Abstract:
Internal communication signals in a stored program controlled system comprising a plurality of units configured to process signals are provided by an optical beam line which is proximal to all of the plurality of units. The system of this invention uses Space Division Multiplexing techniques to provide a plurality of logically independent subchannels over a single, shared free space optical beamline.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is also related to U.S. Patent Application Attorney Docket No. C. C. Byers 39-1, entitled “Interconnecting Processing Units Of A Stored Program Controlled System Using Free Space Optics”, filed concurrently herewith and commonly assigned to Lucent Technologies Inc., and incorporated by reference herein, with priority claimed for all commonly disclosed subject matter.  
         [0002]    This application is related to U.S. Patent Application Attorney Docket No. C. C. Byers 41-3, entitled “Interconnecting Processing Units Of A Stored Program Controlled System Using Time Division Multiplexed Free Space Optics”, filed concurrently herewith and commonly assigned to Lucent Technologies Inc., and incorporated by reference herein, with priority claimed for all commonly disclosed subject matter.  
         [0003]    This application is also related to U.S. Patent Application Attorney Docket No. C. C. Byers 42-4, entitled “Interconnecting Processing Units Of A Stored Program Controlled System Using Wavelength Division Multiplexed Free Space Optics”, filed concurrently herewith and commonly assigned to Lucent Technologies Inc., and incorporated by reference herein, with priority claimed for all commonly disclosed subject matter.  
         [0004]    This application is also related to U.S. Patent Application Attorney Docket No. C. C. Byers 44-6, entitled “Installation Of Processing Units Into A Stored Program Controlled System Wherein The Component Processing Units Are Interconnected Via Free Space Optics”, filed concurrently herewith and commonly assigned to Lucent Technologies Inc., and incorporated by reference herein, with priority claimed for all commonly disclosed subject matter. 
     
    
     
       FIELD OF THE INVENTION  
         [0005]    This invention relates to the field of stored program controlled systems, including, but not limited to, telephone switching offices, data routers, and robotic machine tools; and, more specifically, this invention describes an optical communication path interconnect to provide communications for component processing units of the stored program controlled systems, wherein the data is carried on the optical communication path using space division multiplexing.  
         BACKGROUND OF THE INVENTION  
         [0006]    The invention of U.S. Patent Application Attorney Docket No. Byers 39-1 discloses a system and method for interconnecting processing units of a stored program controlled system using free space optics. According to this disclosure, an optical beam line carries signals among the various processing units. Multiple distinct streams of data are communicated within that system. When multiple distinct communication streams within a system or network share the same physical media, a multiplexing and Media Access Control (MAC) protocol is necessary for optimal system operation. These protocols help insure that the system performs at the desired capacity, performance and reliability levels.  
           [0007]    One important function of any multiplexing and MAC protocol is to divide the limited capacity of the shared physical communication channel among the various logical streams or subchannels that share it. This subdivision of the shared physical channel provides capacity guarantees, so that each of the logical channels receives an equal portion of the available capacity, or a pre-arranged larger or smaller proportion of this capacity. Pre-allocation of capacity provides a guaranteed level of capacity to all users. Such pre-allocation is often wasteful of system capacity because, if a given subchannel has no traffic to send, other subchannels may have more traffic than they can handle.  
           [0008]    Other multiplexing and MAC protocols provide statistical multiplexing of the system&#39;s capacity, wherein all potential transmitters on the shared channel negotiate for an opportunity to transmit. The theoretical maximum load offered, if all transmitters are operating at full rate, often exceeded the capacity of the shared media, which requires various buffering, queuing and priority techniques to govern when each transmitter operates. This system has potential channel utilization efficiency advantages, at the expense of making it more difficult to guarantee a minimum individual subchannel capacity or latency.  
           [0009]    Another important function of any multiplexing and MAC protocol is to direct or route a given channel&#39;s traffic to the intended receiver over the shared medium. Channels in an optical system are often statically mapped so that each subchannel operates in its own space (or similar separators as known in the art), and the receiver is determined by the location in space assigned to the transmitter. In other cases, traffic is offered to the shared medium as packets. These packets contain a destination address, which is used by switching or routing functions to complete the connections.  
           [0010]    The concept of priority is also important to multiplexing and MAC protocols. Some messages have a higher priority than others, based upon the importance or time sensitivity of their contents. The multiplexing and MAC protocol must take priority into account as it manages access to the shared medium.  
           [0011]    Security is often an intrinsic function of multiplexing and MAC protocols. If there is a hazard associated with unauthorized interception of messages over the shared medium, the protocol can take steps to eliminate (or at least greatly reduce the impact of) this hazard. One means for heightened security is physically separating sensitive traffic from all other traffic in the various parallel submedia in the transmission medium. A more common approach is to use some form of encryption at the transmitter, and decryption at only the authorized receiver(s).  
