Abstract:
One embodiment of the present invention provides a system that accommodates multiple optical segments in an Ethernet passive optical network (EPON), wherein the EPON includes a central node and a number of remote nodes, and wherein the remote nodes reside in a number of optical segments. During operation, the system transmits downstream data from the central node to the remote nodes by broadcasting the data to the optical segments. In addition, the system selectively allows an optical segment to communicate with the central node during an upstream transmission period assigned to a remote node residing in that optical segment, thereby accommodating multiple optical segments and hence an increased number of remote nodes within the EPON.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to architectures for communication networks. More specifically, the present invention relates to a method and an apparatus for accommodating multiple optical segments in an Ethernet passive optical network.  
         [0003]     2. Related Art  
         [0004]     In order to keep pace with the increasing Internet traffic, optical fibers and optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this capacity increase in backbone networks has not been accompanied by a corresponding capacity increase in access networks. Despite improved broadband access solutions such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks remains to be a severe bottleneck in delivering high bandwidth to end users.  
         [0005]     Among the different technologies presently being developed, Ethernet passive optical networks (EPONs) are among the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics. They offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. Because of optical fiber&#39;s high bandwidth, EPONs can carry broadband voice, data, and video traffic simultaneously. Such integrated services are difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, because Ethernet frames can encapsulate native IP packets with different sizes. In contrast, ATM passive optical networks (APONs) use fixed-size ATM cells and require packet fragmentation and reassembly.  
         [0006]     Typically, EPONs reside in the “first mile” of the network, which provides connectivity between the service provider&#39;s central offices and business or residential subscribers. This first mile network is often a logical point-to-multipoint network, with a central office servicing a number of subscribers. In a typical tree-topology EPON, one fiber couples the central office to a passive optical coupler/splitter, which divides and distributes downstream optical signals to users (subscribers). The coupler/splitter also combines upstream signals from subscribers (see  FIG. 1 ).  
         [0007]     Transmissions in an EPON are typically between an optical line terminal (OLT) and optical networks units (ONUs) (see  FIG. 2 ). The OLT generally resides in the central office and couples the optical access network to an external network (e.g., a carrier network). An ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are typically coupled to a one-by-N (1×N) passive optical coupler, which is coupled to the OLT through a single optical link. (Note that a number of optical couplers can be cascaded.) This configuration can achieve significant savings in the number of fibers and amount of hardware.  
         [0008]     Communications within an EPON are divided into downstream traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT). In the upstream direction, the ONUs share channel capacity and resources, since there is only one link coupling the passive optical coupler to the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, packets are broadcast by the OLT to all ONUs and are subsequently extracted by their destination ONUs. Each network device is assigned a Logical Link ID (LLID), according to the IEEE 802.3ah standard. A downstream packet is first processed at the OLT, where the packet receives the LLID of its destination, and is then transmitted to the ONUs. Although a packet is broadcast to all the ONUs, only the ONUs with an LLID that matches the one carried by the packet is allowed to receive the packet. Therefore, the OLT switches packets by attaching proper LLIDs to the packets. Note that in certain cases where broadcast or multicast is desired, the OLT attaches a corresponding broadcast/multicast LLID to a downstream packet so that a number of ONUs are allowed to receive the packet.  
         [0009]     One challenge in designing a scalable, cost-effective EPON is to accommodate as many ONUs as possible. Based on the current IEEE 802.3ah standard, one OLT can accommodate up to 256 LLIDs. However, it is not likely that all 256 ONUs can reside in the same optical network segment. This is because the number of ONUs in a tree-topology EPON is limited by the optical power budget and the loss incurred at the optical splitter. A typical optical splitter may have up to 32 ports. A single optical splitter with a higher-port count (e.g., 128 or 256) or a cascaded configuration of multiple splitters inevitably incurs significantly higher loss and leaves little power budget for optical transmission.  
         [0010]     One approach to combat high splitting loss is to use a high-power laser for upstream transmission within each ONU. Alternatively, the system may employ optical amplification. Unfortunately, the costs associated with either of these solutions may be prohibitively high.  
