Patent Publication Number: US-2007121638-A1

Title: Method and system of communicating superframe data

Description:
RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. application Ser. No. 11/291,483 filed Nov. 30, 2005. This application also claims the benefit of U.S. Application No. 60/755,020, filed Dec. 29, 2005. The entire teachings of the above applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      Prior to growth in the public&#39;s demand for data services, such as dial-up Internet access, existing local loop access networks transported mostly voice information. In telephony, a local loop is defined as a wired connection from a telephone company&#39;s central office (CO) to its subscribers&#39; telephones at homes and businesses. This connection is usually based on a pair of copper wires, typically in the form of twisted-pair wires. An existing access network typically includes numerous twisted-pair wire connections between a plurality of user locations and a central office switch (or terminal). These connections can be multiplexed in order to transport voice calls more efficiently to and from the central office. The existing access network for the local loop is designed to carry these voice signals, i.e., it is a voice-centric network.  
      Today, data traffic carried across telephone networks is growing exponentially, and, by many measures, may have already surpassed traditional voice traffic, due in large measure to an explosive growth of dial-up data connections. The basic problem with transporting data over this voice-centric network, and, in particular, the local loop access part of the network, is that it is optimized for voice traffic, not data. The voice-centric structure of the access network limits an ability to receive and transmit high-speed data signals along with traditional quality voice signals. Simply put, the access part of the existing access network is not well-matched to the type of information it is now primarily transporting. As users demand higher and higher data transmission capabilities, the inefficiencies of the existing access network will cause user demand to shift to other mediums of transport for fulfillment, such as satellite transmission, cable distribution, wireless services, etc.  
      An alternative existing local access network that is available in some areas is a digital loop carrier (DLC). DLC systems utilize fiber-optic distribution links and remote multiplexing devices to deliver voice and data signals to and from the local users. In a typical DLC system, a fiber optic cable is routed from the central office terminal (COT) to a host digital terminal (HDT) located within a particular neighborhood. Telephone lines from subscriber homes are then routed to circuitry within the HDT, where the telephone voice signals are converted into digital pulse-code modulated (PCM) signals, multiplexed together using a time-slot interchanger (TSI), converted into an equivalent optical signal, and then routed over the fiber optic cable to the central office. Likewise, telephony signals from the central office are multiplexed together, converted into an optical signal for transport over the fiber to the HDT, converted into corresponding electrical signals at the HDT, demultiplexed, and routed to the appropriate subscriber telephone line twisted-pair connection.  
      Some DLC systems have been expanded to provide so-called Fiber-to-the-Curb (FTTC) systems. In these systems, the fiber optic cable is pushed deeper into the access network by routing fiber from the HDT to a plurality of Optical Network Units (ONUs) that are typically located within 500 feet of a subscriber&#39;s location. Multi-media voice, data, and even video from the central office location is transmitted to the HDT. From the HDT, these signals are transported over the fibers to the ONUs, where complex circuitry inside the ONUs demultiplexes the data streams and distributes the voice, data, and video information to the appropriate subscriber.  
      These prior art DLC and FTTC systems suffer from several disadvantages. First, these systems are costly to implement and maintain due to a need for sophisticated signal processing, multiplexing/demultiplexing, control, management and power circuits located in the HDT and the ONUs. Purchasing, then servicing this equipment over its lifetime has created a large barrier to entry for many local loop service providers. Scalability is also a problem with these systems. Although these systems can be partially designed to scale to future uses, data types, and applications, they are inherently limited by the basic technology underpinning the HDT and the ONUs. Absent a wholesale replacement of the HDT or the ONUs (a very costly proposition), these DLC and FTTC systems have a limited service life due to the design of intermediate electronics in the access loop.  
     SUMMARY OF THE INVENTION  
      According to an embodiment of the present invention, a system or corresponding method provides narrowband communications across a communications link through the processing of a superframe of data into packets. In an embodiment, a first node repackages a superframe of data containing multiple subframes of data in known positions within the superframe into multiple packets. A sequence indicator is inserted into a payload area of the multiple packets, the sequence indicator corresponding to a subframe in the given communications packet and its position within the superframe. The packets are transmitted across a connection to a second node. At the second node, sequence indicators in the payload portion of each of the packets are inspected. The multiple subframes of data are extracted along with corresponding command and control information. Using the sequence indicators, frames of data are formed from the multiple subframes of data.  
      According to an embodiment of the present invention, a system or corresponding method provides narrowband communications across a communications link through the processing of packets into a superframe. In an embodiment, a first node forms a sequence of packets containing subframes of data and inserts a sequence indicator in a payload portion of the packets. The sequence indicator is used to position the respective subframes within a superframe of data formed at a second node receiving the sequence of packets. At the second node, sequence indicators in the payload portion of received packets are inspected. The multiple subframes of data are extracted along with corresponding command and control information. Using the sequence indicators, a superframe of data may be formed from the multiple subframes of data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
       FIG. 1  is a block diagram of a network including a system in which an embodiment of the present invention may be deployed;  
       FIG. 2  is a more detailed diagram of network of  FIG. 1  including components of a remote digital terminal and an optical networking unit according to an embodiment of the present invention;  
       FIG. 3  is a detailed block diagram of a host digital terminal and an optical networking unit of  FIG. 2  according to an embodiment of the present invention;  
       FIG. 4  is a detailed block diagram of internal system interfaces of a remote digital terminal and an optical networking unit of  FIG. 2  incorporating redundant Ethernet Switch Units according to an embodiment of the present invention;  
       FIG. 5  is a functional block diagram of an Ethernet Switch Unit (ESU) of  FIGS. 2, 3  and  4 .  
       FIG. 6  is a functional block diagram of a Quadrature (Quad) Optical Interface Unit (QOIU) of  FIGS. 2, 3  and  4 .  
       FIG. 7  is a functional block diagram of a BroadBand Controller (BBC) of  FIGS. 2,3  and  4 .  
       FIG. 8  is a functional block diagram of a Quad Digital Subscriber Line Card (QDC) of  FIGS. 2, 3  and  4 .  
       FIG. 9  is a signal diagram showing a source specific multicast signal flow, according to principles of the present invention, between an Edge Aggregation Router, various nodes of a remote digital terminal and an optical networking unit, and a subscriber gateway;  
       FIG. 10  is a clock-to-signal timing diagram showing a double data rate transmission, according to an embodiment of the present invention, between a BroadBand Controller and a Quad Digital Subscriber Card;  
       FIG. 11  is a block diagram illustrating internal system interfaces of narrowband communications between a Remote Digital Terminal (RDT) and Optical Networking Unit (ONU);  
       FIG. 12  is an exemplary superframe of data that may be processed for network communications according to an embodiment of the present invention;  
       FIG. 13A  is a block diagram illustrating the processing of packets from a superframe;  
       FIG. 13B  is a detailed exemplary set of packets of  FIG. 13A  processed from the superframe of data in  FIG. 12 , according to an embodiment of the present invention;  
       FIG. 13C  is a block diagram illustrating the formation of a frame of data from packets at a node into a frame of data;  
       FIG. 14A  is a block diagram illustrating the processing of a superframe from packets at a QOIU according to an embodiment of the present invention;  
       FIG. 14B  is a block diagram illustrating the formation of packets from a narrowband signal at a BBC according to an embodiment of the present invention;  
       FIG. 15A  is signal diagram illustrating the detection of a network connection using Virtual Local Area Network (VLAN) identification according to an embodiment of the present invention;  
       FIG. 15B  is a block diagram illustrating a system for detecting a network connection using VLAN identification according to an embodiment of the present invention;  
       FIG. 16  is a flow diagram illustrating detection of a network connection using VLAN identification according to an embodiment of the present invention;  
       FIG. 17  is a block diagram illustrating an embodiment of the present invention within the IPTV system;  
       FIG. 18  is a high level diagram illustrating an embodiment of the present invention for generating a network quality clock signal;  
       FIG. 19  is a system timing block diagram showing use of a digitally controlled oscillator and a voltage controlled oscillator for generating a network clock signal according to an embodiment of the present invention;  
       FIG. 20  is a diagram illustrating jitter reduction using a digital Phase Locked Loop; and  
       FIG. 21  is a diagram illustrating jitter reduction using an analog Phase Locked Loop. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A description of preferred embodiments of the invention follows.  
