Abstract:
An Ethernet passive optical network provides a subscriber with a high speed and large capacity data service and a real time digital broadcast/video service. The network includes an optical line terminal for frame-multiplexing broadcast/video signals, which are obtained by performing a switching operation and a time-slot multiplexing with respect to a plurality of digital broadcast/video data delivered from external broadcasting vendors according to broadcast/video selection information delivered from each user, and communication data delivered through an Internet protocol network.

Description:
PRIORITY 
     This application claims priority to an application entitled “Ethernet PON for Broadcasting Telecommunication Convergence Using Time Division Multiplexing,” filed in the Korean Intellectual Property Office on Jan. 12, 2004 and assigned Serial No. 2004-2017, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical transmission, and more particularly to an Ethernet passive optical network for providing a subscriber with a high speed and large capacity data service and a real time digital broadcast/video service. 
     2. Description of the Related Art 
     A data transmission rate of above 100 Mb/s is required for high speed, real time, service of a combination of large capacity data and digital broadcast/video. Since, however, currently used xDSLs or cable modems have a data transmission rate of 50 Mb/s at maximum, xDSLs and cable modems cannot meet this challenge. Pursuant to studies and research, an optical access network has been suggested as a method for meeting such a requirement. In particular, a passive optical network (PON) has been proposed as an economical optical access network. 
     Such a PON may be an ATM-PON based on the ATM (asynchronous transfer mode) protocol, a WDM-PON based on a WDM (wave division multiplexing), or an Ethernet PON (E-PON) based on Ethernet. A fiber to the home (FTTH) version of an Ethernet PON structure has been suggested and developed for high speed optical transmission to a home. 
     Ethernet PONs have generally been developed to fundamentally process communication data. The Ethernet PON employs a wavelength of 1550 nanometers (nm) to transmit data from an optical line termination (OLT) to a plurality of optical network terminations (ONTs) This entails transferring gigabit Ethernet signals at a data rate of 1.25 Gb/s and at a wavelength of 1310 nm. The Ethernet PON has required broadcasting signals, however, as demand for broadcasting services using an optical access network has increased. 
     To this end, and referring to  FIG. 1 , suggestion has been made for an overlay broadcasting processing method for transmitting broadcasting signals to a plurality of ONTs by using broadcasting signal wavelengths, which are different from communication data wavelengths.  FIG. 1  portrays a conventional Ethernet PON for broadcasting/telecommunication convergence, which includes, positioned between a user and a service node, an optical line terminal (OLT)  100 . The latter receives and electro-optically converts a broadcasting signal and a communication signal delivered from a broadcasting vendor and a communication vendor, respectively, and sends the broadcasting signal and the communication signal as a combined optical signal. The conventional Ethernet PON also includes, at the users&#39; side, optical network terminals (ONTs)  200 - 1  to  200 -N for delivering to users information received from the OLT  100 . A passive optical splitter  118  and an optical cable connect the OLT  100  to the ONTs ( 200 - 1  to  200 -N). 
     The OLT  100  converts optical broadcasting signals delivered through a broadcasting network into optical signals for downstream transmission by means of an opto-electrical converter  115 , followed by an electro-optical converter  116 . The converted optical signals are amplified by an erbium doped fiber amplifier (EDFA)  117 , and the amplified signals are transmitted downstream. The OLT  100  also receives communication data from an internet protocol (IP) network through an IP router  111  and converts the communication data into optical signals by means of an E-PON OLT function processing part  112  so as to transmit the optical signals by means of a transmitter  113 . The OLT  100 , on the other hand, receives data from the ONTs  200 - 1  to  200 -N and transmits the received data through the IP router  111  to the IP network. 
     The ONTs  200 - 1  to  200 -N consequently receive, by means of broadcasting receivers  119 - 1  to  119 -N, the broadcasting signals and deliver the broadcasting signals to users through broadcasting set-top boxes  122 - 1  to  122 -N. As to the communication data, the ONTs  200 - 1  to  200 -N receive it by means of receivers  120 - 1  to  120 -N and deliver it to users through E-PON ONT function processing parts  123 - 1  to  123 -N. In addition, the ONTs  200 - 1  to  200 -N receive upstream communication data from a user by means of the E-PON ONT function processing parts  123 - 1  to  123 -N and forward the received data sent by means of burst mode transmitters  121 - 1  to  121 -N. 
     The EDFA  117  is expensive, and is required to deliver analog broadcasting signals to the ONTs  200 - 1  to  200 -N. Even if only digital broadcasting is processed an expensive EDFA is still required if the number of digital broadcasting channels increases. 
     Since all broadcasting channels are transferred to the ONTs  200 - 1  to  200 -N, the ONTs  200 - 1  to  200 -N require expensive optical receivers featuring great receiving-sensitivity, superior noise-characteristics, etc., in order to receive broadcasting signals transferred from the OLT  100 . Additionally, an optical transmitter required for the OLT. Moreover, although subscribers may require high definition and real-time digital video services as well as digital broadcasting services in the future, it is difficult for the conventional Ethernet PON to process high definition and real-time digital video services. 
     In order to solve the above described problems, an Ethernet PON has been proposed in which broadcast/video channels are time-division multiplexed together with GbE (Gigabit Ethernet) communication data and are transferred to an ONT. Accordingly, it is unnecessary to employ an EDFA and it is unnecessary for an OLT and an ONT to additionally have an optical transmitter and an optical receiver, respectively. In addition, the Ethernet PON ensures quality of service (QoS) for high definition digital video, to be required by subscribers in the future, as well as for digital broadcasting. However, in the proposed Ethernet PON, a broadcast/video time-slot is specifically assigned to every subscriber and is used only for transmitting broadcast/video. Also, when a subscriber does not look at and listen to broadcast/video, a time-slot assigned to the subscriber is not used. For example, if the E-PON has a 1×16 structure and broadcast/video data, e.g., an MPEG transport stream (MPTS) having a data rate of 27 Mb/s, a band assigned for the broadcast/video data has a data rate about 432 Mb/s even if a guard band is excluded from a consideration. The data rate of 432 Mb/s corresponds to 50% of the available bands of GbE. Accordingly, considerable waste of bandwidth occurs if the broadcast/video time-slot is not used even if a user does watch or listen to the broadcast/video. 
