Abstract:
Performance monitoring and equalization of DWDM optical links equipped with optical add/drop multiplexers (OADMs) at one or more sites is effected by selecting a direction of transmission and the end sites from the point of view of equalization.  
     Performance monitoring and equalization of DWDM optical links equipped with optical add/drop multiplexers (OADM) is presented in this application. This approach involves conceptually converting a multiple ends system to a “two-end” system with analogous channels (AC) to simplify the network management and equalize the DWDM channel performance. All analogous channels are treated as originating at a source analogous end and terminating at a destination analogous end for the purpose of equalization. The input power for all ACs, is adjusted at the source AE to obtain equal channel performance at the destination AE.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention is directed to a method for performance monitoring and equalization of wavelength division multiplexed (WDM) optical networks with optical add/drop multiplexers (OADM).  
           [0003]    1. Background Art  
           [0004]    Performance monitoring and equalization is one of the key issues in the development of optical dense WDM (wavelength division multiplexed) networks. In the last decade, transmission rates of data signals have increased progressively, which demands faster and more complex transmission systems.  
           [0005]    To increase the capacity of transmission using the existing fiber plant, many optical network providers adopted WDM/DWDM technology. On the other hand, for high transmission rates, i.e. rates over 40 or 80 Gb/s, system performance degradation caused by noise and optical path distortions are usually difficult to measure, making the performance evaluation complicated.  
           [0006]    In optically amplified WDM/DWDM system, signals travelling on different channels have a different gain and a different signal-to-noise ratio due to the unflat gain profile of the optical amplifiers provided along the path of the signal, different parameters of the transmitter-receiver pairs, optical distortion, etc.  
           [0007]    The bit error rate (BER) and the optical signal-to-noise ratio (OSNR) have usually been used as parameters for the evaluation of the characteristics of an optical fiber communication system.  
           [0008]    BER is defined as the ratio between the number of the erroneously received bits to the total number of bits received over a period of time. The BER includes information on all impairments suffered by the signal between the transmitter and receiver, i.e. both noise and distortion information. OSNR represents the noise characteristics of an optical system and is the ratio between the signal and noise levels.  
           [0009]    Individual channels generally experience different OSNR and BER, even if the input power is equal at the transmitter site. This is because the transmitter-receiver pairs do not have identical characteristics for all channels, the optical amplifiers have unflat gain and noise profiles, the passive components connected over the transmission link have wavelength-dependent characteristics, etc. In addition, the active and passive network elements have different characteristics, further contributing to the differences between the performance of the respective transmission channels.  
           [0010]    It is important to detect accurately the performance of individual optical channels for many reasons, including improved control of optical amplifiers, signal tracking at the optical layer, monitoring accumulation of optical noise in a link with cascaded amplifiers, and most importantly, equalization of the channels. Without equalization, some optical channels may have much more tolerance for noise and loss than other, which may fail to meet the provisioned performance parameters.  
           [0011]    Equalization based on OSNR is a straight-forward method. OSNR may be measured using an optical spectrum analyzer or other methods. The drawback associated with this method is that it does not consider the effects of the optical distortion and electrical noise.  
           [0012]    Another method used currently for equalization is based on measuring the BER margin for each channel. While equalization based on BER margin is generally more sophisticated, it is more accurate than OSNR method. BER margin can be measured using, for example, “noise loading” method disclosed in U.S. patent application Ser. No. 08/934969 (Khaleghi, filed Sept. 19, 1997 and assigned to Northern Telecom Limited).  
           [0013]    The article “Equalization in amplified WDM lightwave transmission systems”, A. R. Chraplyvy et al., IEEE Photonics Technology Letters, Vol. 4, No.8,1992, pp920-922 and U.S. Pat. No. 5,225,922 (issued on Jul. 6, 1993 to Chraplyvy et al., and assigned to AT&amp;T Bell Laboratories) provides a method for adjusting the optical powers at the transmitter end of a WDM transmission link to obtain the same OSNR for all channels at the receiver end. The adjustment of a particular channel takes into account the total power of all channels and the end-to-end gain for that channel normalized by the end-to-end gain for all channels. A telemetry link is necessary for conveying the measurements between the two ends of the link.  
           [0014]    Latest advances in the opto-electronics conducted to replacement of electrical add/drop multiplexers (ADM) with optical ADMs (OADMs), where an entire optical channel is dropped or added at the OADM site according to the wavelength. As OADMs are transparent to the signal rate, they can be used in DWDM networks with different SONET/SDH rates.  
           [0015]    Performance monitoring and equalization of optical links with multiple OADMs becomes more challenging also because a wavelength can be reused in the same link. Furthermore, the OADMs is installed at locations specified by the customer.  
