PON system, upper stage OLT, lower stage OLT, setting method for operational status of communication line, and construction method for the lower stage PON

Abstract

To provide a multistage passive optical network (PON) system with excellent physical robustness.SOLUTION: The system is a multistage passive optical network (PON) system including an upper stage PON and a lower stage PON. The upper stage PON includes an upper stage optical line terminal (OLT) and a plurality of upper optical network units (ONUs). The lower stage PON includes a lower stage OLT and at least one lower stage ONU. The plurality of upper ONUs include a plurality of connection ONUs for redundancy of transmission paths to the lower stage PON. The plurality of connection ONUs are each connected to the lower stage OLT via a predetermined communication line.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a PON system, an upper-stage OLT, a lower-stage OLT, a method for setting the operational state of a communication line, and a method for constructing a lower-stage PON. [Background technology]

[0002] Patent Document 1 describes a PON system in which an upper-stage PON (Passive Optical Network) and a lower-stage PON are configured in multiple stages in order to extend the service area of ​​FTTH (Fiber To The Home). In Patent Document 1, upper and lower PONs are configured in multiple stages by relay nodes that function as ONUs (Optical Network Units) of the upper PON and as OLTs (Optical Line Terminals) of the lower PON.

[0003] Patent document 2 describes a PON system in which a remote node that performs PON communication with subordinate ONUs and a relay node accommodated in an OLT at a station are connected via an optical line, and Ethernet communication is performed between the remote node and the relay node. According to Patent Document 2, since the remote node is installed at a location away from the station, it is possible to extend the distance of the communication service and increase the number of terminals that can be accommodated. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-82393 A [Patent Document 2] JP 2019-92005 A Summary of the Invention [Problem to be solved by the invention]

[0005] As in Patent Document 1, by connecting upper-stage PONs and lower-stage PONs in multiple stages, there is an advantage that more than 128 ONUs can be accommodated when viewed from the upper-stage OLT. However, as the number of ONUs accommodated increases, there is a problem that if an optical fiber break occurs between the upper OLT and the lower OLT, many subscribers may be unable to communicate.

[0006] In view of the above-mentioned problems in the related art, an object of the present disclosure is to realize a multi-stage PON system with excellent physical resistance. [Means for solving the problem]

[0007] A system according to one embodiment of the present disclosure is a multi-stage PON system comprising an upper stage PON including an upper stage OLT and a plurality of upper stage ONUs, and a lower stage PON including a lower stage OLT and at least one lower stage ONU, wherein the plurality of upper stage ONUs include a plurality of connecting ONUs for making a transmission path to the lower stage PON redundant, and the plurality of connecting ONUs are each connected to the lower stage OLT by a specified communication line.

[0008] The present disclosure can be realized not only as a system and device having the above-mentioned characteristic configuration, but also as a program for causing a computer to execute such characteristic configuration. Furthermore, the present disclosure can be realized as a semiconductor integrated circuit that realizes part or all of the system and device. Effect of the Invention

[0009] According to the present disclosure, a multi-stage PON system with excellent physical durability can be realized. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a network configuration diagram showing an example of the configuration of a PON system. [Diagram 2] FIG. 2 is an explanatory diagram of a problem 1 associated with redundancy of communication lines. [Diagram 3]FIG. 3 is an explanatory diagram showing Solution 1 to Problem 1 associated with redundancy of communication lines. [Figure 4] FIG. 4 is an explanatory diagram showing a problem 2 associated with redundancy of communication lines. [Diagram 5] FIG. 5 is an explanatory diagram showing a second solution to the second problem associated with redundancy of communication lines. [Figure 6] FIG. 5 is a state transition diagram showing an example of a monitoring process of a redundant path executed by the frame processing unit of the upper OLT. [Figure 7] FIG. 6 is a state transition diagram showing an example of a monitoring process of a redundant path executed by the frame processing unit of the lower stage OLT. [Figure 8] FIG. 8 is an explanatory diagram showing an example of a method for constructing a lower stage PON. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] <Overview of the embodiment of the present disclosure> Below, an overview of the embodiments of the present disclosure will be listed and described. (1) A system according to one aspect of this embodiment is a multi-stage PON system including an upper stage PON including an upper stage OLT and a plurality of upper stage ONUs, and a lower stage PON including a lower stage OLT and at least one lower stage ONU, wherein the plurality of upper stage ONUs include a plurality of connecting ONUs for making a transmission path to the lower stage PON redundant, and the plurality of connecting ONUs are each connected to the lower stage OLT by a specified communication line.

