Patent Publication Number: US-2011076012-A1

Title: Optical network terminal and method for detecting transmission error in optical network terminal

Description:
TECHNICAL FIELD 
     The described technology relates generally to an optical network terminal (ONT) and a method for the ONT to detect an optical transmission error. 
     BACKGROUND 
       FIG. 1  is a block diagram of a passive optical network (PON). A PON  100  to shown in  FIG. 1  as an example is an optical subscriber network architecture providing an optical-fiber-based high-speed service to companies or even general homes. The PON  100  uses a splitter  130  in an optical cable, thereby enabling one optical line termination (OLT)  110  to access several optical network terminals (ONTs)  120 ,  122  and  124 . PONs include time division multiplexing (TDM)(A)-PONs which employ a TDM scheme and wavelength division multiplexing (WDM)(A)-PONs which employ a WDM technique. The TDM(A)-PONs include asynchronous transfer mode (ATM)-PONs based on ATM, gigabit Ethernet (G)E-PONs, and G-PONs which use a general frame protocol. 
     Meanwhile, in the PON  100  employing the TDM scheme, data is exchanged between the OLT  110  and the ONTs  120 ,  122  and  124  as follows. After the OLT  110  inserts the identifier of a registered ONT in a preamble of a frame and sends the frame to the registered ONT downstream, the ONT sends only the frame having the identifier of the ONT to a user interface (UI). On the other hand, when the OLT  110  dynamically assigns upstream time slots to all the ONTs  120 ,  122  and  124 , each of the ONTs  120 ,  122  and  124  transmits data to the OLT  110  during the time slot assigned to the ONT itself upstream. 
     SUMMARY 
     Embodiments provide an optical network terminal (ONT) and a method for the ONT to detect an optical transmission error. 
     In one embodiment, an ONT connected with an OLT and constituting a passive optical network (PON) is provided. The ONT includes: an optical transmitter configured to transmit an optical signal to the OLT; an error detector configured to detect an error of the optical transmitter; and a controller configured to transmit an error message to the OLT through the optical transmitter when the error detector detects an error of the optical transmitter. 
     In another embodiment, a method for an ONT connected with an OLT and constituting a PON to detect an optical transmission error is provided. The method includes: transmitting an optical signal to the OLT; monitoring the transmitted optical signal to generate an optical output state signal; comparing the generated optical output state signal and an optical transmission enable signal to detect the optical transmission error; and transmitting an error message to the OLT when the optical transmission error is detected. 
     The Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. The Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present disclosure will become more apparent to those of ordinary skill in the art by describing in detail example embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a block diagram of a passive optical network (PON). 
         FIG. 2  is a block diagram illustrating an operation in which an optical line termination (OLT) assigns time slots to respective optical network terminals (ONTs) and receives data from the ONTs upstream; 
         FIG. 3  is a block diagram illustrating an ONT according to an embodiment of the present disclosure; 
         FIG. 4  is a block diagram illustrating an error detector of  FIG. 3 ; 
         FIG. 5  is a flowchart illustrating a method of detecting an optical transmission error according to an embodiment of the present disclosure; 
         FIG. 6  is a flowchart illustrating a method of detecting an optical transmission error according to another embodiment of the present disclosure; and 
         FIG. 7  is a flowchart illustrating a method of detecting an optical transmission error according to still another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It will be readily understood that the components of the present disclosure, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of apparatus and methods in accordance with the present disclosure, as represented in the Figures, is not intended to limit the scope of the disclosure, as claimed, but is merely representative of certain examples of embodiments in accordance with the disclosure. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
     Meanwhile, terms used herein are to be understood as follows. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could to be termed a first element, without departing from the scope of the present disclosure. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It should also be noted that in some alternative implementations, the functions/acts noted in the blocks may occur out of the order noted in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 2  is a block diagram illustrating an operation in which an optical line termination (OLT) assigns time slots to respective optical network terminals (ONTs) and receives data from the ONTs upstream. 
       FIG. 2(A)  illustrates an example in which an OLT assigns time slots  1 ,  2  and  3  to ONTs  1 ,  2  and  3  and receives data from the respective ONTs  1 ,  2  and  3 .  FIG. 2(B)  illustrates an example in which an optical output of the ONT  1  keeps occupying the line of the ONT  1  exclusively due to a problem of an optical module. In this way, when a laser is kept on regardless of a laser control signal due to a problem of an optical module, a laser of an optical module is kept on due to malfunction of a laser control signal of a specific ONT, or a laser control signal and an optical module control signal are set to be opposite to each other, one ONT occupies the entire upstream time slots. Thus, an OLT considers that all ONTs including the problematic ONT do not make correct responses, and releases the registration of all the ONTs to block upstream access. 
     The OLT cannot know in which ONT a problem has occurred. For this reason, the OLT needs to check each of all the ONTs, and thus normal ONTs cannot receive service until the OLT finds the ONT in which the problem has occurred. To solve this problem, an ONT  120  according to an embodiment of the present disclosure detects an error of an optical transceiver module by itself. And, when an error is detected, the ONT  120  transmits an error detection message to an OLT to inform the OLT of the error. 