           [0016]    It is known to use the “glass through” method in end-to-end systems provided with a small number of OADMs. This method involves short-connecting the drop and add ports of the OADM with a patch-cord, so that they appear as a single channel from equalization point of view. The equalization of the channels is then performed generally based on OSNR or BER margin. After equalization, the patchcord between the add and drop ports is removed and the affected channels are reconnected to the respective transmitter and receiver. However, the “glass-through” method ignores entirely the existence of the OADM from the point of view of performance monitoring. As well, after equalization, the add/drop channels will most probably have much more margin-to-failure than the other channels. Furthermore, this method cannot be used for systems that have asymmetric OADMs, where the signal is added or dropped only, or the number of add and drop signals is not equal, or systems where the wavelength of the add and drop channels are different.  
           [0017]    The above prior art methods fall short in providing a reliable solution for monitoring the performance as well as equalizing the channels of WDM or DWDM systems equipped with OADMs.  
         SUMMARY OF THE INVENTION  
         [0018]    An object of the present invention is to provide a method for performance monitoring and equalization of networks with OADMs that alleviates totally or in part the drawbacks of current methods.  
           [0019]    Another object of the invention is to provide a performance monitoring and equalization method that may be used for networks with symmetric or asymmetric OADMs and networks where the wavelength of the dropp and add channels are equal or not.  
           [0020]    A further object of the invention is to provide methods for performance monitoring and equalization of OADM networks that treat a complex WDM network as a “two-end” system. This allows for simplifying the monitoring of the network, as the performance information for all physical sites of the network is available at these two ends.  
           [0021]    Accordingly there is provided a method for performance monitoring and equalization of an optical link between two line terminating equipment (LTE/REGEN) sites of an optical network provided with one or more sites equipped with optical add/drop multiplexers (OADMs) and multi-channel optical amplifiers (MOA), comprising: selecting a direction of transmission and accordingly defining one of the LTE/REGEN sites as a source analogous end (AE) and the other as a destination AE; converting the optical link into an analogous system of J analogous channels (AC), an AC(j) originating at the source AE and terminating at said destination AE; and, adjusting the input power of each AC(j) to obtain a substantially equal performance parameter for all ACs, where J&gt;2, j is the identifier of an AC, and j ε [1, J].  
           [0022]    An advantage of this method is that it provides a reliable equalization method for a WDM/DWDM network equipped with OADMs that could be used in the field.  
           [0023]    Another advantage of the invention is that it provides a simple solution for performance monitoring from the network management point of view. Thus, OAM&amp;P (operations, administration, maintenance and provisioning) information for an entire network comprised between two sites of interest is available at these two sites/ends.  
           [0024]    Still another advantage of the method according to the present invention is that it can used in for equalization of networks provided with symmetric or asymmetric OADMs and also in cases when the add and drop channels have different wavelengths. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments, as illustrated in the appended drawings, where:  
         [0026]    [0026]FIGS. 1 a - d  (Prior Art) show various types of OADMs. FIG. 1( a ) shows a filter-based symmetrical OADM, FIGS.  1 ( b ) and  1 ( c ) show filter-based asymmetrical OADM and FIG. 1( d ) illustrates an OADM build with fiber grating and optical circulators;  
         [0027]    [0027]FIG. 2A is an example of a bidirectional communication network comprising optical amplifiers and OADMs;  
         [0028]    [0028]FIG. 2B shows how the network of FIG. 1A is separated into forward analogous channels;  
         [0029]    [0029]FIG. 2C shows how the network of FIG. 1A is separated into reverse analogous channels;  
         [0030]    [0030]FIG. 3A is an example of a complex bidirectional DWDM network comprising optical amplifiers and OADMs;  
         [0031]    [0031]FIG. 3B shows the reverse analogous channels for the network of FIG. 3A.  
         [0032]    [0032]FIG. 3C shows the forward analogous channels for the network of FIG. 3A;  
         [0033]    [0033]FIG. 4A is a flow-chart of the method of equalization based on OSNR;  
         [0034]    [0034]FIG. 4B illustrates the step of converting the network into a two-ends network of analogous channels;  
         [0035]    [0035]FIG. 4C illustrates the step of setting the input power;  
         [0036]    [0036]FIG. 4D illustrates the step of determining the adjusting amount for the input power;  
         [0037]    [0037]FIG. 5A shows a flow-chart of the method of equalization based on BER margin;  
         [0038]    [0038]FIG. 5B illustrates the step of setting the input power; and  
         [0039]    [0039]FIG. 5C illustrates the step of determining the distance to failure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0040]    [0040]FIG. 1 illustrates various types of optical add-drop multiplexers (OADM), which have been lately used for accessing a multichannel optical signal without conversion to electrical signals. FIG. 1( a ) shows a symmetrical OADM  10  comprising filters  10 ′ and  10 ″. Filter  10 ′ reflects channels λ 2 -λ n  from the input multichannel signal presented at input port In of the OADM towards filter  10 ″, and transmits channel λ 1  to the output port Drop. An add channel λ 1 ′ is presented to Add port and directed to filter  10 ″. Filter  10 ″ reflects channels λ 2 -λ n  towards the output port Out, and also transmits channel λ 1 ′ to port Out, so that the output signal comprises the reflected and the inserted wavelengths λ 1 ′, λ 2 -λ n . The wavelength λ 1  and λ 1 ′ could be equal or not.  