       FIGS. 2A ,  2 B show and OLT and ONT, respectively, using time division multiplexing in a conventional Ethernet PON structure. The conventional Ethernet PON includes, as shown in  FIGS. 2A and 2B , one OLT, an optical splitter  216 , and multiple ONTs, each of the ONTs being assigned, for example, to a single user 
     OLT  300 , referring to  FIG. 2A , includes a broadcast/video channel selection switch  21 , a broadcast/video time-slot multiplexer  22 , a broadcast/video channel selection control part  23 , an IP router  24 , and Ethernet PON OLT function processing part  25 , a scrambler controller  26 , a frame multiplexer  27 , and Ethernet time-slot matching buffer  28 , and electro-optical converter  209  and an opto-electrical converter  210 . The switch  21  performs switching for MPEG (Motion Picture Experts Group) broadcasting and video data. After receiving selection channel information from ONTs  200 - 1  to  200 - 16 , the broadcast/video channel selection control part  23  delivers, to the broadcast/video channel selection switch  21 , control signals for selecting broadcast/video channels. The broadcast/video time-slot multiplexer  22  connected to the broadcast/video channel selection switch  21  performs time division multiplexing for broadcast/video channels selected by each subscriber in one time-slot. IP router  27  is used for routing communication data to an upper layer IP network or an Ethernet PON OLT function processing part  25  for processing Ethernet-PON OLT functions. The Ethernet time-slot matching buffer  28  stores communication data from the Ethernet PON OLT function processing part  25  to be sent to an ONT. The communication data is matched with broadcast/video signals time-division multiplexed so as to deliver matched data to the ONT. Frame multiplexer  27  multiplexes into one frame broadcasting/image signals of the broadcast/video time-slot multiplexer  22  and Ethernet communication signals of the Ethernet time-slot matching buffer  28 . The optical transmitter  209  optically modulates frame multiplexed signals for subsequent transfer of the modulated frame multiplexed signals λ down . The optical receiver  210  receives upstream optical signals from the ONT and converts them into electrical signals. WDM coupler  211  performs combination/division by transmission/reception wavelength. 
     The ONT, referring to  FIG. 2B , includes a WDM coupler  217 , an electro-optical converter  218 , an opto-electrical converter  219 , a frame &amp; time-slot demultiplexer  220 , and E-PON ONT function processing part  221  and a broadcast/video matching unit  222 . The WDM coupler  217  combines/splits wavelengths to be transmitted and wavelengths being received. The optical receiver  219  receives through the WDM coupler  217  optical signals λ down  from the OLT and opto-electrically converts the signals. Optical transmitter  218  transmits data upstream to the OLT. The frame and time-slot demultiplexer  220  receives broadcast/video signals and Ethernet communication signals which have been time-slot multiplexed into respective frames, and separates the broadcast/video signals from the Ethernet communication signals. The Ethernet PON ONT function processing part  221  processes an ONT function, and the broadcast/video matching unit  222  recovers original signals from separated broadcast/video signals. 
       FIG. 3  illustrates the above-mentioned frame format for a single frame  31  and time-slots  32 - 1  to  32 -n associated with that frame, for broadcast/video signals and Ethernet communication signals associated with the respective time slots. This format is employed in the Ethernet PON structure shown in  FIGS. 2A ,  2 B. 
     As shown in  FIG. 3 , the time-slots include broadcast/video sub time-slots  33 - 1 ,  34 - 1 , and  35 - 1  and Ethernet sub time-slots  33 - 2 ,  34 - 2 , and  35 - 2  whose content varies according to selections made by the subscribers. In particular, the broadcast/video sub time-slot within the i th  time-slot includes only broadcast/video signals, if any, selected by the i th  ONT. Therefore, for example, if the broadcast/video signals have not been selected by the user for the i th  ONT, the broadcast/video sub time-slot within the predetermined i th  time-slot is vacant or includes null data. The data rate of the broadcast/video signals is 1.25 G/2k [b/s] (k=0, 1, 2, . . . ), for example, the Ethernet communication signals being 1.25 GbE. 
     Each of Ethernet sub time-slots within all time-slots may, by contrast, include communication data of any of the ONTs. For example, although the broadcast/video sub time-slot  33 - 1  of the first time-slot  32 - 1  is limited to only broadcast/video signals, if any, selected by a first ONT, the Ethernet sub time-slot  33 - 2  within the first time-slot  32 - 1  can be assigned to the Ethernet communication signaling of any of the ONTs. The same applies to other time-slots  32 - 2 ,  32 - 3 . 
     However, in the conventional Ethernet PON shown in  FIGS. 2A ,  2 B, a broadcast/video time-slot is specifically assigned to every subscriber as shown in  FIG. 3  and is used only for transmitting broadcast/video. Also, when a user does not watch or listen to broadcast/video, the time-slot assigned to the user is not used. For example, if the E-PON has a 1×16 structure and broadcast/video data are carried on an MPEG transport stream (MPTS) having a data rate of 27 Mb/s, a band assigned for broadcast/video has a data rate about 432 Mb/s even if a guard band is excluded from a consideration. Herein, the data rate of 432 Mb/s corresponds to 50% of the available bands of GbE. Accordingly, if the broadcast/video time-slot is not used even if a user does watch or listen to the broadcast/video, serious waste of bandwidth incurs. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above-mentioned problems occurring in the prior art, and a first object of the present invention is to provide a time division multiplexing (TDM) Ethernet passive optical network (E-PON) for converging broadcasting/video with data in which the subscriber is provided with high-definition, real-time digital broadcasting/video as well as large-capacity data communication at high speed. 
     A second object of the present invention is to provide a TDM E-PON, for converging broadcasting/video with data, capable of transmitting from the OLT to an ONT broadcasting channels selected by a subscriber without using an EDFA to receive broadcasting services. Accordingly, the ONT may utilize a low-requirement, inexpensive optical receiver for receiving broadcasting. 
     A third object of the present invention is to provide a TDM E-PON, for converging broadcasting/video with data, capable of ensuring quality of service (QoS) when transmitting high-definition digital video to be required by subscribers in the future as well as digital broadcasting. 
     A fourth object of the present invention is to provide a TDM E-PON, for converging broadcasting/video with data, capable of performing bi-directional broadcasting functions by delivering broadcasting information through communication data lines of an Ethernet passive optical network. 
     A fifth object of the present invention is to provide an Ethernet passive optical network for broadcasting/telecommunication convergence using time division multiplexing, capable of efficiently solving a problem of bandwidth waste by utilizing time-slots assigned for broadcasting/video as data communication time-slots when subscribers do not watch or listen to broadcasting/video. 