[0012] According to the PON system of this embodiment, multiple upper-stage ONUs include multiple connecting ONUs for making the transmission path to the lower-stage PON redundant, and the multiple connecting ONUs are each connected to the lower-stage OLT via a specified communication line, thereby obtaining a multi-stage PON system with excellent physical durability.

[0013] (2) In the PON system described above in (1), the operational states of the plurality of communication lines may include active and standby as defined below. Active: An operational state that allows user and control frames to pass through. Standby: An operating state in which user frames are prohibited from passing but control frames are permitted to pass. In this way, for example, by setting one of multiple communication lines as active and the rest as standby, it is possible to prevent downstream BUM traffic from reaching the lower ONU in duplicate.

[0014] (3) In the PON system of (2) above, the upper OLT and the lower OLT may use CCM of Ethernet CFM to perform alive monitoring of the multiple communication lines regardless of the type of the operation state. In this way, alive monitoring is performed on both the active communication line and the standby communication line, so that the operation state can be switched for each communication line.

[0015] (4) In the PON system of (3) described above, when communication on the active communication line is interrupted, the upper OLT and the lower OLT may select the communication line to be switched to active from among the standby communication lines. In this way, if any fault occurs in the active communication line, the standby communication line can be switched to the active line.

[0016] (5) In the PON systems described above in (2) to (4), the plurality of communication lines may be configured to be included in one LAG in the lower OLT. In this way, for example, by sharing MAC learning information between member ports, upstream traffic can be seamlessly sent to active communication lines.

[0017] (6) The device according to one aspect of this embodiment is an upper-stage OLT applied to the upper-stage PON of the PON system described above in (2). Therefore, the upper-stage OLT of this embodiment has the same effects as those of the PON system described above in (2).

[0018] (7) The device according to another aspect of the present embodiment is a lower OLT applied to the lower PON of the PON system described above in (2). Therefore, the lower OLT of the present embodiment has the same effects as those of the PON system described above in (2).

[0019] (8) A method according to one aspect of the present embodiment is a method for setting an operational state of a plurality of redundant communication lines for connecting a plurality of upper stage ONUs and the lower stage OLT, the operational state including the above-mentioned active and standby, the method including a step in which the upper stage OLT and the lower stage OLT set one communication line of the plurality of communication lines to active, and a step in which the upper stage OLT and the lower stage OLT set the remaining communication lines of the plurality of communication lines to standby.

[0020] According to the method of setting the operation state of this embodiment, the upper OLT and the lower OLT set one of the multiple communication lines to active and the remaining communication lines to standby, so that the number of communication lines that become active is narrowed down to one. Therefore, it is possible to prevent downstream BUM traffic from reaching the lower ONU excessively.

[0021] (9) A method according to another aspect of this embodiment is a method for constructing a lower stage PON including a lower stage OLT, comprising: a first step of constructing a plurality of upper stage ONUs in an upper stage PON including an upper stage OLT; a second step of constructing the lower stage OLT; a third step of connecting the plurality of upper stage OLTs to the lower stage OLT via communication lines; and a fourth step of setting information necessary for monitoring a redundant path consisting of the plurality of communication lines in the upper stage OLT and the lower stage OLT.

[0022] According to the construction method of this embodiment, the information required for monitoring the redundant path consisting of multiple communication lines is set in the upper OLT and the lower OLT (fourth step), so that the upper OLT and the lower OLT can autonomously monitor the redundant path after the lower OLT is installed. Therefore, it is possible to construct a multi-stage PON system that is highly resistant to communication failures.

[0023] <Details of the embodiment of the present disclosure> Hereinafter, the details of the embodiments of the present disclosure will be described with reference to the drawings. Note that at least some of the embodiments described below may be combined in any combination.

[0024] [Definition of terms, etc.] Generally, a PDU (Protocol Data Unit) of layer 2 is called a "frame" and a PDU of layer 3 is called a "packet," but in this embodiment, the PDUs of these layers are unified into the name "communication frame."

[0025] In this embodiment, a communication frame traveling in the downstream direction is called a "downstream frame", and a communication frame traveling in the upstream direction is called an "upstream frame". Moreover, a communication frame that includes user data related to a communication service is called a "user frame", and a communication frame that includes control information exchanged between communication nodes for setting communication parameters and controlling communication is called a "control frame".