         [0041]    [0041]FIG. 1( b ) shows an asymmetrical OADMs, where a channel λ 1  is dropped and no channel is added. Fiber terminator  11  prevents the reflection of the signal back to the fiber link. FIG. 1( c ) also shows an asymmetrical OADMs, where a channel λ 1  is added and no channels are dropped, shown by fiber terminator  11  provided at the Drop port.  
         [0042]    [0042]FIG. 1( d ) illustrates an OADM build with three-port optical circulators  12  and  12 ′, and a fiber grating  13 . In this example, the dropped and added channels have the same wavelength λ 1 .  
         [0043]    [0043]FIG. 2A shows an example of a simple bidirectional DWDM network connecting sites A, B and C, and is used to define some terms used in this specification.  
         [0044]    The term “optical span” defines the optical connection between two pieces of section terminating equipment (STE) located at two different sites. STE includes a regenerator (REGEN), an optical amplifier (OA), or a piece of line terminating equipment (LTE). For example, the fiber between site A and site B, denoted with numeral  18  on FIG. 2A is a span. Another span shown on this Figure is denoted by numeral  28 .  
         [0045]    The term “optical channel” refers to the path of a signal of a given wavelength between the respective transmitter and receiver for that wavelength. An optical span can carry a plurality of channels. For example, two optical channels λ r1  and λ r2  are present along span  18  to transport two different signals in one direction, and two optical channels λ b1  and λ b2  are present along the same span  18  to transport two more signals in the opposite direction.  
         [0046]    The type of optical channels can be defined in complex networks with respect to two selected end sites of the optical link as “express”, “add”, “drop” and “add-drop” channels. An “express channel” is an optical channel that bypasses all OADMs between the LTE/REGEN sites, i.e. between the physical ends of the network. As the name indicates, an “add” channel has the transmitter at an OADM site, a “drop” channel has the receiver at an OADM site, and an “add/drop” channel has the transmitter and the receiver at different OADM sites.  
         [0047]    The term “analogous end (AE)” is used in connection with an optical network to define two sites of interest from the point of view of equalization, that are located at two physical ends equipped with a LTE/REGEN. As such, the analogous ends are accessible to the network manager, so that equalization may be performed remotely.  
         [0048]    To convert a network into a two-ends analogous channel system according to this invention, all channels are converted to “analogous channels (AC)”. An analogous channel is an abstraction for defining the path of a signal of a given wavelength between an end site (a LTE/REGEN site) and an OADM site, or between two OADM sites, as viewed with respect to the analogous ends. This term is meaningful only in conjunction with all the analogous channels that can be abstracted between the respective analogous ends.  
         [0049]    From a network management point of view, it can be assumed that an analogous channel comprises, in addition to the spans of “real” fiber, spans of “lossless” fiber that complete the link between the physical ends of the respective optical channel and the analogous ends.  
         [0050]    The arrows noted “forward direction” and “reverse direction” are provided on the drawings for specifying the directions of transmission. It is to be understood that these are relative terms, which apply as such to the particular example of networks illustrated in the appended drawings. In addition, in the examples illustrated and described in this specification, the channels travelling in the forward direction are selected in the red band of the spectrum, while the reverse channels are selected in the blue band. Other associations are equally covered by this invention, and also the forward and reverse bands are not limited to the red and respective blue bands.  
         [0051]    Table 1 lists the acronyms for some of the terms used for describing the AC methods for performance monitoring and equalization.  
                       TABLE 1                       #   Name   Acronym                   1   Analogous Channel   AC       2   Analogous End   AE       3   Bit Error Rate   BER       4   Code Violations   CV       5   Dispersion Compensation Module   DCM       6   Error Seconds   ES       7   Line Terminating Equipment   LTE       8   Multiwavelength Optical Amplifier   MOA       9   Network Manager   NM       10    Optical Add/Drop Multiplexer   OADM       11    Optical Signal-to-Noise Ratio   OSNR       12    Performance Monitor (NM term)   PM       13    Receiver, Transmitter   Rx, Tx       14    Ratio Indicator (CV/ES)   R       15    Regenerator   REGEN       16    First Threshold (OSNR method)   TH1       17    Second Threshold (BER margin method)   TH2       18    Variable Optical Attenuator   VOA                  
 
         [0052]    The network of FIG. 2A shows only two channels in each direction of transmission, but the invention is equally applicable to more than two forward and two reverse channels, and also to networks having a different number of forward and reverse channels.  
