Abstract:
An optical network unit (ONU) of an Ethernet passive optical network (EPON) and a control method thereof eliminates or substantially reduces instances of an ONU transmitting in time slots other than its allocated time slot. The ONU includes: a medium access controller for accessing a medium without temporal overlapping in order to transmit during one or more allocated TDM time slots without collision in upstream transmission to an optical line terminal; a burst-mode optical transceiver having a separately allocated wavelength before outputting the signal in the upstream transmission; and a complex programmable logic device for controlling an optical output of the burst-mode optical transceiver by monitoring an optical-output control signal from the medium access controller. An erroneous output from an ONU malfunction can be prevented from by cutting off the output once the duration of the allocated time slot has been reached.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 (a) from an application entitled “Optical Network Unit Of Ethernet Passive Optical Network And Control Method Thereof,” filed with the Korean Intellectual Property Office on Aug. 17, 2006 and assigned Serial No. 2006-77576, the contents of which are incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical network unit of an Ethernet passive optical network and a control method thereof. More particularly, the present invention relates to an optical network unit of an Ethernet Passive Optical Network (EPON) that monitors optical-output control signals in upstream transmission so as to prevent a malfunction thereof in advance. In addition, the present invention relates to an optical network unit of an EPON that prevents influences from being exerted on other optical network units operating normally when an abnormal operation occurs, and a control method of the optical network unit in an EPON. 
         [0004]    2. Description of the Related Art 
         [0005]    At present, Asymmetric Digital Subscriber Line (ADSL) and cable modem systems are the most widely used for high-speed Internet services. The ADSL system uses existing telephone lines, and provides high-speed Internet services with speeds between 2 Mbps and 10 Mbps through the ADSL modem installed in each subscriber&#39;s computer. Users typically install filters on the telephone line immediately prior to entry into a telephone, to filter the noise from ADSL traffic that can adversely affect operation telephones, faxes, etc. 
         [0006]    On the other hand, cable modems use existing coaxial cables installed for cable TV services in order to provide high-speed Internet services, so a subscriber must install a cable modem within the area of his/her PC when already subscribing to cable TV service in order to be provided with the high-speed Internet services, or would have to purchase additional equipment, such as a wireless router that receives an output of the cable modem, and install a wireless network card into their pc. Otherwise, coaxial cables output from the modem would have to be run throughout one&#39;s house, which is unsightly and labor intensive. 
         [0007]    These high-speed Internet services are satisfactory in performance in providing services such as Internet web surfing (HTTP), E-mail, file transfer (FTP), etc. with much higher transmission capacity of 2 to 10 Mbps, as compared with the existing telephone line modem having a speed of 56 Kbps, but they still have a limitation in meeting the users&#39; emerging requirements such as VoIP (Voice over Internet Protocol), VoD (Video on Demand), Internet broadcasting service, etc. 
         [0008]    Moreover, high-speed Internet service using the cable modem has a disadvantage in that the bandwidth which can be provided decreases as the number of subscribers increases, and the high-speed Internet service using the ADSL scheme has a disadvantage in that the bandwidth which can be provided decreases as the distance between a telephone office and a subscriber network increases. Also ADSL has problems associated with adverse weather conditions, for example, many subscribers to ADSL are aware that during a thunderstorm it is not unusual for the ADSL connection to be interrupted. 
         [0009]    In an attempt to solve these problems, there have been proposed FTTH (Fiber To The Home), FTTB (Fiber To The Building), FTTC (Fiber To The Curb), etc., in which optical cables are installed to the subscriber&#39;s in-home network. In addition, studies are being conducted about an Ethernet passive optical network (E-PON) for the sake of enhancing the price-to-service ratio. 
         [0010]    More particularly, the E-PON is an Ethernet-associated network which is constructed with passive elements without using power-consuming active elements in the optical subscriber network, so as to enhance its price competitiveness. The standard for the E-PON is being established by the IEEE (Institute of Electrical and Electronics Engineers) 802.3ah Ethernet in the first Mile Task Force. An example of the E-PON is illustrated in  FIGS. 1A and 1B  and discussed herein below. 
         [0011]      FIGS. 1A and 1B  are block diagrams illustrating respective flows of upstream and downstream traffic in a conventional E-PON. It can be seen from  FIG. 1A  that an E-PON has a point-to-multipoint structure in which a plurality of Optical Network Units (ONUs)  20 - 1  to  20 - 3 , etc., are connected to one Optical Line Terminal (OLT) port  10  through a splitter  30 , which is a passive element. Data transference between the OLT  10  and ONUs  20  is performed in units of an Ethernet frame. For example, downstream signals from the OLT  10  to the ONUs  20  are transmitted as broadcasting data, and upstream signals from the ONUs  20  to the OLT  10  share bandwidths allocated to the ONUs  20  by the Time Division Multiple Access (TDMA) scheme. 
