Abstract:
A computerized method for modifying an optical wavelength division multiplexing (WDM) network is described. The method includes providing an initial configuration of the optical WDM network. At least one parameter that represents a characteristic of a span in the optical WDM network as defined by the initial configuration is evaluated. The initial configuration of the optical WDM network is modified in response to the evaluation. The method can also include determining if a predetermined set of engineering rules are satisfied.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date of U.S. provisional patent application Ser. No. 60/493,683, filed Aug. 8, 2003, titled “System and Method for Automated Engineering of Optical Networks,” the entirety of which provisional application is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the computer modeling of optical networks. More particularly, the invention relates to a system and method that automatically modifies an optical wavelength division multiplexing (WDM) network to improve performance. 
     BACKGROUND 
     The task of engineering a WDM network consists of identifying the appropriate type of equipment, the appropriate location of the equipment, and specifying the required settings of the equipment so that optical signals transmitted through the network have satisfactory optical characteristics. Computer modeling software can be used to aid in engineering new WDM networks and modifying existing WDM networks. As shown in  FIG. 1 , central to the engineering process is a software modeling environment  10 , including a computer system  12  for running the computer modeling software. A network designer executes the software to generate a computer model  16  of an optical network  14 . The optical network  14  typically includes fiber optic links  18 , or optical spans, through which multiple channels of bi-directional optical signals are multiplexed on a pair of optical fibers. Each signal is defined on a carrier wavelength. The links  18  connect sites  22  in the optical network  14 . A site, as used herein, generally refers to a building or structure in which network equipment and components are maintained and operated. Network equipment and components include, for example, optical amplifiers, regenerators, band equalizers, optical multiplexers, and the like. 
     The software modeling environment  10  allows a network designer to add, remove and/or relocate network components, and simplifies the process of changing equipment settings and topologies. Further, the software modeling environment  10  can include an analysis module for evaluating the operational parameters describing the modeled network  16  and determining their effect on the performance of the optical network  14 . 
     When designing an optical network, a network engineer performs several general tasks. First, the engineer produces a network plan that includes a description of the functionality to be provided by the network and identifies the constraints for the network. Next, the engineer generates an initial configuration for the network. To evaluate the performance of the initial configuration, the engineer uses the analysis module to evaluate various parameters that represent the characteristics of the spans in the network. Such parameters include optical power, optical signal-to-noise ratio (OSNR), chromatic dispersion, polarization mode dispersion (PMD), jitter and crosstalk. If any of the parameters are unsatisfactory, the engineer modifies the network by changing the initial configuration, for example, by adding, removing and/or repositioning network components. After “manually” implementing the configuration modifications, the engineer again analyzes the performance of the optical network. This iterative process of manual modifications and subsequent analysis continues until an acceptable network configuration is achieved, or until it is determined that no satisfactory network configuration is possible and the design process is terminated. 
     The iterative process of manual modifications and analysis is complicated, time consuming and requires highly trained engineers. Consequently, the process is expensive. Moreover, because the process depends on the individual expertise and bias of the network engineers, the network configurations resulting from similar analyses are often inconsistent. The process is complex even for small networks because modifying a single network component can cause numerous changes to optical signal parameters on the connected links or on other links in the network. For example, optical amplifiers are used to extend the range of an optical signal by increasing the signal power, however, optical amplifiers increase the amplified spontaneous emission (ASE) noise of the signal. The ASE noise can be addressed by using equalization techniques, however, additional noise and optical power problems can arise. 
     What is needed are a system and method for designing WDM networks that overcome the cost, complexity, and time disadvantages of the current techniques. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features a computerized method for modifying an optical WDM network. The method includes providing an initial configuration of the optical WDM network and evaluating at least one parameter that represents a characteristic of a span in the optical WDM network as defined by the initial configuration. The method also includes modifying the initial configuration of the optical WDM network in response to the evaluation of the one or more parameters. In one embodiment, the method also includes determining if a predetermined set of engineering rules are satisfied. In other embodiments, the evaluation of the at least one parameter includes evaluating a value of chromatic dispersion, polarization mode dispersion, optical signal to noise ratio or crosstalk for the span. In another embodiment, the evaluation of the at least one parameter includes evaluating an optical power at a receiver in the span. In still other embodiments, the modification of the initial configuration includes adjusting a placement in the span of an optical regenerator, or an optical amplifier, or inserting an optical regenerator or an optical amplifier in the span. In yet another embodiment, the modification of the initial configuration includes adjusting at least one setting of equipment in the optical WDM network. 
