Abstract:
One embodiment of the present invention provides an optical line terminal (OLT) in an Ethernet passive optical network (EPON). The OLT includes a number of bi-direction optical transceivers. At least one bi-direction optical transceiver is coupled to an optical network unit (ONU) group that includes a number of ONUs. The OLT further includes a first downstream media access control (MAC) interface configured to provide a first downstream control signal and a splitter configured to split the first downstream control signal to a number of sub-signals. At least one sub-signal is configured to control downstream transmission of a corresponding bi-direction optical transceiver to a corresponding ONU-group.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/165,770, entitled “MULTIPLE EPONS SHARING COMMON DOWNSTREAM LINK,” by inventors Ryan E. Hirth and Edward W. Boyd, filed 1 Apr. 2009. 
    
    
     BACKGROUND 
     1. Field 
     This disclosure is generally related to an Ethernet Passive Optical Network (EPON). More specifically, this disclosure is related to multiple EPONs sharing a common downstream link. 
     2. Related Arts 
     In order to keep pace with increasing Internet traffic, network operators have widely deployed optical fibers and optical transmission equipment, substantially increasing the capacity of backbone networks. A corresponding increase in access network capacity, however, has not matched this increase in backbone network capacity. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering large bandwidth to end users. 
     Among different competing technologies, passive optical networks (PONs) are one of the best candidates for next-generation access networks. With the large bandwidth of optical fibers, PONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, PONs can be built with existing protocols, such as Ethernet and ATM, which facilitate interoperability between PONs and other network equipment. 
     Typically, PONs are used in the “first mile” of the network, which provides connectivity between the service provider&#39;s central offices and the premises of the customers. The “first mile” is generally a logical point-to-multipoint network, where a central office serves a number of customers. For example, a PON can adopt a tree topology, wherein one trunk fiber couples the central office to a passive optical splitter/combiner. Through a number of branch fibers, the passive optical splitter/combiner divides and distributes downstream optical signals to customers and combines upstream optical signals from customers. Note that other topologies, such as ring and mesh topologies, are also possible. 
     Transmissions within a PON are typically performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. The ONU can reside in the residence of the customer and couples to the customer&#39;s own home network through a customer-premises equipment (CPE). 
       FIG. 1A  illustrates a passive optical network including an OLT (located at a central office) and a number of ONUs (located at customers&#39; premises) coupled through optical fibers and a passive optical splitter (prior art). A passive optical splitter  108  and optical fibers couple ONUs  102 ,  104 , and  106  to an OLT  100 . Although  FIG. 1  illustrates a tree topology, a PON can also be based on other topologies, such as a logical ring or a logical bus. Note that, although in this disclosure many examples are based on EPONs, embodiments of the present invention are not limited to EPONs and can be applied to a variety of PONs, such as ATM PONs (APONs) and wavelength domain multiplexing (WDM) PONs. 
       FIG. 1B  presents a block diagram illustrating the layered structure of a conventional EPON (prior art). The left half of  FIG. 1B  illustrates the layer structure of an Open System Interconnection (OSI) model including an application layer  110 , a presentation layer  112 , a session layer  114 , a transport layer  116 , a network layer  118 , a data link layer  120 , and a physical layer  122 . The right half of  FIG. 1B  illustrates EPON elements residing in data link layer  120  and physical layer  122 . EPON elements include a media access control (MAC) layer  128 , a MAC control layer  126 , a logic link control (LLC) layer  124 , a reconciliation sublayer (RS)  130 , medium interface  132 , and physical layer device (PHY)  134 . 
     In an EPON, communications can include downstream traffic and upstream traffic. In the following description, “downstream” refers to the direction from an OLT to one or more ONUs, and “upstream” refers to the direction from an ONU to the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, data packets are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. Moreover, each ONU is assigned one or more Logical Link Identifiers (LLIDs), and a data packet transmitted by the OLT typically specifies an LLID of the destination ONU. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler to the OLT. 
     In order to avoid collision of upstream transmissions from different ONUs, ONU transmissions are arbitrated. This arbitration can be achieved by allocating a transmission window (grant) to each ONU. An ONU defers transmission until its grant arrives. A multipoint control protocol (MPCP) located in the MAC control layer can be used to assign transmission time slots to ONUs, and the MPCP in an OLT is responsible for arbitrating upstream transmissions of all ONUs coupled to the same OLT. 