         [0053]    [0053]FIG. 2A shows at site A, a WDM  10 , a transmitter unit including transmitters (Tx)  14  and  14 ′, and a receiver unit including receivers (Rx)  15  and  15 ′, and attenuators  191 ,  192 .  
         [0054]    Each transmitter  14 ,  14 ′ transmits a client signal using an optical carrier of a respective wavelength λ r1 , λ r2 . Each receiver  15 ,  15 ′ detects a client signal carried by an optical carrier of a respective wavelength λ b1 , λ b2 .  
         [0055]    Index “r” indicates that the respective wavelength is selected in the red band of the light spectrum, “b” indicates that the wavelengths are selected in the blue band of the light spectrum, while “1” or “2” gives the identifier of a channel in the respective direction/band.  
         [0056]    WDM  10  combines the forward optical channels λ r1  and λ r2  into a bidirectional multichannel optical signal S 1  and delineates the reverse optical signals λ b1  and λ b2  from S 1 .  
         [0057]    A variable optical attenuator (VOA)  17  is also provided at site A for adjusting the power of each signal arriving at receiver  15  and respectively receiver  15 ′ for equalization reasons, as it will be seen shortly.  
         [0058]    [0058]FIG. 2A also shows bidirectional multi-wavelength optical amplifier (MOAs)  16  provided for optically amplifying the forward channels of S 1  (in the postamplifier module), before insertion on span  18 , and also for optically amplifying the reverse channels of signal S 1  (in the preamplifier module), before arriving at the site A.  
         [0059]    At site B, the forward channels of signal S 1  are amplified by MOA  19  and passed to an OADM  20 . In the forward direction, OADM  20  drops the optical signal of wavelength λ r1  to a receiver  25  and inserts (adds) a new client signal of wavelength λ r1 ′ received from a transmitter  24 . Channel λ r2  passes through the OADM; it is an express channel. Since in a general case, λ r1  may not be equal to λ r1 , the multichannel optical signal travelling between sites B and C over span  28  is denoted here with S 2 . In the reverse direction, both channels λ b1 , λ b2  pass through site B towards site A. λ b1  and λ b2  are also express channels.  
         [0060]    Receiver  25  is provided with a VOA  27  for adjusting the received power. A MOA  26  is provided at site B for optically amplifying the forward channels of S 2  after leaving site B, and also for optically amplifying the reverse channels of signal S 2 , before arriving at the site B.  
         [0061]    At site C, forward channels of S 2  are amplified by a MOA  36  and presented to a WDM  30 , which directs channel λ r1 ′ to a receiver  35 , and channel λ r2  to a receiver  35 ′. Reverse channels λ b1  and λ b2  originating at transmitters  34  and respectively  34 ′, are multiplexed by OADM  30  and sent towards site B over span  28 , after amplification by MOA  36 .  
         [0062]    The basic idea of this invention is to construe the network between two sites of interest as an analogous channel system comprised of a plurality of end-to-end analogous channels (AC). This implies selecting the respective end sites of interest from the point of view of equalization, and determining all Tx-Rx pairs at, and between these sites. The sites are defined as a “source AE” and a “destination AE”, respectively, in accordance with the direction of transmission in which equalization is performed.  
         [0063]    When the physical ends of an optical channel coincide with the physical ends of the network, i.e. the transmitter and receiver of the respective channel are at a respective LTE/REGEN site, the analogous channel is identical with the optical channel. This is the case of express channels. When the physical ends of an optical channel do not coincide with the physical ends of the network, the transmitter and/or receiver for the respective physical channel are/is “moved” to the respective source and destination AE, and “lossless” fiber is assumed between the respective physical end and the analogous end.  
         [0064]    The network of FIG. 2A can thus be converted to the ACs shown in FIGS. 2B and 2C. Thus, AC 1  carrying the forward channel of wavelength λ r1  comprises Tx  14 , MOA  16 , fiber  18 , MOA  19  and Rx  25 . Since the source analogous end coincides with the location of Tx  14 , namely is at site A, the physical channel also coincides with the analogous channel between site A and MOA  19  at site B. However, since Rx  25  is not located at the destination analogous end, but at site B, Rx  25  is displaced on FIG. 2B at the destination analogous end, i.e. at site C. The dotted connection  21  between MOA 19  and Rx  25  is considered “lossless” fiber.  
         [0065]    AC 2  carries the forward channel of wavelength λ r1  and comprises Tx  24 , MOA  26 , fiber  28 , MOA  36  and Rx  35 . Since the destination analogous end coincides with the location of Rx  35 , namely is at site C, the physical channel also coincides with the analogous channel AC 2  between MOA  26  at site B and Rx  35  at site C. Tx  24  is displaced on FIG. 2B from site B to the source analogous end (site A) and the dotted connection  31  between Tx  24  and MOA  26  is considered “lossless” fiber.  
