Abstract:
Disclosed herein is a shared Local Area Network (LAN) emulation method and apparatus. The method includes the following four steps. At the first step, a Logical Link Identifier (LLID) management table is set up to assign unique LLIDs to a plurality of Optical Network Units (ONUs) and manage the assigned LLIDs so as to identify the plurality of ONUs connected to a single Optical Line Terminal (OLT). Thereafter, a MAC address table is set up for the LLIDs to learn MAC addresses of the ONUs. Thereafter, the unique LLIDs are assigned to ONUs when the ONUs request registration from the OLT. Finally, data frames, which are received by a Shared LAN Emulation (SLE) layer of the OLT, are bridged using the LLIDs, VIDs of Virtual LANs to which the ONUs belong and destination MAC addresses of the data frames so as to provide a single matched port between a Logical Link Control (LLC) layer and a MAC layer of the OLT.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to Ethernet passive optical networks, and more particularly, to a local area network emulation method and apparatus using logical link identifiers that are used for point-to-point communications and broadcasting in an Ethernet passive optical network.  
         [0003]     2. Description of the Related Art  
         [0004]     Currently, for the economical construction of a network suitable for the provision of broadband services, a Passive Optical Network (PON) based on Ethernet attracts attention. Ethernet PON (EPON) has been standardized by the IEEE 802.3ah Ethernet First Mile (EFM) task force. In general, PON is constructed to use Optical Distribution Networks (ODNs) between Optical Line Terminal (OLT) and Fiber To The Home (FTTH) Optical Network Terminations (ONTs) or Fiber To The Curb/Cabinet (FTTC) Optical Network Units (ONUs), and adapts a network topology in which all ONUs/ONTs are distributed in the form of a bus or tree structure.  
         [0005]      FIG. 1  is a configuration diagram of an EPON system.  
         [0006]     Referring to  FIG. 1 , the EPON system has a point-to-multipoint structure, in which a plurality of ONUs  101  installed on subscriber sides, such as houses or offices, share an Optical Line Terminal (OLT)  103  installed on a central office side and connected to the ONUs  101  through ODNs  102  using optical cables. Accordingly, downstream transmission in which a message is transmitted from the OLT  103  to the plurality of ONU  101  is performed in a broadcasting manner. In contrast, upstream transmission in which a message is transmitted from one of the plurality of ONUs  101  to the OLT  103  is performed in multipoint-to-point manner. In the EPON system constructed as described above, subscribers are provided with a variety of services, such as Internet services, telephone services and interactive video services, through video/audio networks, Internet Protocol (IP) networks, Asynchronous Transfer Mode (ATM) networks, Time Division Multiplexing (TDM)/Public Switched Telephone Networks (PSTN) networks, etc.  
         [0007]     In such a PON scheme, it is possible for users to share exchange equipment and optical resources. The optical distribution network is passive, so that maintenance costs are low and high flexibility is provided to allow a provider to easily add and delete splits. Furthermore, the PON scheme is advantageous in that costs per subscriber are low because intensive resource sharing is realized.  
         [0008]      FIG. 2  is a diagram showing the layered architectures of the OLT  103  and ONU  101  in a conventional EPON. The layered architectures shown in  FIG. 2  have been adapted in IEEE802.3ah standards.  
         [0009]     Referring to  FIG. 2 , the physical layers of the Open Systems Interconnection (OSI) reference model layers of the OLT  103  and the ONU  101  are constructed as described below. The OLT  103  and the ONU  101  are connected to a passive optical medium  210  through two Medium Dependent Interfaces (MDI)  211  and  241 , respectively. Two Physical Medium Dependents (PMDs)  212  and  242 , two Physical Medium Attachments (PMAs)  213  and  243 , and two Physical Coding Sublayers (PSCs)  214  and  244  are connected to two reconciliation sublayers  216  and  246  through two Gigabit Media Independent Interfaces (GMII)  215  and  245 , respectively. The reconciliation sublayers  216  and  246  are connected to two higher layers, that is, two data link layers, respectively.  
