Patent Publication Number: US-2006013260-A1

Title: Optical line termination system, method and apparatus for bus management, and method and apparatus for signal concentration

Description:
FIELD OF THE INVENTION  
      The invention relates to communications networks.  
     BACKGROUND  
      The following acronyms may appear in the description below: APON, asynchronous transfer mode (ATM) passive optical network (PON); ASIC, application-specific integrated circuit; ATM, asynchronous transfer mode; B-PON or BPON (broadband PON); CATV, community access television (cable television); CPU, central processing unit (e.g. microprocessor); EPON (Ethernet PON); FPGA, field-programmable gate array; ISDN, integrated services digital network; PON, passive optical network; POTS, plain old telephone service; PPV, pay per view; PSTN, public switched telephone network; RAM, random-access memory; ROM, read-only memory; TDM, time division multiplexed (or multiplexing); VoIP, voice over Internet Protocol; VoATM, voice over ATM; VoD, video on demand.  
      Optical access systems offer a potentially large bandwidth as compared to copper-based access systems. A broadband optical access system may be used, for example, to distribute a variety of broadband and narrowband communication services from a service provider&#39;s facility to a local distribution point and/or directly to the customer premises. These communication services may include telephone (e.g. POTS, VoIP, VoATM), data (e.g. ISDN, Ethernet), and/or video/audio (e.g. television, CATY, PPV, VoD) services.  
       FIG. 1  shows examples of two optical access network (OAN) architectures. The first example includes an optical line termination (OLT), an optical distribution network (ODN), an optical network unit (ONU), and a network termination (NT). The OLT provides the network-side interface of the OAN (e.g. a service node interface or SNI), and it may be located at a carrier&#39;s central office or connected to a central office via a fibre trunk (e.g. the OLT may include an OC-3/STM-1 or OC-12c/STM-4c interface).  
      The OLT may be implemented as a stand-alone unit or as a card in a backplane. The AccessMAX OLT card of Advanced Fibre Communications (Petaluma, Calif.) is one example of a superior OLT product. Other examples of OLTs include the 7340 line of OLTs of Alcatel (Paris, France), the FiberDrive OLT of Optical Solutions (Minneapolis, Minn.), and assemblies including the TK3721 EPON media access controller device of Teknovus, Inc. (Petaluma, Calif.). The OLT may communicate (e.g. via cable, bus, and/or data communications network (DCN)) with a management system or management entity, such as a network element operations system (NE-OpS), that manages the network and equipment.  
      On the user side, the OLT may be connected to one or more ODNs. An ODN provides one or more optical paths between an OLT and one or more ONUs. The ODN provides these paths over one or more optical fibres. The ODN may also include optional protection fibres (e.g. for backup in case of a break in a primary path).  
      An optical network unit (ONU) is connected to an ODN and provides (either directly or remotely) a user-side interface of the OAN. The ONU, which may serve as a subscriber terminal, may be located outside (e.g. on a utility pole) or inside a building. One or more network terminations (NTs) are connected to an ONU (e.g. via copper trace, wire, and/or cable) to provide user network interfaces (UNIs), e.g. for services such as Ethernet, video, and ATM. Implementations of such an architecture include arrangements commonly termed Fibre to the Building (FTTB), Fibre to the Curb (FTTC), and Fibre to the Cabinet (FTTCab).  
      The second architecture example in  FIG. 1  includes an OLT, an ODN, and one or more optical network terminations (ONTs). An ONT is an implementation of an ONU that includes a user port function. The ONT serves to decouple the access network delivery mechanism from the distribution at the customer premises (e.g. a single-family house or a multi-dwelling unit or business establishment). Implementations of such an architecture include arrangements commonly termed Fibre to the Home (FTTH). In some applications, an ONT may be wall-mounted.  
      The AccessMAX ONT  610  of Advanced Fibre Communications (Petaluma, Calif.) is one example of a superior ONT product. Other examples of ONTs include the Exxtenz ONT of Carrier Access Corporation (Boulder, Colo.), the FiberPath 400 and 500 lines of ONTs of Optical Solutions, the 7340 line of ONTs of Alcatel, and assemblies including the TK3701 device of Teknovus, Inc.  
      As shown in  FIG. 1 , an OAN (including an ODU and the terminals connected to it) may be configured in several different ways, and two or more OANs may be connected to the same OLT. As shown in  FIG. 2 , an ODN may connect an OLT to multiple ONUs. An ODN may also be connected to both ONUs and ONTs. In some applications, the nominal bit rate of the OLT-to-ONU signal may be selected from the rates 155.52 Mbit/s and 622.08 Mbit/s, although other rates are also possible for upstream and downstream communications.  