     In order to accomplish these objects, there is provided an Ethernet passive optical network (E-PON) for broadcasting/telecommunication convergence using time division multiplexing, the Ethernet passive optical network being configured for time division multiplexing, into respective frames, signals with communication data. The signals to be multiplexed include broadcast and video signals and are obtained by performing a switching operation with respect to digital broadcast and/or digital video data for downstream delivery to at least one of multiple users of the E-PON. The communication data to be multiplexed is delivered to the network through an Internet protocol network. The switching operation is carried out according to selection information delivered from one or more of the multiple users. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, 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 the same or similar components in drawings are designated by the same reference numerals as far as possible throughout the several views: 
         FIG. 1  is a view showing a structure of a conventional Ethernet PON for broadcasting/telecommunication convergence; 
         FIGS. 2A ,  2 B are views showing an example of a conventional Ethernet PON structure for broadcasting/telecommunication convergence using time division multiplexing; 
         FIG. 3  is a view representing one a frame and time-slots for broadcast/video signals and Ethernet communication signals employed in the Ethernet PON structure shown in  FIGS. 2   a  and  2   b;    
         FIGS. 4A ,  4 B are views showing an Ethernet PON (E-PON) using time division multiplexing (TDM) to converge broadcasting/video with data according to the present invention; 
         FIG. 5(   a )-( c ) shows an arrangement of broadcast/video signals and Ethernet communication signals by frame according to one embodiment of the present invention; 
         FIG. 6(   a )-( d ) provides conceptual views representing a procedure of demultiplexing multiplexed frames shown in  FIGS. 5   a  to  5   c;    
         FIG. 7  is a view showing a structure of a frame obtained by multiplexing broadcast/video signals and Ethernet communication signals according to a first embodiment of the present invention; 
         FIG. 8  is a view showing a structure of a frame obtained by multiplexing broadcast/video signals and Ethernet communication signals according to a second embodiment of the present invention; 
         FIG. 9  is a view showing a notification packet structure defined according to the present invention; 
         FIG. 10  is a view showing a method of transferring a notification packet; 
         FIG. 11  is a view showing a notification packet structure when all ONT of a system do not receive broadcast/video; 
         FIG. 12  is a view showing a frame formed when an Ethernet PON for broadcasting/communication convergence using time division multiplexing according to the present invention initially sets up; 
         FIG. 13  is a view showing a notification packet structure during initial set-up according to the present invention; 
         FIG. 14  is a view showing a procedure of reserving broadcast/video time-slots in an Ethernet PON for broadcasting/telecommunication convergence using time division multiplexing according to a first embodiment of the present invention; 
         FIG. 15(   a )-( c ) shows an embodiment for a form of notification packets shown in  FIG. 14 ; 
         FIG. 16(   a )-( c ) shows that an ONT receives data and performs demultiplexing in an Ethernet PON for broadcasting/telecommunication convergence using time division multiplexing according to the present invention; 
         FIG. 17  is a view showing a procedure of reserving broadcast/video time-slots in an Ethernet PON for broadcasting/telecommunication convergence using time division multiplexing according to a second embodiment of the present invention; 
         FIG. 18(   a )-( d ) shows notification packet structures in connection with the procedure described in  FIG. 17 ; 
         FIG. 19(   a )-( c ) illustrates a procedure for processing a notification packet shown in  FIG. 17  when each ONT receives a notification packet; 
         FIG. 20(   a )-( c ) demonstrates a case in which an ONT, which has received broadcast/video in an Ethernet PON, stops receiving broadcast/video; and 
         FIG. 21  ( a )-( c ) presents a demultiplexing procedure of a first ONT in a case in which an ONT receiving broadcast/video in an Ethernet PON according to the present invention stops receiving broadcast/video. 
     
    
    
     DETAILED DESCRIPTION 
     Preferred embodiments of the present invention are described in detail with reference to the accompanying drawings. Details of known functions and configurations incorporated herein are omitted for clarity of presentation. 
       FIGS. 4A ,  4 B depict, by way of illustrative and non-limitative example, an Ethernet PON (E-PON) using time division multiplexing (TDM) to converge broadcasting/video with data according to the present invention. 
     As shown in  FIGS. 4A ,  4 B, the TDM E-PON according to the present invention includes one OLT, an optical splitter  427 , and N ONTs. Each of the ONTs is assigned to a single user. 
     The OLT, as seen in  FIG. 4A , includes a broadcast/video channel selection switch  41 , a broadcast/video time-slot multiplexer  42 , a broadcast video channel selection control part  43 , an IP router  44 , an E-PON OLT function processing part  45  having a notification packet generating part  48 , a scrambler controller  46 , a broadcast/video time-slot reservation/cancellation processing part  47 , a broadcast/video &amp; GbE multiplexing control part  49 , a frame multiplexer  410 , and Ethernet time-slot matching buffer  411 , and opto-electrical converter  412 , an electro-optical converter  413  and a WDM coupler  414 . 
     The switch  41  performs switching for MPEG (motion picture experts group) broadcasting and video data. Broadcast/video channel selection control part  43 , after receiving selection channel information from the ONTs, delivers to the broadcast/video channel selection switch  41  control signals used for selecting broadcast/video channels. The broadcast/video time-slot multiplexer  42  is connected to the broadcast/video channel selection switch  41  and performs time division multiplexing for broadcast/video channels selected according to subscribers. IP router  44  is used for routing communication data to an upper layer IP network or an Ethernet PON OLT function processing part  45  for processing Ether-PON OLT functions. The synchronization control unit  46  is used for providing synchronization for the broadcast/video time-slot multiplexer  42  and the broadcast/video and GbE multiplexing control unit  49  by using the control signals delivered from the Ethernet PON OLT function processing part  45 . Ethernet time-slot matching buffer  411  stores communication data to be sent to each ONT from the Ethernet PON OLT function processing part  45  in order to match the communication data with time division multiplexed broadcast/video signals so as to deliver matched data to each ONT. The broadcast/video time-slot reservation/cancellation processing unit  47  is connected to the Ethernet PON OLT function processing part  45  and processes broadcast/video time-slot reservation and cancellation signals transferred from each ONT. The broadcast/video and GbE multiplexing control unit  49  controls frame multiplexing by using broadcast/video time-slot reservation and cancellation information delivered from the broadcast/video time-slot reservation/cancellation processing unit  47  and synchronization control information delivered from the synchronization control unit  46 . Notification packet generating part  48  uses broadcasting/video time-slot reservation and cancellation information delivered from the broadcast/video time-slot reservation/cancellation processing unit  47  to generate notification packets for notifying each of the ONTs of whether or not the ONT receives broadcast/video. Frame multiplexer  410  multiplexes into one frame broadcast/video signals of the broadcast/video time-slot multiplexer  42  and Ethernet communication signals of the Ethernet time-slot matching buffer  411 . The optical transmitter  413  optically modulates frame-multiplexed signals to form modulated signals λ down  and transfers the modulated signals. The optical receiver  412  receives optical signals λ up  from the ONTs and converts the optical signals into electrical signals. WDM coupler  414  performs combination/division for wavelengths being transmitted and received. 