     Due to the splitting loss at passive optical splitter  108 , the number of ONUs coupled to an OLT is limited, thus limiting the number of subscribers within a PON. In order to increase the number of subscribers, the carrier needs to install more OLTs in the central office. Because OLTs are expensive, it is desirable to find an alternative that can allow more subscribers to couple to one OLT. 
     SUMMARY 
     One embodiment of the present invention provides an optical line terminal (OLT) in an Ethernet passive optical network (EPON). The OLT includes a number of bi-direction optical transceivers. At least one bi-direction optical transceiver is coupled to an optical network unit (ONU) group that includes a number of ONUs. The OLT further includes a first downstream media access control (MAC) interface configured to provide a first downstream control signal and a splitter configured to split the first downstream control signal to a number of sub-signals. At least one sub-signal is configured to control downstream transmission of a corresponding bi-direction optical transceiver to a corresponding ONU-group. 
     In a variation on this embodiment, the OLT further includes a number of individual upstream MAC interfaces, and at least one individual upstream MAC interface is configured to communicate with a corresponding ONU-group. 
     In a further variation, the individual upstream MAC interface is configured to arbitrate upstream transmissions from the ONUs belonging to a corresponding ONU-group. 
     In a further variation, different individual upstream MAC interfaces separately arbitrate upstream transmissions from different ONU-groups, thereby facilitating concurrent upstream transmission to the OLT from ONUs belonging to the different ONU-groups. 
     In a further variation, the individual upstream MAC interfaces are configured to allocate discovery slots to respective ONU-groups, wherein the discovery slots for different ONU-groups are aligned in time. 
     In a further variation, the downstream transmission and the upstream transmissions are carried on two different wavelengths. 
     In a further variation, the first downstream MAC interface and at least one individual upstream MAC interface are configured to operate at two different data rates. 
     In a further variation, the first downstream MAC interface and at least one individual upstream MAC interface are configured to operate at a same data rate. 
     In a further variation, the OLT further includes a shared upstream MAC interface configured to interface with more than one of the ONU groups. 
     In a further variation, the OLT further includes a merger configured to merge upstream transmissions from the more than one ONU-groups, and to send the merged transmissions to the shared upstream MAC interface. 
     In a further variation, upstream transmissions to a respective individual upstream MAC interface and the shared upstream MAC interface are carried on two different wavelengths over a single strand of fiber. 
     In a variation on this embodiment, the OLT further includes an optical transmitter and a second downstream MAC interface, wherein the second downstream MAC is configured to control downstream transmission of the optical transmitter to an ONU-group. 
     In a further variation, downstream transmissions from the bi-direction optical transceiver and the optical transmitter are coupled to a single strand of fiber via a wavelength division multiplexing (WDM) coupler. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates a passive optical network including an OLT (located at a central office) and a number of ONUs (located at customers&#39; premises) coupled through optical fibers and a passive optical splitter (prior art). 
         FIG. 1B  presents a block diagram illustrating the layered structure of a conventional EPON (prior art). 
         FIG. 2  presents a block diagram illustrating an OLT that supports multiple ONU-groups in accordance with an embodiment of the present invention. 
         FIG. 3  presents a block diagram illustrating an exemplary configuration of an OLT line card for an asymmetric EPON system in accordance with one embodiment of the present invention. 
         FIG. 4  presents a block diagram illustrating an exemplary configuration of an OLT line card for an asymmetric EPON system in accordance with one embodiment of the present invention. 
         FIG. 5  presents a block diagram illustrating an exemplary configuration of an OLT line card for an asymmetric EPON system in accordance with one embodiment of the present invention. 
         FIG. 6  presents a block diagram illustrating an exemplary configuration of an OLT line card for a symmetric EPON system in accordance with one embodiment of the present invention. 
         FIG. 7  presents a block diagram illustrating an exemplary configuration of an OLT line card for a symmetric EPON system in accordance with one embodiment of the present invention. 
         FIG. 8  presents a block diagram illustrating an exemplary configuration of an OLT line card for a symmetric EPON system in accordance with one embodiment of the present invention. 
         FIG. 9  presents a diagram illustrating a WDM-EPON configuration in accordance with an embodiment of the present invention 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the embodiments, 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 disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
       FIG. 2  presents a block diagram illustrating an OLT that supports multiple groups of ONUs in accordance with one embodiment of the present invention. As shown in  FIG. 2 , an OLT couples to a number of ONU-groups including an ONU-group  210  and an ONU-group  220 . ONU-group  210  includes ONUs  214 - 218 , all coupled to OLT  200  via a passive optical splitter  212 , and ONU-group  220  includes ONUs  224 - 228 , all coupled to OLT  200  via a passive optical splitter  222 . Using the architecture shown in  FIG. 2 , the number of ONUs coupled to an OLT can increase dramatically. For example, a conventional OLT may be able to couple to 32 ONUs via a passive optical splitter. With this configuration that includes eight separate ONU-groups, the number of ONUs coupled to the OLT can reach 32×8 (256). To distinguish one ONU from another, each ONU is assigned one or more LLIDs, which are unique across all ONUs coupled to OLT  200 . 
     Similar to a conventional EPON, the downstream traffic is broadcast from OLT  200  to all ONU-groups including ONU-groups  210  and  220 . In other words, all ONU-groups share the same downstream link. However, each ONU-group has its own upstream link, and the upstream traffic from ONUs of each ONU-groups is arbitrated separately by its own upstream MAC implementing MPCP located in OLT  200 , as explained in more details in the examples shown in  FIGS. 3-9 . In other words, OLT  200  is able to arbitrate upstream traffic for each ONU-group separately and concurrently. As a result, it is possible for two ONUs coupled to the same OLT to have simultaneous transmission. 
     MPCP schedules upstream traffic from ONUs via GATE and REPORT messages. MPCP REPORT messages are used by the ONUs to tell the OLT the amount of data in its buffer to be sent to the OLT and the MPCP GATE message is used by the OLT to grant a time slot for the ONU to transmit a message. To schedule an ONU&#39;s upstream transmission, the OLT sends a GATE message specifying receiving LLID and a time slot. As a result, the ONU with the specified LLID schedules its upstream transmission during the time slot indicated by the GATE message. During a discovery process, in which OLT  200  discovers and initializes coupling ONUs, such as ONUs  214 - 218  and  224 - 228 , OLT  200  broadcasts a discovery GATE message to all coupling ONUs within different ONU-groups. The discovery GATE message specifies a time interval in which OLT  200  enters the discovery mode and allows ONUs to register (this time interval is called the discovery window). To register, ONUs from different ONU-groups can respond to the discovery GATE message within the discovery window. To avoid collision, the multiple upstream MACs that are responsible for scheduling their respective upstream traffic need to synchronize their scheduling of the response to the discovery GATE. 
     Asymmetric EPON 
       FIG. 3  presents a block diagram illustrating an exemplary configuration of an OLT line card for an asymmetric EPON system in accordance with one embodiment of the present invention. OLT line card  300  includes a 10 Gigabit (G) EPON OLT chip  302 , eight optical transceiver modules including module  304  and module  306 , an optional packet buffer  308 , a synchronous-dynamic random-access memory (SDRAM)  310 , and a flash memory  312 . Packet buffer  308  may include a number of SDRAMs, such as SDRAM  332 . OLT line card  300  interfaces with a backplane via a redundant uplink interface  314 , and a management interface  316 . Redundant uplink interface  314  can include one or more 10 G-attachment-unit-interfaces (XAUIs), and management interface  316  can include an asynchronous bus and other Ethernet interfaces. OLT line card  300  couples to eight downstream ONU-groups, such as ONU-group  326  and ONU-group  328 , each includes a number of ONUs. Each ONU-group interfaces with OLT line card  300  via an optical transceiver module. For example, ONU-group  326  interfaces with line card  300  via transceiver module  304 . 
     OLT chip  302  includes an embedded processor  330 , a 10 G downstream MAC interface  318  that controls the downstream transmission to the eight ONU-groups, and eight 1.25 G upstream MAC interfaces, such as MAC interface  322  and MAC interface  324 , that control the individual upstream transmissions from the eight ONU-groups. The output of MAC interface  318  is split by a 1:8 splitter  320  into eight signals; each signal controls the transmission of an optical transceiver module, such as module  304  and module  306 . In other words, all eight optical transceivers modules are transmitting the same signal downstream, thus providing a shared downstream link to all eight ONU-groups. Each of the eight optical transceiver modules has a transmitting port for 10 G downstream transmission at a wavelength of 1577 nm and a receiving port for 1.25 G upstream receiving at a wavelength of 1310 nm. Because the downstream transmission and the upstream transmission have different data rates, the EPON system is said to be asymmetric. Also note that the downstream and the upstream signals are carried at different wavelengths; thus, a single strand of fiber can be used to carry signals to and from an ONU-group. The upstream receiving of the eight optical transceiver modules are independently controlled by eight upstream MAC interfaces, such as MAC interface  322  and MAC interface  324 , all working at a speed of 1.25 G. Each upstream MAC interface is configured to arbitrate upstream transmissions from ONUs within an ONU group. As a result, OLT line card  300  can arbitrate upstream traffic for each ONU group separately and concurrently. 
