Ethernet passive optical network and point-to-point emulation method

CLAIM OF PRIORITY

P-0001[0001] This application claims priority under 35 U.S.C. §119 to an application entitled, “Ethernet—Passive Optical Network and Point-to-Point Emutation Method”, filed in the Korean Industrial Property Office on Apr. 3, 2002 and assigned Serial No. 2002-18267, and an application entitled “Ethernet—Passive Optical Network and Point-to-Point Emulation Method” filed in the Korean Intellectual Property Office on Apr. 30, 2002 and assigned Serial No. 2002-23824, the contents of both being incorporated herein by reference.

BACKGROUND OF THE INVENTION

P-0002[0002] 1. Field of the Invention

P-0003[0003] The present invention relates generally to an optical network, and in particular, to an Ethernet passive optical network.

P-0004[0004] 2. Description of the Related Art

P-0005[0005] A passive optical network is a subscriber network that forms a tree-structured distributed topology by connecting a plurality of optical network units (ONUs) to one optical line terminal (OLT) using a 1×N optical distribution network (ODN). The ITU-T (International Telecommunications Union—Telecommunication Standardization Sector) has recently published ITU-T G982, ITU-T G983.1 and ITU-T G983.3, all of which describe an asynchronous transfer mode-passive optical network (ATM-PON) standard. In addition, IEEE 802.3ah TF organized by IEEE (Institute of Electrical and Electronics Engineers) has been working toward standardization for a gigabit Ethernet-based passive optical network system.

P-0006[0006] In ATM-PON and Ethernet passive optical network systems discussed in the standardization organizations such as ITU-T and IEEE 802.3, transmission capacity depends on a format of data carried on two different wavelengths between an OLT and an ONU. That is, the international standardization organizations of ITU-T and IEEE 802.3 have been discussing a transmission method in which an ATM cell or an Ethernet frame is carried on a 1,550 μm- or 1,490 nm-wavelength signal for downstream transmission from an OLT of the telephone office side to an ONU of the subscriber side, and data is carried on a 1,310 nm-wavelength signal for upstream transmission from an ONU of the subscriber side to an OLT of the telephone office side.

P-0007[0007] Standardization of a point-to-point gigabit Ethernet and a medium access control (MAC) technology for ATM-PON has already been completed and is disclosed in IEEE 802.3z and ITU-T G.983.1. MAC technology for ATM-PON is also discussed in U.S. Pat. No. 5,978,374, issued to Ghaibeh et al., on Nov. 2, 1999, entitled “Protocol for Data Communication over a Point-to-Multipoint Passive Optical Network.”

P-0008[0008]FIG. 1 illustrates a known configuration of an 802.1D bridge in an Ethernet system. Referring to FIG. 1, since, an Ethernet medium is a shared medium in accordance with 802.1D, an Ethernet frame transmitted from a subscriber terminal belonging to a particular region connected to a bridge is transmitted to all subscriber terminals in other regions connected to the bridge.

P-0009[0009] To reduce traffic and consequent collisions, the bridge can be subject to a table-based learning process; Upon receiving a frame, the bridge stores in an address table a source address of the received frame and an identifier of the port from which the frame was received. Through this process, the bridge progressively learns which addresses are assigned to respective ports. Upon receiving a frame after the learning, the bridge transmits the received frame only to the port assigned a destination address of the frame, and does not transmit the received frame to other ports, thereby mitigating the advent of collision in the shared medium.

P-0010[0010] If the bridge fails to detect a destination port, i.e., either the received frame has no destination or a port for a destination address of the received frame has never been learned, the bridge transmits the received frame to all ports except a source port from which the frame is received. Transmission to the source port is unnecessary, since terminals on the source port have already received the frame on a “broadcast and select” basis in light of characteristics of the Ethernet. Likewise, if the address table has a destination port for the received frame, but the destination port is the same as the source port, the bridge discards the frame. Thus, for example, the bridge in FIG. 1 discards an Ethernet frame from subscriber terminal A to subscriber terminal B since both terminals belong to the same region and are therefore connected to the bridge by the same port.

