Abstract:
An entertainment system that exhibits advantages of fiber-to-the-seat systems, has improved failure recovery characteristics, and reduces the connection components is disclosed. In one aspect, an inflight entertainment system comprises a plurality of head end line replaceable units physically interconnected in a ring configuration and a plurality of serially-connected networking line replaceable units physically interconnected in a serial configuration, wherein two of the serially-connected networking line replaceable units at the edge of the serial configuration are physically interconnected with two of the head end line replaceable units, respectively, wherein a loop-free head end data path is maintained between active head end line replaceable units by regulating link participation in the head end data path, and wherein one or more loop-free serially-connected networking data paths are maintained between at least one of the two head end line replaceable units and active serially-connected networking line replaceable units by regulating link participation in the serially-connected networking data paths.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application No. 61/273,584 entitled “SERIAL NETWORKING FIBER-TO-THE-SEAT INFLIGHT ENTERTAINMENT SYSTEM,” filed on Aug. 6, 2009 and U.S. provisional application No. 61/274,726 entitled “SERIAL NETWORKING FIBER-TO-THE-SEAT INFLIGHT ENTERTAINMENT SYSTEM NETWORK MANAGEMENT,” filed on Aug. 20, 2009, the contents of both of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Inflight entertainment systems have evolved significantly over the last 25 years. Prior to 1978, IFE systems consisted of audio-only systems. In 1978, Bell and Howell (Avicom Division) introduced a group viewing video system based on video cassette tapes. In 1988, Airvision introduced the first in-seat video system allowing passengers to choose between several channels of broadcast video. In 1997, Swissair installed the first interactive video on demand (VOD) system. Currently, several inflight entertainment systems provide VOD with full digital video disc-like controls. 
         [0003]    One factor in the commercial viability of an inflight entertainment system is the system&#39;s line replaceable units (LRUs). The term “LRU” is a term of art generally describing a complex component (e.g. “black box”) on an airplane that is designed to be replaced quickly on the flight line or airport ramp area. LRU&#39;s are beneficial because they are generally self-contained units that can be rapidly swapped-out in the event that maintenance is required thus allowing the airplane to continue to operate with little down time. To be installed on an airplane, an LRU design must first be approved by the Federal Aviation Administration by means defined in Title 14 of the Code of Federal Regulations. LRUs of a single hardware design configuration may have different software installed. An inflight entertainment system&#39;s installation costs, operating costs, maintenance costs and passenger comfort depend greatly on the size, form factor, number and weight of its LRUs, as well as the number of distinct LRUs deployed in a single aircraft and across an airline&#39;s entire fleet of aircraft. 
         [0004]      FIG. 1  illustrates conventional inflight entertainment systems that employ a terrestrial-like VOD architecture (i.e. head end, distribution area, seat end). The left side of the figure shows components that are typically found at the head end of the system or in an electronics bay. The right side of the figure shows system components that are typically found at the seat end. The middle section of the figure shows system components that are typically found in a distribution area between the head end and the seat end. These components include area distribution boxes (ADBs) that fan-out data from the head end to the seat end. ADBs are typically connected to seat electronics boxes (SEBs) within each seat column, which distribute data forward and/or backward to an adjacent seat group in the same seat column. 
         [0005]    Inflight entertainment system vendors have recently made attempts to reduce the number of distinct LRUs at the seat end. Four examples of conventional seat end architectures are shown in  FIG. 1 . Boxes A and D show traditional seat architectures wherein SEBs are interposed between ADBs and seat end components such as VDUs and passenger control units (PCUs). Boxes B and C show newer seat architectures that reflect attempts to eliminate or reduce reliance on SEBs by moving SEB functionality to VDUs and/or PCUs. This shift has generally come at the expense of increased VDU and/or PCU size, weight and power. 
         [0006]    The inflight entertainment industry has been slower to reduce the number of distinct LRUs at the head end and in the distribution area. Moreover, conventional inflight entertainment systems require inter-area wiring and seat-to-seat wiring that varies across inflight entertainment system vendors and even across a single inflight entertainment system vendor&#39;s products, raising installation and maintenance costs. 