     In addition to the OLT architecture shown in  FIG. 3 , other asymmetrical variations are also possible. For example, in addition to a shared downstream link, it is also possible for an OLT to provide a dedicated downstream link to each ONU-group.  FIG. 4  presents a block diagram illustrating an exemplary configuration of an OLT line card for an asymmetric EPON system in accordance with one embodiment of the present invention. In  FIG. 4 , OLT line card  400  includes an OLT chip  402 , and is coupled to four individual ONU groups including ONU-group  426  and ONU-group  428 . In addition to an optical transceiver module, each ONU-group is also coupled to an optical transmitter module. For example, ONU-group  426  is coupled to an optical transceiver module  404  and an optical transmitter module  408 , and ONU-group  428  is coupled to an optical transceiver module  406  and an optical transmitter module  410 . As a result, in addition to a shared 10 G downstream link, OLT line card  400  also provides each ONU-group with its own dedicated downstream link. Similar to the one shown in  FIG. 3 , the shared 10 G downstream link is controlled by a common 10 G downstream MAC interface  412 , whose output is sent to a 1:4 splitter  416  and is split into four signals, each controlling the downstream transmission of an optical transceiver module. Optical transmitter modules, such as modules  408  and  410 , provide additional dedicated downstream links to individual ONU-groups. Each optical transmitter module is individually controlled by a dedicated downstream MAC interface, thus resulting in a dedicated downstream link for each ONU-group. For example, optical transmitter modules  408  and  410  are controlled separately by downstream MAC interfaces  422  and  424 . Optical transmitter modules  408  and  410  can transmit at a data rate of 1.25 G or 2.5 G over a wavelength of 1490 nm. The transmission output of an optical transceiver module (10 G, 1577 nm) and the transmission output of a corresponding optical transmitter (1.25 or 2 G, 1490 nm) can be coupled to a single strand of fiber using a wavelength-division-multiplexing (WDM) coupler, such as WDM coupler  430  and  432 . Providing dedicated downstream links in addition to a shared down stream link makes it possible for implementing quality of service (QOS) control. Similar to  FIG. 3 , the 1.25 G, 1310 nm upstream link for each ONU-group is provided by the receiving port of a corresponding optical transceiver module, which is controlled by an individual 1.25 G upstream MAC interface, such as MAC interface  418  and MAC interface  420 . 
       FIG. 5  presents another exemplary configuration of a 10 G OLT line card for an asymmetric EPON system in accordance with an embodiment. In  FIG. 5 , OLT line card  500  is coupled to two ONU-groups  526  and  528 , providing each a shared 10 G downstream link at 1577 nm, a dedicated 2.5 G downstream link at 1550 nm, and a 1.25 G dedicated downstream link at 1490 nm. Similar to  FIGS. 3 and 4 , the output of a common 10 G downstream MAC interface  512 , which is located on an OLT chip  502 , is split in two ways by a 1:2 splitter  516 , and the split signals control downstream transmissions of optical transceiver modules  504  and  506 , thus providing a shared 10 G downstream link to ONU-groups  526  and  528 . In addition, optical transmitter modules  508  and  510 , which are separately controlled by 1.25 G downstream MAC interfaces  522  and  524 , provide dedicated downstream links at a wavelength of 1490 nm to ONU-groups  526  and  528 , respectively. Moreover, optical transmitter modules  538  and  540 , which are separately controlled by 2.5 G downstream MAC interfaces  534  and  536 , provide additional dedicated downstream links at a wavelength of 1550 nm to ONU-groups  526  and  528 , respectively. The three downstream transmissions at different wavelengths are multiplexed together by a WDM multiplexer, such as multiplexers  530  and  532 , to a single strand of fiber before reaching the passive optical splitters for a corresponding ONU-group. Similar to  FIGS. 3 and 4 , the 1.25 G, 1310 nm upstream links for each ONU-group are provided by the receiving ports of optical transceiver modules  504  and  506 . 
