Abstract:
One embodiment provides a pluggable optical line terminal (OLT). The OLT includes a bi-directional optical transceiver configured to transmit optical signals to and receive optical signals from a number of optical network units (ONUs), an OLT chip coupled to the optical transceiver and configured to communicate with the ONUs through the optical transceiver, and a pluggable interface coupled to the OLT chip and configured to electrically interface between the OLT chip and a piece of network equipment. The optical transceiver, the OLT chip, and the pluggable interface are contained in an enclosure complying with a form factor, thereby allowing the pluggable OLT to be directly plugged into the network equipment.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The present disclosure relates to the design of Ethernet passive optical networks (EPONs). More specifically, the present disclosure relates to the design of a pluggable optical line terminal (OLT). 
         [0003]    2. Related Art 
         [0004]    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 is also needed to meet the increasing bandwidth demand of end users for triple play services, including Internet protocol (IP) video, high-speed data, and packet voice. 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. 
         [0005]    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. 
         [0006]    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 (see  FIG. 1 ). Note that other topologies are also possible, including ring and mesh topologies. 
         [0007]    Transmissions within a PON are typically performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT controls channel connection, management, and maintenance, and generally resides in the central office. The OLT provides an interface between the PON and a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. For EPON, such interface is an Ethernet interface. The ONU terminates the PON and presents the native service interfaces to the end users, and can reside in the customer premise and couples to the customer&#39;s network through a customer-premises equipment (CPE). 
         [0008]      FIG. 1  illustrates a passive optical network including a central office and a number of customers coupled through optical fibers and a passive optical splitter (prior art). A passive optical splitter  102  and optical fibers couple the customers to a central office  101 . Multiple splitters can also be cascaded to provide the desired split ratio and a greater geographical coverage. Passive optical splitter  102  can reside near end-user locations to minimize the initial fiber deployment costs. Central office  101  can couple to an external network  103 , such as a metropolitan area network operated by an ISP. 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), gigabit PONs (GPONs), and wavelength division multiplexing (WDM) PONs. 
         [0009]    In conventional EPON configurations, at a carrier&#39;s central office, an OLT line card containing multiple OLTs can aggregate traffic from multiple downstream PONs, each communicating with an OLT. Multiple OLT line cards can be placed in a chassis to interface with the metro backbone via a backplane. To implement such a configuration, a carrier typically purchases custom-designed OLT line cards (based on the requirement of the traffic aggregation equipment and the metro backbone network) from equipment vendors. Such custom-designed OLT line cards can be expensive, and often require large amounts of capital expenditures (CAPEX) even during the initial network deployment stage. For example, each OLT line card includes a fixed number of OLT chips regardless of the number of existing subscribers. Therefore, when the number of subscribers is low, a large portion of the capacity of the OLT line card is wasted without generating revenue for the carrier. In addition, such an approach can also be cost ineffective for future upgrades as the number of subscribers increases. 
       SUMMARY 
       [0010]    One embodiment provides a pluggable optical line terminal (OLT). The OLT includes a bi-directional optical transceiver configured to transmit optical signals to and receive optical signals from a number of optical network units (ONUs), an OLT chip coupled to the optical transceiver and configured to communicate with the ONUs through the optical transceiver, and a pluggable interface coupled to the OLT chip and configured to electrically interface between the OLT chip and a piece of network equipment. The optical transceiver, the OLT chip, and the pluggable interface are contained in an enclosure complying with a form factor, thereby allowing the pluggable OLT to be directly plugged into the network equipment. 
         [0011]    In a variation on this embodiment, the network equipment is an Ethernet line card. 
         [0012]    In a variation on this embodiment, the pluggable interface is a gigabit interface converter (GBIC) interface which can be plugged into a GBIC port on the network equipment, and the form factor is substantially the same as a GBIC transceiver. 
         [0013]    In a variation on this embodiment, the pluggable interface is a small form-factor pluggable (SFP) interface which can be plugged into an SFP port on the network equipment, and the form factor is substantially the same as an SFP transceiver. 
