Abstract:
A system for performing in-band reflection analysis in a passive optical network. The system comprises an optical line terminal (OLT) that includes a transceiver for transmitting continuous downstream data modulated on a first wavelength and receiving upstream burst data modulated on a second wavelength, the OLT further includes a receiver for receiving signals reflected from the PON that are modulated on the first wavelength, wherein the continuous downstream data comprises user data and a test data pattern; and a reflection analysis unit for cross-correlating between a time-shifted version of the transmitted test data pattern and the reflected signals, wherein the test data pattern is time-shifted relatively for an optical location to be tested.

Description:
TECHNICAL FIELD 
     The present invention relates generally to passive optical networks (PONs), and more particularly for performing in band reflection analysis in such systems. 
     BACKGROUND OF THE INVENTION 
     A passive optical network (PON) comprises an optical line terminal (OLT) connected to multiple optical network units (ONUs) in a point-to-multi-point network. New standards have been developed to define different types of PONs, each of which serves a different purpose. For example, the various PON types known in the related art include an Ethernet PON (EPON), a Gigabit PON (GPON), a 10-Gigabit (XGPON), and others. 
     An exemplary diagram of a typical PON  100  is schematically shown in  FIG. 1 . The PON  100  includes N ONUs  120 - 1  through  120 -N (collectively known as ONUs  120 ) coupled to an OLT  130  via a passive optical splitter  140 . In a GPON, for example, traffic data transmission is achieved using GPON encapsulation method (GEM) over two optical wavelengths, one for the downstream direction and another for the upstream direction. Downstream transmission from the OLT  130  is broadcast to all the ONUs  120 . Each ONU  120  filters its respective data according to pre-assigned labels (e.g., GEM port-IDs in a GPON). The splitter  140  is a 1 to N splitter, i.e., capable of distributing traffic between a single OLT  130  and N ONUs  120 . In most PON architectures, the upstream transmission is shared between the ONUs  120  in a TDMA based access, controlled by the OLT  130 . 
     In order to provide reliable operation of the PON, there is a need to identify faults that occur on the optical fibers and/or optical components of the PON, for example, detection of breaks or of major attenuation, due to a bent fiber, dirty connectors, and so on. Additionally, in order to allow repairing of a faulty optical fiber, there is a need to locate the exact location of a fault for a faster, more efficient network repair. 
     Optical faults and their location in the PON can be detected using optical time-domain reflectometers (OTDRs). The principle of an OTDR includes injecting, at one end of the fiber, a series of optical pulses into the fiber under test and also extracting from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. The strength of the return pulses is measured and integrated as a function of time and may be plotted as a function of fiber length. The results may be analyzed to estimate the fiber&#39;s length, the overall attenuation, to locate faults, such as breaks, and to measure optical return loss. 
     OTDR measurement techniques of the PON include “out of band”, “in band”, and different wavelengths. Out-of-band testing requires stopping the normal operation of the network and verifying the fiber using external OTDR tools. This can be performed using, for example, wavelengths and test pulses that are separate and independent from all other wavelengths used to carry customer service traffic. 
     The in-band OTDR testing may be performed when the network is operational. However, such a testing requires dedicated OTDR testing signals. The OTDR testing signals utilized in conventional in-band OTDR solutions are either AM modulated or FM modulated. As such they can be transmitted during a test period of the PON, during which time data signals are not transmitted to the ONUs. Other OTDR solutions utilize a dedicated upstream wavelength for measuring reflection from the fiber. However, such solutions require an additional transceiver. 
     It would be therefore advantageous to provide a solution for performing reflection analysis in a PON while overcoming the deficiencies of prior art testing techniques. 
     SUMMARY OF THE INVENTION 
     Certain embodiments include herein include a system for performing in-band reflection analysis in a passive optical network (PON). The system comprises an optical line terminal (OLT) that includes a transceiver for transmitting continuous downstream data modulated on a first wavelength and receiving upstream burst data modulated on a second wavelength, the OLT further includes a receiver for receiving signals reflected from the PON that are modulated on the first wavelength, wherein the continuous downstream data comprises user data and a test data pattern; and a reflection analysis unit for cross-correlating between a time-shifted version of the transmitted test data pattern and the reflected signals, wherein the test data pattern is time-shifted relatively for an optical location to be tested. 