         [0010]     Each of the two data link layers is divided into one or more Medium Access Control (MAC) layers  220  or  250 , a multi-point MAC control layer  221  or  251 , one or more optionally added Operations, Administration &amp; Maintenance (OAM) layers  222  or  252 , and one or more Logical Link Control (LLC) layers  223  or  253 . The LLC layers  223  or  253  are connected to an upper layer  230  or  260 .  
         [0011]     In this case, the ONU  101  has a single MAC layer  250 . In contrast, a plurality of ONUs  101  are connected to the OLT  103 , so that the OLT  103  should accommodate independent MACs  220  the number of which corresponds to the number of connected ONUs  101 . When the branching ratio between the OLT  103  and the ONU  101  is 1:16, 1:32 or 1:64, the OLT  103  has 16, 32 or 64 MACs  220  according to the corresponding branching ratio.  
         [0012]     In general, the term “bridging” refers to a function of enabling data to be transmitted between networks by interconnecting the networks employing similar MAC protocols. Such bridging function is performed by the LLC layers  223  and  253  and the upper layers  230  and  260 .  
         [0013]     In the conventional layered architectures, the OLT  103  has independent MAC layers  220  and LLC layers  223 , the number of each of which corresponds to the number of ONUs  101  connected to the OLT  103 . In this case, a plurality of physical connection ports are required to connect the plurality of MAC layers  220  with the plurality of LLC layers  223 . However, a problem arises in that it is very difficult to form and match the plurality of independent physical ports.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a shared LAN emulation method and apparatus that provides a single matched interface between MAC layers and a LLC layer by allowing a bridging function to be performed in a layer lower than the LLC layer.  
         [0015]     Another object of the present invention is to provide a shared LAN emulation method and apparatus, which is capable of preventing a collision in the matching of bridging between EPON having a point-to-multipoint structure and IEEE 802.1D having a point-to-point structure.  
         [0016]     In order to accomplish the above object, the present invention provides a shared LAN emulation method, comprising the steps of setting up a Logical Link Identifier (LLID) management table for assigning unique LLIDs to a plurality of Optical Network Units (ONUs) and managing the assigned LLIDs so as to identify the plurality of ONUs connected to a single Optical Line Terminal (OLT); setting up a MAC address table for the LLIDs learning MAC addresses of the ONUs; assigning the unique LLIDs to ONUs when the ONUs request registration from the OLT; and bridging data frames, which are received by a Shared LAN Emulation (SLE) layer of the OLT, using the LLIDs, VIDs of Virtual LANs to which the ONUs belong and destination MAC addresses of the data frames so as to provide a single matched port between a Logical Link Control (LLC) layer and a MAC layer of the OLT.  
         [0017]     The LLID management table may include LLIDs for identifying the ONUs; ONU MAC addresses corresponding to the MAC addresses of the ONUs having requested connection from the OLT; VIDs for identifying virtual LANs of groups of ONUs to which the ONUs belong; authentication information fields for determining whether connection of the ONUs is allowed; connection setup information fields for determining whether the ONUs have been connected to the OLT and links have been set up therebetween; and subscriber MAC table pointer fields for the respective LLIDs, the subscriber MAC table point fields corresponding to pointer values of the MAC table, the MAC table being set up by learning the MAC addresses for the respective LLIDs.  
         [0018]     The MAC address table may include LLIDs, MAC addresses and VIDs.  
         [0019]     The step of assigning LLIDs may comprise the steps of if the ONUs having requested registration have been already registered and LLIDs having been already assigned to the ONUs exist, assigning the LLIDs having been already assigned to the ONUs having requested registration; if the ONUs having requested registration have been already registered but the LLIDs having been already assigned to the ONUs do not exist, assigning vailable LLIDs to the ONUs having requested registration; and if the ONUs having requested registration have been already registered but the LLIDs having been already assigned to the ONUs and the available LLIDs do not exist, assigning new dynamic LLIDs or LLIDs appointed by an operator to the ONUs having requested registration.  
