Abstract:
A method and apparatus for performing optical channel performance management on a WDM system while in operation is disclosed in which each transmitter and receiver uses forward error-correction schemes to improve the BER performance of the channel in a known relation to the performance before error-correction. The receiver measures the BER performance before error-correction in real-time and communicates it to a system manager which determines, from this data, the appropriate launch power of each transmitter in the system, which it commands each transmitter to adopt, thereby ensuring relative launch powers which optimize the overall BER performance of the WDM multiplexed signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for performance management in a multiplexed transmission system, particularly but not exclusively an optical transmission system. 
     BACKGROUND OF THE INVENTION 
     The demand for high speed and high capacity data transmissions has been rising. In long haul transport, as well as in metro ring applications, the use of dense wavelength-division-multiplexing (DWDM) or wavelength-division-multiplexing (WDM)allows for increases in the transmission bandwidth by two, three or more times. 
     DWDM, or equivalently, for the purposes of this specification, WDM, systems permit a number of signals to be carried along a single optical fiber by modulating each signal about a separate optical carrier wavelength. Typically, optical routing and signal regeneration are performed by passive and active optical elements. 
     DWDM optical fiber telecommunication systems can have extremely high overall data capacity since each channel is capable of carrying a high data rate signal. These high capacity signals can be carried cost-effectively over many hundreds of kilometers if Erbium Doped Fiber Amplifiers (EDFA) are used to boost the power of the optical signal periodically and overcome the loss incurred in the optical fiber and the passive optical elements. There is a growing requirement to increase the capacity of the existing communication systems. 
     While optical amplifiers are designed to produce a linear gain profile, as a practical matter, the wavelength dependent profile of EDFAs and other optical elements in the network is non-uniform, so that this objective cannot always be reached across the entire wavelength range over which signals will be transmitted. A significant challenge in carrying such multi-channel signals over many spans of fiber separated by boosting EDFAS has to do with the fact that the wavelength spectrum of the gain of the EDFAs is not flat. In fact, because of the physical properties of the Erbium ions that provide the gain, the shape of the gain spectrum changes from strong gain (about 23.5 dB at 1530 nm) to weak gain (about 21.5 dB at 1560 nm). The fiber span also shows non-uniform loss across the wavelength spectrum. Generally, the higher the wavelength, the higher the loss. 
     Moreover, even with a gain flattening filter, the gain profile of an optical amplifier across the wavelength range still shows ripple and gain tilt. The ripple (the slight variation in the gain) and tilt (the slope of the gain profile) are functions of input power and are intrinsic properties of the amplifier material. 
     Further, due to aging amplifiers and environmental factors, optical signal quality could degrade, resulting in a degradation of the system performance over time. 
     In a long multi-span cascade of fiber spans and EDFA line-amplifiers, the nominal gain of the EDFA is set equal to the span loss, so that a nominal channel does not rise or fall in power as it propagates downstream. This non-ideal gain (due to EDFAs) and loss (due to fiber and component lose) spectrum means that in a long multi-span cascade of fiber spans and EDFA line-amplifiers, some channels will have more gain or higher loss than the average and will grow in relative power as the multi-channel signal propagates down the link. However, some channels have less gain or lower loss than the average, and so the power of that channel will decrease as the multi-channel signal propagates down the link. 
     The non-linear nature of the overall gain or loss profile has a profound impact on the bit error rate (BER) of the optical link. 
     The amount of gain provided by an EDFA is controlled by the amount of pump laser power that is applied to the Erbium doped fiber, and typically covers a range of 15 dB to 35 dB. The amount of output power capability of the EDFA is also influenced by the amount of pump laser power. For any given amount of pump power, there is a certain limit to the total power over all of the channels, with 15 dBm as an example of a typical value. This is a natural physical limit at which the pump photon flux is just sufficient to replenish the depletion of the Erbium population inversion by the high signal output power. As well as this natural physical limit on the total power capability, there can also be an additional lower limit applied by design. For a given number of channels, it might be useful to limit the total power out of the EDFA and launched into the optical fiber in order to avoid certain nonlinearities in the fiber. This total power control (TPC) mode typically is implemented by tapping off a very small but controlled fraction of the light at the output of the EDFA and monitoring that with a photodetector. 
