU. In the present embodiment as well, there is proposed a method in which, using the serial number given to each ONU, the number of ONUs replying to a ranging request message from the OLT is restricted. 
     In the present embodiment, if the need arises because distance measurement signal collisions occur frequently or the like, the SN mask generation part  508  shown in  FIG. 5  restricts the serial numbers of ONUs returning a distance measurement signal. Further, as for the condition of activating SN mask generation part  508 , it may be devised so that the control part controlling the operation of OLT  200  as a whole detects frequent occurrences of collisions and notifies SN mask generation part  508  of the activation, or it may be devised so that distance measurement part  507  counts the number of distance measurement failures and requests an activation when the counted number of failures exceeds a predetermined value. Alternatively, the condition may be devised so that SN mask generation part  508  itself counts the number of distance measurement failures and judges whether an activation is necessary or not by comparing the number against a predetermined value. 
     In  FIG. 11 , the time chart in the present embodiment is shown. OLT  200  transmits a distance measurement request (ranging request)  1101  toward ONU  300 - 1 , ONU  300 - 2 , and ONU  300 - 3 . If ONU  300 - 1  and ONU  300 - 2  respectively transmit distance measurement signals (ranging responses)  1102 - 1  and  1102 - 2 , collisions occur, and a distance measurement failure is detected by means of a CRC error of the received signal in OLT  200 . In the prior art, all the information on a distance measurement signal received in the case where an OLT had failed in a distance measurement was discarded, but in the present embodiment, at least the information including the serial number of the ONU is accumulated temporarily, from among the received distance measurement signals. The place of storing the serial number may be memory space of OLT  200 , or generation part  508  may be devised to hold the serial numbers. 
     Next, SN mask generation part  508  of OLT  200  extracts the first half of the serial number from the aforementioned accumulated distance measurement signals and outputs it to PON frame generation part  510 . PON frame generation part  510  creates a serial number mask message  1104 , described in Ch. 9 of ITU-T Recommendation G.984.3, so as to match the concerned extracted value and transmits it toward the ONU. Serial number mask message  1104  is used to make only those ONUs react whose eight-byte serial numbers with a part masked coincide. Even if it is assumed that distance measurement signals from a plurality of ONUs collide and the distance measurement fails, there is a high probability that it has been possible to correctly receive part of the serial numbers. Consequently, if the ONUs are narrowed down to those whose serial numbers match in part and a ranging request  1105  is transmitted, the reactions of e.g. ONU  300 - 2  and ONU  300 - 3  are masked, so the probability that distance measurement signal  1106  of only ONU  300 - 1  can be received becomes higher. 
     If distance measurement fails here as well, if OLT  200  transmits a ranging request  1108  after serial number mask generation part  508  has extracted the value of the first quarter of the serial number in a serial number mask message  1107  and narrowed down the ONUs, the reactions of ONU  300 - 2  and ONU  300 - 3  are masked, so the probability of being able to receive distance measurement signal  1109  of ONU  300 - 1  only is further increased. In this example, the first half or the first quarter of the serial number is extracted and a narrowing down of the ONUs is implemented, but it is acceptable for SN mask generation part  508  to use an arbitrary position of the serial number to conduct the narrowing down. 
     In the aforementioned embodiment, an explanation was given in accordance with a GPON compliant with ITU-T Recommendation G.984.3, but it can also be applied to other PON systems, e.g. an Ethernet™PON system defined in Ch. 64 of the IEEE 802.3 Standard. 
     In this way, even if it is not possible, in the present embodiment, to obtain the entire eight byte length of a serial number due to a collision, there is used a serial number mask message, defined in Recommendation G.984.3, in which the first half (four bytes) of an eight-byte serial number is input, the ranging candidate ONUs are limited, and a distance measurement is carried out for a second time. Even in the case where a collision of a plurality of distance measurement signals is generated, the probability that a part of the first half of distance measurement signal can be received normally is high if a random delay function is used in combination, it is possible, by limiting the ONUs by means of the serial number mask message, to bring a distance measurement from the limited ONUs to success with a high probability. Even in this processing, it is still possible, in case there occurs a collision of distance measurement signals, to further only use the first quarter (two bytes) of the serial number, use a serial number mask message, and attempt a distance measurement for a second time. As for the serial number limitation methods, a number of them can be considered, such as the method of reducing by one bit at a time, the method of progressively increasing the number of bits to be reduced from 1 to 2, 4, etc., and the method of using a random length for each distance measurement, but the method of gradually decreasing the length of the used serial number from 1 to ½, ¼, ⅛, etc., has the best balance between comprehensiveness and efficiency. 
     Further, the present embodiment is not limited to the second and third embodiments, since it may be used in combination with the first embodiment. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.