Abstract:
An optical line terminal (OLT) determines when an optical network terminal (ONT) has become a rogue ONT, and then identifies the rogue ONT so that service can be restored. The OLT identifies the rogue ONT by comparing the received power levels with the levels that would be expected during normal operation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to optical line terminals and, more particularly, to an optical line terminal that detects and identifies a rogue ONT.  
         [0003]     2. Description of the Related Art  
         [0004]     A fiber-to-the-x (FTTx) passive optical network (PON) is a point-to-multipoint communications network. Examples of an FTTx network include a fiber-to-the curb (FFTC) network and a fiber-to-the-home (FTTH) network. In an FTT-x network, downstream data packets are transmitted from an optical line terminal (OLT) to a number of optical network terminals (ONT) that are located at or near a corresponding number of end users. Upstream data packets, on the other hand, are transmitted from the ONTs back to the OLT.  
         [0005]      FIG. 1  shows a block diagram that illustrates a prior-art example of a point-to-multipoint communications network  100 . As shown in  FIG. 1 , network  100  includes an OLT  110 , a fiber optic cable  112 , and a number of ONTs  114  that are connected to OLT  110  via fiber optic cable  112 . Each ONT  114 , in turn, is connected to an end user  116 .  
         [0006]     As further shown in  FIG. 1 , OLT  110  includes an interface unit  120  that is connected to cable  112 , and an optical transmitter  122  that generates and outputs downstream data packets to the ONTs  114  via interface unit  120 . In addition, OLT  110  includes an optical receiver  124  that receives upstream data packets from the ONTs  114  via interface unit  120 .  
         [0007]     OLT  110  also includes a MAC/processor  126  and a memory  128  that is connected to MAC/processor  126 . MAC/processor  126  outputs downstream data to optical transmitter  122 , and receives upstream data from optical receiver  124 . Memory  128 , in turn, stores the instructions and data required to operate MAC/processor  126 .  
         [0008]     In operation, OLT  110  establishes a sequence of transmission timeslots, and assigns transmission timeslots to the ONTs  114  that are connected to fiber optic cable  112 . During normal operation, the ONTs  114  transmit data packets to OLT  110  during their assigned timeslots and only during their assigned timeslots.  
         [0009]     One problem which can arise is when, due to a hardware or software failure, an ONT  114  transmits at the wrong time. If a first ONT transmits during a time slot which has been assigned to a second ONT and both ONTs attempt to transmit at the same time, a collision results which can prevent OLT  110  from being about to receive the data packets output by either ONT.  
         [0010]     Thus, for example, when the optical transmitter of an ONT  114  “sticks on” and continuously transmits during all of the assigned time slots, the entire FTTx network can be rendered incapable of any communication between the ONTs  114  and OLT  110 . An ONT that transmits in the upstream direction during the wrong time slot, such as a continuously transmitting ONT, is known as a “rogue ONT”.  
         [0011]     Thus, in order to prevent a rogue ONT from incapacitating an entire a FTTx network segment, there exists a need for an OLT that can detect when an ONT has become a rogue and is illegally transmitting, and then identify which ONT is the rogue ONT so that service can be restored.  
       SUMMARY OF THE INVENTION  
       [0012]     An optical line terminal (OLT) is disclosed. The OLT is to be connected to a plurality of optical network terminals (ONTs) via a fiber. In addition, each ONT is to have a timeslot during which data can be transmitted to the OLT. The OLT includes an interface unit to be connected to the fiber, and an optical transmitter to generate and output downstream data packets to the ONTs via the interface unit.  
         [0013]     The OLT also includes an optical receiver to receive upstream data packets, which have power levels, from the ONTs via the interface unit. Further, the OLT includes a processor to output downstream data to the optical transmitter, and receive upstream data from the optical receiver. In addition, the OLT includes an optical power meter connected to the optical receiver to detect the power levels of the upstream data packets that are received during a sequence of transmission time slots, and to the processor to output a plurality of measured power levels.  
         [0014]     A method of operating an optical line terminal (OLT) is also disclosed according to an embodiment of the invention. The OLT is to be connected to a plurality of optical network terminals (ONTs) via a fiber. In addition, each ONT is to have a timeslot during which data can be transmitted to the OLT.  