           [0012]    A further function that multiplexing and MAC protocols provide is fault tolerance and fault recovery. If a failure occurs that disables the shared media or significantly reduces its capacity, the protocol invokes various diagnostic actions to discover the source of the problem and then appropriate recovery actions to attempt to correct the source of the problem. Often, the fault recovery operation involves switching the traffic to a redundant medium or attempting to shed load so only the highest priority traffic is allowed on the remaining capacity.  
           [0013]    One form of multiplexing and MAC protocols usable in a free space optical beam line medium is space division multiplexing. In a Space Division Multiplexed (SDM) system, the shared medium is divided into a set of simultaneously transmitted beams, all occupying different subvolumes of the shared beamline volume. Each subchannel sharing the medium is assigned a specific beam with a specific spatial position on which it carries traffic. The number of beams assigned per subchannel may be equal for all subchannels, or some subchannels may be given higher capacity by assigning them a higher proportion of beam volumes on which to transmit.  
         SUMMARY OF THE INVENTION  
         [0014]    This problem is solved and a technical advance is achieved in the art by a system and method that uses free space optics to interconnect processing units of a stored program controlled system that uses space division multiplexing. The system of this invention uses Space Division Multiplexing techniques to provide a plurality of logically independent subchannels over a single shared freespace optical beamline. The single logical beam is divided among the subchannels, with each subchannel operating in an assigned sub-volume or sub-volumes. Many narrowly collimated laser beams are arranged in parallel (or nearly parallel) such that an individual channel is formed between central hub(s) and each processing unit. Advantageously, the property of light beams to cross one another without interference can be used to provide novel physical arrangements and system topologies. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    A more complete understanding of this invention may be obtained from a consideration of the specification taken in conjunction with the drawings, in which:  
         [0016]    [0016]FIG. 1 is a perspective view of a beam line illustrating the relationship of the beam line and probes according to a general overview of an exemplary embodiment of this invention;  
         [0017]    [0017]FIG. 2 illustrates separation of the beam line of FIG. 1 into sub-volumes, according to an exemplary embodiment of this invention;  
         [0018]    [0018]FIG. 3 is a perspective view of a large system looking down the center of the beamline from one end;  
         [0019]    [0019]FIG. 4 is a block diagram of an exemplary embodiment of this invention illustrating full duplex communication among the processing units;  
         [0020]    [0020]FIG. 5 is a block diagram of a central hub according to one exemplary embodiment of this invention;  
         [0021]    [0021]FIG. 6 is a block diagram of the physical arrangement of probes at a hub of FIG. 5 and a processing unit transmit and receive probes;  
         [0022]    [0022]FIG. 7 is a block diagram of a processing unit&#39;s transmit and receive probe; and  
         [0023]    [0023]FIG. 8 is a block diagram of a quadrant photodetector as used in a receive probe according to an exemplary embodiment of this invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]    Turning to FIG. 1, a perspective view of a beam line  10  according to one exemplary embodiment of this invention is shown. According to this exemplary embodiment, a beam line  10  is generated by a transmitter  12  within a transmitting probe  14  which projects optically encoded signals, as will be described further, below. Transmitting probe  14  produces a beam line  10  with desired spatial characteristics along the length of its path.  
         [0025]    A plurality of receivers  16  within receiving probes  18  are distributed throughout beam line  10  along the outer periphery in the form of a spiral or helix, in this exemplary embodiment. Other possible configurations of probes along the beam line will be apparent to one skilled in the art after studying this disclosure. Receiving probes  18  are distributed in a helix in this exemplary embodiment so that there is a minimal amount of shadowing; that is, one receiving probe  18  being in the shadow of a previous receiving probe  18  in beam line  10  causing the probe in the shadow to receive little or none of the optically encoded signals in beam line  10 . Beam line  10  may be contained within a reserved volume or conduit  22  in an enclosure, such as a cylinder or pipe or, alternatively, may be in the open.  
         [0026]    Conduit  22  includes, in this exemplary embodiment, a first terminal unit  24  and a second terminal unit  26 . First terminal unit  24  includes a transmitting probe  14  and second terminal unit  26  includes a receiving probe  18 , in this exemplary embodiment. As will be discussed further, below, first terminal unit  24  and/or second terminal unit  26  may include both transmitters and receivers, and may be interconnected to recycle the encoded signal.  
