Abstract:
A method is provided for protection switching in an optical network. The method may include communicating a switch request for initiation of protection switching in response to a determination that at least a minimum frequency of interrupts indicating failure of an optical signal has occurred over a first period. The method may also include communicating a switch request for cessation of protection switching in response to a determination that no more than a maximum frequency of interrupts indicating failure of an optical signal has occurred over a second period.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to optical networks and, more particularly, to a method and system for protection switching in an optical system. 
       BACKGROUND 
       [0002]    Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of communicating the signals over long distances with very low loss. 
         [0003]    To ensure high reliability and availability in optical communications networks, protection switching is often used. When implemented, protection switching typically provides a primary or “working” path for a network and a redundant or “protection” path for the network. Accordingly, each path may be monitored, and if a failure is detected on the working path, network traffic may be switched to the protection path. An example of protection switching may be Ethernet Linear Protection Switching (ELPS) as defined by the ITU G.8031 standard. 
         [0004]    With protection switching, an optical signal may be transmitted via two or more optical paths between the same source and destination node. A selector at the destination may include a photodetector per each path to monitor signals received from the two or more paths. Based on such received signals, the selector may select one of the signals to be forwarded to a transponder or receiver at the destination node. For example, the selector may determine, based on the photodetector monitoring, whether one of the paths has experienced a loss of signal or “loss of light.” If a particular path experiences a loss of light, then the selector may select another path to forward to the transponder or receiver. Such selection may be referred to as a “protection switch.” 
         [0005]    The selector may operate in accordance with a protection switching protocol (e.g., ITU G.8031 or other standard). Each protection switching protocol may include a hierarchy for handling user-initiated and auto-failure initiated protection switching requests. Such hierarchy may be implemented via hardware, software, or a combination thereof. If a portion of the hierarchy is implemented in software, then hardware must quickly notify software of any signal loss that has occurred or cleared as switching is a time-sensitive operation. Such notification is typically performed via interrupts. 
         [0006]    Often, an optical signal entering the selector may be unstable, in that the signal failure occurs and clears rapidly and repeatedly. For example, while unstable a signal may fail and clear 20 times per second. This may lead to many interrupts being received by an interface layer in software, which are then translated to switch request messages for a switching engine. The switching engine may receive such requests and apply a switching hierarchy. Frequent switch requests can exhaust the resources available to the switching engine, and may cause software failure. 
       SUMMARY 
       [0007]    In accordance with a particular embodiment of the present disclosure, a method for protection switching in an optical network may include communicating a switch request for initiation of protection switching in response to a determination that at least a minimum frequency of interrupts indicating failure of an optical signal has occurred over a first period. The method may also include communicating a switch request for cessation of protection switching in response to a determination that no more than a maximum frequency of interrupts indicating failure of an optical signal has occurred over a second period. 
         [0008]    Technical advantages of one or more embodiments of the present invention may provide a software-based solution to reduce the frequency of interrupts received by a switching element, thus potentially reducing processing required by a switching engine. 
         [0009]    It will be understood that the various embodiments of the present invention may include some, all, or none of the enumerated technical advantages. In addition, other technical advantages of the present invention may be readily apparent to one skilled in the art from the figures, description and claims included herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a block diagram illustrating an example optical network, in accordance with certain embodiments of the present disclosure; 
           [0012]      FIG. 2  is a block diagram illustrating an example stack for a decision module of a selector, in accordance with certain embodiments of the present disclosure; and 
           [0013]      FIG. 3  is a block diagram illustrating a finite state machine implemented by an interface layer of software to reduce the frequency of switch requests to a switch engine implemented in software. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  illustrates an example optical network  10 . Optical network  10  may include one or more optical fibers  28  operable to transport one or more optical signals communicated by components of the optical network  10 . The components of optical network  10 , coupled together by optical fiber  28 , may include nodes  12   a  and  12   b  and one or more optical add/drop multiplexers (OADMs)  32 . A node  12  and/or an OADM  32  may be generally referred to as a “network element.” Although the optical network  10  is shown as a point-to-point optical network with terminal nodes, the optical network  10  may also be configured as a ring optical network, a mesh optical network, or any other suitable optical network or combination of optical networks, and may include any number of nodes intermediate to nodes  12   a  and  12   b . The optical network  10  may be used in a short-haul metropolitan network, a long-haul inter-city network, or any other suitable network or combination of networks. 