     Each ONT, referring to  FIG. 4B , includes a WDM coupler  428 , an electro-optical converter  429 , an opto-electrical converter  430 , an E-PON ONT function processing part  431 , an Ethernet demultiplexer  432 , and broadcast/video matching unit  433 , a broadcast/video demultiplexer  434 , a broadcast/video matching unit  435  and a notification packet processor  436 . 
     The WDM coupler  428  performs combination/division for wavelengths being transmitted and received. Optical receiver  430  receives from the OLT through the WDM coupler  428  signals delivered as optical signals λ down  so as to opto-electrically convert the received signals. The optical transmitter  429  transmits upstream data to the OLT. The Ethernet demultiplexer  432  extracts Ethernet signals from frame multiplexed signals delivered through the optical receiver  430 . Broadcast/video demultiplexer  434  extracts broadcast/video signals from the frame multiplexed signals delivered through the optical receiver  430 . The E-PON ONT function processing part  431 , in addition to dealing with ONT functions and delivering downstream (i.e., OLT-to-ONT(s)) communication data  440 , receives the Ethernet signals from the Ethernet demultiplexer  432 , and delivers to the optical transmitter  429  upstream signals including communication data  437 , broadcast/video selection data  438 , and broadcast/video selection time-slot reservation/cancellation signals  439 . Notification packet processing part  436  processes a notification packet delivered from the E-PON ONT function processing part  431 . The demultiplexing control unit  433  is connected to the notification packet processing part  436  and controls the Ethernet demultiplexer  432  and the broadcast/video demultiplexer  434  according to notification packet information. Broadcast/video matching unit  435  recovers original signals (digital broadcast/video  445 ) from broadcast/video signals delivered from the broadcast/video demultiplexer  434 . 
       FIG. 5(   a )-( c ) shows an arrangement of broadcast/video signals and Ethernet communication signals by frame according to one embodiment of the present invention. 
     Each of frames is divided into n time-slots (the number of ONTs) in order to perform frame multiplexing for broadcast/video signals and Ethernet communication signals according to the present invention. Each of the time-slots includes one broadcast/video sub time-slot and one GbE frame sub time-slot. 
       FIG. 5(   a ) shows broadcast/video sub time-slots  501 - 1  to  50 N- 2  assigned for n respective ONTs in a frame i and in a frame j. 
       FIG. 5(   b ) shows GbE frame sub time-slots used for transferring GbE frames by using n time-slots assigned to each of the frame i and the frame j. 
     The first GbE frame sub time-slot shown in  FIG. 5   b  contains a notification packet having broadcast/video receiving information of ONTs so as to carry the notification packet. This is represented with reference number  51 . 
       FIG. 5(   c ) is a view showing the frame i and the frame j including multiplexed broadcast/video sub time-slots  501 - 1  to  50 N- 2  assigned to n ONTs and GbE frame sub time-slots used for transferring GbE frames through n time-slots. 
       FIG. 6(   a )-( d ) represents a procedure of demultiplexing multiplexed frames shown in  FIG. 5(   a )-( c ). 
       FIG. 6(   a ) is a view representing a procedure by which a first ONU demultiplexes broadcast/video signals in multiplexed frames shown in  FIG. 5(   a )-( c ). Broadcast/video sub time-slots assigned to the first ONT are first sub time-slots  602 - 1 ,  602 - 2  included in the frame i and the frame j. Therefore, the first ONT receives only broadcast/video data  602 - 1 ,  602 - 2  corresponding to the first ONT through a filtering procedure  601  shown in  FIG. 6(   c ). In  FIG. 6(   a )-( d ), all GbE sub time-slots include notification packets having broadcast/video receiving information of ONTs, these packets being denoted  603 - 1  through  603 - 6 . 
       FIG. 6(   b ) is a view showing that the first ONT performs demultiplexing procedure with respect to GbE frames in the multiplexed frames shown in  FIGS. 5A to 5C . All GbE sub time-slots  605 - 1  to  605 - 6  included in the multiplexed frames are used for all ONTs. Accordingly, the ONTs perform demultiplexing with respect to the GbE sub time slots  605 - 1  to  605 - 6  through a filtering operation  604  the result of which is shown in  FIG. 6(   d ). 
       FIG. 7  is an exemplary view showing a structure of a frame  71  obtained by multiplexing broadcast/video signals and Ethernet communication signals according to a first embodiment of the present invention. The frame  71  includes n time-slots  72 - 1  to  72 -n. The time-slots include broadcast/video sub time-slots  73 - 1  to  73 -n of ONTs and GbE sub time-slots  74 - 1  to  74 -n used for transferring GbE frames (1.25 G Ethernet frames). 
     A broadcast/video sub time-slot of a predetermined i th  time-slot is assigned for a predetermined i th  ONT. If the predetermined i th  ONT receives broadcast/video, broadcast/video data are transferred through the broadcast/video sub time-slot of the predetermined i th  time-slot. However, if the predetermined i th  ONT does not receive broadcast/video, instead of broadcast/video data, GbE communication data are transferred through the broadcast/video sub time-slot of the predetermined i th  time-slot. Accordingly, it is possible to efficiently utilize bandwidth according to the present invention. 
     If an ONT does not receive broadcast/video or, alternatively, does not receive communication data, the otherwise vacant sub time slot can be filled with additional information of a type that the ONT does receive. For example, in the frame  71 , if a first ONT receives broadcast/video and a second ONT does not receive broadcast/video, a broadcast/video sub time-slot  73 - 1  of a first time-slot  72 - 1  has broadcast/video data received by the first ONT, and a broadcast/video sub time-slot  73 - 2  of a second time-slot  72 - 2  has GbE communication data (because the second ONT does not receive broadcast/video). The above description identically applies to all time-slots  72 - 1  to  72 -n. 