[0026] In this embodiment, it is assumed that each communication node constituting the PON system executes Ethernet-based communications ("Ethernet" is a registered trademark), and the PON is an Ethernet-based EPON (for example, 10G-EPON). For example, in EPON, registration and removal of ONUs, bandwidth requests from ONUs, and bandwidth allocation to ONUs in response to the bandwidth requests are performed using MPCP (Multi-Point Control Protocol) frames defined in IEEE802.3ah.

[0027] [Example of PON system configuration] FIG. 1 is a network configuration diagram showing an example of the configuration of a PON system 100. As shown in FIG. As shown in FIG. 1, a PON system 100 of this embodiment is a multi-stage PON system including an upper stage PON 200 located on the upper side (left side in FIG. 1) and a lower stage PON 300 located on the lower side (right side in FIG. 1).

[0028] The upper stage PON 200 includes an upper stage OLT 10 and a plurality of upper stage ONUs 20 . The upper OLT 10 is configured, for example, by a chassis-type OLT having slots capable of accommodating a line concentrator 11 and at least one line card 12. In Fig. 1, a case is illustrated in which one line concentrator 11 and two line cards 12A and 12B are accommodated in the chassis of the upper OLT 10.

[0029] The concentrator 11 is a card-type communication module including a circuit board on which at least one communication IC (Integrated Circuit) is mounted. The communication IC of the concentrator 11 includes a frame processor 11U that performs Ethernet-based relay processing. The line card 12 is a card-type communication module equipped with at least one communication IC. The communication IC of the line card 12 includes a MAC (Media Access Controller) 13U capable of communication according to the PON protocol.

[0030] The line cards 12A and 12B are connected to PON lines L1 and L2, respectively, which are optical fiber networks having a tree structure. The lower end of the PON lines L1 and L2 is connected to the upper-stage ONU 20. The upper-stage ONU 20 includes the following two types of ONUs. Normal ONU20N: Upper ONU20 not connected to lower OLT30 Connected ONU 20C: An upper-stage ONU 20 connected to a lower-stage OLT 30 via a specified communication line

[0031] Typically, ONU20N is installed in the home of a PON subscriber who uses the communication service. The connection ONU 20C is a communication node for multi-stage configuration of the PON system 100, and is installed in, for example, a fixed facility (such as a building or an outdoor pole) under the management of a telecommunications carrier. The fixed facility of the connecting ONU 20C may be a distant or nearby facility different from the fixed facility of the lower OLT 30, or may be the same as the fixed facility of the lower OLT 30.

[0032] The connection form between the connection ONU 20C and the line concentrator 31 may be a direct connection using a metal line or an optical line as a medium, or an indirect connection via a LAN (Local Area Network).

[0033] The lower tier PON 300 includes a lower tier OLT 30 and at least one lower tier ONU 40 . The lower OLT 30 is configured, for example, by a chassis-type OLT having slots capable of accommodating a line concentrator 31 and at least one line card 32. In Fig. 1, a case is illustrated in which one line concentrator 31 and two line cards 32A and 32B are accommodated in the chassis of the lower OLT 30.

[0034] The concentrator 31 is a card-type communication module including a circuit board on which at least one communication IC is mounted. The communication IC of the concentrator 31 includes a frame processor 31D that performs Ethernet-based relay processing. The line card 32 is a card-type communication module having at least one communication IC mounted thereon. The communication IC of the line card 32 includes a MAC (Media Access Controller) 33D capable of communication according to the PON protocol.

[0035] The line cards 32A and 32B are connected to PON lines L3 and L4, respectively, which are made up of an optical fiber network having a tree structure. The lower end of the PON lines L3 and L4 is connected to a lower ONU 40. The lower ONU 40 is installed in the home of a PON subscriber who uses the communication service.

[0036] To the line concentrator 11 of the upper OLT 10, a higher-level device 50 that is connected to a higher-level network (not shown) such as a core network, and a management terminal 60 of a telecommunications carrier are connected. The connection form between the concentrator card 11 and the higher-level device 50 and the management terminal 60 may be a direct connection using a metal line or an optical line as a medium, or an indirect connection via a LAN (Local Area Network).

