DESC:TECHNICAL FIELD
[0001] The present disclosure relates to a telecommunications network. In particular, the present disclosure relates to a system and method for protecting and handling failure of network elements.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, message switching, or packet switching, to pass messages and signals.
[0004] A passive optical network (PON) communication system supports multi-service transmission and needs to provide reliability at a telecommunication level, and protection switching is its important content. The PON communication system can include an optical line terminal (OLT), a trunk optical fiber, an optical splitter network, an optical network unit, and a branch optical fiber.
[0005] Business services and mobile backhaul connectivity services typically require protection in an access layer. Gigabit Passive Optical Network (GPON) (G.984/G.983) standards define two different protection mechanisms including a type B protection, as shown in FIG. 1, and a type C protection, shown in FIG. 2. The type B protection provides protection against fiber cuts in a feeder layer while the type C protection provides protection against cuts in the feeder as well as distribution layers.
[0006] The type B protection as defined in International Telecommunication Union (ITU) Series G Supplement 51, primarily runs on the optical line terminal (OLT) and can work with typical 1-port optical network terminals (ONTs) which are cost effective. It is applicable for scenarios in which subscribers are in a cluster and are close to each other. The feeder fiber can be taken close to the cluster from where the distribution layer can start. If the subscribers are spread over a wider area and there are multiple clusters, each cluster needs a different optical distribution network (ODN) and separate OLT port-pairs. This increases the fiber requirement and the number of OLT ports needed. As shown in the FIG. 1, a typical Type B protection architecture for feeder layer protection includes One 2xN splitter in the feeder layer and only a work port A is active during normal operation. On failures in the work path, protect port B on OLT is made active and the port A is turned off. Switching time can be less than 50ms. In addition, implementations described in G.984 typically assume that the differential delay from one ONT to the two different ports on OLT is constant and is the same for all ONTs.
[0007] FIG. 2 illustrates a typical Type C protection architecture for the feeder layer and OLT node protection. The type C protection architecture provides protection against cuts in the feeder as well as distribution layers. OLT dual homing, as shown in FIG. 2, protects against OLT node failures. It is also possible to have both OLT ports on the same node. Both work and protect GPON OLT ports are active during normal operation. Switching time can be less than 50ms. The type C protection can address the limitations of the type B protection and can cover multiple clusters but requires a 2-port ONT which is more expensive.
[0008] There is, therefore, a need to provide an easy to use, efficient, and cost-effective solution which can overcome abovementioned problems in the art.

BRIEF DESCRIPTION OF DRAWINGS
[0009] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. The diagrams are for illustration only, which thus is not a limitation of the present disclosure.
[0010] In the figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.
[0011] FIG. 1 illustrates a typical Type B protection architecture.
[0012] FIG. 2 illustrates a typical Type C protection architecture.
[0013] FIGs. 3A to 3F illustrate exemplary schematic diagrams of the proposed protection mechanism /Type S protection in ring architecture, in accordance with an embodiment of the present disclosure.
[0014] FIG. 4A illustrates an exemplary schematic diagram of the proposed protection mechanism /Type S protection for first protection scenario, in accordance with an embodiment of the present disclosure.
[0015] FIG. 4B illustrates an exemplary schematic diagram of the proposed protection mechanism /Type S protection for second protection scenario, in accordance with an embodiment of the present disclosure.
[0016] FIG. 4C illustrates an exemplary schematic diagram of the proposed protection mechanism /Type S protection for third protection scenario, in accordance with an embodiment of the present disclosure.
[0017] FIG. 5 illustrates a relationship between time references when backup path is no longer than the working path, in accordance with an embodiment of the present disclosure.
[0018] FIG. 6 illustrates an exemplary flow chart showing working of the proposed protection mechanism /Type S protection, in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION
[0019] In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details.
[0020] Embodiments of the present invention include various steps, which will be described below. The steps may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor programmed with the instructions to perform the steps. Alternatively, steps may be performed by a combination of hardware, software, firmware and/or by human operators.
[0021] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure).
[0022] While embodiments of the present invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the invention, as described in the claim.
[0023] Embodiment explained herein relate to a telecommunications network. In particular, the present disclosure relates to a system and method for protecting and handling failure of network elements.
[0024] In an aspect, the present disclosure provides a protection mechanism (hereinafter, also referred to as Type S protection mechanism), which can support multiple splitters in the feeder layer, thereby servicing users distributed in multiple clusters. This reuses the fiber across multiple clusters, minimizes the number of Optical Line Terminal (OLT) ports needed while using the inexpensive 1-port ONT. It can also provide sub 50ms protection switching.