         [0007]    Some small players in the inflight entertainment industry have made advances at the head end and in the distribution area. For example, one recent inflight entertainment system has no head end or distribution area. However, this system has no parallel in terrestrial VOD systems and cannot easily leverage advancements and technology developments from the terrestrial world. Another recent inflight entertainment system has a simplified head end that integrates the audio, video and application servers in a single LRU. However, this system is proprietary and cannot easily leverage advancements in terrestrial VOD system technology. Moreover, the latter system requires a network of distribution area nodes between the head end and the seat end. 
       SUMMARY OF THE INVENTION 
       [0008]    The fiber-to-the-seat (FTTS) system described in U.S. Patent Application Publication No. 2007/0077998, the contents of which are incorporated herein by reference, and summarized in  FIG. 2  has offered a more modular, scalable, extensible, future proofed, wired inflight entertainment system that leverages terrestrial VOD hardware and software advances and is packaged to minimize the number of distinct LRU not only in a single aircraft but across an airline&#39;s entire fleet of aircraft (i.e. regional jets to jumbo jets). However, this FTTS system has certain drawbacks. First, each server switch unit (SSU) is a single point of failure for all VDUs and any cabin management terminal (CMT) that connects directly to that SSU. Second, the implementation of a star wired network topology wherein each VDU has a dedicated optical fiber “home run” to a head end SSU adds cost and complexity to the system. For example, over two miles of fiber are required on a typical narrow body aircraft installation and over four miles of fiber are required on a typical wide body aircraft installation. The high cost of aircraft grade fiber and fiber optic connectors, coupled with the cost and complexity of installing these fiber components, make this architecture very expensive to implement. 
         [0009]    In some embodiments, the present invention provides an inflight entertainment system that offers advantages of the FTTS system described in U.S. Patent Application Publication No. 2007/0077998 while exhibiting superior failure recovery characteristics and having reduced fiber component requirements. The system, however, is not limited to the field of aviation; indeed other applications are contemplated, such as but not limited to buses, boats, automobiles, trains, and the like. 
         [0010]    In one aspect of the invention, such an inflight entertainment system comprises a plurality of head end line replaceable units physically interconnected in a ring configuration and a plurality of serially-connected networking line replaceable units physically interconnected in a serial configuration, wherein two of the serially-connected networking line replaceable units at the edge of the serial configuration are physically interconnected with two of the head end line replaceable units, respectively, wherein a loop-free head end data path is maintained between active head end line replaceable units by regulating link participation in the head end data path, and wherein one or more loop-free serially-connected networking data paths are maintained between at least one of the two head end line replaceable units and active serially-connected networking line replaceable units by regulating link participation in the serially-connected networking data paths. 
         [0011]    In some embodiments, the loop-free serially-connected networking data paths are maintained by removing a selected link from a previous serially-connected serially-connected networking data path. In some embodiments, the selected link is selected using hop count information. In some embodiments, the selected link is selected to minimize the maximum number of hops between any of the active serially-connected networking line replaceable units and either of the two head end line replaceable units. 
         [0012]    In some embodiments, the loop-free serially-connected networking data paths are maintained by adding a selected link to a previous serially-connected networking data path in response to detecting a failure on the previous serially-connected networking data path. In some embodiments, the failure is a link failure. In some embodiments, the failure is a line replaceable unit failure. 
         [0013]    In some embodiments, the loop-free head end data path is maintained by removing a selected link from a previous head end data path in response to detecting a loop on the previous head end data path. 
         [0014]    In some embodiments, the loop-free head end data path is maintained by adding a selected link to a previous head end data path in response to detecting a failure on the previous head end data path. In some embodiments, the failure is a link failure. In some embodiments, the failure is a line replaceable unit failure. 
         [0015]    In some embodiments, the plurality of serially-connected networking line replaceable units comprises at least one video display line replaceable unit. 