         [0020]     The step of bridging the data frames may comprise the steps of, if each of the received data frames entering the SLE layer corresponds to a broadcast frame and have a VID, transmitting the received data frame to only an ONU group having a VID identical with the VID of the data frame; if the received frame corresponds to the broadcast frame entering the SLE layer from a layer higher than the SLE layer but do not have a VID, transmitting the data frame so that all the ONUs can receive packets; if the received frame corresponds to the broadcast frame entering the SLE layer from a layer lower than the SLE layer but do not have a VID, transmitting the data frame so that only an ONU having a source LLID cannot receive a packet; if the received frame has the VID of the destination LLID and but VID of the received frame is different from the VID of the destination LLID, discarding the received unicast frame; and if the received frame has the VID of the destination LLID and the VID of the received frame is identical with the VID of the destination LLID, setting the LLID of the frame to the destination LLID and transmitting the unicast frame. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0022]      FIG. 1  is a configuration diagram of an EPON system;  
         [0023]      FIG. 2  is a diagram showing the layered architectures of an OLT and an ONU in a conventional EPON;  
         [0024]      FIG. 3  is a diagram showing the layered architectures of an OLT  103  and an ONU in an EPON in accordance with an embodiment of the present invention;  
         [0025]      FIG. 4   a  is a diagram showing a shared LAN emulation apparatus in accordance with an embodiment of the present invention;  
         [0026]      FIG. 4   b  is a diagram showing the format of frames in which a LLID is included in a preamble when the frames are exchanged between the OLT and the ONUs;  
         [0027]      FIG. 5  shows a LLID management information table for respective ONUs, which is used in the shared LAN emulation apparatus according to the embodiment of the present invention;  
         [0028]      FIG. 6  is a MAC address table for respective LLIDs, which is used for bridging in the shared LAN emulation apparatus according to the embodiment of the present invention;  
         [0029]      FIG. 7  is a flowchart showing a process of assigning LLIDs to ONUs according to an embodiment of the present invention; and  
         [0030]      FIG. 8  is a flowchart showing a process of bridging an Ethernet frame at the SLE layer of the OLT according to an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.  
         [0032]      FIG. 3  is a diagram showing the layered architectures of an OLT  103  and an ONU  101  in an EPON in accordance with an embodiment of the present invention.  
         [0033]     Referring to  FIG. 3 , in the embodiment of the present invention, the OLT  103  further includes a Shared LAN Emulation (SLE) layer  310 , which is an optional layer of IEEE 802.3ah EMF standards, above a multipoint MAC control layer  221 . When optional OAM layers  222  are employed, the SLE layer  310  is located above the OAM layers  222 . A single LLC layer  320  is located above the SLE layer  310 . Meanwhile, the other layers and elements are the same as those of  FIG. 2 .  
         [0034]     Even when a plurality of ONUs  101  are connected to the OLT  103  using the shared LAN emulation method and apparatus in accordance with the embodiment of the present invention, the OLT  103  performs a bridging function at the data link layer thereof. That is, the OLT  103  performs the bridging function at a layer lower than the LLC layer  320  or layer higher than the MAC layers  220 . Furthermore, unlike the conventional OLT having a plurality of independent LLC layers, the OLT  103  has a single integrated LLC layer  320 . The plurality of independent MAC layers  220  formed in the OLT  103  are connected to the LLC layer  320  through the multipoint MAC control layer  221  and the single SLE layer  310 . When the optional OAM layers  222  are employed, the plurality of MAC layers  220  are connected to the LLC layer  320  sequentially through the multipoint MAC control layer  221 , the OAM layers  222  and the SLE layer  310 .  
         [0035]     A shared LAN emulation method of providing a single matched interface above the plurality of independent MAC layers  220 , internally performing a bridging function, and registering and managing a plurality of ONUs  101  in the SLE layer  310  of the embodiment of the present invention below.  
