JP5767183B2 - PON system and link disconnection prevention method thereof - Google Patents

Description

  In the present invention, light is transmitted between a plurality of subscriber terminal devices (optical network units, hereinafter abbreviated as “ONU”) and one station side communication device (optical line terminal, hereinafter abbreviated as “OLT”). The present invention relates to a technique for preventing a link disconnection in a PON (passive optical network) system that performs point-to-multipoint communication via a fiber transmission line.

  A configuration example of the PON system is shown in FIG. The PON system includes an OLT (101), a plurality of ONUs (102), a splitter connecting them, a connector (105), and an optical fiber transmission line (106). In the example of FIG. 1, the splitter has two stages of a 4-branch splitter (103) and an 8-branch splitter (104). Point-to-multipoint communication is performed between the OLT (101) and the plurality of ONUs (102). In this configuration diagram, the state of switching the optical fiber transmission line (106-1) to the optical fiber transmission line (106-2) is also shown. When the optical fiber transmission line is switched, the optical signal is momentarily interrupted.

  As a typical standard of the gigabit class PON, there is EPON (Ethernet (registered trademark) PON) standardized by IEEE802.3. Non-Patent Document 1 discloses a configuration of OLT and ONU in EPON. Also, 10G-EPON is standardized as a standard for 10 gigabit class PON. On the other hand, there is GPON (gigabit-capable PON) standardized by ITU-T. Further, XG-EPON is standardized as a standard for 10 gigabit class PON in ITU-T.

  A configuration of a conventional OLT is shown in FIG. The OLT (201) is a PON-IF (PON-interface) port (202) connected to the ONU via the optical fiber transmission line (106), a signal processing for transferring the signal received from the service node to the ONU, and the ONU. It includes a PON signal processing unit (203) that performs signal processing for transferring a received signal to a service node, an SNI (service node interface) port (204) connected to the service node, and a control unit (205) that performs monitoring control. It consists of The downlink main signal transferred from the service node (not shown) is transferred toward the ONU via the SNI port (204), the PON signal processing unit (203), and the PON-IF port (202). In general, a downstream signal of a PON is broadcast to each ONU via an optical fiber transmission line (106), and thus is encrypted by a PON signal processing unit (203). The control unit (205) designates the transmission start time so that the upstream signal from each ONU does not collide due to the PON configuration (configuration in which the optical fiber transmission line (106) is shared by a plurality of ONUs). To do. For example, in EPON, a protocol called MPCP (multipoint control protocol) is used, and the OLT (101) transmits a GATE frame in which the transmission start time of the upstream signal is transmitted to each ONU (102), and each ONU ( 102) transmits the data frame and the REPORT frame to the OLT (101) at the time designated by the GATE frame. The upstream frame transmitted by the ONU is received by the PON-IF port (202) and transferred to a service node (not shown) via the PON signal processing unit (203) and the SNI port (204).

  A functional block diagram of the ONU is shown in FIG. The ONU (301) is connected to the OLT through a UNI (user network interface) port (302) connected to the communication terminal, a PON signal processing unit (303) that performs signal transmission / reception processing, and an optical fiber transmission line (106). It includes a PON-IF port (304) and a control unit (305) that performs ONU monitoring control. The downlink main signal is transferred from the OLT to a communication terminal (not shown) via the PON-IF port (305), the PON signal processing unit (303), and the UNI port (302). When the downstream signal is encrypted by the OLT (201), the PON signal processing unit (303) performs a decryption process. The upstream main signal is transferred from the communication terminal to the OLT via the UNI port (302), the PON signal processing unit (303), and the PON-IF port (304).

  When an OLT (101) and an ONU (102) communicate with each other, and the transmission path is switched for the optical fiber transmission path (106), the optical signal is physically interrupted for an instant (physical signal disconnection). Will occur. At this time, the time during which communication is disabled is not limited to the momentary interruption time, but will be described with reference to FIGS. 4 and 5. FIG. 4 shows a processing flow executed by the control unit (205) of the OLT after an instantaneous interruption of the optical signal occurs. FIG. 5 shows a processing flow executed by the control unit (305) of the ONU after an instantaneous interruption of the optical signal occurs.

  A processing flow of the OLT control unit (205) in FIG. 4 will be described. Here, EPON will be described as an example. First, after the occurrence of a momentary interruption (S101), the OLT determines whether to detect Timestamp Drift Error. (S102). This is because the OLT calculates the round trip time (RTT) between the OLT and each ONU every time a REPORT frame is received, and when the RTT changes over 192 ns, the Timestamp Shift Error is calculated. Detect (IEEE802.3 standard). Generally, when the optical fiber transmission line is switched, the transmission line distance may change greatly. In this case, the RTT between the OLT and the ONU changes. For example, assuming that the speed of an optical signal traveling in an optical fiber is 200,000 km / s, when the transmission path distance changes beyond 19.2 m (= 200,000 km / s × 192 ns / 2), the Timestamp Shift Error It becomes.

  When the OLT detects the Timestamp Drift Error, it deregisters the ONU (S104). It may also occur when the REPORT frame from the ONU does not reach the OLT. When a state where the REPORT frame does not reach the OLT has elapsed for 1 second, MPCP Timeout (S103) is obtained. At this time, the OLT unregisters the corresponding ONU (S104). In order to re-register an ONU that has been deregistered, an MPCP Discovery process (S105) is executed between the OLT and the ONU, and the ONU is in a state in which transmission permission is given.

