Abstract:
In a wavelength division multiplexed (WDM) optical communication system having optical transmitters and receivers communicating via a optical channels, the bit error rate (BER) for the optical channels is tested simultaneously by performing a BER measurement for the cascaded chain. A BER test signal is supplied from a BER tester to a first optical transmitter. The BER test signal passes through the optical channels. The last optical receiver in the cascaded chain supplies the test signal to the BER tester measure the BER. The measured BER is compared to a predetermined system BER threshold to determine if the optical channels meet their specified BER values. Each optical transmitter and receiver includes a performance monitoring circuit that monitors the quality of the BER signal supplied to the optical transmitter/receiver for identifying one or more optical channels that the measured BER exceeds a predetermined system BER threshold.

Description:
BACKGROUND OF THE INVENTION 
   1) Field of the Invention 
   This invention pertains to the field of wavelength division multiplexed optical communication systems and, more particularly, to method of performing bit error rate tests for a wavelength division multiplexed optical communication system having a plurality of optical communication channels. 
   2) Background of the Related Art 
   Optical devices are increasingly being used in communication and information systems. An optical communication system, as used herein, refers to any system which uses optical signals to convey information across an optical waveguiding medium, such as an optical fiber. Examples of such systems include, but are not limited to, telecommunications systems, cable television systems and local area networks (LANs). 
   In the past, optical communication systems were designed to communicate data on an optical fiber via an optical communication channel having a single wavelength. To convey information to and from multiple sources and/or destinations, time division multiplexing (TDM) was frequently employed to share the single-wavelength channel. If multiple communication channels were desired, multiple fibers could be used. 
   More recently, wavelength division multiplexing (WDM) has been employed in optical communication systems to increase the information capacity of existing fiber networks. A WDM optical communication system employs multiple optical communication channels in a single fiber, each channel carrying a different optical signal operating on a different optical wavelength and transmitted over a single optical waveguide, or fiber. 
   As the demand for communications has increased, WDM optical communication systems are being developed with more and more optical communication channels. There are now WDM systems which have 80-100 separate optical communication channels each operating on a different optical wavelengths on a single optical fiber. 
   One standard measure of the quality of an optical communication system is the specified bit error rate (BER) for an optical communication channel in the system. 
     FIG. 1  shows a prior art arrangement for testing the BER for an exemplary optical communication system comprising two optical communication network elements  105 ,  107 . Each optical communication network element  105 ,  107  comprises N optical transmitters  120  and N optical receivers  130  for communicating over N optical communication channels. Each optical communication channel has a specified BER value. If the BER for an optical communication channel is greater than the specified BER value, then the optical transmitter/receiver pair  120 / 130  for that optical communication channel is not within specification and corrective measures are in order. A BER tester  140  is used to measure the BER for each optical communication channel to test whether or not the BER is within specification. Typically, the BER tester  140  comprises a BER test signal generator and a BER detector. 
   To test the BER for an optical communication channel, i, the BER test signal generator is connected to the optical transmitter  120  Tx i  of the first optical communication network element  105 . The output of the optical transmitter Tx i  is connected via an optical waveguide or optical fiber  115  to the corresponding optical receiver Rx i  of the second optical communication network element  107 . The output signal from the optical receiver  130  Rx i  of the second optical communication network element  107  is provided to the BER detector in the BER tester  140 . 
   The BER test signal generator of the BER tester  140  supplies a BER test signal to the optical transmitter  120  Tx i  of the first optical communication network element  105 . The BER detector receives the BER test signal from the optical receiver  130  Rx i  of the second optical communication network element  107  and detects and counts any bit errors produced by passing the BER test signal through the optical communication channel i to produce a measured BER for the optical communication channel i. Finally, the measured BER for the optical communication channel i is compared against a specified BER value for the optical communication channel to determine whether or not the optical communication channel i is within specification. 
   To test BER for the WDM optical communication system, the optical communication channel BER test is repeated for each optical communication channel i, in the optical communication system, where iε(1, N). If the measured BER for each of the optical communication channels is less than the specified BER value, then the optical system meets its BER requirements and passes the BER test. If one or more of the optical communication channels has a measured BER which is greater than the specified BER value, then corrective measures are required, including troubleshooting and repairing the optical transmitter/receiver pair  120 / 130  for the optical communication channels which failed the BER test. 
   The above-described prior art method works well when the number of optical communication channels is small and the specified BER values are relatively large. 