         [0036]      FIG. 4   a  shows a shared LAN emulation apparatus in accordance with an embodiment of the present invention.  
         [0037]     Referring to  FIG. 4   a , the shared LAN emulation apparatus according to the embodiment of the present invention includes a storage unit  410 , a Logical Link Identifier (LLID) management unit  420 , a bridging unit  430  and a control unit  440 , and is formed in the SLE layer  310  of the OLT  103 .  
         [0038]     The storage unit  410  stores a LLID management table for assigning different LLIDs to ONUs and managing the assigned LLIDs so as to identify a plurality of ONUs. The storage unit  410  further stores a MAC address table for the LLIDs learning the MAC addresses of the ONUs  101 . Although, in  FIG. 4   a , the LLID management table and the MAC address table are shown as being stored in the single storage unit  410 , the LLID management table and the MAC address table may be stored in respective storage units.  
         [0039]     The LLID management unit  420  is used to assign a unique LLID to an ONU  101  and manage the assigned LLID when the ONU  101  requests registration from the OLT  103 .  
         [0040]     The bridging unit  430  bridges data frames received by the MAC layers  220  using the LLIDs, the virtual LAN identifiers (VIDs) of certain groups to which the ONUs  101  belong, and the destination MAC addresses of the data frames so as to provide a single matched port to the LLC layer  320  of the OLT  103 .  
         [0041]     The control unit  440  controls the storage unit  410 , the LLID management unit  420  and the bridging unit  430 . Although in  FIG. 4   a , the control unit  440  is shown as being a separate block, the control unit  440  may be formed in the LLID management unit  420  or bridging unit  430 .  
         [0042]      FIG. 4   b  shows the format of frames in which a LLID is included in a preamble when the frames are exchanged between the OLT  103  and the ONUs  101 .  
         [0043]     Each of the data frames exchanged between the OLT  103  and the ONUs  101  includes a preamble/SFD field  451 , a destination address field  452 , a source address field  453 , a type/length field  454 , a data field  455  and a Frame Check Sequence (FCS) field  456 . The preamble/SFD filed  451  includes a SPD field  457 , an LLID field  458 , and a Cyclic Redundancy Check field  459 .  
         [0044]     When a frame is transmitted from the ONU  101  to the OLT  103 , the value of the LLID field  458  is representative of a source ONU  101 . In contrast, when a frame is transmitted from the OLT  103  to the ONU  101 , the value of the LLID field  458  is representative of a destination ONU  101 .  
         [0045]      FIG. 5  shows a LLID management information table for respective ONUs, which is used in the shared LAN emulation apparatus according to the embodiment of the present invention.  
         [0046]     Referring to  FIG. 5 , the LLID management information table required for the performance of the function of the SLE layer  310  according to the embodiment of the present invention includes indexes  510 , LLIDs  520 , ONU MAC addresses  530 , VIDs  540 , authentification information fields  550 , connection setup information fields  560  and subscriber MAC table pointer fields for respective LLIDs  570 .  
         [0047]     In this case, the maximal value of the indexes  510  is the maximal number of ONUs  101  connectable to a port of the OLT, and has different values of 16, 32, 64 and 128 according to the configurations of the system.  
         [0048]     Each of the LLIDs  520  is representative of a specific one of the plurality of ONUs  101  connected to the single OLT  103 , and is a logical link identifier or logical port number. When each of the ONUs  101  transmits the packet to the OLT  103  using the LLID information assigned by the OLT  103 , the ONU  101  transmits a packet with LLID information combined therewith. The values of the LLIDs  520  are different from the unique numbers of the ONUs  101  for identifying the plurality of ONUs  103  connected to a single port of the OLT  103  in that the values of the LLIDs  520  are logical port numbers for logically identifying the plurality of ONUs  101  connected to the single OLT  103 . Each of the LLIDs  520  has a unique value with respect to the ONUs  103 , and each of the logical port numbers has a unique value within the OLT  103 .  
