JP2008099299A - Pon system and ranging method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OLT, ONU and PON system capable of utilizing a band effectively and starting not less than 100 ONUs quickly. <P>SOLUTION: The OLT receives a plurality of distance measurement signals transmitted from a plurality of ONUs in one section of a ranging window region. Immediately after the distance measurement signal is received, a delimiter detection circuit is made effective for resetting an automatic threshold circuit. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a passive optical network (PON) system in which a plurality of subscriber connection devices share an optical transmission line.

  As an optical access system, optical signals such as an optical splitter are passively multiplexed / demultiplexed between an OLT (Optical Line Terminal) arranged on the station side and an ONU (Optical Network Unit) arranged on the subscriber side. There is known a PON that is connected by n to 1 (n is an integer of 2 or more) depending on the device. Each of the plurality of ONUs is connected to a subscriber's terminal (for example, a PC), and converts an electrical signal from the terminal into an optical signal and transmits it to the OLT. The optical splitter that has received optical signals from a plurality of ONUs optically (time-division) multiplex these optical signals and transmit them to the OLT. Conversely, an optical signal from the OLT is branched into a plurality of optical signals by an optical splitter and transmitted to a plurality of ONUs. Each ONU selectively receives a signal addressed to itself from the transmitted signals. To process.

  As described above, upstream optical signals transmitted from a plurality of ONUs to the OLT are time-division multiplexed by the optical splitter. The OLT determines and notifies the transmission timing of the optical signal to each ONU so that the optical signals from the plurality of ONUs do not collide, and each ONU sequentially transmits the optical signal at the notified timing. Since each ONU is arbitrarily installed in the range of optical fiber length 0 to 20 km, 20 km to 40 km to 40 km to 60 km as defined in Chapters 8 and 9 of Non-Patent Document 1, the OLT and each ONU The distances between them and the optical fiber lengths are not necessarily equal, and the transmission delay times of optical signals transmitted from each ONU to the OLT are also different. Therefore, the OLT needs to determine the transmission timing of the optical signal in consideration of the transmission delay time of the optical signal resulting from the difference in distance between the ONUs.

  To achieve this, the OLT uses the technology called ranging described in Chapter 10 of Non-Patent Document 2, which allows the OLT to set the transmission timing of each ONU as if each ONU was installed at an equal distance. Adjustment is made so that optical signals from a plurality of ONUs do not interfere with each other on the optical fiber. In other words, the OLT assumes that all ONUs are equally spaced apart by the same distance, and determines and notifies the timing at which each ONU transmits an optical signal. Further, the OLT actually sets up the assumed distance and each ONU. Each ONU is notified of the delay time of the optical signal resulting from the difference from the distance being transmitted, and each ONU transmits the optical signal at a timing delayed by the notified delay time from the transmission timing notified from the OLT.

  In ranging, the OLT requests the ONU to transmit a distance measurement signal. When the ONU returns a distance measurement frame, the OLT receives the signal, measures the time from the transmission request for the distance measurement signal to the reception of the distance measurement signal, that is, the round trip delay time, and determines how much the ONU has received from the OLT. Know if you are away. Subsequently, in order to make all ONUs appear equidistant, the OLT sends an instruction to each ONU to delay transmission for a time called an equalization delay (EqD: Equalization Delay). For example, in order to have all ONUs have a round trip delay time of 20 km, an equalization delay amount equal to “(20 km round trip delay time) − (measured round trip delay time)” is instructed to the ONU. The ONU has a circuit for transmitting data with a fixed delay by the specified equalization delay amount, and in accordance with the above instruction, upstream data transmission is performed so that all ONUs have a round-trip delay time of 20 km. .

  In the Ethernet (registered trademark) PON system defined in Chapter 64 of Non-Patent Document 3, although the distance measurement is performed, there is no instruction of equalization delay amount. Instead, when the OLT sends a grant to the ONU after the distance measurement, the start value of the grant is corrected based on the measured round trip delay time.

ITU-T Recommendation G. 984.1 ITU-T Recommendation G. 984.3 IEEE 802.3 standard

  In ranging, a distance measurement signal transmitted by the OLT is received by a plurality of ONUs, and each ONU receiving this signal transmits a response signal to the signal to the OLT. Since the timing at which the response signal from each ONU reaches the OLT at this time is not adjusted, the OLT may receive a large number of response signals in a short time. In order to prevent this, the OLT has a non-signal area (ranging window) that prevents a response signal other than the response signal received first from being received for a certain time after the distance measurement signal is transmitted to the ONU. Yes.

