Abstract:
A method, a system and a module are proposed for detecting and locating faults in an optical multi channel network composed of network nodes. The method includes assigning different sub-carrier pilot tones to a number of the nodes comprising Network Elements (NE), applying the different sub-carrier pilot tones to any signal added to the network via the respective nodes, checking presence of one or more of the sub-carrier pilot tones at one or more points of the network, and, based on results of the checking, locating a faulty section of the network in case a fault occurred.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method of detecting and locating faults in optical telecommunication networks.  
         BACKGROUND OF THE INVENTION  
         [0002]    There is quite a known principle in the art of optical telecommunication networks, to detect a fiber cut by analyzing power of a signal transmitted over the span in which the fiber cut may occur. The signal may be, for example, an optical carrier wavelength assigned to a particular optical channel, or a plurality of optical carrier wavelengths transmitted over one and the same optical fiber.  
           [0003]    According to another known approach, a pilot tone is added to the data transmitted over a fiber path in the network. Absence of the pilot tone at a particular location of the network manifests the presence of a fiber cut before this particular location.  
           [0004]    U.S. Pat. No. 6,115,154 describes a method and a system for detecting fiber cuts in an optical network regardless of the number of EDEAs (amplifiers) that are located between the fiber cut and the monitor point. The power of a marker wavelength is compared to the power of a nearby spectral region. Where the comparison indicates that the power ratio is approximately equal to unity, a flag is raised indicating that there is a fiber cut.  
           [0005]    U.S. Pat. No. 6,285,475 B1 proposes detecting optical faults by tapping a portion of a data signal from along a fiber network link at an optical switching site or node. A sub-carrier modulation signal can be added to the high rate data signal prior to transport over the link. The sub-carrier signal is significantly lower in both frequency and amplitude than the main data signal so as to not impact reliable reception of the main data signal. A low-pass filter tuned to the sub-carrier modulation signal filters the tapped data signal. By detecting the sub-carrier data signal, fault can be determined inexpensively and reliably in the presence of optical noise.  
           [0006]    The problem of the above solutions is that neither of them enables precise location of a fiber cut, i.e., pointing out a specific segment of the network where a failure occurred.  
           [0007]    The above problem, in particular, stems from the fact that any optical network comprises add-drop network elements. Any optical carrier wavelength or a pilot sub-carrier tone may disappear just because one or more optical channels marked by them are dropped by an OADM (Optical Add Drop Multiplexer); due to that detection of a fiber cut becomes problematic.  
         OBJECT OF THE INVENTION  
         [0008]    It is therefore an object of the present invention to provide a method and a system for precise location of a fault (such as a fiber cut or an equipment failure) in an optical network.  
         SUMMARY OF THE INVENTION  
         [0009]    A method of detecting and locating faults in an optical multi channel network (such as WDM) composed of network nodes, the method including:  
           [0010]    assigning sub-carrier pilot tones to a number of the nodes comprising Network Elements NE (such as Optical Add-Drop Multiplexers (OADMs) or switches), wherein each of said tones having frequency different from those of multiple optical channels of the network and different from those of sub-carrier pilot tones assigned to other network nodes,  
           [0011]    applying said different sub-carrier pilot tones to any signal added to the network via the respective nodes,  
           [0012]    checking presence of one or more of the sub-carrier pilot tones at one or more points of the network, and,  
           [0013]    based on results of the checking, locating a faulty section of the network in case a fault occurred.  
           [0014]    Preferably, said one or more points of the network are respectively positioned at one or more of said nodes.  
           [0015]    The locating of a faulty section is preferably performed by relating the absent pilot tone, if any, with the network section associated with the Network Element (NE) from which said tone should originate.  
           [0016]    More particularly, the location of the faulty section is selecting a section in the network associated with a particular one of said Network Elements (NE) responsible for “marking” the added optical channels with the sub-carrier which is absent but should be present at the particular point of the network.  
           [0017]    Preferably, the checking is accomplished by filtering a split out portion of the incoming optical signal for determining which sub-carrier pilot tone(s) is absent and which sub-carrier pilot tone(s) are present.  
           [0018]    For the location of a fault, there is a need in comparing the information on the sub-carriers (status) with a so-called correct picture of the network (the one reflecting which sub-carriers should be present at this particular point).  
           [0019]    Preferably, the correct picture of the network is known to a so-called Network Manager (NM) and is formed from at least a) information on the optical channels added and dropped at each of said Network Elements, and b) data on sub-carriers assigned to each of said Network Elements.  