         [0066]    AC 1  and AC 2  have one end at an analogous end, and therefore one segment of “lossless” fiber,  21  and respectively  31 , is assumed between the respective OADM and the respective analogous end. It is to be understood that more than one segment of imaginary “lossless” fiber may be needed to complete an AC of a complex network as it is shown in connection with the example of FIGS.  3 A- 3 C.  
         [0067]    [0067]FIG. 3A shows a more complex transmission network, connecting sites A, B, C and D. The multichannel signal travelling between sites A and B on fiber spans  181  and  182  is denoted with Si, the signal travelling between sites B and C on fiber spans  183  and  184 , is denoted with S 2  and the signal travelling between sites C and D on spans  185  and  186 , with S 3 . FIG. 3A shows bidirectional multichannel optical amplifiers (MOAS)  50 ,  51  and  52  connected onto these spans, but is to be understood that a span may comprises more line optical amplifiers or none may be connected between two adjacent sites, depending on the particular configuration of the network under equalization.  
         [0068]    Site A comprises a plurality M of transmitters  14 ( 1 ) to  14  (M), each for generating a forward optical carrier λ r1  to λ rM , and a multiplexer  10  for combining the forward channels into optical signal S 1 . Postamplifier module of a MOA  16  amplifies the forward channels of signal S 1 . Site A is also provided with a plurality N of receivers  15 ( 1 ) to  15 (N), each for a reverse optical carrier λ b1 ″, λ b2 ′ and λ b3  to λ bM , and a demultiplexer  10 ″ for separating the reverse channels form signal S 1 . The reverse channels are also amplified before separation by the preamplifier module of MOA  16 .  
         [0069]    A VOA unit  17  is provided at site A for independently adjusting the level of the signals at the input of the receivers, and also with dispersion compensation modules (DCM) or optical attenuators  191  and  192 , as is well known in the art.  
         [0070]    Site D comprises transmitters  34 ( 1 ) to  34 (N) for reverse channels λ b1  to λ bn , receivers  35 ( 1 ) to  35 (M) for forward channels λ r1 ′, λ r2 ′ and λ r3  to λ rM , multiplexer  30 , demultiplexer  30 ′, and MOA  33 . DCM or optical attenuators  197 ,  198 , and a VOA unit  37  are also provided for the same reasons as disclosed above.  
         [0071]    Sites B and C are OADM sites, where some channels are dropped and some added. As indicated above, the wavelength of a channel added at a site may, or may not, be the same with that of a dropped channel. An OADM  20  at site B drops channel λ r1  form S 1  and adds a new signal over a forward channel λ r1 ′. Site B is also provided with two OADMs  21  and  22 , each for dropping and adding a channel in the reverse direction. Thus, OADM  21  drops channel λ b2  and adds a new signal over channel λ b2 ′ and OADM  22  drops a channel λ b1 ′ and adds a new channel λ b1 ″.  
         [0072]    Site B is also equipped with a MOA  23  comprising a post and pre amplifying module for amplifying respectively the forward and the reverse channels of Si, and with a MOA  26  for amplifying the respective forward and the reverse channels of S 2 . Each receiver  25 ,  29  and  29 ′ is provided with a VOA (not marked) for adjusting the input power and with DCM or optical attenuators  193 ,  194 .  
         [0073]    Site C is equipped with an OADM  40  that drops channel λ r2  form S 2  and adds a new signal using a forward channel λ r2 ′. An OADM  41 , also located at site C, accesses the reverse channels of S 3  to drop channel λ b1  and to add a new signal over channel λ b1 ′. Each receiver  45  and  47  is provided with a VOA (not marked) for adjusting the input power and with DCM or optical attenuators  195 ,  196 .  
         [0074]    The sites of interest from the equalization point of view are LTE/REGEN sites A and D; therefore the analogous ends are at these sites. Site A is the source AE for the equalization of forward channels, and the destination AE for equalization of reverse channels. Similarly, site D is the source AE for the equalization of reverse channels, and the destination AE for equalization of forward channels.  
         [0075]    The number of ACs is equal to the number of all transmitter—receiver pairs of the network.  
         [0076]    It is to be noted that the analogous channels do not have the same number of amplifiers. This is an important difference between systems with and without OADMs.  
         [0077]    The following detailed description is made in connection with the reverse (blue) channels, but it is to be understood that performance monitoring and equalization for the forward channels can be realized in the same way. The analogous links for the reverse channels are illustrated in FIG. 3B.  
         [0078]    For the express channels, the real optical channel is identical with the analogous channel. In the example of FIG. 3A and 3B, such direct relation can be found for wavelengths λ b3  to λ bM . For the channels being added and dropped with wavelength reuse, the real optical link between sites A and D is divided into several analogous channels.  