         [0012]    Therefore, still referring to  FIGS. 1A and 1B , upon transmitting upstream signals to the OLT  10 , when the conventional ONUs  20  are granted by the OLT  10  so as to collide between each other in a burst mode scheme, the ONUs  20  will transmit IDLE data (data for clock recovery time and code group arrangement in the OLT) to the OLT  10 , and then transmit corresponding data as upstream frames in the TDMA scheme. 
         [0013]    More particularly, each ONU  20 - 1 ,  20 - 2  and  20 - 3  transmits frames upstream to the OLT  10  in the TDMA scheme. Each ONU  20 - 1 ,  20 - 2  and  20 - 3  is allocated with a time period (time slot) from the OLT  10 , and the individual ONU can transmit a corresponding frame only during the time period (time slot) allocated, as other time slots are allocated to other ONUs, etc. Upon an upstream transmission, when the on/off control of a laser diode is operating in a normal state, the individual ONUs do not transmit outside (beyond) their allocated time slot. However, when the on/off control of the laser is not properly performed due to a malfunction of a particular ONU (such as  20 - 2 ), abnormal data is transmitted in excess of a preset time period. 
         [0014]    The extended transmission beyond the respect duration of the time slot by the malfunctioning ONU exerts an influence upon time periods allocated to the other ONUs  20 - 1  and  20 - 3  such that the other ONUs  20 - 1  and  20 - 3  recognize that the malfunctioning ONU  20 - 2  is continuously occupying the transmission line, thereby causing a serious error in data transmitted in the upstream direction. 
         [0015]    While ONU # 1   20 - 1  successfully transmits an ONU # 1  data frame during a preset time, that is, during a first time period, ONU # 2   20 - 2  cannot transmit ONU # 2  data within a second time period due to an error occurring in ONU # 2  and occupies the transmission line, even after the second time period has elapsed. That is, ONU # 2   20 - 2  continuously transmits the erroneous ONU # 2  data in excess of a preset time period, so that data collision occurs in a third time period during which the ONU # 3   20 - 3  must be transmitting ONU # 3  data, and such data collision continuously occurs, thereby causing the overall Gigabit E-PON to be unable to transmit data normally. If the malfunctioning ONU# 2   20 - 2  continues to transmit beyond the allocated time slot, the data collisions can slow down or essentially impair the communication capability of the other ONUs that are not malfunctioning. 
       SUMMARY OF THE INVENTION 
       [0016]    Accordingly, the present invention has been made in part to solve at least some of the above-mentioned problems occurring in the prior art. The present invention provides an optical network unit and a control method thereof, which monitor optical-output control signals upon an upstream transmission in an Ethernet Passive Optical Network (EPON) so as to prevent malfunction of the optical network unit in advance upon occurrence of an abnormal operation by one of the components, such as an Optical Network Unit (ONU). 
         [0017]    Also, the present invention provides an ONU and a control method thereof, which can prevent influences from being exerted on other ONUs operating normally when an abnormal operation occurs in an EPON. 
         [0018]    In accordance with an exemplary aspect of the present invention, there is provided an ONU in an E-PON using a time division multiplexing (TDM) scheme, the optical network unit including: a medium access controller for accessing a medium without temporal overlapping. This medium access controller thus functions to transmit during one or more allocated TDM time slots without collision in upstream transmission to an optical line terminal; a burst-mode optical transceiver for converting a signal transferred from the medium access controller into an optical signal having a separately allocated wavelength before outputting the signal in the upstream transmission; and a complex programmable logic device for controlling an optical output of the burst-mode optical transceiver by monitoring an optical-output control signal from the medium access controller. 
         [0019]    In accordance with another exemplary aspect of the present invention, there is provided a control method of an ONU in an E-PON using a time division multiplexing (TDM) scheme, the method including the steps of: determining whether there is an optical-output control signal being output in upstream transmission to an optical line terminal, and detecting an output time period of the optical-output control signal; comparing the detected output time period with a preset monitoring time period; determining that the optical-output control signal is abnormal when it is determined that the detected output time period is greater than the monitoring time period as a result of the comparison; and cutting off an optical output of the optical network unit as the optical-output control signal has been determined to be abnormal. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above and other exemplary aspects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0021]      FIG. 1A  is a view illustrating flows of downstream traffic in the conventional system; 
           [0022]      FIG. 1B  is a view illustrating flows of upstream traffic in the conventional system; 
           [0023]      FIG. 2  is a block diagram illustrating the configuration of an optical network unit (ONU) of an Ethernet passive optical network (E-PON) operating under a time division multiplexing scheme according to an exemplary embodiment of the present invention; and 
           [0024]      FIG. 3  is a flowchart illustrating exemplary operational steps of the complex programmable logic device according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Hereinafter, an exemplary embodiment of the present invention will be described with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein may be omitted when it may obscure appreciation of the subject matter of the present invention by a person of ordinary skill in the art. It is understood by an artisan that the drawings and explanation have been provided for explanatory and illustrative purposes and the invention is not limited to the descriptions shown and described. 