           [0020]    It is understood that the Network Manager must be continuously updated about changes in the topology and the correct picture of the network.  
           [0021]    The Network Manager is to be understood as a system of software programs and/or hardware equipment ensuring management of the network. The Network Manager may be either fully automatic or include participation of a human operator.  
           [0022]    For example, a number of local decisions on the fault location may be made automatically at local point(s) of the network and displayed on a screen of a control room of the network, so that the final location of the fault be made by the operator who is also responsible for taking steps for eliminating the faults. Alternatively, the NM may be responsible of collecting the status information from the local point(s) of the network and making the fault detection and location decisions.  
           [0023]    For performing the comparison at a local point of the network, the information on the correct network picture can be obtained from the NM or introduced manually. Alternatively, the comparison can be performed by the Network Manager if the information on the checked sub-carrier at a particular local point is transferred to the Network Manager.  
           [0024]    The final fault location is preferably performed in the Network Manager since it requires analysis of the information or local decisions obtained from a number of local points (NE) of the network.  
           [0025]    The fault location preferably comprises discriminating between a fiber cut occurring in a network span associated with the Network Element originating the sub-carrier detected to be absent, and between an equipment fault in said Network Element. Owing to the fact that the sub-carrier pilot tones assigned to different nodes “mark” different segments of the network, the method enables checking one local decision on the fault location by another local decision, thereby locating the fault with maximal accuracy.  
           [0026]    The Network Element (NE) suitable for the above method is a device capable of adding one or more optical channels to the mentioned optical network. According to the above condition, the NE can be a switching device of the kind suitable for interconnecting two or more adjacent networks. Another example of the NE is an Optical Add Drop Multiplexer (OADM). A third example is a terminal network element injecting a number of optical channels into the network.  
           [0027]    The method may additionally comprise a step of measuring power of a particular sub-carrier pilot tone if present at a particular point of the network and comparing its power with a predetermined expected power level. The expected power level value(s) can be part of the correct picture of the network. Preferably, the step of measuring said power is performed periodically and can thereby be used for monitoring the network performance, for example for revealing degradation of a fiber at specific sections of the network and predicting faults. The periodic character of the measurements enables excluding random errors and considering fluctuations caused, for example, by temperature changes.  
           [0028]    There is also provided a system intended for a multi-channel optical network comprising a number of nodes,  
           [0029]    the system comprising Network Elements (NE) respectively located at the nodes, wherein each of the NE being capable of adding one or more optical channels to the network and provided with equipment for modulating all optical channels added at said node by a unique frequency of a sub-carrier pilot tone, the sub-carrier pilot tones at different Network Elements differing from one another and from frequencies of the optical channels,  
           [0030]    the system also comprising at least one checking device located at a particular point of the network and capable of detecting presence of one or more of said sub-carrier pilot tones at said particular point,  
           [0031]    the system being capable of processing the information detected at each of said checking devices, together with a correct picture of the network, and  
           [0032]    based on results of the processing, detecting a fault, if any, and locating the fault in the network.  
           [0033]    The above-mentioned correct picture of the network is usually created in a Network Manager (NM) which can be considered part of the system, said correct picture comprises complete and update data on added and/or dropped optical channels at each of said Network Elements and on the sub-carrier pilot tones over the network.  
           [0034]    The processing can be provided at the checking device(s) upon obtaining the required network picture information from the Network Manager (NM). Fragments of the correct picture of the network, obtained form the NM, can be stored at the checking devices located at the Network Elements.  
           [0035]    However, the processing is preferably provided by the Network Manager upon obtaining information on the detected present/absent sub-carrier pilot tones from the checking devices. Therefore, the NM is preferably responsible for the fault detection and location.  
           [0036]    Actually, the NM locates a fault by pointing out a section in the network associated with a particular NE responsible for introducing such a sub-carrier (pilot tone) which is absent but should be present at the checking device.  
           [0037]    Owing to the capability of comparing the correct picture of the network with information on the present and/or absent sub-carriers at the Network Elements, the system is capable of discriminating between fiber cuts and between equipment faults in the NE associated with the section pointed out.  
           [0038]    The above system is preferably suitable for point-to point and ring network configurations, and for combinations thereof.  
           [0039]    Most preferably, the checking devices are positioned at the respective nodes and form integral part of the Network Elements (NE).  