         [0049]     The ONU MAC addresses  530  represent the addresses of the plurality of ONUs  101  connected to the OLT  103 , respectively. That is, the ONU MAC addresses  530  correspond to the MAC addresses of the ONUs  101  requesting connection from the OLT  103 .  
         [0050]     The VIDs  540  are a list of Virtual IDs (VIDs) for identifying groups of VLANs to which the ONUs  101  belong. Accordingly, in a frame transmitted from an ONU  101  connected to the OLT  103  is included a VID of a VLAN group to which the ONU  101  belongs. The VIDs  540  represent the values of such VIDs.  
         [0051]     The authentication information fields  550  are used to determine whether the connection of the ONUs  101  is allowed or not. This authentication information fields  550  indicate information on whether the connection of the ONU MAC layers  250  is allowed (Y) or not (N).  
         [0052]     The connection setup information fields  560  are used to determine whether the ONUs  101  have been connected to the OLT  103  and links have been set up therebetween. The connection setup information fields  560  indicate information whether the ONUs  101  having specific ONU MAC addresses are connected to the OLT  103 .  
         [0053]     The subscriber MAC table pointer fields  570  for the respective LLIDs are composed of pointer values for the MAC table. When logical ports are distinguished for the respective ONUs  101  connected to the OLT  103 , the subscriber MAC table pointer fields  570  for the respective LLIDs are provided to learn the subscriber MAC addresses for the respective LLIDs. That is, subscriber MAC table pointer fields  570  for the respective LLIDs corresponds to points for the MAC table set up for the respective LLIDs.  
         [0054]      FIG. 6  is a MAC address table for respective LLIDs, which is used for bridging in the shared LAN emulation apparatus according to the embodiment of the present invention.  
         [0055]     Referring to  FIG. 6 , the MAC address table learned for the. respective LLIDs according to the embodiment of the present invention includes LLIDs  610 , MAC addresses  620  and VIDs  630 .  
         [0056]     Referring to  FIG. 7 , a process of the OLT  103  assigning the LLIDs to the ONUs  101  according to an embodiment of the present invention is described below. In the assignment of the LLIDs, available LLIDs are previously provided to the ports of the OLT  103 , so that the available LLIDs can be dynamically assigned. A specific LLID having been assigned to a specific ONU  101  is always assigned only to the same ONU  101 , so that a specific LLID is used as a value representative of a specific ONU  101 . The process of assigning the LLIDs is described in more detail below.  
         [0057]     At step  701 , an ONU  101  requests registration from the OLT  103  to connect with the OLT  103 .  
         [0058]     Then, at step  702 , the OLT  103  determines whether the ONU  101  having requested the registration has been registered. If, as a result of the determination at step  702 , the ONU  101  has been already registered, the process proceeds to step  703 . If the ONU  101  has not been registered, the process proceeds to step  710 .  
         [0059]     At step  703 , it is determined whether a LLID assigned to the ONU  101  exists because the ONU  101  has been already registered. If, as a result of the determination at step  703 , the assigned LLID exists, the process proceeds to step  704 . If the assigned LLID does not exist, the process proceeds to step  705 .  
         [0060]     At step  704 , the assigned LLID is assigned to the ONU  101  because the ONU  101  has been already registered and the LLID was assigned to the ONU  101 .  
         [0061]     Step  705  is performed in the state where the ONU  101  having requested the registration has been already registered but the LLID assigned to the ONU  101  does not exist. Accordingly, the OLT  103  determines whether available LLIDs exist at step  705 . If, as a result of the determination, the available LLIDs exist, the process proceeds to step  706 . If the available LLIDs do not exist, the process proceeds to step  707 .  
         [0062]     At step  706 , the OLT  103  assigns one of the available LLIDs to the ONU  101  because the available LLIDs exist as a result of the determination at step  705 .  
         [0063]     At step  707 , it is determined whether a new LLID can be dynamically assigned because the available LLID does not exist as a result of the determination at step  705 . If, as a result of the determination at step  707 , the new LLID can be dynamically assigned to the ONU  101 , the process proceeds to step  708 . If the new LLID cannot be dynamically assigned to the ONU  101 , the process proceeds to step  709 .  