P-0011[0011]FIG. 2 illustrates a known Ethernet—passive optical network with an 802.1D L2 (Layer 2) switch, which can, in conjunction with a L3 (Layer 3) router, be utilized in place of an 802.1D bridge according to the prior art, and FIG. 3 illustrates a format of a standard Ethernet frame. The Ethernet passive optical network includes an optical line terminal (OLT) <b>110</b>, an optical distribution network (ODN) <b>120</b>, and optical network units (ONUs) <b>131</b>-<b>133</b>.

P-0012[0012] The OLT <b>110</b> accesses data of each of the ONUs <b>131</b>-<b>133</b> by time division multiplexing (TDM). The OLT <b>110</b> includes an L2 switch, and transmits frames received from the ONUs <b>131</b>-<b>133</b> by matching corresponding addresses to corresponding ports. In a learning process that occurs during initialization, the L2 switch learns MAC addresses assigned to respective ports while storing in an address table a source address (SA) of a frame received at a particular port. In this case, MAC addresses of the ONUs <b>131</b>-<b>133</b> are used as the source addresses or the destination addresses (DA). Upon receiving a frame after the address learning process, the L2 switch transmits the received frame only to the port where a destination address of the frame is assigned, using a static filtering entry (see IEEE 802.1d clause 7.9.1).

P-0013[0013] As with the bridge of the previous example, the switch discards any frame whose source port is identical to its destination port. Therefore, any frame whose destination port is identical to its source port is discarded by the bridge or L2 switch without being transmitted downstream, even if that frame must be subject to upstream transmission to the bridge or L2 switch and subsequent downstream transmission through the same port.

SUMMARY OF THE INVENTION

P-0014[0014] The present invention provides for an Ethernet passive optical network configured with an L2 switch or bridge an Ethernet frame format that supports subscriber-to-subscriber communication, multiple subscribers or multiple services and selectively supports QoS (Quality of Service). In another aspect, the present invention features a point-to-point emulation method for use with the inventive frame format on the network.

P-0015[0015] A point-to-point emulation method for operating an Ethernet passive optical network in accordance with the present invention has one optical line terminal (OLT), a plurality of optical network units (ONUs) each connected to the OLT. A tag or augmented Ethernet frame transmitted between the OLT and the ONUs includes a destination address (DA) field representing a destination address, a source address (SA) field representing a source address, and an LLID field in which a unique LLID assigned to an object of the ONU side is recorded. According to the method, LLIDs are assigned on a one-to-one basis to ports of a bridge or an L2 (Layer 2) switch or similar structure of the OLT side. Upon receiving the augmented Ethernet frame, the OLT outputs an LLID field-deleted Ethernet frame to the bridge or L2 switch port to which the unique LLID is assigned. The bridge or the L2 switch transmits the Ethernet frame to the destination port to which a previously learned destination address is assigned. The OLT regenerates an augmented Ethernet frame by inserting into the LLID field-deleted Ethernet frame an LLID field in which an LLID of a destination ONU is recorded. The regenerated augmented Ethernet frame is transmitted to the destination ONU.

BRIEF DESCRIPTION OF THE DRAWINGS

P-0016[0016] The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which same or similar elements are denoted by the same reference numerals throughout the several views:

P-0017[0017]FIG. 1 illustrates a relationship between the 802.3 Ethernet protocol and an 802.1D bridge according to the prior art;

P-0018[0018]FIG. 2 illustrates an Ethernet passive optical network with an L2 switch according to the prior art;

P-0019[0019]FIG. 3 illustrates a format of a standard Ethernet frame;

P-0020[0020]FIG. 4 illustrates an exemplary format of a tag Ethernet frame according to a first embodiment of the present invention;

P-0021[0021]FIG. 5 illustrates an exemplary format of a tag Ethernet frame according to a second embodiment of the present invention;

P-0022[0022]FIG. 6 illustrates an example of a detailed format of the LLID field <b>240</b> shown in FIGS. 4 and 5;

P-0023[0023]FIG. 7 is a block diagram illustrating a structure of an Ethernet PON according to a third embodiment of the present invention;