           [0040]    However, according to yet another aspect of the invention, there can be provided a sub-carrier module suitable for being used in any point of a multi-channel optical network (and in the system according to the invention), and comprising  
           [0041]    a modulating unit for modulating one or more optical channels, to be added to the network at a particular point, by a unique frequency of a sub-carrier pilot tone from a list of tones, said sub-carrier pilot tones differing by frequency from other tones of the list and from the optical channels,  
           [0042]    a checking unit capable of detecting presence of one or more of the sub-carrier pilot tones of the list at a particular point where the checking unit is located,  
           [0043]    wherein said modulating unit and checking unit being operative either separately or together.  
           [0044]    Preferably, the checking unit is capable of communicating with a Network Manager system.  
           [0045]    Such a sub-carrier module can be placed at any point of the network as a stand-alone device, but preferably is adapted to operate at a network node, in conjunction with a switching device capable of adding to the network one or more optical channels. In the last embodiment, the module is already not a stand-alone device, but forms integral part of the Network Element.  
           [0046]    The sub-carrier module is operative to communicate with the Network Manager for information exchange and for detecting and locating faults in the network.  
           [0047]    In case the sub-carrier module is not provided with prior information on the correct picture of the network, its checking unit will be capable of forwarding reports on status of the absent and present sub-carrier pilot tones, say to the Network Manager. If the sub-carrier module is provided with such prior information, the checking module is capable of comparing it with the status and producing a local decision on the fault detection and location which can be either confirmed or corrected.  
           [0048]    One further aspect of the invention is a Network Element (NE) capable of adding one or more optical channels to a multi-channel optical network, the NE being equipped with a modulation unit capable of impressing optical signals of the one or more optical channels added to the network via the NE, with a particular sub-carrier pilot tone having frequency differing from those of the multiple optical channels, and also equipped with a checking unit for determining presence of one or more of the sub-carrier pilot tones differing form those of said particular pilot tone.  
           [0049]    There is also proposed a network node comprising a Network Element (NE) capable of adding to a multi-channel optical network one or more optical channels, the node being equipped with a modulation unit capable of impressing optical signals of the one or more optical channels added at the node with a particular sub-carrier pilot tone having frequency differing from those of other nodes and those of the multiple optical channels, and also equipped with a checking unit for determining presence of one or more of the sub-carrier pilot tones at the node—for further processing the results in order to detect a fault and locate it in the network.  
           [0050]    With respect to the system, the sub-carrier module, the node and the Network Element itself being capable of adding to the network one or more optical channels, said NE may constitute, for example, an OADM or a switching device connecting two or more adjacent networks. A third example of a such NE is a terminal element of the network which transmits a number of optical channels to the network.  
           [0051]    Further details of the invention will be apparent from the figures and the description which follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]    The invention will be further described and illustrated by way of examples and with the aid of the following non-limiting drawings, in which:  
         [0053]    [0053]FIG. 1 schematically illustrates a simplified example of a ring network, adapted for detecting and locating faults at the network sections using various sub-carrier frequencies assigned to different nodes of the network.  
         [0054]    [0054]FIG. 2 schematically illustrates one optical network configuration being a combination of a point-to point architecture and a ring architecture with nodes comprising OADMs, the configuration is adapted for detecting and locating faults at any section of the network.  
         [0055]    [0055]FIG. 3 schematically illustrates another configuration of two adjacent networks, interconnected by a switching device and adapted for detecting and locating faults.  
         [0056]    [0056]FIG. 4 schematically illustrates one exemplary implementation of the checking device according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]    [0057]FIG. 1 shows a ring-like network  10  where each of the network nodes of interest— 12 ,  14 ,  16 —comprises an OADM element and is provided with an ability of impressing the added data streams with a specific sub-carrier pilot tone f 1 , f 2  and f 3 , respectively. The network also comprises a checking unit  18 , which is illustrated as a stand-alone device.  
         [0058]    In this exemplary embodiment, the unit  18  is provided with filters F 1 , F 2 , F 3  which are capable of detecting whether any particular pilot tone exists on the span. The unit  18  will be able to serve as a fault detecting/locating one, if it receives from a Network Manager system (not shown) information called a correct network picture. In other words, the unit  18  is informed (arrow  13 ) whether any of the pilot tones should exist at this particular point and, using this information, decides whether there is a fault and where (alarm  15  being a local decision). The alarms can be forwarded to the Network Manger. For example, if a sub-carrier f 1  must exist but is absent, an alarm signal may manifest either a fault in spans between nodes  12 - 14 ,  14 - 18 , or a fault in the node  12 . If the pilot tone f 2  is present, there is no fiber cut in the span  14 - 18 . To clarify whether there is a cut in the span  12 - 14  or in the node  12  itself, it would be necessary to have more fault detection and location devices.  