         [0014]    In a variation on this embodiment, the pluggable interface is a 10 gigabit small form factor pluggable (XFP) interface which can be plugged into an XFP port on the network equipment, and the form factor is substantially the same as an XFP transceiver. 
         [0015]    In a variation on this embodiment, the pluggable interface is a small form-factor pluggable plus (SFP+) interface which can be plugged into an SFP+ port on the network equipment, and the form factor is substantially the same as an SFP+ transceiver. 
         [0016]    In a variation on this embodiment, the pluggable interface is a XENPAK interface which can be plugged into a XENPAK port on the network equipment, and the form factor is substantially the same as a XENPAK transceiver. 
         [0017]    In a variation on this embodiment, the pluggable interface is an X2 interface which can be plugged into an X2 port on the network equipment, and the form factor is substantially the same as an X2 transceiver. 
         [0018]    In a variation on this embodiment, the bi-directional optical transceiver is a pluggable transceiver and is configured to transmit optical signals into and receive optical signals from a multi-mode or a single-mode optical fiber. 
         [0019]    In a variation on this embodiment, the OLT further includes a power-management module configured to provide power to the OLT chip and the optical transceiver, using power delivered from the network equipment through the pluggable interface. 
         [0020]    In a variation on this embodiment, the OLT further includes a serializer/deserializer (SERDES) module which is coupled between the pluggable interface and the OLT chip, thereby facilitating serial communication through the pluggable interface. 
         [0021]    In a variation on this embodiment, the OLT further includes a printed circuit board (PCB). In addition, the OLT chip includes a die directly attached on the PCB without conventional chip packaging. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0022]      FIG. 1  illustrates an EPON wherein a central office and a number of subscribers are coupled through optical fibers and a passive optical splitter (prior art). 
           [0023]      FIG. 2  presents a diagram illustrating the architecture of an exemplary dual-port OLT chip within a pluggable OLT module in accordance with an embodiment of the present invention. 
           [0024]      FIG. 3  presents a diagram illustrating the architecture of an exemplary pluggable dual-port OLT module with a XENPAK form factor in accordance with an embodiment of the present invention 
           [0025]      FIG. 4  presents a diagram illustrating the architecture of an exemplary pluggable dual-port OLT module with an X2 form factor in accordance with an embodiment of the present invention. 
           [0026]      FIG. 5  presents a diagram illustrating the architecture of an exemplary OLT line card in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    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 (e.g., general passive optical network (PON) architectures). 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. 
       Overview 
       [0028]    In embodiments of the present invention, the problem of the need to use custom-designed OLT line cards is solved by providing an OLT module that is pluggable to an existing off-the-shelf line card. The OLT module complies with the form factor required by the line card interface and includes an OLT chip and a pluggable interface. During operation, the OLT chip within the pluggable OLT module communicates with a number of ONUs via a bi-directional optical transceiver. The OLT chip, the pluggable interface, and the optical transceiver are contained in an enclosure that complies with a certain form factor, thus forming a pluggable OLT module. In some embodiment, the pluggable OLT module can be plugged into an input port on an off-the-shelf (OTS) Ethernet line card. Consequently, a service carrier can utilize any available OTS Ethernet line card to aggregate traffic from a number of downstream PONs instead of developing a custom-designed OLT line card. 
       OLT Chip Architecture 
       [0029]      FIG. 2  presents a diagram illustrating the architecture of an exemplary dual-port OLT chip within a pluggable OLT module in accordance with an embodiment of the present invention. Dual-port OLT chip  200  includes two EPON media access control (MAC) modules  202  and  204 , two EPON scheduler/traffic-shaper modules  206  and  208 , two EPON lookup engines  210  and  212 , a first-in-first-out (FIFO) buffer  214 , two Ethernet lookup engines  216  and  218 , two Ethernet traffic-shaper modules  220  and  222 , two Ethernet MAC modules  224  and  226 , and a management interface  228 . 