  Next, OAM (operations, administration, and maintenance) Discovery processing (S106) is executed, and the OAM frame for maintenance operation management can be transmitted and received. Next, an authentication function process (S107) is executed, and the OLT confirms whether the connected ONU is a device to be connected. After a series of registration processes are executed, communication between the OLT and the ONU becomes possible, communication is resumed (S108), and the normal state is restored (S109). Even when the optical signal is momentarily interrupted, if the instantaneous interruption time is very short and the change in transmission distance is small, the Timestamp Shift Error or MPCP Timeout may not be detected. In this case, the normal state is maintained (processing that transitions from S103 to S109).

  A processing flow of the ONU control unit (305) in FIG. 5 will be described. First, when an instantaneous interruption of an optical signal occurs (S201), the optical signal intensity falls below a specified value, and LOS (loss of signal) is detected at the PON-IF port (S202). Further, even when the optical signal intensity does not decrease sufficiently and LOS is not detected, since the transmission path distance has changed after switching, the Timestamp (OLT local time) described in the GATE frame greatly deviates from the local time of the ONU, and the Timestamp Drift There may be an error (S203). Note that LOS detection is defined by G-PON.

  Even if the transmission line distance does not change, the ONU may enter a free-running mode based on an internal clock when the optical signal is momentarily interrupted. At this time, even when the instantaneous interruption of the optical signal is resolved, a difference between the local time of the OLT and the local time of the ONU occurs in the ONU, and the ONU detects Timestamp Drift Error. Here, IEEE 802.3 stipulates that when the difference between the local time of the OLT and the local time of the ONU exceeds 128 ns, the ONU becomes a Timestamp Drift Error. In IEEE802.3, the clock error of each device is allowed to be + -100 ppm. Therefore, assuming that the clock errors of the OLT and the ONU are each +100 ppm, there is a possibility that the time stamp drift error will occur at the shortest interruption of 640 μs (= 128 ns ÷ (100 ppm + 100 ppm)). On the other hand, if the ONU cannot receive the MPCP frame from the OLT for more than 1 second, it detects the MPCP timeout (S204).

  When any of LOS detection, Timestamp Shift Error, and MPCP timeout is applied, the state transits to a logical link disconnection state (S205). In other cases, the ONU cannot detect that the optical signal is momentarily interrupted, and the process returns to the normal state (S210). In the logical link disconnection state (S205), the optical signal is recovered immediately by only a momentary disconnection, and processing for resuming communication is executed between the OLT and the ONU. This registration process is executed between the OLT and the ONU, and corresponds to the OLT processes S105, S106, and S107. Even in the ONU, the MPCP Discovery sequence (S206), the OAM Discovery sequence (S207), and the authentication function process (S208) are executed. After these processes S206 to S208 are completed, the OLT and the ONU resume communication (S209). When this process is completed, the process returns to the normal state (S210) in which communication is possible.

  After the momentary interruption of the optical signal occurs and the logical link is broken, the communication remains disabled for the time until the conduction of the main signal resumes. The logical link disconnection time is a time required for the processes S206 to S208, and the communication service is stopped for a much longer time than the time when the optical signal is interrupted.

JP 2007-74645 A

Tsutsumu Tatsuta, Noriyuki Oota, Noriki Miki and Kiyomu Kumozaki, “Design philosophy and performance of GE-PON system. 6, no. 6, pp. 689-700, 2007.

  In order to reduce the impact on communication services, even if the optical fiber transmission line switching is automated and the time that the optical signal is momentarily interrupted is reduced to a few milliseconds, the time it takes to determine that the optical signal is interrupted is short. There arises a problem that the communication service is stopped for a time much longer than the instantaneous interruption time. Further, even when an OLT redundancy configuration is adopted, an instantaneous interruption occurs when switching to another OLT, but the ONU re-registration process takes time, and thus there is a problem that high-speed redundancy switching cannot be realized.

  Further, when the switching of the optical fiber transmission line is executed, if the transmission line distance before and after the switching is different, the RTT changes to a specified value or more, thereby generating a Timestamp Shift Error. As a result, even if the switching can be performed at a higher speed than the detection of the LOS signal, the ONT is unregistered and the logical link is broken because the RTT changes to a regulation value or more.

  In addition, when an encryption method (Patent Document 1) using time information for encryption is used at the time of frame encryption, the received frame cannot be decrypted because the RTT has changed. In this case, the GATE frame itself sent from the OLT cannot be decoded, the time information cannot be updated, and the logical link is broken.

  In the present invention, when the optical signal is instantaneously interrupted, even if the optical signal is instantaneously interrupted, a protection time is provided when shifting to the registration deletion / logical link disconnection state. We decided to reduce the impact on communications.

  In addition, the present invention provides an OLT with a link disconnection prevention function for transmitting a control frame in order to immediately correct the RTT from the OLT to the ONU when the length of the optical fiber transmission line changes due to switching. After the control frame is received and the control frame is received, the effect on communication is reduced by executing a link disconnection prevention procedure that promptly corrects the RTT and resumes communication.

