Abstract:
A passive optical network system having a node that is optically coupled to optical line terminals (OLTs), and that is optically coupled to optical network units (ONUs). The node includes at least one fiber link module (FLM), each FLM including an upstream multiplex conversion device (MCD), and a downstream MCD. The upstream MCD receives an upstream optical signal from the ONUs, converts the upstream optical signal to an upstream electrical signal, and transmits a regenerated upstream optical signal to the OLTs. The downstream MCD receives a downstream optical signal from the OLTs, converts the downstream optical signal to a downstream electrical signal, and transmits a regenerated downstream optical signal to the ONUs.

Description:
BACKGROUND 
       [0001]    Many communications networks provide high bit-rate transport over a shared medium, such as a Passive Optical Network (PON), a cable television coaxial or hybrid fiber/coax (HFC) network, or a wireless network. These shared medium networks typically use time, frequency, or code division multiplexing to transport data signals from a central terminal to several remote customer terminals and Time Division Multiple Access (TDMA) to transport data signals from the remote terminals to the central terminal TDMA is characterized by non-continuous or burst mode data transmission. In existing optical networks, especially in a PON architecture, each packet of information from a remote terminal is multiplexed in a time sequence on one fiber and transmitted in a burst-like manner. 
         [0002]    A PON generally uses wavelength division multiplexing (WDM), for example, using one wavelength for downstream traffic and another for upstream traffic on a particular single fiber. WDM can include, for example, Wide Wavelength Division Multiplexing, Coarse Wavelength Division Multiplexing (CWDM), and Dense Wavelength Division Multiplexing (DWDM). 
         [0003]    An Ethernet passive optical network (EPON) is a PON that uses standard IEEE 802.3 Ethernet frames, for example, to encapsulate Internet Protocol (IP) traffic. Conventional EPON varieties include 1G EPON (supporting downstream and upstream speeds of 1 Gbps (gigabit per second)), 10G EPON (supporting downstream and upstream speeds of 10 Gbps), and 10G/1G EPON (supporting downstream speeds of 10 Gbps and upstream speeds of 1 Gbps). Other PON variations include Broadband PON (BPON), Gigabit PON (GPON), XGPON (also known as 10G-PON), and the like. 
         [0004]    An EPON typically supports bidirectional communications between an Optical Line Terminal (OLT) and one or more Optical Network Units (ONUs). Downstream traffic is from the OLT to the ONUs, and upstream traffic is from the ONUs to the OLT. An ONU may, for example, be included in customer premises equipment (CPE), or installed at a customer or end-user site, such as a home or residence, a multiunit residential building, an office building or complex, or a business or workplace. A typical ONU converts optical signals (e.g., transmitted via fiber) to electrical signals, and vice versa. 
         [0005]    In a typical configuration, an EPON hub includes one or more OLTs, each of which includes one or more EPON transceivers for optical signals. Each OLT includes one or more media access control (MAC) instances. The optical signals from each EPON transceiver are combined in a WDM combiner having one or more stages. A power splitter receives a single optical signal from the WDM combiner, and splits the signal to a plurality of optical fibers (each carrying many wavelengths). For example, a one-by-M (1×M) power splitter supports splitting the optical signal to M fibers. In another embodiment, the optical signals from each EPON transceiver bypass the WDM combiner and connected directly to the power splitter. 
         [0006]    There is a need for an optical regeneration device that converts from the WDM/CWDM/DWDM domain to the EPON domain, resides in the node that connects the hub to the CPE, and provides a cost effective solution that is flexibly deployed. The advantages provided by the optical regeneration device of the present invention include enabling coexistence between PON signals and existing HFC services, increasing the optical link budget, increasing the geographic area that each OLT port can serve, increasing the number of subscribers who can be served by a single OLT port, and reducing the cost of the subscriber side optics. The presently disclosed invention satisfies these demands. 