         [0015]     In the method, a database of received power levels is formed for all of the ONTs that are connected to the OLT via the fiber so that an expected power level can be determined for each ONT that is connected to the OLT via the fiber. Further, a rogue ONT is detected if an ONT becomes a rogue. In addition, the rogue ONT is identified after the rogue ONT has been detected.  
         [0016]     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a block diagram illustrating a prior-art example of a point-to-multipoint communications network  100 .  
         [0018]      FIG. 2  is a block diagram illustrating an example of a point-to-multipoint communications network  200  in accordance with the present invention.  
         [0019]      FIG. 3  is a flow chart illustrating an example of a method  300  of operating an OLT in accordance with the present invention.  
         [0020]      FIG. 4  is a flow chart illustrating an example of a method  400  of creating a database of measured power levels in accordance with the present invention.  
         [0021]      FIG. 5  is a flow chart illustrating an example of a method  500  of detecting the presence of a rogue ONT in accordance with the present invention.  
         [0022]      FIGS. 6A-6C  are a flow chart illustrating an example of a method  600  of identifying a rogue ONT in accordance with the present invention.  
         [0023]      FIGS. 7A-7G  are timing diagrams illustrating a number of examples of rogue ONT signals in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 2  shows a block diagram that illustrates an example of a point-to-multipoint communications network  200  in accordance with the present invention. Network  200  is similar to network  100  and, as a result, utilizes the same reference numerals to designate the structures which are common to both networks.  
         [0025]     As shown in  FIG. 2 , communications network  200  differs from network  100  in that OLT  110  of network  200  includes an optical power meter  210  that is connected to optical receiver  124  to detect the power levels of the data packets that are received during a sequence of transmission time slots, and to MAC/processor  126  to output a sequence of measured power levels.  
         [0026]      FIG. 3  shows a flow chart that illustrates an example of a method  300  of operating an OLT in accordance with the present invention. Software implementing method  300  can be executed by MAC/processor  126 , and stored in memory  128 . As shown in  FIG. 3 , at  310 , a database of measured power levels is created so that an expected power level can be determined for each ONT that is connected to the OLT via the fiber optic cable.  
         [0027]      FIG. 4  shows a flow chart that illustrates an example of a method  400  of creating a database of measured power levels in accordance with the present invention. As shown in  FIG. 4 , at  410 , each time an ONT associated with a timeslot transmits, the power level is measured until a predetermined number of power levels measurements have been made for each timeslot.  
         [0028]     As further shown in  FIG. 4 , at  412 , for each timeslot, an expected power level is formed from the predetermined number of power level measurements. For example, the predetermined number of power level measurements can be averaged, or otherwise weighted and combined, to form the expected power level to reduce the effect of any transitory power spikes.  
         [0029]     At  414 , the expected power level for each timeslot is stored in a memory, such as memory  128 . In addition, at  416 , method  400  is periodically repeated to account for changing conditions, such as temperature. When repeated, the old value can be replaced by the new value, or the old and new values can be averaged together or otherwise weighted and combined to form the value stored in memory  128 .  
         [0030]     Referring again to  FIG. 3 , at  312 , if an ONT becomes a rogue ONT, the presence of the rogue ONT is detected. A rogue ONT can be detected in a number of ways. For example, whenever two ONTs with relatively similar power levels, as seen by the OLT, transmit at the same time, the signals received by the OLT are largely unrecoverable. Thus, the regular presence of wholly or partially unrecoverable data packets can be taken to indicate the presence of a rogue ONT.  
         [0031]     A rogue ONT can also be detected from the power levels.  FIG. 5  shows a flow chart that illustrates an example of a method  500  of detecting the presence of a rogue ONT in accordance with the present invention. As shown in  FIG. 5 , at  510 , the measured power level of a data packet from an ONT is compared with the expected power level for the timeslot.  