  Specifically, the present invention is a PON system that performs point-to-multipoint communication between an OLT and a plurality of ONUs via an optical fiber transmission line, and the OLT and the ONU are linked. A PON system is characterized in that a link disconnection is determined with a protection time after detecting an optical signal reception disconnection or a change in the distance of an optical fiber transmission line by setting a disconnection prevention mode.

  Specifically, the present invention is a method for preventing link breakage in a PON system that performs point-to-multipoint communication between an OLT and a plurality of ONUs via an optical fiber transmission line. The ONU detects a link disconnection with a protection time after detecting a reception disconnection of an optical signal or a change in the distance of an optical fiber transmission line by setting a link disconnection prevention mode. It is a prevention method.

  According to the present invention, even if an instantaneous interruption of an optical signal occurs, the influence on communication is reduced.

  In the present invention, the ONU sets the link disconnection prevention mode according to an instruction from the OLT, and the ONU in the link disconnection prevention mode detects an optical signal reception interruption after detecting an optical signal reception interruption. It is preferable to cancel the link disconnection prevention mode when the signal reception recovery is detected or when the local time of the ONU is reset within the protection time after detecting the change in the distance of the optical fiber transmission line.

  According to the present invention, even if an instantaneous interruption of an optical signal occurs, the influence on communication is reduced.

  In the present invention, the OLT preferably sets the link disconnection prevention mode for each ONU.

  According to the present invention, in a PON system that accommodates a plurality of ONUs, even when the optical fiber transmission line between the splitter and the ONU is troublesome and transferred, a measure for preventing a link break can be applied to each individual ONU.

  In the present invention, the OLT determines that the link is broken when a REPORT frame is received from the ONU as a response to the GATE frame that indicates the frame transmission start time to the ONU, and the link is recovered when the REPORT frame is recovered. It is preferable to determine recovery.

  According to the present invention, it is possible to easily determine whether to maintain the link with the ONU.

  In the present invention, it is preferable that the OLT deletes the registration of the ONU when there is no reception recovery of the REPORT frame from the ONU instructed to set the link disconnection prevention mode within the protection time.

  According to the present invention, by deleting the registration of the ONU, an opportunity for reconnection can be given to the ONU at an early stage.

  In the present invention, the OLT transmits an RTT correction command for measuring the RTT between the OLT and the ONU to the ONU, and performs link recovery by receiving the REPORT frame corresponding to the RTT correction command from the ONU. It is preferable to determine.

  According to the present invention, even when the transmission line distance changes before and after the instantaneous interruption, it can be easily handled.

  The present invention preferably uses a Discovery Gate frame of the EPON standard as the RTT correction command.

  According to the present invention, even when the transmission line distance changes before and after the instantaneous interruption, it can be easily handled.

  In the present invention, it is preferable that the OLT transmits the RTT correction command to the ONU in the link disconnection prevention mode while maintaining communication with the ONU in a normal state.

  According to the present invention, a mixture of ONUs in a normal state and ONUs in the link breakage prevention mode can be allowed.

  In accordance with the present invention, in order to maintain the link state and prevent the communication service from being completely stopped when the optical signal is interrupted due to the switching of the transmission path or the replacement of the optical component, the communication service such as the telephone is provided. Line switching is possible even when the user is in use.

It is a figure explaining the structure of a PON system. It is a figure explaining the functional block of OLT. It is a figure explaining the functional block of ONU. It is a figure explaining the conventional OLT flow at the time of a momentary interruption generation | occurrence | production. It is a figure explaining the conventional ONU flow at the time of instantaneous interruption generation | occurrence | production. It is a figure explaining the link disconnection prevention procedure 1 of OLT. It is a figure explaining the link disconnection prevention procedure 1 of ONU. It is a figure explaining the sequence at the time of the switching at the time of a momentary disconnection generation | occurrence | production. It is a figure explaining the link disconnection prevention procedure 2 of OLT. It is a figure explaining the link disconnection prevention procedure 2 of ONU. It is a figure explaining the sequence at the time of the switch which requires RTT correction | amendment. It is a figure explaining the link disconnection prevention procedure 2 of OLT.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(First embodiment)
As a first embodiment of the present invention, a link disconnection prevention procedure will be described. This procedure is executed in the control unit (205) of the OLT (101, 201) and the control unit (305) of the ONU (102, 301) in the PON system of FIG. 1, FIG. 2, or FIG. In this PON system, the link disconnection executed by the OLT control unit (205) against the instantaneous disconnection that occurs when the optical fiber transmission line (106-1) is switched to the optical fiber transmission line (106-2). FIG. 6 shows the prevention procedure, and FIG. 7 shows the link disconnection prevention procedure executed in the control unit (305) of the ONU.

  A procedure for preventing link disconnection of the OLT in FIG. 6 will be described. The OLT instructs to switch from the normal state to the link disconnection prevention mode for each ONU that is affected by the instantaneous disconnection due to the switching before the optical fiber transmission line is switched. The procedure of FIG. 6 shows a case where only one ONU is affected by a momentary interruption due to switching of the optical fiber transmission line. However, when a plurality of ONUs are affected, for all affected ONUs Will be instructed.