      An ODN that contains only passive components (e.g. fibre and optical splitters and/or combiners) may also be referred to as a passive optical network (PON). Depending e.g. on the particular protocol used, a PON may also be referred to, for example, as a B-PON (broadband PON), EPON (Ethernet PON), or APON (ATM PON). A OAN may include different OLTs and/or ONUs to handle different types of services (e.g. data transport, telephony, video), and/or a single OLT or ONU may handle more than one type of service. The OLT and/or one or more of the ONUs may be provided with battery backup (e.g. an uninterruptible power supply (UPS)) in case of mains power failure.  
       FIG. 3  shows an example of a OLT connected to a PON that includes a four-way splitter  20  and four eight-way splitters  30   a - d . In this example, each of up to thirty-two ONUs may be connected to the PON via a different output port of splitters  30   a - d  (where the small circles represent the PON nodes depending from these ports). Other PON configurations may include different splitter arrangements. In some such configurations, for example, a path between the OLT and one ONU may pass through a different number of splitters than a path between the OLT and another ONU.  
      The protocol for communications between the OLT and the ONUs may be ATM-based (e.g. such that the OLT and ONUs provide transparent ATM transport service between the SNI and the UNIs over the PON), for example. Such embodiments of the invention may be applied to optical access systems that comply with one or more of ITU-T Recommendation G.983.1 (“Broadband optical access systems based on Passive Optical Networks (PON),” dated October 1998 and as corrected July 1999 and March 2002 and amended November 2001 and March 2003, along with Implementor&#39;s Guide of October 2003) (International Telecommunication Union, Geneva, CH), and ITU-T Recommendation G.983.2 (“ONT management and control interface [OMCI] specification for B-PON,” dated June 2002 and as amended March 2003, along with Implementor&#39;s Guide of April 2000) (International Telecommunication Union, Geneva, CH). Additional aspects of optical access systems to which embodiments of the invention may be applied are described in the aforementioned Recommendations.  
      An OLT may be capable of delivering one or multiple voice telephony lines to each of a subset of subscribers (possibly to each subscriber) via one or more respective ONTs.  FIG. 4A  shows an architecture example including an OLT with an integrated voice gateway, an ODN, and an ONT. The OLT is connected to an external ATM network and an external PSTN. This example illustrates a dichotomy between packet switched (e.g., ATM data) signals and circuit switched (e.g., PSTN voice) signals. ATM packet switched signals may typically be passed between the external ATM network and the PON without any encapsulation operation, for both networks employ ATM protocols. In contrast, circuit switched voice signals, which may be modulated by the PSTN in accordance with synchronous (e.g. TDM) protocols, may need to be encapsulated over ATM for transmission within the PON. In the PON, the voice signals are carried as packets of ATM cells and transported over a high bandwidth physical medium. Bandwidth for voice transport may be over-engineered (e.g. to reduce voice delay within the PON), and ATM cells that carry voice signals may be only partially filled (e.g. to reduce packetization delay). Outgoing ATM voice signals from the PON are decapsulated into TDM voice signals for transmission over the PSTN.  
      In a TDM “nailed-up” transport approach, the OLT locally decapsulates ATM voice signals to TDM voice signals onto a TDM voice infrastructure that reserves capacity for every subscriber line of the PON. Because transport resources are provided for all possible subscribers, such an approach may be very inefficient in practice. Though OLT systems may have a very high capacity and density of served subscriber lines, in practice subscribers seldom need to concurrently utilize every available voice line.  
      In an ATM transport approach, ATM voice signals are transported through an OLT to an interface (e.g., a gateway external to the OLT), which terminates the packetized voice signals and decapsulates them into TDM voice signals onto the switch interface to the PSTN. However, if partial cell fill is used for circuit emulation, ATM transport facilities at the OLT must support a much higher bandwidth, even more so as they carry voice traffic for multiple PON networks. In addition, the ATM transport facilities may need to transport all circuits—whether active or not—or, alternatively, address the complexity of dynamically setting virtual circuit connections (VCCs) upon call activity. Switched virtual circuits (SVCs) may be employed to accomplish dynamic allocation, but these require implementation of a complex signaling stack. Such additional complexity increases software development costs and requires processing hardware and ATM switching hardware capable of setting up calls quickly enough to meet stringent timing requirements. Other methods for concentrating voice traffic over. ATM cells (e.g. AAL2 idle channel suppression) also involve higher complexity.  