         [0030]    EPON MAC modules  202  and  204  interface with downstream PONs. In the downstream direction, EPON MAC module  202  and EPON MAC module  204  broadcast Ethernet traffic to their respective PONs. In one embodiment, EPON MAC modules  202  and  204  can each operate at dual data rate. In the upstream direction, EPON MAC modules  202  and  204  perform dynamic bandwidth allocation (DBA), which is used for arbitrating upstream traffic from various ONUs. In addition, EPON MAC modules  202  and  204  also perform forward error correction (FEC) in both downstream and upstream directions. 
         [0031]    EPON scheduler/traffic-shaper modules  206  and  208  perform downstream traffic flow control. Similarly, Ethernet traffic-shaper modules  220  and  222  perform flow control in the upstream direction. EPON lookup engines  210  and  212  and Ethernet lookup engines  216  and  218  can perform forwarding-table lookup functionalities such as determining the corresponding logical-link-identifier (LLID) in the downstream direction or virtual local area network (VLAN) ID in the upstream direction for a received packet. FIFO buffer  214  includes a number of FIFO queues corresponding to different traffic priorities. A received packet can be stored in a corresponding FIFO queue based on the lookup result. For example, in the downstream direction the system can maintain separate FIFO queues for unicast traffic, multicast traffic, and broadcast traffic. 
         [0032]    Ethernet MAC modules  224  and  226  couple to the service provider&#39;s network via a number of interfaces, including but not limited to: Media Independent Interface (MII), Gigabit MII (GMII), 10 Gigabit MII (XGMII), and 10 Bit parallel Interface (TBI). Management interface  228  includes an embedded microprocessor that enables PON management and control. 
         [0033]    Note that each dual-port OLT chip  200  can support two downstream PONs. In one embodiment, dual-port OLT chip  200  communicates with the downstream PONs via optical transceivers. In the upstream direction, each port of the dual-port OLT chip  200  is coupled to a serializer/deserializer (SERDES), which provides a digital interface between OLT chip  200  and the service provider&#39;s network. In addition to optical transceivers and SERDES, external memories, such as synchronous dynamic random access memory (SDRAM) and flash memory, can also be used to support packet buffering and PON management. 
         [0034]    The OLT chip and other supporting components, including a module that manages and supplies power, can be placed in a pizza-box type physical enclosure to form an OLT module. However, in the carrier&#39;s central office, where space is precious, it is desirable to place multiple OLT chips on a single line card, which can be plugged into a chassis. Such an approach enables the carrier&#39;s ability to support large numbers of subscribers using cost-effective equipments. Because currently no standard OLT line cards exist, carriers often need to purchase custom-designed OLT line cards from equipment vendors. These custom-designed OLT line cards can be expensive. In addition, because the number of OLT chips on a custom-designed OLT line card is fixed, this solution is less economical during the initial network deployment stage when the subscriber number is low. 
         [0035]    To overcome aforementioned issues, in embodiments of the present invention, OLT chip  200  illustrated in  FIG. 2  is enclosed in a pluggable OLT module which can be directly plugged into a standard off-the-shelf Ethernet line card with a standard interface. This solution can significantly reduce the CAPEX costs since the OTS Ethernet line card is much cheaper than a custom-designed OLT line card. In addition, when the number of subscribers is low, the carrier can choose to leave a number of ports on the Ethernet line card vacant, thus further reducing equipment costs during initial EPON deployment. The carrier can install more pluggable OLT modules as the number of subscribers increases. Consequently, this approach provides a “pay-as-you-grow” solution for service providers. 