[0025] In another aspect, the present disclosure provides mechanism to handle protection in set of ONTs connected in a ring or chain to one or two optical line terminations OLTs respectively.
[0026] In an aspect, the protection mechanism may involve plurality of ONTs connected to one to two hub nodes in a ring or chain format. The mechanism and/or method of the preset disclosure may take care of a failure of forward and reverse path sharing a fiber duct or two failures one each in forward ring and other in backward ring. The mechanism and/or method of the preset disclosure may take care of adjusting the equalization delay experienced between a pair of OLT and ONT. The mechanism and/or method of the preset disclosure may also figures out the set of ONTs connected on the clockwise ring and counterclockwise ring and connect the same through the appropriate port, such as m or p.
[0027] In another aspect, the mechanism and/or method of the preset disclosure can use the concept of each ONT will have exactly one path to one OLT during working and protection scenario and then ensure appropriate equalization delay is applied on each path.
[0028] In another aspect, the mechanism and/or method of the preset disclosure can work when the ring is terminated on two OLTs as well with first OLT having port m and second OLT having port m. These two OLTs are assumed to communicate among each other using Ethernet or IP protocol with a tunnel among them.
[0029] In another aspect, the mechanism and/or method of the preset disclosure can be based on a set of ONTs, which are connected to the port m and another set, which are connected to the port p. This list of ONTs can vary during normal working condition and during failure scenario. The set can then trigger the use of appropriate equalization delay between a given pair of OLT and ONT.
[0030] In another aspect, as in case of the Type B protection only one port is active at any given time, whereas the proposed mechanism and/or method of the preset disclosure can enable to send the traffic on both ports but takes care of equalization issue and ensure that one ONT is handled by one and only one OLT port/OLT at any given point of time. Other mechanism in prior art only allows the whole set of ONTs to switch from one OLT/OLT port to other. The proposed mechanism and/or method of the preset disclosure can allow a part of the ring ONTs to connect to one OLT/OLT port and another set to connect to the other OLT port/OLT simultaneously. The proposed mechanism and/or method of the preset disclosure can also do pre-ranging and ensures that in case of node/ONT augmentation. The proposed mechanism and/or method of the preset disclosure can redo the ranging to ensure this is done in the fastest possible way.
[0031] In another aspect, the proposed mechanism and/or method of the preset disclosure facilitates faster switchover. In case at least one connectivity path between ONT and OLT exist, the internet traffic and all traffic between ONT and OLT would not be affected.
[0032] In another aspect, the proposed mechanism and/or method of the preset disclosure can be used to provide highly reliable enterprise connectivity to offices. In another aspect, the proposed mechanism and/or method of the preset disclosure can provide higher SLA service using cheaper PON based solution than the existing high SLA solution based on routers and L3/ERPS (Ethernet Ring Protection Switching) switches.
[0033] In another aspect, the present disclosure provides a telecommunications network which can include a plurality of ONTs with one or more Ethernet and/or Wi-Fi user ports; and two PON OLT ports in one or two systems. In an embodiment, the first and second OLT ports can be within same card. In another embodiment, the first and second OLT ports can be on different cards within the same system. In another embodiment, the first and second OLT ports can be on different cards on two different systems. The telecommunications network further includes a plurality of optical splitters; a first plurality of 1:2 optical fiber cable (OFC) splitter connected in a ring fashion connected to one OLT port; a second plurality of 1:2 OFC splitter connected in a ring fashion connected to other OLT port; a plurality of 2:1 optical coupler which aggregates signal from primary and secondary paths; and optical network terminals (ONTs) connected to the optical coupler.
[0034] In an embodiment, the telecommunications network can further include a means for detecting a failure of one of the first plurality of optical fibers; and a means for controlling the traffic over the OLT ports to provide an alternate communications route in response to the detection of a failure to a set of ONT/ optical network units (ONUs) through the secondary OLT port.
[0035] In another embodiment, the telecommunications network can further include a means for detecting a failure of one of the plurality of primary interface modules and/or nodes; and a means for controlling the traffic over the OLT ports to provide an alternate communications route in response to the detection of a failure to a set of ONT/ONUs through the secondary route.
[0036] In another embodiment, the telecommunications network can further include a means to do pre-ranging for detecting the equalization delay between any pair of ONT/ONU and OLT ports during the initial phase of installation and commissioning; and a means of re-doing the ranging in case additional nodes with ONTs are added to the network at a later stage after the initial installation.