     SUMMARY  
      A method of communications processing according to an embodiment of the invention includes receiving idle traffic via each of a plurality of voice ports. The method also includes receiving active traffic via at least one of the plurality of voice ports, while continuing to receive idle traffic via the remainder of the plurality of voice ports, and concentrating the active traffic received via at least one of the plurality of voice ports onto a shared bus.  
      A method of communications processing according to another embodiment of the invention includes receiving active traffic via each of a first voice port and a second voice port, and receiving a first allocation of resources of a shared bus and a second allocation of resources of the shared bus. The method also includes concentrating the active traffic received via the first voice port onto the shared bus according to the first allocation, and concentrating the active traffic received via the second voice port onto the shared bus according to the second allocation. At least one of the group consisting of the active traffic received via the first voice port and the active traffic received via the second voice port consists essentially of partially filled cells.  
      A communications apparatus according to an embodiment of the invention includes a shared bus and a cross-connect device. The cross-connect device is configured to receive idle traffic via each of a plurality of voice ports and to transfer, onto an allocated portion of the shared bus, a voice signal based on active traffic received via one of the plurality of voice ports.  
      A communications apparatus according to another embodiment of the invention includes a shared bus and a cross-connect device. The cross-connect device is configured to receive active traffic via each of a first voice port and a second voice port and to transfer, onto a first allocated portion of the shared bus, a voice signal based on active traffic received via the first voice port. The cross-connect device is also configured to transfer, onto a second allocated portion of the shared bus different from the first allocated portion, a voice signal based on active traffic received via the second voice port, At least one of the group consisting of the active traffic received via the first voice port and the active traffic received via the second voice port consists essentially of partially filled cells. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows examples of two OAN architectures.  
       FIG. 2  shows an example of an OAN.  
       FIG. 3  shows an example of an OLT and a PON including splitters.  
       FIG. 4A  shows an example of an OAN architecture.  
       FIG. 4B  shows an example of an OLT system with an integrated voice gateway.  
       FIG. 5  shows a flowchart of a method according to an embodiment of the invention.  
       FIG. 6A  shows a flowchart of a method according to an embodiment of the invention.  
       FIG. 6B  shows a flowchart of a method according to an embodiment of the invention.  
       FIG. 6C  shows a flowchart of a method according to an embodiment of the invention.  
       FIG. 7  shows a system according to an embodiment of the invention.  
       FIG. 8  shows a controller according to an embodiment of the invention.  
       FIG. 9  shows an interface according to an embodiment of the invention.  
       FIG. 10  shows a system according to an embodiment of the invention.  
       FIG. 11  shows an architecture for concentrating ATM signals onto a TDM bus according to an embodiment of the invention.  
       FIG. 12  shows a system according to an embodiment of the invention.  
       FIG. 13  shows interactions between a subscriber line card, control card, and switch interface card according to an embodiment of the invention.  
       FIG. 14  shows an implementation of a switch interface according to an embodiment of the invention.  
       FIG. 15  shows an implementation of a subscriber interface according to an embodiment of the invention.  
       FIG. 16  shows a system according to an embodiment of the invention.  
       FIG. 17  shows a system including a data storage medium according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      In general, OAN systems employ asynchronous transfer mode (ATM) based protocols for voice calls, while external circuit-switched telephone networks (e.g., PSTNs—public switched telephone networks) employ time division multiplexing (TDM) based protocols. Accordingly, for voice calls spanning both OAN systems and circuit-switched networks, adaptation between the protocols may be necessary, whether within an OLT system or at a location between the OLT system and the circuit-switched network.  
      Embodiments of the invention provide methods and systems for facilitating such adaptation for voice calls in a highly efficient, practical, and cost-effective manner that may also be applied to achieve high voice quality. Embodiments herein may be useful, for example, to architects, service providers, and other operators of a passive optical network (PON).  
      According to embodiments of the invention, a common TDM bus is shared between a subscriber interface (e.g., a PON-side interface) and a switch interface (e.g., a PSTN-side interface). The bus may be shared among multiple subscribers (e.g. via subscriber interface line cards), associated with one or more different PONs, and/or among multiple voice switch interface line cards associated with a PSTN. The bus may transport only active calls, and resources otherwise needed for ATM transport may be eliminated. As such, voice capacity aggregated from multiple PONs can be efficiently concentrated. It is estimated that in some cases, the aggregated voice capacity may be statistically reduced by a factor of ten.  