  First, the link disconnection prevention mode is set, and this procedure is started (S301). The OLT monitors whether a REPORT frame is received as a response to the GATE frame sent to the ONU. If the OLT does not receive the REPORT frame within the specified time, it is determined that an instantaneous interruption of the optical signal has occurred with the ONU (S302). In the IEEE 802.3 standard, a report timeout value of 50 ms is specified as the specified time. That is, the ONU is defined to transmit a REPORT frame at least once every 50 ms. Although the IEEE 802.3 standard may be adopted, detection can be performed in a short time if the time is set to a short time considering RTT. When the OLT receives the REPORT frame, the logical link is not broken, so the communication with the ONU is executed in the same way as in the normal state (S303), and the link break that is issued after the switching of the optical fiber transmission line is completed. It is confirmed whether there is an instruction to end the prevention mode (S304). If there is an end command, the process shifts to the normal state and this procedure is terminated. If there is no end command, the processing is re-executed from the REPORT frame reception confirmation (S302).

  In the process S302, when the OLT does not receive the REPORT frame, the OLT determines that there is an instantaneous interruption, and starts a timer for measuring the time until the communication is restored (S306). The OLT continuously transmits a GATE frame to the ONU, and executes communication similar to the normal state (S307). In order to confirm whether or not the communication is possible, it is determined whether a REPORT frame from the ONU has been received as a response to the GATE frame transmitted from the OLT (S308). When the OLT receives the REPORT frame, it is in a communicable state and the process proceeds to S304. If an instruction to end the link disconnection prevention mode is issued in the process S304 (S304), the process transits to the normal state and the series of processes ends (S305). In step S304, if the command to end the link breakage prevention mode is not issued, the process proceeds to step S302 to repeat the process in preparation for a case where a momentary break occurs again.

  In step S308, if the OLT has not received the REPORT frame from the ONU, the communication has not been recovered, so it is confirmed whether the protection time, which is the maximum time waiting for recovery, has elapsed (S309). If the time has not elapsed, the process returns to step S307. When the protection time has elapsed, since the recovery of communication cannot be expected, the OLT cancels the registration of the ONU and disconnects the logical link (S310). The ONU re-registration process is performed, and this process is the same procedure as in the conventional example. Processing for resuming communication is performed between the OLT and the ONU. This registration process is executed between the OLT and the ONU. In the OLT, an MPCP Discovery sequence (S311), an OAM Discovery sequence (S312), and an authentication function process (S313) are executed. After the registration sequence is completed, the OLT and the ONU resume communication (S314). When this process is completed, the process returns to the normal state in which communication is possible (S305), and the series of processes ends.

  Next, a link disconnection prevention procedure executed by the ONU control unit (305) shown in FIG. 7 will be described. The ONU that is affected by the switching of the optical fiber transmission path is instructed by the OLT to shift to the link disconnection prevention mode in advance of the switching of the optical fiber transmission path. When a plurality of ONUs are affected by the switching of the optical fiber transmission line, the OLT instructs each ONU to shift to the link disconnection prevention mode. The normal state is shifted to the link disconnection prevention mode, and this procedure is started (S401). When the optical signal is continuously transmitted from the OLT to the ONU and the optical fiber transmission line is switched, the ONU immediately detects the LOS (S402).

  If the intensity of the optical signal does not decrease below the specified value, or if switching has not been performed, LOS is not detected. In this case, the ONU executes normal communication processing for transmitting the REPORT frame and the data frame at the transmission time described in the GATE frame transmitted from the OLT (S403). Thereafter, the ONU confirms whether an instruction to end the link break prevention mode has been issued (S404), and if an end instruction has been issued, transitions to the normal mode and ends the process (S405). If no end command has been issued, the process restarts from the LOS detection process (S402).

  In the process S402, when detecting any one of the LOS, the time stamp drift error, and the MPCP timeout, the ONU performs a process of waiting for the protection time in order to wait for the recovery of the logical link state (S406). Thereafter, the ONU confirms the logical link state (S407). If the ONU has recovered, the communication process is resumed in order to resume communication (S403). The subsequent processing is the same as described above.

  In step S407, when the ONU confirms that the communication state has not recovered, it checks whether the protection time, which is the maximum time waiting for recovery, has elapsed (S408). If it has not elapsed, the processing is executed again from step S406. If the protection time has elapsed, the ONU determines that the communication state has not been recovered, and disconnects the logical link as in the unregistered state (S409). The ONU re-registration process is performed. Since this process is executed in cooperation with the OLT, the procedure corresponds to the OLT. This registration processing is executed between the OLT and the ONU, and the ONU executes an MPCP Discovery sequence (S410), an OAM Discovery sequence (S411), and an authentication function process (S412). After the registration sequence is completed, the OLT and the ONU resume communication (S413). When this process is completed, the process returns to a normal state in which communication is possible (S405), and the series of processes ends.

  When the OLT link disconnection prevention procedure 1 in FIG. 6 is executed in the OLT, and the ONU link disconnection prevention procedure 1 in FIG. 7 is executed in the ONU, it is affected by the switching of the optical fiber transmission line. The sequence at the time of switching will be described with reference to FIG. FIG. 8 illustrates a state in which a GATE frame, a REPORT frame, and a data frame are exchanged as communication processing between the OLT and the ONU.

