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 Time Division Multiplexing techniques to provide a plurality of logically independent subchannels over a single shared free space optical beam line. The single logical beam is divided temporally among the subchannels, with each subchannel operating for a selected timeslot or timeslots.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [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 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.  
         [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 time 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. Preallocation of capacity provides a guaranteed level of capacity to all users. Such preallocation is often wasteful of system capacity because, if a given subchannel has no traffic to send, its allocation is idle and unavailable to other connections that may be overloaded.  
           [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 the multiplexing and MAC protocols is to direct or route a given channel&#39;s traffic to the intended receiver over the shared medium. Channels are often statically mapped using timeslots (or similar separators as known in the art), and the receiver is determined by the timeslot assigned to the transmitter.  
           [0010]    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.  
           [0011]    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 this priority into account as it manages access to the shared medium.  
           [0012]    Security is often an intrinsic function of multiplexing and MAC protocols. If there is a hazard associated with un-authorized interception of a message 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 to physically separate 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).  
           [0013]    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 the 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.  
           [0014]    One form of multiplexing and MAC protocols is time division multiplexing. In a Time Division Multiplexed (TDM) system, the shared medium is divided temporally into a series of sequentially occurring timeslots. All subchannels sharing the medium are assigned specific timeslots on which they are allowed to carry their traffic. This timeslot assignment may be equal for all subchannels, or some subchannels may be given higher capacity by assigning them a higher number of timeslots on which to transmit.  
           [0015]    Timeslots in TDM systems have various granularities depending upon the application. A timeslot can be as short as a single bit or as long as a full message or packet.  
           [0016]    TDM systems employ a timeslot counter (which is often implemented as a counter at each transmitter and receiver, synchronized by a system-wide clock and Sync signal). This counter counts from 0 to a maximum value corresponding to the number of timeslots in the system, and recycles back to 0. Transmitters and receivers employ a timeslot map storage array, often implemented with RAMs, to hold the database that maps timeslots to transmitters or receivers. System wide control functions coordinate the contents of these map RAM&#39;s among all the transmitters and receivers accessing the shared medium to insure each timeslot is assigned to at most one transmitter. If more than one transmitter map RAM holds an enable value for the same timeslot, a collision occurs, and the shared channel&#39;s data will be corrupted. If no transmitter is using a timeslot, it is considered idle, and is available for assignment to a transmitter with traffic (or additional traffic) to send.  
           [0017]    The rate that a timeslot counter counts through all its values and recycles is referred to as the “frame rate”. Many interesting tradeoffs occur when choosing this value for a system. Choosing too fast a frame rate causes problems such as encountering the physical limits of the channel&#39;s information bandwidth, having too few channels available, and reducing the system&#39;s efficiency because each transmitter cannot access the channel long enough to send a message of meaningful length. Conversely, if the frame rate chosen is too slow, latency becomes an issue. Channels are not served fast enough, leading to perceivable delays to users of the system, and in extreme cases, transmit buffer overflows. Systems that are optimized to carry voice typically use frame rates of 8 KHz, with several thousand timeslots.  
           [0018]    TDM systems of useful physical sizes running at electronic or optical data rates need to take into account the time it takes for a timeslot&#39;s information to pass over the physical medium between the transmitter and receiver. It is important that the synchronization and MAC algorithms take into account the non-zero propagation time of the messages. Often, a technique called “guard bands” is employed to insure that a transmitter on one extreme physical end of the medium is disabled, and its transmission has adequate time to propagate to the opposite extreme end of the medium before any other transmitter is permitted to engage. Guard bands reduce the utilization efficiency of a system, and careful system timing design and physical layout can reduce their efficiency penalty.  
           [0019]    TDM is a useful multiplexing and MAC protocol in systems that use a free space optical beam line as the shared access media to interconnect processing units in a stored program controlled system. 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 time division multiplex system.  
