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 Wavelength Division Multiplexing techniques to provide a plurality of logically independent subchannels over a single shared free space optical beam line.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is 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 also 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 43-5, entitled “Interconnecting Processing Units Of A Stored Program Controlled System Using Space 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 data is carried on the optical communication path using wavelength 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 has 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 sub-channels may have more traffic than they can handle.  
           [0008]    Other multiplexing and MAC protocols provide statistical multiplexing of the system&#39;s capacity, where in 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.  
           [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 using separate wavelengths (or similar separators as known in the art), and the receiver is determined by the wavelength 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 listeners interception of a 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 sub-media 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 wavelength division multiplexing. In a Wavelength Division Multiplexed (WDM) system, the shared medium is divided into a set of simultaneously-transmitted wavelengths. All subchannels sharing the medium are assigned specific wavelengths on which to carry traffic. The number of different wavelengths assigned may be equal for all subchannels, or some subchannels may be given higher capacity by assigning them a higher proportion of wavelengths available on the beam line on which to transmit.  
           [0014]    Therefore, there is a need in the art to provide a free space optical interconnect between units of a stored program controlled system using a wavelength division multiplex system.  
         SUMMARY OF THE INVENTION  
         [0015]    This need is satisfied and a technical advance is achieved in the art by a system and method that provides wavelength division multiplexing in a system that uses free space optics to interconnect processing units of a stored program controlled system. The system of this invention uses Wavelength Division Multiplexing (WDM) techniques to provide a plurality of logically independent subchannels over a single shared free space optical beam line. The single logical beam is divided among the subchannels, with each subchannel operating on a selected wavelength or wavelengths.  
           [0016]    In static WDM mapping, wavelengths are assigned to subchannels when the system is configured, and each processing unit using the beam line has a predetermined wavelength or wavelengths for communication with other processing units. The wavelength assignment may also be determined as the processing units are installed by selecting a laser of an individually designated wavelength at each transmit probe and a wavelength selective filter optical element at each receive probe. An alternate embodiment, dynamic mapping, uses frequency agile lasers and tunable filters at the receive probes to dynamically assign wavelengths to the various subchannels. Dynamic assignment has advantages in versatility, and reliability, but at the cost of more complex hardware. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    A more complete understanding of this invention may be obtained from a consideration of the specification taken in conjunction with the drawings, in which:  
         [0018]    [0018]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;  
         [0019]    [0019]FIG. 2 is a cross-sectional view of the beam line taken along line  2 - 2  of FIG. 1;  
         [0020]    [0020]FIG. 3 is an exemplary embodiment of transmitting and receiving probes of FIGS. 1 and 2;  
         [0021]    [0021]FIG. 4 is a block diagram of a hub of the beam line of FIGS. 1 and 2 according to an exemplary embodiment of this invention;  
         [0022]    [0022]FIG. 5 is a representative wavelength spectrum of the beam line of FIG. 1 operating at full capacity; and  
         [0023]    [0023]FIG. 6 is a flowchart of processing for bandwidth management tasks of increasing and decreasing the capacity of each subchannel. 
     
    
     DETAILED DESCRIPTION  
       [0024]    Turning to FIG. 1, a perspective view of a free space beam line  10  according to an exemplary embodiment of this invention is shown. According to this exemplary embodiment, a free space beam line  10  is generated at hub  11  which projects optically encoded signals, as will be described below in connection with FIG. 4  
         [0025]    A plurality of transmitters  12  within transmitting probes  14  and 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. The probes are distributed in a helix in this exemplary embodiment so that there is a minimal amount of shadowing; that is, one probe being in the shadow of a previous probe in beam line  10 , inhibiting the probe in the shadow from transmitting or to receiving signals in beam line  10 .  
         [0026]    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. If the beam line  10  is contained in a conduit, then the interior surface may be optically absorptive or optically reflective depending upon the length of the pipe, the wavelength of the signal generated by the laser within transmitter  12  and loss budget to provide optimal reception of optically encoded signal by the plurality of receiving probes  18  throughout the length of beam line  10 .  
         [0027]    Conduit  22  includes, in this exemplary embodiment, a first terminal unit  24  comprising a hub  11  and a second terminal unit  26 . First terminal unit  24  originates free space beam line  10  and second terminal unit  26  terminates the portion of free space beam line  10  passing beyond the probes  18 . 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 also interconnected to recycle the encoded signal.  