   However, there is a problem in testing a WDM optical communication system having many optical communication channels, each operating with a low specified bit error rate. To accurately and reliably measure a bit error rate for an optical communication channel, it is generally considered necessary to communicate enough data through the optical communication channel so that, at the specified BER, an average of ten errors will be produced. As technology has improved, WDM optical communication systems with lower and lower specified BERs are being produced. WDM optical communication systems have been developed which specify an optical communication channel BER of less than 10E-15. 
   Accordingly, to measure the BER for an optical communication channel having a specified BER is 10E-15, it is necessary to communicate at least 10E16 bits through the channel, which will on average produce ten errors. In a WDM optical communication system specifying an optical communication channel BER of less than 10E-15 and a data rate of 2.5 Gbps, the BER measurement for a single optical communication channel takes more than 46 days. In the case of a WDM optical communication system having forty (40) optical communication channels each having a specified BER of less than 10E-15 at a specified data rate of 2.5 Gb/s, it would take over 5 years to test the BER for the entire system, if each channel is tested sequentially in time. Clearly, a test taking so long is completely unacceptable. 
   Alternatively, it is possible to use 40 separate BER testers to test the 40 optical communication channels in parallel at the same time. However, the BER testers are somewhat expensive, and this approach is very costly. 
   Accordingly, it would be advantageous to provide an improved method of testing BER for a WDM optical communication system having a plurality of optical communication channels. It would also be advantageous to provide such a method wherein when one or more of the optical communication channels has an actual BER which exceeds a specified BER value, those optical communication channels are identified so that the optical transmitter(s) and/or optical receiver(s) for those optical communication channels can be troubleshot and repaired. Other and further objects and advantages will appear hereinafter. 
   SUMMARY OF THE INVENTION 
   The present invention is therefore directed to a method of testing the bit error rate (BER) for a plurality of optical communication channels in a wavelength division multiplexed (WDM) optical communication system which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art. 
   It is an object of the present invention to provide a quick method of determining whether the BER is acceptable. 
   It is another object of the present invention to provide an inexpensive method for determining whether the BER is acceptable. 
   It is yet another object of the present invention to be able to determine which channel(s) gives rise to an unacceptable BER. 
   These and other objects may be realized by performing a BER measurement for a WDM optical communication system which has been cascaded, and, when the BER measurement is unacceptable, determining the faulty channel(s) using an on-board performance monitor for each channel. 
   In one aspect, the optical transmitters and receivers for “N” optical communication channels within a WDM optical communication system are connected together in a chain to form a single continuous communication path, and a system BER measurement is performed. A BER signal generator is connected to the input of the first optical transmitter in the single continuous communication path, and a BER detector is connected to the output of the last optical receiver for the last optical communication channel in the single continuous communication path. The BER for the single continuous communication path is measured and compared with a predetermined system BER threshold. If the measured BER is less than the predetermined system BER threshold, then each of the optical communication channels meets its specified BER value and the WDM optical communication system passes the system BER test. 
   In another aspect, when the measured BER for the single continuous communication path is greater than the predetermined system BER threshold, one or more optical communication channels whose actual BER exceeds its specified BER value are identified using on-board diagnostic circuits included in the optical transmitters and optical receivers in the WDM optical communication system. 
   These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be described with reference to the drawings, in which: 
       FIG. 1  shows a prior art arrangement for testing the bit error rate for a WDM optical communication system; 
       FIG. 2  shows a preferred embodiment of an arrangement for testing the bit error rate for a WDM optical communication system according to the present invention; and 
       FIG. 3  shows a block diagram of an on-board performance monitor for an optical receiver or an optical transmitter in a WDM optical communication system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred arrangement for testing the BER for optical communication channels in a WDM optical communication system is shown in  FIG. 2 . 
   The WDM optical communication system shown in  FIG. 2  comprises two optical communication network elements  110 , 112  each comprising a plurality of optical transmitters  120  and optical receivers  130 . In a preferred embodiment, each of the optical communication network elements  110 , 112  comprises N optical transmitters  120  and N optical receivers which communicate data over N forward optical communication channels, from the first optical communication network element  110  to the second optical communication network element  112 , and N return optical communication channels, from the second optical communication network element  112  back to the first optical communication network element  110 , on a pair of optical fibers  115 , 117  respectively. 
