Abstract:
A method and corresponding apparatus for diagnosing problems on a time division multiple access (TDMA) optical distribution network (ODN) is provided. An example method may include: (i) measuring no-input signal power level on a communications path configured to carry upstream communications between multiple optical network terminals (ONTs) and an optical line terminal (OLT) in a passive optical network (PON) at a time no upstream communications are on the communications path from the ONTs to the OLT; (ii) comparing the measured no-input signal power level to a threshold; and (iii) generating a notification in an event the threshold is exceeded. Through the use of this method, faults in optical transmitters, such as bad solder joints, can be determined. Such faults may cause errors in parameters, such as ranging or normalization parameters, associated with communications. By determining the faults, the time required to resolve communications errors can be reduced.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/789,357 filed on Apr. 5, 2006, the entire teachings of which are herein incorporated by reference. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    A Passive Optical Network (PON) can contain multiple Optical Line Terminals (OLTs), each connected by a shared optical fiber to a respective Optical Distribution Network (ODN) with multiple Optical Network Terminals (ONTs) on individual optical fibers. ONTs can malfunction and interfere with communications between the ONTs and the OLT on a shared optical fiber. Such malfunctions are generally the result of power outages or typical communication systems errors or failures. Other disruptions in communications can be caused by optical fibers being cut, such as by a backhoe. If ONTs are malfunctioning for any other reason, identifying the issue requires a technician to inspect each ONT, possibly causing costly interruptions to service. 
       SUMMARY OF THE INVENTION 
       [0003]    A method for diagnosing problems on a time division multiple access (TDMA) optical distribution network (ODN) is provided. A method according to an example embodiment of the invention includes: (i) measuring a no-input signal power level on a communications path configured to carry upstream communications between multiple optical network terminals (ONTs) and an optical line terminal (OLT) in a passive optical network (PON) at a time no upstream communications are on the communications path from the ONTs to the OLT; (ii) comparing the measured no-input signal power level to a threshold; and (iii) generating a notification in an event the threshold is exceeded. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0005]      FIG. 1  is a network diagram of an exemplary passive optical network (PON); 
           [0006]      FIG. 2  is a power level diagram illustrating power levels associated with an input signal and a no-input signal in accordance with example embodiments of the invention; 
           [0007]      FIG. 3A  is block diagram illustrating layer 2 communications established between an optical line terminal (OLT) and optical network terminals (ONTs) in accordance with example embodiments of the invention; 
           [0008]      FIG. 3B  is a network block diagram illustrating measuring a no-input signal power level on an upstream communications path prior to establishing layer 2 communications between an optical line terminal (OLT) and an optical network terminal (ONT) in accordance with example embodiments of the invention; 
           [0009]      FIG. 3C  is a network block diagram illustrating measuring a no-input signal power level on an upstream communications path after establishing layer 2 communications between an optical line terminal (OLT) and optical network terminals (ONTs) in accordance with example embodiments of the invention; 
           [0010]      FIGS. 4A-4C  are upstream communications frames illustrating example embodiments of measurements of a no-input signal power level on an upstream communications path being measured during a time there are no upstream communications; 
           [0011]      FIG. 5  is a power level diagram illustrating an extinction ratio and no-input extinction ratio in accordance with example embodiments of the invention; 
           [0012]      FIG. 6A  is a power level diagram illustrating an integrated no-input signal power level ramping over time; 
           [0013]      FIG. 6B  is a timing diagram illustrating an integrated no-input signal power level ramping over a ranging window; 
           [0014]      FIG. 7A  is a block diagram of an exemplary optical line terminal (OLT); 
           [0015]      FIG. 7B  is a block diagram of an exemplary processor supporting example embodiments of the invention; 
           [0016]      FIG. 8A  is a flow diagram of an exemplary process performed in accordance with an example embodiment of the invention; and 
           [0017]      FIG. 8B  is a flow diagram of an exemplary process performed in accordance with an example embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    A description of example embodiments of the invention follows. 
         [0019]    An optical network terminal (ONT) can malfunction in such a way that it sends a continuous stream of light (e.g., low level, such as less than 10 dBm) up to a shared fiber of an optical distribution network (ODN). This can adversely affect communications between ONTs on the ODN and an optical line terminal (OLT). Using existing error detection techniques, such as those described in various passive optical network (PON) protocols, this type of ONT malfunction may not be detected. Even if it is detected (e.g., resulting from system failure), the ONT malfunction (i.e., output of continuous light at a low level) may not be identified, and field service engineers may spend a great deal of time inspecting a receiver in the OLT, fiber optic cables between the ONTs and OLT, and any relays or junctions between the ONTs and OLT. Moreover, the amount of continuously outputted light which can cause communications errors has been found to be very low. So, unless field service engineers are sensitive to the source of the communications errors, hours of lost network services can result. 