       Pluggable OLT 
       [0036]      FIG. 3  presents a diagram illustrating the architecture of an exemplary pluggable dual-port OLT module with a XENPAK form factor in accordance with an embodiment of the present invention. Note that XENPAK is standard for transceivers which are compatible with 10 Gigabit Ethernet standard (IEEE standard 802.3). XENPAK defines a hot-swappable electrical interface and can support a wide range of physical media, including multi-mode and single-mode optical fibers and copper cables. Transmission distances vary from 100 meters to 80 kilometers for optical fiber and up to 15 meters for copper cable. The specification of the XENPAK standard can be obtained at the website of the small form factor (SFF) committee (http://www.sffcommittee.com/ie/Specifications.html). 
         [0037]    In  FIG. 3 , a XENPAK dual-port OLT module  300  includes fiber connectors  302  and  304  for coupling to optical fibers on the plant side, i.e., the EPON fibers. Through connectors  302  and  304 , optical bi-directional transceivers  306  and  308  transmit optical signals to and receive signals from the optical fibers. Note that both transceivers are capable of simultaneously transmitting and receiving. For example, transceiver  306  can transmit a downstream signal to and receive an upstream signal from the same fiber, wherein the two signals are on different wavelengths, and wherein the fiber can be a single-mode or multi-mode fiber. In a further embodiment, optical transceivers  306  and  308  can be pluggable transceivers, such as small form-factor pluggable (SFP) transceivers or 10 gigabit SFP (XFP) transceivers. The specifications of SFP standard and XFP standard can also be found at the website of the SFF committee. 
         [0038]    Each optical transceiver is further coupled to a corresponding EPON SERDES, through a transmission (TX) link and a receiving (RX) link. For example, transceiver  306  is coupled to EPON SERDES  310 . In the upstream direction, an EPON SERDES, such as SERDES  310 , deserializes the PON signals received by a corresponding optical transceiver before sending the deserialized signals to an OLT chip  314  for processing. OLT chip  314  has a similar configuration as the OLT chip shown in  FIG. 2 . Note that EPON SERDES  310  and EPON SERDES  312  are coupled to corresponding EPON MAC modules located on OLT chip  314 . 
         [0039]    In addition to EPON SERDES  310  and EPON SERDES  312  for serializing/deserializing PON signals, XENPAK dual-port OLT module  300  also includes an Ethernet SERDES  316  and an Ethernet SERDES  318 , which provide a high-speed serial interface between the OLT chip and the carrier&#39;s network. In the downstream direction, an Ethernet SERDES, such as SERDES  316 , deserializes network signals received from the carrier&#39;s network before sending the deserialized network signals to OLT chip  314  for processing. Note that Ethernet SERDES  316  and Ethernet SERDES  318  are coupled to corresponding Ethernet MAC modules located on OLT chip  314 . XENPAK dual-port OLT module  300  includes a standard XENPAK interface  320 , which provides serial communication channels between OLT chip  314  and a corresponding Ethernet line card. 
         [0040]    Also included in XENPAK dual-port OLT module  300  are a number of synchronous dynamic random access memories (SDRAM), such as double data rate (DDR2) SDRAMs  322  and  324 , a flash memory  326 , a power management module  328 , a craft port  330 , and a command line interface (CLI) port  332 . DDR2 SDRAMs  322  and  324  are coupled to the FIFO buffer located on OLT chip  314 , thus extending the packet buffering capacity of the FIFO buffer in both the upstream and downstream directions. Flash memory  326  is coupled to the management interface of OLT chip  314 , and supports the network management and control operation of the embedded processor. Power management module  328  draws power from XENPAK interface  320  and provides power for the rest of XENPAK dual-port OLT module  300 , including OLT chip  314 . Craft port  330  and CLI port  332  are both coupled to the management interface of OLT chip  314 , thus enabling various user management functionalities, including remote out-of-band management by a network administrator. 
         [0041]    In one embodiment, the integrated circuits, such as the OLT chip, SERDES modules, flash memory, and the power management modules, can be directly attached to the underlying printed circuit board (PCB) without individual packaging. That is, an IC die can be attached directly to a PCB, and conductive wires are bonded to the IC connectors and conductive regions on the PCB. The die is typically covered with a blob of epoxy. 