         [0059]    It should be mentioned, that the fault detection and location unit  18  may be a separate stand-alone unit in the network (as shown in the drawing), but preferably it forms integral part of a network node. Therefore, each of the nodes  12 ,  14 ,  16  may comprise and preferably comprises both the equipment for adding the assigned sub-carrier to the added channels, and the equipment for analyzing which sub-carriers have arrived to the particular node. In such an embodiment, discrimination of the fault would be provided more accurately. FIGS. 2 and 3 will illustrate the last statement.  
         [0060]    [0060]FIG. 2 shows a combined network configuration  20 , comprising a point-to-point portion  21  between MUX  22  and DEMUX  24  and including OADM  25 , and a ring portion  23  connected to the OADM NODE  32 . The network portions are interconnected via OADMs  25  and  32 . Presence of a checking device  26  on the point-to point portion, and of one or more checking devices  28  on the ring portion of the network allow detection faults at any span of the combined architecture.  
         [0061]    In this example, the MUX  22  multiplexes a number of optical channels λ1-λ40 into an optical signal which can be transmitted via an optical fiber  30  in the point-to point configuration  21  which belongs to a first service provider. The optical channels, entered to the network at the terminal element, can be modulated by a sub-carrier pilot tone f 23 . The OADM  25  drops several optical channels (λ1-λ5) to be forwarded to customers of a second service provider, which are arranged in the ring network  23 . In this embodiment, the OADM  25  adds the same optical channels (λ1-λ5) to the point-to point network  21 , after the information transmitted via these channels from the MUX  22  is distributed in the ring network  23  and replaced by other information to be transmitted to the DMUX  24 .  
         [0062]    In this example, the ring configuration  23  includes OADM nodes  32 ,  34 ,  36  and  38 , each equipped with means for impressing the added optical channels with a characteristic pilot sub-carrier tone: f 32 , f 34 , f 36  and f 38  respectively. In this network, each of the OADM nodes comprises a checking device  28  capable of analyzing which sub-carrier tones are present at the particular point of the network. Each of the checking devices is also informed which sub-carrier pilot tones must exist and which should be absent at each particular point of the network. The manner of informing them is not discussed in the frame of the present application. One of possible options is obtaining this information from a Network Manager (not shown). Another option is to provide each of the nodes with this “correct picture” information in a de-centralized manner, before starting the network.  
         [0063]    If the checking device  26  detects absence of the pilot sub-carrier f 25  (which is to be present), and presence of f 23 , it will be a signal of a fault, but will indicate that there is no fiber cut in the network  21  between the OADM  25  and the device  26 . The fault may be either a fiber cut in the network  23  between OADM  32  and OADM  38 , or a fault in the OADM  25  and/or  32  (a possible fiber cut is marked with a dotted cross). The location of the fault should be exactly pointed out to let a suitable service provider both to take care of it and to bear expenses connected to it. For example, to clarify the position of the fault, the checking device  28  of the OADM  32  should also be activated. In case the pilot sub-carriers f 36 , f 34 , f 38  are absent at the OADM  32 , and the OADM  32  itself is in operative condition (which can be confirmed if f 32  is present in any of OADMs  34 ,  36 ,  38 ) the fault is definitely located in the network  23 , between nodes  32  and  38 . In this case, two local decisions confirm one another. Similarly, a specific fault of a particular NE (and not a fiber cut in the span outgoing the NE) can be located if at least one following NE detects sub-carrier pilot tone(s) which passed through the particular NE but does not detect a pilot tone assigned to the particular NE.  