[0037] The management terminal 60 is, for example, a management computer operated by a communications manager, and is capable of communicating with the line concentrator 11 in accordance with a predetermined communications protocol. The management terminal 60 generates setting information related to communications in the PON system 100 in response to operational input by a communications manager using a command line or the like. The management terminal 60 transmits the generated setting information to the concentrator 11.

[0038] When the line concentrator 11 of the upper OLT 10 receives setting information for its own device (upper OLT 10) from the management terminal 60, it performs setting processing related to communication of the upper OLT 10 in accordance with the received setting information. The configuration process includes, for example, setting the maximum upstream and downstream bandwidths for the upper OLT 10, setting a Quality of Service (QoS) policy for each Logical Link ID (LLID), setting a Virtual LAN (VLAN), and setting domain information for Ethernet CFM.

[0039] When the concentrator 11 of the upper OLT 10 receives the setting information of the lower OLT 30 from the management terminal 60, it transmits the received setting information to the lower OLT 30. The concentrator 31 of the lower OLT 30 performs setting processing related to communication of the lower-upper OLT 30 in accordance with the received setting information. The setting process includes, for example, setting the maximum bandwidth in the upstream and downstream directions for the lower OLT 30, setting a QoS policy for each LLID, setting VLANs, and setting domain information for Ethernet CFM.

[0040] [Redundancy of communication lines] As described above, the PON system 100 of this embodiment is a multi-stage PON system in which the upper stage PON 200 and the lower stage PON 300 are configured in a multi-stage configuration by connecting some (connection OLT 20C) of the multiple upper stage ONUs 20 to the lower stage OLT 30. In this way, by connecting the upper OLT 10 and the lower OLT 30 in multiple stages, for example, by installing the lower OLT 30 in a specified facility located more than 20 km away from the exchange, the FTTH service area can be expanded.

[0041] Also, by connecting the upper OLT 10 and the lower OLT 30 in multiple stages, there is an advantage that it is possible to accommodate more than 128 ONUs 20, 40 (the number of ONUs that can be accommodated in one stage configuration) when viewed from the upper OLT 10. However, if the number of ONUs 20 and 40 accommodated increases, if any kind of failure (such as a break in the optical fiber) occurs in the transmission path between the upper OLT 10 and the lower OLT 30, it could result in a major communications failure in which many subscribers would be unable to communicate.

[0042] Therefore, in this embodiment, a plurality of connection ONUs 20C are connected to the lower-stage OLT 30 (specifically, the line concentrator 31), and the communication line connecting the upper-stage PON 200 and the lower-stage PON 300 is physically redundant. This enhances the resistance of the transmission line to physical failures. Although FIG. 1 illustrates an example in which there are two connected ONUs 20C for redundancy, the number of connected ONUs 20C may be three or more.

[0043] [Problems with redundancy of communication lines 1] FIG. 2 is an explanatory diagram showing a problem 1 associated with redundancy of communication lines. The dashed-dotted arrows in Fig. 2 indicate the flow of downstream BUM (Broadcast, Unknown Unicast, Multicast) traffic. In the following, a downstream BUM communication frame is referred to as a "BUM downstream frame."

[0044] As shown in FIG. 2, the concentrator 11 of the upper OLT 10 copies the BUM downstream frame received from the upper device 50 by the number of line cards 12A and 12B so that the BUM traffic reaches the ONUs 20 under its control. For the same reason, the line concentrator 31 of the lower OLT 30 copies the BUM downstream frame received from the two redundantly connected ONUs 20C, 20C by the number of line cards 32A, 32B.

[0045] For this reason, the same BUM downstream frames reach the lower-stage ONU 40 belonging to the lower-stage PON 300 excessively (twice in the illustrated example). Therefore, if the communication line is simply made physically redundant, the lower-stage ONU 40 will receive multiple BUM downstream frames, increasing the processing load. In this embodiment, in order to solve the problem 1, an operation policy is adopted that allows the passage of user data through only one of a plurality of redundant communication lines (Solution 1).

[0046] [Solution 1 for problem 1 associated with communication line redundancy] FIG. 3 is an explanatory diagram showing Solution 1 to Problem 1 associated with redundancy of communication lines. As shown in FIG. 3, in this embodiment, at least two types of logical link types defined as follows are adopted as the operation states of a plurality of redundant communication lines. Active: An operational state that allows user and control frames to pass through. Standby: An operating state in which user frames are prohibited from passing but control frames are permitted to pass.