      In various embodiments of the invention, high bandwidth on a PON (including partial cell fill and/or transport of traffic for inactive calls) may be employed to achieve high quality for voice calls transported over ATM in the PON without the need to introduce complex bandwidth allocation methodologies. Calls to be carried between the PON and an external TDM-based network are concentrated so that only active calls are transported over a shared TDM bus. Such an approach maximizes bandwidth savings, especially in systems that aggregate multiple PONs and thousands of subscribers.  
      In an embodiment of the invention, a central control module (e.g. a control card) allocates resources on a shared TDM bus. The central control module may be provisioned with the logical mappings from ONT voice ports to OLT voice interface card ports (for example, to enable or facilitate routing of calls to appropriate locations, such as the ONT associated with a particular voice circuit). The subscriber interface line cards decapsulate packetized ATM voice signals onto TDM (e.g., PCM—pulse code modulation) voice signals. Each subscriber interface line card and switch interface line card may enable programmable access to the TDM bus via a programmable TDM cross-connect device. The TDM cross-connect device cross-connects any arbitrary PON side channel to any arbitrary TDM bus channel.  
      Embodiments herein may complement and coexist with techniques applied in PONs to reduce delays in ATM voice traffic, such as bandwidth over-engineering techniques. Embodiments herein may also&#39;avoid shortcomings of other approaches taken to adapt voice traffic between an asynchronous system (e.g., an ATM-based system) and a synchronous system (e.g., a ITDM-based system).  
       FIG. 4B  shows an example of an OLT system. The OLT system includes one or more OLTs (e.g. cards) in communication with an uplink (e.g. a switch interface) to one or more external systems (e.g., ATM-based and TDM-based systems) via a cell (e.g., ATM cell) bus and/or a TDM bus. Each OLT may serve one or more PONs. The number of OLT cards shown in  FIG. 4A  is exemplary. The system includes an integrated voice gateway that serves as an ATM termination and a PSTN interface.  
       FIG. 5  shows a flowchart of a method according to an embodiment of the invention. The method may be performed by, for instance, a control module in an OLT or related entity, which module controls the allocation of resources of a shared synchronous (e.g., TDM-based) bus. Task T 100  receives a notification of a voice call that is to be carried between an asynchronous network (e.g. the PON) and a synchronous network (e.g. the circuit-switched network). For instance, the voice call may have originated from a port of an ONU (e.g., an ONT—optical network termination), with its destination being a subscriber line of a circuit-switched network (e.g., an external PSTN). In this case, the notification may be received, for example, from a TDM cross-connect device with signaling monitoring (e.g. via an OAM channel). Alternatively, the voice call may have originated at a subscriber line of a circuit-switched network, with its destination being a port of an ONU.  
      Task T 110  allocates a timeslot of the synchronous bus to the call. In particular, task T 110  may assign one or more available synchronous bus timeslots to the call. Task T 120  transmits an indication of the allocated bus timeslot to one or both of the synchronous bus terminations associated with the call (e.g. via an OAM channel). (In an embodiment, the transmitting of the indication may be referred to as signaling.) The indication may specify which bus resources should be used at each end of the synchronous bus. The synchronous bus terminations may be respectively associated with, for instance, a subscriber interface and a switch interface.  
       FIG. 6A  shows a flowchart of a method of call setup according to another embodiment of the invention. The method, which is a counterpart to that shown in  FIG. 5 , may be performed by a synchronous bus termination in, for example, a subscriber line interface card (to a PON), such as a programmable TDM cross-connect device. Alternatively, the method may be performed by a switch interface line card (associated with the PSTN). Task T 200  receives an indication of an allocated timeslot of the synchronous bus (e.g. via an OAM channel). The indication may come, for example, from a control module (e.g. a control card) in an OLT. Task T 210  connects (e.g. a port of the cross-connect device) to the synchronous bus in accordance with the received indication. Accordingly, the termination may be configured according to the allocated synchronous bus resources so that the call may be established. It is to be understood that, in some embodiments, the method of  FIG. 6A  may be performed with respect to both synchronous bus terminations associated with the call, such that each termination is configured to connect to the synchronous bus according to a respective received indication.  