  First, an instruction for transition to the link interruption prevention mode (S301) for the OLT and an instruction for transition to the link interruption prevention mode (S401) for the switching ONU affected by the switching are issued, and the OLT and the ONU shift to the link interruption prevention mode. (801). Even after the transition to the link disconnection prevention mode, normal communication processing is performed, but switching of the optical fiber transmission path is executed halfway (802). The GATE frame from the OLT does not reach the switching ONU due to the instantaneous interruption of the optical signal generated by this switching (803). For a non-switching ONU that is not affected by switching, a GATE frame from the OLT arrives as usual and communication is executed (804). The instantaneous interruption caused by the switching is recovered immediately, and the frame returns to the switching ONU (805). Thereafter, the GATE frame transmitted from the OLT to the ONU also arrives at the switching ONU (806). Based on the transmission start time described in the GATE frame, communication between the OLT and the ONU is resumed (807). Thereafter, the operator managing the switching of the optical fiber transmission path instructs the OLT and the switching ONU to return from the link disconnection prevention mode to the normal state after completing the switching work, and completes all the work ( 808).

  The processing described so far is processing for one ONU. However, there are cases where a plurality of ONUs are affected by switching due to switching of optical fiber transmission lines. At that time, the OLT executes the OLT link disconnection prevention procedure 1 of FIG. 6 for each ONU for all ONUs affected by the switching. Each ONU executes the ONU link disconnection prevention procedure 1 in FIG.

(Second embodiment)
In the second embodiment of the present invention, in the link break prevention mode, the procedure when the time stamp drift error is detected because the transmission path distance has changed due to the switching of the optical fiber transmission path and the RTT has changed. Show. When the time information is used for encryption or when the ONU detects the time stamp drift error as a result of the ONU entering the free-running clock mode when the optical signal is momentarily interrupted, the GATE detects the time stamp drift error before detecting the time stamp drift error. The frame cannot be decoded, the transmission start time and current time information cannot be obtained, and communication is disabled. In such a case, a link disconnection prevention procedure including a procedure for transmitting an RTT correction command to update time information from the OLT to the switching ONU will be described.

  The link disconnection prevention procedure 2 executed in the OLT control unit (205) will be described with reference to FIG. This procedure shows OLT processing for an ONU that is affected by switching of an optical fiber transmission line. The ONU that is affected by the switching of the optical fiber transmission line executes this procedure corresponding to each individual. First, the OLT is instructed to transition to the link break prevention mode, and the procedure is started (S501). The OLT monitors the arrival of the REPORT frame as a response to the GATE frame in order to recognize that the communication with the ONU has been instantaneously interrupted. In addition, the OLT also monitors the dying gas of the OAM frame defined in IEEE 802.3 in order to distinguish whether the communication interruption is due to the power interruption of the ONU or the switching of the optical fiber transmission line. When the REPORT frame and the OAM frame (Dying gas notification) are not received within a certain period, it is determined that the communication has been cut off (S502). If it is received and it is determined that the communication is not interrupted, it is confirmed whether an instruction to end the link disconnection prevention mode is issued (S503). If an end command is issued and entered, a series of processing is terminated, and a transition is made to the normal state (S504). If no end command has been issued, the process restarts from step S502.

  In step S502, if the OLT cannot confirm the reception of the REPORT frame or the dying gasp and determines that the communication is disconnected, a process for preventing a link disconnection is executed. First, the normal state communication process (S505) is performed. This is a process for performing communication for ONUs other than the ONU affected by the switching of the optical fiber transmission line. In this process, a communication process between the OLT and the ONU in the normal state is performed. Subsequently, an RTT correction command is sent from the OLT to correct the RTT of the ONU (S506). As a RTT correction command, for example, if a Discovery Gate frame defined in IEEE 802.3 is used, it is a broadcast frame and is not encrypted. Therefore, all ONUs connected to the OLT can be received simultaneously and encrypted. The time information used for conversion can also be corrected. When the ONU side receives the RTT correction command and the correction process is completed, the ONU sends a REPORT frame to the OLT. The OLT confirms the arrival of this REPORT frame (S507). If the OLT can confirm the REPORT frame, a series of processing (S509 and subsequent steps) for resuming communication with the ONU is performed. If the REPORT frame cannot be confirmed, it is confirmed whether the transmission processing of the RTT correction command is performed a predetermined number of times (S508). If the specified number of times has not been implemented, the processing is executed again from step S505. If it has been executed the specified number of times, this process is terminated, and the OLT determines that the communication restart with the ONU has failed, and executes the processes after S511.

  A process after the OLT confirms the REPORT frame in the process S507 will be described. The OLT corrects the RTT of the ONU in order to specify the transmission start time for the ONU (S509). Next, normal communication processing is performed, and the OLT resumes communication with the ONU (S510). Thereafter, it is confirmed whether an instruction to end the link breakage prevention mode has been issued (S503). If an end instruction has been issued, a series of processing is terminated (S504). If no end command has been issued, the process restarts from step S502.

  In the process S508, when the OLT fails to resume communication with the ONU, the OLT deregisters the ONU and determines that the logical link is broken (S511). After determining that the logical link is broken, the ONU re-registration process is performed. This process is the same as the conventional procedure. Processing for resuming communication is performed between the OLT and the ONU. This registration process is executed between the OLT and the ONU, and an MPCP Discovery sequence (S512), an OAM Discovery sequence (S513), and an authentication function process (S514) are executed with the OLT. When the registration sequence is completed, the OLT and the ONU resume communication (S515). When this process is completed, the process returns to the normal state in which communication is possible (S504), and the series of processes ends.