      According to an embodiment of the present invention, a system or corresponding method increases available bandwidth for transmission of data, video, and audio to a customer, or sometimes a curb local to a customer, within a network. The system may include multiple network nodes. In one embodiment, first network node in the system converts a first optical communications signal to a corresponding first electrical signal with an asynchronous, packet-based format. The first network node processes the first electrical signal in a corresponding, asynchronous, packet-based manner, and routes the first electrical signal to a second network node among a group of secondary network nodes. This second network node converts the first electrical signal to a second optical signal and routes the second optical signal to a third network node among a group of tertiary network nodes. The third network node converts the second optical signal to a corresponding second electrical signal with an asynchronous, packet-based format, processes the second electrical signal in a corresponding, asynchronous, packet-based manner, and routes the second electrical signal to a fourth network node among a group of quaternary network nodes. This fourth network node may transmit the second electrical signal to at least one end user node.  
      In one embodiment of the invention, a communications system, such as a Digital Subscriber Line Access Multiplexer (DSLAM), or corresponding method, increases available bandwidth for transmission of data, video, or audio to a customer premise, or curb node, for further distribution to customer premises within a network. In one embodiment, a system comprises a host digital terminal (HDT), including an Ethernet switch unit and multiple optical interface units coupled via at least one communications bus. The optical interface units may be configured to communicate over an optical communications link with broadband cards of optical network units (ONUs). The ONUs also include data cards coupled to the broadband cards via at least one communications bus. The data cards may be configured to communicate over end user communications links to end user nodes.  
      Some embodiments of the present invention provide network access to higher speed video and data transmissions. An example architecture provides Fiber to the Curb (FTTC) that supports higher bandwidth to the customer premise than a Digital Subscriber Line Access Multiplexer (DSLAM) Host Digital Terminal (HDT) or Central Office solution.  
       FIG. 1  illustrates an Internet Protocol Television (IPTV) system  100  according to an embodiment of the present invention within a network  1000 . The IPTV system  100  may serve as an interface between an end user node, such as a residential gateway  52 , and an Edge Aggregation Router (EAR)  20  that may provide voice, video, and/or data services from a media provider.  
      The EAR  20  may provide access to a Video Service Office (VSO)  40 , as well as Internet traffic through an Internet Service Provider (ISP)  30 . A management station  60  may operate as an Element Management System (EMS) server to provide low level management and surveillance functions for the system  100 . The EMS server  60  may host some or all sessions for a client  70  to access the IPTV system  100 . In addition, the EMS server  60  may also communicate with a customer&#39;s network management system  80  for service activation, surveillance, and alarm reporting. These communications may be made through a network, such as an Internet Protocol (IP) network  10 . The network management system  60  may be an application platform used for managing some or all of the systems in a multi-vendor environment, may provide seamless access to some or all IPTV systems, and may provide some or all flow-through capabilities for service activation and maintenance.  
      The EMS server  60  may be a custom or commercial server, such as a Sun Solaris® based server application. The EMS client  70  may be an application program and may be loaded onto Microsoft® Windows® or a Sun Solaris® workstation. The client  70  may provide a Graphical User Interface (GUI) front end to the element management system application and may communicate to the EMS server  60 . The client  70  allows EMS users to make changes to the IPTV system  100 , generate reports, and view status data.  
      The IPTV system  100  may also interface with an end user node, such as a residential gateway  52 , on customer premise(s). In one embodiment, the gateway  52  can provide an interface to customer premises devices  54  for access to the Internet, while also providing an interface to a set top box  56  for providing video services. The IPTV system  100  may provide delivery of voice, video, and/or data services from a central location to multiple homes.  
      In the embodiment of  FIG. 1 , the IPTV system  100  comprises two main components. The first component is a Remote Digital Terminal (RDT)  200  (referred interchangeably with Host Digital Terminal (HDT)), which provides access points from the router  20 . The RDT  200  connects to Optical Networking Unit (ONU)  300  through an optical fiber  255  connection. In a communications system, a single RDT  200  may connect to multiple ONUs through multiple optical fiber connections. The ONU  300  may be located in a local neighborhood to provide the delivery of voice, video, and data services to a number of customer premises  50 .  
       FIG. 2  sets forth a more detailed schematic of the system  1000  shown in  FIG. 1 . As with  FIG. 1 , the IPTV system  100  of  FIG. 2  has both a Remote Digital Terminal (RDT)  200  and an Optical Network Unit (ONU)  300 . Referring to  FIG. 2 , the RDT  200  may receive incoming signals from the Edge Aggregation Router (EAR)  20  through an optical gigabit Ethernet (GigE) connection  1001  at an Ethernet Switch Unit (ESU)  250  of the RDT  200 . The EAR  20  may provide access to a number of Video Service Offices (not shown) through a video network  45 , as well as Internet traffic  35 . A management station (not shown) may connect to the EAR  20  through a management network  65 .  
      The ESU  250  may be responsible for a first layer of multicast replication within the system  100 . The ESU  250  may perform a proxy function for the network elements to track and keep proper multicast channels (not shown) flowing from the EAR  20 , through the IPTV System  100 , and to the end nodes  52 .  
      The RDT  200  may also have a Distribution Processor Unit (DPU)  265 . The DPU  265  may provide the RDT  200  with access to a common shelf  90 , such as a DISC*S® common shelf made by Tellabs Operations, Inc., at a Central Office. The common shelf  90  may perform call processing and provide a TR-008 or GR-303 interface to the voice switch. The common shelf  90  may further include a connection to a narrowband network  92  and a narrowband element management system (EMS)  94 . The narrowband EMS  94  may provide an interface to the system operator&#39;s Operational Support Systems (OSS)  95 . The EMS  94  may manage tasks, such as system configuration, provisioning, maintenance, inventory, performance monitoring, and diagnostics.  
      In an embodiment shown in  FIG. 2 , the ESU  250  connects with fourteen Quad Optical Interface Unit (QOIU) cards  260  within the RDT  200 . Within the system  100 , an Ethernet switch (discussed in detail below with respect to  FIG. 6 ) located in the QOIU  260  performs layer 2 functions. Each QOIU may interface via an optical connection  255  with one or more Broad Band Controllers (BBC)  350 .  
      In the embodiment of  FIG. 2 , the BroadBand Controller  350  (BBC) may be responsible for some or all the operations, administration, management, and provisioning functions within the ONU  300 . Each BBC  350  may support multiple quad digital subscriber line cards (QDC)  360 . The hardware on the BBC  350  is responsible for distributing IP packets or ATM cells to the QDC  360  cards. In addition, the BBC  350  may provide the optical interface (not shown) between the ONU  300  and the QOIU  260 . The QDC  360  serves as the interface to the end user node (e.g., residential gateway  52 ) in a subscriber premises.  
       FIGS. 3 and 4  provide a more detailed diagram of embodiments of an IPTV system  100  of both  FIGS. 1 and 2 .  
       FIG. 3  illustrates an IPTV system  100  comprising an RDT  200  and an ONU  300 . In the embodiment of  FIG. 3 , within the RDT  200  are at least two primary nodes: at least one Ethernet Switch Unit (ESU)  250  and multiple corresponding Quad Optical Interface Units (QOIU)  260  (only one of which is shown in  FIG. 3 ). The Ethernet Switch Unit (ESU)  250  interfaces with the Quad Optical Interface Units (QOIU)  260  along a backplane (not shown). The ESU  250  may provide an uplink to the EAR  20  of  FIGS. 1 and 2 , convert optical signals into an electrical signal, and route the electrical signal to an appropriate QOIU  260  using Ethernet Layer 3 information. Each QOIU can subsequently convert electrical signals back into optical signals and transmit the optical signals via optical fiber link(s)  255  to various Optical Network Units  300  (ONU) using Layer 2 information.  
      In embodiments of the present invention, and as shown in  FIG. 4 , the RDT  200  may employ two or more ESU  250  units to support a redundancy. As shown in  FIG. 4 , the ESU  250  connects to the QOIUs  260  in the RDT  200  enclosure. The multiple ESUs  250  may be configured to operate as a single unit, but introduction of redundancy provides additional reliability in the IPTV system  100  shown in  FIGS. 1 and 2 . Therefore, if one of the ESUs  250  were to fail, the system  100  would lose capacity, but not service. To support this redundancy, the HiGig port from each ESU  250  is cross-connected back to the other ESU  250 . Like the QOIU  260  interface, this port may be physically connected to a redundant switch module via the RDT  200  backplane (not shown). Multiple ESUs may be combined to form a load sharing redundant unit via a mechanism known as trunk aggregation. Trunk aggregation allows Ethernet links on different ESUs to combine to form a single logical link. When an ESU fails, as indicated by loss of Ethernet link, the connected devices each may remove that ESU from its aggregation group.  
