Abstract:
A method and system for identifying faults in a passive optical network (PON), The method comprises acquiring at an optical network terminal of the PON at least one parameter indicative of at least one malfunction in at least one optical network unit of the PON, and identifying each malfunction from the at least one parameter The parameters measured include, for each ONU, laser power, sync-lock and -unlock time and bit error rates. The information at the OLT is acquired remotely and in digital form, without use of any physical probing of any point or element in the PON. The system includes a temporal measurement module and a power laser measurement module coupled to a central processing unit that can extract fault information from digital data processed in the two modules.

Description:
[0001]     This patent application 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 OF THE INVENTION  
       [0002]     The present invention relates generally to passive optical networks (PON) and more particularly to active real time monitoring of such networks  
       BACKGROUND OF THE INVENTION  
       [0003]     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.  
         [0004]     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.  
         [0005]     A PON may suffer from one or mote 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 granit. 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.  
         [0006]     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.  
         [0007]     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.  
         [0008]     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.  
         [0009]     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.  
         [0010]      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:  
         [0011]     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.  
         [0012]     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.  
         [0013]     Unstable laser signal refers to the laser power of a specific ONU being unstable and having random patterns.  
         [0014]     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.  
         [0015]     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.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention discloses a system and method for detecting and isolating problems and malfunctions in a PON without placing any physical test equipment anywhere. The system, centered on the OLT, is also referred to as an “OLT diagnostic system”. In some cases, a malfunction comes into play when errors are detected. In other cases, a measurement can alert a PON operator to a problem before there are errors. For example, a too weak (low) optical power level may work without any problem, but the carrier would be alerted and can send a technician to handle the problem before a subscriber call. Subscriber calls and downtime are very costly to the operator, and any reduction in these decreases the operational expenses (OPEX) of the carrier, providing the carrier with a major benefit.  
         [0017]     According to the present invention there is provided a method for identifying faults in a PON, the method comprising the steps of: at the OLT, acquiring at least one parameter indicative of at least one malfunction in at least one of the ONUs, and identifying each malfunction from the at least one parameter.  
         [0018]     According to one feature of the method of the present invention, the at least one malfunction includes at least one malfunction selected from the group consisting of a temporal malfunction and a laser-power malfunction, and the step of acquiring at least one parameter includes acquiring at least one parameter in digital form.  
         [0019]     According to another feature of the method of the present invention, the step of acquiring at least one parameter in digital form includes acquiring for each ONU at least one BER parameter.  
         [0020]     According to yet another feature of the method of the present invention, the step of acquiring at least one BER parameter includes acquiring a BER parameter indicative of an error, and the step of identifying includes performing an error isolation procedure.  
         [0021]     According to yet another feature of the method of the present invention, the step of acquiring at least one BER parameter includes acquiring a BER parameter not indicative of an error, and the step of acquiring further includes acquiring a parameter selected from the group consisting of a sync-lock time value and a sync-unlock time value.  
         [0022]     According to yet another feature of the method of the present invention, the temporal malfunction is selected from the group consisting of an early burst reception and a late burst reception, the acquiring includes acquiring the sync-lock time value and the step of identifying further includes comparing the acquired sync-lock time value with a known value, whereby the comparison identifies the temporal malfunction.  
         [0023]     According to yet another feature of the method of the present invention, the at least one temporal malfunction is selected from the group consisting of an early end of burst and a late end of burst, the acquiring includes acquiring the sync-unlock time value and the step of identifying further includes comparing the acquired sync-unlock time value with a known value, whereby the comparison identifies the temporal malfunction.  
         [0024]     According to yet another feature of the method of the present invention, the laser-power malfunction is selected from the group consisting of a weak laser signal and a strong laser signal, the step of acquiring at least one parameter in digital form includes acquiring for each ONU at least one laser power value during a respective grant, and the step of identifying includes comparing the measured laser power value with a known power value, whereby the comparison identifies the laser-power malfunction.  