  In this way, the OLT transmits a distance measurement signal and calculates the ONU equalization delay time for each ONU. During ranging of one ONU, other ONUs transmit optical signals to the OLT. Can not do it. Therefore, for example, in order to perform the ranging of the ONU located in the range of the optical fiber length of 0 to 60 km, a non-signal region (ranging window) having a length of 600 microseconds corresponding to a round trip delay time of 60 km is required. As described above, since the distance measurement is performed for one ONU in one ranging window, for example, the distance measurement of 128 ONUs has a no-signal area of 76.8 milliseconds, which is 128 times. is necessary. Furthermore, considering the stability of the system, it is desirable to take multiple averages for distance measurement. For example, if ranging is performed using the average of four distance measurement results, the bandwidth that cannot be used by the user is further quadrupled. Therefore, a total no signal area of 307.2 milliseconds is required.

  In order to suppress the loss of the signal band due to the ranging window, it is only necessary to widen the interval for performing the ranging and sufficiently reduce the lost upstream band. For example, in the above example, if the ranging interval is 30 seconds, the ratio of the total non-signal area of 307.2 milliseconds is about 1% and can be sufficiently ignored. However, in this case, there arises a new problem that it takes 30 seconds to start all ONUs simultaneously. In view of the importance of communication services, it is desirable that the simultaneous activation time of all ONUs be sufficiently small, for example 1 second, in order to minimize the service interruption time caused by a temporary failure.

  An object of the present invention is to provide an OLT, ONU, and PON system that can start up 100 or more ONUs in a short time while effectively using a bandwidth.

  In order to achieve both low bandwidth loss and short start-up time, it is only necessary to enable distance measurement of a plurality of ONUs within one ranging window.

  The above problem is that the OLT includes a plurality of distance measurement circuits, receives a plurality of distance measurement signals transmitted from the plurality of ONUs within one section of the ranging window region, and receives a distance measurement signal immediately after receiving the distance measurement signals. This is realized by a method of enabling the detection circuit and resetting the automatic threshold circuit.

  As another means, the OLT periodically generates an ATC reset pulse multiple times within the ranging window regardless of whether there is an upstream ranging response, and the ONU receives a ranging request at an interval different from an integer multiple of the above period. Send multiple ranging responses. By transmitting multiple ranging responses, at least one signal does not collide with the ATC reset pulse and the distance measurement can be successful.

  According to the present invention, it is possible to provide an optical access system that allows 100 or more ONUs to be activated within one second in a short time while effectively using the bandwidth.

  Hereinafter, embodiments of the present invention will be described.

  FIG. 1 shows the configuration of an optical access network to which the present invention is applied. The PON 10 is an optical splitter 100, an OLT 200 that is a station side device installed in a station building of a communication carrier, etc., a trunk fiber 110 that connects the OLT 200 and the optical splitter, and a subscriber side installed in or near each subscriber premises. The apparatus includes a plurality of ONUs 300 serving as apparatuses, and a plurality of branch fibers 120 respectively connecting the optical splitter 100 and the plurality of ONUs 300. The OLT 200 can be connected to, for example, 32 ONUs 300 via the trunk fiber 110, the optical splitter 100, and the branch fiber 120. Further, user terminals such as a telephone 400 and a personal computer 410 are connected to each of the plurality of ONUs 300. The PON 10 is connected to a PSTN (Public Switched Telephone Networks) or the Internet 20 via the OLT 200, and transmits / receives data to / from an external network.

  FIG. 1 shows five ONUs, each having a different fiber length from the OLT 200. In FIG. 1, the ONU 300-1 has a fiber length of 1 km from the OLT 200, the ONU 300-2 has a fiber length of 10 km from the OLT 200, the ONU 300-3 has a fiber length of 20 km from the OLT 200, and the ONU 300-4 has a fiber length from the OLT 200. The 10 km ONU 300-n has a fiber length of 15 km from the OLT 200. In the signal 130 transmitted from the OLT 200 in the downstream direction of the ONU 300, signals addressed to the respective ONUs 300 are time-division multiplexed. Each ONU 300 receives the signal 130 and determines whether or not the signal is addressed to itself. If the signal is addressed to itself, the ONU 300 sends a signal to the telephone 400 or the personal computer 410 based on the signal destination. To deliver.

  In the upstream direction from the ONU 300 to the OLT 200, the signal 150-1 transmitted from the ONU 300-1, the signal 150-2 transmitted from the ONU 300-2, the signal 150-3 transmitted from the ONU 300-3, and the ONU 300-4 Transmitted signal 150-4. The signal 150-n transmitted from the ONU 300-n is time-division multiplexed after passing through the optical splitter 100 to become the signal 140 and reaches the OLT 200. Since the OLT 200 knows in advance at which timing the signal from which ONU is received, the OLT 200 identifies the signal from each ONU according to the received timing and performs processing.