         [0015]    A node  12  and/or OADM  32  may represent a Label Switching Router (LSR). One or more label switched paths (LSPs) including a sequence of nodes  12  and OADMs  32  may be established for routing packets throughout optical network  10 . For example, traffic may travel from source node  12   a , through zero, one, or more intermediate OADMs  32 , to destination node  12   b.    
         [0016]    Node  12   a  may include transmitters  14 , a multiplexer  18 , an amplifier  26 , and a splitter  24 . Transmitters  14  may include any transmitter or other suitable device operable to transmit optical signals. Each transmitter  14  may be configured to receive information transmit a modulated optical signal at a certain wavelength. In optical networking, a wavelength of light is also referred to as a channel. Each transmitter  14  may also be configured to transmit this optically encoded information on the associated wavelength. The multiplexer  18  may include any multiplexer or combination of multiplexers or other devices operable to combine different channels into one signal. Multiplexer  18  may be configured to receive and combine the disparate channels transmitted by transmitters  14  into an optical signal for communication along fibers  28 . 
         [0017]    Amplifier  26  of node  12   a  may be used to amplify the multi-channeled signal. Amplifier  26  may be positioned before and/or after certain lengths of fiber  28 . Amplifier  26  may comprise an optical repeater that amplifies the optical signal. This amplification may be performed without opto-electrical or electro-optical conversion. In particular embodiments, amplifier  26  may comprise an optical fiber doped with a rare-earth element. When a signal passes through the fiber, external energy may be applied to excite the atoms of the doped portion of the optical fiber, which increases the intensity of the optical signal. As an example, amplifier  26  may comprise an erbium-doped fiber amplifier (EDFA). However, any other suitable amplifier  26  may be used. 
         [0018]    Splitter  24  may represent an optical coupler or any other suitable optical component operable to split an optical signal into multiple copies of the optical signal and transmit the copies to other components within network  10 . In the illustrated embodiment, splitter  24  may receive a signal from amplifier  26  of node  12   a  and split the received traffic into two copies. One copy may be transmitted via path  42   a , while the other copy may be transmitted over  42   b , in order to provide redundancy protection for the signal, as described in greater detail below. 
         [0019]    The process of communicating information at multiple channels of a single optical signal is referred to in optics as wavelength division multiplexing (WDM). Dense wavelength division multiplexing (DWDM) refers to the multiplexing of a larger (denser) number of wavelengths, usually greater than forty, into a fiber. WDM, DWDM, or other multi-wavelength transmission techniques are employed in optical networks to increase the aggregate bandwidth per optical fiber. Without WDM or DWDM, the bandwidth in networks would be limited to the bit rate of solely one wavelength. With more bandwidth, optical networks are capable of transmitting greater amounts of information. Referring back to  FIG. 1 , node  12   a  in optical network  10  may be configured to transmit and multiplex disparate channels using WDM, DWDM, or some other suitable multi-channel multiplexing technique, and to amplify the multi-channel signal. 
         [0020]    As discussed above, the amount of information that can be transmitted over an optical network varies directly with the number of optical channels coded with information and multiplexed into one signal. Therefore, an optical signal employing WDM may carry more information than an optical signal carrying information over solely one channel. An optical signal employing DWDM may carry even more information. 
         [0021]    After the multi-channel signal is transmitted from node  12   a , the signal may travel over one or more paths  42  (e.g., paths  42   a  and  42   b ) to node  12   b . Each path  42  may include one or more OADMs  32 , one or more amplifiers  26 , and one or more fibers  28  coupling such OADMs  32  and amplifiers  26 . 
         [0022]    An OADM  32  may include any multiplexer or combination of multiplexers or other devices operable to combine different channels into one signal. An OADM  32  may be operable to receive and combine the disparate channels transmitted across optical network  10  into an optical signal for communication along fibers  28 . In addition, an OADMs  32  comprise an add/drop module, which may include any device or combination of devices operable to add and/or drop optical signals from fibers  28 . An OADM  32  may be coupled to an amplifier  26  which may be used to amplify a WDM and/or DWDM signal as it travels through the optical network  10 . After a signal passes through an OADM  32 , the signal may travel along fibers  28  directly to a destination, or the signal may be passed through one or more additional OADMs  32  before reaching a destination. 