         [0064]     At step  708 , the OLT  103  dynamically assigns the new LLID to the ONU  101  because the new LLID can be dynamically assigned to the ONU  101  as a result of the determination at step  707 .  
         [0065]     At step  709 , the OLT  103  is provided with a separate LLID by an operator and assigns the separate LLID to the ONU  101  because the new LLID cannot be dynamically assigned to the ONU  101  as a result of the determination at step  707 .  
         [0066]     At step  710 , the OLT  103  receives information on whether the registration of the ONU  101  is allowed from the operator because the ONU  101  has not been registered as a result of the determination at step  702 . If, as a result of the determination at step  710 , the registration of the ONU  101  is allowed, the process proceeds to step  705 . If the registration of the ONU  101  is not allowed, the process proceeds to step  711 .  
         [0067]     If, as a result of the determination at step  710 , the registration of the ONU  101  is allowed, the process performs steps  705  to  709 . That is, it is determined whether available LLIDs exist at step  705 . If the available LLIDs exist, one of the available LLIDs is assigned to the ONU  101  at step  706 . In contrast, if the available LLIDs do not exist, it is determined whether a new LLID can be dynamically assigned at step  707 . If the new LLID can be dynamically assigned, the new LLID is assigned to the ONU  101  at step  708 . In contrast, if the new LLID cannot be dynamically assigned, a separate LLID appointed by the operator is assigned to the ONU  101  at step  709 .  
         [0068]     At step  711 , the OLT  103  transmits a negative registration response to the ONU  101  having requested the registration because the registration of the ONU  101  is not allowed as a result of the determination at step  710 .  
         [0069]     The LLIDs assigned to the ONUs  101  connected to the OLT  103  as the logical port numbers thereof are managed or appointed by the operator.  
         [0070]     When the unique LLIDs are assigned to all the ONUs  101  connected to the OLT  103  as described above, the LLIDs are widely used to identify all the ONUs connected to a single OLT  103  rather than identifying ONUs  101  connected to a single port of a single OLT  103 . Furthermore, the LLIDs assigned as described above together with the ONU MAC addresses are used to update and refer to the information of the subscriber MAC table pointer field for respective LLIDs  570  shown in  FIG. 5 .  
         [0071]      FIG. 8  is a flowchart showing a process of bridging an Ethernet frame at the SLE layer  310  of the OLT  103  according to an embodiment of the present invention. The process of bridging an Ethernet frame is performed by the bridging unit  430  located in the SLE layer  310  of the OLT  103 .  
         [0072]     Referring to  FIG. 8 , a process of transmitting an Ethernet frame in the SLE layer  310  using a LLID, a VID and a destination Ethernet address is described in detail. In this case, a frame entering the SLE layer  310  from a layer higher than the SLE layer  310  is defined as an external frame. Additionally, a frame entering the SLE layer  310  from a layer lower than the SLE layer  310  is defined as an internal frame.  
         [0073]     At step  801 , a new frame reaches the SLE layer  310  of the OLT  103 .  
         [0074]     At step  802 , it is determined whether the frame received by the SLE layer  310  corresponds to an external frame. If, as a result of the determination at step  802 , the received frame corresponds to the external frame, the process proceeds to step  803 . If the received frame does not correspond to the external frame, the process proceeds to step  803 .  
         [0075]     At step  803 , it is determined whether the external frame corresponds to a broadcast frame. In this case, the term “broadcast frame” refers to a frame used to transmit a single data packet to an entire network. In this sense, the broadcast frame is distinguished from a unicast frame that is used to transmit a copy of data to each recipient. If the received frame corresponds to the broadcast frame, the process proceeds to step  804 . If the received frame does not correspond to the broadcast frame, the process proceeds to step  807 .  