         [0016]    In some embodiments, the plurality of serially-connected networking line replaceable units comprises at least one cabin management terminal line replaceable unit. 
         [0017]    In some embodiments, the plurality of serially-connected networking line replaceable units comprises at least one on board network interface line replaceable unit. In some embodiments the on board network interface line replaceable unit provides connectivity to a public address system. In some embodiments the on board network interface line replaceable unit provides connectivity to a flight management system. 
         [0018]    In some embodiments, the plurality of serially-connected networking line replaceable units comprises at least one off board network interface line replaceable unit. 
         [0019]    In some embodiments, the plurality of serially-connected networking line replaceable units comprises at least one data loader line replaceable unit. 
         [0020]    In some embodiments, the plurality of head end line replaceable units comprises at least one application server. 
         [0021]    In some embodiments, the plurality of head end line replaceable units comprises at least one audio server. 
         [0022]    In some embodiments, the plurality of head end line replaceable units comprises at least one video server. 
         [0023]    In some embodiments, the plurality of head end line replaceable units comprises at least one file server. 
         [0024]    In some embodiments, the plurality of head end line replaceable units comprises at least one game server. 
         [0025]    In some embodiments, the plurality of head end line replaceable units comprises at least one passenger flight information system server. 
         [0026]    In another aspect of the invention, a first head end line replaceable unit for an inflight entertainment system comprises a plurality of fiber optic transceivers and a processor communicatively coupled with the transceivers, wherein under control of the processor in response to failure of a first link to a second head end line replaceable unit via a first one of the transceivers the first head end line replaceable unit activates a second link to a third head end line replaceable unit via a second one of the transceivers whereby a loop-free head end data path between a plurality of head end line replaceable units is restored. 
         [0027]    In another aspect of the invention, a serial networking line replaceable unit for an inflight entertainment system comprises a plurality of fiber optic transceivers and a processor communicatively coupled with the transceivers, wherein under control of the processor in response to failure of a first data path to a first head end line replaceable unit via a first one of the transceivers the serial networking line replaceable unit activates a second data path to a second head end line replaceable unit via a second one of the transceivers. 
         [0028]    In another aspect of the invention, a head end line replaceable unit for an inflight entertainment system comprises a plurality of fiber optic transceivers and a processor communicatively coupled with the transceivers, wherein under control of the processor the head end line replaceable unit transmits a presence message on a link via a first one of the transceivers, receives the presence message on a link via a second one of the transceivers, and in response to receiving the presence message removes one of the links from participation in a loop-free head end data path between a plurality of head end line replaceable units. 
         [0029]    In some embodiments, under control of the processor the head end line replaceable unit transmits to a serial networking line replaceable unit via a third one of the transceivers a second presence message having a hop count. 
         [0030]    In yet another aspect of the invention, a serial networking line replaceable unit for an inflight entertainment system comprises a plurality of fiber optic transceivers and a processor communicatively coupled with the transceivers, wherein under control of the processor the serial networking line replaceable unit receives a presence message having a received hop count on a link via a first one of the transceivers, increments the hop count and transmits the presence message having an incremented hop count on a link via a second one of the fiber optic transceivers, and wherein under control of the processor the serial networking line replaceable unit regulates participation of one of the links in a loop-free serial networking data path between a head end line replaceable unit and a plurality of serial networking line replaceable units based at least in part on the received hop count. 
         [0031]    These and other aspects of the invention will be better understood by reference to the following detailed description taken in conjunction with the drawings that are briefly described below. Of course, the invention is defined by the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  shows known inflight entertainment systems. 
           [0033]      FIG. 2  shows a known FTTS inflight entertainment system. 
           [0034]      FIG. 3  shows an inflight entertainment system with serially-connected networking line replaceable unit chains and a head end line replaceable unit ring in accordance with some embodiments of the invention. 
           [0035]      FIG. 4  shows a head end line replaceable unit for an inflight entertainment system with serial networking line replaceable unit chains and a head end line replaceable unit ring in accordance with some embodiments of the invention. 