         [0023]    Similar to amplifier  26  of node  12   a , other amplifiers  26  or optical network  10  may be used to amplify the multi-channeled signal communicated by OADMs  32 . Amplifiers  26  may be positioned before and/or after certain lengths of fiber  28 . Amplifiers  26  may comprise an optical repeater that amplifies the optical signal. This amplification may be performed without opto-electrical or electro-optical conversion. In particular embodiments, amplifiers  26  may comprise an optical fiber doped with a rare-earth element. When a signal passes through the fiber, external energy may be applied to excite the atoms of the doped portion of the optical fiber, which increases the intensity of the optical signal. As an example, amplifiers  26  may comprise an erbium-doped fiber amplifier (EDFA). However, any other suitable amplifiers  26  may be used. 
         [0024]    An optical fiber  28  may include, as appropriate, a single, unidirectional fiber; a single, bi-directional fiber; or a plurality of uni- or bi-directional fibers. Although this description focuses, for the sake of simplicity, on an embodiment of the optical network  10  that supports unidirectional traffic, the present invention further contemplates a bi-directional system that includes appropriately modified embodiments of the components described below to support the transmission of information in opposite directions along the optical network  10 . Furthermore, as is discussed in more detail below, the fibers  28  may be high chromatic dispersion fibers (as an example only, standard single mode fiber (SSMF) or non-dispersion shifted fiber (NDSF)), low chromatic dispersion fibers (as an example only, non zero-dispersion-shifted fiber (NZ-DSF), such as E-LEAF fiber), or any other suitable fiber types. 
         [0025]    Node  12   b  may be configured to receive signals transmitted over optical network  10 . For example, as shown in  FIG. 1 , a portion of the multi-channel signal through path  42   a  may be dropped to node  12   b  by OADM  32   a , and a portion of the multi-channel signal through path  42   b  may be dropped to node  12   b  by OADM  32   b . Node  12   b  may include a selector  82  and a receiver  22 . Selector  82  may be configured to receive at least a portion of the multi-channel signal from each of path  42   a  and  42   b  and selects which of the two signals to pass to receiver  22 . Such selection may be made on any suitable criteria, including bit error rate and/or power levels of the individual signals. 
         [0026]    Selector  82  may include a photodetector  86  (e.g., photodetectors  86   a  and  86   b ) associated with each path  42 , a decision module  88 , and a switch  84 . A photodetector  86  may be any system, device or apparatus configured to detect an intensity of light and convert such detected intensity into an electrical signal indicative of such intensity. Such electrical signals from photodetectors  86  may be communicated to decision module  88 . Based on analysis of the electrical signals from photodetectors  86 , decision module  88  may determine whether to pass the signal dropped from path  42   a  or the signal dropped from path  42   b . A signal indicative of such determination may be communicated from decision module  88  to switch  84 , and switch  84  may pass either the signal from path  42   a  or the signal from path  42   b  to receiver  22  based on the signal received from decision module  88 . For example, decision module  88  may be configured such that the signal received from path  42   a  is passed to receiver  22  unless the intensity of signal received via path  42   a  falls below a particular threshold relative to a baseline power level (thus indicating a loss of light condition), in which case switch  84  may protection switch such that the signal received via path  42   b  is passed to receiver  22 . 
         [0027]    Receiver  22  may include any receiver or other suitable device operable to receive an optical signal. Receiver  22  may be configured to receive one or more channels of an optical signal carrying encoded information and demodulate the information into an electrical signal. 
         [0028]      FIG. 2  is a block diagram illustrating an example stack for decision module  88  of selector  82 , in accordance with certain embodiments of the present disclosure. As shown in  FIG. 2 , decision module  82  may include hardware  102  and software  104 . Hardware  102  of decision module  88  may be communicatively coupled to photodetectors  86  and based on signals received from photodetectors  86 , may communicate an interrupt  110  to software  104 . Such interrupt may indicate a failure of an optical signal received at a photodetector  86 , or a clearing of such failure. 
         [0029]    Software  104  may include a program of instructions carried on a computer-readable medium and executable by a processor, the program of instructions operable to, when executed, carry out the functionality described herein. As shown in  FIG. 2 , software  104  may include interface later  106  and switch engine  108 . Interface layer  106  may be a software abstraction layer that translates commands, messages, and requests between switch engine  108  and hardware  102 . Interface layer  106  may receive an interrupt  110  from hardware  102  and translate interrupt  110  to a switch request message  112  for switch engine  108 . 