         [0025]     According to yet another feature of the method of the present invention, the laser-power malfunction includes an unstable laser power signal, the step of acquiring at least one parameter in digital form includes acquiring for each ONU measuring the transmission power of each ONU during several grant transmissions, and the step of identifying includes comparing a deviation of these measurements with a configurable measurement.  
         [0026]     According to the present invention there is provided a method for identifying faults in a PON, the method comprising the steps of: at the OLT, obtaining at least one of temporal and laser power data from each ONU, and identifying a malfunctioning ONU from the at least one of temporal and laser power data.  
         [0027]     According to one feature of the method of the present invention, the step of obtaining temporal and laser power data includes obtaining, for each ONU, transmission error data.  
         [0028]     According to another feature of the method of the present invention, the obtaining for each ONU transmission error data includes obtaining at least one parameter selected from the group consisting of a bit error rate, a sync-lock time, a sync-unlock time and a laser-power parameter.  
         [0029]     According to yet another feature of the method of the present invention, the step of identifying a malfunctioning ONU includes identifying, for a given ONU, a malfunction selected from the group consisting of early burst reception, late burst reception, early end of burst, late end of burst, weak laser signal, strong laser signal, unstable laser signal and laser stuck at 1.  
         [0030]     According to the present invention there is provided a system for identifying faults in a passive optical network comprising: a timing measurement module coupled to the OLT and operative to provide at least one temporal parameter for each GNU from a comma sync data stream, a laser power measurement module coupled to the OLT and operative to provide at least one laser power parameter for each ONU and a central processing unit operative to extract an ONU status indicative of a fault based on at least one parameter selected from the group consisting of the at least one temporal parameter, the at least one laser power parameter, and a combination thereof.  
         [0031]     According to one feature of the system of the present invention, the system further comprises a BER measurement module operative to periodically sample transmission errors for each ONU from a BER detection data stream.  
         [0032]     According to another feature of the system of the present invention, the system further comprises a grant monitor unit coupled and operative to notify the timing measurement, laser power measurement and BER measurement modules of an expected granted ONU.  
         [0033]     According to yet another feature of the system of the present invention, the system further comprises a MAC unit used to detect code errors in an incoming data stream and to provide the BER detection data stream to the BER measurement module and the comma sync data stream to the timing measurement module.  
         [0034]     According to yet another feature of the system of the present invention, the at least one temporal parameter includes a parameter selected from the group of sync-lock time and sync-unlock time.  
         [0035]     According to yet another feature of the system of the present invention, the laser power measurement module is coupled to and operative to receive a digital laser power signal from an analog laser power measurement subsystem.  
         [0036]     According to yet another feature of the system of the present invention, the incoming data stream is an optical data stream provided by an optical transceiver.  
         [0037]     These and further embodiments will be apparent from the detailed description and examples that follow 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     For a better understanding of the present invention and to show more clearly how it could be applied, reference will now be made, by way of example only, to the accompanying drawings in which:  
         [0039]      FIG. 1A  shows a transmission timing diagram of several ONUs;  
         [0040]      FIG. 1B  shows a transmission power level diagram of several ONUs;  
         [0041]      FIG. 2  shows an OLT diagnostic system of the present invention;  
         [0042]      FIG. 3  shows a Receive Signal Strength Indication (RSSI) measurement subsystem;  
         [0043]      FIG. 4  shows a timing measurement subsystem;  
         [0044]      FIG. 5  shows major steps of a process for fault isolation of the present invention;  
         [0045]      FIG. 6  shows a detailed example for the process of fault isolation of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0046]     The present invention discloses a PON diagnostics system and method that provide an operator with the ability to identify and isolate problems in teal 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 to an operator one or more of the following parameters:  
         [0047]     “Laser power”, presented per ONU and measured with the subsystem of  FIG. 3 .“ 
         [0048]     “Sync-lock and -unlock time”, i.e. the actual time the ONU starts and ends its transmission relative to an expected time, measured with the subsystem of  FIG. 4 .  