     Symmetric EPON 
     In addition to asymmetric EPON solutions, embodiments of the present invention also include symmetric EPON solutions, where the downstream and upstream transmissions have the same bandwidth.  FIG. 6  presents a block  10  diagram illustrating an exemplary configuration of an OLT line card for a symmetric EPON system in accordance with one embodiment of the present invention. Similar to  FIG. 3 , OLT line card  600  couples to eight ONU-groups, such as ONU-group  626  and ONU-group  628 , via eight optical transceivers modules, such as transceiver modules  604  and  606 . A common 10 G downstream MAC interface  618  controls the transmission of all eight transceiver modules via a 1:8 splitter  620 , thus providing a shared 10 G downstream link at a wavelength of 1577 nm to all ONU-groups. In  FIG. 6 , the upstream transmission links from each ONU-group include a 10 G transmission link at a wavelength of 1270 nm and a 1.25 G transmission link at a wavelength of 1310 nm. Both transmissions are received by the receiving port of the corresponding transceiver module, which is capable of dual-rate receiving. For example, the receiving port of transceiver module  604  receives the two upstream transmissions from ONU-group  626 . To avoid collisions of the two upstream transmissions, each transmission is assigned a time-division-multiple-access (TDMA) time slot. Similar to OLT chip  302  shown in  FIG. 3 , OLT chip  602  includes a group of 1.25 G upstream MAC interfaces, such as upstream MAC interfaces  622  and  624 , each controls and processes a 1.25G, 1310 nm signal received from each individual ONU-group, thus providing a dedicated upstream link to each ONU-group. Such dedicated upstream link allows 1.25 G, 1310nm upstream traffic from each ONU-group to be scheduled separately and concurrently. As a result, it is possible to for two ONUs coupled to OLT line card  600  to have simultaneous upstream transmission. Note that the summed bandwidth of all eight dedicated upstream links can be 10 G. In one embodiment, each dedicated upstream MAC interface, such as MAC  622  and MAC  624 , has a flexible capacity of up to 10 G. Thus, it is possible for an individual ONU-group to have a 10 G upstream transmission on such a dedicated link. However, the aggregate uplink bandwidth is limited by an aggregate shaper to a sum of 10 G among all ONU-groups. Consequently, a switch behind the MACs only sees a limited bandwidth of 10 G. 
     On the other hand, the 10 G, 1270 nm upstream transmission from all ONU-groups are merged together by a 8:1 merger  610 , and the merged signal is sent to a common 10G upstream MAC interface  608  for control and processing. As a result, in addition to dedicated upstream links, OLT line card  600  also provides a shared upstream link to all ONU-groups. The 10G upstream transmissions from all ONUs within all ONU-groups are arbitrated by the MPCP implemented in the common 10G upstream MAC interface  608 . 
       FIG. 7  presents a block diagram illustrating an exemplary configuration of an OLT line card for a symmetric EPON system in accordance with one embodiment of the present invention. In  FIG. 7 , the structure of OLT line card  700  is similar to that of OLT line card  400  illustrated in  FIG. 4 . In  FIG. 7 , OLT line card  700  includes an OLT chip  702  and couples to four ONU-groups including ONU-groups  726  and  728 . OLT line card  700  provides a 10 G, 1577 nm shared downstream link and a 1.25 G (or 2.5 G), 1490 nm dedicated downstream link to each ONU-group. The 10 G shared downstream link is provided by a common 10 G downstream MAC interface  712 , which controls the downstream transmission of optical transceiver modules, such as modules  704  and  706 , via a 1:4 splitter  716 . Dedicated downstream links to individual ONU-groups are provided by dedicated 1.25 G (or 2.5 G) downstream MAC interfaces, such as MAC interfaces  722  and  724 , each controlling the transmission of an optical transmitter module, such as transmitter modules  708  and  710 . The shared and dedicated downstream transmissions to each ONU-group are coupled to a single strand of fiber via a WDM coupler, such as coupler  730  and  732 . The difference between the system shown in  FIG. 7  and the one shown in  FIG. 4  is that, in addition to a dedicated 1.25 G, 1310 nm upstream link, each ONU-group in  FIG. 7  is also provided with a 10 G, 1270 nm shared upstream link. Both the dedicated upstream transmission and the shared upstream transmission from each ONU-group are received by the receiving port of an optical transceiver module, such as modules  704  and  706 , and each upstream transmission occupies a TDMA time slot. The 10 G, 1270 nm upstream transmissions received from all four ONU-groups are merged together by a 1:4 merger  736 , and the merged signal is sent to a common 10 G upstream MAC interface  734  for control and processing. As a result, common 10 G upstream MAC interface  734  arbitrates the 10 G upstream transmissions from all ONUs within all four ONU-groups. 