       SUMMARY 
       [0007]    Aspects of the present invention provide a multiplex conversion module, an EPON system with a multiplex conversion module, and systems and methods for EPON multiplex conversion. Aspects of the present invention also provide a passive optical network system having a node that is optically coupled to optical line terminals (OLTs), and that is optically coupled optical network units (ONUs). The node includes at least one fiber link module (FLM), each FLM including an upstream multiplex conversion device (MCD), and a downstream MCD. The upstream MCD receives an upstream optical signal from the ONUs, converts the upstream optical signal to an upstream electrical signal, and transmits a regenerated upstream optical signal to the OLTs. The downstream MCD receives a downstream optical signal from the OLTs, converts the downstream optical signal to a downstream electrical signal, and transmits a regenerated downstream optical signal to the ONUs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram that illustrates an exemplary EPON system in accordance with an embodiment of the present invention. 
           [0009]      FIG. 2  is a block diagram that illustrates a fiber link module according to an embodiment of the present invention. 
           [0010]      FIG. 3  is a block diagram that illustrates an exemplary 10G EPON OLT system in accordance with an embodiment of the present invention. 
           [0011]      FIG. 4  is a block diagram that illustrates an exemplary 10G EPON OLT system in accordance with an embodiment of the present invention. 
           [0012]      FIG. 5  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. 
           [0013]      FIG. 6  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. 
           [0014]      FIG. 7  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. 
           [0015]      FIG. 8  is a block diagram that illustrates an exemplary 1G EPON OLT system in accordance with an embodiment of the present invention. 
           [0016]      FIG. 9  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. 
           [0017]      FIG. 10  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The figures described below depict an EPON system, but the reader should understand that embodiments of the invention are applicable to any point-to-multipoint optical network topology, for example, BPON or GPON. 
         [0019]      FIG. 1  is a block diagram that illustrates an exemplary EPON system in accordance with an embodiment of the present invention. The EPON system shown in  FIG. 1  depicts a system level implementation that includes a hub  110  that connects to one or more nodes  130 , where each node  130  connects to ONUs installed at one or more customer sites. For illustrative purposes,  FIG. 1  depicts an exemplary ONU at a cellular tower  151  (e.g., to support backhaul), another exemplary ONU at a business or office building  152 , and a third exemplary ONU at a home or residence  153 . The connection from the node  130  to each ONU is a fiber connection. 
         [0020]    The hub  110  shown in  FIG. 1  depicts one or more PON OLTs that can be combined together with other HFC operator services onto a single network fiber from the hub  110  to each node  130 . The single network fiber from the hub  110  to each node  130  may carry many wavelengths.  FIG. 1  depicts the hub  110  including one or more OLTs that connect to a WDM combiner  120 , where the combined side of the WDM combiner  120  connects to a 1×M power splitter  125 . For illustrative purposes,  FIG. 1  depicts an exemplary 1G EPON OLT  111  that includes a 1G EPON transceiver (XCVR)  112 , an exemplary 10G EPON OLT  113  that includes a 10G EPON XCVR  114 , an exemplary 1G EPON OLT  115  that includes a DWDM/CWDM 1G XCVR  116 , and an exemplary 10G EPON OLT  117  that includes a DWDM/CWDM 10G XCVR  118 . The 1G EPON OLT  111 , 10G EPON OLT  113 , 1G EPON OLT  115 , and 10G EPON OLT  117  shown in  FIG. 1  further include a MAC (not shown) instance. 
         [0021]    The WDM combiner  120  may include one or more stages, for example, wide WDM, CWDM, or DWDM. In various embodiments, the WDM combiner  120  may include a WDM filter to support fiber conservation where the desire is to minimize the number of fibers used in a multiple service system; however, the present invention can operate without a WDM filter. In one embodiment, the 1×M power splitter  125  is a 1×M splitter where the desire is to use a single OLT port to support multiple nodes  130  in the field. In an alternative embodiment, the 1×M power splitter  125  is not present, and the invention supports the use of a 1×M splitter. The output of the splitter can then feed one or more of fiber nodes  130 . Thus, a single OLT port can service multiple nodes  130 . By connecting the nodes  130  in this manner, one or more instantiations of embodiments of the invention can operate independently, or together, to feed an access optical network. The subscriber (downstream) side of the node  130  can support any split and distance architecture that a standard PON architecture would support. The downstream side of the node can include, in some embodiments, a WDM filter, or WDM combiner  120 , for recombining signals using EPON wavelengths. 