         [0011]     Hence, what is needed is a method and an apparatus for accommodating an increased number of ONUs in an EPON without incurring significant costs.  
       SUMMARY  
       [0012]     One embodiment of the present invention provides a system that accommodates multiple optical segments in an Ethernet passive optical network (EPON), wherein the EPON includes a central node and a number of remote nodes, and wherein the remote nodes reside in a number of optical segments. During operation, the system transmits downstream data from the central node to the remote nodes by broadcasting the data to the optical segments. In addition, the system selectively allows an optical segment to communicate with the central node during an upstream transmission period assigned to a remote node residing in that optical segment, thereby accommodating multiple optical segments and hence an increased number of remote nodes within the EPON.  
         [0013]     In a variation of this embodiment, the optical segments are coupled to a number of inputs of a multiplexer. The output of the multiplexer is coupled to the central node. In this variation, selectively allowing the optical segment to communicate with the central node involves configuring the multiplexer so that the upstream data from that optical segment can be received by the central node.  
         [0014]     In a further variation, the system periodically broadcasts discovery windows to the optical segments. By responding during the discovery window, a newly joined remote node may register with the central node and receive a logical link identifier (LLID). Furthermore, the system configures the multiplexer to allow only one optical segment to communicate with the central node during a given discovery window. The system then associates the LLID assigned to a remote node which is registered during this discovery window with the optical segment which is allowed to communicate with the central node during the same discovery window. In this way, the system can properly configure the multiplexer during the registered remote node&#39;s subsequent upstream transmission.  
         [0015]     In a further variation, selectively allowing the optical segment to communicate with the central node involves detecting a special bit pattern transmitted from that optical segment.  
         [0016]     In a further variation, selectively allowing the optical segment to communicate with the central node involves detecting the signal power level received from that optical segment. In a variation of this embodiment, broadcasting the downstream data to the optical segments involves broadcasting the data electrically to a number of optical transmitters and transmitting the data with one optical transmitter for each optical segment.  
         [0017]     In a variation of this embodiment, broadcasting the downstream data to the optical segments involves transmitting the data through one optical transmitter and broadcasting the data to all the optical segments with an optical splitter.  
         [0018]     In a variation of this embodiment, the system protects an optical segment by using another optical segment as a backup segment. When a failure occurs in the protected optical segment, the system allows the backup optical segment to replace the failed optical segment.  
         [0019]     In a variation of this embodiment, the system deserializes upstream bits received from an optical segment subsequent to selectively allowing that optical segment to communicate with the central node. In addition, the system serializes downstream bits transmitted from the central node prior to broadcasting the data to the optical segments. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0020]      FIG. 1  illustrates a passive optical network wherein a central office and a number of subscribers are coupled through optical fibers and a passive optical splitter.  
         [0021]      FIG. 2  illustrates an EPON in normal operation mode.  
         [0022]      FIG. 3  illustrates an OLT configuration which uses an electrical multiplexer to accommodate multiple optical segments in accordance to one embodiment of the present invention.  
         [0023]      FIG. 4  illustrates a multi-optical segment OLT configuration where downstream data is transmitted by a single high-power laser in accordance to one embodiment of the present invention.  
         [0024]      FIG. 5  presents a flow chart illustrating the process of associating an ONU&#39;s LLID with an input port of the multiplexer during a discovery process in accordance with an embodiment of the present invention.  
         [0025]      FIG. 6  presents a flow chart illustrating the process of protection switching using multiple optical segments in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0026]     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.  
         [0027]     The data structures, operations, and processes described in this detailed description are typically stored on a digital-logic-readable storage medium, which may be any device or medium that can store code, data, instructions, and/or operation sequences for use by a digital-logic system such as a computer system. This includes, but is not limited to, application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), semiconductor memories, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated).  
         [0000]     Passive Optical Network Topology  
         [0028]      FIG. 1  illustrates a passive optical network, wherein a central office and a number of subscribers form a tree topology through optical fibers and a passive optical splitter. As shown in  FIG. 1 , a number of subscribers are coupled to a central office  101  through optical fibers and a passive optical splitter  102 . Passive optical splitter  102  can be placed near end-user locations, so that the initial fiber deployment cost is minimized. The central office is coupled to an external network, such as a metropolitan area network operated by an ISP.  