  Next, the link disconnection prevention procedure 2 executed in the control unit (305) of the ONU will be described with reference to FIG. This procedure shows processing in an ONU that is affected by switching of an optical fiber transmission line. The OLT instructs the ONU to transition to the link break prevention mode. In response to this instruction, the ONU starts the first procedure (S601). The ONU monitors whether LOS is detected (S602), whether Timestamp Drift Error is not detected, and whether MPCP timeout is exceeded (S603). In any case, in order to perform the logical link recovery processing, the processing from S606 is executed. If none of these applies, the communication is continued, and it is confirmed whether a command to end the link disconnection prevention mode has been issued (S604). If an end command is issued and entered, a transition is made to the normal state, and a series of processing is completed (S605). If no termination command has been issued, the process restarts from step S602 in preparation for the occurrence of an instantaneous interruption due to switching of the optical fiber transmission line.

  A logical link recovery process that corresponds to one of the processes S602 and S603 and is executed after the communication is cut off will be described. The ONU whose communication has been cut off due to the effect of switching of the optical fiber transmission line waits for the protection time in order to wait for the communication to recover (S606), and confirms whether the optical signal has recovered (S607). ). If the optical signal is recovered, processing for communication recovery after S609 is executed. If the optical signal has not been recovered, it is checked whether the recovery of the optical signal has been waited for the maximum protection time. If the maximum protection time has not elapsed, the processing is executed from step S606. If the maximum protection time has elapsed, the process of deleting the registration (S615) and subsequent steps are executed.

  When the optical signal is recovered, the ONU waits for reception of an RTT correction command transmitted from the OLT (S609). If the RTT correction command cannot be received, the next timing is waited. It is confirmed whether the reception confirmation of the RTT correction command (S609) has been executed a specified number of times (S610). If it has not been executed the specified number of times, it waits for the specified time, which is the time for the OLT to communicate with another ONU, in order to wait for the reception of the RTT correction command again (S611). Thereafter, the process is executed again from the process S609. If the specified number of times has been executed in step S610, it is determined that reception of the RTT correction command has failed, registration is deleted in step S615 and subsequent steps, and a procedure for executing re-registration processing to resume communication is resumed.

  When receiving the RTT correction command in S609, the ONU corrects the time based on the information transferred by the correction command (S612). Next, a REPORT frame is transmitted to the OLT to notify the OLT that communication is possible. (S613). Thereafter, a GATE frame is transmitted from the OLT, and the ONU starts processing for performing normal communication (S614). It is confirmed whether an instruction to end the link break prevention mode has been issued (S604). If an end command is issued and entered, a transition is made to the normal state, and a series of processing is completed (S605). If no end command has been issued, the process restarts from step S602 in preparation for the occurrence of an instantaneous interruption due to switching.

  If the optical signal is not recovered in process S608, or if the RTT correction command is not received in process S610, the ONU is deregistered and the logical link is disconnected (S615). The ONU re-registration process is performed, and this process is the same procedure as in the conventional example. Processing for resuming communication is performed between the OLT and the ONU. This registration process is executed between the OLT and the ONU, and an MPCP Discovery sequence (S616), an OAM Discovery sequence (S617), and an authentication function process (S618) are executed with the OLT. After the registration sequence is completed, the OLT and the ONU resume communication (S619). When this process is completed, the process returns to the normal state in which communication is possible (S605), and the series of processes ends.

  The OLT link disconnection prevention procedure 2 in FIG. 9 is executed in the OLT and the ONU link disconnection prevention procedure 2 in FIG. 10 is executed in the ONU, so that the switching ONU is affected by the switching of the optical fiber transmission line. A state in which the logical link disconnection is prevented by the correction will be described with reference to FIG. FIG. 11 illustrates a state in which a GATE frame, a REPORT frame, and a data frame are exchanged as communication processing between the OLT and the ONU.

  First, an instruction for transition to the link break prevention mode for the OLT (S501) and an instruction for transition to the link break prevention mode for the switching ONU affected by the switching (S601) are issued, and the mode shifts to the link break prevention mode (1101). Even after the transition to the link disconnection prevention mode, normal communication processing is performed, but switching of the optical fiber transmission line is executed at the timing of (1102). In this switching, an instantaneous interruption of the optical signal and a change in the transmission path distance occur due to the switching (1103). In the PON system using the common key cipher communication of Patent Document 1, the downlink frame cannot be decrypted by the switching ONU, and the encrypted GATE frame cannot be received in the PON system using the common key encryption communication of Patent Document 1 ( 1104). The switching ONU cannot read the encrypted GATE frame and cannot transmit the REPORT frame. Therefore, the OLT determines that no REPORT frame has been received (1105). At this time, the OLT stops transmitting the GATE frame to all connected ONUs. Then, until the transmission of the uplink frame assigned to the non-switching ONU is waited immediately before the transmission of the GATE frame is stopped, the RTT of the switching ONU is corrected to enter the RTT correction state (1106). This is to prevent the uplink frame allocated to the non-switching ONU and the REPORT frame responding to the RTT correction command from colliding immediately before stopping the transmission of the GATE frame.