           [0036]      FIG. 5  shows a generic serial networking line replaceable unit for an inflight entertainment system with serial networking line replaceable unit chains and a head end line replaceable unit ring in accordance with some embodiments of the invention. 
           [0037]      FIGS. 6A through 6D  show serial networking data path maintenance in accordance with some embodiments of the invention. 
           [0038]      FIGS. 7A through 7D  show head end data path maintenance in accordance with some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0039]      FIG. 3  shows an inflight entertainment (IFE) system with serial networking line replaceable unit (SN-LRU) chains  311 - 313  and a head end line replaceable unit (HE-LRU) ring  310  in some embodiments of the invention. As illustrated, SN-LRU chain  311  and HE-LRU ring  310  are positioned outside of the seats, while SN-LRU chains  312 ,  313  are positioned at the seats. In these embodiments, multiple HE-LRUs  309  are physically connected by ring via fiber optic links  308 . Multiple chains of SN-LRUs  301 - 305  are physically connected to HE-LRUs  309  at their edges (ends) via links  307 , for example fiber optics, such that the two edges of each chain are physically connected to a different one of HE-LRUs  309 . Many types of SN-LRUs can be employed, for example serial networking onboard network interface unit  301 , serial networking offboard network interface unit  302 , serial networking data loader  303 , serial networking CMT  304  (generally positioned in the galley), and serial networking VDUs  305 . 
         [0040]    Each SN-LRU  301 - 305  discovers through topology messaging the nearest HE-LRU  309 . In the illustrated embodiment, starting on one SN-LRU chain  311  edge, unit  301  is connected to one of HE-LRUs  309  via a link  307  in the upstream direction while connecting to unit  302  in the downstream direction via another link  306 . Unit  301  receives from the HE-LRU  309  in the upstream direction a presence message including a hop count to the HE-LRU  309 , increments the hop count, and passes the updated presence message along in the downstream direction to unit  302 . As the presence message progresses in the downstream direction, each successive SN-LRU in the chain (e.g.  302 ,  303 ,  304 ) increments the hop count. Continuing on this chain, unit  302  is connected in the downstream direction to data loader  303  over another link  306 . Data loader  303  is connected in the downstream direction to CMT  304  over another link  306 . In the final link of this SN-LRU chain  311 , CMT  304  at the edge of the chain is connected back to a different one of HE-LRUs  309  over yet another link  307 . In the other direction, CMT  304  receives from that HE-LRU  309  a presence message including a hop count, increments the hop count, and passes the updated presence message along in the upstream direction to data loader  303 . Each successive SN-LRU in the chain  311  increments the hop count accordingly. 
         [0041]    The IFE system can include at least one additional SN-LRU chain  312  and probably at least two additional SN-LRU chains  312 ,  313 . The additional SN-LRU chain or chains  312 ,  313  can consist of most any type of SN-LRU, such as VDUs  305 . On each edge of these additional SN-LRU chains  312 ,  313 , one of serial networking VDUs  305  is connected to one of HE-LRUs  309  over a link and within each of these SN-LRU chains serial networking VDUs  305  are connected over links. These additional SN-LRU chains  312 ,  313  generally disseminate presence messages and hop count information in the same manner as the previously described SN-LRU chain  311 . 
         [0042]    Separately, each HE-LRU discovers through topology messaging whether the HE-LRU ring  310  is closed or open. Each HE-LRU  309  receives a presence message from neighboring HE-LRUs  309  and relays the presence message on its non-ingress port until it is determined whether presence message loops-back to the originating HE-LRUs  309 , in which case it is revealed that the HE-LRU ring  310  is closed, or does not loop-back to the originating HE-LRUs  309 , in which case it is revealed that the HE-LRU ring  310  is open. 