         [0022]    The node  130  (e.g., a remote node in an HFC network) shown in  FIG. 1  separates the hub-to-node link from the node-to-CPE link. The node  130  includes a WDM demultiplexer  135  that receives a fiber connection from the 1×M power splitter  125 . The WDM demuliplexer  135  is paired with the WDM combiner  120 , where the demultiplexed side of the WDM demultiplexer  135  connects to one or more segments, where each segment includes a fiber link module  140 . In the embodiment shown in  FIG. 1 , each fiber link module  140  includes a 1 Gbps/10 Gbps upstream optical-to-electrical-to-optical (O-E-O) conversion  141 , and a 1 Gbps/10 Gbps downstream O-E-O conversion  142 . The downstream side of each fiber link module  140  connects to a 1×N power splitter  145 , where each downstream side of the 1×N power splitter  145  is an optical fiber connection to an ONU installed at a customer site. 
         [0023]    Advantageously, embodiments of the invention can allow fiber conservation between the hub  110  and the node  130 , and can coexist with existing HFC services. Further embodiments of the invention can advantageously increase the split ratio that a single OLT port can support when operated at full distance. Further embodiments of the invention can advantageously provide a significant reduction in the cost of 10G PON optics. Further embodiments of the invention can advantageously allow a single OLT port to address a much wider geographic radius than conventional PON systems. Still further embodiments of the invention can be fully IEEE/EPON compliant on the CPE-side, and can use off-the-shelf ONUs. Additional embodiments of the invention can be fully ITU compliant in a G.983/G.984 version, allowing the use of off-the-shelf ONTs. Further embodiments can offer significantly less power consumption when compared to a remote OLT or Intelligent PON Node, and/or significantly better mean time between failures (MTBF) and reliability when compared with a remote OLT or Intelligent PON Node. 
         [0024]      FIG. 2  is a block diagram that illustrates a fiber link module according to an embodiment of the present invention. The fiber link module  140  shown in  FIG. 2  provides conversion between a WDM domain (e.g., wide WDM, CWDM, DWDM, and the like) and a PON domain (e.g., EPON, BPON, GPON, 10GEPON, XGPON, and the like). In an embodiment, a fiber link module  140  comprises an optical regeneration device, for O-E-O conversion. In an embodiment, the fiber link module  140  comprises an upstream multiplex conversion device (MCD)  210  as the upstream O-E-O conversion  141 , and a downstream MCD  220  as the downstream O-E-O conversion  142 . The upstream MCD  210  converts optical signals from a WDM domain to a PON domain, and the downstream MCD  220  converts optical signals from a PON domain to a WDM domain. 
         [0025]    In addition, unlike a conventional  3 R (reamplify, re-shape, re-time) reach extender, an embodiment of the fiber link module  140  does not need to include clock detection and recovery circuits, does not need to include a MAC function in the node, and does not require an ONU system on a chip (SOC). 
         [0026]    The downstream MCD  220  shown in  FIG. 2  includes a photodiode  221 , downstream receiver  222 , downstream transmitter  223 , and laser  224 . The photodiode  221  and the circuitry comprising the downstream receiver  222  function as an optical receiver that receives downstream signals from the OLTs. The photodiode  221  converts the optical power on a fiber optic cable into a proportionate electrical current that is the input to the downstream receiver  222 . The output of the downstream receiver  222  is a digital logic voltage signal that is the input to the downstream transmitter  223 . The laser  224  converts the output of the downstream transmitter  223  to optical power on a fiber optic cable. The circuitry comprising the downstream transmitter  223  and laser  224  function as an optical transmitter to transmit downstream signals to the ONUs. 