         [0049]     “Bit error”, included in the IEEE802.3ah standard and presented per ONU on a time scale from grant start or grant end.  
         [0050]     The type of malfunction is identified, per ONU or group of ONUs, from an analysis of one parameter or a combination of these parameters.  
         [0051]     The bit error rate for each ONU is sampled periodically. If there are errors (in terms of BER), then further measurements (BER, sync-lock and -unlock 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.  
         [0052]      FIG. 2  shows an OLT diagnostic system  200  of the present invention. 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 . The temporal (timing) measurements are performed using a subsystem  400  shown in  FIG. 4 . System  200  comprises subsystems  300  and  400 .  
         [0053]     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 and -unlock measurements based on comma sync data received in a 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 or unlock 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 comma sync data stream  218  to module  206 .  
         [0054]     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.  
         [0055]      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 “&#39;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  12 C). 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 GNU for which the measurement was taken (“ON”) 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 .  
         [0056]     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 .  
         [0057]      FIG. 4  shows in more detail timing measurement subsystem  400  comprising a  8 B/ 10 B decoder  402 , a sync state machine  404 , a sync gain/loss (lock/unlock) detection unit  406  and a counters module  408 . Serial comma sync data stream  218  ( FIG. 2 ) received from the ONUs is aligned by  8 B/ 10 B decoder  402 , which is responsible for finding the correct phase of the serial stream based on the existence of a comma code word. Sync state machine  404  then searches for the number of repetitions as defined by the IEEE802.3 standard (currently three consecutive comma code-words) and outputs a lock indication signal. Detection unit  406  detects any chance in the level of the lock indication signal, moving from lock to unlock and vice-versa. The counters module includes a counter to count the time since expected grant time (lock time) and another counter for measuring the unlock time from end of grant.  
         [0058]     “Lock event” and “unlock event” refer to respective pulses during the event of the change. A SW database (not shown) stores the relative time of attaining the sync-lock event, measured from the start of a grant, and the relative time of the sync-unlock event measured from the end of a grant, per each ONU A “grant window” input is used to measure each event relative to the expected grant, and 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 “CPL interface” is used for accessing the database.  
         [0059]     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.  
         [0060]     In summary, the subsystem of  FIG. 4  is used to measure the sync-lock (as well as the sync-unlock) time for each ONU. The value of the sync-lock time can be used as pass/fail decision. The specific measurement(s) to detect each type of malfunction and their analysis is (are) now described in more detail  
         [0000]     Early Burst Reception  
         [0061]     There are two measurements performed to detect this malfunction. Their order is interchangeable.  
         [0062]     Scenario 1: Measurement of the sync-lock time of each ONU. The result of each measurement is compared with a predetermined value and a previous value a value stored in the SW database (both values referred to henceforth as “known” values). If the measured sync-lock time value is smaller than the known value over several measurements, 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.  
         [0063]     Scenario 2: Measurement of transmission errors (BER) during the transmission of each is 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.  
         [0000]     Late Burst Reception  
         [0064]     The same two measurements serving early burst reception are also used in late burst reception:  
         [0065]     Scenario 1: As above, measurement of the sync-lock time of each ONU and comparison with a known value. If the measured sync-lock time value is larger than the known value over several measurements, an alarm is raised in SW,  
         [0066]     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.  
         [0000]     Early End of Burst  
         [0067]     There are two measurements to detect this malfunction:  
         [0068]     1. Measuring the sync-unlock time.  
         [0069]     2. Measuring the BER that exists during the transmission of each ONU.  
         [0070]     As before, there are two scenarios.  
         [0071]     Scenario 1: Measurement of the sync-unlock time of each ONU and comparison with a known value. If the currently measured value indicates an earlier sync-unlock time for several measurements than the known stored predetermined/previously measured value, an alarm is raised.  
         [0072]     Scenario 2: Measurement of the transmission errors during the transmission of each ONU The transmission of the probed ONU for which errors were detected (identified from the BER measurement) is set apart from that of other ONUs. If the probed ONU transmission errors persist and the power level seems to be as needed, then the probed ONU should be replaced as it suffers from either early end of burst or late start of burst, see below.  
         [0000]     Late End of Burst  
         [0073]     The same measurements serving to detect an early end of burst are also used in a late end of burst malfunction.  
         [0074]     Scenario 1: As above, measurement of the sync-unlock time of each ONU and comparison with a known value.. If the currently measured value indicates a later sync-unlock time for several measurements than the stored predetermined/previously measured value an alarm is raised.  
         [0075]     Scenario 2: If transmission errors are detected at ONU N, the scheduled transmission of the following ONU is delayed. If errors persist at ONU N, the scheduled transmission of the following ONU is delayed from that of the previous ONU. If the errors disappear, then the ONU granted before ONU N is faulty  
         [0000]     Weak Laser Signal  
         [0076]     There are two measurements to detect this malfunction:  
         [0077]     1. Measuring the laser power during a grant and detecting a low result.  
         [0078]     2. Measuring the overall BER and obtaining a high BER.  
         [0079]     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.  
         [0080]     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..  
         [0000]     Strong Laser Signal  
         [0081]     The same measurements serving to detect a weak laser signal are also used in a strong laser signal malfunction.  
         [0082]     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.  
         [0083]     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.  
         [0000]     Laser Stuck at 1  
         [0084]     There are three measurements to detect this malfunction, and all three need to be evaluated together:  
         [0085]     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.  
         [0086]     2. Measuring the BER and discovering a high BER—the other ONUs will suffer from BER distributed randomly throughout the grant.  
         [0087]     3. Measuring the laser signal at the time signal should have no power—the result would be higher than expected.  
         [0000]     Unstable Laser Signal  
         [0088]     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 .  
         [0089]      FIG. 5  shows major steps of the method for detecting and isolating problems and malfunctions in a PON according to the present invention. The diagnostic process starts in step  500 , where a periodical timer triggers periodic checks. The transmission errors of all ONUs are read in step  502 . If, based on the BER, one or more ONUs are found to suffer from transmission errors, the system performs a transmission error isolation procedure in step  506 . If none of the ONUs suffer from transmission errors, the power level and sync time of each ONU is read in step  508 , and all “bad” (“ill”), values are checked in step  510  in order to notify the system of a potential fault.  
         [0090]     The flow diagram in  FIG. 6  presents a detailed exemplary fault isolation procedure (details of step  506 ). The operation begins after transmission errors are detected in step  600  (equivalent to step  504  in  FIG. 5 ) for ONU N. The first action taken in step  602  is to increase the “time to grant” until the ONU after ONU N is granted.. In step  604 , 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  608 , 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  606 , where the time to grant from the ONU granted before ONU N is increased. Errors are rechecked in step  610  for ONU N. If the errors stop, then the operation completes in step  614 , where an alarm is raised for the ONU granted before ONU N as suffering from a late end of bursts If the errors persist in step  610 , the power level of ONU N is checked in step  612 . If the power level is good, the operation completes in step  618  with an alarm raised for ONU N for a timing problem. If the power level is not good, execution continues from step  616  The performance of other ONUs in the same leaf is checked starting from step  612 . If any of those ONUs suffer from errors, then all the leaf is tested in step  620 . If only ONU N experiences problems, then ONU N and its feed are checked physically (egg by a technician) in step  622 .  
         [0091]     Note that the flow in  FIG. 6  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 giants the ONU before tile one with transmission errors, the OLT can first grant the ONU after the ONU with transmission errors.  
         [0092]     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 to the present invention.  
         [0093]     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.