P-0024[0024]FIG. 8 illustrates a procedure for transmitting an Ethernet frame in a downstream direction in the Ethernet PON of FIG. 7;

P-0025[0025]FIG. 9 illustrates a procedure for handling an upstream Ethernet frame received at the OLT in the Ethernet PON of FIG. 7;

P-0026[0026]FIG. 10 illustrates a procedure for transmitting an Ethernet frame in an upstream direction in the Ethernet PON of FIG. 7;

P-0027[0027]FIG. 11 illustrates a procedure for handling a downstream Ethernet frame received at the ONU in the Ethernet PON of FIG. 7;

P-0028[0028]FIG. 12 is a flowchart illustrating procedures for transmitting/receiving upstream/downstream Ethernet frames in the Ethernet PON of FIG. 7; and

P-0029[0029]FIG. 13 is a block diagram illustrating a structure of an Ethernet PON according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

P-0030[0030] Several preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings.

P-0031[0031]FIG. 4 illustrates a format of a tag Ethernet frame according to a first embodiment of the present invention. The tag Ethernet frame includes a 6-byte destination address (DA) field <b>210</b> indicating a destination address, a 6-byte source address (SA) field <b>220</b> indicating a source address, a 2-byte length/Ethernet type (LEN/ETYPE) field <b>250</b>, a data field <b>260</b>, and a frame check sequence (FCS) field <b>270</b> which constitute the existing or conventional Ethernet frame. Typically, MAC (Medium Access Control) addresses are used as the destination address and as the source address.

P-0032[0032] The tag Ethernet frame additionally includes an Ethernet type (ETYPE) field <b>230</b> and a logical link identifier (LLID) field <b>240</b>, interposed between the source address field <b>220</b> and the length/Ethernet type field <b>250</b>. The Ethernet type field <b>230</b> and the LLID field <b>240</b> are inserted using a tagging method when downstream transmission to the ONU side or upstream transmission to the OLT side is performed. The LLID represents a unique identifier (ID) assigned to an object of the ONU side, where each ONU is an object and, likewise, each of one or more service ports or subscriber ports connected to the ONUs is an object. The LLIDs are assigned to ports of a bridge or an L2 switch of the OLT side on a one-to-one basis. The LLID field <b>240</b> records the LLID assigned to the sending port of the bridge or the L2 switch, the same LLID that has been assigned to the destination object, during downstream transmission. During upstream transmission, the LLID field <b>240</b> records the LLID of the source object on the ONU side, the same LLID that has been assigned to the receiving port of the bridge or L2 switch. The Ethernet type field <b>230</b> records a new Ethernet type to distinguish the tag Ethernet frame from the existing Ethernet frame. The tag Ethernet frame is compatible with the existing system, since it can be transmitted to an upper layer after deletion of the LLID field <b>240</b> and the Ethernet type field <b>230</b>.

P-0033[0033] To deliver the tag Ethernet frame of FIG. 4 to an upper layer at the source or destination, the LLID field <b>240</b> is deleted and the Ethernet type field <b>230</b> is changed to one of previously defined Ethernet frame types. That is, the Ethernet type field <b>230</b> is so compositely defined as to indicate the existing Ethernet frame type and the tag Ethernet frame.

P-0034[0034]FIG. 5 illustrates a format of a tag Ethernet frame according to a second embodiment of the present invention, which differs from the first embodiment in that the Len/Etype field <b>250</b> is not implemented. The two bytes thereby conserved compensate for storage utilized in augmenting the Ethernet frame with an Ethernet type (ETYPE) field <b>230</b> and a logical link identifier (LLID) field <b>240</b>.

P-0035[0035]FIG. 6 illustrates an example of a detailed format of the LLID field <b>240</b> shown in FIGS. 4 and 5. If it is assumed that the LLID field <b>240</b> is comprised of 2 bytes, the LLID field <b>240</b> may include a 3-bit class <b>242</b>, a 1-bit mode <b>244</b>, and a 12-bit LLID <b>246</b>. The LLID <b>246</b> represents a unique ID assigned during registration of an object of the ONU side, i.e., a new ONU, a subscriber port or classified service port.