         [0027]    [0027]FIG. 2 illustrates the separation of the beamline  10  volume into sub-volumes  150  in the context of the exemplary embodiment of FIG. 1. Each sub-volume is reserved for carrying a bi-directional connection between a single processing unit and the central hub at the end of the beamline. Reserved volumes  150 A- 150 G are arranged around the circumference of beamline  10  in this example, but it is appreciated that other arrangements are possible, including many concentric rings of volumes, or a spiral arrangement (as is illustrated in FIG. 1). Each reserved volume  150  is further divided into transmit signals  160 A- 160 H, and receive signals  170 A- 170 H. In practice, these volumes can be side by side as shown, or coincident with one another, or overlapping to some degree. Since there is no equivalent of a “short circuit” in the freespace optical domain, the beam volumes can overlap and cross one another as they traverse the beamline  10 .  
         [0028]    Turning now to FIG. 3, FIG. 3 is a representation of what one would see if one removed the end cap and central hub from the system of FIG. 1, and looked straight down the center of the beamline. The concentric circles are the edges of the bounding bulkheads between the frames holding the processing units. Transmit probes  14 A- 14 P and receive probes  18 A- 18 P on all sixteen processing units in this exemplary embodiment are visible from this perspective, illustrating the advantages of the helical arrangement. If a central hub (not shown, but explained further, below, in connection with FIG. 5), containing an array of transmit and terminal receive probes  135  is located in the end position, it is possible to create unobstructed beam paths between all transmit probes and all receive probes.  
         [0029]    Turning now to FIG. 4, a further exemplary embodiment of this invention is shown. In this exemplary embodiment, beam line  10  is uni-directional, i.e., signals flow in the direction from unidirectional first terminal unit  132  to second unidirectional terminal unit  134  and are then recirculated, as will be described further below. In this exemplary embodiment, a processing unit controller  136  and processing unit  138 ,  140 ,  142  and  144  are each connected to a transmitting probe  14 . Processing units  138 ,  140 ,  142  and  144  are connected to receiving probes  18 . Terminal  134  uses proble  135 .  
         [0030]    In the exemplary embodiment of FIG. 4, processing unit or controller  136  originates electrical control signals for processing units  138 ,  140 ,  142  and  144  and communicates such signals to router  146 . Router  146  comprises a conventional router as is known in the art. Router  146  communicates signals for processing units  138 ,  140 ,  142  and  144  to a signal generator  56 . Transmitter  14  in unidirectional first terminal unit  132  optically encodes the signals onto a plurality of subvolumes and assembles these into beamline  10 . Receiving probes  18  receive the optically encoded signals and convey them to their respective processing unit  138 ,  140 ,  142  and  144 . Each processing unit  138 ,  140 ,  142  and  144  may send feedback or other information to controller  136  by creating beams and injecting the beams into beam line  10 , which are all received at terminal receiving probe  135  in uni-directional second terminal unit  134 . The signals are then fed back to router  146  where they may be further circulated in beam line  10  or delivered to controller  136 .  
         [0031]    Systems using many of the embodiments of this invention (i.e., FIG. 4) must include features to prevent messages from recirculating in the beam line  10 . If these features are not included, infinite feedback loops are possible, where a single message is continuously relayed between two endpoints and/or probes, quickly absorbing all available bandwidth. To prevent this, a means to break these loops is provided. Router  146  is programmed (or programmed in conjunction with the probes or endpoints) to detect addresses that lead to looping behavior, and not forward those messages back into the beam line. Alternately, the optical characteristics of the beam line, transmitters and receivers are controlled to prevent messages from a given source from circulating indefinitely. For example, the diameter or directionality of the beams from probe  14  are designed to be fully intercepted by probes  18 , with no optical power flowing past probes  18  to receive probe  135 .  
         [0032]    [0032]FIG. 5 illustrates the details of the central hub used in this space division implementation. Since a plurality of individual, highly collimated beams are used (at least one in each direction between the hub and each processing unit), the hub has many receive and transmit probes, each probe dedicated to a single beam. Shared beamline  40  is subdivided into individual beam paths  150 A- 150 C, each directed to a specific processing unit some distance down the beamline. A plurality of transmit probes  14 A- 14 C and receive probes  18 A- 18 C are arranged in a hub array  310  occupying some or all of the inner surface of the end cap of the beamline  10 . Router  320  accepts signals from each of the receive probes  18 A- 18 C, decodes their address, and relays them to the appropriate destination processing unit over transmit probes  14 A- 14 C. Some of the traffic is routed through router  320  to processing unit controller  330  that performs centralized control and administration for all elements of the system connected through beamline  10 .  
         [0033]    In FIG. 6, the physical relationship among the probes associated with a processing unit and the probes on the hub is shown. Central hub electro-optical assembly  24  is located at the end cap of the beamline enclosure  22 . Included in electro-optical assembly  24  is hub array  310 , which includes a plurality of transmit probes  14 A- 14 F and receive probes  18 A- 18 F. These probes are shown arranged in an annular ring around the periphery of circular hub array  310 , but other arrangements are possible.  