       FIG. 6B  shows a flowchart of a method of call detection and setup according to an embodiment of the invention. The method, which is another counterpart to that shown in  FIG. 5 , may be performed by a synchronous bus termination in, for example, a subscriber line interface card (to a PON), such as a programmable TDM cross-connect device. Alternatively, the method may be performed by a switch interface line card (associated with the PSTN). Task T 300  detects a call. Task T 310  transmits a notification associated with the call. For example, task T 310  may transmit a notification to a control module in an OLT, which may then allocate a timeslot of the synchronous bus to the call. Task T 200  receives an indication of the allocated timeslot of the synchronous bus. Task T 210  connects (e.g. a port of the cross-connect device) to the synchronous bus in accordance with the received indication.  
       FIG. 6C  shows a flowchart of a method of call setup and teardown according to an embodiment of the invention. The method, which is a further counterpart to that shown in  FIG. 5 , may be performed by a synchronous bus termination in, for example, a subscriber line interface card (to a PON), such as a programmable TDM cross-connect device. Alternatively, the method may be performed by a switch interface line card (associated with the PSTN). Task T 200  receives an indication of an allocated timeslot of a synchronous bus (e.g. via an OAM channel). The indication may come, for example, from a control module (e.g. a control card) in an OLT. Task T 210  connects to the synchronous bus in accordance with the received indication. Task T 320  detects a termination of the call. Task T 330  transmits a notification of the termination of the call, such as to a control module in an OLT (e.g. via an OAM channel). Task T 340  disconnects from the synchronous bus. In some embodiments, task T 340  may disconnect from the synchronous bus in response to a signal received from a control or other module (e.g. via an OAM channel). In other embodiments, task T 340  may disconnect from the synchronous bus after a predetermined time period beginning with termination of the call.  
       FIG. 7  shows a system according to an embodiment of the invention. The system includes an interface  710 , an interface  730 , and a controller  740 .  
      Interface  710  is an interface between an asynchronous (e.g., ATM-based) system and a synchronous (e.g., TDM-based) bus  720 . For example, interface  710  may be included in a subscriber line interface (e.g. a card providing an interface to a PON). Interface  730  is an interface between a shared synchronous bus  720  and a synchronous (e.g., TDM-based) network. For example, interface  730  may be included in a switch interface (e.g. a card or other module providing an interface to the PSTN). Controller  740  (e.g. a control card) allocates resources of synchronous bus  720 . Interfaces  710  and  730  connect to shared synchronous bus  720  to use the resources allocated by controller  740 .  
       FIG. 8  shows a controller  800  according to an embodiment of the invention. Controller  800  may be employed, for example, in an OLT system to direct usage of a shared TDM bus for voice calls. In this example, controller  800  includes a receiver  100 , an allocator  110 , and a transmitter  120 .  
      Receiver  100  receives (e.g. over a circuit trace or control bus) an indication of a voice call to be carried between a PON and a circuit switched network (e.g. a call setup request). Receiver  100  may also receive other call information such as an indication of the origination and/or desired destination for the call (e.g. an originating circuit number, which may be associated with an ONT port). Allocator  110  allocates, to the call, at least one time slot of a shared TDM bus associated with the PON (and possibly with other PONs). Allocator  110  may also identify (e.g. via a port-to-port mapping as described herein, possibly stored in nonvolatile memory such as flash) an appropriate synchronous bus termination to receive the call. Allocator  100  may include an array of logic elements (e.g. an application-specific integrated circuit or programmable device). Transmitter  120  transmits (e.g. over a circuit trace or control bus) an indication of the allocated timeslot to one or both synchronous bus terminations associated with the call.  
      Controller  800  may be a part of a PON card, management system device, and/or control card internal or external to an OLT. In some embodiments, such a device may be inserted into a backplane of an OLT, and the OLT may include other cards or card assemblies inserted into the same or a different backplane. Such a backplane may include a standardized bus (e.g. ISA, PCI, VME, VxI) and/or a proprietary or otherwise non-standardized bus. For example, the backplane may include a control bus over which controller  800  communicates with interfaces to a shared synchronous bus (e.g. via OAM channels). Alternatively, the management system or entity may be external to the OLT and associated equipment and may also include, for example, a command-line interface (CLI) or operational support system (OSS).  
       FIG. 9  shows an interface  900  according to an embodiment of the invention. Interface  900  may be employed as an interface to a synchronous bus (e.g. a shared TDM bus for voice calls). For example, interface  900  may be associated with the PON side or the PSTN side of a voice call. Interface  900  includes a detector  300 , a receiver  200 , and a connection mechanism  210 .  