P-0036[0036] The class <b>242</b> represents a class determined according to SLA (Service Level Agreement) for a subscriber or a type of multiple services supported. Priority can be assigned according to the class <b>242</b>. For example, priority can be given to a queue of a corresponding port of a bridge or an L2 switch of the OLT side, or top priority can be given to an object on the ONU side during frequency assignment, in order to guarantee high QoS (Quality of Service) while supporting multiple services. Class <b>242</b> can also be classified according to a class of SLA for the subscriber. The mode <b>244</b> distinguishes a point-to-point link mode P for a transmission frame from a point-to-multipoint broadcast mode B.

P-0037[0037]FIG. 7 is a block diagram illustrating a structure of an Ethernet PON according to a third embodiment of the present invention. Referring to FIG. 7, the Ethernet PON is comprised of one OLT <b>310</b> and a plurality of ONUs <b>340</b>. The OLT <b>310</b> includes an 802.3 physical (PHY) layer <b>311</b>, an 802.3 MAC layer <b>312</b>, a bridge <b>330</b>, and an LLID multiplexer/demultiplexer (MUX/DEMUX) layer <b>320</b>. The bridge <b>330</b> includes ports <b>331</b>-<b>334</b> assigned to the ONUs <b>340</b> on a one-to-one basis for point-to-point emulation, and a normal port <b>335</b> for handling the existing Ethernet frame rather than the tag Ethernet frame. The number of ports of the bridge <b>330</b> and the number of the ONUs <b>340</b> are variable, where if the number of ONUs <b>340</b> is n, the number of ports of the bridge <b>330</b> is n+1.

P-0038[0038] The LLID MUX/DEMUX layer <b>320</b> inserts, into an existing Ethernet frame, an Ethernet type field in which a tag Ethernet frame type is recorded and an LLID field in which LLID assigned to a corresponding port of the bridge <b>330</b> is recorded, during downstream transmission to the ONUs <b>340</b>. The LLID MUX/DEMUX layer <b>320</b> also determines whether an Ethernet frame received from the ONUs <b>340</b> is a tag Ethernet frame.

P-0039[0039] Each of the ONUs <b>340</b> includes an <b>802</b>.<b>3</b> PHY layer <b>341</b>, an <b>802</b>.<b>3</b> MAC layer <b>342</b>, an LLID MUX/DEMUX layer <b>350</b>, and a logical link control (LLC) layer <b>360</b>. The LLID MUX/DEMUX layer <b>350</b> checks an Ethernet type field of an Ethernet frame received from the OLT <b>310</b>. If the Ethernet type field indicates a tag Ethernet frame, the LLID MUX/DEMUX layer <b>350</b> checks an LLID field and transmits the Ethernet frame to the LLC layer <b>360</b> only if the LLID is identical to its own LLID, and discards the other frames whose LLIDs are not identical to its own LLID. During upstream transmission to the OLT <b>310</b>, the LLID MUX/DEMUX layer <b>350</b> inserts, into the existing Ethernet frame, an Ethernet type field in which a tag Ethernet type is recorded and an LLID field in which its own LLID is recorded, to thereby convert the existing Ethernet frame into the tag Ethernet frame.

P-0040[0040]FIG. 8 illustrates a procedure for transmitting an Ethernet frame in a downstream direction in the Ethernet PON of FIG. 7. Upon receiving an Ethernet frame, the bridge <b>330</b> of the OLT <b>310</b> analyzes a destination address of the received Ethernet frame, and provides the Ethernet frame to the LLID MUX/DEMUX layer <b>320</b> through a port <b>331</b>-<b>335</b> of the bridge <b>330</b> to which the destination address is assigned. The LLID MUX/DEMUX layer <b>320</b> converts an existing Ethernet frame to a tag Ethernet frame by inserting into the existing Ethernet frame an Ethernet type field in which a tag Ethernet type is recorded and an LLID field in which an LLID assigned to a corresponding port of the bridge <b>330</b>, at which the Ethernet frame is received, is recorded.