         [0032]     Following this, at  512 , the presence of a rogue ONT can be detected when the measured power level is substantially greater than the expected power level for the timeslot. This is because when a rogue ONT is transmitting, two ONTs are transmitting at the same time (the ONT which is supposed to transmit during the timeslot and the rogue ONT).  
         [0033]     Next, at  314 , when the presence of a rogue ONT has been detected, the identity of the rogue ONT is determined. Once the identity of the rogue ONT has been determined, the ONT can be turned off if it is still obeying commands, or a service truck can be dispatched if it is no longer obeying commands. As a result, the amount of time that the network is impacted by the rogue ONT can be minimized.  
         [0034]      FIGS. 6A-6C  show a flow chart that illustrates an example of a method  600  of identifying a rogue ONT in accordance with the present invention. As shown in  FIGS. 6A-6C , at  610 , all ONTs are commanded to turn off. For example, MAC/processor  126  can command all ONTs  114  connected to fiber  112  to turn off.  
         [0035]     After the ONTs have been commanded to turn off, the power level is measured during each timeslot in  612 . Since the ONTs have been turned off, the detected power should be zero, i.e., no power should be detected. However, when a rogue ONT is present, the detected power can also be less than the expected power level, equal to the expected power level, or more than the expected power level for the timeslot.  
         [0036]     At  614 , the detected power levels for the timeslots are evaluated to identify one of a number of potential power conditions. One potential power condition which can be identified is when no power is measured during any of the timeslots. This condition can arise if the rogue ONT turns off in response to the OLT command to turn off which, in turn, indicates that the rogue ONT is still responding to commands from the OLT.  
         [0037]     In this case, at  616 , the ONTs are all turned back on, and one at a time each of the ONTs is turned off. When an ONT is turned off, it is determined whether or not the presence of the rogue ONT can still be detected. When the presence of the rogue ONT can no longer be detected, the rogue ONT can be identified to be the ONT that is currently turned off.  
         [0038]     Another potential power condition which can be identified is when the rogue ONT continuously transmits. When the rogue ONT continuously transmits, power is received during each timeslot.  FIGS. 7A-7G  are timing diagrams that illustrate a number of examples of rogue ONT signals in accordance with the present invention. In each of the figures, the ONT that transmits during the fifth timeslot is the rogue ONT.  
         [0039]      FIG. 7A  shows a timing diagram that illustrates a continuously transmitting rogue ONT in accordance with the present invention. As shown in  FIG. 7A , although all of the ONTs have been commanded to turn off, power is nonetheless received during each timeslot of the sequence of timeslots.  
         [0040]     When in  614  it is determined that the rogue ONT is continuously transmitting (power was received in each timeslot), the ONTs are sequentially turned on and off, and the received power levels are measured in  620 . In  622 , for each timeslot, the received power level is compared to the expected power level for the timeslot.  
         [0041]     In this case, the timeslots will have a measured power level that is either greater than the expected power level (the sum of the ONT commanded to transmit and the rogue ONT which is continuously transmitting), or substantially equal to the expected power level (only the rogue ONT transmits during its&#39; assigned timeslot). Thus, in  622 , the ONT which has a power level that is substantially equal to the expected power level is identified as the rogue ONT.  
         [0042]     Referring again to  614 , another potential power condition which can be identified is when the rogue ONT transmits so that an expected level of power is received during one or more, but less than all, of the timeslots when the ONTs were commanded to be off. In addition, in one case the rogue ONT transmits during its&#39; own timeslot, while in another case the rogue ONT does not transmit during its&#39; own timeslot.  
         [0043]      FIGS. 7B-7C  show timing diagrams that illustrate a rogue ONT that transmits an expected level of power during one or more, but less than all, of the timeslots in accordance with the present invention.  FIG. 7B  shows the case where the rogue ONT transmits during its&#39; own timeslot, while  FIG. 7C  shows the case where the rogue ONT does not transmit during its&#39; own timeslot. Thus, as shown in  FIGS. 7B-7C , although all of the ONTs have been commanded to turn off, an expected level of power is nonetheless received during a number of timeslots of the sequence of timeslots.  