     In another aspect, the invention features a computer system for modifying the configuration of an optical WDM network. The computer system includes a WDM network software modeling environment, a network analysis software module and an automated network engineering software module. The WDM network software modeling environment models a WDM network and the network analysis software module evaluates at least one parameter of the modeled WDM network. The automated network engineering software module automatically modifies the WDM network in response to the at least one parameter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is graphical representation of a software modeling environment that is used to create a computer model of an optical wavelength division multiplexing network. 
         FIG. 2  is a flowchart of an embodiment of a process to configure a wavelength division multiplexing network that includes an automated link engineering process according to the principles of the invention. 
         FIG. 3  is a block diagram depicting an example of a wavelength division multiplexing network. 
         FIG. 4  is a block diagram depicting network equipment present at a site in the wavelength division multiplexing network of  FIG. 3 . 
         FIG. 5  is a flowchart illustrating an embodiment of the automated link engineering process of  FIG. 2 . 
         FIG. 6  is a block diagram depicting a modified network generated by applying the automated link engineering process of  FIG. 2  to the network of  FIG. 3 . 
         FIG. 7  is a block diagram depicting a portion of the modified network of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows an embodiment of a process  80  that automatically modifies a wavelength division multiplexing (WDM) network design. In one embodiment, the process is performed by a software module that is integrated into a software engineering environment. The process  80  includes generating (step  42 ) a high level network plan. Typically, the high level network plan is generated by identifying the required performance parameters of the network such as the data rates, channel densities and the number of access nodes. An initial configuration for a network is then generated (step  44 ) by selecting various network equipment and components, and determining where to locate them in the network. This step generally relies on the experience of the individual network engineer. Thus different engineers do not usually select the same initial configuration for the network. In some instances, the initial configuration can be a previously derived network configuration. 
     The initial configuration of the network is analyzed (step  46 ) by evaluating a set of network parameters and comparing these values with a set of link engineering rules. The link engineering rules specify acceptable parameter ranges and configuration requirements. A network configuration is acceptable if the configuration does not violate any of the link engineering rules. If the performance of the network is determined (step  52 ) to be acceptable, the process terminates (step  54 ) without modifying the initial configuration. However, if the performance is determined (step  52 ) not to be acceptable, an automated link engineering (ALE) process is executed (step  84 ). 
     The ALE process evaluates the initial network configuration to determine (step  86 ) whether the configuration can be modified to comply with the link engineering rules. If the configuration cannot be made to satisfy the link engineering rules, an error report is generated (step  88 ) and the process  80  is terminated (step  90 ). The error report includes a list of the link engineering rules for which acceptable performance could not be achieved. In addition when appropriate, the error report indicates the equipment yielding the unacceptable performance. Depending on the information provided in the error report, the network engineer may reassess the high level plan and generate a new initial network configuration (return to step  44 ). If changes can be made to the configuration to satisfy the link engineering rules, a modified network configuration is generated (step  92 ). In a modified configuration, the position and settings of some of the network components and equipment in the initial configuration are changed. In addition, modified configurations can include additional network components and equipment not present in the prior configuration. Moreover, some of the network components and equipment present in the prior configuration may be absent from the modified configuration. 
     The ALE process determines the behavior of the optical layer based on various parameters, including the optical characteristics and location of the network components and equipment in the network. For example, the parameters can include the location of all network sites, including glass-through sites (i.e., sites at which the signals are not processed), characteristics of the fiber spans between network sites, information identifying the origination and termination points of wavelength bands and channels, line-side interface bit rates, previously designed network equipment used as part of the initial network configuration, and user preference parameters. The modified network configuration specifies the location of amplifiers, regenerators, and associated equipment, and identifies equipment settings, including changes to the settings of pre-existing network equipment. Because the ALE process is deterministic, identical initial configurations always yield the same modified network configuration, if one can be generated, that satisfies the link engineering rules. In one embodiment, generation of the modified network configuration does not change the location or settings of previously defined equipment, but only includes the addition of new equipment. In another embodiment, the user generates a “profile” that defines specific preferences and parameters for various equipment and equipment configurations. In this embodiment the ALE process is constrained by the profile when generating the modified network configuration. 