      Detector  300  detects a call status (e.g. presence of a call and/or a termination of a call). Detector  300  may also detect other call information such as an indication of the desired destination for the call (e.g. a voice circuit). Transmitter  220  transmits the detected information to, e.g., a control device such as controller  800  (for example, as a call setup or teardown request). In some implementations, the information is transmitted via an OAM channel. Receiver  200  receives an indication of an allocated timeslot of a synchronous (e.g. TDM) bus (for example, from a control device, possibly via an OAM channel). Connection mechanism  210  (e.g. a programmable cross-connect device) connects to the synchronous bus in accordance with the received indication. Connection mechanism  210  also may disconnect from the synchronous bus (or otherwise release the allocated timeslot(s)) based on the detected termination of the call and/or the occurrence of another event or condition, such as receipt of an indication from a control module (e.g. via an OAM channel). In another embodiment, connection mechanism  210  also performs signaling monitoring operations.  
       FIG. 10  shows a system according to an embodiment of the invention. The system includes a PON interface  1000 , a switch interface  1010 , and a controller  800 .  
      PON interface  1000  is an interface between shared TDM bus  1020  and one or more PONs utilizing ATM protocols for voice calls. PON interface  1000  includes a detector  300 , a transmitter  220 , a receiver  200 , and a connection mechanism  210 . PON interface  1000  is an implementation of interface  900  of  FIG. 9  and may be embodied as or within a card or card assembly inserted into a backplane. The system may also include other instances of PON interface  1000  configured to connect other PONs to the shared bus.  
      Switch interface  1010  is an interface between shared TDM bus  1020  and a PSTN utilizing TDM protocols for voice calls. Switch interface  1010  includes a detector  300 , a transmitter  220 , a receiver  200 , and a connection mechanism  210 . Switch interface  1010  is an implementation of interface  900  of  FIG. 9  and may be embodied as or within a card or card assembly inserted into a backplane. The system may include multiple instances of switch interface  1010 .  
      Controller  800  allocates timeslots of shared TDM bus  1020  for voice calls and directs usage of such timeslots by PON interface  1000  and switch interface  1010  in order that voice calls may be transferred between the PON(s) and the PSTN. Controller  800  includes a receiver  100 , an allocator  1110 , and a transmitter  120  and may be embodied as or within a card or card assembly inserted into a backplane.  
      In an example scenario, a call arrives at either the PON side or PSTN side with respect to shared TDM bus  1020 . Assuming, for illustrative purposes, that the call is originated at the PON side, then detector  300  in subscriber interface  300  detects the call. Subscriber interface  1000  transmits an indication thereof (e.g. a call setup request, possibly via an OAM channel) to controller  800 , which indication is received by receiver  100 . Allocator  110  of controller  800  allocates one or more timeslots of shared TDM bus  1020  to the call. Transmitter  120  transmits an indication thereof to both subscriber interface  1000  and switch interface  1010  (e.g. via OAM channels), whose respective receivers  200  receive such indication. Their respective connection mechanisms  210  connect to shared TDM bus  1020  in accordance with the indication. As such, a call is established over shared TDM bus  1020 .  
      One or both of the respective detectors  300  of subscriber interface  1000  and switch interface  1010  detect a termination of the call. Subscriber interface  1000  and/or switch interface  1010  respectively transmit an indication thereof (e.g. a call teardown request, possibly via an OAM channel) to controller  800 . In response, transmitter  120  of controller  800  may transmit an indication (e.g. via OAM channels) that subscriber interface  1000  and switch interface  1010  should disconnect from shared TDM bus  1020  (or otherwise release the timeslot(s) associated with the terminated call) to free the bus resources. After receipt of the indication by the respective receivers  200 , the respective connection mechanisms  210  disconnect from the shared TDM bus  1020  (release the associated timeslot(s)).  
       FIG. 11  shows another architecture for concentrating ATM signals onto a TDM bus according to an embodiment of the invention. The architecture includes a shared TDM bus  1110  to transport active calls, a PON subscriber line card  1120  with a TDM cross connect device  1130 ; a control channel  1150 , and one or more ONTs  1140 . It is to be appreciated that the call capacities and voice port specifications identified in  FIG. 111  and discussed below are merely exemplary and are not limiting. Moreover, ONTs  1140  may be replaced with other types of ONUs.  