         [0079]    For example, AC 1  (λ b1 ) originates at transmitter  34 ( 1 ) at site D, which is also the source analogous end (AE) for reverse direction of transmission, and ends at receiver  47  at site C. This analogous channel includes reverse module of MOA  33 , MOA  52  and reverse module of MOA  46 . Receiver  47  is displaced on FIG. 3B from site C to site A, where is the destination analogous end, and a hypothetical length of “lossless” fiber  21 ′ is assumed between MOA  46  and receiver  47 .  
         [0080]    AC 2 (λ b1 ′) originates at transmitter  49  at site C and ends at receiver  29  at site B. This channel includes reverse module of MOA  43 , MOA  51  and reverse module of MOA  26 . Since none of the physical ends of this AC coincide with an analogous end, transmitter  49  is displaced on FIG. 3C at the source AE, and receiver  29  is shown at the destination AE. This AC is then completed with two lengths of “lossless” fiber, namely  31 ′ between transmitter  49  and MOA  43 , and  31 ″ between MOA  26  and receiver  29 .  
         [0081]    AC 3 (λ b1 ″) originates at transmitter  27  at site B and ends at receiver  15 ( 1 ) at site A. This channel includes reverse module of MOA  23 , MOA  50  and reverse module of MOA  16 . The AC is completed with lossless fiber  36  between transmitter  27  and MOA  23 . Transmitter  27  is shown on FIG. 3B at the source AE.  
         [0082]    Similar transformation of the second blue channel results in AC 4 (λ b2 ) and AC 5 (λ b2 ′), where the respective AC are completed with lossless fibers  46  and respectively  56 .  
         [0083]    [0083]FIG. 3C shows the ACs for the forward direction.  
       1. Analogous Channel Method for Performance Monitoring and Channel Equalization Based on OSNR  
       [0084]    Equalization based on OSNR is disclosed in connection with FIG. 3B and FIGS.  4 A- 4 D.  
         [0085]    Step  100  is an initialization step, showing selection of the source and destination analogous ends (AE) and also of the direction of traffic (forward or reverse) for which the equalization is being performed. In this specification, the source AE is selected at site D and the destination AE is selected at site A, the direction of transmission for which the methods of equalization are described and illustrated being the reverse direction, from site D to site A.  
         [0086]    In step  105 , the network is converted into an analogous channel system comprising a plurality of two-end analogous channels, as shown in further detail on FIG. 4B.  
         [0087]    To convert the physical network into ACs, all physical channels are determined, by identifying the Tx-Rx pairs for the reverse direction and the corresponding wavelength, as shown in step  105 ( 1 ). The type of channel is then determined in steps  105 ( 2 ) and  105 ( 5 ) of FIG. 4B. For an express or drop channel originating at the source AE, the source analogous end is identical to the physical end, shown in step  105 ( 3 ). If the transmitter for the channel under consideration is not at the source analogous end, the transmitter is “moved” at the source AE, shown in step  105 ( 4 ). Also, all receivers are “moved” at the destination AE, step  105 ( 5 ) and  105 ( 7 ).  
         [0088]    In the meantime, the total number of ACs comprised in the real optical link between sites A and D is counted and recorded as N_ac in step  105 ( 8 ). In FIG. 3D, the network is converted into N+3 analogous channels.  
         [0089]    Returning to FIG. 4A, the initial input power Pin_j_ini_dBm is set for each AC, shown on step  110  and detailed in FIG. 4C.  
         [0090]    In step  110 ( 1 ), a target value for the total input power, denoted with Pin_tot_target_dB, is set at the input of a respective amplifier module at the LTE/REGEN site, which is in this case at the input of MOA  33 .  
         [0091]    An initial input power parameter Pin_ini_dBm is determined for all AC(j), where j is the range of the AC. The initial input power for the axpress and drop ACs is determined in a different way from that for the add ACs, as shown in steps  110 ( 2 ),  110 ( 3 ) and  110 ( 4 ).  
         [0092]    Thus, the initial power for add channels is calculated according to equation EQ(1): 
           P in —   j   —   ini _dBm= P peak_Loss_before —   OADM   EQ(1) 
         [0093]    Ppeak is a provisioned value and gives the maximum permitted output power for a given amplifier and a given channel, in FIG. 3B pre-amplifier module of MOA  33 .  
         [0094]    Loss_before_OADM is the total optical component loss measured between the output of the last optical amplifier at the OADM site where channel AC(j) is added, and before the respective OADM. For example, Loss_before_OADM for channel ACλ′ b1  added at site C is the optical loss between the output of the pre-amplifier module of MOA  46  and OADM  41 , and is given by the insertion loss of DCM/optical attenuator  197 . Loss_before_OADM for channel ACλ″ b1  added at site B is the sum of the loss given by the DCM/optical attenuator  194  and the loss introduced by OADM  21 .  
         [0095]    If the value given by EQ( 1 ) is higher than the maximum output power for the respective transmitter, Pin_j_ini_dBm should be set at the maximum power for the respective transmitter.  