  Note that the time to wait to transmit all the uplink frames allocated to the non-switching ONU is the maximum value of the transmission permission start time allocated in one GATE frame (implementation-dependent) and the maximum value of the transmission permission amount (implementation-dependent). It should be larger than the sum of.

  After entering the RTT correction state (1106), the OLT transmits an RTT correction command to the switching ONU (1107). In FIG. 11, the above Discovery GATE frame is transmitted as the RTT correction command. Since the Discovery GATE frame is a broadcast frame, the time of a plurality of switching ONUs can be corrected with one frame. The switching ONU receives the RTT correction command and corrects the time (1108). The switching ONU transmits a REPORT frame to the OLT as a response to the Discovery GATE frame. Here, since the non-switching ONU is already registered in the OLT, the Discovery GATE frame is discarded. The OLT that has received the REPORT frame corrects the RTT of the switching ONU and becomes ready for communication (1109). From this RTT correction state, a normal communication state is established in which other ONUs and the OLT communicate (1110). However, in the case of this figure, the communication state of the switching ONU has already been recovered, and the switching ONU can receive the GATE frame in which the OLT has been transmitted (1111). At the transmission start time described in the GATE frame, communication is resumed from the switching ONU to the OLT (1112). Thereafter, when a series of switching processes is completed, a command to stop the link disconnection prevention process is instructed to the OLT and the switching ONU, and the normal communication state is resumed, and all operations are completed (1113). .

  The processing described so far is processing for one ONU. However, there are cases where a plurality of ONUs are affected by switching due to switching of optical fiber transmission lines. At that time, the OLT performs the OLT link disconnection prevention procedure 2 of FIG. 9 for each ONU for all ONUs affected by the switching. Each ONU executes the ONU link disconnection prevention procedure 2 of FIG.

(Third embodiment)
In the third embodiment of the present invention, there is shown a link disconnection prevention method for simultaneously performing RTT correction of ONUs whose transmission path distances have changed before and after switching in switching of optical fiber transmission lines shared by a plurality of ONUs. In the case of switching of optical fiber transmission lines shared by a plurality of ONUs, it is considered that the amount of change in transmission line distance before and after switching is the same for all ONUs that have been switched. That is, if the change amount of the RTT of one ONU among the ONUs that have been switched can be measured, the RTTs of the other ONUs that have been switched can be corrected based on the RTT change amount. In this embodiment, the RTT correction command transmission performed for each ONU in the second embodiment is performed only for one ONU, the amount of change in the RTT is calculated, and for other ONUs, This is a method of making a transition to the normal state earlier than the link break prevention procedure of the second embodiment by performing RTT correction using the amount of change of the RTT.

  The link disconnection prevention procedure 2 executed in the OLT control unit (205) will be described with reference to FIG. This procedure shows the OLT process for an ONU that is affected by the switching of the optical fiber transmission line, and this procedure is executed for each ONU that is affected by the switching. First, the OLT is instructed to transition to the link disconnection prevention mode, and the procedure is started (S701). The OLT monitors the arrival of a REPORT frame serving as a response to the GATE frame so that it can be determined that the communication with the ONU has been disconnected. The OLT also monitors the dying gas of the OAM frame defined in IEEE 802.3 in order to distinguish whether the communication disconnection is due to ONU power supply disconnection or switching. When the REPORT frame and the OAM frame (Dying gasp notification) do not arrive within a certain period, it is determined that the communication has been cut off (S702). If it is determined that the communication is not interrupted, it is checked whether an instruction to end the link disconnection prevention mode has been issued (S703). If an end instruction is issued, a series of processing is terminated (S704). The process restarts from step S702.

  In process 702, if the OLT cannot confirm the reception of the REPORT frame and determines that the communication is disconnected, a process for preventing the link disconnection is executed. First, normal communication processing (S705) is performed. This is a process for performing communication for ONUs other than the ONU affected by the switching. In this process, a normal communication process between the OLT and the ONU is performed. Subsequently, it is confirmed whether the RTT correction has already been performed by another ONU (S706). If the RTT correction has not been performed yet, an RTT correction command is transmitted from the OLT (S707).

  When the ONU side receives the RTT correction command and completes the correction process, a REPORT frame is sent from the ONU to the OLT. The OLT confirms the arrival of this REPORT frame (S708). If the REPORT frame is confirmed, the RTT of the ONU affected by the switching is corrected. Further, the amount of change in RTT before and after correction of the ONU is calculated, and correction is performed by adding the amount of change in RTT to all ONUs affected by switching in the ONU (S709). When the correction is completed, a series of processing (S710) for resuming communication with the ONU is performed.

  If the OLT cannot confirm the REPORT frame in step S708, RTT correction is performed individually for each ONU in steps S712 to S721 (steps S505 to S515 in FIG. 9). If the RTT correction is not successful within the specified number of times, it is determined that the communication restart with the ONU has failed, and the processing after S717 is executed.

  Processing in the case where the RTT has already been corrected by another switching ONU in processing S706 will be described. In order to confirm whether the RTT corrected by another switching ONU is really correct, the OLT allocates an upstream band for the REPORT frame of the ONU once using the corrected RTT, and sets the transmission start time and length. It is described in the GATE frame and transmitted to the ONU. Then, the OLT confirms that a REPORT frame is returned from the ONU (S711). When the REPORT frame does not collide with an upstream signal of another ONU and the OLT receives the REPORT frame, the RTT is updated using the Timestamp described in the REPORT frame and the Timestamp described in the GATE frame (S715).