         [0076]     At step  804 , it is determined whether a VID exists in the received frame because the received frame corresponds to the broadcast frame as a result of the determination at step  803 . If the VID exists in the received frame, the process proceeds to step  805 . If the VID does not exist in the received frame, the process proceeds to step  806 .  
         [0077]     Step  805  is performed when the received frame corresponds to both the external frame and the broadcast frame and has the VID. Accordingly, in this case, setting is performed so that only ONUs belonging to an ONU group with the same VID and having designated LLIDs can receive packets (mode=0, llid=LLIDs having same VID assigned), and the received frame is sequentially transmitted. In this case, ‘mode=0’ indicates a unicast transmission method.  
         [0078]     Step  806  is performed when the received frame corresponds to both the external frame and the broadcast frame but does not have the VID. Accordingly, in this case, setting (mode=1, llid=Broadcast_LLID assigned) is performed so that all the ONUs can receive packets, and the received frame is transmitted. In this case, ‘mode=1’ designates a broadcasting transmission method.  
         [0079]     Step  807  is performed when the received frame does not correspond to the broadcast frame as a result of the determination at step  803 . In this case, the bridging unit  430  determines whether a destination MAC address and a LLID exist by searching the MAC table stored in the storage unit  410 . Accordingly, using the destination address included in the received frame, it can be learned that the frame is transmitted to which of the ONUs  101 , and a LLID representative of the ONU  101  can be assigned. The MAC table is set up by referring to source addresses. If a frame under examination does not have a destination address contained in the MAC table, this frame corresponds to the broadcast frame. If the frame has the destination address, the frame corresponds to the unicast frame.  
         [0080]     At step  808 , using the results obtained by the search of the MAC table, it is determined whether the destination LLID exists in the MAC table. If the destination LLID exists in the MAC table, steps  804  to  806  are performed. If not, the process proceeds to step  809 .  
         [0081]     If the received external frame does not correspond to the broadcast frame and doesn&#39;t have the destination LLID, it is determined whether a VID exists in the received frame at step  804 . If the VID exists in the received frame, setting (mode=0, llid=LLIDs with same VID assigned) is performed so that only an ONU belonging to a VLAN group with the same VID and having a designated LLID can receive a packet, and the received frame is transmitted, at step  805 . In contrast, if the VID does not exist in the received frame, setting (mode=1, llid=Broadcast_LLID assigned) is performed so that all the ONUs can receive packets, and the received frame is transmitted, at step  806 .  
         [0082]     Step  809  is performed when the received external frame does not correspond to the broadcast frame but has the destination LLID. In this case, it is determined whether a VID exists in the received frame. If the VID exists in the received frame, the process proceeds to step  810 . If the VID does not exist in the received frame, the process proceeds to step  812 .  
         [0083]     At step  810 , it is determined whether the VID of the frame is identical with the VID of the destination LLID because the received frame has the VID of the destination LLID. If the VID of the received frame is not identical with the VID of the destination LLID, the process proceeds to step  811 . If the VID of the received frame is identical with the VID of the destination LLID, the process proceeds to step  812 .  
         [0084]     Step  811  is performed when the received frame has the VID of the destination LLID but the VID of the received frame is different from the VID of the destination LLID. In this case, the received unicast frame is discarded. The reason for this is to allow communication between the ONUs  101  of an ONU group having the identical VID and prevent the leakage of information to a LAN group having a different VID.  
         [0085]     Step  812  is performed when the received frame has the VID of the destination LLID and the VID of the frame is identical with the VID of the destination LLID. In this case, the LLID of the frame is set to the destination LLID (mode=0, llid=destination_LLID assigned) and, thereafter, the unicast frame is transmitted. With this step, a bridging function is performed so that the received frame is bridged to the destination ONU  101 .  
         [0086]     Subsequent steps  813  to  822  process frames received by the SLE layer  310  in a manner similar to that of steps  803  to  812 .  
         [0087]     At step  813 , it is determined whether the received frame corresponds to a broadcast frame because the received frame corresponds to the internal frame. If the received frame corresponds to the broadcast frame, the process proceeds to step  814 . If the received frame does not correspond to the broadcast frame, the process proceeds to step  817 .  