      PON subscriber line card  1120  includes 100 voice ports and has the requisite bandwidth to service  100  ATM-based voice calls passing through ONTs  1140 . TDM cross-connect device  1130  is configured to couple PON subscriber line card  1120  to shared TDM bus  1110  in order to utilize allocated timeslots of shared TDM bus  1110 . In the example of  FIG. 11 , TDM cross-connect device  1130  and shared TDM bus  1110  have a 200-call capacity. In addition, TDM cross-connect device  1130  has signaling monitoring capabilities. Accordingly, TDM cross-connect device  1130  can detect a call that is to be established between an ONT  1140  and another endpoint, such as another ONU within a PON or an endpoint in an external TDM-based network (e.g. PSTN), and transmit an indication (e.g. a call setup request) to a common control module (not shown) over control channel  1150  (e.g. via an OAM channel).  
      In a representative mode of operation, TDM cross-connect device  1130  detects a call and transmits a notification, via control channel  1150 , to the common control module. The common control module allocates one or more appropriate timeslots of shared TDM bus  1110  to the call and transmits, via control channel  1150 , an indication of the allocated timeslot(s) to TDM cross-connect device  1130 . TDM cross-connect device  1130  then appropriately couples PON subscriber line card  1120  to shared TDM bus  1110  so that the call may be established. Thus TDM cross-connect device  1130  is configured to concentrate active voice calls onto the shared TDM bus  1110 .  
       FIG. 12  shows a network including an OAN according to an embodiment of the invention. The OLT system may be employed to concentrate voice signals carried between an external circuit-switched (TDM-based) network and a (ATM-based) PON onto a shared TDM bus. In the system of  FIG. 12 , ATM signals from the PON are concentrated onto a TDM bus. Similarly, TDM signals from the circuit-switched network are carried over the TDM bus. The system includes a shared TDM bus  1110 , switch interface line cards  1220 , PON subscriber line cards  1120 , a common control module  1210 , a control channel  1150 , and ONTs  1140 . The call capacities and voice port specifications identified in  FIG. 12  and discussed below are merely exemplary and are not limiting. Further, ONTs  1140  may be replaced with other types of ONUs.  
      In this example, PON subscriber line cards  1120  each include 100 voice ports and have the requisite bandwidth to service  100  ATM-based voice calls that involve ONTs  1140 . PON subscriber line cards  1120  and switch interface line cards  1220  each can connect to shared TDM bus  1110  in order to utilize allocated timeslots of shared TDM bus  1110 . Shared TDM bus  1110  has a 200-voice-call capacity in the example of  FIG. 12 . Control channel  1150  is coupled to each PON subscriber line card  1120  and switch interface line card  1220 , as well as to common control module  1210 .  
      In an example scenario, common control module  1210  allocates one or more appropriate timeslots of shared TDM bus  1110  to a detected call and transmits, via control channel  1150  (e.g. over OAM channels), an indication of the allocated timeslot(s) to the appropriate switch interface line card  1220  and PON subscriber line card  1120 . The call is established when the cards  1220  and  1120  connect to shared TDM bus  1110 .  
      Two pairs of established voice calls  1230 ,  1235  are shown in  FIG. 12 . For calls  1230 , TDM voice concentration points  1240  at a PON subscriber line card  1120  are identified. For established voice calls  1235 , TDM voice concentration points  1245  at a switch interface line card  1220  are identified.  
       FIG. 13  shows one example of a sequence of interactions between a subscriber line card  1320 , a common control card  1310 , and a switch interface card  1330  according to an embodiment of the invention. At PON subscriber line card  1320 , task T 400  transmits a call setup request to common control card  1310 . Task T 410  finds and allocates unused resources (e.g. timeslots) of a shared TDM bus. Tasks T 420  and T 430  respectively transmit TDM bus resource assignments to switch interface card  1330  and PON subscriber line card  1320 . The call is established when the cards  1330  and  1320  connect to the shared TDM bus. When the call is completed, task T 440  at switch interface card  1330  may transmit a call teardown request to common control card  1310 . Task T 450  at common control card  1310  deallocates the shared TDM bus resources. Tasks T 460  and T 470  respectively transmit TDM bus resource un-assignments to switch interface card  1330  and PON subscriber line card  1320 . The resources of the shared TDM bus are freed for future use when the cards  1330  and  1320  disconnect from the shared TDM bus.  