[0047] In addition, in this embodiment, the logical link of one arbitrarily selected communication line from among multiple physically redundant communication lines is set to "active," and the logical links of the other communication lines are set to "standby." In this way, the BUM downstream frame passes only through the active link and is transmitted to the lower OLT 30, and the concentrator 31 does not copy the BUM downstream frame more than necessary. Therefore, the load of the reception process in the lower ONU 40 can be suppressed.

[0048] [Problems with redundancy of communication lines 2] FIG. 4 is an explanatory diagram showing a problem 2 associated with redundancy of communication lines. 4, in the PON system 100 of this embodiment, the communication between the upper OLT 10 and the connected ONU 20C is "PON communication", and the communication between the connected ONU 20C and the lower OLT 30 is "Ethernet communication". Since these communications are separate and independent, even if the PON communication is linked down, the Ethernet communication continues to be linked up.

[0049] In this way, a link down of PON communication that occurs in the upper stage PON 200 is not notified to the lower stage OLT 30 of the lower stage PON 300, so a mechanism is required in which the three communication nodes, the upper stage OLT 10, the connecting ONU 20C, and the lower stage OLT 30, work together to detect the fault. On the other hand, IEEE802.3ah includes "LACP" (Link Aggregation Control Protocol) as a protocol for building and managing redundant paths.

[0050] The above LACP is the same slow protocol (Destination MAC Address: 0180:c200:0002, Ether Type: 0x8809) as the OAM (Operation, Administration, and Maintenance) used in PON communications. Therefore, in order to enable both to be used simultaneously on the PON communications path, the connected ONU 20C must have a subtype determination function and be able to distinguish between LACP communication frames and OAM communication frames.

[0051] However, depending on the ONU model, a simplified determination may be made that slow protocols are always regarded as OAM, on the assumption that LACP communication frames do not pass through the PON communication path. Therefore, in this embodiment, in order to solve problem 2 regardless of the type of ONU, redundant paths are managed using control frames that can also be applied to PON communication paths (Solution 2).

[0052] [Solution 2 for problem 2 associated with communication line redundancy] FIG. 5 is an explanatory diagram showing a second solution to the second problem associated with redundancy of communication lines. As shown in FIG. 5, in this embodiment, a CCM (Continuity Check Message) of Ethernet CFM (Connectivity Fault Management) defined in IEEE802.3ag is used as a control frame for managing a redundant path.

[0053] Specifically, the maintenance end points (MEPs) and maintenance intermediate points (MIPs) that make up the Ethernet CFM maintenance domain are configured as follows: Upper MEP: Frame processing unit 11U of concentrator card 11 of upper OLT 10 Upper MIP: MAC13U of line cards 12A and 12B of upper OLT10 Lower MIP: Connected ONU20C, 20C Lower MEP: Frame processing unit 31D of concentrator card 31 of lower OLT 30

[0054] Ethernet CCM has a keep-alive function as a kind of alive monitoring of logical links. Therefore, each MEP checks the keep-alive regardless of the type of active or standby, and checks the normality of all redundant paths. In addition, if a problem occurs with the normality of the active logical link, and the standby logical link is in a normal state, the standby logical link is switched to active to ensure a data communication route.

[0055] However, simply switching the link using a communication frame relay device (a device with a layer 2 switch function) will cause traffic to continue to flow through the old link. Therefore, to switch traffic seamlessly, it is necessary to share the MAC learning information of the logical port between the active logical link and the standby logical link.

[0056] Therefore, for the frame processing unit 31D of the lower stage OLT 30, an active logical link and a standby logical link are set in one LAG (Link Aggregation Group), and MAC learning information is shared between member ports. As a result, if a failure occurs in the communication line between the upper OLT 10 and the lower OLT 30, the upstream traffic can be seamlessly sent to the active logical link.

[0057] Regarding the downstream direction, when the upstream traffic is switched, the downstream MAC learning information in the upper OLT 10 is switched to active. For this reason, in Internet communications, which generally occur in the order of upstream to downstream, the occurrence of upstream traffic will naturally result in the downstream route also being switched to an appropriate route.

[0058] [Redundant Route Monitoring Process] FIG. 6 is a state transition diagram showing an example of a monitoring process of a redundant path executed by the frame processing unit 11U of the upper OLT 10 (upper-level MEP). FIG. 7 is a state transition diagram showing an example of a monitoring process of a redundant path executed by frame processing unit 31D (lower-stage MEP) of lower-stage OLT 30. In FIG.