  FIG. 2 shows an example of a downstream PON signal frame transmitted from the OLT 200 to each ONU 300. The downstream frame includes a frame synchronization pattern 201, a PLOAM area 202, a grant indication area 203, and a frame payload 204. The grant indication area 203 corresponds to an area called US Bandwidth MAP shown in the recommendation 8.1.3.6, and the OLT uses this area to specify the uplink transmission permission timing of each ONU. The US Bandwidth MAP area includes a Start value that specifies the start of transmission permission and an End value that specifies the end, and each byte is specified. This value is also called a grant value in the sense that transmission is permitted.

  Each ONU can be assigned a plurality of band allocation units called T-CONT (Trail Container), and the uplink transmission permission timing is designated for each T-CONT. In the grant indication area 203, for each T-CONT, a start value indicating the timing for starting transmission of the optical signal and an End value indicating the timing for ending transmission of the optical signal are stored. T-CONT is a bandwidth allocation unit in DBA. For example, when an ONU has a plurality of transmission buffers, T-CONT ID, which is T-CONT identification information, is assigned to each buffer, and buffered from the OLT. It is also possible to control every time.

  A ranging time message in FIG. 13 to be described later is stored in the PLOAM area 202, and a grant request request signal 320 including information on the timing of the ranging request signal 310-1 and an optical signal to be transmitted to each ONU is granted. It is stored in the instruction area 203. In the frame payload 204, a user signal or the like from the OLT 200 to the ONU 300 is stored. For details, see ITU-T Recommendation G. 984.3.

  FIG. 3 shows an example of an upstream PON signal frame transmitted from the ONU to the OLT. The upstream signal 150-1 from the ONU 300-1 includes a preamble area 301, a delimiter area 302, a PLOAM area 303, a queue length area 304, and a frame payload 305. The Start value indicates the start position of the PLOAM area 303, and the End value 313 indicates the end position of the frame payload 305. A guard time is set immediately before each upstream signal to prevent collision with the previous burst signal. The difference between the above End value and the next Start value is an upstream no-signal area and corresponds to the guard time. That is, the guard time is from the end position of the frame payload 305 of the upstream signal to the start position of the preamble area 301 of the next upstream signal. In the present embodiment, the data after the delimiter area is identified as new data by detecting the signal of the delimiter area. That is, the delimiter area is used as information for identifying a signal and a signal delimiter.

  FIG. 4 shows a configuration example of the OLT 200 according to the present invention. The ONU transmission / reception unit 401 transmits / receives an optical signal to / from the ONU 300, converts the optical signal received from the ONU by the optical signal reception processing unit 403, and converts the optical signal in the apparatus by the optical signal transmission processing unit 404. Processing such as transmitting a signal to an optical signal and transmitting it to the ONU is performed. The network transmission / reception unit 402 transmits / receives signals to / from a higher-order network such as the PSTN or the Internet 20. The control unit 409 performs processing according to the PON protocol for input / output signals. The reception signal processing unit 405 performs processing such as dividing the electrical signal received from the optical signal reception processing unit 403 into PON frames. The ranging processing unit 405 performs a ranging process described later. The transmission permission unit 407 sets the Start value and End value of each ONU from the value of the communication band assigned to each ONU by DBA processing, and notifies each ONU of these values. The transmission signal processing unit 408 generates a PON frame to be transmitted to each ONU.

  FIG. 14 shows an example of the hardware configuration of the OLT 200. The OLT 200 includes a control board 1400 that manages the operation of the entire apparatus, and a plurality of network interface boards 1440, 1450, and 1460 that are connected to a network and transmit and receive signals. The control board 1400 includes a memory 1410 and a CPU 1420, and controls each network interface board via the HUB 1430. Each network interface board includes an ONU transmission / reception unit 401, a network transmission / reception unit 402, and a CPU 1470 and a memory 1480 that perform processing necessary for transmission / reception of signals between the ONU and the Internet or PSTN. Various processes in this embodiment function by, for example, the CPU 1470 executing a program stored in the memory 1480. Alternatively, dedicated hardware (LSI or the like) specialized for each process may be prepared as necessary, and the process may be executed by this. Note that the hardware configuration of the OLT is not limited to this, and various implementations may be performed as necessary.

  FIG. 13 shows a ranging signal in the optical access network of this embodiment. The OLT 200 transmits a ranging request signal 310-1 to the ONU 300-1. After receiving the ranging request signal 310-1, the ONU 300-1 transmits a ranging response signal 311-1 after a predetermined fixed time. The OLT 200 determines the distance to the ONU 300-1 from the difference between the transmission timing of the ranging request signal 310-1 and the reception timing of the ranging response signal 311-1. Subsequently, the OLT 200 transmits a ranging time message 312-1 to set an equalization delay amount 330-1 for the ONU 300-1. By the action of the equalization delay amount 330-1, the ONU 300-1 is adjusted so that the distance from the OLT 200 is 20 km regardless of the physical installation position. Thereafter, distance measurement of the ONU 300-2 and ONU 300-3 is similarly performed.