         SUMMARY OF THE INVENTION  
         [0020]    This need is satisfied and a technical advance is achieved in the art by a system and method that provides time 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 Time Division Multiplexing techniques to provide a plurality of logically independent subchannels over a single shared free space optical beam line. The single logical beam is divided temporally among the subchannels, with each subchannel operating for a selected timeslot or timeslots. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    A more complete understanding of this invention may be obtained from a consideration of the specification taken in conjunction with the drawings, in which:  
         [0022]    [0022]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;  
         [0023]    [0023]FIG. 2 is a cross-sectional view of the beam line of the exemplary embodiment of FIG. 1;  
         [0024]    [0024]FIG. 3 is an exemplary embodiment of transmitting and receiving probes of FIGS. 1 and 2;  
         [0025]    [0025]FIG. 4 is a block diagram of an exemplary embodiment of this invention wherein each of the processing units communicate with each other;  
         [0026]    [0026]FIG. 5 is a block diagram of a signal generator of FIG. 4;  
         [0027]    [0027]FIG. 6 is a block diagram of a transmit and receive protocol handler according to an exemplary embodiment of this invention;  
         [0028]    [0028]FIG. 7 is a timing diagram of a time division multiplex system according to an exemplary embodiment of this invention; and  
         [0029]    [0029]FIG. 8 is a flowchart of operation of bandwidth management for increasing or decreasing the capacity of each subchannel. 
     
    
     DETAILED DESCRIPTION  
       [0030]    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 below in connection with FIG&#39;s.  3  and  4 . Transmitting probe  14  produces a beam line  10  of desired diameter along the length of its path.  
         [0031]    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  20  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 .  
         [0032]    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 .  
         [0033]    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. First terminal unit  24  originates optical beam line  12  and second terminal unit  26  terminates the portion of optical beam line  12  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 interconnected to recycle the encoded signal.  
         [0034]    [0034]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 receiving probes  18  around its inner edge. In the illustration of FIG. 2, the laser of transmitter  12  (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 may also include a transmitter  12 , not shown.) Alternatively, second terminal unit  26  may comprise 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.  
         [0035]    Turning now to FIG. 3, exemplary embodiments of a transmitting probe  14  and a receiving probe  18  are shown. In this exemplary embodiment, transmitting probe  14  includes a transmitter  12  comprising a laser  30  (i.e., a laser diode  32  and a feedback photodetector  34 , as known in the art), which converts electronically encoded signals into optical beam line  10 . Optical beam line  10  is projected through a concave lense  36  and a convex lense  38  (which form a reverse Galilean telescope, as is known in the art). A laser driver  40  feeds electrically encoded signals to, and receives feedback from, laser  30 , as known in the art. Feedback amplifier  42  regulates the input to laser  30 . Laser  30  and laser driver  40  are both known to those skilled in the art. Laser  30  and laser driver  40  are illustrated herein as two separate units, but may be one unit.  
         [0036]    Beam line  10  is received at a receiving probe  18  at a receiver  16 , which includes a convex lense  44  that focuses beam line  10  on a photodetector  46 . Photodetector  46  receives a portion of beam line  10  and generates an electrical signal in response thereto. The electrical signal is fed into a receiver circuit  48  comprising a trans-impedance amplifier (TIA)  50 , clock recovery circuit  52  and decision circuit  54 . embodiment. First terminal unit  24  originates optical beam line  12  and second terminal unit  26  terminates the portion of optical beam line  12  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 interconnected to recycle the encoded signal.  
         [0037]    [0037]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 receiving probes  18  around its inner edge. In the illustration of FIG. 2, the laser of transmitter  12  (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 may also include a transmitter  12 , not shown.) Alternatively, second terminal unit  26  may comprise 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.  
         [0038]    Turning now to FIG. 3, exemplary embodiments of a transmitting probe  14  and a receiving probe  18  are shown. In this exemplary embodiment, transmitting probe  14  includes a transmitter  12  comprising a laser  30  (i.e., a laser diode  32  and a feedback photodetector  34 , as known in the art), which converts electronically encoded signals into optical beam line  10 . Optical beam line  10  is projected through a concave lense  36  and a convex lense  38  (which form a reverse Galilean telescope, as is known in the art). A laser driver  40  feeds electrically encoded signals to, and receives feedback from, laser  30 , as known in the art. Feedback amplifier  42  regulates the input to laser  30 . Laser  30  and laser driver  40  are both known to those skilled in the art. Laser  30  and laser driver  40  are illustrated herein as two separate units, but may be one unit.  