         [0000]     EPON Operation  
         [0029]     An ONU typically can accommodate one or more networked devices, such as personal computers, telephones, video equipment, network servers, etc. Note that an ONU can identify itself by using a Logical Link Identifier (LLID), as defined in the IEEE 802.3ah standard. To allow ONUs to join an EPON at arbitrary times, an EPON has two modes of operation: a discovery (initialization) mode and a normal operation mode. The discovery mode allows newly joined ONUs to register with the OLT and receives an LLID from the OLT. The normal operation mode allows regular upstream data transmissions, where transmission opportunities are assigned to all initialized ONUs.  
         [0030]     In a discovery process, an OLT broadcasts a discovery solicitation message to all the ONUs, including a newly joined unregistered ONU. The discovery solicitation message typically specifies the start time of a discovery window during which an unregistered ONU may register with the OLT. When the discovery window arrives for the unregistered ONU, the ONU sends a response message which contains the ONU&#39;s MAC address. The OLT subsequently assigns an LLID to the ONU.  
         [0031]      FIG. 2  illustrates an EPON in normal operation mode. As shown in  FIG. 2 , in the downstream direction, an OLT  201  broadcasts downstream data to ONU  1  ( 211 ), ONU  2  ( 212 ), and ONU  3  ( 213 ). While all ONUs receive the same copy of downstream data, each ONU selectively forwards only the data destined to itself to its corresponding users, which are user  1  ( 221 ), user  2  ( 222 ), and user  3  ( 223 ), respectively.  
         [0032]     For the upstream traffic, OLT  201  first schedules and assigns transmission windows to each ONU according to the ONU&#39;s service-level agreement. When not in its transmission window, an ONU typically buffers the data received from its user. When its scheduled transmission window arrives, an ONU transmits the buffered user data within the assigned transmission window. Since every ONU takes turns in transmitting upstream data according to the OLT&#39;s scheduling, the upstream link&#39;s capacity can be efficiently utilized.  
         [0000]     Accommodating Multiple Optical Segments in EPON  
         [0033]     A challenge in designing a scalable and cost effective EPON is to accommodate a large number of ONUs. Currently, the IEEE 802.3ah standard allows over 32,000 LLIDs in an EPON. However, these LLIDs are not all used. This is because the number of optical branches fanning out from an optical splitter is limited by the splitting loss and the optical power budget. Optical splitters commercially available today can have up to 32 ports. Although a single splitter with a higher port count or a cascaded splitter configuration provide an increased number of output ports, these configurations incur excessive splitting loss and quickly deplete the optical power budget in the EPON.  
         [0034]     It is possible to use high-power lasers to compensate for the excessive splitting loss. However, using a high-power laser in every ONU for upstream transmission inevitably increases the ONU cost. Consequently, the overall cost of the entire EPON can be prohibitively high.  
         [0035]     One embodiment of the present invention effectively increases the total number of ONUs in an EPON by accommodating multiple optical segments. In the downstream direction, data is broadcast to all the optical segments. In the upstream direction, different optical segments are interfaced with an electrical multiplexer which allows one segment to communicate with the OLT at a time.  
         [0036]      FIG. 3  illustrates an OLT configuration which uses an electrical multiplexer to accommodate multiple optical segments in accordance to one embodiment of the present invention. In this example, the EPON includes four optical segments  332 ,  334 ,  336 , and  338 . Each optical segment has a tree topology and can accommodate up to 64 ONUs with a 1×64 optical splitter. Within an optical segment, the ONUs are coupled to the branch optical fibers which are coupled to a main fiber through the optical splitter, such as splitter  306 . The main fibers are coupled to OLT transceivers (XCVR)  320 ,  322 ,  324 , and  326 , respectively. The OLT transceivers perform the optical-to-electrical and electrical-to-optical signal conversion.  