         [0028]    [0028]FIG. 2 illustrates a view looking down a cross-section of beam line  10  taken along line  2 - 2  of FIG. 1. Conduit  22  includes a plurality of transmitting probes  14  and receiving probes  18  around its inner edge. In the illustration of FIG. 2, the laser of hub  11  (FIG. 1) focuses beam line  10  to encompass the interior circumference of conduit  22  whereby each probe  18  receives the encoded optical signal. Second terminal unit  26  is illustrated herein as comprising a receiving probe  18 . (Second terminal unit  26  may also include a transmitter  12 , not shown.) Alternatively, second terminal unit  26  may comprise another hub or an end cap. An end cap may be absorptive in order to stop the beam line  10  or may be reflective (i.e., a mirror or retroreflector) to recycle beam line  10  in the opposite direction.  
         [0029]    Turning now to FIG. 3, exemplary embodiments of a transmitting probe  14  and a receiving probe  18  configured for wavelength division multiplexing (WDM) of the bandwidth of a beam line are shown. Transmitting probe assembly  14  includes transmit electro-optical components  12 , and the receiving probe assembly  18  includes receive electro-optical components  16 . Light of a pre-specified, closely controlled wavelength is transmitted over free space beam line  10  from transmit probe  14  to receive probe  18  to effect communication between two processing units in the system.  
         [0030]    Transmit electro-optical assembly  12  includes a laser  30  that operates at a controlled wavelength, a condensing lens  80 , a fiber  82  to carry the light to the beam line and a diverging lens  36  and converging lens  38 . Lenses  36  and  38  represent a reverse Galilean telescope that expands the small diverging beam from fiber  80  into a larger, less divergent beam, and directs it into free space beam line  10 .  
         [0031]    Laser driver  40  energizes laser  30 . Laser driver  40  includes circuits to provide the correct bias current and modulation to laser  30 , but also includes elements necessary to control and monitor the emission wavelength of laser  30 , as known in the art. Feedback is maintained between laser  30  and laser driver  40  to establish servo loops needed to precisely control the emission power, emission wavelength, and temperature of laser  30 . Protocol handler  58  provides a serial bitstream used by laser driver  40  to modulate laser  30 . Mux  60  merges data from all the various sources of bearer, control and administrative information arriving on links  310 , which are to be transmitted to other processing units.  
         [0032]    Receive electro-optical assembly  16  includes a converging lens  306  to focus light from beam line  10  onto fiber  86 . Fiber  86  carries the light from the beam line to wavelength selective photodetector  46 . Lens  88  focus the light from fiber  86  to photodetector  46 .  
         [0033]    Wavelength selective photodetector  46  includes a tunable optical bandpass filter, as known in the art, to select a single wavelength from the plurality of wavelengths present in beam line  10  (and on fiber  86 ). This filtering function must be very selective, as the wavelength spacing may be very close. The photoreceiver element, (i.e., a photodiode in the preferred embodiment) contained in  46  must be illuminated by only a single wavelength to reliably receive information from a single subchannel.  
         [0034]    Receiver circuit  48  includes all the circuits typical of known photoreceivers, including a transimpediance amplifier, clock recovery logic, and a decision circuit. In addition, receiver  48  includes elements needed to tune the wavelength selective filter contained in  46 . Protocol handler  64  receives the serial stream and clock recovered in receiver circuit  48 , and reassembles it into a data stream. This data stream (bearer, control and administrating data) is then sent to router/demux  66 , which routes it to one of a plurality of possible destinations over links  68 .  
         [0035]    [0035]FIG. 4 is a block diagram of hub  11 , which may be first terminal unit  24  and/or second terminal unit  26  (FIG. 1). Hub  11  must simultaneously terminate all wavelengths on the free space beam line in each direction. Beam line  40  enters hub electro-optical assembly  400  through focusing lens  405 . Light from beam line  40  passes through dichoric beam splitter  410 , which divides the optical energy passing through into two bands: a band of longer wavelengths that pass straight through the beam splitter  410 , and a shorter band of wavelengths that turn 90 degrees. Channels from the processing units are in the longer wavelength band, so they pass straight through the beam splitter  410 , and are directed onto fiber  420 . Wavelengths being sent to the processing units are in the shorter band, and therefore emerge from fiber  422 , turned 90 degrees by beam splitter  410 , and are transmitted through lens  405  and onto beam line  40 .  