P-0041[0041]FIG. 9 illustrates a procedure for handling an upstream Ethernet frame received at the OLT in the Ethernet PON of FIG. 7. The upstream Ethernet frame is applied to the LLID MUX/DEMUX layer <b>320</b> through the 802.3 PHY layer <b>311</b> and the 802.3 MAC layer <b>312</b>. If the tag Ethernet frame type was recorded in the Ethernet type field of a received Ethernet frame, the LLID MUX/DEMUX layer <b>320</b> determines a corresponding port of the bridge <b>330</b> by analyzing the LLID field, deletes the Ethernet type field and the LLID field, and recalculates FCS, thereby restoring the received Ethernet frame to an existing Ethernet frame. The LLID MUX/DEMUX layer <b>320</b> provides the restored or regenerated existing Ethernet frame to the bridge <b>330</b> through the corresponding port <b>331</b>-<b>334</b>. In contrast, however, if the existing Ethernet frame type was recorded in the Ethernet type field of a received Ethernet frame, the LLID MUX/DEMUX layer <b>320</b> delivers the Ethernet frame intact to the bridge <b>330</b> through the normal port <b>335</b>.

P-0042[0042]FIG. 10 illustrates a procedure for transmitting an Ethernet frame in an upstream direction in the Ethernet PON of FIG. 7. If the LLC layer <b>360</b> transmits an Ethernet frame to the LLID MUX/DEMUX layer <b>350</b>, the LLID MUX/DEMUX layer <b>350</b> inserts, into the existing Ethernet frame, an Ethernet type field in which the tag Ethernet frame type is recorded and an LLID field in which its own LLID is recorded.

P-0043[0043]FIG. 11 illustrates a procedure for handling a downstream Ethernet frame received at the ONU in the Ethernet PON of FIG. 7. The downstream Ethernet frame is applied to the LLID MUX/DEMUX layer <b>350</b> through the 8023 PHY layer <b>341</b> and the 802.3 MAC layer <b>342</b> of the ONU <b>340</b>. The LLID MUX/DEMUX layer <b>350</b> first examines the Ethernet type field. As a result of the examination, if the Ethernet type field indicates a tag Ethernet frame, the LLID MUX/DEMUX layer <b>350</b> analyzes the LLID field, to thereby transmit to the LLC layer <b>360</b> the Ethernet frame only if the LLID is identical to its own LLID and to otherwise discard the frame. The LLID MUX/DEMUX layer <b>350</b> also restores the existing Ethernet frame by deleting the Ethernet type field and the LLID field and recalculating FCS, and then transmits the restored Ethernet frame to the LLC layer <b>360</b>. In contrast, however, if the existing Ethernet frame type was recorded in the Ethernet type field of a received Ethernet frame, the LLID MUX/DEMUX layer <b>350</b> transmits the Ethernet frame intact to the LLC layer <b>360</b>.

P-0044[0044]FIG. 12 is a flowchart illustrating procedures for transmitting/receiving upstream/downstream Ethernet frames based on the structures depicted in FIGS. <b>8</b> to <b>11</b>, and specifically for the case in which a particular ONU transmits an Ethernet frame to another ONU in the Ethernet PON of FIG. 7. Reference numerals <b>410</b>, <b>420</b>, <b>430</b> and <b>440</b> denote processes that correspond to the above discussion of FIGS. <b>8</b> to <b>11</b>, respectively.

P-0045[0045] If the LLC layer <b>360</b> of the ONU <b>340</b> transmits an Ethernet frame to the LLID MUX/DEMUX layer <b>350</b> (Step <b>431</b>), the LLID MUX/DEMUX layer <b>350</b> inserts, into an existing Ethernet frame, an Ethernet type field in which a tag Ethernet frame type is recorded and an LLID field in which its own LLID is recorded (Step <b>432</b>). The LLID MUX/DEMUX layer <b>350</b> then transmits a tag Ethernet frame to the OLT <b>310</b> in an upstream direction.