       FIG. 3  illustrates a WDM network  138  having a ring topology that is frequently used in a metropolitan environment. The illustration resembles a graphical display provided in the software modeling environment to enable an engineer to view the network configuration. The network  138  includes sites  140   a ,  140   b ,  140   c ,  140   d  and  140   e  (generally  140 ) that are connected by five spans of optical fiber pairs  150   a  to  150   e  (generally  150 ) and  152   a  to  152   e  (generally  152 ) for bi-directional communications. The outer optical fiber loop  150  carries optical signals that propagate in a clockwise direction and the inner loop  152  carries optical signals that propagate in a counter-clockwise direction. 
     Each fiber supports a maximum of 32 signal channels. The 32 channels are divided into eight bands of four channels each. Each site  140  contains equipment that allows optical signals to be independently added or dropped from the network. The identifiers B 1  through B 8  refer to equipment for processing a respective signal band. Bands B 1  though B 4  are conventional bands (C-bands) and include wavelengths between 1535 nm and 1565 nm. Bands B 5  through B 8  are long bands (L-bands) and include wavelengths between 1570 nm and 1620 nm. 
     For example, site  140   e  contains equipment for processing the bands B 2 , B 4 , and B 8 . The existence of processing equipment at the sites  140  determines the access points for various bands in the network  138 . For example, optical signals generated at a location not shown in  FIG. 3  can be added to the network  138  at site  140   a  and travel counter-clockwise to the site  140   d . At site  140   d , such optical signals can be extracted from the network  138  and routed to a destination site or node. Thus, the optical path traversed by such optical signals includes the sites  140   a ,  140   e , and  140   d ; the links  152   e  and  152   d ; and optical equipment and links external to the network. In traversing the network  138 , the optical signals can be carried in either band B 1  or band B 3  as processing equipment is available for both of these bands at the sites  140   a  and  140   d.    
     Some of the network sites  140  include C-band amplifiers  160   a  to  160   h  (generally  160 ). The direction (i.e., pointing of the triangle) of an amplifier  160  indicates whether it operates on a signal received from an optical fiber  150 ,  152  or a signal to be transmitted through an optical fiber  150 ,  152 . The location of an amplifier  160  indicates whether the amplification occurs before or after band processing. For example, one amplifier  160   a  in site  140   d  amplifies optical signals after band specific processing for transmission through the optical fiber  152   c . The amplifier  160   b  in site  140   d  amplifies optical signals received from optical fiber  150   c  before band specific processing occurs. As discussed below, the sites  140  include additional processing equipment such as regenerators, equalizers, and other optical circuitry. 
       FIG. 4  shows an illustrative example of the equipment present at the site  140   e . The illustration resembles a graphical display generated by a software modeling environment to represent to a WDM network. The site  140   e  includes equipment  162  that processes optical signals propagating in a counter-clockwise direction. The equipment  162  includes a splitter  166  and a coupler  180 , that are coupled to the optical fibers  152   e  and  152   d , respectively. The splitter  166  and the coupler  180  are internally coupled through one optical path including an optical circuit  172 , a per band equalizer (PBE)  174  and the C-band amplifier  160   c , and through a second optical path including an optical circuit  182 . The site  140   e  also includes equipment  164  that processes optical signals propagating in a clockwise direction. The equipment  164  includes a splitter  184  and a coupler  190 , that are coupled to the optical fibers  150   d  and  150   e , respectively. The splitter  184  and the coupler  190  are internally coupled through two optical paths  169  and  171 . One optical path  169  includes an optical circuit  186 , a PBE  188  and the C-band amplifier  160   d . The other optical path  171  includes an optical circuit  192 . 