      A link layer is a standardized part of the line level Ethernet protocol which determines the presence of a device on the distant end of an Ethernet link. It is a complex protocol which requires that the line interface be fully functional and, as such, provides a significant level of diagnostic insight into the distant end. The devices at the edge of the switching subsystem each make their own determination vis a vis the viability of the switching subsystem, and, therefore, do not require to communicate or coordinate the redundancy failover event with each other. As such, this mechanism is inherently simpler and more reliable than currently offered reliability strategies, both by its inherent simplicity and its ability to absorb multiple failures.  
      Consistent with the principles of the present invention, systems may be configured to have only one ESU  250  active at any one time, or they may be configured whereby both ESUs  250  are active. Spare slots at the QOIU  260  may also be provided to adapt the RDT  200  for future services  266 .  
      Continuing to refer to  FIG. 3 , the ONU  300  also has two primary nodes, at least one BroadBand Controller (BBC)  350  and multiple corresponding Quad Digital Subscriber Line Cards (QDCs)  360 . Within the ONU  300 , the BBC  350  terminates the RDT interface and may split the narrowband traffic to the Quad Channel Units  380  from the broadband traffic to the QDCs  360 . The BBC  350  of  FIG. 3  also shows a connection with a narrowband common card (NCC)  370 . The BBC  350  may receive optical signals from a QOIU  260 , convert them into an electrical signal, and switch the electrical signal to the appropriate QDC  260  (for narrowband communications, the NCC  370 ) using Layer 2 information.  
       FIG. 4  also illustrates the DPU  265  and QOIU  260  interface which may transport the narrowband traffic between the RDT and common shelf (shown as  90  in  FIG. 2 ). The narrowband traffic may be transported over a superframe format that may include Pulse Code Modulation (PCM), Channel Unit Data Link (CUDL), ISDN 2 B channels (64 kb/s) and D channels (16 kb/s) Pulse Code Modulation and High Level Data Link Control (HDLC) data for up to twenty-four channels in each of four ONUs  300 . This interface may also include the DPU  265  BUS (not shown) that may be used by the DPU  265  to control the QOIU  260  narrowband interface.  
      The QOIU  260  interfaces with a BroadBand Controller (BBC)  350  at the ONU  300  over an optical connection  255 . The ONU  300  may have a spare slot at the BBC  350  that may also be provided to adapt the ONU  300  for future services  356 .  
      In one embodiment, the ESU  250  may be responsible for the first layer of multicast replication within the system  100 . The ESU  250  may perform an Internet Group Management Protocol (IGMP) proxy function to track and keep all of the proper multicast channels flowing from the Edge Aggregation Router (EAR)  20 .  
      Elements within the RDT  200  and ONU  300 , such as the ESU  250 , QOIU  260 , BBC  350 , and QDC  360 , may be referred to as “nodes” or “network nodes.” Through use of these nodes, some embodiments of the present invention may be employed. It should be understood that the nodes may be physically separated from each other.  
      With reference to  FIGS. 2, 3 , and  4 , the optical link  255  between the QOIU  260  and BBC  350  may have a 1.25 Gbps symmetrical interface rate. The interface rate may allow the QOIU  260  switch to be connected to the BBC  350  switch without additional glue logic. The BBC  350  may convert an optical signal (not shown) through a line card aggregator function. Optical circuitry may be provided on a printed circuit board (not shown) in the BBC  350 .  
      The BBC  350  processor may be responsible for some or all of the DSP management functions in the ONU  300 . The BBC  350  may support ADSL, ADSL2+, VDSL2, and Quad DS1 line cards.  
       FIG. 4  shows internal data interfaces between the various components of the IPTV system according to an embodiment of the present invention. A QOIU  260  in the RDT  200  may connect to an ESU  250  gigabit port. In embodiments of the present invention, this interface may comply with the IEEE 802.3 standard. The physical connection between the modules may be via an interface across the RDT  200  backplane (not shown). In embodiments of the present invention, the SerDes signals may connect the Ethernet switch devices on the ESU  250  to the QOIU  260  without the need for external glue logic. The transmission between the two points may employ 8B/10B encoding.  
      The interface between the QOIU  260  and the BBC  350  provides the link between the RDT  200  and the ONU  300 . This interface may be an optical connection  255 . In embodiments of the present invention, this optical connection uses a 1490 nm wavelength for downstream transfers and 1310 nm for upstream transfers. In such an embodiment, the raw bit rates for this interface may be 1.25 Gbps downstream and 1.25 Gbps upstream. This connection may support a distance of 12,000 feet between the RDT  200  and ONU  300 .  
      As shown in  FIG. 5 , in one embodiment of the present invention, the ESU  250  is a 24-Port GigE Layer ⅔ Ethernet switch  2503 , such as a Broadcom® BCM56500 24-Port Gigabit Ethernet Multilayer Switch by Broadcom Corporation of Irvine, California. In this embodiment, the ESU  250  supports four Small Form-factor Pluggable (SFP) gigabit uplink ports  2502  for optical-to-electrical conversion, and twenty gigabit SerDes interfaces  2504  to its backplane I/O (not shown).  
      The switch  2503  shown in  FIG. 5  connects to a management module  2505  which may support a 10/100BT port  2506   a  and a serial port  2506   b  for craft. The management module  2505  may also interface with a data storage unit  2508  and an inventory storage unit  2509 . A clock  2507  provides timing for both the switch  2503  and the management module  2505 . The ESU  250  of  FIG. 5  also has a power converter  2501  that interfaces with the backplane (not shown). The ESU  250  may operate primarily as a Layer 2 Ethernet switch for unicast traffic, but may also have significant Layer 3 capabilities in hardware for multicast traffic.  
      The RDT  200  may also house one or more Quad Optical Interface Units (QOIU)  260 . Each QOIU  260  may connect with an ESU  250  through GigE SerDes links to a backplane (not shown) or through Small Form-factor Pluggable (SFP) ports. The QOIU  260  is specifically designed to support the IPTV architecture with the hardware capability to maintain narrowband (i.e., voice channels) interfaces (shown below with respect to  FIG. 6 ) in existing systems.  
      In an embodiment of the present invention, as shown in  FIG. 6 , the QOIU may be equipped with a 12-port Layer ⅔ Ethernet switch  2601 , such as either a Broadcom® BCM5695 or the BCM5696 12-Port Gigabit Ethernet Multilayer Switch. In  FIG. 6 , the Ethernet switch  2601  performs layer 2 functions. Signals  2610   a  and  2610   b  may be exchanged between the switch  2601  and both a primary and secondary ESU over a backplane  210 . The switch  2601  may also have an interface with a control plane processing module  2602 , which in turn interfaces with a data storage unit  2604   a  and an inventory storage unit  2603 . The switch  2601  may also directly interface with a data storage unit  2604   b . The switch  2601  may interface with a narrowband processing module  2605 , which connects to the backplane  210  through a distribution processing unit  2606 .  
      A clock  2607  may provide timing for both the switch  2601  and the narrowband processing module  2605 . In this embodiment, electrical signals  2611   a  transmit directly with the switch  2601  and four Small Form-factor Pluggable (SFP) gigabit uplink ports  2609  for optical-to-electrical conversion, providing optical connections  2611   b  with downstream ONUs (not shown in  FIG. 6 ). The switch  2601  may have a port for an Ethernet Aggregation Switch (EAS) interface that provides an additional link for signals  2610   c  in an upgrade configuration. The QOIU  260  of  FIG. 6  also has two power converters  2608   a  and  2608   b  that interface with the backplane  210 .  
      Each QOIU  260  may also serve as an interface to a BroadBand controller (BBC)  350  at one or more ONU devices  300  over a multi-wavelength optical connection. In the embodiment shown in  FIG. 6 , each optical interface of the QOIU  260  provides a bidirectional, symmetrical, 1.25 Gbps link using a 1490 nm wavelength in the downstream path and a 1310 nm wavelength in the upstream path.  