[0037] In another embodiment, the telecommunications network can further include a means to find a set of nodes with ONT/ONUs which are connected to the primary path; and a means to find a disjoint set of nodes with ONT/ONUs connected to the secondary path.
[0038] In another embodiment, the telecommunications network can further include a means to adjust the equalization delay for the set of ONT/ONUs connected on the secondary path after the detection of failure.
[0039] In an aspect, the present disclosure provides an optical line terminator that can include a control and switching card; a plurality of OLT cards coupled to the control and switching card; a plurality of OLT ports being associated with the plurality of OLT cards; and a memory storing operational data associated with the card. The optical line terminator can be configured for detecting one or more failures in the network, associating one or more of the OLT ports with respective one or more of the failures after the detection of the failures, copying the operational data from one OLT card associated with one of the failures to another OLT card in case the OLT ports are on different cards or system, and switching control of communications from the primary port whose operation data was copied to the secondary port within the same card or across different cards.
[0040] In an aspect, the present disclosure provides a method for ranging network interface modules, the method can include steps of: (a) selecting a primary network interface or port from a plurality of OLT cards; (b) calculating relative distance data comprising differences in distances from this port to the plurality of ONU/ONTs; (c) selecting a secondary network interface or port from a plurality of OLT cards within a system or across two different systems; and (d) determining the distance from this secondary port to the selected ONT/ONUs.
[0041] In an aspect, the present disclosure provides a method for ranging network interface modules for a network, the method can include steps of: (a) determining range data for each OLT port associated with a plurality of primary OLT cards during a period when the system is installed for the first time or when a new node with ONTs are added at a later stage; (b) determining range data for each OLT port associated with the plurality of primary OLT cards during a period when the system is relatively having lower amount of traffic to correct the ranging information periodically to compensate for aging etc. and (c) storing the determined range data in a memory of a control card which is coupled to all of the primary interface modules and a protection interface module.
[0042] Referring to FIG. 3A, in an embodiment, in a distributed architecture, there are multiple splitters in a feeder layer. Typically, two ring splitters at each ring node, one from each fiber can protect against any single link failure scenarios.
[0043] In any of the links, there can be single or dual fiber cuts; e.g. between Node 4 and Node 5 only outer fiber or both outer and inner fibers can be cut and will be addressed by Type S protection mechanism.
[0044] In case of ring splits, e.g., both the fibers being cut between Nodes 3 and 4, both the OLT transmitters, i.e., ports m and p, are turned on. This is unlike Type B protection in which only one of the transmitters can be ON.
[0045] Referring to FIG. 3B, in an embodiment, in a distributed architecture, there are multiple splitters in the feeder layer. This is a new architecture, not discussed in the G.824 standards. Fundamental difference is that the differential delay from an ONT to the main and protect OLT ports can be different for different ONTs.
[0046] Two ring splitters at each location: one from each fiber. These ring splitters are deployed in a ring configuration.
[0047] In an embodiment, for protection against fiber cuts in the ring, both single and dual fiber cuts can be addressed. Parallel rings can be used to increase the customers covered.
[0048] In another embodiment, there is no splitter needed at the last building on the tree.
[0049] Referring to FIG. 3C, in the traditional Type-B architecture, the differential delay from an ONT to the main and protect OLT ports will be the same for all ONTs. This implies that the uplink signal received on the protect port is a delayed/advanced (based on the diff delay in the feeder link) version of the signal received on main/working OLT port. So, the protect port can actively monitor the uplink transmissions for ONT alarms and latencies. This info can be used to optimize the switching times.
[0050] In an embodiment, the ring architecture can introduce some challenges, such as challenge 1, since the delay compensation for the uplink is done with respect to the working OLT port, the uplink signals at the protect OLT port will not be aligned and the signal overlap from different ONTs may make the uplink signals not decipherable. The protect port cannot be actively monitored in normal operating state.
[0051] Referring to FIG. 3D, traditional Type B protection switching, as defined in the GPON standards, requires the transmission on one of the OLT ports (Port m or Port P) to be off at any given time. However, in case of ring architecture, it is possible that both the fibers on a span are routed along the same duct thereby both are cut at the same time.
[0052] Challenge 2, this results in the ring is being split and some of the nodes will lose service. As shown in the FIG. 3D, if the protection switching happens to port p with port m switched off, Nodes 1-3 will be out of service.
[0053] Referring to FIG. 3E, challenge 3: some of the failure scenarios that could be addressed by Type-C protection cannot be handled by Type-S protection since each ONT sees a signal combined from both OLT ports.