         [0042]    The connection interface between a pluggable OLT module and the Ethernet line card can be based on any open-standard or proprietary format. In one embodiment, the OLT module complies with the XENPAK standard. In addition to XENPAK, other form factors are also applicable to the inventive pluggable OLT, including, but not limited to: gigabit interface converter (GBIC), small form-factor pluggable (SFP), SFP+, 10 gigabit small form-factor pluggable (XFP), and X2. A pluggable OLT module generally can have any form factor, so long as its size allows the OLT module to be plugged into a piece of OTS network equipment located at the carrier&#39;s central office. Particularly, the pluggable OLT can have a form factor which is substantially similar to any fiber-optical transceivers. The specifications of the addition form factors are also available at the SFF committee website. 
         [0043]      FIG. 4  presents a diagram illustrating the architecture of an exemplary pluggable dual-port OLT module with an X2 form factor in accordance with an embodiment of the present invention. X2 defines a 10 GHz optical module that is slightly smaller than a XENPAK module. Similar to XENPAK dual-port OLT module  300  shown in  FIG. 3 , X2 dual-port OLT module  400  also includes fiber connectors  402  and  404 , optical bi-directional transceivers  406  and  408 , EPON SERDES  410  and EPON SERDES  412 , an OLT chip  414 , Ethernet SERDES  416  and Ethernet SERDES  418 , DDR2 SDRAMs  422  and  424 , a flash memory  426 , a power module  428 , a craft port  430 , and a CLI port  432 . However, instead of a standard XENPAK interface, X2 dual-port OLT module  400  includes a standard X2 interface  420 , which provides serial communication channels between OLT chip  414  and a corresponding Ethernet line card. Note that the corresponding Ethernet line card is equipped with interfaces that are compatible with the X2 standard. Also note that, although the examples herein use Ethernet line cards as the exemplary carrier&#39;s network equipment, embodiments of the present invention can be readily applied to a variety of network equipment, such as routers, switches, crossconnects, and multiple-layer switches. In general, the inventive pluggable OLT modules can be plugged into any network equipment which provides a compatible interface. 
       OLT Line Card 
       [0044]    In one embodiment, a number of pluggable OLT modules can be plugged into a standard OTS Ethernet line card to form an OLT line card.  FIG. 5  presents a diagram illustrating the architecture of an exemplary OLT line card in accordance with an embodiment of the present invention. OLT line card  500  includes an OTS Ethernet line card  502  and a number of XENPAK OLT modules including XENPAK OLT modules  504 - 514 . OTS Ethernet line card  502  can be a standard Ethernet line card fabricated by any equipment vendor. To be able to interface with XENPAK OLT modules  504 - 514 , Ethernet line card  502  includes corresponding XENPAK interfaces and slots. 
         [0045]    OTS Ethernet line card  502  also includes a power module  516 , a field-programmable gate array (FPGA) module  518 , an Ethernet switch  520 , a complex programmable logic device (CPLD) module  522 , a central processing unit (CPU)  524 , a flash memory  526 , and a DDR2 SDRAM  528 . 
         [0046]    Power module  516  receives external power and provides power to the rest of Ethernet line card  502 . Ethernet switch  520  provides standard switch functionality, including aggregating traffic from all coupled Ethernet ports which are coupled to pluggable OLT modules  504 - 514 . Programmable logic, including FPGA module  518  and CPLD module  522 , enables control and management of Ethernet line card  502 . CPU  524  manages local components on Ethernet line cards  502 , aggregates management and control signals from OLT modules  504 - 514 , and communicates with a routing engine located on the chassis. In one embodiment, CPU  524  configures OLT modules  504 - 514 . Flash memory  526  stores the programs and the initial boot-up configurations for CPU  524 . DDR2 SDRAM  528  can provide memory space for CPU  524  processing and/or packet buffering. 
         [0047]    The foregoing descriptions of embodiments of the present invention 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. The scope of the present invention is defined by the appended claims.