      In addition to broadband data traffic, this interface between the QOIU and the BBC may transport narrowband payload and maintenance information encapsulated in IP Packets. This interface is symmetrical in that the same types of packets are transmitted in both the downstream path as well as the upstream path. In the downstream path, the narrowband payload is received by the QOIU  260  from the DPU  2606  as in  FIG. 6 . The QOIU collects the narrowband traffic and forms the payload in a narrowband processing module  2605 , and the payload is encapsulated in an Ethernet packet. In the upstream direction, the QOIU switches all narrowband packets to the narrowband processing function  2605 . The payload is extracted and sent to the DPU  2606 .  
       FIG. 7  is an embodiment of a BBC in accordance with an embodiment of the present invention. With reference to  FIG. 7 , the BBC  350  includes a Line Card Aggregator (LCA)  3502 , such as the Broadcom® BCM6550A. An optical-to-electrical converter  3501  interfaces with the DSP  3502  to provide an optical connection  3511  with an upstream QOIUs (not shown in  FIG. 6 .). The LCA  3502  may also have a program storage module  3503  and a data storage module  3504 . The BBC  350  may also have a power converter  3505  that interfaces with the backplane  3510 .  
      The BBC  350  may use a Field Programmable Gate Array (FPGA)  3507  that interfaces with the LCA  3502  and a backplane  3510 . In such an embodiment, the FPGA implements some of the functions on the BBC that cannot be handled by the LCA Digital Signal Processor (DSP), such as: Medium Access Control (MAC) address translation between provisioned network MACs and learned subscriber MACs; Virtual Local Area Network Identification (VLAN ID) translation as cell or Packet Transfer Mode (PTM) traffic passes through the device; UTOPIA 2 conversion to/from the ONU backplane UTOPIA architecture; and termination of the narrowband traffic and conversion from the fiber format to that required by the NCC backplane interface and narrowband line cards. A narrowband interface module  3509   c  is shown on the FPGA  3507 . The FPGA  3507  also has a QDC interface module  3509   b  and a spare interface  3509   a . A clock  3506  provides timing for both the DSP  3502  and the FPGA  3507 . The FPGA  3507  also interfaces with an inventory storage module  3508 .  
      As shown in  FIG. 7 , signals from the FPGA  3507  may be exchanged with the QDC (not shown in  FIG. 7 ) over an asymmetrical UTOPIA-like backplane interface  3510 . UTOPIA describes a Universal Test &amp; Operations Physical Interface for ATM level 1 data path interface, as defined in technical specifications by the ATM Forum. UTOPIA describes the interface between the Physical Layer and upper layer modules, such as the ATM Layer, and various management entities. The UTOPIA bus is a standard interface between asynchronous transfer mode (ATM) link and physical layer devices. It covers rates from sub-100 Mbit/s to 155 Mbit/s and gives guidance for 622 Mbit/s. 8-bit wide data paths use octet-level/cell-level handshake at 25 MHz. UTOPIA Level 2 is an addendum to Level 1 and describes support of a data rate of 622 Mbit/s over a 16-bit wide data path at 33 and 50 MHz.  
      The interface to the QDC  360  may be a point-to-multipoint interface. In an embodiment according the principles of the present invention, the downstream transfers may be accomplished on a double-data rate 16-bit bus  3511  while the upstream is an 8-bit UTOPIA bus  3512 . The transfer clock rate for both the downstream and upstream data transfers may be 25 MHz.  
      The Quad Digital Subscriber Line Card (QDC)  360  is a subscriber line card in the ONU. This card may support four ports of ADSL/ADSL2+ or VDSL2 service. As shown in  FIG. 8 , a QDC  360  may consist of a FPGA  3601  that provides the glue logic functions needed to support the interface between the BBC  350  switch and a QDC  360  DSP  3604 . A DSP used in a QDC in accordance with the present invention may be the Broadcom® BCM6510. The FPGA  3601  may handle the ATM operations, administration and management functions, as well as the downstream bus  3611  translation from 16 bits double data rate to the DSP&#39;s 8-bit single data rate bus  3613 . A QDC  360  may be capable of supporting the various XDSL modes of service (e.g. ADSL, ADSL2, ADSL2+, VDSL2 and T1.413). In an embodiment according to the principles of the invention shown in  FIG. 8 , the card may support four ports of ADSL/ADSL 2 + or VDSL service. In embodiments of the present invention, the FPGA may also interface with an inventory storage module  3602 . A clock  3605  provides timing between the FPGA  3601  and the DSP  3604 . The DSP  3604  may also interface with a data storage module  3606 .  
      In addition to the DSP  3604 , the QDC  360  may also comprise analog front ends (AFEs)  3607 , line drivers (not shown) and low-pass filters (not shown) for DSL service. As an example, an AFE used in a QDC in accordance with the present invention may be the Broadcom® BCM6505. Management of the QDC  360  may be performed in-band by the BBC  350 .  
      In one embodiment, due to the limitations of existing hardware in ONU backplanes, the interface between the BBC  350  and a QDC  360  is a 16-bit UTOPIA 2 downstream bus  3611  operating at approximately 25 MHZ for all control timing and double data rate for all data bus timing. The QDC  360  may also have a power converter  3603  that interfaces with the backplane (not shown).  
      The IPTV system  100  of an embodiment of the present invention as described above allows a service provider to provide a source specific multicast of a signal. According to principles of the present invention, a source specific multicast may be performed in a network, by inspecting a signal for a source specific multicast channel identifier. The source specific multicast identifier signal can be then mapped to a frame switching identifier. The frame switching identifier can be mapped to the signal, allowing the signal to be directed a location based on the frame switching identifier.  FIG. 9  is a high level diagram that shows the signal flow for an exemplary source specific multicast according to an embodiment of the present invention.  
      A subscriber gateway device  52  makes a request to “Join” a particular multicast channel. This “Join” request  910  includes the Media Access Control (MAC) address of the specific device  52 , as well as the request for the specific channel. This request  910  travels upstream through the IPTV system. The signal first arrives at the QDC  360 , where the signal  912  is forwarded to the BBC  350 . From the BBC  350 , the signal  914  is forwarded to the QOIU  260 . At the QOIU, the signal  916  is forwarded to the ESU  250 .  
      At the ESU  250 , an Edge Aggregator Router (EAR)  20  may feed a source specific multicast signal  900  to the ESU  250 . The ESU  250  inspects the signal  916  for a source specific multicast channel identifier. The ESU  250  then maps the multicast signal  900  to a frame switching identifier, such as an Ethernet frame, and then applies the frame switching identifier to the signal  916 . Once the signal is mapped, the multicast signal  900  may be switched back to the subscriber gateway  52  through the various port assignments through a switching stream  920 ,  922 ,  924 , and  926 . At the subscriber gateway  52 , the frame switching identifier of the received signal  926  may be translated to a different identifier for processing. This different identifier may include the original source specific multicast channel identifier, including an Internet Protocol (IP) address, or some unique predefined channel identifier. The source specific multicast channel identifier may be mapped using a destination address, or a destination address and some combination of a source address or VLAN address.  
      The signal flow allows for the inspection of a multicast signal  900  with Ethernet Layer 3 information to be mapped to Layer 2 frames for delivery through a switching stream  920 ,  922 ,  924 , and  926 . In some instances, intermediary nodes, such as the QDC  360 , the BBC  350 , or the QOIU, may already be aware of a particular VLAN assignment made to the requested channel  910 , and may assign the switching port, accordingly.  
      In an embodiment of the present invention, the system provides a Layer 2 MAC bridge between the network  100  and the subscriber  52 , with a VLAN  950  separation of traffic (e.g., different Virtual Local Area Networks (VLANs) may be used for different Internet Service Providers (ISPs)). In one embodiment, there is no bridging provided between subscribers. This may be referred to as “forced forwarding” from the subscriber to the network. Further, the system may provide replication of multicast streams from the network to subscribers based on subscriber Internet Group Management Protocol (IGMP) requests. At any point in the system, multicast signals can be replicated and directed to a number of different nodes within a different downstream switching stream (alternative switching streams not shown).  
      Data traffic on the network side may fall within various VLANs. These VLANs may include: 
          Management VLAN—may contain management traffic from an element management system.     IPTV VLAN—may contain the IPTV source specific multicast streams     IPTV Internet VLAN—may contain traffic to the internet for IPTV subscribers in a separate VLAN from the multicast video traffic.     Legacy VLAN—may carry traffic from legacy subscribers with ADSL Internet and no IPTV.     Other ISP VLANs—may carry traffic from other third-party ISPs     Point to Point VLAN—may provide a Point-to-Point service as a VLAN per port.        