         [0027]    In an exemplary embodiment, in the downstream path, the downstream MCD  220  comprises a simple O-E-O regeneration device with an optical receiver driving an optical transmitter. The optical receiver is designed to operate in standard continuous mode and receive the WDM/CWDM/DWDM signal from the OLT transmitter. The wavelength is selected based upon the constraints in a particular plant, in this example an HFC plant, and is based upon co-existence with existing services in that plant. The optical receiver simply converts the optical WDM/CWDM/DWDM non-return-to-zero (NRZ) signal into an electrical NRZ bit stream. This NRZ bit stream is then fed into a continuous mode optical transmitter that transmits at the appropriate wavelength for the data-rate and PON type. For example, in the case of a 1.25 Gbps EPON system, the nominal transmitter wavelength would be at 1490 nm. In the case of a 10 Gbps EPON system, the nominal transmitter wavelength would be at 1577 nm. 
         [0028]    The upstream MCD  210  shown in  FIG. 2  includes a photodiode  211 , upstream receiver  212 , upstream transmitter  213 , and laser  214 . The photodiode  211  and the circuitry comprising the upstream receiver  212  function as an optical receiver that receives upstream signals from the ONUs. The photodiode  211  converts the optical power on a fiber optic cable into a proportionate electrical current that is the input to the upstream receiver  212 . The output of the upstream receiver  212  is a digital logic voltage signal that is the input to the upstream transmitter  213 . The laser  214  converts the output of the upstream transmitter  213  to optical power on a fiber optic cable. The circuitry comprising the upstream transmitter  213  and laser  214  function as an optical transmitter to transmit upstream signals to the OLTs. In an alternative embodiment, the upstream MCD  210  includes an optional laser  214  enable/disable signal  215  from the upstream receiver  212  to the upstream transmitter  213 . The optional laser  214  enable  215  may be generated using an optical signal detect on the upstream receiver  212  photodiode  211 , or by using an RF signal detect function on the electrical data signal generated by the upstream receiver  212 . 
         [0029]    In the exemplary embodiment, in the upstream path, an upstream MCD  210  is provided. The upstream MCD  210  includes additional complexity that is not found in the downstream MCD  220 . In the upstream MCD  210 , a burst mode receiver  212  is used that does not require any kind of MAC information, such as a receiver described in U.S. Pat. No. 6,420,928 and U.S. Pat. No. 6,963,696, the disclosures of which are hereby fully incorporated by reference as if set forth herein. This burst mode receiver  212  is provided because the node  130  and the OLT will not always be co-located and because the PON MAC will be contained within the OLT. The burst mode receiver  212  is designed to receive at the appropriate wavelength for the data-rate and PON type that is being used. For example, in the case of a 1.25 Gbps EPON system, the nominal receiver wavelength would be at 1310 nm. In the case of a 10 Gbps EPON system, the nominal receiver wavelength would be at 1270 nm. The burst mode receiver  212  is designed to be stable over extended strings of logic “1&#39;s” and logic “0&#39;s”, as well as inter-packet gaps and burst dead times. In the case of inter-packet gaps and burst dead times, the optical signal will return to the logic “0” state, in which case the receiver will resolve to a logic “0” state on the electrical side as well. 
         [0030]    In the exemplary embodiment, the return transmitter  213  on the upstream side requires additional design consideration as well. The Automatic Power Control (APC) loop must be capable of handling the burst-mode nature of the upstream multi-point to point architecture. This means that the APC loop must be able to be activated during the burst, but then have its measuring and monitoring function frozen between bursts. Alternatively, a look-up table approach could also be used, as this would be immune to the challenges of operating in a burst-mode environment. In addition to the concerns around the APC loop operating in burst mode, the Laser bias current operation must also be addressed when operating in a point-to-multipoint configuration. Embodiments of the invention may be deployed under a number of different scenarios, consequently, a number of different bias current/power management options would need to be handled, dependent upon the deployment architecture. In some cases, no provisions would need to be made, with the exception of the APC loop control described above. In other cases, the bias power would need to be lowered, and consequently the Extinction Ratio (ER) at the transmitter would need to be increased in order to allow the design to operate in a system where multiple nodes are all serviced by a single OLT port. In other cases, where multi-point-to-point, many node to single hub architectures require the noise from adjacent upstream transmitters to be minimized, a signal will need to be provided between the burst mode receiver and the burst mode transmitter that logically enables and disables the transmitter portion of the design. This would allow the designer to reduce the competing transmitter noise to a level that is inconsequential and would enable the use of now industry standard burst mode transmitter architectures, such as those described in U.S. Pat. No. 6,738,401 and U.S. Pat. No. 7,031,357, the disclosures of which are hereby fully incorporated by reference as if set forth herein. The wavelength chosen for the upstream transmitter  213  located in the fiber link module  140  can be any WDM, CWDM, DWDM or other wavelength that is most compatible with the existing services running on the fiber, depending upon the exact deployment scenario. 