   A single BER tester  140  is used to test BER for the optical communication system. The BER tester  140  comprises a BER test signal generator and a BER detector. In a preferred embodiment, the BER tester is a synchronous optical network (SONET) BER tester. The BER test signal generator generates a BER test signal for testing the BER of an optical communication channel. Typically, the BER test signal will comprise a psuedorandom bit stream (PRBS). Preferably, the BER test signal generator formats the PRBS into a series of SONET packets which comply with standardized formatting protocols. 
   As shown in  FIG. 2 , all of the optical communication channels are connected together with each other in a cascaded arrangement to form a single continuous communication path from the input of a first optical transmitter  120 , Tx 1 , of the first optical communication network element  110 , to the output of a last optical receiver  130 , Rx N , of the first optical communication network element  110 . The BER test signal generator of the BER tester  140  is connected to, and supplies the BER test signal to, the first optical transmitter  120 , Tx 1 , of the first optical communication network element  110 . The BER test signal passes through the single continuous communication path comprising the cascaded arrangement of all of the N forward optical communication channels and the N return optical communication channels. The BER test signal is then supplied from the output of the last optical receiver  130 , Rx N , of the first optical communication network element  110  to the BER detector of the BER tester  140 . 
   The BER detector detects and counts a number of errors in the received BER test signal which were produced by passing the BER test signal through all of the forward and return optical communication channels in the WDM optical communication system. Thereby, the BER tester  140  generates a measured BER for the single continuous communication path comprising a cascaded arrangement of all of the forward and return optical communication channels in the WDM optical communication system. 
   Each optical communication channel in the WDM optical communication system has a specified BER value. If the actual BER for the optical communication channel is greater than this specified BER value, then the optical transmitter/receiver pair  120 / 130  for that optical communication channel is not within specification and corrective measures are in order. Since the cascaded system will be carrying the errors all the way through, with no way of determining where an error occurred, detection of an error in excess of the minimum error will result in diagnostic testing for the cascaded system to determine the potentially unsatisfactory communication path. 
   Even using such a cascaded system and even assuming a transmission rate of 2.5 Gbps, it would take too long, i.e., 4.6 days to measure a very low bit error rate e.g., 1E-15 or 1E-14. In accordance with the present invention, the bit error rate testing for such low bit error rates is to occur over a shorter time period, e.g., 40,000 seconds, than would be required to determine the actual bit error rate, and it is to be assumed that no errors are to be allowed over this shorter time period. Thus, for the system, if even a single error is detected by the bit error rate tester  140  testing for very low bit error rates, it is necessary to identify the optical communication channel(s) in which the error(s) occur so that corrective measures may be taken, including troubleshooting and repairing the corresponding optical transmitter/receiver pair(s)  120 / 130 . 
   Accordingly, as shown in  FIG. 2 , each optical transmitter  120  and optical receiver  130  includes an on-board diagnostic circuit for monitoring the quality of the signal received by the corresponding optical transmitter  120  or optical receiver  130 . For each optical transmitter  120 , the on-board diagnostic circuit provides a transmitter diagnostic output signal  225  indicating a signal quality for the signal received by the corresponding optical transmitter  120 . For each optical receiver  130 , the on-board diagnostic circuit provides a receiver diagnostic output signal  235  indicating a signal quality for the signal received by the corresponding optical receiver  130 . Each transmitter diagnostic signal  225  and receiver diagnostic signal  235  indicates a number of bit errors present in the signal received by the corresponding optical transmitter  120  or optical receiver  130 . 
   That is, each transmitter diagnostic signal  225  and receiver diagnostic signal  235  indicates a number of bit errors which were produced by that portion of the single continuous communication path in the WDM optical communication system from the first optical transmitter, Tx 1 , of the first optical communication network element  110  up to and including the output of the corresponding optical transmitter  120  or optical receiver  130 , respectively. Each of the transmitter diagnostic output signals  225  and receiver diagnostic output signals  235  are provided to a diagnostic analyzer  250 , which may include a computer processor. 
   While this simultaneous BER testing allows determination of an acceptable overall BER, it will not indicate which channel(s) are faulty when the system BER is unacceptable. Since the WDM system is not cascaded in operation, when the BER is unacceptable, the individual channel(s) giving rise to the BER must be ascertained and corrected. 
   Accordingly, when the measured system BER for the WDM optical communication system exceeds the predetermined BER threshold for any of the communication channels, the diagnostics analyzer  250  analyzes the transmitter diagnostic output signals  225  and receiver diagnostic output signals  235  from each optical transmitter  120  and optical receiver  130  in the WDM optical communication system. The diagnostics analyzer  250  identifies which optical communication channel(s) are unsatisfactory by determining where excessive bit errors were detected by the on-board diagnostic circuits in the optical transmitters  120  and optical receivers  130  in the cascaded chain of optical communication channels. By this method, the optical communication channel(s) which are not within specification are identified so that corrective measures may be taken. 