     Certain embodiments include herein also include a method for performing in-band reflection analysis in a passive optical network (PON). The method comprises establishing a dedicated receive path at an optical line terminal (OLT) of the PON to receive signals reflected from the PON, wherein the reflected signals are modulated on a downstream wavelength of the OLT; transmitting continuous downstream data modulated on the downstream wavelength, wherein the downstream data includes user data and a test data pattern; and cross-correlating between a time-shifted version of the transmitted test data pattern and the reflected signals, wherein the test data pattern is time-shifted relative to an optical location to be tested. 
     Certain embodiments include herein also include an optical line terminal (OLT) operative in a passive optical network (PON) and configured to perform in-band reflection analysis. The OLT comprises a transceiver for transmitting continuous downstream data modulated on a first wavelength and receiving burst upstream data modulated on a second wavelength; a receiver for receiving signals reflected from the PON and being modulated on the first wavelength; a MAC module for generating the continuous downstream data, wherein the continuous downstream data includes user data and a test data pattern; and a reflection analysis unit for cross-correlating between a time-shifted version of the transmitted test data pattern and the reflected signals, wherein the test data pattern is time-shifted relatively for an optical location to be tested. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic diagram of a PON. 
         FIG. 2  is a block diagram of a system for performing reflection analysis according to an embodiment of the invention. 
         FIGS. 3A and 3B  show graphs of the power of a returned signal over time as generated using an embodiment of the invention. 
         FIG. 4  is a block diagram of a system for performing reflection analysis according to another embodiment of the invention. 
         FIG. 5  is a flowchart describing a process for detecting faults in an optical path of a PON in accordance with an embodiment of the invention. 
         FIG. 6  is an example demonstrating the reflection analysis. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is important to note that the embodiments disclosed are only examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present disclosure do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality. In the drawings, like numerals refer to like parts through several views. 
     According to certain exemplary embodiments of the invention an in-band OTDR of PONs is performed using a reflection analysis method. Accordingly, continuous downstream data including a test data pattern is transmitted by an OLT to ONUs of the PON. The test data pattern is time-shift cross-corrected with a signal received through a dedicated receiver in the OLT, and a reflection analysis is performed on the cross-corrected results. The results of the reflection analysis can be processed to at least detect faults in their locations in the optical path of the PON. The downstream data including the test data pattern is fully compliant with the communication specifications employed by the respective PON. Thus, it should be appreciated that the reflection analysis can be performed during normal operation of the PON and it is not limited to a testing period any time a downstream data signal can be transmitted through the PON for analysis. 
     In an embodiment of the invention, the test data pattern is a continuous high rate data pattern that is characterized by low frequency components. The high rate is the transmission rate of the PON (e.g., 2.5 Gbit/sec for GPON or 9.9 Gbit/sec in XG-PON). The low frequency components are generated using a low rate polynomial and may be at a rate of, for example, 58.32 Mbit/sec to 155.52 Mbit/sec. Various techniques for generating the test data pattern and encapsulating the pattern in downstream data frames can be found in a co-pending patent application entitled “TECHNIQUES FOR GENERATING LOW RATE DATA PATTERNS COMPLIANT WITH PASSIVE OPTICAL NETWORKS”, assigned to the common assignee of the present application. 
       FIG. 2  shows an exemplary block diagram of a system  200  for in-band OTDR of a PON by performing reflection analysis according to an embodiment of the invention. The system  200  includes an OLT  210  that is connected to an optical fiber  220  of the PON, and a reflection analysis unit  230 . The OLT  210  communicates with a plurality of ONUs (not shown in  FIG. 2 ) through one or more splitters (not shown) and the fiber  220 . 
     In an embodiment of the invention, the system  200  also includes an OTDR processor  240  adapted for detecting faults and their locations in the PON. Such faults include, for example, flattened fiber, bends, bad splices, dirty connectors, and a fiber cut. 
     The OLT  210  includes, among other components, an optical module  250  and a MAC module  260 . The optical module  250  includes a continuous optical transmitter  251  adapted to transmit continuous optical signals at a wavelength λ 1  and an optical burst receiver  252  that receives optical signals (sent from ONUs) at a wavelength λ 2 . The wavelengths λ 1  and λ 2  are defined by the respective standard of the PON for carrying downstream and upstream data respectively. For example, according to GPON standard, the OLT generates an optical signal of 1480 nm to 1500 nm (referred to as 15XY) at the downstream direction, and at the upstream direction each ONU transmits an optical signal of 1260 nm to 1330 nm (also referred to as 13XY). 
     In accordance with an embodiment of the invention, the optical module  250  also includes a continuous optical receiver  253  adapted to receive optical signals at the wavelength λ 1 , i.e., the wavelength of signals transmitted in the downstream direction by the transmitter  251 . As there is no connection between the optical transmitter  251  and the continuous optical receiver  253 , the latter receives optical signals reflected from the fiber  220 . The reflected signals are analyzed by the reflection analysis unit  230  as will be described below. It should be noted that the reflected signal as received at the optical receiver  253  corresponds to the downstream data transmitted by the transmitter  251 . Thus, the reflected signals include low frequency (rate) components similar to the test pattern injected to the downstream data. As will be further described below, the optical receiver  253  is also a bandwidth limiter, thus acts as a low pass filter. This ensures that high frequency components and reflected signals are filtered out, thus does not affect the reflection analysis. 
     The MAC module  260  generates continuous downstream data that is transmitted to the ONUs. The MAC module  260  operates according to the type of the PON. That is, the MAC module  260  may be compliant with the communication standard including, but not limited to, Ethernet PON (EPON), Gigabit (PON), XGPON, and the like. The MAC module  260  is also adapted to process burst upstream data sent from ONUs of the PON. 
     The reflection analysis unit  230  performs a continuous time-shift cross-correlation between the transmitted downstream data and data signals received through the continuous optical receiver  253 . With this aim, the reflection analysis unit  230  includes a low pass filter (LPF)  231  coupled to a delay line  232  that can be set to a predefined delay value ‘x’, a multiplier  233 , an analog to digital converter (ADC)  234  coupled to the output of the receiver  253 , and an accumulator  235 . 
     The LPF  231  filters out high frequency components from the transmitted continuous downstream data (output by the MAC module  260 ). This is performed in order to ensure that the cross-correlation will be performed between low frequency components of the transmitted data and the reflected signal to guarantee accurate results. The cutoff frequency of the LPF  231  is set to match the bandwidth of the receiver  253 . In an embodiment of the invention, the output of the LPF  231  is down-sampled to meet the sampling rate of the ADC  234 . The filtered data is delayed by the delay line  232 . The time delay value ‘x’ is predetermined according to a location on the optical path to be tested. The delay value x can be determined using the following equation:
 
Optical distance=( x*C *refraction−index)/2  [1]
 
where, C is the light speed.
 
The delay value ‘x’ is a configurable parameter and a reflection analysis can be computed for different values of x. The output of the delay line  232  is a time-shifted low-rate (frequency) data.
 
     The output of the delay line  232  is scaled to prevent zero values. The scaling is by a user defined constant value and may be performed by any scaling technique discussed in the related field of the art of signal scaling. The ADC  234  converts the received continuous optical signals (output by receiver  253 ) to digital signals. In an embodiment of the invention, the ADC  234  is a ‘k’ bit ADC, where ‘k’ represents an integer number equal to or greater than 1. The output of the ADC  234  is multiplied with the output of the data delay line  232  by means of a multiplier  233 . 
     Then, the multiplication result is accumulated by means of the accumulator  235 . The accumulation is performed over N times, where N is a function of the length of the test data pattern in the downstream data or a length of low frequency components in the downstream data. It should be appreciated that the value of the parameter N defines the dynamic range of the test. In an embodiment of the invention, the value of N is predetermined based on a typical length of binary sequence characterized by low frequency components that typically exist in continuous data sent in the downstream direction. 
     The output of the accumulator  235  is the cross-correlation of the reflected signal (as received by the receiver  253 ) and a time-shifted (delayed) signal transmitted in the downstream direction. In an embodiment of the invention, the reflection analysis for a delay x (i.e., a certain location at the PON) is performed P times, where P is a configurable parameter. This allows averaging the computed cross-correlation results P times, thus increasing the accuracy of the analysis. 
     In an embodiment of the invention, the cross-correlation results, produced by the reflection analysis unit  230 , can be processed by the OTDR processor  240  to detect optical faults in their locations in the PON. With this aim, the OTDR processor  240  compares the value of the cross-correlation at delay (location) x to a value that indicates normal or acceptable behavior at a location on the optical path, respective of the delay x. The OTDR processor  240  generates a fault indication for any deviation from the norm. 
     In an embodiment of the invention, the OTDR processor  240  can also generate a graph representative of the cross-correlation results for different values of delay ‘x’, each such value representing a different optical distance on the fiber  220 . Any non cross-correlated results are noticeable on the graph (e.g., as blips) and are indicative of faults in the optical path. 
       FIG. 3A  shows graphs  1010  and  1020  that are examples for possible normal cross-correlations between the transmitted test data pattern and the signal received by the continuous receiver  253 .  FIG. 3B  depicts a graph  1030  showing a blip at time T (relative to the time delay x). This indicates a fault in the optical path that may be a result of, for example, a bad connector that returns some of the power. The location of the fault is at (C*T)/2. Examples for other faults that can be detected include a cut fiber, a flattened fiber, bends in the fiber, dirty connectors, and so on. 
       FIG. 4  shows an exemplary block diagram of a system  400  for in-band OTDR of PON by performing reflection analysis according to another embodiment of the invention. The system  400  includes an OLT  410  that is connected to an optical fiber  420  of the PON, a reflection analysis unit  430 , and an OTDR processor  440 . The OLT  410  includes an optical module  450  and a MAC module  460 . The structure and functionality of the OLT  410 , optical fiber  420 , OTDR processor  440 , optical module  450 , and MAC module  460  are similar to the OLT  210 , optical fiber  220 , OTDR processor  240 , optical module  250  and a MAC module  260  described in detail above. For the sake of brevity the description of these components will not be repeated herein. 
     The reflection analysis unit  430  includes a data scanner  436  coupled to the output of the MAC module  460  and a LPF  431 . The data scanner  436  samples the downstream data output by the MAC module  460  and identifies data patterns that should not be cross-correlated by the reflection analysis unit  430 . As mentioned above, the techniques described herein are operable during normal operation of the PON, thus user data can be transmitted together with the test data pattern as part of the continuous downstream data. That is, some frames may include user data while others may include the test patterns. The data scanner  436 , in an embodiment of the invention, asserts a skip indication when the density of the high frequency components in the downstream data is above a predefined threshold. 
     In another embodiment, the skip decision may be based on the output of the LPF  431 , an example of which is provided below. In this embodiment, the data scanner  436  is coupled to the output of the LPF  431 . In yet another embodiment of the invention, the skip indication may be generated based on the value of a target ONU field&#39;s value in the transmitted downstream frames (e.g., GEM frame). If the field&#39;s value includes an identifier that is not associated with any ONU in the PON, then the skip indicator is asserted. It should be noted that the skip indication may be generated based on one or more, or a combination thereof, of the techniques discussed herein. 
     The skip indication is added to the filtered data output by the LPF  431 . In an embodiment of the invention, the combined data is down-sampled to meet the sampling rate of the ADC  434 . The combined data is delayed by a time delay value x by means of a delay line  432 . The output of the delay line  432  is scaled to prevent zero values. In an embodiment of the invention, such scaling is performed by the delay line  432 . As mentioned above, the scaling of the delayed signal is by a user defined constant value and may be performed by any scaling technique discussed in the related field of the art of signal scaling. The delayed and scaled signal is further multiplied, by means of a multiplier  433 , with a digital data respective of an optical signal received by the receiver  453 . The result of the multiplication is accumulated by means of the accumulator  435 . The accumulation is performed only for multiplication results that do not include a skip indication. Thus, data indicated as skip is not included in the output of the reflection analysis, hence is not part of the cross-correlation. The total number of accumulations cannot exceed N times, where N is the length of a pattern as defined above. It should be appreciated that by ignoring data that includes high frequency components, the reflection analysis unit  430  does not cross-correlate such signals. This ensures accurate results of the refection analysis. 
     In an embodiment of the invention, the outcome of the accumulator  435  is processed by the OTDR processor  440  to detect faults and their locations in the optical path of the PON. The operation of the OTDR processor  440  is described above with a reference to  FIGS. 3A and 3B . 
       FIG. 5  is a non-limiting and exemplary flowchart  500  illustrating the method for detecting faults and their exact locations in an optical path in a PON according to an embodiment of the invention. At S 510 , a dedicated receive path is established at the OLT to receive continuous optical signals at wavelengths of the OLT&#39;s continuous transmitter. Thus, through the receive path optical signals reflected from the optical fiber are received. The reflected signals correspond to the downstream data transmitted by the OLT. 
     At S 520 , a delay x is set to a value receptive of an optical location requested to be tested (see for example equation [1] above). In addition, an accumulation parameter is set to zero. At S 525 , downstream data generated by the OLT is scanned to detect a pattern that should not be cross-correlated. For example, a pattern that is characterized by a high density of the high frequency components should not be cross-correlated. At S 530 , it is checked if such a pattern was detected; and if so, at S 535 , a skip indication is generated; otherwise, execution continues with S 540  where the downstream data is low-pass filtered to remove high frequency components. At S 550 , the filtered data is combined with the skip indication, if such indication was generated. Optionally, the data is down sampled to meet the sampling rate of the ADC. At S 560 , the combined data is multiplied with the output of the ADC which is a value respective of the reflected optical signal (received through the receive path). It should be noted that the value generated by the ADC can be a rational number or a binary number. At S 570 , the multiplication result of the combined data is accumulated with a previous multiplication result according to the skip indication. That is, if the skip indication indicates that the current value should be skipped, the respective multiplication result is not accumulated. At S 580 , it is checked if the number of the accumulation parameter is equal to N, where N is the length of the test data pattern for which the reflection analysis is performed. If so, execution continues with S 590 ; otherwise, the accumulation parameter is increased by one (S 585 ) and execution returns to S 560 . 
     At S 590 , the cross-correlated value for the delay x (output at S 580 ) is compared to a normal cross-correlation (or reflected power) value for a location in the optical path respective of the delay x. Any deviation from the normal is an indication for a fault in the optical path. For example, a higher cross-correlation value may be indicative of a flattened fiber, while a value lower than the norm is indicative of attenuations in the fiber. 
     In an embodiment of the invention, the reflection analysis illustrated in  FIG. 6  for a specific delay x is performed P times (P is an integer number greater than 1), where P is a configurable parameter. This allows averaging the cross-correlation results computed P times, thus increasing the accuracy of the analysis. 
       FIG. 6  shows a non-limiting example for the operation of the of reflection analysis unit  430 . A portion of the downstream data is shown in column  610 . The column  620  includes the output of the LPF  431 . In this non-limiting example, the LPF  431  averages 10 bits and generates a rational number. For instance, the sum of the 10 first bits is 1 and dividing such sum by 10 results with number 0.1. The output of the LPF  431  is down sampled to meet the sampling rate of the ADC  434 . In the example shown in  FIG. 6 , the down sampling ratio is 1 to 10, thus every tenth value of the LPF  431  is selected. The skip indicator is generated based on the value of the LPF by comparing the value to a threshold. In this example, for every LPF&#39;s value less than 0.3 and higher than 0.7 the skip indicator is set to ‘1’, i.e., such a value will be included in the cross-correlation; otherwise, the indicator is set to zero. For example, the value rows  601  and  602  will be included in the accumulation, while value in rows  604  and  605  will not be part of the cross-correlation results. It should be noted that the ADC  434  also generates a rational number respective of the signal input by the receiver  453 . Typically, the receiver  453  filters noises and high frequency components in the optical signal. It should be further noted that without departing from the scope of the invention, in the example shown in  FIG. 6 , the delay x is set to ‘0’ merely for the sake of brevity of the description. 
     The various embodiments disclosed herein can be implemented as hardware, firmware, software, or any combination thereof. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage unit or computer readable medium. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPUs”), a memory, and input/output interfaces. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU, whether or not such computer or processor is explicitly shown. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.