      In accordance with certain embodiments of the present invention, the subscriber interface to the IPTV system may be an ADSL, ADSL2+ or VDSL interface. For example, the primary protocol stack may be (i) Ethernet over ATM Adaptation Layer 5 (AAL5) for Asymmetric Digital Subscriber Line (ADSL) and (ii) Ethernet over EFM for VDSL. Specific layers above the primary protocol stack may depend on the type of subscriber and network device(s) to which the subscriber is connected. In an Ethernet system, traffic may be bridged before it can reach a Broadband Remote Access Server function.  
      A simple VLAN implementation may involve a Transparent LAN service (TLS). The implementation is a standard Ethernet switch in which a network VLAN is added at the subscriber port. If the subscriber port contains a VLAN, the network VLAN is stacked on top of the subscriber VLAN. Within the access network (e.g., Matrix (MX) or Fiber-in-the-Loop (FITL)), the BBC&#39;s DSP (shown in  FIG. 7 ) in the ONU may be configured as a network VLAN endpoint. Ethernet traffic may be passed with no filtering. Virtual MACs may not be allowed in this configuration. If the subscriber connection is ATM, there may be multiple Permanent Virtual Circuits (PVCs) on the connection, and each PVC may be mapped to a separate network VLAN. Some embodiments do not allow for multiple PVCs to be mapped to the same VLAN. Internal routing to the PVC may be based on the VLAN ID only. This VLAN configuration is sometimes referred to as 1:1 or port-based VLANs.  
      In embodiments of the present invention, legacy ATM Internet subscribers may use a similar implementation as Transparent LAN services (TLS) with some exceptions. With legacy ATM, only one PVC is used. Further, in such embodiments, all network traffic may be Point to Point Protocol over Ethernet (PPPoE). This means it may be possible to apply a filter to allow only PPPoE traffic. This VLAN configuration is N:1, meaning that multiple subscribers map to the same network VLAN, and routing to a port is based on VLAN and MAC. Finally, with a Legacy ATM service, it may be possible to configure Virtual MACs (i.e., up to eight), if desired.  
      In connection with an embodiment of the present invention, IPTV subscribers can have two paths to the network. One path is for Internet (ISP) traffic, and the second path for the video network. In this configuration, the IPTV system may perform some additional routing beyond a standard Ethernet switch. In particular, the IPTV system may separate the Video and ISP traffic into two separate network VLANs. Network to subscriber routing may be standard. Both VLANs may be merged to a single port. In one embodiment, multicast traffic and Internet Group Multicast Protocol (IGMP) queries may be routed from the video VLAN to the subscriber. There may be no unicast traffic on the video network in some networks. The subscriber-to-network routing may be more complicated. The following operation occurs at the subscriber edge. Depending on the service, the IPTV system according to some embodiments of the present invention either (i) translates VLAN identifiers or (ii) inserts on subscriber ingress and removes on subscriber egress. When inserting a tag, the priority may also be specified. The translation values or insertion values may be provisioned on a per circuit (port or ATM VC) basis.  
      In embodiments according to the present invention, MAC address translation may be provided on the subscriber ports. The addresses to use for translation may be assigned as a block to the IPTV system. The simplest implementation is to assign a block equal to the number of ports times eight and to use a fixed mapping per port. MAC address translation provides certain the benefits, such as prevention of certain attacks (e.g., MAC routing table spoiling, impersonation, etc.). Protection may also be provided from duplicate MAC addresses with different customers (e.g., due to manufacturer errors or user misconfiguration). Other embodiments may be used for IP address assignment and additional security in the network (e.g., MAC address identifies the port).  
      Although the BBC/QDC interface is a UTOPIA level 2-like interface, the clock-to-data and control signal timing relationship may be modified to increase performance of the interface. In particular, data may be transmitted at a “double data rate” between the BBC  350  and QDCs  360  at the ONU  300  in order to improve system bandwidth. According to embodiments of the present invention, data is transmitted between a first node, e.g. a BBC  350 , and at least one second node, e.g. a QDC  360  of an optical networking unit. Data transmission begins at the first node, which polls at least one second node for availability of a data transfer. The polling occurs at a first rate, typically based on a rise and a fall of a clock cycle generated from the first node. Once the first node receives a signal indicating a second node&#39;s availability to receive data, the first node sends an initiating signal to the second node and begins transferring data to the at least one available address at twice the first rate used for the polling. An overall interface signal timing is specified in  FIG. 10 .  
       FIG. 10  shows a signal timing between a BBC  350  (not shown in  FIG. 10 ) and a QDC  360  (not shown in  FIG. 10 ). A clock signal  1210  provides synchronization between the BBC  350  and the QDC  360 , and a given rate may be based on the rising and falling edges of the clock cycle for which a data transfer may be associated. In one embodiment, the BBC  350  continually transmits a polling signal  1220  at every other clock cycle to the QDCs  360  for availability of a data transfer, sending a source address  1222 ,  1224 . In-between polling transmissions, the BBC  350  may transmit an idle signal  1221 . The BBC  350  may have any number of signal source addresses to send in a polling signal. The BBC  350  may select to transmit any one of those source addresses based on various types of networking algorithms. For example, the BBC  350  may select the signal source address sequentially, using a priority queue method, or a round robin method.  
      In one embodiment, a QDC  360  communicates with the BBC by providing a signal that indicates availability  1230 . When the QDC is available to receive a data transmission from an available address, the transmission signal  1230  indicates availability to receive a particular address  1232 . As shown in  FIG. 10 , the BBC  350  continues to send polling requests  1220  while it is transmitting data  1250 . Once the BBC  350  completes a transmission  1252 , having previously received an availability signal  1232  from a QDC  360 , the BBC sends a transmission initiation signal  1242  to the particular QDC  360 . Subsequently, the BBC may simultaneously send a “start of cell” (or alternatively “start of packet”) signal  1260  and begin transferring data  1254  to the at least one available address at twice the first rate. By receiving the initiation signal  1242 , the QDC  360  knows that the subsequent data transmission from the BBC  350  occurs at a double data rate.  
      As mentioned briefly above in referenced to  FIG. 6 , the IPTV system of an embodiment of the present invention allows communication of narrowband traffic between a remote digital terminal (RDT) and a number of Optical Network Units (ONUs).  
      According to embodiments of the present invention, a system or corresponding method provides narrowband communications across a communications link through processing a superframe of data into packets. In one embodiment, a first node, such as a Quadrature (Quad) Optical Interface Unit (QOIU) in an RDT, repackages a superframe of data, containing multiple subframes of data in known positions within the superframe, into multiple packets where the payload area may include narrowband data (e.g., voice data). A sequence indicator may be inserted into a payload area of the multiple packets. The sequence indicator may correspond to a subframe in the given communications packet and its position within the superframe.  
      The packets may be transmitted across a connection to a second node, such as a BroadBand Controller (BBC) of an ONU. The transmission may occur at a rate of 500 μsecs, for example, optionally as part of broadband data packets transmitted at higher rates where the multiple subframe packets are carried on an as-available basis, causing a jitter in a received rate. At the second node, sequence indicators in the payload portion of each of the packets may be inspected. The multiple subframes of data may be extracted along with corresponding command and control information. Using the sequence indicators, frames of data may be formed from the multiple subframes of data.  
       FIG. 11  illustrates an embodiment of the narrowband communications system interfaces between an RDT  200  and four ONUs  300   a - d . In this embodiment of the present invention, a common shelf  90 , such as a DISC*S® common shelf made by Tellabs Operations, Inc., at a Central Office (not shown) may perform call processing and provide an interface such as a TR-008 or GR-303 interface, to communication narrowband traffic. The narrowband traffic may be configured in a superframe format and transported using Time Division Multiplexing (TDM), which may include timeslots of data encoded using, for example, Pulse Code Modulation (PCM), Differential Pulse Code Modulation (DPCM) Channel Unit Data Link (CUDL), or ISDN 2 B channels (64 kb/s) and D channels (16 kb/s) Pulse Code Modulation. The superframe format may include High Level Data Link Control (HDLC) data for up to twenty-four channels in each of four ONUs  300 .  
      The common shelf  90  of  FIG. 11  sends a superframe  1110  to a data processing unit (DPU)  265 . The DPU  265  sends the superframe  1110  to a Quadrature (Quad) Optical Interface Unit (QOIU)  260 , which processes the superframe  1110  into multiple packets  1120   a ,  1120   b ,  1120   c ,  1120   d  to send to respective ONUs  300   a - d.    
      In the embodiment of  FIG. 11 , the QOIU  260  interfaces with BroadBand Controllers (BBC)  350  at four individual ONUs  300   a - d  over optical connections  255 . After processing the superframe  1110  into individual packets  1120   a - d , the QOIU sends the packets to the particular ONU based on identifiers in the packets. As shown in  FIG. 11 , the QOIU  260  sends packets  1120   c - 0  through  1120   c - 5  ( 1120   c - 0  . . .  5 ) to a BBC  350  at ONU 3   300   c . Because the narrowband packets share the same optical connections  255  with broadband communications, these narrowband packets  1120   c - 0  . . .  5  are interleaved at a particular frequency with broadband communications occurring between the QOIU  260  and the BBC  350 . As illustrated, the QOIU  260  also sends narrowband packets  1120   a - 0  . . .  5 ,  1120   b - 0  . . .  5 , and  1120   d - 0  . . .  5  to other ONUs  300   a ,  300   b , and  300   d , respectively.  
      In an embodiment of the present invention, the narrowband packets  1120   a - d  are sent from the QOIU  260  to the corresponding BBC  350  every 500 μsecs. The BBC  350  may process the packets and send the narrowband communications to a narrowband common card (NCC)  370 , and subsequently to appropriate one(s) of the Quad Channel Units (QCUs)  380 .  
       FIG. 12  is an exemplary superframe  1110  according to an embodiment of the present invention. In the embodiment of  FIG. 12 , the superframe  1110  may be organized in twenty-four subframes, indicated as rows  1 - 24 . Across each row (i.e., subframe), the superframe  1110  contains data organized for four superframe groups, designated DA, DB, DC and DD. These designates may provide a unique source address that identifies a unique communications path of the sequence of packets. In a 24-Channel mode, the superframe groups are associated with one of the four ONUs  300   a - d  connected to the QOIU  260  (i.e., group DA corresponds to ONU 1   300   a , group DB corresponds to ONU 2   300   b , and so forth). Within each group there are four timeslots, designated as TA, TB, TC, and TD. As indicated within the superframe format, PCM, DPCM, CUDL, and HDLC data provide twenty-four channels to the four ONUs ( 300   a - d  in  FIG. 11 ). For example, viewing the superframe  1100  from left to right, the first few bytes of data for all twenty-four subframes are allocated to PCM TA/DA, which is the Pulse Code Modulation data for timeslot TA for group DA (e.g., ONU 1   300   a ).  
      The superframe  1110  of  FIG. 12  is split in half, such that groups DA and DB&#39;s format is mirrored for groups DC and DD, where each half supports two ONUs. The particular superframe  1110  shown in  FIG. 12  is organized to allocate CUDL bytes to six subframes. Further, groups DA and DC are each allocated six CUDL groups per timeslot (e.g., CUDL 1  TA/DA, CUDL 2  TA/DA . . . etc.), whereas groups DB and DD are each allocated only one CUDL group per timeslot. One of ordinary skill in the art will understand that a superframe may be organized in other ways consistent with embodiments of the present invention. For example, in 12-Channel mode (not shown in the figures), an odd numbered ONU may share its group with an even numbered ONU. Accordingly, group DA is shared across ONU  1   300   a  and ONU  2   300   b  while group DC is shared across ONUs  3  and  4 ,  300   c  and  300   d , respectively.  
      The columns  1301 - 1303 ,  1311 - 1313 ,  1304 - 1306  and bytes  1309  are described below in reference to  FIG. 13B .  
       FIG. 13A  illustrates an exemplary “downstream” flow of a superframe  1110  through a QOIU  260 , resulting in multiple communications packets  1120   a - d  transmitted to the multiple ONUs  300   a - d . Based on the provisioned mode described above with respect to  FIG. 12 , the superframe  1110 , having twenty-four subframes, is processed by the QOIU  260  into twenty-four packets  1120   a - 0  through  1120   a - 5  (collectively  1120   a ),  1120   b - 0  through  1120   b - 5  (collectively  1120   b ),  1120   c - 0  through  1120   c - 5  (collectively  1120   c ), and  1120   d - 0  through  1120   d - 5  (collectively  1120   d ).  
      The QOIU  260  processes the superframe  1110  to repackage the superframe of data containing multiple subframes of data in known positions within the superframe into multiple communications packets. This may occur in a repackaging unit  261  of a QOIU  260 .  
      An insertion unit  262  may insert a sequence indicator into the payload area of each packet to  1120   a - d  identify the position of the respective subframe within the superframe  1110 . For example, the first four subframes of the superframe may be repackaged into four packets  1120   a - 0 ,  1120   b - 0 ,  1120   c - 0 , and  1120   d - 0 . Similarly, the next four subframes may be repackaged into four packets  1120   a - 1 ,  1120   b - 1 ,  1120   c - 1 , and  1120   d - 1 . In this example, the packets relating to superframe group DA are processed into six packets  1120   a - 0 ,  1120   a - 1 ,  1120   a - 2 ,  1120   a - 3 ,  1120   a - 4 , and  1120   a - 5  and directed to ONU 1   300   a  at a transmission rate λ This transmission rate may be a packet every 500 μsec. Each ONU  300   a - d  may collect its corresponding packet in a buffer (not shown). Through use of the sequence indicators, each ONU can repackage the six packets in a manner that preserves the position of the subframe data from the original superframe  1100 .  
      The repackaging of subframes and insertion of sequence indicators may occur on a processor (not shown) executing software instructions. The software may be stored on any form of computer readable media, such as RAM, ROM, CD-ROM, and so forth, loaded by the processor, and executed. The processor may be a general purpose processor or an application specific processor. Alternatively, the repackaging and insertion of sequence numbers may be implemented in hardware, firmware, or a combination of software and either or both hardware or firmware.  
       FIG. 13B  provides a detailed illustration of the superframe data contained in two exemplary packets processed by the QOIU  260 . The first of the two packets, packet  1120   a - 0 , contains data relating to superframe group DA from the first four subframes of the superframe  1110  of  FIG. 12 . Inspecting the first subframe of superframe  1110  of  FIG. 12  along the time axis (horizontal of  FIG. 12 , vertical  1122 - 1  of  FIG. 13B ), the first byte content includes PCM data  1301  for group DA in timeslot TA, followed by CUDL data  1302  and DPCM data  1303  for the same ONU group and timeslot. This content is placed into the first packet, packet  1120   a - 0 .  
      Continuing across the first subframe  1122 - 1  of the superframe  1110  (of  FIG. 12 ), the subsequent byte content includes PCM data  1311  for group DB (corresponding to ONU  2 ) in timeslot TA, followed by CUDL data  1312  and DPCM data  1313  for the same ONU group and timeslot. This content is placed into the second of the two packets, packet  1120   b - 0 . The QOIU  260  continues to build the packets  1120   a - 0  and  1120   b - 0  by extracting the data relating to each particular ONU for each subframe. For example, in PCM data  1304 , CUDL data  1305 , and DPCM data  1306  of the Superframe are organized into the first subframe  1122 - 1  of the first packet  1120   a - 0  processed by the QOIU  260 . The second subframe  1122 - 2  of the same packet is built using the PCM data  1301 , CUDL data  1302 , and DPCM data  1303  from the second subframe of the superframe  1110  (of  FIG. 12 ). As shown in  FIG. 12 , the data for each superframe group may be interleaved within the superframe  1110 , and reorganized when repackaged into packets.  
      In the embodiment shown in  FIG. 13B , the subframes of each packet are structured to include a second CUDL byte location after the DPCM data. As shown in both subframe  1  of the first packet  1120   a - 0  for ONU 1  and subframe  1  of the first packet  1120   b - 0  for ONU 2 , an empty register 0xFF follows the DPCM data. In subsequent subframes of packets, such as subframe  13  (not shown in  FIG. 13B ), additional CUDL bytes  1309  are allocated to ONU 1 , consistent with the data format of the superframe  1110  of  FIG. 12 .  
      In the example of  FIG. 13B , the packets  1120   a - 0  and  1120   b - 0  contain four subframes of data  1122 - 1  through  1122 - 4  and  1123 - 1  through  1123 - 4 , respectively. Each narrowband packet  1120   a - 0 ,  1120   b - 0  may also contain a standard Ethernet header. As packets  1120   a ,  1120   b , etc. are processed from the superframe  1110 , they are tagged with a sequence number (e.g.,  1125 ,  1126 , etc. in a payload area). Further, the source MAC address and narrowband VLAN ID may be loaded from an internal register, optionally preloaded by a control processor (not shown) in the QOIU  260 . Other values in the header may be predefined in the narrowband packet format.  
