Abstract:
A scalable fault tolerant cable headend switching system using modular switching or multiplexing devices. A radio frequency switch connects a series of adjacent cable headend element to a series of output cables. At least one spare cable headend element is connected along with the series. Each cable headend element is configured to be readily swapped with adjacent elements. Output cables are cascaded away from a faulty element toward a spare element when a fault is detected.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to headend devices and more particularly to devices for providing high availability and fault tolerance in a cable system. 
     Cable communication systems typically route signals from a headend through trunk cables or fibers from which cables branch to individual users. The headend is the originating point in a communication system. Demand for connections through cable headends has greatly increased as cable usage has increased. Increased usage of cable modems and cable telephony is expected to place further demand upon headend operators. Cable headends therefore require high availability platforms having maximum fault tolerance for routing signals. 
     Providing a high availability platform in a cable headend environment is difficult. Typical cable headend units have multiple headend elements, each driving a separate cable wire. Physical limitation on the length of cable wire which can be driven by one headend element requires cable networks to be divided into smaller connection networks, each driven by a separate headend element. Even within these limits, cable headend units suffer high burnout, in part due to the power requirements and heat generated from driving a radio frequency (RF) signal. Failures often suddenly occur without prior indication, thereby causing an interruption in service. Typical radio frequency (RF) interfaces at a cable headend are switched using RF matrix switches which are well known to persons of ordinary skill in the art. Fault tolerance is achieved by using a matrix switch to transfer signals from a failed cable element to a good element. Any number of spare elements are typically provided for any set of cable elements that are attached to the matrix switch. A failed element is switched out when a fault is detected and an appropriate spare element is switched into its place while the fault is swapped out or repaired. 
     Fault tolerant systems have been developed to provide a single spare element which is capable of taking on the role of any one of a set of other elements in the cable headend. The number of spare elements and switching interfaces is reduced, thereby reducing cost and space requirements. Such fault tolerant systems are not scalable because they are typically switched using matrix switches which are available only in fixed N×N configurations. Incremental addition of a single spare element or a small number of spare elements in such systems may therefore require the expensive purchase and installation of a large matrix switch. Scalability is thus limited to installation of new element in blocks based on added switch matrixes. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an apparatus and method for efficiently replacing failed elements in a cable headend. Each cable element in a fault tolerant cable headend system is configured to serve as a spare for at least one of its adjacent elements. Upon the failure of any element, an adjacent element is switched into the circuit of the failed element. The next adjacent element is switched into the circuit of the first adjacent element to take its place. A rippling sequence of shifting elements propagates until a spare unused element is switched into its neighbors place. The failed element may then be repaired or replaced (swapped out) and becomes a spare element, or the elements are shifted to their original position. No further switching is required until another failure occurs. 
     Switching may be implemented according to the present invention by using simple multiplexing devices. Each RF switch or multiplexing device may be installed as an individual module. A systems according to the present invention is therefore scalable to very large configurations in an economical and nearly linear manner. 
     The system according to the present invention is useful for applications where each elements is capable of performing the function of the adjacent elements that it may be called upon to replace. State information (if any) for an element should be available for at least one adjacent element. A system according to the present invention may be implemented using 3 to 1 multiplexing devices wherein any element is capable of replacing either of its adjacent elements. 
     Advantages of the present invention include automatic fault tolerance in a system which may be easily scaled up or down. A system according to the present invention also requires minimal real estate for extra components such as spare boards. Other advantages include simplicity in monitoring and fail over, in that an element need only monitor the functioning and state of an adjacent element, without a need for centralized monitor. Other advantages include ease of repair by technicians in that only a failed board in a system need be pulled out and replaced, instead of having to pull out several boards for reconfiguration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system according to the present invention; 
         FIG. 2   a  is a block diagram showing an illustrative, embodiment according to the present invention; 
         FIG. 2   b  is a block diagram showing how the illustrative embodiment in  FIG. 2   a  compensates for a failed element; 
         FIG. 2   c  is a block diagram showing the illustrative embodiment of  FIG. 2   b  after the failed element has been repaired or replaced; and 
         FIG. 2   d  is a block diagram showing how the illustrative embodiment of  FIG. 2   c  compensates after another element has failed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a typical system according to the present invention including a headend element  14  and a switch  10 . The headend element  14  is any type of transmitting or receiving unit used in a system where multiple similar headend elements  14  are in connection with separate circuits or networks  16 . Examples include RF cable transmitters or receivers, ethernet or ATM drivers, optical drivers, repeaters, telephone circuits or terminators, modems including telephonic or cable, etc. Such elements  14  are typically on PC boards mounted close together in a backplane system. The switch  10  is any type of switch which selects between two (or more) connections, including relays, both mechanical and solid state, and multiplexers or demultiplexers. A headend element  14   a  is connected to one input of one switch or multiplexing device  10   a  and an adjacent headend element  14   b  is connected to another input of the multiplexing device  10   a . The multiplexing device  10   a  is switchable to connect a line  16  to either of two adjacent headend elements  14 , as shown by arrow  13 . 
     Each element  14  has access to state information  15  regarding an adjacent element  14  that is connected to another input on a shared multiplexing device  12 , as shown by arrow  17 . State information  15  (if any) may be accessible either by maintaining individual element  14  state information in a centralized location separate from the elements  14  (not shown), or by providing element  14  with appropriate storage for both its own state information  15 , and for it&#39;s adjacent neighbor. Each element  14  is thereby continuously prepared to be swapped into a role of an adjacent element  14 . 
     Upon detection of a fault in any headend element  14  according to the present invention, the switch  10  is signaled to switch over so the adjacent element  14  is now driving the line  16  connections over to an adjacent element  14  in the direction of the nearest spare element. The elements  14  participating in a shift then refresh their status information in preparation for a later shift. 