         [0031]    The forward or downstream MCD  220  uses a continuous mode receiver  222  circuit with an appropriate WDM/CWDM/DWDM front end. This receiver  222  receives input from, or comprises, a photodiode  221  that converts the optical power on the fiber into a proportionate electrical current. The receiver circuit then converts the current into an NRZ digital logic voltage signal. This logical signal can be any number of digital logic families, including, but not limited to, LVPECL or CML. The receiver  222  can be, for example, PIN or APD based. 
         [0032]    The downstream transmitter  223  functions using the appropriate wavelength for the data rate chosen. For example, in the case of a 1.25 Gbps EPON system, the nominal transmitter wavelength would be at 1490 nm. In the case of a 10 Gbps EPON system, the nominal transmitter wavelength would be at 1577 nm. It should also be noted that if the design is capable of 10 Gbps data rates, then it will also work at a 1.25 Gbps data rate. The laser  224  used in the downstream side, on the subscriber side, can be an FP, DFB or externally modulated laser  224 . In the case of a 10 Gbps application, where solution cost is very sensitive and where the distance between each node and the end customers allows, a lower cost DFB laser could be used in place of much more expensive externally modulated laser, even when the distance from the hub  110  to the customer ONU is too great to allow for such an implementation. The laser bias current is shown external of the transmitter block, though this could also be included in the laser driver block. 
         [0033]    In the upstream MCD  210 , a burst mode receiver  212  is used that does not require any kind of MAC level information (i.e., a reset-less burst mode receiver) that can be chatter-free, such as the receiver described in U.S. Pat. Nos. 6,420,928 and 6,963,696. This is because the node and the OLT will not always be co-located and because the PON MAC will be contained within the OLT. The burst mode receiver  212  is designed to receive at the appropriate wavelength for the data-rate and PON type that is being used. For example, in the case of a 1.25 Gbps EPON system, the nominal receiver wavelength would be at 1310 nm. In the case of a 10 Gbps EPON system, the nominal receiver wavelength would be at 1270 nm. The burst mode receiver is designed to be stable over extended strings of logic “1&#39;s” and logic “0&#39;s”, as well as inter-packet gaps and burst dead times. In the case of inter-packet gaps and burst dead times, the optical signal will return to the logic “0” state, in which case the receiver will resolve to a logic “0” state on the electrical side as well. The burst mode receiver  212  may employ either a PIN or APD based receiver, depending upon the cost/network architecture trade-offs. 
         [0034]    In an embodiment, the upstream transmitter  213  (return transmitter on the upstream side) requires additional design consideration as well. The return transmitter  213  includes an Automatic Power Control loop (APC) that must be capable of handling the burst-mode nature of the upstream multi-point-to-point architecture. This means that the APC loop must be able to be activated during the burst, but then have its measuring and monitoring function frozen between bursts. Alternatively, a look-up table approach could also be used, as this would be immune to the challenges of operating in a burst-mode environment. In addition to the concerns around the APC loop operating in burst mode, the Laser bias current operation must also be addressed when operating in a point to multipoint configuration. Embodiments of the invention may be deployed under a number of different scenarios, consequently, a number of different bias current/power management options would need to be handled, dependent upon the deployment architecture. In some cases, such as a point-to-point network between the node and the hub, no provisions would need to be made, with the exception of the APC loop control described above. In other cases, the bias power would need to be lowered, and consequently the Extinction Ratio (ER) at the transmitter would need to be increased in order to allow the design to operate in a system where multiple nodes are all serviced by a single OLT port. In other cases, where multi-point-to-point, many node to single hub architectures require the noise from adjacent upstream transmitters to be minimized, a signal will need to be provided between the burst mode receiver and the burst mode transmitter that logically enables and disables the transmitter portion of the design. This would allow the designer to reduce the competing transmitter noise to a level that is inconsequential and would enable the use of now industry standard burst mode transmitter architectures, such as those described in U.S. Pat. Nos. 6,738,401 and 7,031,357. The wavelength chosen for the upstream transmitter located in the node can be any WDM, CWDM, DWDM or other wavelength that is most compatible with the existing services running on the fiber, depending upon the exact deployment scenario. The control connection between the burst mode receiver and the burst mode transmitter is also illustrated for cases when this signal is required. 