  Thereafter, the OLT 200 transmits a grant and request report signal 320 to request the ONU 300-1, ONU 300-2, and ONU 300-3 to give upstream transmission permission and notify the transmission request amount. In response to this signal, the ONU 300-1 transmits user data and a report 321-1. In the report, the amount of upstream signal waiting for transmission in the ONU 300-1 is displayed in bytes and notified to the OLT 200. The transmission of the user data and the report 321-1 is performed at a timing delayed by the equalization delay amount 330-1 from the instruction timing 331-1 by the grant after receiving the user data and the report 321-1. The same applies to the transmission control of the ONU 300-2 and ONU 300-3. With this operation, when the OLT 200 receives an uplink signal, the user data and report 321-1 from the ONU 300-1 and the user data and report from the ONU 300-2 are received. 321-2, user data from ONU 300-3 and report 321-3 are efficiently aligned and received by OLT 200 without colliding with each other and without significantly separating. In this way, dynamic bandwidth allocation (DBA) is performed by changing the amount of uplink transmission permission based on the transmission requests of the ONU 300-1, ONU 300-2, and ONU 300-3.

  When performing distance measurement of a plurality of ONUs within one ranging window, ITU-T Recommendation G. Since the length of the synchronization preamble signal allowed for the uplink burst signal specified in 984.3 is only a few bytes, the identification threshold of the uplink signal and the clock are drawn with such a short preamble. The operation of resetting the receiver at a known timing is indispensable. Actually, in the steady state after the ONU is activated, the arrival time of the upstream burst signal is controlled by the designation of the OLT, so that it is easy to reset the receiver. However, in the ranging process, since the arrival time of the distance measurement signal differs depending on the distance between the OLT and the ONU, the receiver cannot be reset at a timing known in advance. In the case of an Ethernet (registered trademark) PON defined by IEEE 802.3ah that allows a long preamble of several hundreds of bytes, it is possible to receive a signal without using a reset using an AGC that follows a high speed. No method has been proposed for measuring the distance of a plurality of burst signals within a single ranging window with a short preamble defined in 984.3. In the present embodiment, in order to realize this, the OLT ranging process is improved.

  Details of functional blocks related to the ranging process of the OLT 200 in this embodiment will be described with reference to FIG. The optical signal received from the trunk fiber 110 is converted into an electric signal by the O / E conversion unit 501, and 0 value or 1 value is identified by an appropriate threshold value by an ATC (Automatic Threshold Control) 503. Thereafter, clock extraction and retiming are performed, and the delimiter detection unit 504 detects the delimiter area 302 shown in FIG. The PON frame decomposition unit 505 decomposes the upstream PON frame described with reference to FIG. 3 and sends the queue length report stored in the queue length area 304 to the grant generation unit 509. The distance measurement unit 507 performs distance measurement in the ranging operation described with reference to FIG. 13 and calculates an equalization delay amount for each ONU. The grant generation unit 509 performs DBA processing using the queue length report from the PON frame decomposition unit, determines a communication band to be allocated to each ONU, and generates a Start value and an End value. In addition, the Start value and the End value are passed to the reset timing generation unit 506 and used for resetting the ATC 208. The PON frame generation unit 510 stores the signal from the grant generation unit 509 in the grant indication area 203 based on the PON downlink frame signal format described with reference to FIG. The equalization delay amount calculated by the distance measurement unit 507 is also stored in the format of the Raising time message by the PON frame generation unit 510 and transmitted to each ONU. The driver 511 converts the electrical signal from the PON frame generation unit 510 from voltage to current, and the E / O unit conversion unit 502 converts the current signal into an optical signal and transmits it to the trunk fiber 110.

  FIG. 6 shows a configuration example of the optical signal receiving portion of the OLT in the present invention. In the O / E conversion unit 501, an APD (Avalanche Photo Diode) connected to a high voltage bias source 601 is reverse-biased with a high voltage, and a received optical signal is amplified by an avalanche effect and converted into a current. The converted current is converted into a voltage by a TIA (Trans Impedance Amplifier) 244 including a resistor 604 and an amplifier 605. The voltage of the received signal is digitally output by the A / D converter, and the ATC 503 sets a threshold value to ½ of the amplitude, and outputs a signal identified as 0 or 1 value. The output of the amplifier 606 is detected by a peak value using a diode function from the base to the emitter of the transistor 607 and held in the capacitor 608, and provided as a threshold value of the amplifier 609. Immediately before receiving a signal from each ONU, a reset signal is given to the transistor 609, and the threshold value held in the capacitor 608 is discharged and reset to 0 level.