         [0043]    Server functionality (e.g. application server, audio server, video server, game server, file server, passenger information system server) is integrated into HE-LRUs  309  in a modular, scalable, robust fashion that minimizes the impact on the IFE system in the event one or more of HE-LRUs  309  fails. Network management processors within HE-LRUs  309  and SN-LRUs restore network access of live SN-LRUs  301 - 305  to HE-LRUs  309  under the following scenarios: (a) a connection break along an SN-LRU chain  311 - 313 ; (b) failure of an SN-LRU  301 - 305  in an SN-LRU chain  311 - 313 ; (c) failure of an HE-LRU  309  at one end of an SN-LRU chain  311 - 313 . Moreover, network management processors within HE-LRUs  309  restore network access of SN-LRUs  301 - 305  as described with respect to  FIGS. 6A-7D  to live HE-LRUs  309  under the following scenarios: (a) a connection break between any two HE-LRUs  309 ; (b) failure of an HE-LRU  309 . While the number of SN-LRUs  301 - 305  in an SN-LRU chain  311 - 313  will vary, relatively short SN-LRU chains generally offer a higher level of redundancy and failover bandwidth to SN-LRUs. In some embodiments, error indications are provided, e.g. error codes, to facilitate identification, diagnosis, and/or location of the error. In some embodiments the error indications are transmitted to offboard monitoring and/or maintenance systems. 
         [0044]      FIG. 4  shows a representative HE-LRU  400  adapted for use in an IFE system with SN-LRU chains and an HE-LRU ring in some embodiments of the invention. In these embodiments, HE-LRU  400  has integrated servers of six different types, including one or more application servers  401 , video servers  402 , audio servers  403 , game servers  404 , file servers  405  and passenger flight information system servers  406 , all of which are connected to an integrated network management processor  407  over internal connections. By way of example, network management processor  407  may be a managed switch. 
         [0045]    Application servers  401  are system controllers that provide the following types of services: content management; channel packaging; transaction processing; billing system integration; services management; provisioning integration; system administration and management; encryption management (key servers, authentication etc.); software client management; server integration for audio, video, gaming and file servers or the like. Video servers  402  provide the following types of services: VOD, near VOD; pay per view; network personal video recorder; broadcast video or the like. Audio servers  403  provide the following types of services: audio on demand; broadcast audio or the like. Game servers  404  provide the following types of services: logic and programming for games; dynamically delivered web pages for browser based games or the like. File servers  405  provide the following types of services: cached Internet content; cached user data and user profile data or the like. Passenger flight information system servers  406  use inputs from the aircraft navigation system and compute various flight information including time to destination, speed, altitude, outside air temperature, time at destination, aircraft location for display to passenger either in text form or graphically such as a moving map display or the like. 
         [0046]    Processor  407  has N ports reserved for physical connections to SN-LRUs on the edges of chains and K ports reserved for physical connections to other HE-LRUs in a ring. The K ports reserved for HE-LRU ring connections are connected to K HE-LRU port transceivers  408  over internal connections. Port transceivers  408  are in turn connected to a fiber optic panel connector  420  over K internal fiber optic connections. Similarly, the N ports reserved for SN-LRU chain connections are connected to N SN-LRU port transceivers  410  over internal connections. Port transceivers  410  are in turn connected to panel connector  420  over N internal fiber optic connections. In some embodiments, the internal fiber optic connections are simplex by the time they connect to panel connector  420  (e.g. port transceivers  408  and  410  are bidirectional or a coupler is used to convert a unidirectional duplex transceiver output to bidirectional simplex format). Panel connector  420  blind mates with a connector  421  when HE-LRU  400  is installed in a rack at the head end. Connector  421  has K external fiber optic cables reserved for HE-LRU ports that connect to the corresponding HE-LRU internal fiber optic connections when HE-LRU  400  is installed in the rack. Similarly, connector  421  has N external fiber optic cables reserved for the SN-LRU chain ports that connect to the corresponding SN-LRU internal fiber optic connections when HE-LRU  400  is installed in the rack. K and N are each greater than one. Moreover, HE-LRU  400  has T data ports, where K+N is less than or equal to T Under control of processor  407 , HE-LRU  400  provides presence information to any SN-LRU that is connected directly to HE-LRU  400  over one of the N external fiber optic cables reserved for SN-LRU chain ports (i.e. any edge SN-LRU). Under control of processor  407 , HE-LRU  400  also provides its own presence information to any HE-LRU that is connected directly to HE-LRU  400  over one of the K external fiber optic cables reserved for HE-LRU ports and relays on its non-ingress port any presence information (that HE-LRU did not originate) received on these ports from neighboring HE-LRUs. 