     Since all of the wavelength channels can carry revenue generating traffic, it is of interest to ensure that all of the channels meet a certain standard of performance. In a digital system, BER is typically used as a figure of merit, and 10 −12  is a common objective for BER. One of the main influences which will degrade the BER of multi-span EDFA links is the noise known as the Amplified Spontaneous Emission (ASE) which is generated inside the EDFAS. The amount of total noise (ASE, signal-to-spontaneous beat noise, spontaneous-to-spontaneous noise, etc.) relative to the signal power is typically quantified by the Optical Signal to Noise Ratio (OSNR), defined as: 
     
       
           OSNR =Signal Power/(noise density* BW   OSNR )  (1) 
       
     
     where BW OSNR  is the spectral band over which the OSNR is defined (for example 0.1 nanometers). 
     To optimize the OSNR of any given channel in a multi-span link, the input powers to each EDFA should be kept as high as possible at all of the amplifiers. This influences the design of multi-channel links where some channels will be increasing in power going down span, and some channels will be decreasing in power. The simplest case to consider is one in which all of the channels are initially launched at the same power. In the case of a channel which has more than average EDFA gain, it increases in power after that initial launch point, up until the receiver. With such high powers going into the EDFAs, that channel will have a good OSNR and will then have a good BER, provided that fiber nonlinearities are not provoked. However, a channel which has less than average gain will drop in power at every span as it propagates down-link. This channel will have a poor OSNR and thereby will have a high BER, which may not meet an objective like 10 −12 . 
     At first, it might be thought that the simplest way to ensure that the weak channels do not severely hamper the system would simply be to turn all transmitters up to their highest launched power achievable. However, constraints (either natural or by design) on the total power available from the EDFA rule out this simple approach. Given that the total EDFA power is limited, the solution in the past has traditionally been to turn up all transmitters only by the appropriate amount such that the end performance (either OSNR or BER) is balanced between all channels. If any transmitters were launching more than the power necessary to achieve this balanced performance condition, then they would necessarily be taking more power than they need from at least one of the EDFAs. Because of the constraint on total EDFA power, this removal of power would then reduce the power available to the weaker channels. This means that the performance of the weaker channels would suffer if the strong channels were allowed to get better end performance than the average. In conclusion, when operating under total power constraints, adjusting the channel launched power of the transmitters to achieve equalization of the end performance of all of the channels is the optimum solution. 
     Therefore, it is important to have a method to adjust the launching power of the channels in order to equalize the BER performance of all the channels. Since aging and optical degradation happens over time, it is important to develop the equalization method so it can be used during the operation of the network and not simply during system set up. 
     One solution to this problem would be to physically measure the BER value generated at a receiver and use this information to adjust the launching power of all the channels to provide equal output BER. Such an approach, has not been heretofore practical, however, because of the long monitoring times that would be required in order to calculate the receiver BER, for BER values representative of a channel with any practical value. 
     Another solution to this problem would be to determine the OSNR values corresponding to each DWDM channel of the DWDM signal being received at a DWDM receiver and to subsequently attenuate the input power of the DWDM channels with high OSNR at the transmitter prior to multiplexing the channels. 
     In U.S. Pat. No. 5,225,922, (Chraplyvy et al.) issued Jul. 6, 1993 to AT&amp;T Bell Laboratories entitled “Optical Transmission System Equalizer”, the OSNRs at the output of an amplified WDM system are measured directly and the input powers are iteratively adjusted to achieve equal OSNRs. 
     However OSNR values alone do not accurately characterize the system performance. Rather, OSNR is only one of several parameters that affect the performance of an optical transmission system, which by definition, is fully expressed by the BER. 
     In European published Patent Application No. EP 0926 854 A2 (Barnard et al,) laid-open for publication on Jun. 30, 1999 and entitled “Methods for Equalizing WDM Systems”, there is disclosed a method of equalizing the channels of a WDM link by identifying for each optical channel in the link an error threshold level for the BER of the optical channel and the attenuation of the channel&#39;s power along the link and then adjusting the input powers of the weaker channels in accordance with the measured attenuations of all of the channels to obtain substantially equal BER for all of the channels. 
     With both of these approaches, however, the system is unable to operate while data is being sent on the network since both of these algorithms must at times make sure channels are accessible (i.e. taken off service) in order to determine constants used within the attenuation calculations. 
     SUMMARY OF THE PRESENT INVENTION 
     Accordingly, it is desirable to provide an improved method and apparatus for optical channel performance management. 
     It is further desirable to provide a method and apparatus for optical channel performance management that can be implemented while the system is in operation. 
     The present invention accomplishes these aims by providing a forward error correcting (FEC) element within a WDM receiver, which provides as an output, a signal having a BER which provides an improvement over the BER of the signal provided at its input in a known relationship. As a result, the BER at the input is sufficiently large that it can be measured and used for performance management purposes in real-time while maintaining the low BER performance required for practical operation of a WDM system. 
     According to a broad aspect of an embodiment of the present invention, there is disclosed a method of equalizing the performance of a plurality of multiplexed transmission channels comprising: encoding signals for transmission in the channels using a forward error-correcting code; receiving the encoded signals; determining the BERu (bit error rate prior to forward error correction) of each of the received signals; decoding the received signals using the forward error-correcting code to retrieve output data signals; and adjusting the transmission powers of the channels according to the determined BERu for each channel thereby to equalize the BER u  across the channels. 
     According to a second broad aspect of an embodiment of the present invention, there is disclosed a communications system comprising at least two transmitters for transmitting respective signals to respective receivers in a multiplexed signal across a communications channel, the transmitters and receivers respectively coding and decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in the receiver in a known relation to the BER before forward error-correction, a system manager comprising: a status module for receiving, from the receivers in the communications system, the BER of their respective signals before forward error-correcting decoding; a calculation module for determining the relative launch power of each transmitter which will provide optimal BER performance of the multiplexed signal along the communications channel; and a command module for issuing commands to the transmitters in the communications system to adjust their launch powers in accordance with the relative launch powers determined by the calculation module. 
     According to a third broad aspect of an embodiment of the present invention, there is disclosed a communications system comprising a system manager and at least two transmitters for transmitting respective signals in a multiplexed signal across a communications channel, the transmitters coding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, a receiver associated with each transmitter for receiving its respective signal comprising: a decoder for decoding the received signal using the forward error-correcting code; a BER calculator for determining the BER of its respective signal before forward error-correcting decoding; and a communications module for providing the BER to the system manager; whereby the system manager may determine, from the BER values provided to it, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel; and each transmitter in the communications system may adjust its launching power in response to commands from the system manager. 
     According to a fourth broad aspect of an embodiment of the present invention, there is disclosed a communications system comprising a system manager and at least two receivers for receiving respective signals from a multiplexed signal send across a communications channel, the receivers decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, a transmitter associated with each receiver for transmitting its respective signal comprising: a encoder for encoding its respective signal using the forward error-correcting code; and a launch power adjustment module for adjusting the launch power used to transmit the signal in response to commands from the system manager, whereby each receiver may determine the BER of its respective signal before forward error-correcting decoding and provide the BER value so determined to the system manager; and whereby the system manager may determine, from the BER values provided to it by each receiver, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel and issue commands to each transmitter in accordance therewith. 
     According to a fifth broad aspect of an embodiment of the present invention, there is disclosed a computer-readable medium storing computer-executable program instructions which, when executed by a processor in a system manager in a communications system comprising at least two transmitters for transmitting respective signals to respective receivers in a multiplexed signal across a communications channel, the transmitters and receivers respectively coding and decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in the receiver in a known relation to the BER before forward error-correction, cause the system manager to: receive, from the receivers in the communications system, the BER of their respective signals before forward error-correcting decoding; determine the relative launch power of each transmitter which will provide optimal BER performance of the multiplexed signal along the communications channel; and issue commands to the transmitters in the communications system to adjust their launch powers in accordance with the relative launch powers determined by the calculation module. 
     According to a sixth broad aspect of an embodiment of the present invention, there is disclosed a computer-readable medium storing computer-executable program instructions which, when executed by a processor in a receiver in a communications system comprising a system manager and at least two transmitters for transmitting respective signals in a multiplexed signal across a communications channel, the transmitters coding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, cause the receiver, being associated with one of the transmitters for receiving its respective signal, to: decode the received signal using the forward error-correcting code; determine the BER of its respective signal before forward error-correcting decoding; and provide the BER to the system manager; whereby the system manager may determine, from the BER values provided to it, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel; and each transmitter in the communications system may adjust its launching power in response to commands from the system manager. 
     According to a seventh broad aspect of an embodiment of the present invention, there is disclosed a computer-readable medium storing computer-executable program instructions which, when executed by a processor in a transmitter in a communications system comprising a system manager and at least two receivers for receiving respective signals from a multiplexed signal send across a communications channel, the receivers decoding their respective signals using a forward error-correcting code which provides an improvement in the BER after forward error-correcting decoding in a known relation to the BER before forward error-correction, cause the transmitter, being associated with each receiver for transmitting its respective signal, to: encode its respective signal using the forward error-correcting code; and adjust the launch power used to transmit the signal in response to commands from the system manager, whereby each receiver may determine the BER of its respective signal before forward error-correcting decoding and provide the BER value so determined to the system manager; and whereby the system manager may determine, from the BER values provided to it by each receiver, the relative launching power of each transmitter required to optimize the BER performance of the multiplexed signal along the communications channel and issue commands to each transmitter in accordance therewith. 
     According to a eighth broad aspect of an embodiment of the present invention, there is disclosed a method of adjusting the performance of a channel in a multiplexed transmission environment comprising: encoding signals for transmission in the channel using a forward error-correcting code; receiving the encoded signal; determining the BERu (bit error rate prior to forward error correction) of the received signal; decoding the received signal using the forward error-correcting code to retrieve the output data signal; and adjusting the transmission power of the channel according to the determined BERu for the channel thereby to establish a BERu proportionate to the relative importance of the signal to other signals in the multiplexed transmission environment. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which: 
     FIG. 1 is a block diagram of a WDM system in accordance with an embodiment of the present invention; 
     FIG. 2 is a flow chart of exemplary logic applied by the system manager in the embodiment of FIG. 1 to adjust launching power of the transmitters in the system; 
     FIG. 3 is a block diagram of a receiver used in the embodiment of FIG. 1; 
     FIG. 4 is a block diagram of the BER calculation unit in the embodiment of FIG. 3 to calculate the BER of the received data stream; and 
     FIG. 5 is a block diagram of a transmitter used in the embodiment of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a block diagram of a WDM system, generally at  100 . The WDM system  100  comprises a plurality of each of transmitters  110 ,  113 ,  116 , receivers  120 ,  123 ,  126  and amplifiers  150 , The WDM system  100  also comprises a system manager  160  and at least one multiplexer  130  and at least one demultiplexer  140 . 
     Each transmitter  110 ,  113 ,  116  is connected, at its input, to an electrical cable  111 ,  114 ,  117  and at its output, to an optical fiber  112 ,  115 ,  118 . (Those familiar with this art will readily recognize that, depending on the configuration of the network, incoming signals to the transmitters  110 ,  113 ,  116  may be in the optical domain rather than in the electrical domain, for example, it the transmitter  110 ,  113 ,  116  is part of a regeneration station, or if the input signal was originally transmitted along a SONET or other optical network. In such a case, the electrical cable  111 ,  114 ,  117  at the input of the transmitter  110 ,  113 ,  116  would be replaced by an optical fiber (not shown) and corresponding changes made to the structure of the transmitter  110 ,  113 ,  116 .) Additionally, each transmitter  110  is connected by a control line  161 - 163  to the system manager  160 . 
     The transmitter, say  110 , converts an incoming data stream which arrives at its input along electrical cable  111 , into a corresponding optical domain signal which it transmits at its output along optical fiber  112 . 
     Each receiver  120 ,  123 ,  126  is connected, at its input, to an optical fiber  121 ,  124 ,  127  and at its output to an electrical cable  122 ,  125 ,  128 . (Those familiar with this art will readily recognize that, depending on the configuration of the network, signals output by the receivers  120 ,  123 ,  126  may be in the optical domain rather than in the electrical domain, for example, if the receiver  120 ,  123 ,  126  is part of a regeneration station, or if the signal is to be output along a SONET or other optical network. In such a case, the electrical cable  112 ,  115 ,  118  would be replaced by an optical fiber (not shown) and corresponding changes made to the structure of the receiver  120 ,  123 ,  126 .) Additionally, each receiver  120 ,  123 ,  126  is connected by a control line  164 - 166  to the system manager  160 . 
     The receiver, say  120 , converts an optical domain signal containing data which arrives at its input along optical fiber  121 , into a corresponding data stream which it transmits at its output along electrical cable  122 . 
     The multiplexer  130  has a plurality of inputs and a single output. It is connected, at each input, to a respective optical fiber  112 ,  115 ,  118  and, at its output, to a WDM compatible optical fiber  131 . 
     The multiplexer  130  combines a plurality of separate optical domain signals, each containing a data stream, which arrive at its inputs, into a single WDM signal which it transmits at its output along the WDM compatible optical fiber  131 . The WDM signal which it generates is comprised of each of the input data streams encoded about a separate optical wavelength. 
     The demultiplexer  140  has a single input and a plurality of outputs. It is connected, at its input, to a WDM compatible optical fiber  141  and, at each output, to a respective optical fiber  121 ,  124 ,  127 . 
     The demultiplexer  140  breaks up a single WDM signal which arrives at its input, in which a plurality of data streams are each encoded about a separate optical wavelength, into a plurality of separate optical domain signals each containing one of the data streams which it transmits at its output. 
     The amplifier  150  has a single input and a single output. It is connected at its input to a WDM compatible optical fiber  131 , connected to multiplexer  130  and at its output to a WDM compatible optical fiber  151  connected to another amplifier which may be amplifier  152 . 
     The amplifier  152  also has a single input and a single output. It is connected at its input to a WDM compatible optical fiber  153  connected to another amplifier which may be amplifier  150 , and at its output to a WDM compatible optical fiber  141  connected to demultiplexer  140 . Additionally, each amplifier  150 ,  152  is connected by a control line  167 ,  168  to the system manager  160 , used for purposes not related to the present invention. 
     Each amplifier  150 ,  152  accepts as an input, a WDM signal which arrives at its input and amplifies the signal, which it outputs. The amplifier  150 ,  152  may be an erbium-doped fiber amplifier (EDFA) which has a largely flat gain profile across the range of wavelengths used in DWDM systems. 
     The system manager  160  manages the operations, administrative and maintenance(OAM) functions of the WDM system  100 . In this embodiment of the present invention, this includes, inter alia, functionality by which it receives performance monitoring (PM) data from the receivers  120 ,  123 ,  126  and the amplifiers  150 ,  152  in the WDM system  100 , along control lines  164 - 168  respectively. The control lines  164 - 168  are shown for schematic purposes. Those skilled in this art will recognize that such control lines may be implemented by use of an optical service channel (OSC) carried by the optical fiber along a wavelength dedicated to this purpose. The control information carried by each of the control lines  164 - 168  may in fact be transmitted through the OSC data stream back to the transmitters  110 ,  113 ,  116 . 
     As is discussed below, the performance monitoring data received by the system manager  160  includes an actual BER value calculated on a per channel basis, in real-time, from the data stream transmitted along each channel. 
     These BER values are compared against each other and processed to determine how, if at all, the launching power of each of the transmitters  110 ,  113 ,  116  in the WDM system  100  should be adjusted. 
     To the extent that adjustment is required, the system manager  160  transmits launching power adjustment data to the appropriate transmitters  110 ,  113 ,  116 , along the associated control line  161 - 163 . (The control lines  161 - 163  are shown for schematic purposes. As with control lines  164 - 168 , each of control lines  161 - 163  may in fact be transmitted through the OSC data stream.) 
     In so doing, the gain of the transmitters  110  in the WDM system  100  can be adjusted relative to one another to ensure an optimal BER performance across the entire WDM system  100 . 
     Turning now to FIG. 2, a flow chart of an exemplary system for determining the launch power adjustment data to be provided to transmitters  110  in the WDM system  100  from the calculated BER values obtained from the receivers  120  is shown. 
     Upon start-up  200 , the system manager  160  sets the equalization target for all channels to a predetermined value designated BERfinall, . . . BERfinaln  205 . 
     The system manager  160  then obtains the BERu for each channel, which, for a channel n is denoted as BERn  215 . The system manager  160  then calculates a ratio, denoted RATIOn, of the equalization target to the BERu for channel n where RATIOn equals BERfinaln/BERn  220 . 
     The system manager  160  compares each of the ratios so calculated and determines whether they are all greater than 1 225. If so, equalization is achieved  230  and the process terminates  235 . If not, the system manager identifies the channel k with the lowest RATIOk, which is accordingly the worst performing channel  240 . If RATIOk is larger than 1 245, the system manager  160  reverts to step  210 . Note that on the first iteration of this step, this will not take place. 
     If the value of RATIOk is not larger than 1, the channel output power for channel k is increased by a predetermined amount, for example 0.5 dB  250 . The system manager  150  thereupon reads the BERu for channel k (BERk&#39;)  255  and calculates an updated value of RATIOk designated RATIOk&#39; which equals BERfinalk/BERk&#39;. The system manager  160  then compares RATIOk&#39; to the original RATIOk value  260 . If RATIOk is greater than RATIOk&#39;, than equalization cannot be achieved  270  and the process terminates  235 . 
     However, if RATIOk&#39; is greater than RATIOk, the RATIOk is set equal to RATIOk&#39;  265  and the process reverts to step  245 . 
     Turning now to FIG. 3, a block diagram of the receiver  120  used in the embodiment of the present invention is shown. The receivers  123  and  126  would have a similar structure. The receiver  120  comprises an optical to electrical converter  300 , which may be a PIN detector or an avalanche photodiode, a clock and data recovery unit  310 , a linear channel  320 , a forward error-correcting decoding unit  330  and a BER calculation unit  340 . 
     The optical to electrical converter  300  is connected at its input to the input optical fiber  121 . It is connected at its output to the clock and data recovery unit  310  by an electrical cable  301 . The optical to electrical converter  300  converts the optical domain signal arriving at the receiver  120  along the input optical fiber  121  into an electrical domain signal containing the same data, which it outputs along electrical cable  301 . The function of the decoding avalanche photodiode  300  is entirely conventional. 
     The clock and data recovery unit  310  is connected at its input to the output of the optical to electrical converter  300  by electrical cable  301 . It is connected at its output to the linear channel. The clock and data recovery unit  310  accepts the electrical domain signal from the optical to electrical converter  300  and applies processing to recover the clock signal embedded in the electrical domain signal and to clock out the data stream contained in the electrical domain signal using the recovered clock signal. The data stream is output along the linear channel  320 . The function of the clock and data recovery unit  310  is entirely conventional. 
     The linear channel  320  is an electrical cable connected to the output of the clock and data recovery unit  310  and the input of the forward error-correcting decoding unit  330 . 
     The forward error-correcting decoding unit  330  is connected at its input to the linear channel  320 . It is connected at one of its outputs to output electrical cable  122 , and at the other output to the BER calculation unit  340 , by a control line  331 . The forward error-correcting decoding unit  330  decodes the data stream which arrives at its input and outputs the decoded but uncorrected data stream along the electrical cable  331 . The forward error-correcting decoding unit  330  applies error-correcting processing to it to correct errors which may have developed during the course of transmission of the data along the WDM system  100 . The corrected data stream is output by the forward error-correcting decoding unit  320  along the electrical cable  122 . 