       FIG. 13C  illustrates the “downstream” flow of multiple narrowband packets  1120   a - 0 ,  1120   a - 1 ,  1120   a - 2 ,  1120   a - 3 ,  1120   a - 4  and  1120   a - 5  through a BBC  350 . When the narrowband packets arrive at a BBC  350 , an inspection unit  351  inspects the respective sequence indicators of the packets in the payload portion of the packets. An extraction unit  352  extracts multiple subframes of data contained in the packets, and then a formation unit  353  forms a frame of data  1130  from the multiple subframes of data using the sequence indicators from the sequence of packets  1120   a - 0 ,  1120   a - 1 ,  1120   a - 2 ,  1120   a - 3 ,  1120   a - 4  and  1120   a - 5  to maintain organization of the data. The inspection of packets, the extraction of multiple subframes of data, and the formation of a frame of data may occur in processor readable instructions executable by a processor.  
      Further, in other embodiments of the present invention, control bits corresponding to the multiple subframes of data may be extracted and directed to a processing unit, such as a narrowband control card (not shown). Embodiments of the present invention may provide forming multiple frames of data from the multiple subframes of data extracted from the packets. These multiple frames may be directed towards various destination nodes committed to the BBC  350  or may be transmitted through a buffer (not shown) in the QOIU  260  configured to queue multiple frames.  
      In the event that one of the packets  1120   a - 0 ,  1120   a - 1 ,  1120   a - 2 ,  1120   a - 3 ,  1120   a - 4  and  1120   a - 5  is lost in the transmission to the BBC  350 , a loss of synchronization may occur. In this situation, the BBC  350  may form the frame of data using signaling bytes of other received packets from the sequence of packets and either reuse previous subframes of data or use a silence code in place of missing subframes of data. In doing so, the BBC  350  can maintain a call associated with a particular sequence of packets or alternatively drop the call in the event a next sequence of packets associated with the call dropping is received in a given length of time.  
      Similarly, according to embodiments of the present invention as shown in  FIGS. 14A and 14B , communications between the QOIU  260  and the BBC  350  may occur in the “upstream” direction (i.e., from the BBC  350  to the QOIU  260 ). It should be apparent to those of ordinary skill in the art that similar principles of superframe processing can be applied in the upstream direction to provide narrowband traffic to the QOIU  260  in a manner that allows the formation of a superframe of data at the QOIU  260  using subframes contained in the upstream traffic. According to an embodiment of the present invention, a system or corresponding method provides narrowband communications across a communications link through processing packets into a superframe. In an embodiment, a node, such as an ONU, forms a sequence of packets containing subframes of data and inserts a sequence indicator in a payload portion of the packets. The sequence indicator may be used to position the respective subframes within a superframe of data formed at a second node, such as a Remote Data Terminal (RDT), receiving the sequence of packets. At the second node, sequence indicators in a payload portion of the packets may be inspected. The multiple subframes of data may be extracted along with corresponding command and control information. Using the sequence indicators, a superframe of data may be formed from the multiple subframes of data.  
       FIG. 14A  is a block diagram of a QOIU  260  that includes an inspection unit  267 , an extraction unit  268 , and a formation unit  269  that may inspect respective sequence indicators in a payload portion of packets in a sequence of packets  1132   a ,  1132   b ,  1132   c ,  1132   d  ( 1132   a . . . d ), extract multiple subframes of data from the packets  1132   a . . . d , and form a superframe  1111  of data with the multiple subframes of data based on the sequence indicators, respectively.  
       FIG. 14B  is a block diagram of a BBC  350  that includes a formation unit  354 , which may form multiple packets  1132  and of data containing multiple subframes of data, and an insertion unit  355 , which may insert a sequence indicator in a payload portion of the packets  1132   a  used to position the respective subframes within a superframe formed at a node receiving the sequence of packets. As illustrated in this example embodiment, a narrowband signal  1130  arriving at the BBC  350  is formed into packets  0  through  5   1132   a  output by the BBC  350  for ease of reforming the superframe  1111 .  
      In order to transmit the narrowband data from a QOIU  260  to a BBC  350 , a network connection is first established. According to an embodiment of the present invention, a method or corresponding system may detect a network connection in a communications system, such as a narrowband communications system, using Virtual Local Area Network (VLAN) identification. In one embodiment, a first node transmits a message to a specific second node among a group of second nodes. The message from the first node may include a source Medium Access Control (MAC) address, a broadcast address, and a unique VLAN identification corresponding to a port on the first node. The specific second node may process the message and responsively transmit its own MAC address to the first node, along with the unique VLAN identification received in the original message from the first node. The first node may update locally or remotely stored information about the second node.  
       FIG. 15A  is a signal diagram illustrating one embodiment of the present invention for detecting a network connection using VLAN identification. In order for the QOIU  260  to start narrowband communications with a BBC  350 , a QOIU control processor (not shown) enables narrowband communications. In one implementation, an FPGA (shown in  FIG. 6 . as  2601 ) or other electronics device may monitor the narrowband enable bits in an internal control register. In this example, the control processor enables the narrowband process for each ONU once the QOIU  260  source MAC address is loaded into the FPGA registers, as well as the narrowband VLAN ID for the corresponding ONU port.  
      The QOIU  260  may synchronize its Data Processing Unit (DPU) interface to a DPU synchronization signal (not shown). In one embodiment, until the QOIU  260  receives the synchronization signal, no narrowband packets are constructed for transmission to the QOIU  260 . During the time that the QOIU is waiting for ONU port(s) (not shown) to be enabled for narrowband communications, the DPU interface may support processing of a downstream superframe from the DPU.  
      To enable the narrowband communications between the QOIU  260  and a BBC  350  of an ONU, the QOIU  260  may generate and transmit  1510  a broadcast signal  1515  containing (i) a broadcast address  1517   a  as a destination address, (ii) the MAC address  1517   b  of the QOIU  260 , and (iii) the port VLAN ID  1517   c  at a regular interval, such as approximately every 500 μsecs.  
      Upon receiving a narrowband packet (not shown), the BBC  350  checks the packet&#39;s destination MAC address. A broadcast destination MAC address or a destination MAC address that matches the BBC&#39;s MAC address may cause the BBC  350  to write the packet&#39;s source MAC address and VLAN ID into the narrowband packet&#39;s destination MAC address and VLAN ID registers (not shown). If the destination MAC address is not a broadcast address or is not the same as the BBC&#39;s address, the BBC  350  may discard the packet.  
      Once a valid narrowband packet is received by the BBC  350 , the BBC transmits  1520  an upstream packet  1525  to the QOIU  260 . The upstream packet  1525  may contain the MAC address  1527   a  of the BBC  350  and the VLAN ID  1527   b  (same as  1517   c ) assignment. Subsequently, packets  1535  from the QOIU  260  to the BBC  350  are transmitted  1530  with the BBC&#39;s MAC address  1537   a  (same as  1527   a ) identified as the destination address, the QOIU&#39;s MAC address  1537   b  (same as  1517   b ) identified as the source address, and the VLAN ID  1537   c  (same as  1517   c ) to identify the QOIU&#39;s port assignment for the particular BBC  350 .  
      As illustrated in  FIG. 15B , according to an embodiment of the present invention, a QOIU  260  may have port  2621 , a memory  2626 , a transmission unit  2622 , and an update unit  2624 . The memory may store a MAC address of the QOIU  260  and a unique VLAN identification that corresponds to the port  2621 . The transmission unit  2622  is coupled to the port  2621  may be configured to transmit a message (not shown) across an optical connection or link to a BBC  350  connected to that port. In one embodiment, the message includes the MAC address, a broadcast address (since the MAC address of the BBC  350  is unknown), and the unique VLAN identification as discussed above. When the QOIU  260  receives a message from the BBC  350 , the update unit  2606  may be configured to use the information in the message to update stored information about the BBC  350  in the memory  2626 .  
      At the BBC  350 , when an initial message is received at a port  3531 , a parsing unit  3532  may parse the message to determine the MAC address of the QOIU  260  and the VLAN identification associated with the originating port  2621 . A transmission unit  3534  may be configured to transmit a return message to the BBC  350 , the return message including the BBC  350 &#39;s MAC address, and the VLAN identification associated with the originating port  2621 . A memory  3536  may store the MAC address of the BBC  350  and information it receives relating to the QOIU  260 , such as a MAC address and VLAN identification.  