     The failed element  14  may then be repaired or replaced to serve as the next spare element  18 . Alternatively the system may shift back to employ the repaired or replaced element  14  and disconnect the original spare element  18 . 
     Referring to  FIG. 2   a , an exemplary embodiment for a cable headend system according to the present invention is illustrated. The RF switch  10  includes a cascaded set of five 2–1 multiplexing devices  12 . The RF switch  10  of the exemplary embodiment connects five elements  14  in a cable headend to five cable outputs  16  and includes one spare element  18 . It can be seen by reference to the  FIG. 2   a  that the multiplexing devices  12  are modular so that any number of multiplexing devices  12  may be cascaded to switch a corresponding number of elements  14 . It will be apparent to persons of skill in the art that 3–1 multiplexing devices may be used in place of the 2–1 multiplexing devices  12  of the illustrated exemplary embodiment of  FIGS. 2   a – FIG. 2   d.    
     The 2–1 multiplexing devices  12  may be switched to connect any cable output  16  to any of two headend elements  14 . Accordingly each 2–1 multiplexing devices  12  provide a connections from a single cable output to a single headend element  14  or its adjacent element  14  on one side. Systems according to the present invention that employ 3–1 multiplexing devices are capable of switching a single cable output  16  from a single headend element  14  to either of its two adjacent elements  14 . 
     Referring now to  FIG. 2   b , the condition of the exemplary embodiment is illustrated after a failure of a single element  14 ′ has occurred. Upon detection of the failure, the 2–1 multiplexing devices  12  for the failed element  14 ′ switches over so the adjacent element  14  takes over driving the cable output of the failed element  14 ′. Either simultaneously, or in a ripple fashion, the 2–1 multiplexing devices  12  for each adjacent element  14  switches over until all cable outputs  16  are driven, with the spare element  18  now filling in for its adjacent element  14 . It can be seen that three of the 2–1 multiplexing devices  12  have changed state to shift connections of three cable outputs  16  to adjacent elements. Each element  14  from the failed element  14 ′ to the spare element  18  is functionally replaced by the element  14  to its left. 
     Referring now to  FIG. 2   c , the condition of the exemplary embodiment is illustrated after the failed element  14 ′ of  FIG. 2   b , is repaired or replaced. The repaired or replaced element becomes the new spare element  18 . Alternatively, the 2–1 multiplexing devices  12  all switch back to their original settings, whereupon the end element  14  will again be the spare, as shown in  FIG. 2   a . However, it is not necessary to “reset” the system in this way, as will be described. 
     Referring to  FIG. 2   d , the condition of the exemplary embodiment of  FIG. 2   c  is illustrated after a second failure has occurred, this time in element  14 ′  FIG. 2   d . It can be seen that two 2–1 multiplexing devices  12  are switched back to their initial state thereby cascading two cable outputs  16  toward the right and connecting them to good elements  14 . A feature of the present invention is that as elements  14  fail and are replaced, the position of the spare element  18  can move around the system, and the 2–1 multiplexing devices  12  will always be able to configure the system so that all cable outputs  16  are driven. 
     The control of monitoring the system switching headend elements  14  may be performed by a separate monitoring and control system (not shown), or may be performed in a autonomous fashion by the individual headend elements  14 . This autonomous monitoring ability as a useful feature of the present invention. All a headend element  14  has to do is know the state (status information) of it&#39;s adjacent element  14  (which can be determined by noting which element  14  it would replace if the switch device  12  switches over), and if it&#39;s adjacent element  14  is responding. If the adjacent element  14  stops responding, then the headend element  14  informs the switch device  12  to switch over, and updates it&#39;s state to be that of the failed adjacent element  14 . The headend element finally stops responding to its neighbor&#39;s  14  monitoring, thereby causing the neighbor  14  to perform this same switchover, thereby rippling down the system until the spare element  18  is reached. Therefore, the system can shift over autonomously, without the need for central monitoring and control. The spare element  18  monitors the state  15  of its adjacent element  14 , the only difference is that the spare element  18  is offline until it is switched in and takes over for its adjacent element. 
     If a higher level of fault tolerance is desired, then 3–1 multiplexing devices  12  can be employed, whereby a failed element  14 ′ can be replaced an adjacent element on either side (not shown). The advantage to this configuration of the present invention is two (or more) spare elements  18  may be used, with the spare elements  18  initially positioned at either end of the system. This configuration of the present invention allows for multiple element  14  failures. Both spare elements  18  may then move around the system, which has higher reliability because multiple elements  14  can fail before the system must be serviced. 
     Any number of modular multiplexing devices  12  are cascaded to form an RF switch  12  for a fault system at a cable headend. Each element  14  is capable of performing the functions of at least one adjacent element  14  and stores relevant status information regarding the at least one adjacent element  14  to facilitate rapid substitution when a failure of an element  14  occurs. Multiplexing devices  12  may be added or removed in a modular fashion to facilitate scaling of the system. 
     In at least one embodiment of the present invention, a cable headend element is a cable modem termination system such as a model CAS 2000 produced by Motorola Corporation. It should be apparent to persons of skill in the art that any number of headend elements may be switched according to the present invention to provide a low cost scalable fault tolerance system. Persons having ordinary skill in the art will recognize that an RF switch  10  according to the present invention may also be manually or automatically controlled to disconnect an element  14  and shift specific output cables  16  toward a spare element  18 . The invention may thereby be used to facilitate routine maintenance operations as well as to provide fault tolerance. 
     Although exemplary embodiments have been described, other embodiments and modifications of the invention are intended to be covered by the spirit and scope of the appended claims.