     Typically, such error-correction involves the use of error-correcting codes which are known in the art, such as Reed-Solomon codes. 
     The use of such error-correcting codes will improve the BER of the WDM system. At present, the BER improvement resulting from the use of forward error-correction known in the art can be calculated using the expression 
     
       
           BERc=A ×( BERu ) n   (2) 
       
     
     where BERc is the BER after error-correction, BERu is the BER before error-correction, A and n are constants. 
     Thus, for example, with a BERu of 10 −7 , A=4 and n=2, BERc=4×10 −14 . Accordingly, the use of forward error-correction can improve the BER performance of a lossy channel sufficiently to permit it to be used in a practical WDM system. 
     Put another way, the BER improvement occasioned by forward error-correction will permit lossier (and hence less expensive) components to be introduced into a practical WDM system. 
     For the purposes of the present invention, it will be shown below that the scale of BER improvement using forward error-correction is such that performance management using actual BER values before correction can be used to equalize the launching power of transmitters in the WDM system  100 . 
     The forward error-correcting decoding unit  330  generates, in the course of its processing, a code violation every time an error is detected and corrected. These code violations are output to the BER calculation unit  340  along control line  331 . 
     The BER calculation unit  340  is connected at its input to the forward error-correcting decoding unit  330  by control line  331  and at its output to the system manager  160  by control line  164 . The BER calculation unit  340  receives code violations from the forward error-correcting unit  330  along control line  331 . 
     The BER calculation unit  340  thereupon calculates, in real time, the BER before forward error-correction using the formula: 
     
       
           BER =# of errors/(bit rate of channel*monitoring time)  (3) 
       
     
     Thus, for example, assuming an OC-48 channel having a bit rate of 2.5×10 9  b/s, which produces 5 errors in 5 minutes of monitoring time, the BER would be 5/(2.5×10 9 ×5×60) or 6.67×10 −12 . 
     The exponential improvement in the BER as a result of forward error-correction permits the calculation of BER on the uncorrected data, for the purposes of adjusting the launching power of each data signal, while at the same time maintaining a performance with appropriately low BER to permit practical use of the channel. 
     This is amply demonstrated by the following example. Assume a value of A=1, n=2 for the error-correction scheme, and a minimum permissible BER of 10 −14  bit errors/bit, on an OC-48 channel (2.5×10 9  b/s bit rate) Without forward error-correction, the BER calculation unit  340 , in order to calculate the BER, would require a monitoring time of more than 11 hours (1/((2.5×10 9  b/s 10 −14  errors/bit*(1/3600))) to detect a single bit error (and considerably longer, in order to detect sufficient bit errors as to make the calculation statistically significant). 
     If, however, forward error-correction is used to achieve a corrected BER of 10 −14  errors/bit, an uncorrected BER of only 10 −7  errors/bit would be required. If the uncorrected data stream were used to calculate the BER (which could then be converted into the corrected BER rate using the known values of A and n), it would only require 0.4 seconds (100/(2.5×10 9  b/s×10 −7  errors/bit)) to detect 100 bit errors, which is more than adequate to achieve a statistically significant BER value. 
     The BER calculation unit  340  forwards the calculated corrected BER rate to the system manager  160 , as required. This data is sent to the system manager  160  according to any number of methods well known in the art, whether by an analog signal which is sampled by the system manager  160 , by a periodic digital value supplied to the system manager  160 , or a digital value calculated and supplied to the system manager  160  upon receipt of a request from the system manager  160 . 
     Turning now to FIG. 4, a block diagram of an exemplary system for calculating the BERu of the received data stream and forwarding this data to the system manager  160  is shown. 
     The BER calculation unit  340  comprises a counter  400 , a timer  405  and a calculator  410 . 