       FIG. 3  is a block diagram illustrating an ONT according to an embodiment of the present disclosure. Referring to  FIG. 3 , the ONT  120  includes an optical transmitter  310 , an optical receiver  320 , an error detector  330 , and a controller  340 . As shown in  FIG. 1 , the ONT  120  is connected with an OLT  110  and constitutes a passive optical network (PON)  100 . The OLT  110  assigns a line to the ONT  120  according to a time division multiplexing (TDM) scheme, as shown in  FIG. 2 . 
     The optical transmitter  310  transmits an optical signal to the OLT  110  according to the control of the controller  340 . In an embodiment, the optical transmitter  310  can be implemented by a laser diode which outputs an optical signal and a laser driving unit which drives the laser diode according to an optical transmission enable signal received from the controller  340 . 
     The optical receiver  320  receives an optical signal from the OLT  110 . In an embodiment, the optical receiver can be implemented by a photodiode which receives and converts the optical signal into an electrical signal and an amplifier which amplifies the electrical signal and transfers the amplified electrical signal to the controller  340 . 
     In an embodiment, the optical transmitter  310  and the optical receiver  320  may be implemented by one optical transceiver. 
     The error detector  330  detects an error of the optical transmitter  310 . An error is detected, for example, when a laser is continuously output from the laser diode due to a problem of the optical transmitter  310 , when the optical signal output from the optical transmitter  310  is opposite to the optical transmission enable signal transferred from the controller  340 , and so on. Like this, the error detector  330  detects an error in which a laser output from the optical transmitter  310  is not the same as the optical transmission enable signal. 
     When the error detector  330  detects an error, the controller  340  transmits an error message to the OLT  110  through the optical transmitter  310 . The ONT  120  can detect an error by itself and also informs the OLT  110  of the error, so that the error can be rapidly handled. When the ONT  120  detects an error by itself and informs the OLT  110  of the error, the processing speed is improved compared to a conventional method in which the OLT  110  detects an error of the ONT  120 . When the OLT  110  attempts to detect an error, the OLT  110  needs to periodically monitor the respective ONTs  120  connected to the OLT  110  itself, and thus much time and effort are required. 
     Also, the OLT  110  can recognize which ONT has what problem through the error message, and rapidly take appropriate steps such as immediately repairing or changing a problematic ONT. Although the ONT  120  can rapidly detect its own error, if the ONT  120  does not inform the OLT  110  of the error, the OLT  110  cannot know a reason for which the ONT  120  in which the error has occurred is cut off and it is difficult to cope with the error. 
     The error message indicates that an error has occurred at the optical transmitter  310  of the specific ONT  120 , and may include details of the error and the identifier of the ONT  120 . As an example, the error message may be a dying gasp message which can be transmitted without external power supply. As another example, the error message may be defined in a standard for an immediate report to the OLT  110  and transmitted according to an ONT Management and Control Interface (OMCI) used in a PON or Physical Layer Operations, administration and Maintenance (PLOM) used in a PON. However, the error message is not limited to the examples and can have any forms. 
     The controller  340  according to an embodiment can power down the optical transmitter  310  when the error detector  330  detects an error. For example, the controller  340  can generate a power supply cut-off signal for the optical transmitter  310  or control signal to prevent a signal from being transmitted from the optical transmitter  310  in response to an error detection signal, which indicates that an error has been detected, received from the error detector  330 . When an error occurs, the controller  340  cuts off power supply of the optical transmitter  310  by itself, thereby preventing the optical transmitter  310  in which the error has occurred from continuously outputting an optical signal and a communication problem of the other normal ONTs  122  and  124  sharing the same optical line with the ONT  120 . In an embodiment, the controller  340  may cut off power supply of the ONT  120  instead of the optical transmitter  310 . 
     The controller  340  according to another embodiment can power down the optical transmitter  310  when the optical transmitter  320  cannot receive a response message to the error message from the OLT  110 . When the error message is received, the OLT  110  according to an embodiment can transmit a response message to the error message to the ONT  120 . The response message may be a control message to cut off power supply of the optical transmitter  310  or a control message to prevent a signal from being transmitted from the optical transmitter  310 . The controller  340  receiving the response message can control the ONT  120  according to the response message. When the response message is not received from the OLT  110  for a threshold time, the controller  340  can cut off power supply of the optical transmitter  310  by itself. The threshold time can be set or changed (e.g., three seconds) in advance by a network administrator according to system requirements, etc., and a value fixed when the ONT  120  is produced may be set as a default. 
       FIG. 4  is a block diagram illustrating an error detector of  FIG. 3 . Referring to  FIG. 4 , the error detector  330  includes an optical transmission sensor  410  and a signal comparator  420 . 