  On the other hand, in the conditional branch S711, the ONU cannot receive the GATE frame and cannot transmit the REPORT frame, or can receive the GATE frame, but the REPORT frame is changed to another ONU due to the RTT corrected by another switching ONU. If the OLT cannot receive the REPORT frame as a result of collision with an upstream signal, RTT correction is performed individually for each ONU in the processing of S712 to S721 (the same processing as S505 to S515 in FIG. 9). If the RTT correction is not successful within the specified number of times, it is determined that the communication restart with the ONU has failed, and the processing after S717 is executed.

  The process described so far is the OLT process for a plurality of ONUs. The process of each ONU may be performed by executing the ONU link disconnection prevention procedure 2 of FIG. 10 of the second embodiment.

  The present invention can be applied to the information communication industry.

101, 201: Station side terminal equipment (OLT)
102, 301: Subscriber termination unit (ONU)
103, 104: Branch splitter 105: Connector 105, 106: Optical fiber transmission line 202: PON-IF port 203: PON signal processing unit 204: SNI port 205: Control unit 302: UNI port 303: PON signal processing unit 304: PON -IF port 305: control unit

Claims (14)

One station side communication device (optical line terminal, hereinafter abbreviated as “OLT”) and a plurality of subscriber terminal devices (optical network unit, hereinafter abbreviated as “ONU”) are connected via an optical fiber transmission line. A PON (passive optical network) system that performs point-to-multipoint communication,
The OLT and the ONU determine the link break with a protection time after detecting the reception break of the optical signal or the change in the distance of the optical fiber transmission line by the setting of the link break prevention mode .
The ONU sets the link disconnection prevention mode according to an instruction from the OLT,
The ONU in the link disconnection prevention mode detects an optical signal reception interruption, detects an optical signal reception recovery within the protection time, or detects a change in the distance of the optical fiber transmission line and then within the protection time. When the local time of the ONU is reset, the link disconnection prevention mode is canceled . The PON system according to claim 1 , wherein the OLT sets the link break prevention mode for each ONU. The OLT determines that the link is disconnected when the REPORT frame is received from the ONU as a response to the GATE frame instructing the ONU to start frame transmission, and determines that the link recovery is performed when the REPORT frame is recovered. The PON system according to claim 1 or 2 , wherein The OLT within the protection time, the link if no reception recovery of the REPORT frame from ONU that indicate the setting of the cross-sectional prevention mode, claim 3, characterized in that deletes the registration of the ONU The PON system described in 1. The OLT transmits an RTT correction command for measuring a transmission trip time (RTT) between the OLT and the ONU to the ONU, and receives a REPORT frame corresponding to the RTT correction command from the ONU. PON system according to any one of claims 1 to 4, characterized by determining with the recovery and link recovery. 6. The PON system according to claim 5 , wherein a Discovery Gate frame of EPON standard is used as the RTT correction command. The PON system according to claim 5 or 6 , wherein the OLT transmits the RTT correction command to the ONU in the link disconnection prevention mode while maintaining communication with the ONU in a normal state. One station side communication device (optical line terminal, hereinafter abbreviated as “OLT”) and a plurality of subscriber terminal devices (optical network unit, hereinafter abbreviated as “ONU”) are connected via an optical fiber transmission line. A PON (passive optical network) system that performs point-to-multipoint communication,
The OLT and the ONU determine the link break with a protection time after detecting the reception break of the optical signal or the change in the distance of the optical fiber transmission line by setting the link break prevention mode .
The ONU sets the link disconnection prevention mode according to an instruction from the OLT,
The ONU in the link disconnection prevention mode detects an optical signal reception interruption, detects an optical signal reception recovery within the protection time, or detects a change in the distance of the optical fiber transmission line and then within the protection time. In the PON system, the link disconnection prevention mode is canceled when the local time of the ONU is reset . The link failure prevention method of the PON system according to claim 8 , wherein the OLT sets the link failure prevention mode for each ONU. The OLT determines that the link is disconnected when the REPORT frame is received from the ONU as a response to the GATE frame instructing the ONU to start frame transmission, and determines that the link recovery is performed when the REPORT frame is recovered. 10. A method for preventing a link break in a PON system according to claim 8 or 9. The OLT within the protection time, claim 10, characterized in that the when the received recovery of the REPORT frame from the link to indicate the setting of the cross-sectional prevention mode ONU did not, deletes the registration of the ONU 2. A method for preventing link breakage of the PON system described in 1. The OLT transmits an RTT correction command for measuring a transmission trip time (RTT) between the OLT and the ONU to the ONU, and receives a REPORT frame corresponding to the RTT correction command from the ONU. link disconnection prevention method of the PON system according to claim 8 11, characterized by determining with the recovery and link recovery. The link failure prevention method of the PON system according to claim 12 , wherein a Discovery Gate frame of an EPON standard is used as the RTT correction command. The OLT, while maintaining communication with the ONU in the normal state, the link disconnection of the PON system according to claim 12 or 13, characterized in that transmitting the RTT correction command to the ONU in the link disconnection prevention mode Prevention method.

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