         [0026]    First, the configuration of an E-PON using the Time Division Multiple Access (TDMA) scheme, to which the present invention is applied, can have a network configuration as shown in the conventional examples in  FIGS. 1A and 1B . Accordingly, the description of the E-PON to which the present invention is applied, will continue to refer to the configuration having the OLT  10 , a plurality of ONUs  20 - 1  to  20 - 3  connected with the OLT  10 , and optional connection with a plurality of end users (user network apparatuses)  40 - 1  to  40 - 3 . However, a person of ordinary skill in the art understands and appreciates that the EPON can have a number of different and sometime more complex configurations than shown and the invention is applicable to these other configurations as well. 
         [0027]    The pieces of data transmitted by the end users  40 - 1  to  40 - 3  are transferred to the OLT  10  via the ONUs  20 - 1  to  20 - 3 , and the pieces of data transmitted by the OLT  10  are transferred to the end users  40 - 1  to  40 - 3  via the ONUs  20 - 1  to  20 - 3 . 
         [0028]    With regard to transmission in a typical E-PON, such data, i.e., Ethernet frames are transmitted at a transmission speed greater than 1 Gbps. Upon an upstream transmission, the OLT  10  accesses data of the ONUs  20 - 1  to  20 - 3  multiplexed in the time division multiplexing (TDM) scheme. Also, upon a downstream transmission, each ONU  20 - 1  to  20 - 3  selects and receives only data which should be received by the ONU, from among data broadcasted by the OLT  10 . 
         [0029]      FIG. 2  is a block diagram illustrating an exemplary configuration of an ONU  200  in an E-PON using the TDM scheme according to an exemplary embodiment of the present invention. 
         [0030]    Still referring to  FIG. 2 , an ONU  200  of an E-PON using the TDM scheme according to an exemplary embodiment of the present invention includes an E-PON medium access controller (E-PON MAC)  100 , a burst-mode optical transceiver  110 , and a complex programmable logic device (CPLD)  120 . 
         [0031]    The E-PON MAC  100  has a Time Division Multiple Access Medium Access Control Protocol (TDMA MAC) for accessing a medium without temporal overlapping in order to transmit during one or more allocated TDM time slots without collision in upstream transmission to the OLT  10 . 
         [0032]    The burst-mode optical transceiver  110  converts a signal transferred from the E-PON MAC  100  into an optical signal having a separately allocated wavelength before outputting the signal in upstream transmission. 
         [0033]    The CPLD  120  monitors optical-output control signals output from the E-PON MAC  100 , and controls an optical output of the burst-mode optical transceiver  110  according to the optical-output control signal. 
         [0034]    The CPLD  120  includes a monitoring-interval clock unit  130 , an optical output monitoring unit  140 , a switch unit  150 , a register  160  and a central processing unit (CPU)  170 . 
         [0035]    The monitoring-interval clock unit  130  outputs a clock having a monitoring interval preset by a system operator. 
         [0036]    The optical output monitoring and control unit  140  measures an elapsed time of a time period from when the output of an optical-output control signal starts from the E-PON MAC  100 , and determines whether a duration of the optical-output control signal is normal or abnormal by comparing the elapsed time with a monitoring time period according to a clock output by the monitoring-interval clock unit  130 . The optical output monitoring unit  140  performs a control function by outputting an optical-output control signal according to a result of the determination to the switch unit  150 . 
         [0037]    Still referring to  FIG. 2 , the switch unit  150  is turned on/off according to the optical-output control signal being transferred from the optical output monitoring unit  140  so as to control an optical output of the burst-mode optical transceiver  110 . In other words, turning on the switch unit  150  corresponds to continuing the optical output of the burst-mode optical transceiver  110 , and turning off the switch unit  150  corresponds to cutting off the optical output of the burst-mode optical transceiver  110 . 
         [0038]    The register  160  stores a status indicating whether a duration of an optical-output control signal is normal or abnormal, which has been determined by the optical output monitoring unit  140 . The register  160  stores normal/abnormal determination signals, which are read under the control of the central processing unit  170 . Note that inventive step involves determining whether the duration of the optical output control signal is abnormal, as opposed to some other measurement regarding the output control signal (phase, intensity, etc) to indicate there is a problem. In other words, so long as the signal has a duration within the allocated timeslot, it is always considered normal. 