         [0020]    Detection of an ONT sending a low level continuous stream of light up to a shared fiber of an ODN may be done several ways. One method may involve individually disconnecting ONTs from the ODN to determine if there is a single ONT or multiple ONTs causing the problem. With this method, however, the problem may not be corrected in a timely fashion. Additionally, this method requires considerable customer downtime. In another method, the OLT may be disconnected from the ODN, and the ODN may be examined with additional test equipment. 
         [0021]    Accordingly, what is needed is a method or corresponding apparatus for diagnosing problems on an ODN which detects, prior to establishing layer 2 communications, a malfunctioning ONT by looking for an inappropriate presence of a modulated or unmodulated upstream optical signal when no signal should be present on the upstream communications path. Furthermore, after establishing layer 2 communications with any number of ONTs, a malfunctioning ONT may be detected by looking for an inappropriate presence of an unmodulated or very low level modulated upstream optical signal when no signal should be present on the upstream communications path. 
         [0022]    As used herein, a modulated upstream optical signal is a signal which conveys information (i.e., communicates upstream communications data) and is interchangeably referred to herein as an “input signal”). The input signal may be either a “zero-bit input signal” (i.e., communicates a zero-bit) or a “one-bit input signal,” i.e., communicates a one-bit. In contrast, an unmodulated upstream optical signal is a signal which does not convey information (i.e., communicates no upstream communications data) and is interchangeably referred to herein as a “no-input signal.” 
         [0023]    Further, power levels associated with a zero-bit input signal or a one-bit input signal are referred to herein as a “zero-bit input signal power level” or a “one-bit input signal power level,” respectively. Additionally, a power level associated with a no-input signal is referred to herein as a “no-input signal power level.” 
         [0024]    In a PON system, multiple ONTs transmit data to an OLT using a common optical wavelength and fiber optic media. Field experience has demonstrated that a malfunctioning ONT can send an optical signal up to the OLT at inappropriate times, resulting in the OLT not being able to communicate with any of the ONTs on the ODN. A typical PON protocol provides some functionality for detecting this problem, but is limited only to inappropriate modulated signals. Consequently, the following ONT malfunctions are not being detected. 
         [0025]    An example ONT malfunction not being detected involves an ONT sending a continuous upstream signal (modulated or unmodulated) up the fiber prior to attempting to establish communications with an OLT on an ODN. Another example ONT malfunction occurs when an ONT sends an unmodulated light signal up the fiber at an inappropriate time while attempting to establish communications or after having established communications with an OLT on an ODN. Consequently, an ability to detect whether a network contains an ONT with such a malfunction may depend on an ability to detect an unmodulated light signal. 
         [0026]    While an OLT must be able to detect the presence of a modulated signal (or an input signal) in order to function as a node in a communications path, the ability to detect an unmodulated signal (or a no-input signal), however, is not required for operation. In accordance with example embodiments of the invention, the ability to detect an unmodulated upstream signal may improve the ability of the OLT to detect error conditions in upstream communications between ONTs and the OLT, as discussed hereinafter. 
         [0027]    As such, in part, a difference between detecting a modulated versus an unmodulated upstream signal is that an optical receiver (or transceiver) does not have the ability to detect an unmodulated signal. In some cases, the optical receiver may not be able to detect or communicate the presence of an unmodulated upstream signal. 
         [0028]    In other cases, even though the presence of an unmodulated signal may indicate a system problem, the presence of an unmodulated signal may not actually result in a problem in upstream communications between ONTs and an OLT. Sometimes the presence of an unmodulated upstream signal is removed by signal conditioning circuitry on the optical receiver (or transceiver). The unmodulated upstream signal adds a “DC” offset to a modulated upstream signal. The “DC” offset may be subsequently removed from the modulated upstream signal without corrupting it. Current experience, however, indicates that the effect of an unmodulated upstream signal on a modulated upstream signal varies from optical receiver to optical receiver. Additionally, the effect of the unmodulated upstream signal depends on the brightness or amplitude of the unmodulated upstream signal 
         [0029]      FIG. 1  is a network diagram of an exemplary passive optical network (PON)  101 . The PON  101  includes an optical line terminal (OLT)  102 , wavelength division multiplexers  103   a - n , optical distribution network (ODN) devices  104   a - n , ODN device splitters (e.g.,  105   a - n  associated with ODN device  104   a ), optical network terminals (ONTs) (e.g.,  106 - n  corresponding to ODN device splitters  105   a - n ), and customer premises equipment (e.g.,  110 ). The OLT  102  includes PON cards  120   a - n , each of which provides an optical feed ( 121   a - n ) to ODN devices  104   a - n . Optical feed  121   a , for example, is distributed through corresponding ODN device  104   a  by separate ODN device splitters  105   a - n  to respective ONTs  106   a - n  in order to provide communications to and from customer premises equipment  110 . 