         [0047]      FIG. 5  shows a generic SN-LRU  500  adapted for use in an IFE system with SN-LRU chains and an HE-LRU ring in some embodiments of the invention. In these embodiments, SN-LRU  500  includes an LRU core  501  having hardware and software elements, a first fiber optic transceiver  503 , a second fiber optic transceiver  504  and a network management processor  502 , which may be a managed switch. Processor  502  is communicatively coupled with first transceiver  503  and second transceiver  504  via internal copper connections. Processor  502  is communicatively coupled with LRU core  501  via an internal connection, such as a copper connection. First transceiver  503  is physically connected via, for example, an external fiber optic link to an upstream HE-LRU or SN-LRU. Second transceiver  504  is similarly physically connected via an external fiber optic link to a downstream HE-LRU or SN-LRU. Processor  502  provides LRU core  501  network access to an upstream HE-LRU through first transceiver  503  or to a downstream HE-LRU through second transceiver  504 . The upstream and downstream directions have been arbitrarily assigned to the network path on the left and the right of the LRU respectively. 
         [0048]    The structure and function of LRU core  501  varies by SN-LRU type. An LRU core for on board network interface unit  301  enables access to public address audio and data for passenger convenience features such as reading light control, flight attendant call and flight information for applications such as moving maps, etc. An LRU core for off board network interface unit  302  enables communication with terrestrial networks generally through satellite- or ground-based radio frequency networks. This LRU core may enable bidirectional or unidirectional communication depending on implementation. Bidirectional versions enable connectivity with terrestrial networks (broadband connectivity). Unidirectional versions enable access to off aircraft broadcast data sources such as television (broadcast video). An LRU core for data loader  303  enables media content updates (movies, audio, games, Internet web pages, files, etc.), key updates and transaction data transfers. This LRU core enables data transfer using one of the following mechanisms: removable disk or tape inserted into data loader  303 , portable disk drive or tape drive carried on board and temporarily connected to the IFE system, wireless LAN, or other wireless link. An LRU core for CMT  304  enables flight attendants to perform system management and administration functions such as: LRU reboot, video channel preview, flight attendant override, attendant call status, reading light status, built in test, interrogation and system test. LRU cores for VDUs  305  each include a physical display device (e.g. flat panel display) that enables a passenger to view video content and navigate an IFE menu. These LRU cores may additionally provide PCU functionality, such as volume control, channel control, lighting control, attendant call button, menu buttons and/or menu selection buttons, via a display device touch screen or mechanically actuated buttons. LRU cores for display interface units (not shown) include a physical interface to an external display device (e.g. flat panel display) that enables a passenger to view video content and navigate an IFE menu. Like the LRU cores for VDUs, these LRU cores may additionally provide PCU functionality, such as volume control, channel control, lighting control, attendant call button, menu buttons and/or menu selection buttons, via a display device touch screen or mechanically actuated buttons. 