  The operation of the ATC when performing distance measurement of a plurality of ONUs within one ranging window will be described with reference to FIG. ITU-T Recommendation G. The length of the synchronization preamble signal allowed for the upstream burst signal specified in 984.3 is only a few bytes. A circuit called ATC (automatic threshold control) shown in FIG. 6 is required to perform upstream signal identification threshold and clock pull-in with such a short preamble. The ATC 503 detects the amplitude of the received signal at high speed for each input burst, and inputs and holds the threshold value in the capacitor, so that even data with continuous zero values can be stably received. On the other hand, as shown in FIG. 12, it is indispensable to reset the ATC 503 at a known timing after the burst signal ends. If there is no reset, the value of the previous signal is kept as it is, and if a smaller signal is subsequently received, the threshold value is too large for correct signal identification.

  In particular, during ranging, a signal is returned with a larger amplitude at an earlier time as the ONU is closer, so that the amplitude of a subsequently received signal is gradually decreasing. In the steady state after the ONU is activated, the arrival time of the upstream burst signal is controlled by the designation of the OLT, so that it is easy to reset the receiver. However, in the ranging process, since the arrival time of the distance measurement signal differs depending on the distance between the OLT and the ONU, the OLT 200 resets the ATC 503 because the receiver cannot be reset at a known timing. Timing needs to be determined.

  FIG. 7 shows a configuration diagram of a reset timing generation unit 506 that supplies a reset signal to the ATC 503 in this embodiment. The start edge detection unit 701 receives the Start value / End value from the grant generation unit 509 and generates a start edge signal when it is time to start receiving an optical signal from the ONU. This signal is used for delimiter detection validation and ATC reset in the normal operation state after the ONU is activated.

  On the other hand, the period timing generation unit 702 is notified of the start and end timings of the ranging window output from the grant generation unit 509, or the signal is turned on during the range of the ranging window. A ranging window signal indicating the period is input. Further, a delimiter detection notification signal indicating that the delimiter included in the signal from the ONU output from the delimiter detection unit 504 is detected is input to the cycle timing generation unit 702. When the delimiter detection notification signal is input while the ranging window signal indicates that the period is in the range of the ranging window, the cycle timing generation unit 702 activates the process of the delimiter detection unit 504 and performs a new delimiter detection process. A delimiter detection enabling signal for urging to perform and an ATC reset signal for resetting the ATC 503 are generated.

  The reason that the delimiter detection unit 504 is validated together with the reset of the ATC 503 is that if the delimiter detection is always performed, a signal in the payload, which is random data in the distance measurement signal, may be erroneously recognized as a delimiter. In order to prevent such erroneous recognition, the delimiter detection unit 504 temporarily stops the subsequent delimiter detection operation once the delimiter is detected. When the delimiter detection unit 504 receives the delimiter detection validation signal, it starts delimiter detection again. As a result, even during the ranging window period, each time a delimiter signal from a different ONU is detected, the ATC 503 is reset to receive and process ranging request signals from multiple ONUs within one ranging window. It becomes possible to do. After the ATC reset, the next distance measurement signal can be received immediately. Since the distance measurement signal has a length of about several tens of nanoseconds at most, the possibility of collision of distance measurement signals from different distances is small, and distance measurement of a plurality of ONUs is performed within one ranging window.

  The OR operation unit 703 receives the delimiter detection validation and ATC reset signal in the normal operation state from the start edge detection unit 701, and the delimiter detection validation signal and ATC in the ranging window from the period timing generation unit 702. The reset signals are merged and output to reset the ATC 503 and enable the delimiter detection unit 504.

  FIG. 8 shows a time chart of the ranging process of the present embodiment. A signal generated by the cycle timing generation unit 702 described with reference to FIG. 7 is indicated as an ATC reset 802. The OLT 200 transmits a distance measurement request (ranging request) 804 to the ONUs 300-1, 300-2, and 300-3, and the ONUs 300-1, 300-2, and 300-3 transmit random delays 805, 806, and 807, respectively. It is generated independently and a distance measurement signal (ranging response) is transmitted. If there are ONUs of approximately equal distance in one PON section, G. By providing a random delay specified in Chapter 10 of 984.3 and transmitting the distance measurement signal, the arrival time at the OLT is randomized to avoid the collision of the distance measurement signal in a probabilistic manner. Within a distance measurement of a plurality of ONUs.

  The OLT 200 performs ATC reset 802-1 at the start position of the ranging window 808 by the cycle timing generation unit 702, and resets the threshold value of the previous burst signal 803. Subsequently, the OLT 200 receives the first distance measurement signal 809 and immediately thereafter performs an ATC reset 802-2 to reset the threshold value of the first distance measurement signal 809. Further, the OLT 200 receives the second distance measurement signal 810, and immediately after that, performs an ATC reset 802-3 to reset the threshold value of the second distance measurement signal 810. Subsequently, the OLT 200 receives the third distance measurement signal 811 and immediately thereafter performs an ATC reset 802-4 to reset the threshold value of the third distance measurement signal 811.