         [0039]    The central processing unit  170  receives a normal/abnormal determination signal from the optical output monitoring unit  140 , and outputs a control signal for representing a normal/abnormal durational status of the optical-output control signal in accordance with the received normal/abnormal determination signal. 
         [0040]    As described above with reference to  FIG. 2 , according to an exemplary embodiment of the present invention, the CPLD  120  outputs a clock for determining a monitoring interval. The optical output monitoring unit  140 , which determines whether an optical-output control signal output from the E-PON MAC  100  is normal or abnormal in duration, detects an optical-output control signal at a monitoring interval and stores information regarding whether the detected optical-output control signal is normal or abnormal in the n-bit register  160 . The central processing unit  170  reads what numbered bit of n bits stores an abnormal signal, and then generates a signal informing the OLT  10  that an error has occurred in the optical-output control signal corresponding to the bit storing the abnormal signal. For example, the central processing unit  170  may generate a control signal to flicker a light-emitting diode (LED). Note that the OLT  10  does the CPU  170  in the CPLD of the ONU to inform about the error. 
         [0041]    According to an exemplary embodiment of the present invention, an optical-output control signal output from the E-PON MAC  100  is input to the CPLD  120 , and the optical-output control signal is monitored in a predetermined sequence at regular intervals. Thus, when a signal representing an abnormal durational state is defined as “High,” the central processing unit  170  reads what numbered bit is “High” in the n-bit register, and informs the OLT  10  that an error occurs in the optical-output control signal corresponding to the bit defined as “High.” 
         [0042]      FIG. 3  is a flowchart illustrating exemplary operational steps of the CPLD  120  according to an exemplary embodiment of the present invention. 
         [0043]    In step  301 , the CPLD  120  determines whether there is an optical-output control signal output to the burst-mode optical transceiver  110 , and proceeds to step  303  when there is an optical-output control signal output to the burst-mode optical transceiver  110 . In step  303 , the optical output monitoring unit  140  detects an elapsed time from when the output of the optical-output control signal starts, and proceeds to step  305 . 
         [0044]    In step  305 , the optical output monitoring unit  140  compares the duration of an output time period detected in step  303  with a preset monitoring time period. As a result of the comparison, when the detected output time period is greater than the preset monitoring time period, step  307  is performed, and when the detected output time period is smaller than the monitoring time period, step  311  is performed. 
         [0045]    In step  307 , it is determined that the optical-output control signal is abnormal because the detected output time period is greater than the monitoring time period, which means that an error may occur in the burst-mode optical transceiver  110 . In step  311 , it is determined that the optical-output control signal is normal because the detected output time period is smaller than the monitoring time period, which means that the optical output of the burst-mode optical transceiver  110  is being properly performed. 
         [0046]    In addition, when it is determined that the optical-output control signal is abnormal, the CPLD  120  controls the switch unit  150  to be turned off so as to fully cut off the optical output of the burst-mode optical transceiver  110  in step  309  in order to prevent subsequent data collisions with other ONUs. In contrast, when it is determined that the optical-output control signal is normal, the CPLD  120  controls the switch unit  150  to be in an ON state so that the burst-mode optical transceiver  110  can continue the optical output. 
         [0047]    Accordingly, in the E-PON using the TDMA scheme, the ONU  20  monitors an optical-output control signal so as to detect the occurrence of an abnormal operation in upstream transmission, thereby preventing the ONU  20  from causing a malfunction that can impact the network, and preventing the malfunction of the ONU  20  from exerting influences upon other ONUs operating normally. 
         [0048]    According to the present invention as described above, an ONU in the E-PON can be prevented from malfunctioning in advance when an abnormal operation occurs by monitoring an optical-output control signal in upstream transmission, and can prevent the malfunction of the ONU from exerting influences upon other ONUs operating normally. 
         [0049]    While the present invention has been shown and described with reference to a certain exemplary embodiment 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 spirit and scope of the invention as defined by the appended claims. Accordingly, the scope of the invention is not to be limited by the above embodiments but by the claims and the equivalents thereof. For example, while the duration of the output signal is being monitored and controlled by being cut off, there are other features regarding transmission that can be the basis for cutting off an output signal, all of which lie within the spirit of the invention and the scope of the appended claims. The CPLD can be configured for monitoring whether the allocated time slot corresponds to the time the optical-output control signal is on, so if the burst mode transceiver is receiving a signal at an incorrect time slot as compared to the allocated time slot, the control signal can also be cut off.