     The counter  400  is connected at its input to control line  331  from the forward error-correcting decoding unit  330  and at its output to the calculator  410  by a control line  401 . The counter is also connected at its reset by a control line  406  output by timer  405 . The output of the timer  405  is also fed back into the reset of the timer by a control line  407 . Thus, a continuous period determined by the settings of the timer  405 , is used to perpetually reset the counter  400 . 
     The calculator  410  receives, along control line  401 , from the output of the counter  400 , the total number of code violations that have taken place since the last reset. Using this data, together with the data bit rate of the received data stream, which is known and the period of the timer  405  which is predetermined, it can calculate the BER using the formula set out in equation (3) above. 
     The calculated BERu value is thereupon output along control line  164  to the system manager  160 , as required. 
     Turning now to FIG. 5, a block diagram of the transmitter  110  used in the embodiment of the present invention is shown. The transmitters  113  and  116  would have similar structures. The transmitter  110  comprises a forward error-correcting encoding unit  500 , an electrical to optical converter laser diode  510 , which may be a laser diode/driver or an MZ modulator, and a launching power adjustment unit  520 . 
     The forward error-correcting encoding unit  500  is connected at its input to the input electrical cable  111 . It is connected at its output to the electrical to optical converter  510  by an electrical cable  501 . The forward error-correcting encoding unit  500  encodes the data stream which arrives at its input with the chosen error-correcting code in a manner complementary to the decoding performed by the forward error-correcting decoding unit  330  in the receiver  120 . The encoded data stream is output by the forward error-correcting encoding unit  500  along the electrical cable  501 . 
     The electrical to optical converter  510  is connected at its input to the output of the forward error-correcting encoding unit  500  by electrical cable  501 . It is connected at its output to the output optical fiber  112 . 
     The electrical to optical converter  510  converts the electrical domain signal arriving at its input along electrical cable  501  into an optical domain signal containing the same data, which it outputs along the output optical fiber  112 . The function of the electrical to optical converter  510  is entirely conventional. 
     The launching power adjustment unit  520  is connected at its input to the system manager  160  by the control line  161  associated with the transmitter  110 . It is connected at its output to an external variable optical attenuator  530  interposed between the electrical to optical converter  510  and the output optical fiber  112 . 
     The variable optical attenuator  530  attenuates the optical domain signal arriving along optical fiber  531  in accordance with the signals transmitted along the biasing signal line  522  from the launching power adjustment unit  520 . 
     The launching power adjustment unit  520  receives launching power adjustment data along the control line  161 , and converts this data into biasing signals which it outputs along biasing signal line  521 . 
     The present invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combination thereof. Apparatus of the invention can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and methods actions can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The invention can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. Suitable processors include, by way of example, both general and specific microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing can be supplemented by, or incorporated in ASICs (application-specific integrated circuits). 
     Examples of such types of computers are programmable processing systems contained in the transmitters  110 ,  113 ,  116  receivers  120 ,  123 ,  126  and system manager  160 , shown in FIG. 1, suitable for implementing or performing the apparatus or methods of the invention. The system may comprise a processor, a random access memory, a hard drive controller, and an input/output controller coupled by a processor bus. 
     It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiments disclosed herein, consistent with the present invention, without departing from the spirit and scope of the present invention. 
     For instance, the invention may be applicable to other multiplexed communications systems, whether or not in domains other than the optical domain, such as electrical or radio frequency, where the amplifiers used in the communications system have a limited input range. 
     The invention may be applied to all WDM network architectures, whether point-to-point or optical ring. 
     Further, the invention may be applicable to adjust the relative performance of individual channels. By using the BER information from the forward error-correcting decoding unit  330 , the BER performance can be managed according to the relative importance of the data associated with a particular channel. For example, a channel carrying internet traffic can be set to run at a relatively higher BER, eg. 10 −9 , while a channel carrying banking information can run at a much lower BER, eg 10 −14 . 
     Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein. 
     Accordingly, the specification and the embodiments are to be considered exemplary only, with a true scope and spirit of the invention being disclosed by the following claims.