       FIG. 8  illustrates one example of a frame  81  obtained by multiplexing broadcast/video signals and Ethernet communication signals according to a second embodiment of the present invention. The frame  81  obtained by multiplexing broadcast/video signals and Ethernet communication signals according to the present invention includes a broadcast/video frame  82  and a communication data frame  83 . The broadcast/video frame  82  has broadcast/video sub time-slots  84 - 1  to  84 -n assigned for ONTs. As described with reference to  FIG. 7 , the broadcast/video sub time-slots  84 - 1  to  84 -n contain GbE communication data rather than broadcast/video data in the event that the predetermined i th  ONT does not receive broadcast/video. The above described frames and time-slots can be variously defined depending on the number of broadcast/video channels, etc., selected by a user. 
     In order to load the broadcast/video slots with GbE communication data and not broadcast/video data and transfer the GbE communication data when the predetermined i th  ONT does not receive broadcast/video, the present invention defines a notification packet notifying the ONTs of whether or not the ONT is to receive broadcast/video, thereby allowing ONTs to discriminate between broadcast/video signals and communication data. 
       FIG. 9  is a view showing a notification packet structure defined according to the present invention. The notification packet, defined as an Ethernet frame, includes an Ethernet header  91 , a layer  3 /layer  4  (L 3 /L 4 ) header  92 , notification fields  93 , a zero padding  94  for achieving the minimum length of 64 bytes, and a layer  2  (L 2 ) trail  95 . 
     Herein, the Ethernet header  91  consists of several fields. A logical link identifier (LLID)  96 , a destination MAC address (DA)  97 , and a type  98  for representing data types are utilized primarily as notification fields. 
     Since the notification packet is transferred from an OLT to all ONTs, the LLID  96  and the DA  97  are assigned as broadcasting addresses. The ONT determines the type  98 , which is information used for delivering the notification packet received by an ONT to the notification packet processing part  436  shown in  FIG. 4B . 
     The notification packet is used mainly for recording receiving states of the broadcast/video in all ONTs and for transferring the receiving states to each ONT. To this end, the broadcast/video receiving states of all ONTs are marked on the notification fields  93  of the notification packet. 
     The notification fields  93  are represented as n bits  99 - 1 ,  99 - 2 ,  99 - 3 , . . ,  99 -n, corresponding to the number of ONTs. A ‘0’ value of a predetermined i th  bit means that a predetermined i th  ONT does not receive broadcast/video, and the value ‘1’ of the predetermined i th  bit means that the predetermined i th  ONT receives broadcast/video. 
     For example, in an 1×16 EPON, values of the notification field  93  within the notification packet ‘1111001100111100’ represent that first, second, third, fourth, seventh, eighth, eleventh, twelfth, thirteenth, and fourteenth ONTs receive broadcast/video, and fifth, sixth, ninth, tenth, fifteenth, and sixteenth ONTs do not receive broadcast/video. 
     Although the notification fields are defined as n bits according to the present invention shown in  FIG. 9 , the notification fields may, if necessary, be defined as n×m (herein, m is a predetermined natural number) bits. 
     The notification packet employing the Ethernet frame described above can be variously defined within the Ethernet frame. As shown in  FIG. 10 , the notification packet defined above is updated in each frame and is positioned at a first time-slot for every frame (as seen from reference numbers  1010 ,  1011 ,  1012 , and  1013 ) so as to be transferred to all ONTs. 
     Positioning the notification packet is preferably at a first time-slot of each frame, because it is necessary for a received notification packet to be processed before the corresponding frame elapses. 
     With reference to  FIGS. 4A ,  4 B in illustrating basic operation of the invention, N digital broadcasting channels and N digital video channels are inputted to the broadcast/video channel selection switch  41 . The number of broadcasting and video channels may differ, however. 
     On the user side, one or more subscribers (ONTs) specify a respective broadcast/video channel by remote controller. Signals of the specified broadcast/video channel are inputted, as broadcast/video channel selection data  438 , from the ONT to the OLT . The broadcast/video channel selection data  438  inputted to the OLT are transmitted through the EPON OLT function processing part  45  to the broadcast/video channel selection part  43 . The latter uses the inputted broadcast/video channel selection data  438  to control the broadcast/video channel selection switch  41 , thereby selectively switching into digital broadcast/video channels  417 - 1  to  417 -N as required by a subscriber. The digital broadcast/video channels  417 - 1 ,  417 - 2 , and  417 -N are selected, for example, by a first ONT, a second ONT, and an nth ONT, respectively. 
     The broadcast/video time-slot multiplexer  42  receives the switched broadcast/video channels in order to form the time-slots defined in  FIG. 7 . The broadcast/video time-slot multiplexer  42  performs time slot multiplexing for broadcast/video data inputted to the broadcast/video time-slot multiplexer  42  with a data rate of R [b/s] after converting a data rate of the broadcast/video data into a data rate of 1.25/2 k [b/s]. According to one embodiment of the present invention, since it is assumed that all subscribers (ONTs) receive broadcast/video channels, a broadcast/video channel selected by each ONT is positioned at a broadcast/video sub time-slot assigned for each ONT as defined in  FIG. 7 . 
     At this time, synchronization for multiplexing time-slots is controlled by the synchronization controlling unit  46  and the broadcast/video and GbE multiplexing control unit  49 . Through ranging, which is a function of the PON, the EPON OLT function processing part  45  supplies the synchronization control unit  46  with a basic signal for maintaining synchronization. 
     The EPON OLT function processing part  45  likewise applies EPON functions to communication data transferred from an upper-layered IP network. Thereafter, the communication data are inputted to the Ethernet time-slot matching buffer  411 , so that the communication data are matched with time-slots defined in  FIG. 7 . The EPON OLT function processing part  45  generates notification packets with respect to every frame in order to notify all ONTs of whether or not each ONT receives broadcast/video. The notification packets are generated in the notification packet generating part  48  of the EPON OLT function processing part  45 . Since it is assumed that all ONTs receive broadcast/video as described above, the notification packets transferred to all ONTs from the OLT are formed as shown in  FIG. 11 . That is, the notification packets are formed in such a manner that the LLID  96  and the DA  97  are assigned as broadcasting addresses, and the type  98  represents a notification packet type. Also, all bits  1104 - 1  to  1104 -n of the notification fields are set as ‘1’. 