[0054] For example, as shown in FIG. 3E, if both the ports are ON, signals to Node 3 and Node 4 will be corrupted. If the OLT port m is turned off, Node 1 and Node 2 will be out of service. If the OLT port p is turned off, Node 5 and Node 6 will be out of service. Type C protection can handle this scenario since each ONT can select which path to use for communication to OLT port m or port p. However, this is a dual failure scenario, Type S protection can handle all single failure scenarios.
[0055] Referring to FIG. 3F, in an embodiment, the fiber that is used for the ring is the feeder fiber. The fiber that connects the ONT to the ring is called the distribution fiber.
[0056] For instance, consider C7 which is serviced by Node 5, assume that the length of the distribution fiber is d7 and feeder fiber to the OLT ports m and p respectively is: f7m and f7p.
[0057] Assume that the circumference of the ring is f:
f7m + f7p = f
f7p = f-f7m
Differential fiber length from C7 to the OLT ports m and p = (f7m+d7) – (f7p+d7) = f7m-f7p
This is not dependent on the feeder length.
This demonstrates two key properties: (a) the differential delay, which is determined by the differential fiber lengths, is the same for all the users serviced by the same ring node, and
(b) once the delays to one of the ports mand p, such as port m, is known, the delays to the other port p can be computed by subtracting from fixed ring delay.
[0058] In an exemplary embodiment, in the Type S protection switching procedure, assume that there are R nodes in a ring. If there is a failure in reaching one of the R nodes of the ring due to fiber cuts, the protection procedure tries to identify an alternate path through the OLT protection port. If a node is not reachable due to multiple fiber cuts, then none of the ONTs serviced by that node are reachable. Conversely, even one of the ONTs at a node responds to a request from OLT, this implies there is a path to that node from an OLT port through the feeder fiber.
[0059] If none of the ONTs attached to a node are reachable, it can be inferred that both the primary and backup feeder fibers are cut. The alarms raised on the ONTs can be correlated with the ring topology to detect which nodes are not reachable and which is the farthest node reachable. In another embodiment, at any given time, this can be computed only for the active OLT ports.
[0060] In another exemplary embodiment, on a node failure, determine if that node is reachable through the backup OLT port. If there is a single fiber cut that brought down one or more nodes, the traffic can be brought back up by switching to the backup path. If there is a dual fiber cut on a link, then it splits the ring. In this case, traffic can be brought back up by turning ON both the working and protect OLT ports. If, however, there are multiple single-fiber cuts in the ring, there may be some nodes that are reachable from both OLT ports. In this case, turning on both the OLT ports corrupts the signal to such nodes. It is better to turn on only one of the OLT port and turn off the other. This can be the port that serves the largest number of ONUs.
[0061] In another exemplary embodiment, for restoration after fault rectification, we may first consider only the single failure scenarios which are rectified. For instance, there are two scenarios to be considered, a first scenario, when there is a unidirectional fiber cut on a span and the protect path is active, and a second scenario, where there is bi-directional fiber cut on a span which is addressed by turning both the transmitters ON.
[0062] For the first scenario, when the fiber cut on the protect path is restored, it does not impact the working traffic. This repair is known at the OLT only when it does a periodic audit of the backup path. At that time, it can choose to revert. If, however, after the fiber cut is repaired and there is a failure on the current active path, the OLT triggers an audit of the protect path and then switches.
[0063] For the second scenario, since the protection switching times are in ms, assume that the fibers are repaired one at a time. Once the first fiber is repaired, all nodes are reachable by the corresponding OLT port, and this causes a momentary corruption to the optical network units (ONUs) serviced by alternate ONTs. The protection algorithm kicks in and switches off the transmitter on the alternate OLT port and all the traffic is restored. Second fiber repair does not impact the traffic as that is used as the backup path now.
[0064] In another exemplary embodiment, for protection switching considerations, we first consider the scenario when both the OLT ports (main and protect) are on the same node. For instance, when a new ONU is brought up, during the ranging process, round trip delays (RTDs) to both the main and protect ports are computed and stored. Since the differential delays, between the main and backup ports, to all the ONUs at a node are the same, the topology of the nodes can also be created at the OLT on both the ports.
[0065] The protection algorithm just needs to (a) switch to the protect path in case of single uni-directional fiber cuts, (b) check when the ring split has happened and turn both the OLT ports ON in that case, (c) ensure that even under multiple failure scenarios, it turns ON one or both the OLT transmitters as needed, and (d) address transition scenarios during repairs which can cause corruption.