      It should be understood that the QOIU  260  may include a port, memory, and processor as illustrated in  FIG. 6 . The memory may store a MAC address of the QOIU  260  and a unique VLAN identification corresponding to the port. The processor may be coupled to the memory and the port. The processor may transmit a message that includes the MAC address, a broadcast address and a unique VLAN identification and also update stored information about a BBC  350 , upon the receipt of a return message from the second node that includes a MAC address of that node.  
       FIG. 16  provides a basic flow diagram of the detection of a network connection using VLAN identification according to an embodiment of the present invention. The connection initializes when either the QOIU  260  or the BBC  350  power(s) up  1610 . In this embodiment, the QOIU  260  sends  1620  a broadcast message indicating (i) its MAC address as the source address and (ii) a VLAN ID corresponding to a port on the QOIU. The QOIU continues to generate and transmit this broadcast message until an upstream narrowband packet is received from the BBC. The BBC sends  1630  a response message to the QOIU indicating the BBC&#39;s MAC address and acknowledging the VLAN ID. The QOIU updates  1640  information in its database about the BBC for use in future transmissions. In some instances synchronization between the nodes may be lost, for example, if the unique VLAN ID is lost, the VLAN ID becomes invalid, or either MAC address becomes invalid. In embodiments of the present invention, if synchronization is lost between the nodes, messages may be retransmitted using the broadcast address to reestablish a connection.  
      Established digital loop carrier (DLC) systems may use the traditional telephony technique of passing 8 kHz network timing via optical or electrical links interconnecting the components of the system. These systems typically use phase locked loops (PLLs) having voltage controlled crystal oscillators (VCXOs). Lower voltages used for digital design has tightened the specifications on off-the-shelf VCXOs. A minimum “pull” range (i.e., a parameter used to define the maximum frequency pull from the actual operating frequency under a given set of operating conditions) has decreased as power rails have dropped. Frequencies that the VCXOs are required to generate have gone higher to track higher link rates. This increases board layout complexity, as shorter runs are required to ensure a clean clock.  
      Embodiments of present invention provide an opportunity to use a different timing architecture. An example IPTV system of the present invention may be dominated by transmission of frame-based data. Frame-based data platforms use asynchronous bidirectional links. Data recovery occurs by using a clock/data recovery (CDR) circuit that has a local crystal oscillator as a timing reference. The data is sampled and retimed to a local clock domain. This local crystal oscillator may also be used to source the outgoing link.  
      According to an embodiment of the present invention, a method or corresponding system generates a network quality clock signal in a communications system by synthesizing a first clock signal based on arrival rate of packets transmitted via a network link at a rate according to a network clock. The system then synthesizes a second clock signal based on the first clock signal. The second clock signal may have a frequency substantially the same as the network clock. In embodiments of the present invention, the first clock signal may be synthesized by using a phase locked loop, such as a digital PLL configured to synchronize with the arrival rate of narrowband packets. This phase locked loop may include a proportional and integral controller configured to integrate frequency error and control overshoot of the first clock signal. The arrival rate of the packets may be detected by an optical detection module. The second clock signal may also be synthesized using a phase locked loop based on the first clock signal. In embodiments of the present invention, the second phase locked loop is an analog PLL. The second clock signal may be used for narrowband data services and time division multiplexing communications networks.  
       FIG. 17  is a block diagram illustrating an embodiment of the present invention within the IPTV system. A QOIU  260  in a Remote Digital Terminal  200  (RDT) provides narrowband communications to a BBC  350  in an Optical Networking Unit  300  (ONU). A first module  1710   a  synthesizes a first clock signal  1715   a  based on arrival rate (e.g., every 500 μsec) of packets  1705  transmitted via a network link  1720  at a rate according to a network clock (not shown). A second module  1710   b  receives the first clock signal  1715   a  and synthesizes a second clock signal  1715   b , based on the first clock signal. The second clock signal  1715   b  may remove jitter created by the first module  1710   a , by the QOIU  260 , or communications path  1720  and provide a frequency substantially the same as the network clock.  
       FIG. 18  is a high level diagram illustrating an embodiment of the present invention for generating a network quality clock signal. Use of local clock demands on the QOIU  260  and the BBC  350  may require that the 8 kHz network timing be available at the BBC  350 . Because of the optical communications link between the QOIU  260  and the BBC  350  with packet-based communications using non-synchronous communications protocols, the network timing is transferred by a different means than in cases the communications links use synchronous communications protocols. Thus, the local clock is synthesized to provide the network quality clock signal.  
      As shown in  FIG. 18 , the BBC narrowband interface system  1950  is designed in such a way as to attenuate jitter of packet arrival, upon which an output clock is based, that appears on the output clock. An embodiment of system  1950  contains both a first in-first-out (FIFO) buffer  1820  and a system of PLLs  1810 .  
      In embodiments of the present invention, a narrowband interface  2600  on the QOIU transmits the narrowband information to the BBC narrowband interface  1950  every 500 μsecs on both the QOIU  260  and the BBC  350 . The PLLs  1810  and FIFO  1820  of the BBC narrowband interface  1950  provide the narrowband data along with a clock signal to the ONU narrowband interface  3500  in a narrowband common card (NCC)  370 .  
      In one embodiment, sequence number imbedded in the narrowband packet allows logic to insert a duplicate of the previous packet&#39;s PCM into a FIFO  1820 . This prevents the system of PLLs  1810  from changing the digitally controlled oscillator (DCO) (not shown) output frequency in the event that a limited number of packets are lost due to errors caused by Ethernet delay variation  1840 . Duplication of the previous PCM minimizes a voice frequency (VF) customer perceived noise. In some embodiments of the present invention, a FIFO  1830  may also be included to buffer upstream data, even though the upstream data received by the QOIU narrowband interface  2600  is looped timed to the backplane timing.  
       FIG. 19  illustrates a more detailed diagram of the BBC narrowband interface  1950  in an ONU. The BBC narrowband interface  1950  uses a digitally controlled oscillator (DCO)  1920  and a voltage controlled oscillator (VCO)  1910 . The narrowband cell interface  1960  receives the narrowband signals and the BBC clock signal. The narrowband cell interface  1960  buffers the incoming packets in its FIFO buffer  1720 . The narrowband cell interface  1960  sends the local BBC clock signal (BBClk) and a FIFO status signal (NB FIFO STAT) to the DCO  1920 , which generates a clock output based on the frequency of the incoming narrowband packets to the BBC.  
      In one embodiment, the edge jitter caused by the DCO  1920  output is minimized by using an analog phase locked loop  1910  that uses a low power voltage controlled oscillator (VCO) that provides the required jitter attenuation. The BBC narrowband PLL recovery range allows for an approximation of a network Stratum clock.  
       FIG. 20  illustrates the reduction of delay jitter as provided by use of the DCO  1920  ( FIG. 19 ) in the example system of the present invention. A first curve  2005  is a simulation output that represents jitter of a clock signal produced by a model of a clock synthesizer found in systems that do not synthesize a system clock, as described in reference to  FIGS. 16 and 17 . A second curve  2010  is a simulation output that represents jitter of a clock signal produced by a model of a clock synthesizer as described in reference to  FIGS. 16 and 17 .  
       FIG. 21  illustrates the reduction in edge jitter as provided by the use of the VCO  1910  ( FIG. 19 ) in the system of the present invention. A “noisy” curve  2105  is a simulation output that represents narrowband packets  1120   a - d  ( FIG. 13A ) received by respective ONUs  300   a - d  every 500 μsecs. A “smooth” curve  2110  is a simulation output that represents a twice synthesized clock signal as described in reference to  FIGS. 17 and 18 . The twice synthesized clock signal may be generated by at least one synthesizer with a Proportional-Integral (PI) controller, so the curve  2110  does not overshoot to any appreciable level (i.e., the synthesized clock signal reaches its operating frequency without going much higher in frequency). This level of stability may be useful to ensure quality sound output for a listener at a receiving end of the narrowband portion of the system described herein.  
      It should be apparent to those of ordinary skill in the art that methods involved in the present invention may be embodied in a computer program product that includes a computer usable medium. For example, such a computer usable medium may consist of a read-only memory device, such as a CD-ROM disk or convention ROM devices, or a random access memory, such as a hard drive device or a computer diskette, having a computer readable program code stored thereon.  
      While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.