         [0030]    The PON  101  may be deployed for fiber-to-the-business (FTTB), fiber-to-the-curb (FTTC), and fiber-to-the-home (FTTH) applications. The optical feeds  121   a - n  in PON  101  may operate at bandwidths such as 155 Mb/sec, 622 Mb/sec, 1.25 Gb/sec, and 2.5 Gb/sec or any other desired bandwidth implementations. The PON  101  may incorporate asynchronous transfer mode (ATM) communications, broadband services such as Ethernet access and video distribution, Ethernet point-to-multipoint topologies, and native communications of data and time division multiplex (TDM) formats. Customer premises equipment (e.g.,  110 ) which can receive and provide communications in the PON  101  may include standard telephones (e.g., Public Switched Telephone Network (PSTN)), Internet Protocol telephones, Ethernet units, video devices (e.g.,  111 ), computer terminals (e.g.,  112 ), digital subscriber line connections, cable modems, wireless access, as well as any other conventional device. 
         [0031]    A PON  101  includes one or more different types of ONTs (e.g.,  106   a - n ). Each ONT  106   a - n , for example, communicates with an ODN device  104   a  through associated ODN device splitters  105   a - n . Each ODN device  104   a - n  in turn communicates with an associated PON card  120   a - n  through respective wavelength division multiplexers  103   a - n . Wavelength division multiplexers  103   a - n  are optional components which are used when video services are provided. Communications between the ODN devices  104   a - n  and the OLT  102  occur over a downstream wavelength and an upstream wavelength. The downstream communications from the OLT  102  to the ODN devices  104   a - n  may be provided at 622 megabytes per second, which is shared across all ONTs connected to the ODN devices  104   a - n . The upstream communications from the ODN devices  104   a - n  to the PON cards  120   a - n  may be provided at 155 megabytes per second, which is shared among all ONTs connected to ODN devices  104   a - n.    
         [0032]    Error conditions in upstream communications between an optical line terminal (OLT) and optical network terminals (ONTs) often result in layer 2 communication errors, for example, errors in ranging or normalization parameters. One such error condition in upstream communications is the presence of an unmodulated signal (or a no-input signal) on an upstream communications path. An example solution to this problem may include detecting the presence of an unmodulated signal on the upstream communications path, identifying whether the detected unmodulated signal leads to a layer 2 communications error, and communicating the error condition so that it may be corrected. An unmodulated signal on the upstream communications path may be detected by measuring a power level associated with the unmodulated signal. For the sake of readability, the power level associated with the unmodulated signal is referred to herein as a “no-input signal power level” and is used throughout this disclosure. 
         [0033]      FIG. 2  illustrates three power levels: a minimum logical one input signal power level  220 , a maximum logical zero input signal power level  225 , and a maximum no-input signal power level  230 . The terms logical one and logical zero are interchangeably referred to herein as a one-bit and a zero-bit. 
         [0034]    In general, when the power level of an input signal is above the minimum logical one input signal power level  220 , the input signal is designated as a logical one input signal. When the power level of an input signal is below the maximum logical zero input signal power level  225 , the input signal is designated as a logical zero input signal. When the power level of an input is below the minimum logical one input signal power level  220  but above the maximum logical zero input signal power level  225 , the input signal is indeterminate, i.e., the input signal is neither a logical one input signal nor is the input signal a logical zero input signal. 
         [0035]    In this way, by modulating or otherwise changing the power level of an input signal, the input signal can either convey a logical one input signal or a logical zero input signal. Moreover, by modulating the power level of an input signal, the input signal conveys information. Accordingly, upstream communications between an ONT and OLT on an upstream communications pathway is accomplished by modulating the power level of an input signal to an optical transmitter generating optical signals. 