         [0064]    [0064]FIG. 3 illustrates a configuration  40 , where a ring network  42  and a ring network  44  (shown partially) are connected via a switching device  46  which is shown most schematically. Optical channels, added and dropped in the network  42 , are shown as (λ1-λ5) at the nodes of the network: OADM  45 , switch  46 , OADM  47  and OADM  48 . The switching device adds to the network  42  an optical channel with λ4 which is switched from the network  44 ; in turn, λ1 is switched from the network  42  and added to the network  44 . Optical channels, added at the nodes  45 ,  46 ,  47  and  48  are respectively “marked” (modulated) by sub-carriers f 11 , f 12 , f 13  and f 14 , by suitable equipment situated at the nodes. A sub-carrier f 15  is impressed to an optical channel carrying λ1 switched from the network  42  to the network  44  via the switch  46 . Each of the nodes is provided with a monitoring block (not shown) for detecting pilot tones at the node. The system includes a Network Manager  50  in communication with the monitoring blocks of the nodes, for the information exchange. For example, each of the nodes  45 ,  46 ,  47 ,  48  comprises a memory in the monitoring block and holds a look-up table where information about the present and absent pilot tones can be registered. The Manager  50  will receive reports from each of the monitoring blocks about the current status of the nodes from the point of pilot tones.  
         [0065]    For example, node  47  detected absence of the pilot tone f 12  and presence of the tones f 11  and f 14  and reported it to the Manager  50 . The Manager checks whether, according to the correct picture of the network, the pilot tone f 12  must be present at the node  47 . If not, it is a normal situation and no action is taken. If f 12  must be present but is absent, there are two options—it is either a fiber cut between nodes  46  and  47 , or a fault in the node  46 . Further, the fault in the node  46  may be a pass fault (the switch does not output any signals) or a switching fault (the switch fails to perform the switching operation). The Manager&#39;s  50  decision is as follows: since the pilot tones f 11  and f 14  are present at the node  47 , there is no pass fault in the node  46  and no fiber cut in the span between the nodes  46  and  47 . Therefore, there is a switching fault in the switch  46 . Upon detecting the fault and locating it for the network  42 , the following actions may be taken. The Manager  50  may contact a Manager (not shown) of the network  44  to report on the absence of f 12  (and therefore, the absence of λ4). The Manager of the network  44  may then check whether there is a fault in its network, and not obligatory in the switch  46 . Any way, an alarm should be produced by the Manager  50  to inform the network&#39;s  42  operator on switching problems in the node  46 . This alarm may be confirmed or complemented by another alarm which possibly will be received by an operator of the network  44  from its Manager.  
         [0066]    [0066]FIG. 4 illustrates one possible embodiment of a checking device (unit), or a monitoring block  60  which preferably forms part of the sub-carrier module and the node according to the invention. The optical signal arrives to the node via a fiber  62  and, upon splitting by a splitter  64 , a portion of the optical signal is converted into an electric signal by an O/E converter  66 . The obtained electric signal is fed to an adaptive filter  68  provided with a voltage control input  70 . The adaptive filter is controlled so that it becomes sensitive to a number of different frequencies of the signal. In other words, it can be adapted to be ready to detect particular sub-carrier pilot tones, as desired at the particular node, by scanning the bandwidth and sequentially skipping from one frequency to another with changing the control voltage  70 . Thus, whenever a particular pilot tone is detected by the adaptive filter  68 , this fact is synchronously registered in a memory block  72 . The detection can be performed using a threshold voltage Vth. The memory holds a sub-carrier pilot tones status table (schematically shown as  74 ; the registered pilot tones are marked by “V” in the table). Reporting the status table to the Network Manager would constitute a status report.  
         [0067]    The checking device may optionally comprise a look-up table  76  located in the memory and reflecting a desired status of sub-carriers at the node. Such a table is actually a fragment of the correct picture of the network and can be either obtained from the Network Manager, or manually introduced in the unit. The checking device  60  can be designed to compare tables  74  and  76  and analyze the result, thus obtaining a local decision (alarm) on the fault and its location. The local decision can be further reported to the Network Manager. Optionally, both the status table  74  and the look-up table  76  can be reported to the Network Manager.  
         [0068]    As shown in FIG. 4, the checking device determines the presence of a sub-carrier pilot tone using the threshold Vth. In addition, the checking device may comprise a circuitry (not shown) for measuring power of the detected particular sub-carrier pilot tone and comparing its power with a suitable predetermined expected power level. If such measurements are provided simultaneously with detecting the sub-carriers, the comparison results can be stored in a specific “power” table in the memory. Such a table can be periodically reported to the Network Manager and used for the network performance monitoring, for example for revealing a fiber degradation at specific sections of the network. Such tables obtained over the network can also be used for estimation and predicting effects caused by temperature changes.