     GbE communication data stored in the Ethernet time-slot matching buffer  411  are outputted in a manner that varies with broadcast/video receiving states of ONTs as set forth above with regard to  FIG. 7   
     Broadcast/video signals outputted from the broadcast/video time-slot multiplexer  42  and GbE communication data outputted from the Ethernet time-slot matching buffer  411  are frame-multiplexed in the frame multiplexer  410  as shown in  FIG. 5   c.    
     The frame multiplexer  410  and the Ethernet time-slot matching buffer  411  are controlled by the broadcast/video and GbE multiplexing control unit  49 . In particular, when the frame multiplexer  410  performs frame multiplexing with respect to broadcast/video time-slots and Ethernet communication data time-slots, synchronization control information regarding each time-slot, and information as to whether the associated data type is to be replaced by the other data type, are delivered to the frame multiplexer, so that, for example, broadcast/video time-slots are replaced with the Ethernet communication data time-slots. 
     The broadcast/video time-slot reservation/cancellation processing unit  47  sends the broadcast/video and GbE multiplexing control unit  49  information (that is, information of an ONT not receiving broadcast/video) representing time-slots, from among broadcast/video time-slots, to be replaced with Ethernet communication data time-slots. The processing unit  47  also transmits, to the notification packet generating part  48 , delivers information used by ONTs in reserving a time for receiving the broadcast/video, and information which is used for canceling the reservation time for broadcast/video. 
     The frame multiplexed broadcast/video channels and communication data are optically modulated in the optical transmitter  413  onto a wavelength of λ down  and transferred on downstream optical signals (λ down ) to ONTs in a path through the WDM coupler  414 , the 1× n optical splitter  427 , the WDM coupler  428  and the optical receiver  430 . The received signals are split into Ethernet communication data and broadcast/video channels selected by each of ONTs and are inputted to the Ethernet demultiplexing part  432  and the broadcast/video demultiplexing part  434 , respectively. 
     When notification packets exist in GbE communication data demultiplexed by the Ethernet demultiplexing part  432 , the EPON ONT function processing part  431  detects the notification packets and delivers the notification packets to the notification packet processing part  436 . 
     The notification packet processing part  436  analyzes a broadcast/video receiving state of each ONT and transfers information representing the broadcast/video receiving state of each ONT to the demultiplexing control unit  433 . 
     The demultiplexing control unit  433  controls the Ethernet demultiplexing part  432  and the broadcast/video demultiplexing part  434  to separate Ethernet communication data from broadcast/video channels. 
     According to one embodiment of the present invention, since it is assumed that all ONTs receive broadcast/video, the notification packets are formed as shown in  FIG. 11 . Therefore, as shown in  FIG. 6(   a ), since the broadcast/video channels  602 - 1  and  602 - 2  selected by a first ONT are positioned at a first time-slot of each frame, the first ONT performs filtering in response to a control signal and GbE communication data  605 - 1  to  605 - 6  are likewise extracted in response to a control signal. 
     Problems in relation to synchronization occurring when separating the broadcast/video channels and the communication data are solved by performing synchronization through ranging which is a function of EPON. The extracted GbE communication data shown in  FIG. 6   d  are transferred to terminal units of the ONT such as a computer, etc., or to the notification packet processing part  436  through the EPON ONT function processing part  431 . 
     The extracted broadcast/video channels shown in  FIG. 6   c  are converted into broadcast/video channels  445  having an original data rate of R [b/s] in the broadcast/video matching unit  435  and are transferred to an MPEG decoder, etc. 
     For upstream operation, each subscriber generates IP communication data created in a computer, etc., and broadcast/video channel selection data  438  for looking at and listening to broadcast/video channels. 
     Each subscriber additionally generates broadcast/video time-slot reservation/cancellation signals  443  for selecting broadcast/video channels or canceling selection of the broadcast/video channels. 
     The data generated by a user are optically modulated in the optical transmitter  429  onto a wavelength of λ up  after the EPON ONT function processing part  431  solves problems such as data collision, and are transferred to an OLT by way of the WDM coupler  428  and the optical splitter  427 . 
     Upstream data signals transferred from each ONT are opto-electrically converted in the optical receiver  412  and inputted to the EPON OLT function processing part  45 . The broadcast/video channel selection data  438  are delivered to the broadcast/video channel selection control part  43  and the IP communication data  441  are delivered to an IP network through the IP router  44  or Ethernet backbone switches. 
     The broadcast/video time-slot reservation/cancellation signals  443  are delivered to the broadcast/video time-slot reservation/cancellation processing unit  47  from the EPON OLT function processing part  45 . These reservation/cancellation signals  443  include information representing which ONT starts or stops receiving a broadcast/video channel. For example, when a first ONT, while receiving a broadcast/video channel, stops its receiving operation for the channel, efficiency suggests that the first ONT be afforded GbE communication data by means of the broadcast/video sub time slot of the first time slot. Therefore, the broadcast/video time-slot reservation/cancellation processing unit  47  is required. 
     The broadcast/video time-slot reservation/cancellation processing unit  47  analyzes which ONTs start or stop receiving broadcast/video channels and delivers analyzed data to the broadcast/video and GbE multiplexing control unit  49 . 
     The broadcast/video and GbE multiplexing control unit  49  allows each ONT to use a broadcast/video channel sub time-slot as a GbE communication sub time-slot depending on a broadcast/video receiving state of each ONT by controlling the broadcast/video time-slot multiplexer  42 , the Ethernet time-slot matching buffer  411 , and the frame multiplexer  410 . Description about this will be given in detail later with reference to  FIGS. 15 to 21 . 
       FIG. 12  shows notification frame format at initial set-up for the TDM E-PON according to the present invention. All sub slots of an i th  frame  1201 , a j th  frame  1202 , and a k th  frame  1203  are utilized for GbE communication data, since none of ONTs receive broadcast/video during initial set-up. As seen in  FIGS. 12 and 13 , ‘0’ is set for bits  1305 - 1  to  1305 -n of the notification fields in notification packets  1204 - 1  to  1204 - 3  positioned at the frames  1201  to  1203 . 
       FIG. 14  is a view showing a procedure of reserving broadcast/video time-slots in the Ethernet PON for broadcasting/telecommunication convergence using time division multiplexing according to a first embodiment of the present invention. 