         [0039]    Beam line  10  is received at a receiving probe  18  at a receiver  16 , which includes a convex lense  44  that focuses beam line  10  on a photodetector  46 . Photodetector  46  receives a portion of beam line  10  and generates an electrical signal in response thereto. The electrical signal is fed into a receiver circuit  48  comprising a trans-impedance amplifier (TIA)  50 , clock recovery circuit  52  and decision circuit  54 . In TDM systems, clock recovery function  52  must have special properties. For example, it must maintain lock through guard bands and quickly recover phase alignment as different transmitters activate. Receiver  16  and receiver circuit  48  are well known in the art. Receiver  16  and receiver circuit  48  are illustrated herein as two separate units, with receiver driver  48  contained within a signal receiver  55 . However, these two units may be one unit, as is known in the art.  
         [0040]    Laser  30  is driven by an electrical signal from signal generator  56 . Signal generator  56  comprises laser driver  40 , protocol handler  58  and multiplexer  60 . Multiplexer receives multiple inputs  62  from one or more processing units, which are multiplexed according to a predetermined algorithm (many algorithms for multiplexing are known in the art and are thus not discussed here). Signals are then passed to protocol handler  58 . Protocol handler  58  encapsulates the signals with the protocol used by the optical beam  10 , as will be described further below. The signal generated by protocol handler  58  is fed into laser driver  40 , which controls laser  30 .  
         [0041]    On the receiver side, when a signal is received by photo detector  46 , it is delivered to signal receiver  55 , which comprises receiver circuit  48 , protocol handler  64  and demultiplexer/router  66 . The received signal is decoded in receiver circuit  48 , as known in the art. The receiver circuit  48  is connected to a protocol handler  64  which de-encapsulates the signal received according to the protocol used by protocol handler  58 . Protocol handler  64  passes the signal to a demultiplexer and router  66  which demultiplexes the signal and then sends signals  68  to the receiving processing unit or units.  
         [0042]    [0042]FIG. 4 illustrates an exemplary embodiment of a stored program controlled system that uses an optical beam line to provide communication among processing units wherein a time division multiplex (TDM) protocol according to this invention is used. In this exemplary embodiment, beam line  10  is uni-directional, i.e., signals flow in the direction from uni-directional first terminal unit  132  to second uni-directional 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 , as well as second uni-directional terminal unit  134  are connected to receiving probes  135 .  
         [0043]    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  (as described above in connection with FIG. 3). Transmitter  14  in uni-directional first terminal unit  132  optically encodes the signals, and transmits optical beam line  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 injecting signals 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 .  
         [0044]    Turning to the block diagram of FIG. 5, details of signal generator  56  (FIG. 4) for multiplexing and media access control capabilities at endpoint  132  are illustrated. Precise timing is especially important to TDM-based systems, therefore signal processing begins with a master clock source  560 . Master clock source  560  comprises, in this exemplary embodiment, an accurate crystal oscillator producing a bit-clock signal on line  564 .  
         [0045]    Bit-clock signal  562  is modified to a lower frequency timeslot clock by divider  562 , producing a timeslot clock signal on line  566 . The division ratio is determined by the desired number of payload bits per timeslot and the length of the guard bands. Timeslot counter  568  divides the timeslot clock signal  566  by a factor related to a predetermined number, based on the desired number of timeslots in a frame. Timeslot counter  568  then produces a frame sync signal  570 , and a timeslot count value  572 . Each time the timeslot counter reaches its terminal (maximum) value and recycles, a special frame synchronization pattern  575  is inserted into the signal generator&#39;s  56  transmit stream by frame sync pattern generator  574 . The timeslot clock signal  566  is processed by the timeslot sync pattern generator  576  to create a timeslot sync pattern  577 , which is inserted into signal generator&#39;s  56  transmit stream by timeslot sync pattern generator  576 .  
         [0046]    Timeslot count  572  is applied as an address to one port of a timeslot map RAM  582 . Timeslot map RAM (as is known in the art of TDM and thus not further described) typically dual ported, with a second address and data port communicating with a central control processor over control bus  584 . The output of the timeslot map RAM  582  is an enable signal  583 , which is processed by guard band insertion logic  586  and enables parallel-to-serial register  588 . Guard band logic  586  inserts a predetermined amount of time between enable signals, as is known in the art, to allow for signal propagation times.  