  As described above, the distance measurement signal is received and immediately after that, the ATC reset and the delimiter detection circuit are validated, so that a plurality of distance measurements can be successfully performed within one ranging window.

  As another embodiment, it can be considered that the cycle timing generator 702 periodically generates and outputs an ATC reset signal and a delimiter detection enable signal during the ranging window period. Also in this case, the periodic timing generation unit 702 can output the reset signal and the like periodically only during the ranging window period by the ranging window signal received from the grant generation unit 509.

  FIG. 9 shows a time chart of this embodiment. A signal generated by the cycle timing generation unit 702 is indicated as an ATC reset 902. The OLT 200 transmits a distance measurement request (ranging request) 905 to the ONUs 300-1, 300-2, and 300-3, and each of the ONUs 300-1, 300-2, and 300-3 transmits a distance measurement signal (ranging response). Send. Specifically, the ONU 300-1 transmits a distance measurement signal 910, the ONU 300-2 transmits a distance measurement signal 911, and the ONU 300-3 transmits a distance measurement signal 912.

  The OLT 200 performs an ATC reset 902-1 at the starting position of the ranging window to reset the threshold value of the previous burst signal 904. Subsequently, the OLT 200 periodically performs ATC resets 902-2, 902-3, 902-4, 902-5, 902-6, and 902-7 at equal intervals 903. If a distance measurement signal from each ONU can be received at an ATC reset interval as in the example shown in FIG. 9, ranging processing for a plurality of ONUs can be performed within one ranging window.

  In the method of the second embodiment described above, there is a possibility that the ATC reset and the distance measurement signal from the ONU collide, and at this time, the ranging process for the ONU fails. As yet another example, a method is conceivable in which each ONU that receives a ranging request from the OLT 200 returns a plurality of ranging responses at intervals.

  FIG. 10 shows a time chart of this embodiment. The same parts as those in the time chart of FIG. 9 are denoted by the same reference numerals. A signal generated by the cycle timing generation unit 702 is indicated as an ATC reset 902. The OLT 200 transmits a distance measurement request (ranging request) 905 to the ONUs 300-1, 300-2, and 300-3. Each of the ONUs 300-1, 300-2, and 300-3 transmits a plurality of distance measurement signals (ranging responses). Specifically, after transmitting the distance measurement signal 910-1, the ONU 300-1 transmits the distance measurement signal 910-2 with an interval 906-1, and after the interval 906-2, the distance measurement signal 910- 3 is transmitted. Similarly, after transmitting the distance measurement signal 911-1, the ONU 300-2 transmits the distance measurement signal 911-2 after an interval 907-1, and further transmits the distance measurement signal 911-3 after the interval 907-2. To do. The ONU 300-3 also transmits distance measurement signals 912-1, 912-2, and 912-3 while sandwiching the intervals 908-1 and 908-2.

  Of the distance measurement signals 910-1, 910-2, and 910-3 transmitted from the ONU 300-1, the distance measurement signal 910-1 is normally received after the ATC reset 902-1. The distance measurement signal 910-2 collides with the ATC reset 902-2 and reception fails. The distance measurement signal 910-3 is insufficiently reset because the distance measurement signal 902-2 collides with the ATC reset 910-2, but the signal having the same amplitude is transmitted from the same ONU 300-1 as the previous signal. Therefore, there is a possibility that the threshold value is received normally even if it is insufficiently reset. Thus, at least one of the three distance measurement signals is normally received.

  Similarly, among the distance measurement signals 911-1, 911-2, and 911-3 transmitted from the ONU 300-2, the distance measurement signal 911-1 is normally received after the ATC reset 902-3, and the distance measurement signal 911 is received. -3 collides with a distance measurement signal 912-2 from another ONU 300-3, and reception fails. Again, at least one of the three distance measurement signals is normally received. Furthermore, among the distance measurement signals 912-1, 912-2, and 912-3 transmitted from the ONU 300-3, the distance measurement signal 912-1 collides with the ATC reset 902-5 and reception fails, and the distance measurement signal 912-2 collides with a distance measurement signal 911-3 from another ONU 300-2 and reception fails, and the distance measurement signal 912-3 is normally received after ATC reset 902-7. Again, at least one of the three distance measurement signals is normally received.

  Although each ONU transmits a plurality of distance measurement signals, the total number of distance measurement signals increases and the possibility of a collision occurring somewhere increases. However, the interval between the plurality of distance measurement signals transmitted by each ONU is increased. By changing, it is possible to avoid the occurrence of repeated collisions with the same ONU combination, and it is highly possible that at least one distance measurement will be successful. That is, the interval 906 at which the ONU 300-1 transmits the distance measurement signal, the interval 907 of the ONU 300-2, and the interval 908 of the ONU 300-3 are different from each other. By making 906-2 different, the possibility of a collision can be lowered.