[0066] Referring to FIG. 4A, where first protection scenario is shown. In this scenario, at least one cut impacts both the fibers of a span. Both the fibers between nodes N3 and N4 are cut. This creates a ring split. Both the OLT transmitters at ports m and p can be turned on at the same time.
[0067] Referring to FIG. 4B, where second protection scenario is shown. In this scenario, at least one cut impacts both the fibers of a span. Both the fibers between nodes N3 and N4 are cut. This creates a ring split. Both the OLT transmitters at ports m and p can be turned on at the same time.
[0068] Referring to FIG. 4C, where third protection scenario is shown. In this scenario, one or more uni-fiber cuts.
[0069] Identify the longest working path from the ODNs starting from port m and port p. As shown in FIG. 4B, they are OLT_m – Node 1 for the port m , and OLT_p – Node 6 – Node 5 – Node 4 – Node 3 for the port p.
[0070] For instance, if there are no link/node overlaps between these two paths, transmitters from both ports m can be active.
[0071] Referring to FIG. 4C, where third protection scenario is shown In this scenario, the longest working paths from the ODNs starting from Port m and Port p:
OLT_m – N1 – N2 – N3 – N4
OLT_p – N6 – N5 – N4 – N3
[0072] There are two overlapping Nodes between these paths N3 and N4. Only one of the transmitters between Port m and Port p can be active, or else the downlink signal to node N3 and node N4 may be corrupted.
[0073] FIG. 5 illustrates a relationship between time references when backup path is no longer than the working path, in accordance with an embodiment of the present disclosure. This scenario can be addressed by turning on both the ports m and p selectively. FIG. 6 illustrates an exemplary flow chart showing working of the proposed protection mechanism /Type S protection, in accordance with embodiments of the present disclosure. Various aspect of FIG. 5 have been explained in conjunction with FIG. 6.
[0074] Notations:
RTD – Time interval at the OLT between transmission of a downstream frame and reception of a corresponding upstream burst from the given ONU. This time is composed of the round-trip propagation delay, and the ONU response time.
?Rtd – The time difference between the RTD(working) and RTD(backup).
Teqd – The "zero distance" equalization delay, equal to the offset between the downstream and upstream frames at the OLT location. The OLT adjusts the equalization delay of each ONU such that, for all ONUs, the start of the upstream frame at the OLT occurs Teqd seconds after the start of the downstream frame.
EqD – The requisite delay assigned by the OLT to an individual ONU as a result of ranging. By adjusting their local transmission times with this value, all the ONUs are viewed as at the same distance from the OLT.
[0075] In another exemplary embodiment, for delay compensation in failure scenarios, let N1 to Nr denote the r nodes forming the ring; where N1 denotes the node closest to the OLT port m.
[0076] Let the number of ONTs serviced at Nj be denoted as Lj and each ONT be denoted as U(k,l); where k denotes the node Nk and ranges from 1 to r, and l denotes the user at Node j, l=1 to Lk.
[0077] Let {Sj} represent the set of Lj ONTs at Node Nj: {U(j,l), l=1 to Lj}.
[0078] Let the RTD (Round Trip Delay) of ONT U(k,l) from OLT port m be denoted RTDm(k,l); and RTD of ONT U(k,l) to OLT port p be denoted as RTDp(k,l).
[0079] Each node Nj, j=1,…r consists of: two ring splitters, each of which provide a drop-and-continue functionality for each ring; a combiner that mixes the signals from the east and west directions of the ring; and a local tree that extends the GPON service at Node j to a set of Lj ONTs denoted as set {Sj}.
[0080] Let the differential RTD seen by ONT U(k,l) to ports m and p be denoted as Delta[U(k,l)]:
Delta[U(k,l)] = RTDp(k,l) – RTDm(k,l)
[0081] It is easy to see that Delta[U(k,l)] = Delta[k], for all l; that is all the Lk ONTs serviced from node Nk have the same differential delay Delta[k]. The differential delay is only dependent on the ring node to which the specific ONT is attached.
[0082] In another exemplary embodiment, in the normal working conditions only the transmitter of the main port m of the OLT, is turned on while the transmitter at the protect port p is turned off. Both the receivers of the OLT serving as work and protect ports are always on.
[0083] At the time of initial ranging of an ONT, the signal is received at both the ports m and p and RTDm and RTDp for that ONT can be computed and stored. The main and protect path EqD can be calculated during Pre-ranging and sent to the ONTs using PLOAM.
[0084] Using these RTD values, the OLT can construct the sets of ONTs {Sj}, where each set consists of nodes with identical diff delay Dj.