     As shown in  FIG. 14 , according to the present invention, the procedure of reserving broadcast/video time-slots will be described with three frames including a first frame  1401 , a second frame  1402 , and a third frame  1403 . 
     When the first ONT and the second ONT turn on set-top boxes in order to receive broadcast/video, the first ONT and the second ONT generate the broadcast/video time-slot reservation signal  439  and transfer the signal to the OLT. This signal may issue at initial network set-up at which time all sub time-slots are devoted to GbE communication data as shown in  FIG. 12 . 
     As shown in  FIG. 14 , when a first ONT broadcast/video time-slot reservation signal is received by the OLT at a first time-slot of a first frame  1401  (step  1404 ) and a second ONT broadcast/video time-slot reservation signals is received by the OLT at a second time-slot (step  1405 ), the EPON OLT function processing part  45  transfers the first ONT and the second ONT broadcast/video time-slot reservation signals to the broadcast/video time-slot reservation/cancellation processing unit  47 . The latter transfers to the notification packet generating part  48  information representing that the first ONT and the second ONT start receiving broadcast/video channels. Although the notification field for a notification packet  1406  has been cleared to zero at initial set-up, the notification packet generating part  48 , in reaction to the received information, changes a value of first and second bits within the notification fields from ‘0’ to ‘1’, so as to create a new notification packet  1407 . The created notification packet B  1407  is positioned at a first time-slot of a second frame  1402  and is transferred to all ONTs. 
     Since broadcast/video channel bands are assigned for the first ONT and the second ONT, the broadcast/video time-slot reservation/cancellation processing unit  47  issues to the broadcast/video and GbE multiplexing control unit  49  broadcast/video channel band assigning signals for the first and second ONTs. 
     The broadcast/video and GbE multiplexing control unit  49  controls the broadcast/video time-slot multiplexer  42 , the Ethernet time-slot matching buffer  411 , and the frame multiplexer  410  so as to assign broadcast/video sub time-slots  1409 ,  1410  of first and second time-slots within a third frame as the broadcast/video channel bands for the first and second ONTs (step  1411 ). Accordingly, broadcast/video channels selected by the first ONT and the second ONT by means of the broadcast/video channel selection data  438  are positioned at the broadcast/video sub time-slots  1409 ,  1410  and are transferred to ONTs. The notification packet  1409  within the frame  1403  is identical to the notification packet  1407  within the frame  1402 , assuming, for simplicity of demonstration, that broadcast/video time-slot reservation/cancellation signals of other ONTs are not received. 
     The notification packets  1406 ,  1407 ,  1409  are shown in  FIG. 15(   a )-( c ), respectively. Comparing  FIG. 15  with  FIG. 13 , with reference to the above discussion relating to  FIG. 14 , 
     If a broadcast/video time-slot reservation signal is received at frame i, a notification packet of frame i+1 is changed and transferred and a broadcast/video time-slot is allotted at frame i+2. Accordingly and advantageously, sufficient time required for processing a notification packet is available to avoid or solve synchronization problems. 
       FIG. 16(   a )-( c ) illustrate demultiplexing in the ONT of an Ethernet PON for broadcasting/telecommunication convergence using time division multiplexing according to the present invention. Data received by the optical receiver  430  of the first ONT are inputted to the Ethernet demultiplexer  432  and the broadcast/video demultiplexer  434 . Referring to  FIG. 16 , since the first ONT does not receive broadcast/video in an initial state, all data of frame  1 ′ are inputted to the EPON ONT function processing part  431 . The latter delivers a notification packet  1601  to the notification packet processing part  436 . Since all values of notification fields within the notification packet  1601  are ‘0’s, the notification packet processing part  436  determines that none of ONTs receive broadcast/video and transfers information to the demultiplexing control unit  433  representing that none of ONTs receive broadcast/video. 
     The demultiplexing control unit  433  controls the broadcast/video demultiplexer  434  and the Ethernet demultiplexer  432  according to information of the notification packet A  1601  with respect to frame  2 ′. This is shown in  FIG. 16(   b ) (step  1606 ). Similarly, a control in relation to frame  3 ′ is achieved according to information of the notification packet  1602  (step  1607 ). 
     When it comes to the frame  2 ′, since none of ONTs receive broadcast/video, all data within the frame  2 ′ are recognized as GbE communication data. Also, the notification packet  1602  exists in the frame  2 ′ and first and second bits  1502 - 1 ,  1502 - 2  of the notification fields within the notification packet B has been set as ‘1’. That is, since the first ONT and the second ONT start receiving broadcast/video, broadcast/video data in relation to the first ONT and the second ONT exist in the frame  3 ′. Accordingly, the notification packet processing part  436 , which has received the notification packet  1602 , notifies the demultiplexing control unit  433  of information representing that the first ONT and the second ONT are to start receiving broadcast/video. Thereafter, the demultiplexing control unit  433  generates a broadcast/video demultiplexing control signal  1604  shown in  FIG. 16(   b ) and an Ethernet demultiplexing control signal  1608  shown in  FIG. 16(   c ) so as to demultiplex to separate the broadcast/video data and the GbE communication data arriving within frame  3 ′. 
     In particular, since a broadcast/video channel  1605  selected by the first ONT exists within the frame  3 ′, the broadcast/video channel is, referring to  FIG. 16(   b ), extracted. GbE communication data are extracted from bands used for GbE communication data, i.e., excluding parts  1609 ,  1610  used for first and second ONT broadcast/video signals. 
     As seen from  FIG. 16(   b )-( c ), the broadcast/video demultiplexer  434  extracts the first ONT broadcast/video band  1609  of a first time-slot within the frame  3 ′ and the Ethernet demultiplexer  432  excludes broadcast/video sub time-slot parts  1609 ,  1610  of the first and second time-slots. 
     As described above, when ONTs do not receive broadcast/video, bands are used for GbE communication data. Thereafter, when specific ONTs start receiving broadcast/video, bands corresponding to the specific ONTs, which have used for GbE communication data, can be utilized for broadcast/video channel bands. Therefore, it is possible to efficiently assign bands in the Ethernet PON. 
       FIG. 17  is a view showing a procedure of reserving broadcast/video time-slots according to a second embodiment of the present invention. 