         [0047]    Parallel data to be transmitted arrives over transmit data input  580 , and is held in transmit queue  581  until the enable signal moves (gates) it through the parallel to serial register  588  and out the serial bit stream port  589 . When enabled, the parallel to serial register  588  transmits one data bit from the transmit queue  581  per bit clock signal  562  interval. Three serial signals, the frame sync pattern  575 , the timeslot sync pattern  577 , and the transmit bit stream  589 , are combined in multiplexer  590  to produce a composite output signal  591  that is sent to laser driver  40  (FIG. 3). Thus, the clock signals, timeslot sync and frame sync patterns controlling operation of all transmitters and receivers are injected into the beam line  10 , according to this exemplary embodiment of this invention. Delivering these control signals in the manner described above obviated the need for electrical (or other fibre optic) signal paths, which would otherwise increase the wiring complexity of this system. However, such external means for communicating control signals comprise another exemplary embodiment that will be understood by one skilled in the art after studying this disclosure.  
         [0048]    [0048]FIG. 6 is a block diagram of the functional components of transmit protocol handler  58  and receiver protocol handler  64  associated with each of the processing units  138 ,  140 ,  142  and  144  (FIG.4). A clock signal local to each transmit protocol handler  58  and receiver protocol handler  64  are extracted from the received signals arriving over the beam line through clock recovery logic (not shown, but well known in the art). The recovered clock is delivered over clock line  666  to synchronize a local Phase Locked Loop circuit  640 , which generates a local reference clock signal  641 . The local reference clock signal  641  is divided by the same factor as master clock source  560  (FIG. 5) in divider  642  to produce local timeslot clock signal  643 . Timeslot interval synchronization with signal generator  56  is achieved by a timeslot sync detector  644  responding to the timeslot sync pattern the signal generator  56  periodically sends. A frame sync pattern detects circuit  646  detects the frame synchronization pattern and generates a local frame synchronization pulse  647 .  
         [0049]    The local timeslot clock signal  643  is used to increment the local timeslot counter  648 , and the local frame synchronization pulse  647  periodically resets the local timeslot counter to its base value. Through this mechanism of clock and sync signal distribution from a central source operating many distributed timeslot counters on each of the board level interfaces, system wide distribution of synchronized local copies of the master timeslot counter can be achieved without physical interconnection cables.  
         [0050]    The timeslot counter&#39;s output is used to provide the address to a timeslot map RAM  650 . The contents of map RAM  650  is arranged to provide both a transmitter enable signal  652  and receiver enable signal  654 . The transmit enable signal  652  only allows the transmitter&#39;s laser to emit energy during the specific timeslot(s) allocated to this particular interface by acting through a transmit enable logic  656 . When a transmitter is enabled and transmit data  658  from the processing unit is available for transmission, data is removed from the transmit queue (buffer)  660 , sent through a parallel-to-serial register  662  and transmitted to the laser driver  40  (FIG. 3) over link  657 . The transmit serial bit stream  663  is synchronized with the local timeslot clock signal  641 . If no data is available during an active timeslot (i.e., transmit queue  660  is empty), an idle signal is generated in transmit enable logic  656  and is transmitted in its place.  
         [0051]    Receive enable signal  654  regulates the data stream  665  from the receive probe&#39;s receiver circuit  48  (FIG. 3) into the receiver serial-to-parallel register  664  only during the specific timeslot(s) allocated to this particular processing unit. Frequently, more receive timeslots will be active than transmit timeslots in order to accommodate broadcast or multicast traffic, which originates at the signal generator  56  (FIG. 3) but may be received on many (or all) of the processing units. The received parallel data is temporarily stored in receive queue  670  until it can be transmitted to the processing unit over receive data link  671 . Control message extraction logic  668  detects update control messages, and updates the timeslot map RAM  650  over its second port.  
         [0052]    Control of the contents of all the timeslot RAM&#39;s associated with a beam line resides with central controller  136  (FIG. 4). Central controller  136  performs system wide bandwidth allocation functions to control which proportion of the total bandwidth available over a beam line will be dedicated to the sub channels of the processing units.  