         [0036]    Erbium Doped Fiber Amplifier  424  boosts the amplitude of the receive optical signal, which passes through wavelength selective fiber splitter  428  and onto photoreceivers  432 A- 432 C, which contain photodiodes, transimpediance amplifiers, clock recovery circuits, and decision circuits, as known in the art. One photoreceiver is required per wavelength received at hub  11 . Electrical signals from the photoreceivers then pass to router  435 . Router  435  receives one electrical stream per wavelength from photoreceivers  432 A- 432 C, decodes their addresses, and routes them as electrical signals to the appropriate transmitter assembly  434 A- 434 C.  
         [0037]    Transmitter assemblies  434 A- 434 C include laser drivers, laser diodes, and feedback circuits to stabilize the laser&#39;s power and wavelength, as known in the art. Each of the optical transmitter assemblies  434 A- 434 C operates on a specific wavelength, with one transmitter required per wavelength transmitted into the beam line  40 . Optical signals from transmitter assemblies  434 A- 434 C are combined in coupler  430 , boosted by Erbium Doped Fiber Amplifier  426 , and sent through beamsplitter  410  and lens  405  onto beam line  40 .  
         [0038]    [0038]FIG. 5 represents a wavelength spectral plot of a WDM system according to an exemplary embodiment of this invention in operation. Eighty distinct wavelengths are used to interconnect  32  processing units in this exemplary embodiment. Each processing unit is assigned a single wavelength for transmission toward central hub  11 , and a second wavelength over which hub  11  transmits messages back to the processing unit. Wavelengths used for transmission towards hub  11  are grouped in the longer wavelength spectrum, and wavelengths used to transmit from hub  11  back to the processing units occupy a group in the shorter wavelength spectrum.  
         [0039]    This plot represents output from a spectrophotometer monitoring the entire light spectrum in free space beam line  10 . The horizontal axis  360  represents the wavelength of the monitored light, and the vertical axis  370  represents the power monitored on each wavelength. The group of 32 longer wavelength peaks  380  is the set of subchannel signals from the processing units toward the hub  11 . The group of shorter wavelength peaks  382  is the optical signal set from the hub to the individual processing units. Unused wavelengths  384  and  386  are spare; to be used for system expansion or in case a failure requires reconfiguration of the wavelength assignments. Unused wavelengths  384  and  386  could also be used to balance the load of the system. Each processing unit transmits and receives its primary traffic on its assigned wavelengths  382  and  384 , but if the traffic load exceeded the capacity of these channels, spare wavelengths  384  and  386  can be allocated to supplement the channels that are in overflow. Using this overload control scheme necessitates each of the processing unit probes being able to transmit and receive on two or more wavelengths, its primarily assigned wavelength, and one or more supplemental wavelengths. The supplemental wavelengths can be a shared, managed free resource pool. Of course, as spare wavelengths  384  and  386  are activated, the amplitude of their peaks on FIG. 5 would increase.  
         [0040]    [0040]FIG. 6 is a flowchart for managing the dynamically allocated bandwidth available if more than one wavelength agile laser and photoreceiver is used at each processing unit. Dynamic bandwidth allocation operation starts at  450 . In decision diamond  452 , a determination is made if any of the processing units have excessive bandwidth allocations. This determination is made by polling all processing units with more than one wavelength assigned in each direction, and calculating whether the total load has fallen below 90% of the capacity of a single channel. If so, the supplementary wavelength channel is de-allocated from the processing unit in action box  454 . Any traffic that may be queued for transmission is allowed to clear in action box  456 . Processing proceeds to action box  458 , where the newly freed wavelength is added to the free list. In decision diamond  460 , a determination is made if there are any remaining processing units to check for excess bandwidth.  
         [0041]    The determination of decision diamond  462  comprises determining if any subchannel is approaching overload. Each processing unit is polled to determine if its transmit queue is over 90% full. If so, decision diamond  564  determines if a supplementary wavelength is available to take the added traffic. If not, a system overload error is sent to the Operations Administration, Maintenance and Provisioning system in action box  466 . If a supplementary wavelength is available, a wavelength is reserved in action box  468 . The lasers and photoreceivers associated with both transmitters and both receivers are programmed to tune their wavelengths in action box  470 . Tests are performed in action box  472  to determine if the new channels are performing correctly. In action box  474 , the wavelength is allocated, and traffic begins to flow to relieve the overloaded primary wavelength. Decision diamond  478  tests to see if other processing units are in overload. The entire bandwidth management loop repeats infinitely as long as the system operates.  
         [0042]    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.