[0085] Arrange the sets {S1}, {S2}, ….. {Sj} …. such that D1 < D2 < D3 …., i.e. ascending order of their diff delays. Based on these RTD measurements, the ring topology with the Nodes N1, N2 …. Nj, …. can be arrived. This also gives the sets of users serviced at each Node, {S1} by N1, {S2} by N2 etc; where {Sj} consists of all the users attached to Node Nj.
[0086] In another exemplary embodiment, in Type S protection procedure for ring topology, if the failure is indeed because of fiber cuts in the ring, trigger topology updation as follows: (a) identify the farthest node reachable on port OLT_m using the fault-free topology that is available prior to the fiber cut and correlating that with the alarm info; (b) switch off OLT_m and turn on the backup port OLT_p; (c) send a pop-up broadcast message from OLT_p to all the ONTs to get in to Operational State O5; (d) send a downlink message to all the ONTs to switch to EqD values for the protect path; and (e) send a test message to all the ONUs: PLOAMu = '1', StartTime = X and StopTime = X + 12
All the ONUs which are reachable from OLT_p respond with a PLOAMu transmission at the defined start time.
[0087] In addition, based on the ONU reachability information, identify the list of Nodes on the ring that are accessible from OLT_p. If all the ring nodes are accessible from OLT_p, the procedure stops as the ring is now restored. However if there are some ring nodes that are not reachable from OLT_p, check if there is a common set of nodes that are accessible from OLT_m as well as OLT_p. If this set is empty, that is there are no ring nodes accessible from both OLT ports, turn on OLT_m as well. Further, fxchange the node reachability info between OLT_m and OLT_p so both have knowledge about which ring nodes are accessible by each of them.
[0088] For instance, assume that there are new alarms and one or more of the nodes hitherto reachable are out of service. This can be due to two reasons: (a) there is an additional fiber cut in the ring that made these nodes inaccessible or (b) one of the earlier fiber cuts is restored making these nodes reachable from both the work and protect OLT ports. Since both of the transmitters are ON, the downlink signal to these nodes got corrupted and hence they have become unreachable:
[0089] 1). If and when some of the nodes that are being served by OLT_p become unreachable from OLT_p, and there is no change in the reachability status from OLT_m, turn off OLT_p (i.e. OLT_m is the only active port now). Get updated reachability information from the OLT_m; if all the nodes are now reachable, the procedure end and ring is now restored to original configuration. Send a Pop-up message to all the ONTs that were hitherto unreachable by OLT_m, but now can be serviced from OLT_m to transition to State O5, followed by the PLOAM message with the EqD values. If this process has not increased the number of Nodes reachable from OLT_m than when OLT_p was also active, this means that there was an additional cut in the protect ODN which made these nodes lose their connectivity to OLT_p. If this is the case, switch ON OLT_p again.
[0090] 2). If and when some of the nodes that are being served by OLT_m become unreachable from OLT_m, and there is no change in the reachability status from OLT_p, turn off OLT_m (i.e. OLT_p is the only active port now). Get updated reachability information from the OLT_p; if all the nodes are now reachable, the procedure ends and ring is now restored to original config.
[0091] 3). If, after (1) or (2) above, all the nodes are not reachable from the active OLT port, there is an option of checking the reachability from the non-active port and switching to it if more ONUs can be served from it.
[0092] In another exemplary embodiment, for sending a downlink message to all the ONTs to switch to EqD values for the protect path, several options exist: ITU Series G. Supplement 51 talks about pre-ranging based switch-over and G.984.3 standards defined a downlink message that can be used to communicate the EqD for the protect path to the ONTs and the provision that can be made in the ONTs to store both main and protect EqD, but (a) they have not defined downlink messages to inform the ONT to switch to using EqD, (b) the standards did not envisage using ring topology with Type B.
[0093] We can explore using PST messages defined to check OLT-ONU connectivity and also to perform APS. The PST is again described in G.983.5 for use with Type C protection, so this needs to be adapted for our current use.
[0094] Second option is to define a new downlink PLOAM broadcast message to affect the switchover Send a downlink Ranging_Time PLOAM messages from OLT_p to all the ONTs asking them to use the EqD for the protect path. Even if there are 128 ONTs in the ring, these messages can be accommodated in one downlink GTC frame which has a duration of 128 microsecond.
[0095] In an embodiment, if there is a dual-fiber cut between any adjacent nodes Nj and Nj+1, then the ONTs which are reachable by OLT ports m and p form disjoint sets:
{S1} …. {Sj} are reachable on OLT port m and {Sj+1} …. {Sr} on OLT port p Assuming that there are no other failures.
[0096] Any failure in the ring affects one or more sets of ONTs {S1} to {Sr}. It is not possible that some of the ONTs in {Sj} is down while some others are up (assuming there are no other failures). If one of the Nodes on the ring is not reachable due to a failure on a feeder fiber, that implies that none of the ONTs attached to that node are not available. Conversely, if even one ONT attached to a node is serviceable.
[0097] In an embodiment, for protection switching logic: start the protection state machine in a normal working scenario, OLT port m is active, OLT port p is on standby and its downlink transmitter is off. If the alarms indicate that some ONTs are down, check if any of the sets {Sj} are out of service. A set {Sj} is out of service if all the ONTs belonging to that set {Sj} are down. If the sets {Sj}, j>t are down, that indicates that there is a cut between Nt and Nt+1. Number of nodes reachable by OLT_m is t and it forms a contiguous set: {N1, N2 … Nt}. There could be zero or more cuts beyond Nt+1.
[0098] Turn off the transmitter at OLT port m and switch transmissions to the port p. All the ONTs are brought up with the pre-computed ranging values to the port p. After giving sufficient time for the network to converge, check if there are still alarms indicating that some ONTs that are still down, check if any of the sets {Sj} is/are out of service; list the set of nodes that can be still serviced by OLT_p: {Nk, Nk+1 to Nr}.
[0099] If k>=(t+1), make port m also active and turn on its transmitter. If k=(t+1), that indicates a dual fiber-cut between Nt and Nt+1.
[0100] In an embodiment, for protection reversion logic, this applies to the case when both the transmitters at ONT ports m and p are turned ON due to a dual fiber-cut. Maintain the sets of Nodes {Qm} & {Qp} reachable from OLT_m & OLT_p and update the lists as and when the failures are corrected and more ONTs and Nodes that were hitherto offline come online, without any additional ‘Node alarms’. A node is considered reachable from an OLT port if at least one ONT serviced by that node is online and its traffic is normal.
[0101] If there are new alarms from ONTs that brings some of the working nodes in {Qm} is down, list the number of nodes that went down and the lowest numbered node among these, say t. If r-t+1 > t-1, turn off OLT_m or else turn off OLT_p.
[0102] If there are new alarms from ONTs that brings some of the working nodes in {Qp} is down, list the number of nodes that went down and the highest numbered node among these, say t. If t> r-t, turn off OLT_p or else turn off OLT_m.
[0103] For example, assume that the fiber cut on the tree connected to port m (between nodes Nt and Nt+1) is first restored. Nodes Nt+1 to Nr receive downlink traffic from both ports m and p will be corrupted and they will be reported as down on both these ports m and p. ONTs connected to Node N1 to Nt will be up through port m. Turn off port p; keep only port m on; all traffic is restored.
[0104] If the fiber cut on the tree connected to OLT port p is restored first, ONTs connected to Nodes N1 to Nt will be down while Nodes Nt+1 to Nr will be up, and turn off port m and turn on port p.
[0105] Notations:
RTD – Time interval at the OLT between transmission of a downstream frame and reception of a corresponding upstream burst from the given ONU. This time is composed of the round-trip propagation delay, and the ONU response time.
?Rtd – The time difference between the RTD(working) and RTD(backup). Teqd – The "zero distance" equalization delay, equal to the offset between the downstream and upstream frames at the OLT location. The OLT adjusts the equalization delay of each ONU such that, for all ONUs, the start of the upstream frame at the OLT occurs Teqd seconds after the start of the downstream frame.
EqD – The requisite delay assigned by the OLT to an individual ONU as a result of ranging. By adjusting their local transmission times with this value, all the ONUs are viewed as at the same distance from the OLT.
EqD(m,j): EqD for ONTj to OLT_m
EqD(p,j): EqD for ONTj to OLT_p
RTD(m,j): RTD for ONTj to OLT_m
In addition, OLT_m: main port of OLT that is serving the ONTs in the ring
OLT_p: backup OLT port
R: number of nodes in the ring
{Wm}: the set of Nodes that are reachable from OLT_m , where{Wm} is a contiguous set starting from the connect node closest to OLT_m
Vm: number of members of the set {Wm}, when there are no failures and the ring is serviced by OLT_m, Vm=R
{Wp}: the set of Nodes that are reachable from OLT_p, where {Wp} is a contiguous set starting from the connect node closest to OLT_p
Vp: number of members of the set {Wp}, when there are no failures and the ring is serviced by OLT_p, Vp=R.