       FIG. 14  shows an implementation of a switch interface  1400  according to an embodiment of the invention. Switch interface  1400  includes one or more switch interface line cards  1420  configured to interface with an external circuit-switched network (e.g. a PSTN). Each card  1420  may have an associated cross-connect device  1410  (e.g. a programmable TDM cross-connect device) that interfaces one of a plurality of channels at one side of the device (e.g. lines to the external network) to a selected one of a plurality of channels on the other side of the device (e.g. timeslots of a shared TDM bus  1020 ). In some embodiments, a cross-connect device  1410  is a programmable module within a card  1420 . In other embodiments, a cross-connect device  1410  is a separate programmable device that interfaces with a card  1420 . A cross-connect device  1410  may also perform signaling monitoring operations (e.g. to support transmission of call set up and/or teardown requests). The cards  1420  and/or cross-connect devices  1410  interact with a central control module  1430 . For each call detected at a subscriber interface side, control device  1430  may transmit a timeslot allocation to a selected one of the switch interface line cards based on, e.g., a load balancing algorithm. Embodiments of the invention may also be implemented to include one or more standard switch interfaces such as TR-57, TR-08, GR-303, and V5.  
       FIG. 15  shows an implementation of a subscriber interface  1500  according to an embodiment of the invention. Subscriber interface  1500  includes one or more subscriber interface line cards  1510 . For example, each card  1510  may be associated with a respective PON. Each card  1510  may have an associated cross-connect device  1520  that is configured to interface one of a plurality of channels at one side of the device (e.g. voice ports) to a selected one of a plurality of channels on the other side of the device (e.g. timeslots of a shared TDM bus  1020 ). In some embodiments, a cross-connect device  1520  is a programmable module integrated into a card  1510 . In other embodiments, a cross-connect device  1520  is a programmable device that interfaces with a card  1510 . The cards  1510  and/or cross-connect devices  1520  may interact with a central control module  1430  (e.g. via respective OAM channels).  
       FIG. 16  shows a system according to an embodiment of the invention. The system includes a subscriber interface  1500  as shown in  FIG. 15 , a switch interface  1400  as shown in  FIG. 14 , and a central control module  1430 . Interactions among these components may be as described in connection with the examples of  FIGS. 10-15 .  
      Certain embodiments herein illustrate interactions between a central control module, a subscriber interface including subscriber interface line cards, and a switch interface including switch interface line cards. It is to be appreciated that, in practice, the actual number of such cards in an implementation is arbitrary, depending on the number of PONs utilizing the shared TDM bus, the capabilities of the cards, and/or the number of served subscriber lines, for example.  
      It is expressly contemplated that alternative operations and/or configurations of such elements, and that apparatus including additional elements, are disclosed by and may be constructed according to the description provided herein. For instance, the subscriber interface line cards and/or switch interface line cards of  FIGS. 11-16  may be implemented by hardware and/or software not contained in a card, or in another equivalent fashion as is or may become known in the art of circuit design. In addition, various system components herein may interface discrete modules that perform respective functions. Alternatively or additionally, system components may utilize integrated modules that perform multiple functions.  
      The foregoing presentation of the described embodiments is provided to enable any person skilled in the art to make or use the present invention. While specific embodiments of the invention have been described above, it will be appreciated that the invention as claimed may be practiced otherwise than as described. Various modifications to these embodiments are possible, and the generic principles presented herein may be applied to other embodiments as well.  
      An embodiment of the invention may be implemented in part or in whole as a hard-wired circuit (e.g. implemented on a computer interface card) and/or as a circuit configuration fabricated into one or more arrays of logic elements arranged sequentially and/or combinatorially and possibly clocked (e.g. one or more integrated circuits (e.g. ASIC(s)) or FPGAs). Likewise, an embodiment of the invention may be implemented in part or in whole as a firmware program loaded or fabricated into non-volatile storage (such as read-only memory or flash memory) as machine-readable code, such code being instructions executable by an array of logic elements such as a microprocessor or other digital signal processing unit.  
      Further, an embodiment of the invention may be implemented in part or in whole as a software program loaded as machine-readable code from or into a data storage medium (e.g., as shown in  FIG. 17 ) such as a magnetic, optical, magnetooptical, or phase-change disk or disk drive; or some form of a semiconductor memory such as ROM, RAM, or flash RAM, such code being instructions (e.g. one or more sequences) executable by an array of logic elements such as a microprocessor or other digital signal processing unit, which may be embedded into a larger device. Thus, the present invention is not intended to be limited to the embodiments shown above but rather is to be accorded the widest scope consistent with the principles and novel features disclosed in any fashion herein.