         [0035]      FIG. 3  is a block diagram that illustrates an exemplary 10G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 3  is an embodiment of the system architecture shown in  FIG. 1 . The hub  310  shown in  FIG. 3  includes a single OLT, a 10G EPON OLT  313  that includes a 10G EPON XCVR  314 , that connects to a 1×M power splitter  125 . The output of the 1×M power splitter  125  feeds one or more fiber nodes  330 . The node  330  includes a WDM demultiplexer  335  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  335  connects to one or more segments, where each segment includes a fiber link module  340  that includes an upstream O-E-O conversion  341  and a downstream O-E-O conversion  342 . The downstream side of each fiber link module  340  connects to a combiner  336 , that connects to a 1×N power splitter  145 , where each downstream side of the 1×N power splitter  145  is an optical fiber connection to an ONU installed at a customer site. 
         [0036]      FIG. 4  is a block diagram that illustrates an exemplary 10G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 4  is an embodiment of the system architecture shown in  FIG. 1 . The hub  410  shown in  FIG. 4  includes a single OLT, a 10G EPON OLT  413  that includes a 10G EPON XCVR  414 , that connects to a 1×M power splitter  125 . The output of the 1×M power splitter  125  feeds one or more fiber nodes  430 . The node  430  includes a WDM demultiplexer  435  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  435  connects to one or more segments, where each segment includes a fiber link module  440  that includes a 10 Gbps upstream O-E-O conversion  441  at the 1270 nm wavelength, a 1.25 Gbps upstream O-E-O conversion  442  at the 1310 nm wavelength, a 10 Gbps downstream O-E-O conversion  443  at the 1577 nm wavelength, and a 1.25 Gbps downstream O-E-O conversion  444  at the 1490 nm wavelength. As shown in  FIG. 4 , the four paths in the fiber link module  440  run in parallel. The downstream side of each fiber link module  440  connects to a combiner  436 , that connects to a 1×N power splitter  145 , where each downstream side of the lxN power splitter  145  is an optical fiber connection to an ONU installed at a customer site. In an alternative embodiment, in an application where it is desired to operate the 10 Gbps EPON and 1.25 Gbps EPON upstream links  441 ,  442  in TDMA mode, the 1.25 Gbps upstream O-E-O conversion  442  at the 1310 nm wavelength may be eliminated, and the 10 Gbps upstream O-E-O conversion  441  at the 1270 nm wavelength may use a broadband detector that is capable of receiving both 1310 nm and 1270 nm signals. In this alternative embodiment, the upstream O-E-O conversion  442  will run at both 10 Gbps and 1.25 Gbps, but a reduction in dynamic range and link budget may be observed. 
         [0037]      FIG. 5  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 5  is an embodiment of the system architecture shown in  FIG. 1 . The hub  510  shown in  FIG. 5  includes a single OLT, a 10G/1G EPON OLT  513  that includes a 10G EPON XCVR  514 , that connects to a 1×M power splitter  125 . The output of the 1×M power splitter  125  feeds one or more fiber nodes  530 . The node  530  includes a WDM demultiplexer  535  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  535  connects to one or more segments, where each segment includes a fiber link module  540  that includes a 1.25 Gbps/10 Gbps upstream O-E-O conversion  541  at the 1310 nm and 1270 nm wavelengths, a 10 Gbps downstream O-E-O conversion  542  at the 1577 nm wavelength, and a 1.25 Gbps downstream O-E-O conversion  543  at the 1490 nm wavelength. As shown in  FIG. 5 , the three paths in the fiber link module  540  are optimized for cost and power consumption within the node, rather than for link budget on the access network side of the node. The downstream side of each fiber link module  540  connects to a combiner  536 , that connects to a 1×N power splitter  145 , where each downstream side of the lxN power splitter  145  is an optical fiber connection to an ONU installed at a customer site. 