         [0036]    In contrast, when the power level of a signal is not modulated, the signal conveys no information. This is the case when there are no upstream communications between an ONT and an OLT on an upstream communications pathway. In this disclosure, the term no-input signal is used to describe a signal whose power level is not modulated. Furthermore, the terms unmodulated signal and no-input signal are used interchangeably throughout this disclosure. 
         [0037]    When the power level of a no-input signal is below the maximum no-input signal power level  230 , a no-input signal is said to be valid or non-faulty. More specifically, a no-input signal with a power level less than the maximum no-input signal power level  230  does not or is less likely to cause an error condition. On the other hand, when the power level of a no-input signal is above the maximum no-input signal power level  230 , the no-input signal is said to be invalid or faulty. In contrast to a no-input signal with a power level less than the maximum no-input signal power level  230 , a no-input signal with a power level greater than the maximum no-input signal power level  230  does or is more likely to cause an error condition (described later in greater detail). 
         [0038]    Still referring to  FIG. 2 , consider the following illustrative example. The minimum logical one input signal power level  220  is +3 dBm (decibel-milliwatt), the maximum logical zero input signal power level  225  is −5 dBm, and the maximum no-input signal power level  230  is −40 dBm. 
         [0039]    An input signal  232  with a series of power levels  235  is received during a grant timeslot  240 . During the grant timeslot  240 , the input signal  232  has power levels which at times are greater than +3 dBm and at times are less than −5 dBm. Thus, the series of power levels  235  in the input signal  232  designates a series of logical ones and logical zeros. Before the grant timeslot  240 , a first no-input signal portion  245   a  of the input signal  232  has a power level less than −40 dBm. As such, the first no-input signal portion  245   a  of the input signal  232  is not faulty, i.e., validly conveys no information. 
         [0040]    In contrast, after the grant timeslot  240 , a second no-input signal portion  245   b  of the input signal  232  has a power level greater than −40 dBm, e.g., a “faulty no-input signal level”  250 . In this case, the second no-input signal portion  245   b  of the input signal  232  is faulty, i.e., invalidly conveys no information. Discussed later in greater detail, a no-input signal having a power level, such as the faulty no-input signal power level  250 , may lead to problems in upstream communications, e.g., errors in ranging and normalization parameters. 
         [0041]      FIG. 3A  illustrates upstream communications between an OLT  305  and communicating ONTs  310   a - n  over an upstream communications path  315 . Upstream communications begins when the communicating ONTs  310   a - n  transmit upstream communications data  320   a - n  on the upstream communications path  315 . Upstream communications data  320   a - n  are then combined on the upstream communications path  315  by a splitter/multiplexer  325 . Upstream communications data  320   a - n  are transmitted by the communicating ONTs  310   a - n  at respective predefined times and in the case of a time division multiplexing (TDM) communications protocol, placed into individual timeslots  330   a - n  of an upstream communications frame  335 . 
         [0042]    The OLT  305 , via the upstream communications path  315 , receives the upstream communications frame  335 . The OLT  305  may then demultiplex (i.e., separate) the upstream communications frame  335  into individual timeslots  330   a - n . As a result, the OLT  305  receives respective upstream communications data  320   a - n  from each communicating ONT  310   a - n.    
         [0043]      FIG. 3B  is a network block diagram illustrating how an OLT  1305  may measure a power level of a no-input signal (or a no-input signal power level) on an upstream communications path  1315  at a time there are no upstream communications between the OLT  1305  and communicating ONTs  1310   a - n . The no-input signal power level on the upstream communications path  1315  may be measured at a time the OLT  1305  is ranging an ONT  1320  or at another time there are no upstream communications on the upstream communications path  1315 , e.g., when the OLT  1305  is immediately rebooted and before any ONTs are ranged. 
         [0044]    In an example embodiment, the OLT  1305  may instruct all communicating ONTs  1310   a - n  to halt upstream communications in order to range the ONT  1320 . With upstream communications from the communicating ONTs  1310   a - n  halted, the no-input signal power level on the upstream communications path  1315  should be small, (e.g., a power level below the maximum no-input signal power level  230  of  FIG. 2 ) or have no value. Typically, once halted, any power present on the upstream communications path  1315  is caused by, for example, very low level leakage of optical transmitters (e.g., laser diodes) in transmitter units of the communicating ONTs  1310   a - n  or due to typical optical noise developed or imparted onto the upstream communications path  1315 . 