     The optical transmission sensor  410  monitors an optical signal output by the optical transmitter  310  and generates an optical output state signal indicating an optical output state. For example, the optical transmission sensor  410  outputs a first signal (e.g., high) as the optical output state signal when the optical transmitter  310  outputs an optical signal, and a second signal (e.g., low) as the optical output state signal when the optical transmitter  310  is not outputting an optical signal. In an embodiment, the optical transmission sensor  410  may include a photodiode which senses an optical signal emitted from the laser diode of the optical transmitter  310  and converts the optical signal into an electrical signal. 
     The signal comparator  420  compares the optical output state signal received from the optical transmission sensor  410  and an optical transmission enable signal received from the controller  340 , thereby detecting an error. Since the optical transmitter  310  outputs an optical signal according to the optical transmission enable signal, an optical output state of the optical signal actually output from the optical transmitter  310  and sensed by the optical transmission sensor  410  is the same as a state of the optical transmission enable signal when there is no error at the optical transmitter  310 . Thus, the signal comparator  420  compares the optical output state signal and the optical transmission enable signal, and determines that there is an error to output an error detection signal indicating that an error has occurred when the optical transmission enable signal has an enable (e.g., high) value but the optical output state signal has a second signal (low) value, or when the optical transmission enable signal has a disable (e.g., low) value but the optical output state signal has a first signal (high) value. In an embodiment, the signal comparator can be implemented by an exclusive OR (XOR) gate which has the optical output state signal and the optical transmission enable signal as inputs and the error detection signal as an output. Here, the error detection signal has a high value (1) when there is an error, and a low value (0) when there is no error. In an embodiment, the error detection signal may have reverse values. The signal comparator  420  provides the output error detection signal to the controller  340 . 
       FIG. 5  is a flowchart illustrating a method of detecting an optical transmission error according to an embodiment of the present disclosure. A method for the ONT  120  to detect an optical transmission error will be described below with reference to  FIGS. 3 to 5 . Since the ONT  120  of  FIG. 3  implemented in time series also corresponds to this embodiment, the above description of the ONT  120  is applied to this embodiment. 
     The ONT  120  is one of a plurality of ONTS connected with the OLT  110  and constituting the PON  100 , as shown in  FIG. 1 . The OLT  110  assigns lines to the ONTs according to the TDM scheme. The ONT  120  transmits an optical signal to the OLT  110  (S 510 ). For example, the ONT  120  can output the optical signal using a laser diode. The ONT  120  monitors the transmitted optical signal and generates an optical output state signal (S 520 ). Using a photodiode, the ONT  120  can sense and convert the transmitted optical signal into an electrical signal. For example, the ONT  120  can convert an optical output state into an electrical logic signal having a high value (1) when an optical signal is output, and an electrical logic signal having a low value (0) when an optical signal is not output. The ONT  120  compares the optical output state signal converted into the electrical logic signal and an optical transmission enable signal, thereby determining whether the two values are the same (S 530 ). When the two values are not the same, the ONT  120  determines that an error has occurred in optical transmission. When an optical transmission error is detected, the ONT  120  transmits an error message to the OLT  110  (S 540 ). 
       FIG. 6  is a flowchart illustrating a method of detecting an optical transmission error according to another embodiment of the present disclosure. The method according to the other embodiment includes a step (S 610 ) in which the ONT  120  powers down its optical transmitter  310  when an optical transmission error is detected in step  530 . Here, the ONT  120  may transmit an error message to the OLT  110  before power supply of the optical transmitter  310  is cut off, or the error message may be a dying gasp message which can be transmitted without external power supply. 
       FIG. 7  is a flowchart illustrating a method of detecting an optical transmission error according to still another embodiment of the present disclosure. The method according to this embodiment includes a step (S 710 ) in which the ONT  120  determines whether a response message to the error message is received from the OLT  110  when an optical transmission error is detected in step  530 . Here, when the response message is not received, the ONT  120  powers down its optical transmitter  310  (S 720 ). On the other hand, when the response message is received from the OLT  110 , the ONT  120  is controlled according to the received response message (S 730 ). 
     The above-described embodiments of the present disclosure have effects including the following merits. However, the embodiments of the present disclosure need not include all of the merits, and it is to be understood that the scope of the present disclosure is not limited by the merits. 
     In an embodiment of the present disclosure, a time for an OLT to detect a transmission error of an ONT is reduced. Thus, the OLT can rapidly cope with the error, and it is possible to provide stable service to users. 
     Also, in an embodiment of the present disclosure, when an error occurs, an ONT cuts off power supply of its optical transmitter, thereby preventing a communication problem from occurring at other ONTs even before an OLT takes action. Furthermore, in an embodiment of the present disclosure, even when an ONT cuts off power supply of its optical transmitter, an error message is transmitted to an OLT so that the OLT or a network administrator can correctly recognize details of the error and take appropriate steps. 
     The foregoing is illustrative of the present disclosure and is not to be construed as limiting thereof. Although numerous embodiments of the present disclosure have been described, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the present disclosure as defined in the claims. Therefore, it is to be understood that the foregoing is illustrative of the present disclosure and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The present disclosure is defined by the following claims, with equivalents of the claims to be included therein.