     As described above, at the early stage of setting up the Ethernet PON, since all bands are used for GbE communication data, all bits of notification fields of a notification packet  1705  within the frame  1701  are set as ‘0’ as shown in  FIG. 18(   a ). Thereafter, if the first and second ONT broadcast/video time-slot reservation signals are received at the frame  1701  (steps  1709 ,  1710 ), notification fields of a notification packet  1706  within the frame  1702  are changed as shown in  FIG. 18   b  and the packet is transmitted (step  1711 ). 
     Broadcast/video time-slots of first and second time-slots within the frame  1703  are assigned as bands for broadcast/video receiving on behalf of the first ONT and the second ONT (step  1713 ). 
     Since a broadcast/video time-slot reservation signal of the n th  ONT is received at the frame  1702  (step  1712 ), a notification packet  1707  to be transferred at the frame  1703  is changed as shown in  FIG. 18   c  and transferred. Specifically, an n th  bit  1801  of notification fields is set as ‘1’. As a consequence, broadcast/video sub time-slot  1715  of an n th  time-slot within frame  1704  is assigned as a broadcast/video receiving band of the n th  ONT (step  1716 ). 
     A notification packet  1708  of the frame  1704  has values of the notification packet  1707  as shown in  FIG. 18(   c )-( d ). 
       FIG. 19(   a )-( c ) illustrates processing the notification packet shown in  FIG. 17  when each ONT receives the notification packet. 
     When each ONT receives the notification packet shown in  FIG. 17 , the notification packet is processed as shown in  FIG. 19(   a )-( c ). 
     If the first ONT receives a notification packet  1901  in frame  1 ′, the first ONT analyzes the notification packet and controls frame  2 ′, so that broadcast/video channels and GbE communication data of the first ONT are separated. 
     The first ONT separates a broadcast/video channel and GbE communication data in frame  3 ′ by analyzing a notification packet  1902  in the frame  2 ′ as shown in  FIG. 19(   b ). Since first and second bits of notification fields of the notification packet  1902  are set as ‘1’s, the first ONT generates a control signal  1905 , so as to extract the broadcast/video channel  1906  of the first ONT, and the first ONT generates a control signal  1911 , so as to extract the GbE communication data. 
     Since the first and second ONTs receive broadcast/video channels, when the GbE communication data are extracted, broadcast/video sub time-slots corresponding to reference numbers  1912 ,  1913  are excluded. 
     Broadcast/video channels and communication data within the frame  4 ′ are extracted by considering the notification packet  1903  within the frame  3 ′. 
     Since the first ONT continuously receives broadcast/video, the broadcast/video demultiplexing control signal  1905  is not changed, and a broadcast/video channel  1907  of the first ONT is extracted from the frame  4 ′ by means of the control signal  1905  is not changed. In addition, since a broadcast/video channel of the n th  ONT is included in the frame  4 ′, broadcasting/video channels  1914 ,  1915 ,  1916  corresponding to the first, second, and n th  ONTs, respectively, are not extracted and only communication data are extracted through the GbE demultiplexing control signal  1911  shown in  FIG. 19(   c ). 
       FIG. 20(   a )-( c ) shows a case in which a first ONT, which has received broadcast/video together with second and n th  ONTs, stops receiving broadcast/video. 
     Since the first, second, and n th  ONTs receive broadcast/video, a notification packet  2004  of frame  2001  is identical to the notification packet shown in  FIG. 20(   b ). In particular, first, second, and n th  bits of notification fields of the notification packet P are set as ‘1’s. 
     If a broadcast/video time-slot cancellation signal of the first ONT is received at the frame  2001  (step  2007 ), values of notification fields of a notification packet  2005  within the frame  2002  are changed into values of the notification fields shown in  FIG. 20(   c ) so as to be transferred (step  2008 ). In particular, a first bit  2010  of the notification fields of the notification packet  2005  is changed from ‘1’ into ‘0’ so as to be transferred. 
     The broadcast/video channel of the first ONT is changed into GbE communication data in the first time-slot of the frame  2003  (step  2009 ). 
     In addition, since none of broadcast/video time-slot reservation signals or broadcast/video time-slot cancellation signals are received, a notification packet  2006  within the frame  2003  is identical to the notification packet  2005 . 
       FIG. 21(   a )-( c ) depict a demultiplexing procedure of the first ONT in a case in which an ONT receiving broadcast/video in the Ethernet PON according to the present invention stops receiving broadcast/video. 
     Hereinafter, when the first ONT receives notification packets identical to the notification packets shown in  FIG. 20(   a )( c ), signals for demultiplexing of the first ONT will be described with reference to  FIG. 21(   a )-( c ). 
     The notification packet processing part  436  of the first ONT analyzes a notification packet  2101  so as to generate a broadcast/video demultiplexing control signal and an Ethernet demultiplexing control signal identical to control signals  2104 ,  2110  within frame  3 ′″, so that broadcast/video data  2105 ,  2106  and GbE data are separated. 
     Broadcast/video data and GbE data of the frame  3 ′ are separated by analyzing the notification packet  2102 . Therefore, since a first bit of notification fields of the notification packet is set as ‘0’, broadcast/video channels of the first ONT do not exist after the frame  3 ′. Accordingly, as shown with reference number  2109  in  FIG. 21(   b ), data of broadcast/video sub time-slots  2105 ,  2106 , which have used for the first ONT, are processed as GbE communication data. A GbE demultiplexing formation of the frame  3 ′ obtained when data of broadcast/video sub time-slots used for the first ONT are processed as GbE communication data is shown in  FIG. 21(   c ). 
     As described above, according to the present invention, required broadcasting channels selected by a subscriber can be transmitted to an ONT from an OLT without using an EDFA used for receiving plural broadcasting services. An inexpensive, low-requirement optical receiver for receiving broadcasting may therefore be used in the ONT. 
     Also, according to the present invention, quality of services (QoS) can be ensured when transmitting high definition digital video to be required by subscribers in the future as well as digital broadcasting. 
     In addition, according to the present invention, bi-directional broadcasting functions can be performed by delivering broadcasting information through communication data lines of an Ethernet passive optical network. 
     Furthermore, according to the present invention, it is possible to efficiently manage network resources by utilizing time-slots assigned for broadcast/video as data communication time-slots when subscribers do not receive broadcast/video. 
     While the invention has been shown and described with reference to certain preferred 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 spirit and scope of the invention. Consequently, the scope of the invention should not be limited to the embodiments, but should be defined by the appended claims and equivalents thereof.