         [0045]    The OLT  1305  may send the ONT  1320  a ranging request  1325 . The ONT  1320 , in turn, may respond with a ranging response  1330 . During the ranging, the no-input signal power level on the upstream communications path  1315  is measured during period(s) the ranging response  1330  is not on the upstream communications path  1315 . As such, the no-input signal power level is not increased by a signal representing the ranging response  1330 . If the no-input signal power level is greater than, for example, the maximum no-input signal power level  230  of  FIG. 2 , the ONT  1320  is faulty. 
         [0046]    The ranging exchange between the OLT  1305  and the ONT  1320  may occur over a period of time known as a ranging window (not shown, but discussed below in reference to  FIG. 6B ). The measured no-input signal power level on the upstream communications path  1315  may be averaged over an un-allocated grant window (not shown). In addition to measuring a no-input signal power level during the un-allocated grant window, a no-input signal power level may also be measured before any ONTs have been ranged, e.g., when the OLT  1305  is rebooted. 
         [0047]      FIG. 3C  is a network block diagram in which upstream communications between an OLT  2305  and communicating ONTs  2310   a - n  are carried over an upstream communications path  2315 . In addition to the communicating ONTs  2310   a - n , there is a non-communicating ONT  2313 . Upstream communications begin with the communicating ONTs  2310   a - n  sending upstream communications data  2320   a - n  via the upstream communications path  2315 . The non-communicating ONT  2313  may have no-data to send. Consequently, rather than sending upstream communications data  2320 , nothing is sent, denoted by a “no-data” indicator  2323 . For purposes of explaining aspects of the invention, the “no-data” indicator  2323  indicates a timeslot portion that is neither filled with an “idle” signal or a substantive upstream communications signal. The upstream communications data  2320   a - n  and the no-data  2323  are then combined by splitter/multiplexer  2325 . The upstream communications data  2320   a - n  and the no-data  2323  are transmitted in their respective timeslots  2330   a - n  of upstream communications frame  2335 . 
         [0048]    The OLT  2305 , via the upstream communications path  2315 , receives the upstream communications frame  2335 . The OLT  2305  then demultiplexes (or separates) the upstream communications frame  2335  into individual timeslots  2330   a - n . Consequently, the OLT  2305  receives from each communicating ONT  2310   a - n  upstream communications data  2320   a - n . The OLT  2305  also “receives” the no-data  2323  from the non-communicating ONT  2313 . 
         [0049]    While the OLT  2305  is “receiving” the no-data  2323  in the timeslot  2330   c  of the upstream communications frame  2335 , a no-input signal power level on the upstream communications path  2315  may be measured. In another example embodiment, a no-input signal power level may be measured on an upstream communications path at a time there are no upstream communications for least a portion of at least one timeslot in an upstream communications frame. 
         [0050]    In contrast to the previous example, the non-communicating ONT  2313  may send an “idle” signal (not shown) or a message indicating there is no data to be sent (not shown). In this situation a no-input signal power level on the upstream communications path  2315  cannot be measured. 
         [0051]      FIG. 4A  is an example embodiment of the invention in which an upstream communications frame  405  has n number of timeslots  410   a - n . Each timeslot  410   a - n  grants (or allocates) a time for upstream communications  415  (referred to herein as t slot ). It is during the t slot    415  that upstream communications data is communicated from an ONT to an OLT. In the upstream communications frame  405 , an “unused” timeslot (i.e., a timeslot without upstream communications data) defines a time for no-upstream communications  420  (referred to herein as t quiet ). It is during the t quiet    420  that a no-input signal power level on an upstream communications path may be measured. An unused timeslot such as t quiet    420  may occur in networks with more timeslots than ONTs. 
         [0052]    In this example embodiment, the t quiet    420  is equal to the t slot    415 . As such, if the t slot  is 1.2 μs, for example, the no-input signal power level on an upstream communications path may be measured for as long as 1.2 μs. 
         [0053]      FIG. 4B  is another example embodiment illustrating a time for no-upstream communications  1420  (referred to herein as t quiet ) optionally equal to some whole multiple of a time for upstream communications  1415  (referred to herein as t slot ). For example, if the t slot    1415  is 1.2 μs, the t quiet    1420  may be two, three, etc., times the length of the t slot    1415 . Accordingly, a no-input signal power level on an upstream communications path is measured for 2.4 μs, 3.6 μs, etc., where the longer time typically results in improved accuracy of the power level measurement. 