     In operation, the equipment  162  receives optical signals from the fiber  152   e . The optical signals are provided to the splitter  166  that directs C-band signals to optical circuit  172  and L-band signals to optical circuit  182 . The optical circuit  172  processes C-bands B 2  and B 4 , for example, by adding or removing signals, bridging signals, regenerating signals and the like. Subsequently, the optical circuit  172  transmits the C-band signals to the PBE  174  which equalizes the power levels of the bands B 1  through B 4 . The C-band amplifier  160   c  amplifies the equalized signals and provides the amplified signals to the coupler  180 . Providing analogous functionality to the optical circuit  172 , the optical circuit  182  processes L-band B 8  signals and provides L-band signals to the coupler  180 . The coupler  180  multiplexes the C-band and L-band signals onto the fiber  152   d.    
     Equipment  164  receives optical signals from the fiber  150   d . The optical signals are provided to the splitter  184  that directs the C-band signals to optical circuit  186  and the L-band signals to optical circuit  192 . The optical circuitry  186  processes the C-bands B 2  and B 4  and transmits the C-band signals to the PBE  188 . The PBE  188  provides equalized C-band signals to the C-band amplifier  160   d . Amplified C-band signals are then transmitted to the coupler  190 . The optical circuit  192  processes L-band B 8  signals and provides the L-band signals to the coupler  190 . The coupler  190  multiplexes the C-band and L-band signals onto the fiber  150   e.    
       FIG. 5  shows a flowchart representation of an embodiment of the ALE process (see step  84  of  FIG. 2 ) of the invention. Performing the ALE process  84  includes evaluating parameters representing characteristics of spans in the optical network and applying the parameters to predetermined engineering rules. The process  84  operates on an initial configuration for a network. During the execution of the ALE process  84 , the network is repeatedly analyzed to confirm compliance with the engineering rules. If execution of the ALE process is unable to generate a network configuration with satisfactory performance, the execution terminates after reporting information describing the problem. 
     Referring to  FIG. 3  and  FIG. 5 , the ALE process includes examining (step  100 ) an initial network configuration to determine, for example, whether the initial network  138  has the proper size and topology. Next, the spans  150 ,  152  are analyzed to determine (step  102 ) whether the jitter in the optical signals exceeds a predetermined value. Each regenerator can introduce jitter to the optical signals due to time displacements between regenerated optical pulses and original optical pulses. Consequently, the maximum number of regenerators permitted in a signal path is limited. As part of the jitter analysis, the numbers of regenerators in the signal paths are compared against a predetermined maximum. 
     Subsequently, the spans  150 ,  152  are analyzed to determine (step  104 ) if the temporal dispersion of the optical signals is acceptable. Dispersion arises, for example, from chromatic variations (e.g., refractive index) in the optical fibers  150 ,  152 , and results in the broadening of optical pulses as they propagate through an optical fiber. The broadening of the optical pulses can limit, for example, the maximum data rates. If the chromatic dispersion is determined to be unacceptable, regenerators are added at the appropriate sites  140  to replace the degraded optical pulses with “clean” optical pulses. 
     The polarization mode dispersion (PMD) introduced by the spans  150 ,  152  is evaluated and compared with predetermined acceptable values to determine (step  106 ) whether the effect of PMD on the network  138  is acceptable performance. PMD results in the broadening of an optical pulse due to the time delay between orthogonal polarization components of an optical pulse. Physical characteristics of the optical fiber, such as birefringence, cause PMD. As with chromatic dispersion, the pulse broadening introduced by PMD degrades the optical pulses. If the PMD exceeds a specified value, regenerators are added to the appropriate sites  140  to recreate clean optical pulses. 
     The network  138  is examined (step  108 ) to quantify crosstalk between optical signals. Crosstalk, as used herein, refers to the coupling of optical energy between optical signals. Thus, the presence of crosstalk degrades the quality of the optical signals. If crosstalk exceeds an acceptable predetermined level, the operational parameters (i.e., settings) of one or more optical amplifiers are adjusted to achieve acceptable network performance. 