         [0049]      FIGS. 6A through 6D  illustrate serial networking data path maintenance in some embodiments of the invention.  FIG. 6A  shows physical wiring of an IFE system having a ring of four HE-LRUs and a single chain of four SN-LRUs physically wired to HE-LRU  1  and HE-LRU  2 . SN-LRUs keep apprised of the nearest HE-LRU through topology messaging and regulate link participation in serial networking data paths to establish and maintain loop-free data paths that minimize the maximum number of network hops of any SN-LRU to an HE-LRU.  FIG. 6B  shows the serial networking topology when there are no faults in the chain. The link between SN-LRU  2  and SN-LRU  3  has been removed from the data path, resulting in establishment of two loop-free data paths wherein the maximum number of hops to an HE-LRU is two.  FIG. 6C  shows the serial networking topology after reconfiguration upon detecting that the link between SN-LRU  1  and SN-LRU  2  has failed. This reconfiguration is made by adding the link between SN-LRU  2  and SN-LRU  3  to the data path to provide all SN-LRUs a least hop data path to an HE-LRU wherein the maximum number of hops to an HE-LRU is three.  FIG. 6D  shows the serial networking topology after reconfiguration upon detecting that SN-LRU  4  has failed. This reconfiguration is made by adding the link between SN-LRU  2  and SN-LRU  3  to the data path to provide all SN-LRUs that remain active a least hop data path to an HE-LRU wherein the maximum number of hops to an HE-LRU is three. The additions and subtractions of links illustrated in  FIGS. 6B through 6D  are made under control of the network management processor in SN-LRU  1 , SN-LRU  2  and/or SN-LRU  3  using hop count and/or presence information gleaned from topology messaging. For example, each SN-LRU may under control of its network management processor determine whether it is a middle SN-LRU of a chain by comparing the hop counts received on both of its ports. If the hop counts for both ports is the same or differ by only one hop, the SN-LRU self-identifies as a middle LRU; otherwise, the SN-LRU does not self-identify as a middle LRU. If the SN-LRU self-identifies as a middle LRU, the SN-LRU breaks the chain to create a loop-free network topology. If the hop counts for both ports differ by one hop, the SN-LRU under control of its network management processor blocks the port with the higher hop count (i.e. the port that has a longer path to the nearest HE-LRU) and unblocks the other port. If the hop count for both ports is identical, the SN-LRU under control of its network management processor blocks a predetermined one of the ports and unblocks the other port. 
         [0050]      FIGS. 7A through 7D  illustrate head end data path maintenance in some embodiments of the invention.  FIG. 7A  shows physical wiring of an IFE system having a ring of four HE-LRUs and a single chain of four SN-LRUs physically wired to HE-LRU  1  and HE-LRU  2 . When HE-LRUs detect a closed HE-LRU ring as a result of topology messaging, a designated HE-LRU removes one of its links from the data path to create loop-free data path between HE-LRUs, which link may later be restored to the data path to maintain the data path if an HE-LRU or a link fails.  FIG. 7B  shows the head end network topology after HE-LRU loop detection. In that topology, the link between HE-LRU  1  and HE-LRU  4  has been removed from the data path to eliminate the loop.  FIG. 7C  shows the head end network topology after reconfiguration upon detecting that the link between HE-LRU  3  and HE-LRU  4  has failed. This link between HE-LRU  1  and HE-LRU  4  has been restored to the data path to maintain network access to all HE-LRUs.  FIG. 7D  shows the head end network topology after reconfiguration upon detecting that HE-LRU  2  has failed. This reconfiguration similarly results in restoration of the link between HE-LRU  1  and HE-LRU  4  to the data path to maintain network access to all live HE-LRUs. The additions and subtractions of links illustrated in  FIGS. 7B through 7D  are made under control of the network management processor in HE-LRU  1 , HE-LRU- 3 , and/or HE-LRU  4  using loop information gleaned from topology messaging. In some embodiments, at least two of the HE-LRUs in an HE-LRU ring are of a single hardware design configuration. 
         [0051]    In some embodiments, links are added and removed from data paths by dynamically regulating the state of transceivers in HE-LRUs and SN-LRUs between a data forwarding state and a data blocking state under control of the network management processors. Naturally, transceivers and their associated ports and links will continue to carry presence messages and other management information even when they are not participating in a data path. 
         [0052]    In one embodiment, an important distinguishing feature of the present invention from conventional spanning tree protocols is that in the present invention networks in which the loop-free data path between HE-LRUs passes through an SN-LRU are not formed. 
         [0053]    It is to be understood that the word “serial” as used herein describes the way the devices described are networked together and does not refer to the type of communications or way that communications are sent over the network links. 
         [0054]    It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character hereof. The present description is therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come with in the meaning and range of equivalents thereof are intended to be embraced therein.