         [0054]      FIG. 4C  is yet another example embodiment in which a time for no-upstream communications  2420  (referred to herein as t quiet ) is equal to some fraction of a time for upstream communications  2415  (referred to herein as t slot ). For example, if the t slot    2415  is 1.2 μs, the t quiet    2420  may be a quarter, one and half, etc. times the length of the t slot    2415 . Accordingly, a no-input signal power level on an upstream communications path may be measured for 0.3 μs, 1.8 μs, etc. 
         [0055]    In still yet other example embodiment, a no-input signal power level on an upstream communications path may be measured during a time there are no upstream communications (e.g., t quiet    1420  or when no communications frames are communicated in an upstream direction) and then averaged, resulting in an averaged measurement, to increase noise immunity. By measuring a no-input signal power level on an upstream communications path at a time there are no upstream communications, an error condition of very small optical power levels can be detected. Having detected such an error condition, a determination may be made as to whether the error condition may lead to layer 2 communications errors, such as errors in the ranging or normalization parameters. 
         [0056]      FIG. 5  illustrates a ratio between a one-bit input signal power level  505  and a zero-bit input signal power level  510 . This ratio is referred to herein as an extinction ratio  515 . The extinction ratio  515  is a measure of a contrast (or a distinction) between power levels of input signals designating a one-bit input signal and a zero-bit input signal. For example, if the extinction ratio  515  is large, the distinction between a one-bit input signal power level and a zero-bit input signal power level is also large. Because the distinction between the power levels is large, an optical receiver has an easier task in detecting an input signal as either a one-bit input signal or a zero-bit input signal. In contrast, if the extinction ratio  515  is small, the distinction between a one-bit input signal power level and a zero-bit input signal power level is also small, and an optical receiver has a more difficult task in detecting an input signal as either a one-bit input signal or a zero-bit input signal. 
         [0057]    A similar ratio may be said to exist between the zero-bit input signal power level  510  and a no-input signal power level  520 . This ratio is referred to herein as a no-input extinction ratio  525 . Like the extinction ratio  515 , the no-input extinction ratio  525  is a measure of a contrast (or a distinction) between a power level of an input signal designating a zero-bit input signal and a power level of a no-input signal. For example, if the no-input extinction ratio  525  is large, the distinction between a zero-bit input signal power level and a no-input signal power level is also large. Because the distinction between power levels is large, an optical receiver has an easier task in detecting a zero-bit input signal or a no-input signal. In contrast, if the no-input extinction ratio  525  is small, the distinction a zero-bit input signal power level and a no-input signal power level is also small, and an optical receiver has a more difficult task in detecting a zero-bit input signal or a no-input signal. 
         [0058]    Difficulties in distinguishing between a no-input signal and a zero-bit input signal may also lead to difficulties in distinguishing between a one-bit input signal and a zero-bit input signal. As a consequence, there may be an increase in the number of bit errors which occur during normal communications. As such, it desirable to have a no-input extinction ratio which is sufficiently large enough to prevent such bit errors. 
         [0059]      FIG. 6A  is a power level diagram illustrating a no-input signal  605  which has a power level at time t initial    610  equal to a power level at time t final    615 . The power level of the no-input signal  605  (i.e., no-input signal power level) may be integrated (or added) by an integrator  620  (or other electronics) in an optical power receiver (or transceiver) to produce an integrated no-input signal power level  625 . The integrator  620  integrates from time t initial  to time t final , resulting in an integrated no-input signal power level at t final    630  being greater than an integrated no-input signal power level at t initial    635 , as is expected. The longer the period of integration time, the higher the integrated no-input signal power level  625  is ramped (or increased). Consequently, over time, a no-input extinction ratio (see  FIG. 5 ) becomes smaller, and it is more difficult to distinguish a no-input signal from a zero-bit input signal. Further, the higher the integrated no-input signal power level at t initial    635 , the more significant the resulting integrated no-input signal power level  625  becomes over time and the smaller a no-input extinction ratio becomes over the same time. 
         [0060]      FIG. 6B  is a diagram illustrating how a transmitted optical power level from a faulty ONT affects measurement during ranging of an ONT by an OLT. A message diagram  1600   a  illustrates an exchange of ranging messages between an OLT  1601  and an ONT  1602  during a ranging window  1620 . A transmitted power level versus time plot  1600   b  illustrates the ONT  1602  transmitting a no-input signal power level  1603  during the ranging window  1620 . A received power level versus time plot  1600   c  illustrates the OLT  1601  receiving the no-input signal power level  1603 , which has been integrated by an integrator  1604  in a receiver (not shown) of the OLT  1604 , as an integrated no-input signal power level  1605 . 