     The optical signal-to-noise ratio (OSNR) at the network receivers is evaluated (step  110 ). Each amplifier  160  that is placed in the network  138  amplifies the optical signal and associated noise and also introduces additional noise, thus decreasing the OSNR of the respective signals. Although only C-band amplifiers are present in the illustrated network  138 , it should be recognized that L-band amplifiers can also degrade the OSNR. Attempts to correct unacceptable OSNR values are made by relocating amplifiers  160  or regenerators in the affected paths. 
     Lastly, the optical power at each receiver is compared (step  112 ) to a predetermined power level to determine whether the optical power is acceptable. An acceptable power level is one, for example, that exceeds a threshold power. The threshold power is defined as a minimum optical power at the receiver plus a penalty (i.e., margin) based on parameters of the light at the receiver. For example, the light at the receiver may have jitter that is within an acceptable range, but the penalty that is included in the threshold power is based on the amount of jitter which may cause the optical power incident on the receiver to be insufficient. Other optical parameters, such as PMD, crosstalk and OSNR, can also contribute to the penalty which also varies according to the bitrate of the optical signal. For example, 10 Gbps optical signals are significantly more affected by penalty than 1.25 Gbps and 2.5 Gbps optical signals. Pulse broadening introduced by chromatic dispersion and PMD can cause the power level of the optical signals in the network  138  to decrease. Alternatively, attenuation and scattering in the optical fibers  150 ,  152  decrease the optical power in the signals. Modifications to the network  138  to avoid low optical power at the receivers include the insertion of optical amplifiers  160  at various positions in the optical paths. The insertion of optical amplifiers  160  is applied to both optical fibers  150 ,  152  in a span on a span by span basis, starting with the span having the lowest optical power relative to the respective predetermined optical power. Amplifiers  160  are generally inserted at the farthest possible location from a transmitter that does not result in an unacceptable OSNR. 
     As mentioned above, an execution of the ALE process determines at each step whether the network  138  still conforms to engineering rules and the user profile constraints evaluated at prior steps. For example, inserting, removing and/or adjusting the settings of components and equipment to address a specific parameter can have adverse effects on other parameters previously examined in the ALE process. If at step  112  the one or more engineering rules or profile constraints are violated, various actions to resolve the problem are performed. In one embodiment, OSNR and crosstalk violations are addressed by changing the locations of the newly placed amplifiers  160 . If no suitable amplifier locations are available to resolve an OSNR failure, then regenerators are added to the system. 
       FIG. 6  shows a configuration of a modified network  238  that results from applying the ALE process to the initial network configuration  138  of  FIG. 3 . The configuration resembles a graphical display generated by a software modeling environment. The modifications include the addition of seven L-band amplifiers  242   a  to  242   g  (generally  242 ) with accompanying PBEs and the insertion of regeneration units for L-band B 6  at site  140   e . The modifications also include the alteration of various component settings described in more detail below with respect to  FIG. 7 . 
     Referring to  FIG. 2  and  FIG. 3 , analyzing the links (step  46 ) indicates that equipment in the initially configured network  138  is receiving inadequate input power. For example for site  140   e , analyzing the links indicates that the optical multiplexers in the optical circuit  172  and  186  are underpowered. L-band amplifiers  242   a  and  242   b  are included in the modified network configuration  238  to improve the optical power at the optical circuits  172 ,  186 . 
       FIG. 7  shows detail of the counter-clockwise  262  and clockwise  264  equipment at the site  140   e  in the modified network configuration  238 . The figure resembles a graphical display generated by a software modeling environment. The components added include two L-band amplifiers  242   c  and  242   f  and two PBEs  244  and  246 . The optical circuitry  182 ,  192  in the modified network  248  includes additional optical multiplexers for band B 6  with regenerators to address the unacceptable OSNR for the B 6  band at the site  140   e . The modified network configuration  238  also includes changes to the settings of network components in the initial configuration  138 . For example, the C-band PBEs at site  140   a  and  140   e  and band drop attenuators of the optical multiplexers at the sites  140  are changed. 
     While the invention has been shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims. For example, the invention contemplates that different steps and alternative orderings of steps can be employed by the ALE process described above.