     FIG. 3  shows block diagram of a preferred embodiment of an internal performance monitor  300  as the on-board diagnostic circuit for an optical transmitter  120  and optical receiver  130  which may be used in a BER testing method according to the present invention. 
   When the network is in SONET format, several bytes in the SONET frame overhead are reserved for a method of error monitoring called Bit Interleave Parity (BIP). Each SONET frame carries the BIP of the previous frame. The internal performance monitor  300  calculates the parity of every frame and compares this parity with the parity stored n the next frame received. If the parities are different, the internal performance monitor  300  assumes there is a bit error. Rather than calculate whether an error occurs for each bit, it is assumed that the system being monitored is prone to random errors, not bursts of errors. Since only parity is being monitored, if an even number of errors occurs, the parity will remain the same, resulting in a false positive. However, checking only parities allows checking for errors without knowledge of the bit pattern being sent by the tester. 
   As can be seen in  FIG. 3 , the internal performance monitor  300  includes an optical-to-electrical (O-E) converter  305 , a signal conditioning unit  310 , an analog-to-digital converter (ADC)  315 , a microprocessor  320 , a clock and data recovery unit  325 , a decision circuit  330  and an error monitoring unit  335 . 
   The O-E converter  305  converts the input optical signal into an electrical signal. This signal is then output to the signal conditioning unit  310  and the ADC  315 . The ADC  315  converts the analog signal from the O-E converter  305  into a digital signal which is then forwarded to the microprocessor  320 . The signal conditioning unit  310  takes the output from the O-E converter  305  and converts it into a form the clock and data recovery unit  325  desires. The clock and data recovery unit  325  separates the clock signal and BIP data from the overhead information and forwards them to the decision circuit  330 . The decision circuit also receives the digital signal from the microprocessor  320 . 
   Since, as noted above, each SONET signal carries the BIP of the previous frame, when a frame is received its actual BIP is calculated and saved as the received BIP in the microprocessor  320 . When the next frame is received, the BIP stored therein is recovered by the clock and data recovery unit  325 . The microprocessor  320  supplies the received BIP and the clock and data recovery unit supplies the stored BIP to the decision circuit  330 . If there is a difference, a counter in the error monitoring unit  335  is incrementally increased. The output from the error monitoring unit is supplied to the diagnostic analyzer  250  in  FIG. 2 . 
   In addition to the bit error rate, the quality or Q of the system may be monitored by adjusting the decision level threshold provided by the microprocessor  320  to the decision circuit  330 . 
   Referring to  FIG. 2 , the internal performance monitors  300  are configured on each optical receiver  130 . Further internal performance monitors  260  are configured on each optical transmitter  120 . The internal performance monitors  260  operate similar to the internal performance monitors  300 , except for converting an electrical signal into an optical signal. The internal performance monitors  260  and  300  are actually employed in the system everywhere a signal goes from being optical to electrical back to being optical, i.e., the performance monitor is included to check the electrical signal. In a preferred embodiment of the system, this results in the internal performance monitors  260  and  300  actually being employed in two places for each channel in the system. As can be seen in  FIG. 2 , each input signal is tested by the internal performance monitors  260  and  300 . 
   For a simple example, assume there are only two channels. The performance monitors  260  and  300  receive the following inputs: 
   Input to Tx1 in optical communication network element  112   
   Input to Rx1 in optical communication network element  110   
   Input to Tx1 in optical communication network element  110   
   Input to Rx1 in optical communication network element  112   
   Input to Tx2 in optical communication network element  112   
   Input to Rx2 in optical communication network element  110   
   Input to Tx2 in optical communication network element  110   
   Input to Rx2 in optical communication network element  112   
   According to the above-described method, simultaneous testing of the BER for the entire WDM optical communication system may be realized, regardless of how many optical communication channels it has. Thus, in the case where a WDM optical communication system has many channels (e.g. 80), the time required to perform the BER test for the WDM optical communication system is substantially reduced from the prior art method wherein all of the optical communication channels were tested sequentially. Moreover, since only a single BER tester  140  is required, a substantial cost savings is achieved over the prior art method wherein all of the optical communication channels were tested simultaneously using many BER testers. 
   While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the concept and scope of the appended claims.