         [0096]    The initial power for an express and drop channel AC(j) is calculated according to equation EQ(2) 
           P in —   j _ini_dBM=10*log10 (Pin     —     tot     —     target     —     dBm/10)/K   EQ(2) 
         [0097]    where K is the number of express and drop channels (transmitters) at LTE/REGEN site, which is N in this example.  
         [0098]    In step  110 ( 5 ) the Pin_j_ini calculated as above is set at the transmitter of each AC. The total power Pin_tot_ini is then measured in step  110 ( 6 ) and compared with the target in step  110 ( 7 ). If the target and the measured total powers are different, the input power for all transmitters is increased or decreased by the same amount, until the two parameters become equal, steps  110 ( 8 ),  110 ( 6 ) and  110 ( 7 ).  
         [0099]    Returning now to FIG. 4A, the initial value for the OSNR, denoted with OSNR_j, is measured at each receiver where the respective reverse AC terminates, as shown in step  115 .  
         [0100]    OSNR_min and OSNR —max  are thereafter determined in step  120 , as the minimum, maximum value, respectively of all OSNRs measured in the previous step, and the difference between these two values is denoted with ΔOSNR.  
         [0101]    If ΔOSNR is less than a provisioned value, herein referred to as the first threshold TH1, the power at the input of each receiver is adjusted in step  130 , for operation according to the specification, and the equalization is accomplished. This is shown by branch “YES” of decision block  125 . The threshold TH1 is typically 1 dB, but other values may be selected for a particular network.  
         [0102]    If ΔOSNR is greater or equal to TH1, an adjustment value ADJ_j is determined in step  135 , as shown in further detail in FIG. 4D. Namely, in step  135 ( 1 ), the difference ADJ_j_offset between the OSNR_min and the OSNR measured in step  115  is calculated for every AC according to equation EQ(3) 
           ADJ   —   j _offset= OSNR _min− OSNR   —   j   EQ(3) 
         [0103]    In the case when the channel is an add channel, step  135 ( 2 ), the ADJ_j is set at the value of ADJ_j_offset in step  135 ( 7 ). For the remaining channels, shown by branch “NO” of decision block  135 ( 2 ), a new input power Pin_j_new is calculated in step  135 ( 3 ) based on the ADJ_j_offset obtained in step  135 ( 1 ), according to the equation EQ(4): 
           P in —   j _new_dBm= P in_j_ini_dBm+ ADJ _offset  EQ(4) 
         [0104]    A new total input power Pin_tot_new_dBm is calculated in step  135 ( 4 ) using equation EQ(5): 
           P in_tot_new_dBm=10*log(SUM(10 Pin     —     new     —     dBm/10)) ))  EQ(5) 
         [0105]    and this total power is used to determine a compensation adjustment value ADJ_comp, step  135 ( 5 ), using equation EQ(6) below: 
           ADJ _comp= P in_tot_new_dBm− P in_tot_target_dBm  EQ(6) 
         [0106]    Adjust value ADJ_j is determined for express and drop channels using equation EQ(7), as shown in step  135 ( 6 ): 
           ADJ   —   j=ADJ _offset− ADJ _comp  EQ(7) 
         [0107]    Returning now to FIG. 4A, step  140 , input power for all transmitters is reset using ADJ_j. ADJ_j calculated using EQ(7) is applied to all transmitters at the LTE/REGEN site to ensure an optimum total input power to the post amplifier is maintained. This adjustment value is not applied to the add channels, as can be seen from FIG. 4D, for which ADJ_j is set equal to ADJ_j_offset.  
         [0108]    The equalization continues by repeating steps  115  to  140  until ΔOSNR becomes less than TH1 for all ACs.  
       2. Analogous Channel Method for Channel Equalization Based on BER Margin  
       [0109]    The failure criterion is a parameter provisioned for a network at installation. In general, a failure in channel performance is considered to occur when the bit error rate (BER) is higher than 10 −9 , that is the failure criterion is BER_fail=10 −9 . A BER lower than this value indicates an acceptable performance for the respective link. As well, the failure criterion could be set using rate R, which is the ratio between the line code violations (CV) and line error seconds (ES) for channels between two LTEs, and is the ratio between the section CV to section ES for channels between a LTE and a REGEN. A channel is considered failed when R is outside a specified range provisioned for the respective channel. The range for R depends on the signal rate for the respective channel. For networks that are equipped with a performance monitor (PM), R may be calculated or read on the PM screen.  
         [0110]    AC method for performance monitoring and channel equalization based on BER margin is illustrated in FIG. 5A. Steps  200  and  205  are similar to steps  100  and  105  described in connection with the method of equalizing the link using OSNR and therefore are not described here in detail.  