  The interval at which the ONU 300-1 transmits the distance measurement signal may be set autonomously by the ONU using the serial number or the ONU-ID specified by the OLT, or the interval of the distance measurement signal from the OLT to the ONU. May be instructed. Alternatively, it is also possible to use a dynamically changing random value as an interval between a plurality of distance measurement signals. In this case as well, the ONU may be provided with a random counter and set the interval autonomously, or the OLT may be random. A counter may be provided to indicate the distance measurement signal interval to the ONU.

  In the above-described methods according to the second and third embodiments, when ONUs that are substantially equidistant exist within one PON section, for example, G.I. By providing a random delay specified in Chapter 10 of 984.3 and transmitting the distance measurement signal, the arrival time at the OLT is randomized to avoid the collision of the distance measurement signal in a probabilistic manner. Within a distance measurement of a plurality of ONUs. However, since collision prevention by random delay is a probabilistic avoidance measure, it is good if there are a small number of ONUs. However, every time hundreds of ONTs perform distance measurement simultaneously, distance measurement is attempted. A collision can occur somewhere. In order to avoid this situation, an embodiment will be described in which the number of ONUs that send back distance measurement signals is limited by the SN mask generation unit 508 subsequent to the delimiter detection unit 507 shown in FIG.

  ITU-T Recommendation G. In 983.1, the OLT specifies one 8-byte serial number, which is the individual identification number of the ONU, and performs distance measurement, or after requesting a distance measurement signal without specifying the serial number When a collision of signals from a plurality of ONUs is detected, a method for requesting a distance measurement signal again while designating a part of the serial number and adjusting so that only one distance measurement signal is transmitted Is stipulated. In addition to these, GPON defines a mechanism called a random delay in which ONU adds a random time delay and transmits a distance measurement signal. First, a distance measurement signal received first by a random delay is provided. A method is defined in which a serial number of an ONU is acquired from the ID, and then a distance is measured after one ONU is specified using the acquired serial number. This embodiment also proposes a method of limiting the number of ONUs returned to the ranging request message from the OLT using the serial number assigned to each ONU.

  In this embodiment, when a need arises due to frequent collisions of distance measurement signals, the SN mask generation unit 508 shown in FIG. 5 limits the serial number of the ONU that returns the distance measurement signals. The condition for starting the SN mask generation unit 508 may be such that the control unit that controls the overall operation of the OLT 200 detects the occurrence of frequent collisions and notifies the SN mask generation unit 508 of the start, or distance measurement. The unit 507 may count the number of distance measurement failures, and request activation when the counted number of failures exceeds a predetermined value. Alternatively, the SN mask generation unit 508 itself may count the number of distance measurement failures and determine whether or not the activation is necessary by comparing with a predetermined threshold value.

  FIG. 11 shows a time chart in the present embodiment. The OLT 200 transmits a distance measurement request (ranging request) 1101 to the ONUs 300-1, 300-2, and 300-3. When the ONUs 300-1 and 300-2 transmit distance measurement signals (ranging responses) 1102-1 and 1102-2, a collision occurs, and the OLT 200 detects a distance measurement failure due to a CRC error of the received signal. In the prior art, the OLT discards all information of the received distance measurement signal when the distance measurement fails, but in this embodiment, information including at least the serial number of the ONU is temporarily stored in the received distance measurement signal. To do. The place where the serial number is stored may be the memory space of the OLT 200, or the generation unit 508 may hold the serial number.

  Subsequently, the SN mask generation unit 508 of the OLT 200 extracts the previous half value of the serial number from the accumulated distance measurement signal and outputs it to the PON frame generation unit 510. The PON frame generation unit 510 sets the ITU-T recommendation G.3 so as to match the extracted value. The serial number mask message 1104 described in Chapter 9 of 984.3 is created and transmitted to the ONU. The serial number mask message 1104 is used for masking a part of the 8-byte serial number and causing only the matching ONU to respond to the distance measurement instruction. Even if distance measurement signals from a plurality of ONUs collide and distance measurement fails, it is highly possible that some serial numbers have been received correctly. Therefore, if the ONU that matches a part of the serial number is narrowed down and the ranging request 1105 is transmitted, for example, the reactions of the ONU 300-2 and ONU 300-3 are masked, and there is a probability that the distance measurement signal 1106 of only the ONU 300-1 can be received. Rise.