         [0038]      FIG. 6  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 6  is an embodiment of the system architecture shown in  FIG. 1 . The hub  610  shown in  FIG. 6  includes a single OLT, a 10G/1G EPON OLT  613  that includes a 10G EPON XCVR  614 , that connects to a 1×M power splitter  125 . The output of the 1×M power splitter  125  feeds one or more fiber nodes  630 . The node  630  includes a WDM demultiplexer  635  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  635  connects to one or more segments, where each segment includes a fiber link module  640  that includes a 1.25 Gbps upstream O-E-O conversion  641  at the 1310 nm wavelength, and a 10 Gbps downstream O-E-O conversion  642  at the 1577 nm wavelength. As shown in  FIG. 6 , the two paths in the fiber link module  640  are an asymmetric implementation that uses a 10 Gbps downstream link in conjunction with a 1.25 Gbps upstream link. The downstream side of each fiber link module  640  connects to a combiner  636 , that connects to a 1×N power splitter  145 , where each downstream side of the 1×N power splitter  145  is an optical fiber connection to an ONU installed at a customer site. 
         [0039]      FIG. 7  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 7  is an embodiment of the system architecture shown in  FIG. 1 . As shown in  FIG. 7 , the hub  110  includes legacy radio frequency (RF) HFC services  719  that connect to the WDM combiner  120 . The output of the 1×M power splitter  125  in the hub  110  shown in  FIG. 7  feeds one or more fiber nodes  730 . The node  730  includes a WDM demultiplexer  135  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  135  connects to one or more segments, where each segment includes a fiber link module  140  similar to that shown in  FIG. 1 . In addition, the node  730  includes a link module  740  that provides upstream RF-to-optical conversion  741 , and downstream optical-to-RF conversion  742 , to support the legacy RF HFC services  719 . The fiber link modules  140  co-exist with the link module  740  because the WDM/CWDM/DWDM link between the node  730  and the hub  110  allow these services to operate independently of each other and with the legacy RF HFC services  719 . 
         [0040]      FIG. 8  is a block diagram that illustrates an exemplary 1G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 8  is an embodiment of the system architecture shown in  FIG. 1 . The hub  810  shown in  FIG. 8  includes one or more 1G EPON OLTs, where one 1G EPON OLT  817  that includes a DWDM/CWDM XCVR  818  connects to a WDM combiner  120 , where the combined side of the WDM combiner  120  connects to a 1×M power splitter  125 . The output of the 1×M power splitter  125  feeds one or more fiber nodes  830 . The node  830  includes a WDM demultiplexer  835  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  835  connects to one or more segments, where each segment includes a fiber link module  840  that includes an upstream O-E-O conversion  841 , and a downstream O-E-O conversion  842 . As shown in  FIG. 8 , a single OLT port services multiple fiber nodes and a variety of services, for example, residential applications, business customers or Cell Tower Backhaul (CTBH) traffic. The downstream side of each fiber link module  840  connects to a combiner  836 , that connects to a 1×N power splitter  145 , where each downstream side of the 1×N power splitter  145  is an optical fiber connection to an ONU installed at a customer site. 