         [0061]    The transmitted power level versus time plot  1600   b  indicates that the no-input signal power level  1603  may be constant during the ranging window  1620 , where the constant level may be a normal low level (e.g., −40 dBm) or a faulty high level (e.g., between −30 dBm and −25 dBm, or higher). The integrated no-input signal power level  1605  ramps up from an integrated no-input signal power level at time t inital    1610  to an integrated no-input signal power level at time t final    1615 , over the ranging window  1620 . 
         [0062]    In operation, while the no-input signal power level  1603  is being integrated over the ranging window  1620 , the OLT  1601  sends a ranging request  1625  to the ONT  1602 . The ONT  1602 , in turn, responds with a ranging response  1630 . The OLT  1601 , having sent the ranging request  1625 , receives the ranging response  1630  from the ONT  1602  during the ranging window  1620  or it reports a ranging error. 
         [0063]    Typically, the receiver of the OLT  1601  is reset between adjacent upstream timeslots to accommodate power levels which vary from ONT to ONT. During ONT ranging, however, an upstream timeslot is effectively enlarged to accommodate variability in supported fiber lengths, i.e., more than one timeslot is used for the ranging window  1620 . For example, the ONT  1602  may be located up to 20 kilometers away from the OLT  1601 . To accommodate this distance, the duration of the ranging window  1620  is set sufficiently long enough to allow the ONT  1602  located 20 kilometers away from the OLT  1601  to receive the ranging request  1625  and the OLT  1601  to receive the ranging response  1630 . 
         [0064]    When the duration of the ranging window  1620  is set for a long period of time, the receiver of the OLT  1601  is not reset during this period of time. As a result, no-input signal power levels from non-transmitting ONTs on the ODN have more time to be integrated by the receiver of the OLT  1601 , thus increasing the integrated no-input signal power level  1605 . This increase has a negative impact on a signal condition circuitry in the receiver of the OLT  1601 . In other words, the longer the duration of the ranging window  1620 , the greater the effects of a small no-input extinction ratio (see  FIG. 5 ). Consequently, it may be difficult to distinguish between a zero-bit input signal power level and a one-bit input signal power level possibly leading to upstream communications problem(s). 
         [0065]    In one embodiment of the present invention, prior to ranging an ONT, an OLT instructs communicating ONTs to halt upstream communications. Despite upstream communications being halted, there still may be a no-input signal from one or more halted ONTs causing a “faulty no-input signal power level” (see  FIG. 2 ). Consequently, the faulty no-input signal power level may be integrated, causing the integrated no-input signal power level  1605  to increase further. 
         [0066]      FIG. 7A  is a block diagram of an exemplary OLT  705  in communication with an ONT  710 . In this particular example, the OLT  705  has a PON card  715 . The PON card  715  includes a processor  720  communicatively coupled to a receiver  725  and a transmitter  730 . Alternatively, the receiver  725  and the transmitter  730  may be integrated into a single transceiver (not shown). In the direction toward from the OLT  705 , the receiver  725  (or transceiver) receives upstream communications  735 . The processor  720  subsequently processes the upstream communications  735 . In the opposite direction toward the ONT  710 , the processor  720  sends, via the transmitter  730  (or transceiver), downstream communications  740 . 
         [0067]      FIG. 7B  is a block diagram which illustrates an exemplary processor  1705 , supporting example embodiments of the invention, operating in a PON card of an OLT. The processor  1705  may include a measurement unit  1710 , a comparison unit  1715 , and a notification generator  1720 . Alternatively, some or all of the aforementioned components may not be co-located with the processor  1705 , but may be remotely located connected via a communications bus (not shown). 
         [0068]    In operation of this example embodiment, the measurement unit  1710  may measure a power level of a no-input signal  1701  on an upstream communications path. The measurement unit  1710  may include an integrator, such as the integrator  620  of  FIG. 6A , or other electronics to measure the power level of the no-input signal  1701 . A measured no-input signal power level  1702  may be compared against a threshold value  1703  by the comparison unit  1715 . A result  1704  from the comparison unit  1715  is communicated to the notification generator  1720 . The notification generator  1720  may generate a notification if the communicated result  1704  indicates the measured no-input signal power level  1702  exceeds the threshold  1703 . Keeping the integrated no-input signal power levels of  FIGS. 6A and 6B  in mind, it should be understood that the comparison unit  1715  may compare a maximum, an average (at multiple times or over a length of time), or a portion of the measured no-input signal power level  1702  against the threshold  1703 . 