         [0044]     When in  614  it is determined that the expected level of power was received in a number of timeslots, in  630  each ONT with a timeslot that received substantially the expected level of power is marked as a selected ONT. In  630 , the selected ONTs are sequentially turned on and off so that the received power levels can be measured. If in  630  the selected ONTs each also has an increased power level when sequentially turned on, then the rogue ONT is not transmitting during its&#39; own timeslot as shown in  FIG. 7C .  
         [0045]     When in  630  it is determined that the rogue ONT is not transmitting during its&#39; own timeslot (an increased power level is measured from each selected ONT), all of the ONTs (not just the selected ONTs) are sequentially turned on and off in  632  to measure the received power levels. Since the rogue ONT is not transmitting during its&#39; own timeslot, the rogue ONT can be identified as the ONT which has no power when the ONT is turned on in  632 .  
         [0046]     On the other hand, if one of the timeslots in  630  has the expected level of power, then the rogue ONT is transmitting during its&#39; own timeslot as shown in  FIG. 7B , and is identified as the ONT that has the expected level of power in the timeslot. This is because only the rogue ONT transmits during its&#39; own timeslot, while both the rogue ONT and the commanded ONT transmit together during the remaining timeslots.  
         [0047]     Referring again to  614 , another potential power condition which can be identified is when the rogue ONT transmits so that less than an expected level of power is received during one or more, but less than all, of the timeslots when the ONTs were commanded to be off. In addition, in a first case, the rogue ONT transmits during its&#39; own timeslot, while in a second case the rogue ONT does not transmit during its&#39; own timeslot. Further, in a third case, the rogue ONT transmits more in an adjacent timeslot and less in its&#39; own timeslot, while in a fourth case the rogue ONT transmits less in an adjacent timeslot and more in its&#39; own timeslot.  
         [0048]      FIGS. 7D-7G  show timing diagrams that illustrate a rogue ONT that transmits less than an expected level of power during one or more timeslots in accordance with the present invention.  FIG. 7D  shows the case where the rogue ONT transmits during its&#39; own timeslot, while  FIG. 7E  shows the case where the rogue ONT does not transmit during its&#39; own timeslot.  FIG. 7F  shows the case where the rogue ONT transmits more in an adjacent timeslot and less in its&#39; own timeslot, while  FIG. 7G  shows the case where the rogue ONT transmits less in an adjacent timeslot and more in its&#39; own timeslot.  
         [0049]     When in  614  it is determined that less than the expected level of power was received in a number of timeslots, in  640  each ONT with a timeslot that received less than or substantially the expected power level are marked as a selected ONT. In  640 , the selected ONTs are sequentially turned on and off so that the received power levels can be measured. If in  640  the selected ONTs each also has an increased power level when sequentially turned on, then the rogue ONT is not transmitting during its&#39; own timeslot as shown in  FIG. 7E .  
         [0050]     When in  640  it is determined that the rogue ONT is not transmitting during its&#39; own time slot (an increased power level is measured from each selected ONT), all of the ONTs are sequentially turned on and off to measure the received power levels in  632 . Since the rogue ONT is not transmitting during its&#39; own timeslot, the rogue ONT is identified as the ONT which has no power when the ONT are sequentially turned on in  632 .  
         [0051]     In addition, if one of the timeslots in  640  has the expected level of power, then the rogue ONT is transmitting during its&#39; own timeslot as shown in  FIG. 7D , and is identified as the ONT that has the expected level of power in the timeslot. This is because only the rogue ONT is transmitting during its&#39; own timeslot, while both the rogue ONT and the commanded ONT transmit together during a portion of the remaining timeslots.  
         [0052]     Further, if one of the timeslots in  640  has less than the expected level of power, then the rogue ONT is only transmitting during a portion of its&#39; own timeslot as shown in  FIGS. 7F and 7G , and is identified as the ONT that has less than the expected level of power in the timeslot. This is because only the rogue ONT is transmitting during its&#39; own timeslot, and when the rogue ONT stops early or starts late, less than the expected power level is detected.  
         [0053]     Thus, an OLT and a method of operating an OLT have been disclosed that detects when an ONT has become a rogue, and then identifies the rogue ONT so that service can be restored.  
         [0054]     It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.