       FIG. 8  presents a block diagram illustrating an exemplary configuration of an OLT line card for a symmetric EPON system in accordance with one embodiment of the present invention. In  FIG. 8 , the structure of OLT line card  800  is similar to that of OLT line card  500  illustrated in  FIG. 5 . In  FIG. 8 , OLT line card  800  includes an ONU chip  802  and couples to two ONU-groups  826  and  828 . OLT line card  800  provides a 10 G, 1577 nm shared downstream link, a 1.25 G, 1490 nm dedicated downstream link, and a 2.5 G, 1550 nm dedicated downstream link to each ONU-group. The 10 G shared downstream link is provided by a common 10 G downstream MAC interface  812 , which controls the downstream transmission of optical transceiver modules  804  and  806 , via a 1:2 splitter  816 . 1.25 G, 1490 nm dedicated downstream links to individual ONU-groups are provided by dedicated 1.25 G downstream MAC interfaces  822  and  824 , which control the optical transmitter modules  808  and  810 . 2.5 G, 1550 nm dedicated downstream links to individual ONU-groups are provided by dedicated 2.5 G downstream MAC interfaces  838  and  840 , which control the optical transmitter modules  842  and  844 . The shared and dedicated downstream transmissions to each ONU-group are multiplexed to a single strand of fiber via a WDM multiplexer, such as multiplexers  830  and  832 . The difference between the system shown in  FIG. 8  and the one shown in  FIG. 5  is that, in addition to a dedicated 1.25 G, 1310 nm upstream link, each ONU-group in  FIG. 8  is also provided with a 10 G, 1270 nm shared upstream link. Both the dedicated upstream transmission and the shared upstream transmission from each ONU-group are received by the receiving port of an optical transceiver module, such as modules  804  and  806 , and each upstream transmission occupies a TDMA time slot. The 10 G, 1270 nm upstream transmissions received from both ONU-groups are merged together by a 1:2 merger  836 , and the merged signal is sent to a common 10 G upstream MAC interface  834  for control and processing. As a result, common 10 G upstream MAC interface  834  arbitrates the 10 G upstream transmissions from all ONUs within the two ONU-groups  826  and  828 . 
     WDM EPON 
     The systems illustrated in  FIGS. 6-8  rely on TDMA for scheduling transmissions between the shared upstream link and the dedicated upstream link. It is also possible to utilize a WDM demultiplexer that demultiplexes the two upstream links and sends them to two separate receivers.  FIG. 9  presents a diagram illustrating a WDM-EPON configuration in accordance with an embodiment of the present invention. In  FIG. 9 , two OLT line cards  900  and  902  are coupled to eight ONU-groups, such as ONU-group  918 . OLT line cards  900  includes a 10 G EPON OLT chip  904 , which is coupled to the eight ONU-groups via eight optical transceivers, such as transceivers  908  and  910 . OLT line card  900  provides a 10 G, 1577 nm shared downstream link and a 10 G, 1270 nm shared upstream link to each of the eight ONU-groups. OLT line card  902  includes a 1 G EPON OLT chip  906 , which is also coupled to the eight ONU-groups via eight optical transceivers, such as transceivers  912  and  914 . Instead of shared links, OLT line card  902  provides each of the eight ONU-groups with a dedicated 1.25 G (or 2.5 G), 1490 nm downstream link and a dedicated 1.25 G, 1310 nm upstream link. The shared and the dedicated links are combined using a WDM multiplexer/demultiplexer. For example, a WDM multiplexer/demultiplexer  916  combines the 10 G shared downstream transmission from transceiver  910  together with the dedicated 1.25 G (or 2.5 G) downstream transmission from transceiver  912 , and sends the combined signal to ONU-group  918 . Similarly, multiplexer/demultiplexer  916  demultiplexes the upstream transmissions from ONU-group  918  by sending the 10 G shared upstream transmission to transceiver  910  and the 1.25 G dedicated upstream transmission to transceiver  912 . 
     The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing computer-readable media now known or later developed. 
     The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium. 
     Furthermore, the methods and processes described above can be included in hardware modules. For example, the hardware modules can include, but are not limited to, application-specific integrated circuit (ASIC) chips, field-programmable gate arrays (FPGAs), and other programmable-logic devices now known or later developed. When the hardware modules are activated, the hardware modules perform the methods and processes included within the hardware modules. 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. 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.