         [0030]    Switch engine  108  may be configured to receive switch request messages  112  and based on such messages and a protection switching hierarchy (e.g., ITU G.8031 or other standard) may generate a switching command  114  to be communicated to interface layer, which interface layer may translate and forward as switching command  116  to hardware  102 , such switching command  116  ultimately destined for switch  84 . 
         [0031]    As mentioned previously, an optical signal entering selector  82  may be unstable, in that the signal failure occurs and clears rapidly and repeatedly. This may lead to many interrupts  110  being received by interface layer  106  and many switch requests  112  being processed by switching engine  108 . These frequent switch requests  112  can exhaust the resources available the switching engine  108 , possibly leading to failure of software  104  or other undesirable effects. 
         [0032]    To prevent such frequent switch requests  112 , interface layer  106  may implement a state machine such that translates an interrupt  110  into a switch request  112  indicating failure to switching engine  108  only upon receipt of at least a minimum frequency of interrupts  110  over a particular period, and may communicate a switch request  112  indicating clearance of a failure to switching engine  108  only upon receipt of nor more than a maximum frequency of interrupts  110  over another particular period. Finite state machine  200  of  FIG. 3  illustrates such a state machine. 
         [0033]    At state  202 , state machine  200  is in a normal state in which no failure exists (e.g., selector  82  selects working path). While in state  202 , if interface layer  106  receives a minimum number of interrupts (e.g., X or more) for each polling interval (e.g., T 1 ) for a particular number (e.g., Y) of consecutive polling periods, interface layer  106  may transition state machine  200  to state  204 , wherein state  204  indicates a failed state. In connection with transitioning to state  204 , interface layer  106  may communicate a switch request  112  to switch engine  108  indicating a failure. In response to the switch request  112 , switch engine  108  may initiate protection switching for node  12   b  (e.g., switch from working path  42   a  to protection path  42   b ). The minimum number of interrupts (X), polling interval (T 1 ), and/or the number of polling intervals (Y) may be set to any appropriate value(s). Such values may be set automatically or manually, and may be determined by a manufacturer, user, administrator, and/or other suitable person. As a specific example, the polling interval T 1  may be set to 500 ms. As another specific example, the number of polling intervals may be set to 20. 
         [0034]    While in state  204 , if interface layer  106  receives a maximum number of interrupts or fewer (e.g., W or fewer) for each polling interval (e.g., T 2 ) for a particular number (e.g., Z) of consecutive polling periods, interface layer  106  may transition state machine  200  to state  202 . In connection with transitioning to state  202 , interface layer  106  may communicate a switch request  112  to switch engine  108  indicating clearance of a failure. In response to the switch request  112 , switch engine  108  may cease protection switching for node  12   b  (e.g., switch from protection path  42   b  to working path  42   a ). The maximum number of interrupts (W), polling interval (T 2 ), and/or the number of polling intervals (Z) may be set to any appropriate value(s). Such values may be set automatically or manually, and may be determined by a manufacturer, user, administrator, and/or other suitable person. In certain embodiments, the maximum number of interrupts W may be zero. In these and other embodiments the polling interval T 2  may be equal to the polling interval T 1 . In the same or alternative embodiments, the number of polling intervals Z may be equal to the number of polling intervals Y. 
         [0035]    A component of optical network  10  may include an interface, logic, memory, and/or other suitable element. An interface receives input, sends output, processes the input and/or output, and/or performs other suitable operation. An interface may comprise hardware and/or software. 
         [0036]    Logic performs the operations of the component, for example, executes instructions to generate output from input. Logic may include hardware, software, and/or other logic. Logic may be encoded in one or more tangible computer readable storage media and may perform operations when executed by a computer. Certain logic, such as a processor, may manage the operation of a component. Examples of a processor include one or more computers, one or more microprocessors, one or more applications, and/or other logic. 
         [0037]    A memory stores information. A memory may comprise one or more tangible, computer-readable, and/or computer-executable storage medium. Examples of memory include computer memory (for example, Random Access Memory (RAM) or Read Only Memory (ROM)), mass storage media (for example, a hard disk), removable storage media (for example, a Compact Disk (CD) or a Digital Video Disk (DVD)), database and/or network storage (for example, a server), and/or other computer-readable medium. 
         [0038]    Modifications, additions, or omissions may be made to optical network  10  without departing from the scope of the invention. The components of optical network  10  may be integrated or separated. Moreover, the operations of optical network  10  may be performed by more, fewer, or other components. Additionally, operations of optical network  10  may be performed using any suitable logic. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
         [0039]    Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.