         [0111]    Pini_j is the notation used for the initial input power for an analogous channel AC(j) when describing the BER margin method. Pini_j is set in step  210 , as shown in further details in FIG. 5B.  
         [0112]    Namely, a total target input power Pin_tot_target_dBm is provided at the postamplifier module of the MOA, shown in step  210 ( 1 ). The value of the initial input power Pini_j is calculated using EQ(1) for the case of an add channel, as shown in step  210 ( 3 ), or using EQ(2), for the case of an express or drop channel, as shown in step  210 ( 4 ).  
         [0113]    Ppeak and Loss_before_OADM in EQ(1) have the same significance as disclosed in connection with FIG. 4C.  
         [0114]    In step  210 ( 5 ) the initial power Pini_j is set for each AC to the respective value calculated in step  210 ( 3 ) or  210 ( 4 ). In step  210 ( 5 ) it is determined if there is any failed channel, using the selected fail criterion. For example, if BER_fail is the criterion for determining failure of a channel, the channel is declared failed if BER measured at the receiver of that channel is greater than BER_fail. Other criteria may be used to determine failure of a channel.  
         [0115]    The initial power Pini_j of a failed channel is increased, as shown in step  210 ( 7 ). If the channel still fails after increasing power to maximum, the power of all other channels is reduced until the failed channel becomes operational, e.g. until the BER measured at the receiver becomes less than BER_fail.  
         [0116]    If all channels operate according to the specification, e.g. at a BER over the BER_fail, as shown by branch “NO” of decision block  210 ( 6 ), the method of equalization continues with step  215  shown on FIG. 5A, where a distance to failure D_j is calculated for each channel. This step is illustrated in further detail on FIG. 5C.  
         [0117]    Step  215 ( 1 ) on FIG. 5C shows that a fail criterion is set for the respective link, which in general is 10 −9 .  
         [0118]    In step  215 ( 2 ), the received power Prx_j is adjusted at each receiver using the associated VOA to a value within the dynamic range of the respective receiver, according to the receiver specification. In the example of FIG. 3A, the power of receiver  15 ( 2 ) is adjusted using the corresponding VOA of unit  17 . Prx_j may be measured from the transmitter site, by remote log-ing into the receiver site and bringing up the performance monitor (PM) screen for each channel.  
         [0119]    Then, in step  215 ( 4 ) and  215 ( 5 ) the initial power Pini_j is adjusted until the respective channel fails. When BER is used for determining a failure in the channel performance, the actual BER at which the channel operates is measured, for each channel and the measured BER value is compared to BER_fail. If the measured BER is greater than BER_fail, the input power Pini_j for the respective failed channel is decreased until the channel fails. When rate R is used, R is calculated or read on the PM screen and compared against a range to determine if the channel is failed. To fail a channel, the input power is increased or decreased until R falls within the range.  
         [0120]    Next step  215 ( 6 ) comprises recording the power of each transmitter in the point of channel failure as the attenuated power Patt_j. It is to be noted that when Patt_j is measured for AC(j), the initial input power Pini_j of the remaining ACs, other than that currently under adjustment, should remain unchanged. This is shown in step  215 ( 7 ), i.e. the power of each transmitter is returned to the Pini_j after Patt_j has been recorded.  
         [0121]    The distance to failure D_j for the respective channel is calculated In step  215 ( 7 ) as the difference between Pini_j and Patt_j: 
           D   —   j=P ini —   j−P att —   j   EQ(8) 
         [0122]    Returning now to FIG. 5A, the minimum and maximum values of the distances to failure calculated in step  215 ( 7 ) for all channels are determined in step  220  and a difference ΔD between these values is calculated in step  225 :  
         Δ D=D   —   j _max− D   —   j _min  EQ(9) 
         [0123]    If ΔD is less than a specified value, herein referred to as a second threshold TH2, as shown in step  230 , the equalization is considered terminated. TH2 could be 1 dB or other value, according to the requirement for the respective network. The power input to the receivers may be now adjusted using the respective VOA, as shown in step  235 .  
         [0124]    If ΔD is greater than threshold TH2, a new input power P_j is calculated, using an adjustment coefficient, noted Coeff. The adjustment coefficient is selected based on experiments and simulations.  
         [0125]    The new input power P_j is determined in step  240  according to EQ(10) 
           P   —   j=P ini —   j— Coeff*( D   —   j−D _min)  EQ(10), 
         [0126]    Input power of all ACs is re-set to the values calculated with EQ(10) and steps  215  to  245  are repeated until ΔD becomes less than the threshold TH2.  
         [0127]    These results are preferably recorded in an equalization table. An example of an equalization table for recording data and calculation is shown as Table 2 below:  
               TABLE 2                                                                                
 
         [0128]    Finally, as a precautionary measure, the R values may be measured again for each AC for verifying if the system performance is acceptable. All ACs must be practically error-free after step  235 , i.e. must have a BER lower than 10 −12 .