         [0034]    Highly collimated beams  160  and  170  pass through specific reserved portions of the beamline volume, and terminate on processing unit probe ring  340 . The processing unit probe ring  340  is a movable structure associated with each processing unit on the periphery of the beamline, with a large aperture in the center through which beams directed to more distant processing units pass. Probe ring  340  can revolve to any position permitting the positioning of processing unit probes  14 G and  18 G at any position around the circumference of the beamline. This permits fine adjustment of the arrangement of the individual beams in the beamline volume. Drive roller  350  moves the position of the beam in response to actuator  360 . Drive roller  350  may be a metallic or plastic roller, tire, or gear. Actuator  360  may be a stepper motor, servomotor, or a simple hand operated knob or crank.  
         [0035]    [0035]FIG. 7 is a block diagram of the transmit probes  14  and receive probes  18 . Transmit probe  14  consists of a laser  616  that produces a modulated light beam. Diverging lens  614  and converging lens  610  form a reverse Galilean telescope to expand and collimate the beam. Beam divergence control is particularly important in the space division multiplexed implementation, because the more control there is on beam divergence, the more information channels can be put through a beamline of given diameter without cross-talk. While lens system  614  and  618  do result in a beam with expanded diameter compared to the beam generated by laser  616 , this expanded beam has much smaller divergence. The far-field beam size that results after traversing tens or hundreds of meters of freespace beamline will be significantly less using this arrangement. In receive probe  18 , lens  612  gathers light from the beamline, and focuses it at the center of quadrant photodiode  618  (described below).  
         [0036]    In some embodiments of the system, especially those with very long beamlines, it may be necessary to perform active alignment to precisely aim the pairs of transmit and receive probes. Electro-mechanical components perform these beam steering functions. Rigid enclosures  640  and  650  provide a stable base for the electrical optical components of each probe to ride on. One end of these enclosures is anchored through a flexible coupling  624  and  634  to a rigid mechanical support (either the hub array  310  or the probe ring  340  in FIG. 6). The other end of the enclosures is positioned by actuators to permit fine control of the direction that the probes point. Actuators  620 ,  630  control the vertical position of the ends of the probes, influencing the elevation of the beams. Actuators  622 ,  632  control the horizontal position, influencing the beam&#39;s azmuth. Actuators  620 ,  622 ,  630 , and  632  may be stepper motors with lead screws, servomotors, piezoelectric actuators if automatic control of beam direction is desired, or simply hand adjustment knobs to be used during system set-up.  
         [0037]    [0037]FIG. 8 describes the operation of a quadrant photodetector in this application. This is the view through section  8  on FIG. 7. The back bulkhead portion of the rigid support structure  650  supports the quadrant photodetector component  618 . Photodetector component  618  comprises five separate photodiodes, the central receiver photodiode  660 , and four pie-shaped error photodiodes  652 ,  654 ,  656 , and  658 . Central receiver  660  is optimized for high speed operation, and detects the data from the modulated beam. Error photodiodes  652 ,  654 ,  656 , and  658  are configured to detect which direction and to what degree the spot that illuminates the photodetector is off center. A feedback loop is established between the actuators and the error photodiodes to move the probes until the spot of light is centered on receiver photodiode  660 . Depending upon the mechanical characteristics of the system, this feedback loop may be fast acting and able to compensate for vibrations of the base on which the system is anchored. Alternatively, it may be slow acting, designed to compensate for the slower effects of thermal expansion and variable floor loads.  
         [0038]    During system installation and adjustment of the preferred embodiment, the frame containing a new processing unit is moved into position into the beam line  10 , as described in U.S. Patent Application attorney docket Number Byers 44-6, entitled “Installation Of Processing Units Into A Stored Program Controlled System Wherein The Component Processing Units Are Interconnected Via Free Space Optics”, incorporated by reference above. The probe ring is rotated to position the probes at a position around the beam line to provide an unobstructed path to the central hub. A free receive and transmit probe on the hub array are allocated to the new processing unit. The receive and transmit probes actuators on both the processing unit end and central hub end are driven to a coarse position determined by a geometric analysis of the various element positions. Then, a fine positioning step is entered, where the error signals from the error photodiodes are processed, and the actuators are moved until the error signals are minimized. Notice that position error monitoring is only possible on receive probes. The positioning commands for the transmit probes are derived by processing the receive probe errors through an algorithm that takes the overall geometry of the system into account, and estimates the pointing error of the transmit beam.  
         [0039]    It is to be understood that the above-described embodiments are merely illustrative principles of the invention and that many variations may be devised by those skilled in the art without departing from the scope of this invention. It is, therefore, intended that such variations be included within the scope of the following claims.