[0059] As described above, the frame processing unit 11U and the frame processing unit 31D monitor the logical links of the redundant paths by regular monitoring based on the CCM of the Ethernet CFM. The monitoring process is executed by the frame processing unit 11U and the frame processing unit 31D periodically exchanging CCMs and judging the status of each logical link. In this embodiment, the frame processor 11U of the upper OLT 10 takes the lead in the monitoring process, but the frame processor 31D of the lower OLT 30 may take the lead.

[0060] In the CCM of the Ethernet CFM, a predetermined value (hereinafter, referred to as a "status value") can be stored in an Interface Status TLV (Type-Length-Value). In addition, since the contents of the status values ​​can be freely determined by the vendor, in this embodiment, three status values ​​(Up, Down, and Testing) are defined as follows, for example: Note that in the following, the status value may be abbreviated as "SV."

[0061] The frame processing unit 31D of the lower OLT 30 judges the received status value as follows. Lower judgment 1: If the status value is "Up" = "Active" notification Lower judgment 2: If the status value is "Down" = "Standby or status not set" notification Lower judgment 3: If the status value is "Testing" = "Active candidate" notification However, if an active logical link already exists in the lower step decision 3, the next active candidate is notified.

[0062] The frame processing unit 11U of the upper OLT 10 judges the received status value as follows. Upper judgment 1: If the status value is "Up" = Notification that "active agreement" has been reached Upper judgment 2: If the status value is "Testing" = "Standby or status not set" notification

[0063] (Monitoring process of redundant routes in upper OLT) As shown in Table T1 in FIG. 6, the frame processing unit 11U of the upper OLT 10 transmits a CCM with SV=Down to a logical link whose status is not set (None). When a CCM with SV=Testing is received in a logical link of None (event E11), the frame processing unit 11U switches the operation state of the logical link to standby.

[0064] The frame processing unit 11U does not directly transition the operational state of a logical link of None to Active (event E14). When a CCM other than SV=Testing is received in a None logical link (event E17), frame processing unit 11U maintains the operational state of the logical link.

[0065] As shown in Table T1 in FIG. 6, the frame processing unit 11U of the upper OLT 10 executes the following transmission process for the standby logical link. Transmission process 1: If the logical link has the highest priority, a CCM with SV=Testing is transmitted. Transmission process 2: In cases other than those mentioned above, a CCM with SV=Down is sent.

[0066] When the frame processing unit 11U receives a CCM with SV=UP in a standby logical link when there are no other active logical links (event E12), it switches the operation state of the logical link to active. When the keep-alive is terminated in the standby logical link, or when a CCM other than SV=UP / Testing is received (event E16), the frame processing unit 11U switches the operation state of the logical link to None.

[0067] When the frame processing unit 11U receives a CCM with SV=Up on a standby logical link when there is another active logical link, or when it receives a CCM with SV=Testing (event E18), it maintains the operational state of the logical link.

[0068] As shown in Table T1 in FIG. 6, the frame processing unit 11U of the upper OLT 10 transmits a CCM with SV=Up to an active logical link. When the keep-alive is terminated in an active logical link, or when a CCM other than SV=UP / Testing is received (event E13), the frame processing unit 11U switches the operation state of the logical link to None.

[0069] When a CCM with SV=Testing is received in an active logical link (event E15), the frame processing unit 11U switches the operation state of the logical link to standby. When a CCM with SV=Up is received in an active logical link (event E19), frame processing unit 11U maintains the operational state of the logical link.

[0070] (Monitoring process of redundant routes in lower OLT) As shown in Table T2 in FIG. 7, frame processing unit 31D of lower OLT 30 transmits a CCM with SV=Testing to a link whose status is not set (None). When a CCM with SV=Down is received in a None logical link (event E21), frame processing unit 31D switches the operation state of the logical link to standby.

[0071] Frame processing unit 31D does not directly transition the state of a logical link from None to Active (event E24). When frame processing unit 31D receives a CCM other than SV=Testing in the None logical link (event E27), it maintains the operational state of the logical link.

[0072] As shown in Table T2 in FIG. 7, the frame processing unit 31D of the lower OLT 30 transmits a CCM with SV=Testing to the standby logical link. When a CCM with SV=Testing is received in a standby logical link when there are no other active logical links (event E22), frame processing unit 31D switches the operation state of the logical link to active.