         [0053]    In order to write the contents of the timeslot map RAM  650  on a particular processing unit, central controller  136  produces a special message pattern (similar to the frame synchronization pattern) containing the data to be written and an address and injects it into the signal generator&#39;s transmit stream. All of the processing units receive and decode these messages and compare the board address field contained in the address to their geographic address  669  (which is derived from the particular processing unit and slot the interface board is connected to). If the board address field matches the geographic address, the message is addressed to this board and it uses the map address field and data field to write the selected address in the timeslot map RAM  650 . The next time the timeslot counter recycles, the new data will cause the updated timeslot assignment to take effect. This above scheme permits reliable and efficient transmission of data between the transmit and receive buffers on the central hub and all the receive and transmit buffers on all the distributed interface boards served by the common beam line.  
         [0054]    [0054]FIG. 7 illustrates a timing diagram for this system according to an exemplary embodiment of this invention. Bit-clock signal  700  governs the high frequency bit rate of the signals in the beam line. Timeslot clock signal, generated by divider  564  (FIG. 5) from bit-clock signal  700 , increments timeslot counters system wide. The frame synchronization pulse  704  is the frame sync pattern and is used to reset all the timeslot counters to their base value (zero in this case). The timeslot counter output signal  706  is the address bus of the timeslot map RAM. Optical data on the beam line is shown as signal  708 , the active data intervals as signal  710 , and the inactive guard bands  712 . During the guard bands  712 , the timeslot synchronization pattern is transmitted. During inter-frame guard band  713 , the frame synchronization pattern is broadcast. Several different enable signals for the various distributed receivers and transmitters are shown as signals  714 ,  715 ,  716 ,  717 ,  718 , and  719 . Notice that at most one transmit enable signal is active during a given timeslot. In timeslot  2 , multiple receivers are active in this example, denoting a multicast transmission.  
         [0055]    Turning to FIG. 8, a flowchart for the process by which bandwidth is allocated and deallocated in the system is shown. Two paths from decision block  400  exist through this flowchart, depending upon whether a static, fixed multiplexing or dynamic statistical multiplexing strategy is selected.  
         [0056]    In Static timeslot mapping, bandwidth in the shared channel is allocated in fixed quanta, equal to the throughput of a single timeslot active in each frame. One or more of these quanta are allocated to a given subchannel, depending upon the bandwidth required. In step  402 , an Operations, Administration, Maintenance and Provisioning (OAM&amp;P) system is consulted to determine the bandwidth needs of each sub channel. The number of timeslots per frame required to satisfy the bandwidth need from each processing unit in both directions is calculated in  404 . Next, the timeslot interleave pattern is calculated in  406 . This pattern is designed to achieve approximately equally temporal spacing between allocated timeslots throughout the frame. Next, in  408 , central control creates tables with the timeslot map RAM contents for each processing unit. Control messages are created in step  410 . In  412 ,  414 ,  416 ,  418 , and  420 , a loop is entered in which messages are transmitted over the OAM&amp;P subchannel of the beam line, received at the addressed processing unit and written into the timeslot map RAMs. At this point, the system is initialized  422  and communication in the shared medium commences. The system now enters a loop  424  where it only processes OAM&amp;P orders to change bandwidth.  
         [0057]    In Dynamic timeslot mapping, bandwidth between the various endpoints is altered in response to the dynamic demands of the traffic. System initialization is completed as in static mapping using steps  402  through  424  and then the dynamic bandwidth allocation loop is entered. In decision block  430 , all receive timeslots are monitored by central control to determine if any currently-assigned timeslot has been idle for more than N timeslots. If so, the idle timeslots are deallocated and returned to a free timeslot pool in  432 . In block  434 , timeslot rearrangement takes place in order to preserve the timeslot interleave pattern established in  406 . Block  435  transmits the messages to the timeslot map RAMs.  
         [0058]    In block  436 , all the distributed transmit queues are polled to determine if any queue is in danger of overflowing. If so, test  438  is made to determine if any timeslots are available for allocation from the free timeslot list. If a timeslot is available to increase the channel bandwidth to a given processing unit, the new timeslot interleave pattern is calculated in  440 . The messages to update the timeslot map RAMs are sent in  442 . In case no timeslots are available for allocation, decision  444  determines if there is any low priority traffic that can be temporarily removed from the system to recover some free timeslots. In block  436 , the low priority timeslots are identified, and added to the idle list, for processing by steps  432 ,  434 , and  435 . If load shedding is not an option, an overload control message is sent to the OAM&amp;P system by  448 . The OAM&amp;P system modifies the network parameters at a higher level to reduce the load on this particular node.  
         [0059]    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.