         [0088]     At step  814 , it is determined whether a VID exists in the received frame because the received frame corresponds to the broadcast frame as a result of the determination at step  813 . If the VID exists in the received frame, the process proceeds to step  815 . If the VID does not exist in the received frame, the process proceeds to step  816 .  
         [0089]     Step  815  is performed when the received frame corresponds to both the internal frame and the broadcast frame and has the VID. Accordingly, in this case, setting is performed so that only ONUs belonging to an ONU group with the same VID and having designated LLIDs can receive packets (mode=0, llid=LLIDs with same VID assigned), and the received frame is sequentially transmitted.  
         [0090]     Step  816  is performed when the received frame corresponds to both the internal frame and the broadcast frame but does not have the VID. Accordingly, in this case, setting (mode=1, llid=source_LLID assigned) is performed so that only a source ONU cannot receive a packet, and the received frame is transmitted.  
         [0091]     Step  817  is performed when the received frame does not correspond to the broadcast frame as a result of the determination at step  813 . In this case, it is determined whether a destination MAC address and a LLID exist by searching the MAC table stored in the storage unit  410 .  
         [0092]     At step  818 , using the results obtained by the search of the MAC table, it is determined whether the destination LLID exists in the MAC table. If, as the result of the determination at step  818 , the destination LLID exists in the MAC table, steps  814  to  816  are performed. If not, the process proceeds to step  819 .  
         [0093]     That is, if the received frame does not correspond to the broadcast frame but has the destination LLID, it is determined whether a VID exists in the received frame at step  814 . If the VID exists in the received frame, setting is performed so that only ONUs belonging to an ONU group with the same VID and having designated LLIDs can receive packets (mode=0, llid=LLIDs with same VID assigned), and the received frame is sequentially transmitted. In contrast, if the VID does not exist in the received frame, setting (mode=1, llid=source_LLID assigned) is performed so that only a source ONU cannot receive a packet, and the received frame is transmitted, at step  816 .  
         [0094]     Step  819  is performed when the received internal frame does not correspond to the broadcast frame but has the destination LLID. In this case, it is determined whether a VID exists in the received frame. If the VID exists in the received frame, the process proceeds to step  820 . If the VID does not exist in the received frame, the process proceeds to step  822 .  
         [0095]     At step  820 , it is determined whether the VID of the frame is identical with the VID of the destination LLID because the received frame has the VID of the destination LLID. If the VID of the received frame is not identical with the VID of the destination LLID, the process proceeds to step  821 . If the VID of the received frame is identical with the VID of the destination LLID, the process proceeds to step  822 .  
         [0096]     Step  821  is performed when the received frame has the VID of the destination LLID but the VID of the received frame is different from the VID of the destination LLID. In this case, the received unicast frame is discarded. The reason for this is to allow communication between the ONUs  101  of an ONU group having the identical VID and prevent the leakage of information to a LAN group having a different VID.  
         [0097]     Step  822  is performed when the received frame has the VID of the destination LLID and the VID of the frame is identical with the VID of the destination LLID. In this case, the LLID of the frame is set to the destination LLID (mode=0, llid=destination_LLID assigned) and, thereafter, the unicast frame is transmitted. With this step, a filtering function is performed so that the received frame is bridged to the destination ONU  101 .  
         [0098]     As described above, the present invention is advantageous in that the SLE layer corresponding to a layer higher than the MAC layers of the OLT in EPON is provided, and a procedure of assigning LLIDs corresponding to the logical port numbers of respective ONUs to the ONUs and a table for managing the assigned LLIDs are provided, so that functions confined to EPON having a point-to-multipoint architecture are processed at the SLE layer and interface with the upper layer is processed by a single matched interface compatible with IEEE 802.1D.  
         [0099]     The present invention is advantageous in that a physical port can be easily implemented between the MAC layers and LLC layer of an OLT in EPON.  
         [0100]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.