         [0069]    The threshold  1703  against which the measured no-input signal power level  1702  is compared may be determined or defined in multiple ways. For example, the threshold  1703  may be set to a value equal to a “tolerable no-input signal power level” multiplied by a number of ONTs in communication with the OLT. Field experience may indicate a no-input signal power level of −20 dBm to −30 dBm per ONT often leads to problems in upstream communications. Based on such experience, the tolerable no-input signal power level may be −40 dBm. Therefore, in an example network having thirty-two ONTs communicating with an OLT, the threshold may be calculated as −40 dBm multiplied by thirty-two. Additionally, losses between the ONTs and the OLT (i.e., ODN losses) may be accounted for in calculating the threshold. In another example embodiment, the tolerable no-input signal power level may be less than a zero-bit input signal power level specified for the ONTs. One skilled in the art will readily appreciate that the value of the tolerable no-input signal power level may not be fixed (i.e., set to the same level for all communications networks, but rather may depend on characteristics of a communications network. 
         [0070]    The threshold  1703  may alternatively represent a maximum power level corresponding to a fault associated with upstream communications in a non-communicating state. In another example embodiment, the threshold  1703  may be less than a sum of a zero-bit input signal power level of each ONT offset by respective losses between the ONTs and the OLT. It should be understood that the threshold  1703  may be predetermined based on a configuration of a passive optical network or determined based on some other metric. 
         [0071]    Continuing to refer to  FIG. 7B , the notification generator  1720  may generate a remote notification  1725  which is sent over a network  1726  to, for example, a remote user or remote management system  1727 . Alternatively, the notification generator  1720  may generate a local notification  1730 , which is presented locally to, for example, a local user or local management system  1731 . It should be understood that the remote notifications  1725  may be any form of signal (e.g., analog, digital, packet, and so forth), data values, including in header or load portions of packets, and so forth. The local notification  1730  may also be any form of signal or may be audio or visual alarms to alert an operator at a console at the OLT that an error as described herein had occurred. 
         [0072]      FIG. 8A  is a flow diagram illustrating an exemplary process  800  for diagnosing a problem on an ODN. A no-input signal power level on an upstream communications path may be measured ( 805 ) at a time no upstream communications are on the upstream communications path. The measured no-input signal power level may be compared ( 810 ) against a threshold. If the measured no-input signal power level on the upstream communications path is greater than the threshold, a notification may be issued ( 815 ) to alert an operator (or management system) that the threshold is exceeded. If, however, the measured no-input signal power level on the upstream communications path is not greater than the threshold, the process  800  may return to begin measuring ( 805 ) the no-input signal power level. 
         [0073]      FIG. 8B  is a flow diagram illustrating a process  1800  for diagnosing a problem on an ODN in accordance with an example embodiment of the invention. A no-input signal power level on an upstream communications path may be measured ( 1805 ) at a time no upstream communications are on the upstream communications path. In this example embodiment, the no-input signal power level is measured during a time for no upstream communications (t quiet ). In reference to  FIGS. 4A-4C , the time for no upstream communications (t quiet ) may be equal to a time for upstream communications (t slot ). Alternatively, the time for no upstream communications (t quiet ) may be equal to a whole multiple or fraction of the time for upstream communications (t slot ). 
         [0074]    Next, a threshold may be calculated ( 1810 ). In this example embodiment, the threshold is equal to a number of ONTs on the ODN multiplied by a tolerable no-input signal power level. The tolerable no-input signal power level may be estimated based on system modeling, equal to a value measured at a time known not be experiencing an error condition (e.g., initial system set-up), and so forth. 
         [0075]    The measured no-input signal power level on the upstream communications path may be compared ( 1815 ) against the calculated threshold. If the measured no-input signal power level is greater than the calculated threshold, a notification may be issued ( 1820 ) that the calculated threshold is exceeded. If, however, the measured no-input signal power level on the upstream communications path is less than the calculated threshold, the process  1800  may wait ( 1825 ) for the time for no upstream communications (t quiet ) to reoccur. After waiting, the process  1800  may once again measure ( 1805 ) the no-input signal power level on the upstream communications path. 
         [0076]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 
         [0077]    For example, although described as “cards” herein, it should be understood that PON cards, OLT cards, or ONT cards may be systems or subsystems without departing from the principles disclosed hereinabove. 
         [0078]    Further, although described in reference to a passive optical network, the same or other example embodiments of the invention may be employed in an active optical network, data communications network, wireless network (e.g., between handheld communications units and a base transceiver station), or any other type of network.