[0106] In an exemplary implementation, 1) initially OLT_m is active, and 2) do the ranging for all the ONTs in the ring for both the work and protect paths. Compute {Wm}, Vm, RTD(m,j), EqD(m,j) for all ONT_j, EqD(p,j) for all ONT_j. The ranging parameters for the protect path can be computed based on the information received from the OLT_p receiver.
[0107] Thus, it will be appreciated by those of ordinary skill in the art that the diagrams, schematics, illustrations, and the like represent conceptual views or processes illustrating systems and methods embodying this invention. The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing associated software. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the entity implementing this invention. Those of ordinary skill in the art further understand that the exemplary hardware, software, processes, methods, and/or operating systems described herein are for illustrative purposes and, thus, are not intended to be limited to any particular named.
[0108] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.
,CLAIMS:1. A telecommunications network comprising:
a plurality of ONTs with one or more network ports;
a first and a second PON OLT ports in one or two systems;
a plurality of optical splitters;
a first plurality of 1:2 optical fiber cable (OFC) splitter connected in a ring fashion connected to one OLT port;
a second plurality of 2:1 optical coupler, wherein the optical coupler is to aggregate signal from primary and secondary paths; and
optical network terminals (ONTs) connected to the optical coupler.
2. The telecommunications network as claimed in claim 1, wherein the one or more network ports is one of a Ethernet and a Wi-Fi user port.
3. The telecommunications network as claimed in claim 1, wherein the first and the second OLT ports are within a same card.
4. The telecommunications network as claimed in claim 1, wherein the first and the second OLT ports are on different cards within the same system.
5. The telecommunications network as claimed in claim 1, wherein the first and the second OLT ports are on different cards on two different systems.
6. The telecommunications network as claimed in claim 1, further comprising:
a means for detecting a failure of one of the first plurality of optical fibers; and
a means for controlling the traffic over the OLT ports to provide an alternate communications route in response to the detection of a failure to a set of ONTs through the secondary OLT port.
7. The telecommunications network as claimed in claim 1, further comprising:
a means for detecting a failure of one of the plurality of primary interface modules; and
a means for controlling the traffic over the OLT ports to provide an alternate communications route in response to the detection of a failure to a set of ONTs through the secondary route.
8. The telecommunications network as claimed in claim 1, further comprising:
a means to do pre-ranging for detecting the equalization delay between any pair of ONT and OLT ports during the initial phase of installation and commissioning; and
a means of re-doing the ranging in case additional nodes with ONTs are added to the network at a later stage after the initial installation.
9. The telecommunications network as claimed in claim 1, further comprising:
a means to find a set of nodes with ONTs, wherein the ONTs are connected to the primary path; and
a means to find a disjoint set of nodes with ONTs connected to the secondary path.
10. The telecommunications network as claimed in claim 1, further comprising:
a means to adjust the equalization delay for the set of ONTs connected on the secondary path after the detection of failure.
11. An optical line terminator comprising:
a control and switching card;
a plurality of OLT cards coupled to the control and switching card;
a plurality of OLT ports being associated with the plurality of OLT cards; and
a memory storing operational data associated with the card,
wherein the optical line terminator is configured to:
detect one or more failures in the network;
associate one or more of the OLT ports with respective one or more of the failures;
copy the operational data from one OLT card associated with one of the failure to another OLT card; and
switch control of communications from the primary port whose operation data was copied to the secondary port.
12. The optical line terminator as claimed in claim 11, wherein the plurality of OLT ports is on different OLT cards.
13. A method for ranging network interface modules, the method comprising:
selecting a primary port from a plurality of OLT cards;
calculating relative distance data comprising differences in distances from the primary port to the plurality of ONTs;
selecting a secondary port from the plurality of OLT cards; and
determining a distance from the secondary port to the selected ONTs.
14. The method as claimed in claim 13, wherein the plurality of OLT cards are across two different systems.
15. A method for ranging network interface modules for a network, the method comprising:
determining range data for each OLT port associated with a plurality of primary OLT cards;
determining range data for each OLT port associated with the plurality of primary OLT cards; and
storing the determined range data in a memory of a control card, wherein the control card is coupled to all of the primary interface modules and a protection interface module.
16. The method as claimed in claim 15, wherein the range data for each OLT port associated with a plurality of primary OLT cards is determined during a period when the system is installed for the first time or when a nee node with ONTs are added at a later stage.

17. The method as claimed in claim 15, wherein the range data for each OLT port associated with the plurality of primary OLT cards is determined during a period when the system is relatively having lower amount of traffic to correct the ranging information periodically to compensate for aging.