[0073] When the keep-alive is terminated in the standby logical link, or when a CCM other than SV=UP / Testing is received (event E26), the frame processing unit 31D switches the operation state of the logical link to None. When the frame processing unit 31D receives a CCM with SV=Down on a standby logical link when there is another active link, or when it receives a CCM with SV=Testing (event E18), it maintains the operational state of the logical link.

[0074] As shown in Table T2 in FIG. 7, the frame processing unit 31D of the lower OLT 30 transmits a CCM with SV=Up to an active logical link. When the keep-alive is terminated in an active logical link, or when a CCM other than SV=UP / Testing / Down is received (event E23), the frame processing unit 31D switches the operation state of the logical link to None.

[0075] When a CCM with SV=Down is received in an active logical link (event E25), frame processing unit 31D switches the operation state of the logical link to standby. When frame processing unit 31D receives a CCM with SV=Up / Testing in an active logical link (event E29), it maintains the operational state of the logical link.

[0076] [How to build the lower PON] FIG. 8 is an explanatory diagram showing an example of a method for constructing the lower stage PON 300. In FIG. Here, it is assumed that a new lower stage PON 300 is constructed in an already operating upper stage PON 200. As shown in Fig. 8, the construction method of this embodiment includes the following steps ST11 to ST14.

[0077] (Step ST11: Installing the connecting ONU) Step ST11 is a process of adding a plurality of connecting ONUs 20C to the upper OLT 10 and installing the plurality of connecting ONUs 20C in predetermined installation locations. The connecting ONU 20C is preferably selected from the ONUs 20 connected to different line cards 12A and 12B. The installation location of the connecting ONU 20C may be a building that can be managed by a telecommunications carrier.

[0078] (Step ST12: Installation of the lower OLT) Step ST12 is a process of installing the lower OLT 30 in a predetermined installation location. The installation location of the lower OLT 30 may be a building that can be managed by a telecommunications carrier, as in the case of the connecting ONU 20C, but may be the same as or a different location from the connecting ONU 20C. When the connecting ONU 20C and the lower OLT 30 are connected by an optical line, the installation location of the lower OLT 30 may be selected to be a location far away from the connecting ONU 20C.

[0079] (Step ST13: Wiring of communication lines) Step ST13 is a process of connecting each of the plurality of connection ONUs 20C to the line concentrator 31 of the lower OLT 30 via a predetermined communication line. The predetermined communication line constitutes a transmission path that provides physical redundancy to the Ethernet communication between the upper OLT 10 and the lower OLT 30. The type of the communication line may be either a metal line or an optical line.

[0080] (Step ST14: Setting monitoring information for redundant paths) Step ST14 is a process of setting information required for monitoring the redundant paths (hereinafter referred to as "monitoring information") in the upper OLT 10 and the lower OLT 30. The monitoring information includes, for example, domain information of the Ethernet CFM shown in Fig. 5, and a control program for causing each of the concentrators 11 and 31 to execute the state transitions in Fig. 6 and Fig. 7. Setting of the monitoring information can be performed, for example, by the management terminal 60 of the telecommunications carrier.

[0081] After completing the setting of the above monitoring information, the PON lines L3 and L4 are connected to the line cards 12A and 12B of the lower OLT 30, and the lower ONUs 40 are connected to the PON lines L3 and L4. In the construction method of FIG. 8, it is assumed that the upper stage PON 200 is already in operation, but the upper stage PON 200 and the lower stage PON 300 may be newly installed at approximately the same time, and both may start operation at the same time.

[0082] [Other Modifications] The embodiments disclosed herein are illustrative and not restrictive in all respects. The scope of the present invention is not limited to the above-described embodiments, but includes all modifications within the scope of the equivalents of the configurations described in the claims.

[0083] In the above-mentioned embodiments, the concentrators 11 and 31 and the line cards 12 and 32 may be not only card-type but also communication devices unitized in a housing. Therefore, in the above-mentioned embodiments, the "concentrator card" and the "line card" can be read as the "concentrator unit" and the "line unit", respectively.

[0084] In the above embodiment, the case where the total number of stages N of the multi-stage PON system 100 is "2" has been exemplified, but the total number of stages N may be three or more. In this case, if the variable representing the ranking from the top to the bottom is defined as "n" (n = 1, 2 ... N), then multiple connected ONUs (n) belonging to PON (n) can be connected to the OLT (n+1) of the next lower PON (n+1) via a communication line.