         [0037]     The optical transceivers are in communication with serializers/deserializers (SERDES)  312 ,  314 ,  316 , and  318 . A SERDES is responsible for converting a serial bit stream received from the fiber side (upstream) to a stream of n-bit wide words (e.g., 10-bit wide words) which can be received by digital interfaces typically used by an OLT chip. Similarly, the SERDES can receive n-bit wide words from the OLT and convert them into a serial bit stream which can be transmitted downstream by an OLT transceiver. Note that, in this example, a transceiver is a combination of an optical transmitter (e.g., a laser) and a receiver, and is therefore capable of both transmitting and receiving optical signals.  
         [0038]     The upstream outputs of the four SERDES&#39; are coupled to a 4×1 electrical multiplexer  304 . Multiplexer  304  can be configured to allow one of these inputs to communicate to its output which is coupled to OLT  300 . Because different optical segments share the same upstream link to OLT  300 , only one optical segment can be allowed to transmit upstream data to OLT  300  at any time. Therefore, the use of an electrical multiplexer is compatible with the existing mode of operation of an EPON.  
         [0039]     In the downstream direction, data from OLT  300  (typically n-bit wide words) is first amplified by an electrical transmission buffer  302  and then broadcast to SERDES&#39;  312 ,  314 ,  316 , and  318 . The SERDES&#39; convert the downstream data into serial bit streams which are subsequently transmitted to the optical segments by the OLT transceivers.  
         [0040]     The configuration in  FIG. 3  effectively adopts an additional level of aggregation in the electrical domain to accommodate multiple optical segments. In the upstream direction, the system uses electrical multiplexer  304  to allow one segment to communicate with OLT  300  at a time. In the downstream direction, the system electrically broadcasts the data to all the optical segments, which further broadcast the data to their ONUs through the optical splitters.  
         [0041]     The advantage of this configuration is that from OLT  300 &#39;s perspective, there is no difference between coupling to a single optical segment and coupling to multiple optical segments through an electrical multiplexer. In addition, the costs of electrical multiplexers, SERDES&#39;, and optical transceivers are significantly lower than those of high-power lasers or optical amplifiers. Therefore, the configuration disclosed herein provides unprecedented scalability, seamless interoperability, and excellent cost-effectiveness.  
         [0042]     It is important for multiplexer  304  to switch between its inputs at proper times so that each optical segment can successfully transmit upstream data to OLT  300  during its assigned transmission windows. In one embodiment of the present invention, the configuration of multiplexer  304 &#39;s switching state is based on the presence of signals on its inputs. For example, the system can use an electrical signal detection mechanism at the upstream outputs of the SERDES, and configure multiplexer  304  to turn on the input port whose signal level exceeds a given threshold. Alternatively, the system can use an optical signal detection mechanism at the OLT transceivers to detect the level of optical power and configure multiplexer  304  accordingly. Furthermore, when an optical segment is communicating with OLT  300 , the system may prohibit multiplexer  304  from changing its switching state to ensure uninterrupted communication from that optical segment.  
         [0043]     It is also possible for multiplexer  304  to implement some intelligence and to configure itself based on received data. In one embodiment of the present invention, multiplexer  304  may include a mechanism which scans the incoming n-bit words on every input. Whenever an incoming word matches a special bit pattern which is designated to mark the beginning of an upstream transmission from an ONU, multiplexer  304  may automatically switch to that input and allows its upstream transmission to pass through.  
         [0044]     Another approach to configuring multiplexer  304  is to allow OLT  300  to control multiplexer  304 . In one embodiment of the present invention, OLT  300  maintains knowledge of which optical segment is allowed to transmit upstream data at any given time. OLT  300  can send a control signal to multiplexer  304  to switch to a proper optical segment when it is time for OLT  300  to receive from that segment.  
         [0045]     For OLT  300  to properly configure multiplexer  304 , OLT  300  ideally learns which ONU/LLID corresponds to which optical segment. In this way, OLT  300  can predict at the beginning of each upstream transmission window from which optical segment the data is sent. One way for OLT  300  to map LLIDs to optical segments is to direct its discovery process to individual optical segments. Conventionally, an OLT broadcasts a discovery window to every ONU and accepts registration requests from any newly joined ONUs. Conversely, in one embodiment of the present invention, OLT  300  selectively listens to a particular optical segment during a discovery window by configuring multiplexer  304  to switch to that segment. Hence, any newly joined ONU registered during this discovery window is associated with that optical segment. Note that the discovery window may still be broadcast to all the optical segments. However, only registration requests from one segment are received by OLT  300 .  