P-0046[0046] Upon receiving the tag Ethernet frame (Step <b>421</b>), the LLID MUX/DEMUX layer <b>320</b> of the OLT <b>310</b> determines whether a tag Ethernet frame type is recorded in the Ethernet type field (Step <b>422</b>). If so, the LLID MUX/DEMUX layer <b>320</b> determines a corresponding port of the bridge <b>330</b> by examining the LLID field (Step <b>423</b>). The LLID MUX/DEMUX layer <b>320</b> thereafter deletes the Ethernet type field and the LLID field from the Ethernet frame, recalculates FCS (Step <b>424</b>), and then transmits the resulting Ethernet frame to the bridge <b>330</b> through a corresponding port.

P-0047[0047] After receiving the Ethernet frame (Step <b>411</b>), the bridge <b>330</b> determines a destination address of the Ethernet frame and transmits the Ethernet frame to the LLID MUX/DEMUX layer <b>320</b> through the particular one of the ports <b>331</b>-<b>335</b> that corresponds to the destination address (Step <b>412</b>). The LLID MUX/DEMUX layer <b>320</b> inserts, into the existing Ethernet frame, an Ethernet type field in which a tag Ethernet frame type is recorded and an LLID field into which is recorded the LLID assigned to that particular port (Step <b>413</b>). The resulting Ethernet frame is then transmitted to the ONU <b>340</b> in a downstream direction.

P-0048[0048] Upon receiving the Ethernet frame from the OLT <b>310</b> (Step <b>441</b>), the LLID MUX/DEMUX layer <b>350</b> of the ONU <b>340</b> determines whether a tag Ethernet frame type is recorded in the Ethernet type field (Step <b>442</b>). If so, the LLID MUX/DEMUX layer <b>350</b> analyzes an LLID field to determine if the LLID is identical to its own LLID. If they are identical, the LLID MUX/DEMUX layer <b>350</b> deletes the Ethernet type field and the LLID field and recalculates FCS (Step <b>443</b>). The LLID MUX/DEMUX layer <b>350</b> then transmits the resulting Ethernet frame to the LLC layer <b>360</b>. However, the LLID MUX/DEMUX layer <b>250</b> discards a tag Ethernet frame whose LLID is not identical to its own LLID (Step <b>444</b>).

P-0049[0049]FIG. 13 is a block diagram illustrating a structure of an Ethernet PON according to a second embodiment of the present invention. The Ethernet PON is comprised of one OLT <b>510</b> and a plurality of ONUs <b>550</b>, <b>610</b>, <b>650</b>, all of which are connected to the OLT <b>510</b> in a tree structure.

P-0050[0050] Referring to FIG. 13, each of the ONUs <b>550</b>, <b>610</b>, <b>650</b> includes one or more of service ports or subscriber ports S<b>1</b>-S<b>9</b>. Each of the subscriber/service ports S<b>1</b>-S<b>9</b> is assigned a unique LLID. The LLID assignment can be performed in an initial registration process, and the OLT <b>510</b> assigns an LLID to an object of the ONU side, i.e., a new ONU, while a subscriber port or classified service port is registered. When such ONU, subscriber port or classified service port is deregistered as a cold ONU or cold port, the LLID assigned thereto is released and reused when a new ONU or subscriber/service port is registered. The LLIDs are assigned to ports of an L2 switch <b>540</b> on a one-to-one basis.

P-0051[0051] During upstream transmission to the OLT <b>510</b>, LLID MUX/DEMUX layers <b>555</b>, <b>615</b>, <b>655</b> of the ONUs <b>550</b>, <b>610</b>, <b>650</b> insert an Ethernet type field, in which a tag Ethernet frame type is recorded and an LLID field in which their own LLIDs are recorded, into existing Ethernet frames generated by LLC layers <b>580</b>, <b>640</b>, <b>680</b>, and then transmit the resulting tag Ethernet frames. The tag Ethernet frames are transmitted to the OLT <b>510</b> in an upstream direction through PHY layers <b>575</b>, <b>635</b>, and <b>675</b>. An LLID MUX/DEMUX layer <b>535</b> of the OLT <b>510</b> checks an LLID of the received tag Ethernet frame, deletes an Ethernet type field and an LLID field from the tag Ethernet frame, and transmits the resulting Ethernet frame to the port P<b>1</b>-P<b>3</b> of the L2 switch <b>540</b>, to which the LLID is assigned. The L2 switch <b>540</b> analyzes a destination address through an address learning process, and then transmits the Ethernet frame to the assigned port.

P-0052[0052] The Ethernet frame is transmitted to the ONUs <b>550</b>, <b>610</b>, <b>650</b> in a downstream direction. The LLID MUX/DEMUX layer <b>535</b> of the OLT <b>510</b> inserts, into the existing Ethernet frame, an Ethernet type field in which a tag Ethernet frame type is recorded and an LLID field in which an LLID assigned to the destination address is recorded, and then transmits the resulting Ethernet frame to the ONUs <b>550</b>, <b>610</b>, <b>650</b> in a downstream direction. Upon receiving the downstream Ethernet frame, each of LLID MUX/DEMUX layers <b>555</b>, <b>615</b>, <b>655</b> of the ONUs <b>550</b>, <b>610</b>, <b>650</b> determines whether the received Ethernet frame is a tag Ethernet frame. If so, the LLID MUX/DEMUX layer determines whether the LLID of the Ethernet frame is identical to its own LLID before deleting an LLID field. If the LLID is identical to its own LLID, the LLID MUX/DEMUX layer transmits the Ethernet frame through a corresponding one of the subscriber/service ports S<b>1</b> to S<b>9</b>. Otherwise, if the LLID is not identical to its own LLID, the LLID MUX/DEMUX layer discards the Ethernet frame.

P-0053[0053] Determining whether the Ethernet frame is used for broadcast can be achieved by examining the mode <b>242</b> of FIG. 6. In the case of a point-to-multipoint mode B, the Ethernet frame is directly transmitted from the LLID MUX/DEMUX layer <b>535</b> of the OLT <b>510</b> to an L3 router <b>545</b> without passing through the L2 switch <b>540</b>, and then routed by the L3 router <b>545</b> and transmitted to the ONUs <b>550</b>, <b>610</b>, <b>650</b>. Bypassing the L2 switch avoids multiple broadcasts, where the frame is transmitted to all ports except the one from which the switch received the frame, which would include the ports of all of the other L2 switches on the L3 router.

P-0054[0054] Prior to transmitting the frame to the ONUs <b>550</b>, <b>610</b>, <b>650</b>, the LLID MUX/DEMUX layer <b>535</b> inserts, into the existing Ethernet frame, a default LLID field designated for broadcast or an LLID field in which an LLID of the OLT <b>510</b> is recorded. The resulting Ethernet frame, in being transmitted in a downstream direction, is broadcasted to each of the ONUs <b>550</b>, <b>610</b>, <b>650</b>. Each of the LLID MUX/DEMUX layers <b>555</b>, <b>615</b>, <b>655</b> of the ONUs <b>550</b>, <b>610</b>, <b>650</b> recognizes the received Ethernet frame as a broadcast frame by examining the mode, and deletes the LLID field from the received Ethernet frame since an LLID of the received Ethernet frame is not identical to its own LLID, and then transmits the resulting Ethernet frame to an upper layer.

P-0055[0055] A determination as to whether the received Ethernet frame is a tag Ethernet frame can be performed by a layer other than the MAC layers <b>565</b>, <b>625</b>, <b>665</b>, in which case only the LLID field is delivered to the MAC layers <b>565</b>, <b>625</b>, <b>665</b>.

P-0056[0056] As described above, the present invention enables peer-to-peer transmission without modification of an existing 802.1D bridge by providing a tag Ethernet frame for securing 802.1D bridge compatibility with an Ethernet passive optical network, and a point-to-point emulation method. In addition, the present invention enables not only ONU-to-ONU communication but also subscriber-to-subscriber communication in the L2 switch. Furthermore, the present invention can support multiple services by assigning LLIDs to service ports according to service types and realizing a point-to-point emulation technique for the service ports. In addition, it is possible to specify class according to the SLA of the subscriber or a type of the multiple services. It is possible to guarantee QoS during multiple services or enable accounting according to the SLA by giving priority according to class.

P-0057[0057] While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, the bridge can be configured separate from rather than part of the OLT.