[0085] This provides an N-stage PON system 100 in which the transmission path between PON(n) and the next lower PON(n+1) is made redundant by a plurality of communication lines. Furthermore, with regard to the monitoring process for the redundant path of each rank n, OLT(n) and the OLT(n+1) one level lower may execute the same monitoring process (FIGS. 6 and 7) as in the above-described embodiment. [Explanation of symbols]

[0086] 10 Upper OLT 11 Concentrator card (concentrator unit) 11U Frame Processing Unit 12 Line card (line unit) 12A, 12B Line Card (Line Unit) 13U MAC (Media Access Controller) 20 Upper ONU 20N normal ONU 20C Connection ONU 30 Lower OLT 31 Concentrator card (concentrator unit) 31D Frame Processing Unit 32 Line card (line unit) 32A, 32B Line Card (Line Unit) 33D MAC (Media Access Controller) 40 Lower ONU 50 Upper device 60 Management terminal 100 PON Systems 200 Upper PON 300 Lower PON

Claims

1. an upper stage PON including an upper stage OLT and a plurality of upper stage ONUs; A multi-stage PON system including a lower stage PON including a lower stage OLT and at least one lower stage ONU, The plurality of upper stage ONUs include: A plurality of connected ONUs for making a transmission path for the lower stage PON redundant; The plurality of connected ONUs include: A PON system, each of which is connected to the lower-stage OLT via a predetermined communication line.

2. The operation status of the plurality of communication lines is The PON system of claim 1 , comprising an active and a standby as defined below. Active: Operational state that allows user frames and control frames to pass Standby: An operating state that prohibits the passage of user frames and allows the passage of control frames.

3. The upper OLT and the lower OLT are 3. The PON system according to claim 2, further comprising: a CCM of an Ethernet CFM, which performs alive monitoring of the plurality of communication lines regardless of the type of the operation state.

4. The upper OLT and the lower OLT are 4. The PON system according to claim 3, wherein when communication is interrupted on the active communication line, the communication line to be switched to active is selected from among the standby communication lines.

5. The plurality of communication lines include The PON system according to claim 2 , wherein the PON system is configured to be included in one LAG in the lower-stage OLT.

6. An upper-stage OLT including a plurality of line units that perform PON communication with subordinate upper-stage ONUs, and a concentrator unit to which the plurality of line units are connected, The line concentrator unit includes: The operation state of a plurality of redundant communication lines connected to the upper ONU in order to connect to the lower PON can be switched; The operation status of the plurality of communication lines is Upper OLT, including active and standby as defined below. Active: Operational state that allows user frames and control frames to pass Standby: An operating state that prohibits the passage of user frames and allows the passage of control frames.

7. A lower-stage OLT including a plurality of line units that perform PON communication with subordinate lower-stage ONUs, and a concentrator unit to which the plurality of line units are connected, The line concentrator unit includes: The operation state of a plurality of redundant communication lines connected to a plurality of upper-stage ONUs included in the upper-stage PON can be switched; The operation status of the plurality of communication lines is Lower OLT, including active and standby as defined below. Active: Operational state that allows user frames and control frames to pass Standby: An operating state that prohibits the passage of user frames and allows the passage of control frames.

8. A method for setting an operation state of a plurality of redundant communication lines for connecting a plurality of upper stage ONUs and the lower stage OLT, the method being executed by an upper stage OLT and a lower stage OLT, comprising: The operational state is: Includes Active and Standby as defined below, The method comprises: The upper OLT and the lower OLT set one communication line among the plurality of communication lines to active; a step in which the upper stage OLT and the lower stage OLT set remaining communication lines among the plurality of communication lines to standby. Active: Operational state that allows user frames and control frames to pass Standby: An operating state that prohibits the passage of user frames and allows the passage of control frames.

9. A method for constructing a lower stage PON including a lower stage OLT, comprising: A first step of installing a plurality of upper stage ONUs in an upper stage PON including an upper stage OLT; A second step of installing the lower OLT; a third step of connecting the plurality of upper stage OLTs to the lower stage OLT via a communication line; a fourth step of setting, in the upper OLT and the lower OLT, information required for monitoring a redundant path consisting of the plurality of communication lines.

Images