         [0046]     Note that the downstream broadcasting and upstream multiplexing may also occur between the optical transceivers and a SERDES. In this case, an upstream multiplexer is placed between the optical transceivers and one SERDES. The input ports of this multiplexer ideally operate at a higher serial bit rate (i.e., line rate). The output of this multiplexer then enters the SERDES and the bit stream is then parallelized. In the downstream direction, the broadcasting occurs after the downstream bits from the OLT are serialized. This configuration allows the electrical broadcasting and multiplexing to occur in the serial domain and therefore reduces the number of SERDES&#39;.  
         [0047]     In the example in  FIG. 3 , the system electrically broadcasts downstream data to all the optical segments. Alternatively, the system can use a single high-power laser and optically broadcast the downstream data.  FIG. 4  illustrates a multi-optical segment OLT configuration where downstream data is transmitted by a single high-power laser in accordance to one embodiment of the present invention.  
         [0048]     As shown in  FIG. 4 , an OLT  400  transmits its downstream data to a SERDES  410  which converts n-bit wide words into a serial bit stream. The serial bit stream is then transmitted to an optical transmitter (TX)  411 , which is a high-power laser. The output of optical transmitter  411  then enters a 1×4 optical splitter  408 , which optically broadcasts the downstream data to four optical segments. Within one optical segment, for example segment  432 , the output of splitter  408  enters a main fiber  407  through a 2×1 optical combiner  406 . 2×1 combiner  406  is used here to facilitate both upstream and downstream transmission through main fiber  407 . After propagating through main fiber  407 , the downstream data enters optical splitter  405  which broadcasts the optical signal to all the ONUs within optical segment  432 .  
         [0049]     In the upstream direction, data from an ONU within segment  432  is transmitted upstream through splitter  405  (working as a combiner), main fiber  407 , and combiner  406  (working as a splitter) to reach optical receiver  420 . The output of receiver  420  is transmitted to SERDES  412 , which converts a serial bit stream in to n-bit wide words. The outputs of the four SERDES&#39; (corresponding to four optical segments) subsequently enter electrical multiplexer  404 , which selects one of the optical segments to communicate with OLT  400 .  
         [0050]      FIG. 5  presents a flow chart illustrating the process of associating an ONU&#39;s LLID with an input port of the multiplexer during a discovery process in accordance with an embodiment of the present invention. The system begins by broadcasting a discovery solicitation message to all the optical segments (step  502 ). The system then configures the multiplexer to allow upstream data communication from one given optical segment during the assigned discovery window (step  504 ).  
         [0051]     Next, the system receives a discovery response from an ONU within that optical segment during the discovery window (step  506 ). The system subsequently assigns an LLID to the requesting ONU (step  508 ). The system also associates the ONU&#39;s LLID with the multiplexer&#39;s input port which is coupled to the optical segment (step  510 ).  
         [0052]     A multiple-optical segment configuration in an EPON can also be used for protection switching. For example, one optical segment can be used as a backup for a primary optical segment. When a failure (e.g., an ONU failure or a fiber cut) occurs in the primary segment, the OLT can quickly switch to the backup segment and minimize transmission interruption. Such fast protection switching provides valuable quality of service (QoS) in critical applications, such as voice communications.  
         [0053]      FIG. 6  presents a flow chart illustrating the process of protection switching using multiple optical segments in accordance with an embodiment of the present invention. During operation, the system first detects a failure in an optical segment (step  602 ). The system then configures the multiplexer to switch to the backup optical segment (step  604 ). Next, the system updates the LLID-to-multiplexer port mapping information to reflect that the backup segment has replaced the primary segment (step  606 ). The system subsequently issues an alarm message to alert the network operator (step  608 ).  
         [0054]     The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.