         [0041]      FIG. 9  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 9  is an embodiment of the system architecture shown in  FIG. 1 .  FIG. 9  depicts a hub  910  that includes four OLTs that connect to a WDM combiner  120 , where the combined side of the WDM combiner  120  connects to a 1×M power splitter  125 . For illustrative purposes,  FIG. 9  depicts an exemplary 1G EPON OLT  911  that includes a DWDM/CWDM XCVR  912 , another exemplary 1G EPON OLT  913  that includes a DWDM/CWDM XCVR  914 , an exemplary 10G EPON OLT  915  that includes two DWDM/CWDM XCVRs  916 ,  917 , and an exemplary 10G/10G/1G/1G Multi-Rate EPON OLT  918  that includes a DWDM/CWDM XCVR  919 . The output of the 1×M power splitter  125  in the hub  910  shown in  FIG. 7  feeds one or more fiber nodes  930 . The node  930  includes a WDM demultiplexer  935  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  935  connects to one or more segments, where each segment includes a fiber link module  140 . The first fiber link module  940  shown in  FIG. 9  includes a 1.25 Gbps upstream O-E-O conversion  941 , and a 1.25 Gbps downstream O-E-O conversion  942 . The second fiber link module  950  shown in  FIG. 9  includes a 10 Gbps upstream O-E-O conversion  951 , and a 10 Gbps downstream O-E-O conversion  952 . The third fiber link module  960  shown in  FIG. 9  includes a 1 Gbps/10 Gbps upstream O-E-O conversion  961 , and a 1 Gbps/10 Gbps downstream O-E-O conversion  962 . A combiner  936  receives the output from the first fiber link module  940  and the second fiber link module  950 . Another combiner  937  receives the output from the third fiber link module  960 . The three fiber link modules  940 ,  950 ,  960  shown in  FIG. 9  run in parallel to provide 1 Gbps to some of the ONUs and 10 Gbps to other ONUs. The system implementation shown in  FIG. 9  provides multi-rate PON service delivered over a single access network. The implementation inside of the node  930  can consist of multiple independent conversion modules, or a single condensed conversion module. Architecturally these approaches are the same as each path is independent. Combination and splitting occurs in the WDM filters on either side of the module, either in a single stage or cascade of stages. Additional, independent modules, can also be populated in the same node. 
         [0042]      FIG. 10  is a block diagram that illustrates an exemplary 10G/1G EPON OLT system in accordance with an embodiment of the present invention. The system architecture shown in  FIG. 10  is an embodiment of the system architecture shown in  FIG. 1 .  FIG. 10  depicts a hub  1010  that includes five OLTs that connect to a WDM combiner  120 , where the combined side of the WDM combiner  120  connects to a 1×M power splitter  125 . For illustrative purposes,  FIG. 10  depicts three exemplary 1G EPON OLTs  1011 ,  1012 ,  1013 , an exemplary 10G EPON OLT  1014  that includes two DWDM/CWDM XCVRs  1015 ,  1016 , and an exemplary 10G EPON OLT  1017  that includes a DWDM/CWDM XCVR  1018 . The output of the 1×M power splitter  125  in the hub  1010  shown in  FIG. 10  feeds one or more fiber nodes  1030 . The node  1030  includes a WDM demultiplexer  1035  that receives a fiber connection from the 1×M power splitter  125 . The demultiplexed side of the WDM demultiplexer  1035  connects to one or more segments, where each segment includes a fiber link module  1040  that includes a 1G/10G upstream O-E-O conversion  1041 , and a 1G/10G downstream O-E-O conversion  1042 . The downstream side of each fiber link module  1040  connects to a combiner  1036 , that connects to a 1×N power splitter  145 , where each downstream side of the 1×N power splitter  145  is an optical fiber connection to an ONU installed at a customer site.  FIG. 10  illustrates an architecture that allows for future flexibility, according to further embodiments. In the case of deployment scenarios where service “take rates” may be low and it is desirable to maximize the geographic area that can be supported per OLT port, many nodes can be supported by a single OLT port and consequently, the service radius of each node can be added together. If, in the future, the service usage at a given node or set of nodes increases to the point where it is desired to move one or a group of nodes to a single OLT port, this can be accomplished by re-routing the fiber to a free OLT port or an additional optical splitter/combiner  1021 ,  1022  that feeds a free OLT port. In these scenarios, service would only be temporarily affected for the users on the nodes that are being moved, the remaining nodes would continue to operate unaffected. As shown in the diagram, a WDM filter combiner would only be required if additional services are required to be multiplexed or if fiber conservation is important to the implementer. 
         [0043]    In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims.