Abstract:
A system, for identifying faults in a GPON that includes an OLT and a plurality of ONUs, including: a global error-counter, coupled to the OLT, for counting FEC-correctable errors, for each ONU, from a data stream from the GPON; and a CPU for extracting an ONU status, indicative of a faulty ONU, contingent on the errors from the global error-counter. A system, for identifying faults in a GPON that includes an OLT and a plurality of ONUs, including: a grant-start error-counter, coupled to the OLT, for counting grant-start errors, for each ONU, from a data stream from the GPON; a grant-end error-counter, coupled to the OLT, for counting grant-end errors for each ONU; and a CPU for extracting an ONU status, indicative of a faulty ONU, contingent on a parameter selected from the group consisting of the grant-start errors, the grant-end errors, and a combination thereof.

Description:
The present application claims the benefit of U.S. Provisional Patent Application No. 60/778,401 filed Mar. 3, 2006. The present application is also a continuation-in-part (CIP) application to U.S. patent application Ser. No. 11/564,299, filed Nov. 29, 2006, which claims the benefit of PCT Patent Application No. PCT/IL05/001358, filed Dec. 19, 2005, which claims the benefit of U.S. Patent Application No. 60/699,879 filed Jul. 18, 2005. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates generally to passive optical networks (PON) and more particularly to active real time monitoring of such networks, and to detection of a rogue optical-network unit (ONU) in gigabit PON (GPON) environments. 
     Passive optical networks, and in particular Ethernet PONs (EPONs) are known, and described for example in U.S. Patent Application No. 20020196801 by Haran et al. The debugging of a running/active network in a shared media network environment is difficult. At the same time, the ability to proactively monitor the network and to verify that its behavior is correct is valuable. Isolating transmission errors (or simply “errors”), detecting the cause of an error and providing debugging tools are highly desired features in a network environment. 
     A major goal in a PON that comprises an optical line terminal (OLT) and a plurality of optical network units (ONUs) is to detect degradation in the network behavior before customer complains, namely before errors are evident on the line. The most critical aspect is fault isolation, i.e. finding a faulty ONU before it harms the performance of other ONUs. The specific fault of the ONU is less important, because the faulty ONU is likely to be replaced by the network operator. 
     A PON may suffer from one or more of a number of failure modes (malfunctions or problems), either time-related (“temporal”) or laser-power related (“power”), as shown respectively in  FIGS. 1A and 1B .  FIG. 1A  shows potential temporal malfunctions in a PON comprising 3 ONUs X, Y and Z. In  FIG. 1A , the transmission pattern includes two collision zones, a zone  102  between ONU X and ONU Y and a zone  104  between ONU Y and ONU Z. Zone  102  represents a case in which either ONU X stopped transmission after its expected stop time, or ONU Y began transmission after its expected start time. Zone  104  represents a case in which either ONU Y stopped transmission before its expected stop time, or ONU Z started transmission before its expected start time. T 1  is the time needed to reach the “sync-lock” or “gain” state of the grant to ONU Y (grant Y), this time also referred to herein as “sync-lock time”. T 2  is the time needed to reach the end of grant Y and of the “sync-unlock” or “loss” state (also referred to herein as “sync-unlock time”). “Head overlapping” and “Tail overlapping” refer to heads and tails of grant Y and their overlap with, respectively, a previous and a following grant. In effect, these illustrate the temporal malfunctions of early or late burst reception and early or late end of burst, explained in more detail below: 
     Early burst reception refers to the case in which an ONU turns-on its laser before the expected time. The outcome may be a bit error rate (BER) in the grant to an ONU immediately preceding the suspected ONU. 
     Late burst reception refers to the opposite of early burst reception, the reasons being similar. The outcome may be a BER detected in the transmission of the suspected ONU. 
     Early end of burst refers to the case in which an ONU turns off its laser before the expected time. The outcome could be a BER at the end of its grant. The reasons for an early end of burst may be a faulty ONU or bad ONU timing. 
     Late end of burst refers to the opposite of early end of burst. The reasons are similar. In this case, the outcome may be a BER at the grant start of the next ONU. 
       FIG. 1B  shows the power transmission pattern of several ONUs a, b, c and d and illustrates potential laser power malfunctions in a PON. The figure shows a normal laser power signal (“ONU burst”)  110  for ONUa as well as three possible power level malfunctions: a weak laser signal  112  for ONUb, a strong laser signal  114  for ONUc and an unstable laser signal  116  for ONUd, all explained in more detail below: 
     Weak laser signal refers to a failure in which the strength of the ONU signal is lower than expected. This can result from an increase in attenuation or degradation in the ONU&#39;s laser power. 
     Strong laser signal refers to a failure in which the strength of the ONU laser signal is higher than expected. This can result from a faulty operation of the ONU&#39;s laser power control. 
     Unstable laser signal refers to the laser power of a specific ONU being unstable and having random patterns. 
     A fourth power malfunction is defined as “Laser stuck at 1”, which refers to the situation in which an ONU does not turn off its laser. The laser can transmit random data, idles, or “1”s, with the most likely events being idles and data. This malfunction can have a high impact on the network operation. It also has no specific characterization measurement and its existence is deduced from the behavior of the system. 
     At present, there are no known methods to detect these malfunctions/problems without intrusive access to the fiber infrastructure and without testing a suspected ONU component in a lab. It would therefore be advantageous to have methods and systems for active real time monitoring (diagnostics) of a PON, which provide information on various failure modes. Preferably, this monitoring should be done without placing any physical equipment at test points of the PON. 
     Rogue-ONU detection is one of the biggest challenges for carriers in deploying a time division multiplexed-passive optical network (TDM-PON). The challenge results from malfinctioning or malicious ONUs transmitting at different time periods than the ONUs are assigned to transmit. An ONU is supposed to transmit during, and only during, time intervals dynamically allocated to the ONU. This can lead to a degradation of service for properly-functioning ONUs. A major goal in a PON that includes an OLT and a plurality of ONUs is to detect degradation in the network behavior before a customer complains; in other words, before errors are evident on the line. The most critical aspect is fault isolation (i.e. finding a faulty ONU before it harms the performance of other ONUs). The specific fault of the ONU is less important, because the faulty ONU is likely to be replaced by the network operator. 
     The ITU-T (ITU Telecommunication Standardization Sector) GPON standard lacks several of the Ethernet PON (EPON) features that allow for simpler detection. Such EPON Features Include:
         (1) “8B/10B” line-coding, which is a coding scheme that translates 8-bit data into 10-bit data and prevents long sequences of 1&#39;s and 0&#39;s; allowing the system to:
           (a) check the DC balance;   (b) check the comma sync-lock and -unlock time; and   (c) check for code errors; and   
           (2) a cyclic redundancy check (CRC) for every packet.       

     In contrast, in GPON environments, a scrambler is used to transmit the data without any redundancy. CRC exists only for the last frame field of a packet, and packets may spread over several grants (i.e. uplink transmission from an optical network terminal (ONT)). Furthermore, the sync-unlock time is not available in GPON environments. 
     In the GPON standard, there is no method to detect a rogue ONU in the network. Received signal-strength indication (RSSI) measurement is a powerful tool for identifying ONU transmission power. RSSI measurements can be used to help detect a rogue ONU, but RSSI measurements cannot be the only source of information. Limitations associated with such means of detection include:
         (1) inaccurate power-level measurement, which can be up to 3 dB, prohibiting accurate interference identification;   (2) averaged RSSI measurement (taken over ˜50-100 nanosecond interval), which prevents detection of timing errors; and   (3) limited A/D sampling rate, which prohibits having multiple data points within ONU transmission, preventing real-time response to other indicators.       

     It would be desirable to have systems and methods for detecting a rogue ONU in GPON environments. It would be further desirable to have such systems configured for simple hardware (HW) implementation, and preferably flexible software (SW) implementation as well. 
     SUMMARY OF THE INVENTION 
     It is the purpose of the present invention to provide systems and methods for detecting a rogue ONU in GPON environments. 
     For the purpose of clarity, several terms which follow are specifically defined for use within the context of this application. The term “rogue” is used in this application to refer to a malfinctioning ONU, which may or may not be interfering with network performance. The terms “frame” and “packet” are used interchangeably herein. The term “sync-lock time” is used in this application to refer to the actual time an ONU starts transmitting relative to an expected time. The term “sync-unlock time” is used in this application to refer to the actual time an ONU finishes transmitting relative to an expected time, which cannot be measured in GPON. The term “bit error” is used in this application to refer to error per ONU on a time scale from grant start or grant end. The term “delimiter” is used in this application to refer to a predefined sequence to indicate the start of a grant. 
     The present invention provides two general approaches for rogue-ONU detection that rely on:
         (1) a byte error-counter (based on forward error correction (FEC) correctable errors); or   (2) determining an error density from a grant-start and grant-end error-counter that can operate:
           (a) with FEC (based on counting correctable errors at specific locations); or   (b) without FEC (based on test pattern sequences).   
               

     Therefore, according to the present invention, there is provided for the first time a system for identifying faults in a GPON that includes an OLT and a plurality of ONUs, the system including: (a) a global error-counter, operationally connected to the OLT, for counting FEC-correctable errors, for each ONU, from a data stream from the GPON; and (b) a CPU for extracting an ONU status, indicative of a faulty ONU, contingent on the errors from the global error-counter. 
     Preferably, the system further includes: (c) a grant monitor operationally connected to, and operative to notify, the global error-counter of an expected granted ONU. 
     Most preferably, the grant monitor is configured to send a pre-determined PRBS to the OLT, and wherein the global error-counter is configured to count the errors based on the PRBS. 
     Preferably, the system further includes: (c) an FEC decoder for calculating a BER of the data stream, and for sending the BER to the global error-counter, wherein a BER threshold is indicative of a transmission error. 
     Preferably, the system further includes: (c) a MAC unit operative: (i) to detect a delimiter, indicative of a grant start, in an input data stream; (ii) to determine a timing of the grant start, and (iii) to send the timing to the global error-counter. 
     Most preferably, the input data stream is an optical data stream provided by an optical transceiver. 
     According to the present invention, there is provided for the first time a system for identifying faults in a GPON that includes an OLT and a plurality of ONUs, the system including: (a) a grant-start error-counter, operationally connected to the OLT, for counting grant-start errors, for each ONU, from a data stream from the GPON; (b) a grant-end error-counter, coupled to the OLT, for counting grant-end errors for each ONU; and (c) a CPU for extracting an ONU status, indicative of a faulty ONU, contingent on at least one parameter selected from the group consisting of the grant-start errors, the grant-end errors, and a combination thereof. 
     Preferably, the system further includes: (d) a grant monitor operationally connected to, and operative to notify, the grant-start error-counter and the grant-end error-counter of an expected granted ONU. 
     Most preferably, the grant monitor is configured to send a pre-determined PRBS to the OLT for determining locations of the grant-start errors and locations of the grant-end errors. 
     Preferably, the system further comprising: (d) an FEC decoder for calculating a BER of the data stream, and for sending the BER to the grant-start error-counter and the grant-end error-counter, wherein a BER threshold is indicative of a transmission error. 
     Preferably, the system further comprising: (d) a MAC unit operative: (i) to detect a delimiter, indicative of a grant start, in an input data stream; (ii) to determine a timing of the grant start; and (iii) to send the timing to the grant-start error-counter and the grant-end error-counter. 
     Most preferably, the input data stream is an optical data stream provided by an optical transceiver. 
     Preferably, the grant-start error-counter is configured to count errors in a first block of a FEC grant, and wherein the grant-end error-counter is configured to count errors in a last block of the FEC grant. 
     Preferably, the grant-start error-counter is configured to count errors in a first N-bytes of a FEC grant, and wherein the grant-end error-counter is configured to count errors in a last N-bytes of the FEC grant. 
     According to the present invention, there is provided for the first time a method for identifying faults in a GPON that includes an OLT and a plurality of ONUs, the method including the steps of: (a) counting FEC-correctable errors, for each ONU, from a data stream from the GPON using a global error-counter operationally connected to the OLT; and (b) contingent on the errors from the global error-counter, extracting an ONU status indicative of a faulty ONU. 
     Preferably, the method further includes the steps of: (c) prior to the step of counting, calculating a BER of the data stream using an FEC decoder; and (d) sending the BER to the global error-counter, wherein a BER threshold is indicative of a transmission error. 
     Preferably, the method further includes the steps of: (c) prior to the step of counting, detecting a delimiter in an input data stream using a MAC unit, wherein the delimiter is indicative of a grant start; (d) determining a timing of the grant start using the delimiter; and (e) sending the timing to the global error-counter. 
     According to the present invention, there is provided for the first time a method for identifying faults in a GPON that includes an OLT and a plurality of ONUs, the method including the steps of: (a) counting grant-start errors, for each ONU, from a data stream from the GPON using a grant-start error-counter, operationally connected to the OLT; (b) counting grant-end errors for each ONU using a grant-end error-counter, operationally connected to the OLT; and (c) extracting an ONU status, indicative of a faulty ONU, contingent on at least one parameter selected from the group consisting of the grant-start errors, the grant-end errors, and a combination thereof. 
     Preferably, the method further includes the steps of: (d) prior to the step of counting grant-start errors, calculating a BER of the data stream using an FEC decoder; and (e) sending the BER to the grant-start error-counter and the grant-end error-counter, wherein a BER threshold is indicative of a transmission error. 
     Preferably, the method further includes the steps of: (d) prior to the step of counting grant-start errors, detecting a delimiter in an input data stream using a MAC unit, wherein the delimiter is indicative of a grant start; (e) determining a timing of the grant start using the delimiter; and (f) sending the timing to the grant-start error-counter and the grant-end error-counter. 
     These and further embodiments will be apparent from the detailed description and examples that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1A  shows a transmission timing diagram of several ONUs; 
         FIG. 1B  shows a transmission power level diagram of several ONUs; 
         FIG. 2  shows an OLT diagnostic system; 
         FIG. 3  shows a Receive Signal Strength Indication (RSSI) measurement subsystem; 
         FIG. 4  shows the major steps of a process for fault isolation; 
         FIG. 5  shows a detailed example for the process of fault isolation; 
         FIG. 6  is a simplified schematic block diagram of a system for rogue-ONU detection in a GPON, according to the present invention; 
         FIG. 7  shows the major data/information-collecting steps of a process for fault isolation, according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to systems and methods for detecting a rogue ONU in GPON environments. A PON diagnostics system and method is disclosed that provide an operator with the ability to identify and isolate problems in real time in a PON. The system includes software (SW) that can analyze the collected results and provide information about existing or potential malfunctions/problems. The diagnostics method provides an operator with one or more of the following parameters: 
     “Laser power”, presented per ONU and measured with the subsystem of  FIG. 3 . “Sync-lock time”, i.e. the actual time the ONU starts its transmission relative to an expected time. “Byte error”, an addition to the standard based on FEC-corrected error and presented per ONU on a timescale from grant start or grant end. 
     The type of malfunction is identified, per ONU or group of ONUs, from an analysis of one parameter or a combination of these parameters. 
     The BER for each ONU is sampled periodically. If there are errors (in terms of correctable bytes), then further measurements (correctable bytes, sync-lock time and/or laser power) are made, also periodically. These measurements follow one (or both) of two scenarios: “Scenario 1”, used if no more errors are detected and “Scenario 2”, used after further errors are detected. Scenario 1 is a simple go/no-go scenario. 
       FIG. 2  shows an OLT diagnostic system  200 . The system is operative to perform diagnostics of each ONU through both temporal measurements and laser power measurements. The laser power measurements are performed using a subsystem  300  shown in  FIG. 3 . System  200  includes subsystem  300 . 
     In more detail, system  200  comprises a grant monitor  202 ; a BER measurement module  204  operative to perform transmission error measurements from received BER data  216 ; a timing measurement module  206  (identical with subsystem  400 ) operative to perform sync-lock measurements based on delimiters received in a data stream  218 ; and a Receive Signal Strength Indication (RSSI=laser power) measurement module  208  operative to perform laser power measurements. Grant monitor  202  is coupled to all three modules  204 ,  206 , and  208  and operative to notify them of the expected granted ONU through notifications  210 . System  200  further comprises a central processing unit (CPU)  212  coupled to all modules/units and used for running the algorithms (scenarios) described in more detail below. CPU  212  acts essentially as a logic device operative to extract an ONU status indicative of a fault based on at least one temporal parameter (sync-lock time), laser power parameter or a combination thereof. System  200  further comprises a media access control (MAC) unit  214  used to detect code errors in an incoming data stream  224 . MAC unit  214  provides a BER detection data stream to module  204  and the sync data stream  218  to module  206 . 
     System  200  is coupled to an optical transceiver  220  and to an RSSI analog measurement subsystem  230 . The connection to subsystem  230  is through an RSSI control interface  228 . Transceiver  220  is operative to receive optical signals from each ONU and to provide data stream  224  to MAC  214  and a RSSI analog data stream  226  to subsystem  230 . Subsystem  230  comprises an analog-to-digital (A/D) converter (“ADC”, see also ADC  304  in  FIG. 3 ) and a RSSI sampling element (not shown). Data  224  is converted to digital signals before its input to the MAC unit. RSSI analog data stream  226  is converted to a digital data stream by ADC  304  in module  230 . RSSI control interface  228  also controls the RSSI sampling element. 
       FIG. 3  shows in more detail RSSI measurement subsystem  300 . System  300  comprises a RSSI estimator module  302  operative to receive a data input (“Fiber input”) and to output an analog RSSI (laser power) signal; an ADC  304  operative to hold the RSSI signal stable during the time the analog value is converted to a digital RSSI value; and an OLT measurement logic  306  that resides in the OLT and is operative to provide “sample” inputs to the ADC and to receive the digital RSSI value from ADC  304  at a programmable time from grant start through a digital interface Read I/F (typically I2C). Logic  306  is further operative to track the expected transmission of the ONUs, indicating which ONU is expected to transmit and when. In essence, logic  306  comprises grant monitor  202  plus a timing element (not shown) that controls the relative timing of the sample within the grant. The results are provided with a validity indication (“RSSI valid”) and the number of the ONU for which the measurement was taken (“ONU number”). Logic  306  is controlled by SW in the OLT (not shown), which processes the received RSSI value. The RSSI value is stored per ONU, and allows the SW to do the processing mentioned below. Note that subsystem  300  essentially includes elements  208 ,  220 , and parts of module  230  in  FIG. 2 . 
     To clarify, subsystem  300  is used to measure the “laser power” parameter. The measurement samples the transmission from the ONU at a programmable time from grant start. RSSI is measured during the grant. Several measurements can be collected and averaged. The values are analyzed by the SW by comparison with absolute allowed values or with previous values measured for the specific ONU. The determination of the measurement point is performed using a sample-and-hold mechanism (not shown) inside A/D converter  304 . 
     “Lock event” refers to respective pulses during the event of the change. A SW database (not shown) stores the relative time of receiving the delimiter, per each ONU. An “expected granted ONU” input is used later to identify the ONU on which the measurement was taken, and use the ONU index to store the measurement in the correct entry of the database. A “CPU interface” is used for accessing the database. 
     It will be apparent to one skilled in the art that some of the modules/functions described above can be implemented in hardware, some in software and some in combinations of hardware and software. 
     Early Burst Reception 
     There are two measurements performed to detect this malfunction. Their order is interchangeable. 
     Scenario 1: Detection of the uplink delimiter of each ONU. If the delimiter arrives early, then an alarm is raised in SW. Several methods of raising alarms are known in the art and may be employed for the purposes set forth herein. 
     Scenario 2: Measurement of transmission errors (BER) during the transmission of each ONU. When transmission errors are detected for an ONU (e.g. ONU N), the time until the next granted ONU (also referred to herein as “distance to next grant”) is increased by the OLT. The distance increase represents an increase in the gaps between timeslots allotted to two consecutively transmitting ONUs. If errors are not detected anymore for the same ONU N, then the ONU granted following ONU N is suffering from a too early burst reception. In essence, the fault detection is done by checking ONU N, with the deduction being that the faulty ONU is the ONU granted immediately following ONU N. 
     Late Burst Reception 
     The same two measurements serving early burst reception are also used in late burst reception: 
     Scenario 1: As above, the delimiter of each ONU is detected. If the delimiter arrives late, then an alarm is raised in SW. 
     Scenario 2: Measurement of the transmission errors during the transmission of each ONU. When transmission errors are detected in ONU N, the distance to the next grant is increased. If errors are still detected, the distance from an immediately preceding grant is increased. If errors are still detected, then the problem resides in ONU N, which suffers from either late burst reception or early end termination. Differentiation between these problems is not required, since the faulty ONU module needs to be replaced in both cases. 
     Weak Laser Signal 
     There are two measurements to detect this malfunction: 
     (1) Measuring the laser power during a grant and detecting a low result. 
     (2) Measuring the overall corrected FEC bytes and obtaining a high value. 
     Scenario 1: A database (not shown) connected to OLT measurement logic  306  holds the power level transmitted from each ONU. If the results received during several power measurements are lower than a minimal configurable threshold or a previous result (referred to henceforth as a “known minimal power value”), an alarm is raised. 
     Scenario 2 kicks in if the transmission errors of any ONU were above the minimal configurable threshold and/or the previous result. The grant of the probed ONU, for which errors were detected, is kept apart from other grants. If errors are still observed, then the laser power of each ONU is measured. If the power is low only for the specific (probed) ONU, then the specific fiber drop connected to the probed ONU needs to be checked and the ONU may need to be replaced. If several ONUs are suffering from low power, then this indicates an infrastructure problem, which can be identified based on the ONUs sharing the same fiber leafs. 
     Strong Laser Signal 
     The same measurements serving to detect a weak laser signal are also used in a strong laser signal malfunction. 
     Scenario 1 uses the same database described above that holds the power level transmitted from each ONU. If the laser power results received during several measurements are higher than a maximal configurable threshold or a previous result (referred to henceforth as a “known maximal power value”), an alarm is raised. 
     Scenario 2 is identical with scenario 2 of a weak laser signal, except that the checking is done for a high power value of an ONU. 
     Laser Stuck at 1 
     There are three measurements to detect this malfunction, and all three need to be evaluated together:
         (1) Measuring which ONUs are logically connected, as indicated from the ONU registration state machine. When all ONUs are disconnected due to an “interrupting” ONU, only a single, “suspicious” ONU is still connected.   (2) Measuring the corrected bytes and discovering a high value—the other ONUs will suffer from corrected bytes distributed randomly throughout the grant.   (3) Measuring the laser signal at the time signal should have no power—the result would be higher than expected.       

     Unstable Laser Signal 
     There is one measurement to detect this malfunction: measuring the transmission power of an ONU during several grant transmissions, and comparing a deviation of these measurements with a configurable power level. If the deviation exceeds an allowed value, an alarm is raised, see item  208  in  FIG. 2 . 
       FIG. 4  shows the major steps of the method for detecting and isolating problems and malfunctions in a PON. The diagnostic process starts in Step  400 , where a periodical timer triggers periodic checks. The transmission errors of all ONUs are read in Step  402 . If, based on the corrected bytes, one or more ONUs are found to suffer from transmission errors in a check Step  404 , the system performs a transmission error isolation procedure in Step  406 . If none of the ONUs suffer from transmission errors in Step  404 , the power level and sync time of each ONU is read in Step  408 , and all “bad” (“ill”) values are checked in Step  410  in order to notify the system of a potential fault. 
     The flow diagram in  FIG. 5  presents a detailed exemplary error isolation procedure (details of Step  406 ). The operation begins after transmission errors are detected in Step  500  (equivalent to Step  404  in  FIG. 4 ) for ONU N. The first action taken in Step  502  is to increase the “time to grant” until the ONU after ONU N is granted. In Step  504 , a check is made again for errors in ONU N. If the transmission errors disappear, ONU N is not faulty, but the ONU granted after ONU N is. The operation moves to Step  508 , where an alarm is raised for the faulty ONU as suffering from an early burst reception. If the errors persist in ONU N, the operation resumes from Step  506 , where the time to grant from the ONU granted before ONU N is increased. Errors are rechecked in Step  510  for ONU N. If the errors stop, then the operation completes in Step  514 , where an alarm is raised for the ONU granted before ONU N as suffering from a late end of burst. If the errors persist in Step  510 , the power level of ONU N is checked in Step  512 . If the power level is good, the operation completes in Step  518  with an alarm raised for ONU N for a timing problem. If the power level is not good, execution continues from Step  516 . The performance of other ONUs in the same leaf is checked starting from Step  512 . If any of those ONUs suffer from errors, then all the leaf is tested in Step  520 . If only ONU N experiences problems, then ONU N and its feed are checked physically (e.g. by a technician) in Step  522 . 
     Note that the flow in  FIG. 5  is for illustrative purposes only, and that the order of some steps may be changed and reversed. For example, the order of the checks can be swapped, i.e. instead of separating the grants in the time domain so that the OLT first grants the ONU before the one with transmission errors, the OLT can first grant the ONU after the ONU with transmission errors. 
       FIG. 6  shows a simplified schematic block diagram of a system  600  for rogue-ONU detection in a GPON, according to the present invention. System  600  includes essentially all the components of system  200 , with a GPON grant-monitor  202  also coupled to a global error-counter (GEC)  604 , to an optional module  606  and to an FEC decoder  614 , in addition to its previous connections. Module  606  includes a grant-start error-counter (GSEC)  608  and a grant-end error-counter (GEEC)  610  and is used to implement the GSEC/GEEC approach described below. In the GSEC/GEEC approach embodiment, module  606  provides additional information to the GEC approach for detecting rogue ONUs. GPON grant-monitor  202  is operative to notify GEC  604 , GSEC  608 , and/or GEEC  610  of the expected granted ONU via an ONT match  612  (i.e. a match between the expected granted ONU and ONT). FEC decoder  614  calculates the BER (as opposed to system  200  of  FIG. 2  which measures the BER), and provides the BER in an encoded results stream  616  to GEC  604 , GSEC  608 , and/or GEEC  610 . Like system  200 , system  600  includes a CPU (CPU  212 ) coupled to all modules/units and used for executing the protocols in the approaches described below. To clarify, the general architecture of system  600  of  FIG. 6  is similar to system  200  of  FIG. 2 , except for modifications required to implement rogue-ONU detection in GPON. Note that in system  600 , MAC  214  determines the grant-start time from the arrival of the delimiter (as opposed to MAC  214  in system  200  of  FIG. 2 , which measures the timing of the start of the grant via timing measurement  206 ). System  600  is used to implement two approaches for detection of a rogue ONU. 
       FIG. 7  shows the major data/information-collecting steps of a process for fault isolation, according to the present invention. The diagnostic process is started (Step  700 ), and a BER is calculated for all ONUs (Step  702 ). The BER indicates whether there is a transmission error in any of the ONUs (Step  704 ). If there are transmission errors, then an error-isolation procedure is initiated (Step  706 ). If there are no transmission errors, then the power levels of all ONUs are read (Step  708 ) and checked for ill values (Step  710 ). Up to Step  710 , the scheme of  FIG. 7  closely resembles the scheme of  FIG. 4 . 
     The error-isolation procedure (Step  706 ) includes two approaches. Both approaches begin by the MAC determining the grant-start time from the arrival of the grant delimiter (Step  712 ). The first approach (named “GEC approach”) utilizes a global byte-error-counter (GEC  604 ) to count FEC-correctable errors (Step  714 ) and to identify rogue ONUs (Step  716 ). GPON frameworks utilize a bit-interleaved parity (BIP) mechanism for error detection (code BIP-8 for code words of one byte). The performance of the BIP-8 mechanism is limited by the error probability and the number of bytes participating in the calculation. For a high error-probability and high number of bytes participating in the calculation, more than one error can occur in the parity associated with one bit. 
     Specific to GPON environments, the uplink grant transmission is typically up to 125 microseconds long, which equals 155,520, bits or 19,440 bytes. Given such an uplink grant transmission, the maximum error rate is approximately equal to the number of bytes squared or (19,440) 2 . A BER higher than 2.7×10 −9  will be inaccurate, and a BER higher than 5×10 −5  will be totally unreliable. Since the BER threshold for adequate accuracy and reliability is 10 −4 , a different mechanism is required. The BER for each ONU is sampled periodically. If there are errors (in terms of BER), then further measurements (e.g. BER, sync-lock time, and/or laser power) are made, also periodically. These measurements follow one (or both) of two cases: The first case occurs when no more errors are detected, and the second case occurs after further errors are detected. BER is based on either corrected bytes when FEC is activated, or on a pseudo-random binary sequence (PRBS)  618  feedback pattern when FEC is not activated (optional Step  718  in  FIG. 7 ). 
     The FEC provides, as a by-product, a count of the corrected errors. This gives a good indication as to the number of errors on the line when the FEC is activated. The maximum number of errors detected in one block is eight. Beyond this error limit, uncorrectable block indication will be asserted by the FEC block. In this case, the GEC  604  should be incremented by nine (i.e. the most-likely number of errors), although GEC  604  can be set to a higher value as well. To be more precise, the higher the BER, the higher the weight assigned for uncorrectable blocks, but since such a BER is not a normal threshold, there is no need for better accuracy. 
     The GEC approach of the present invention involves counting and analyzing such errors, providing an indication for signal degradation. GEC  604  sums the number of errors in the downlink at any given ONU. At the OLT, a respective GEC  604  for each ONU sums the number of errors from each ONU. 
     A second approach (named “GSEC/GEEC approach”) utilizes the determination of an error density to detect rogue ONUs. Returning to  FIG. 7 , the error-isolation procedure (Step  706 ) continues with the MAC determining the grant-start time from the arrival of the grant delimiter (Step  712 ). An error density is determined (Step  720 ) from a grant-start and grant-end error-counter (GSEC  608  and GEEC  610 , respectively, in optional module  606  of  FIG. 6 ). The location of error in the grant can indicate the presence of a rogue ONU (Step  716 ). For example, if the errors are located mostly at the start of a grant, then the errors are most likely a result of a slow laser “turn-on” time, a slow OLT lock mechanism, or a slow laser “turn-off” time of a previous ONU. If the errors are located at the end of the grant, then the errors are most likely a result of a clock drift of the ONU, or the laser turn-on time for the next ONU occurred prematurely. It is noted again that the GSEC/GEEC approach serves as a “second-level” rogue-ONU detection method to the GEC approach. 
     In order to provide a simple implementation, two different embodiments are presented for the GSEC/GEEC approach: the first embodiment is used when the FEC is activated, and the second one is used when the FEC is not activated. 
     The GSEC/GEEC approach with FEC activated is based on counting correctable error at specific locations. As mentioned above, the FEC indicates the number of corrected symbols in a block, where the GPON FEC block is 255 bytes long. In addition to the already defined general counter (GEC  604 ), two additional counters are defined:
         (1) first block counter (GSEC  608 ) that counts the number of errors only at a first FEC block  620  from GPON grant-monitor  202 , and   (2) last block counter (GEEC  610 ) that counts the number of errors only at a last FEC block  622  from GPON grant-monitor  202 .
 
Using this information, the error density at the start or end of a grant can be analyzed and indicate potential problems such as a rogue ONU.
       

     A preferred embodiment of this approach involves counting the errors in the first N-bytes and last N-bytes of the grant (Step  722  of  FIG. 7 ). The FEC issues a vector with the serial number of corrected symbols. For the first N-bytes in a packet, GSEC  608  counts the number of corrected errors in first FEC block  620 . For the last N-bytes in a packet, GEEC  610  counts the number of corrected errors in last FEC block  622 . In order to avoid complicating the approach, N should be smaller than 18 bytes; otherwise, two blocks could be potentially checked. Such a constraint is not a significant limitation. 
     The preferred embodiment of the “N-byte approach” described above has higher granularity than the general GSEC/GEEC approach. Nevertheless, if the implementation does not provide the FEC vector, the general GSEC/GEEC approach is a reasonable compromise. 
     The GSEC/GEEC approach without the FEC being activated is based on test pattern sequences. When FEC is not activated, there is no indication for errors beside the BIP mechanism. Thus, the error density is unknown. 
     A simple solution involves sending pre-determined PRBS  618  from GPON grant monitor  202  (Step  718  of  FIG. 7 ). The OLT receiving the sequence is able to identify precisely an error&#39;s location by comparing the received data to internally-generated PRBS  618 . For that reason, the OLT is configured to accept the packet even when the CRC fails. In this case, the OLT simply ignores the FEC check (i.e. the CRC). Alternatively, such a packet may not have CRC at all. 
     For simplicity, the packet can be as long as the entire grant, and can be requested by the OLT. A similar GSEC/GEEC approach can also be used when FEC is activated, but the approach requires a bypass of the packet in the MAC layer to avoid error correction. It is noted that once the errors are corrected, the PRBS content is useless. 
     All patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications, and other applications of the invention may be made.