  If the distance measurement fails again, the SN mask generation unit 508 further extracts the value of the previous quarter by the serial number mask message 1107 and narrows down the matching ONU, and then the OLT 200 sends the ranging request 1108. When transmitted, the reactions of the ONU 300-2 and ONU 300-3 are masked, and the probability that the distance measurement signal 1109 of only the ONU 300-1 can be received is further increased. In this example, the ONU is narrowed down by extracting the previous half or the previous quarter of the serial number, but the SN mask generation unit 508 narrows down using an arbitrary position of the serial number. good.

  The above embodiments are described in ITU-T Recommendation G. Although described along the 984.3 GPON, it is also applicable to other PON systems, for example, an Ethernet (registered trademark) PON system defined in Chapter 64 of the IEEE 802.3 standard.

  As described above, in this embodiment, even when the serial number of all 8 bits length cannot be acquired due to the collision, the first half (4 bytes) of the 8-byte serial number is input. The range candidate ONU is limited using the serial number mask message defined in 984.3, and distance measurement is performed again. Even if multiple distance measurement signal collisions occur, if the random delay function is used together, it is highly possible that the first half of the distance measurement signal can be received normally. Distance measurement from a given ONT can be made with high probability. If the distance measurement signal collision still occurs in this process, the distance measurement can be attempted again by using the serial number mask message using only the first quarter part (2 bytes) of the serial number. There are many ways to limit the serial number, such as a method of reducing one bit at a time, a method of increasing the number of bits to be reduced to 1, 2, 4, and a method of using a random length for each distance measurement. The method of reducing the serial number length to 1, 1/2, 1/4, 1/8,... Has the best balance between completeness and efficiency.

  The present embodiment is not limited to the second and third embodiments, and may be used in combination with the first embodiment.

The figure which shows one Example of the network structure of PON. The figure which shows one Example of a downstream PON signal frame. The figure which shows one Example of an upstream PON signal frame. The figure which shows one Example of the functional block of OLT. The figure which shows one Example of the functional block relevant to a ranging process. The figure which shows one Example of an optical signal receiving part. The figure which shows one Example of the functional block which supplies reset to ATC. The figure which shows the time chart of a 1st Example. The figure which shows the time chart of a 2nd Example. The figure which shows the time chart of a 3rd Example. The figure which shows the time chart of a 4th Example. The figure which shows operation | movement of the optical signal receiving part of OLT. The figure which shows an example of the ranging operation | movement in PON. The figure which shows one Example of the hardware constitutions of OLT.

Explanation of symbols

10 PON
200 OLT
300 ONU
501 O / E conversion unit 504 delimiter detection unit 506 reset timing generation unit 507 distance measurement unit 508 SN mask generation unit 702 period timing generation unit

Claims (6)

A distance for connecting a station-side communication device and a plurality of subscriber-side communication devices via an optical multiplexer / demultiplexer, and for measuring a distance between the station-side communication device and each of the plurality of subscriber-side communication devices By transmitting a measurement request signal to the plurality of subscriber side communication devices, the station side communication device receives a distance measurement signal that is a response to the distance measurement request signal from the plurality of subscriber side communication devices, individually In the optical communication system for calculating the transmission delay time of the optical signal from the subscriber side communication device,
The station side communication device is:
A threshold control unit for identifying a voltage level of the distance measurement signal converted from an optical signal to an electrical signal;
A signal detection unit for detecting a break of the distance measurement signal from the threshold control unit;
A transmission permission unit that determines a timing at which transmission of an optical signal is permitted for each of the subscriber side communication devices;
A plurality of reset signals instructing the threshold value control unit to reset the voltage level while being notified that distance measurement is being performed with the subscriber side communication device from the transmission permission unit. A reset timing generation unit for sending
The subscriber-side communication device returns a plurality of the distance measurement signals when receiving the distance measurement request signal from the station-side communication device. The optical communication system according to claim 1,
The optical communication system, wherein the plurality of reset signals transmitted by the reset timing generation unit are periodically transmitted. The optical communication system according to claim 2,
The subscriber side communication device periodically returns a plurality of the distance measurement signals,
The optical communication system, wherein a period of the reset signal transmitted from the reset timing generation unit and a period of the distance measurement signal returned from the subscriber side communication device are different from each other. The optical communication system according to claim 1,
The station side communication device has means for storing identification numbers given to the plurality of subscriber side communication devices, and each of the plurality of distance measurement signals is sent to each of the subscriber side communication devices. An optical communication system, wherein a reply cycle is determined based on the identification number, and the determined cycle is notified to each of the subscriber communication devices. The optical communication system according to claim 1,
The station side communication device has a calculation means for outputting a random value, and the random value output by the calculation means is used as a period for returning each of the plurality of distance measurement signals one by one. An optical communication system characterized by notifying a person side communication device. The optical communication system according to claim 4 or 5,
The optical communication system is PON (Passive Optical Network),
The station-side communication apparatus notifies the subscriber-side communication apparatus of the period using a grant indication area of a downstream PON frame.

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