Abstract:
A personal communications system has a plurality of personal communications units defining a network. The personal communications units are configured to send and receive digital communications among one another via the network. A wireless interface adaptor further defines the network and is configured to function as a repeater. The personal communications units are configured to cooperate with the wireless interface adaptor when the wireless interface adaptor is available. The wireless interface adaptor coordinates access of each personal communications unit to at least one other personal communications unit. The personal communications units are further configured to cooperate with one another to effect communications therebetween when no wireless interface adaptor is available.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to radio communications and more particularly to a personal communications system utilizing digital communications for use in both wired intercoms and wireless point-to-point communications. 
     BACKGROUND OF THE INVENTION 
     Wireless communications are well known. Such systems as walkie talkies, CB radios, and cellular telephones utilize wireless communications to facilitate point-to-point communications between individuals at different locations. 
     Such wireless communications systems typically utilize well known half duplex or talk-then-listen radio methodology wherein a user can listen to an incoming communication, or can speak, but not both simultaneously. Such half duplex wireless communication systems use either a push button control or the like or alternatively use a voice operated switch (VOX) to change the mode of the transceiver from receive to transmit. 
     While such contemporary wireless communication systems have generally been suitable for their intended purposes, they possess the inherent deficiency of requiring explicit actuation of the transmit mode via such a manually operated or automatic switch and also suffer from the inherent deficiency of not permitting an incoming communication when the transceiver is in the transmit mode. 
     Of course, requiring an operator to manually actuate the transmit mode, typically via a push button switch, necessitates that the operator use a hand (or possibly a foot) to key the microphone. Such explicit operation of the transceiver is not only a distraction, but may also be extremely undesirable in instances where the operator&#39;s hands (and possibly feet) are otherwise occupied. For example, tank drivers, aircraft pilots, helicopter pilots, etc., particularly when engaged in demanding maneuvers, may not be able to perform such manipulations, or may do so only at the risk of neglecting some other task which requires immediate attention. 
     Voice operated switches have been developed in an attempt to mitigate the problems associated with manually operated half duplex transceivers. However, such voice operated switches introduce an altogether new set of problems. Such problems include the operation of a voice operated switch in a high noise environment and the necessity of properly adjusting the sensitivity of the voice operated switch in such a high noise environment. As those skilled in the art will appreciate, high levels of ambient noise frequently result in the undesirable and inadvertent keying or actuation of the voice operated switch, such that no actual voice transmission is broadcast and the transceiver is prevented from accepting incoming transmissions. 
     Also, the user of such a voice operated switch in a high noise environment must speak louder than normal, so as to actuate even a properly adjusted voice operated switch. Such loud speaking can be fatiguing and may even result in hoarseness or other voice-related problems. 
     Regardless of what type of half duplex transceiver is utilized (manually actuated or VOX), another problem associated with such half duplex systems is the inadvertent keying thereof. Manually operated switches have an undesirable tendency to stick in the actuated position, thereby resulting in constant transmission and the inability to receive broadcasts from other transceivers. Thus, the operator who has such a stuck key can not even be notified by other individuals, who are listening to the inadvertent broadcast, that his key is stuck in the actuator position, since the individual who has the stuck key is incapable of receiving broadcasts due to half duplex operation of the transceiver. Further, as discussed above, voice operated switches may become inadvertently actuated due to high ambient noise levels. 
     As such, it is clear that a full duplex transceiver for point-to-point communications would be desirable. 
     Wire intercoms are also well known. Frequently, such intercoms are configured such that a plurality of users may talk simultaneously with respect to one another and each user may talk while listening to the conversations of a plurality of users. Thus, conversations via such wired intercoms tend to be much more natural than those taking place via wireless, half-duplex wireless communication systems. 
     It would further be desirable to provide intercom-like operation of the radio transceivers, such that they are capable of receiving a plurality of separate transmissions simultaneously, while the user is speaking. In this manner, each transceiver will pick up the broadcast of all other transceivers so as to provide a much more natural means for communication. 
     It would further be desirable to provide a comprehensive communications system which integrates wireless communications with wired intercom communications, such that persons utilizing a wired communications system, such as that of a tank, aircraft, helicopter, etc., may readily communicate among one another, and may also, simultaneously if desired, communicate with persons who are not part of the wired intercom system. 
     SUMMARY OF THE INVENTION 
     The present invention specifically addresses and alleviates the above mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a personal communications systems comprising a plurality of wireless personal communications units defining a network and configured to send and receive digital communications among one another via the network. Each personal communications unit comprises a radio transceiver. Optionally, the network further comprises a wired intercom system. 
     A first wireless interface adaptor further defines the network. Each wireless interface adapter comprises a repeater. As those skilled in the art will appreciate, a repeater comprises a transceiver configured to receive and re-transmit radio signals on predetermined, generally different frequencies. Such repeaters are used to effectively extend the range of transceivers. Sometimes a plurality of repeaters will be required to relay a signal from a sender to a recipient. The personal communications units are configured to cooperate with the wireless interface adaptor when the wireless interface adaptor is available. The wireless interface adaptor coordinates access of each personal communications unit to at least one other personal communications unit. The personal communications units are further configured to cooperate with one another to effect communications therebetween when no wireless interface adaptor is available. 
     Thus, according to the present invention, the wireless interface adaptor determines which personal communications unit, as well as which intercoms, if any, are party to a particular conversation. Thus, an initiator of a conversation may specify, either via voice commands, switch setting, or any other desired manner, which other personal communications units and/or intercoms are to receive his transmission and are to be able to communicate back with him. 
     The personal communications units are configured to communicate among themselves when a wireless interface adaptor is unavailable. The personal communications units are configured to use distributed control of the network defined thereby when a wireless interface adaptor is not available. According to the preferred embodiment of the present invention, the communications units and the wireless interface adapters are configured to facilitate communications of both voice and data. 
     The wireless interface adaptor is configured to transmit communications received from either a personal communications unit or another wireless interface adaptor to another personal communications unit, a wired intercom, and/or a third wireless interface adaptor. 
     The wireless interface adaptor is configured to provide centralized control of the network. Each personal communications unit is configured to request network access from a wireless interface adaptor and to commence communication only after access is granted by the wireless interface adaptor, when a wireless interface adaptor is available. 
     Each personal communications unit is configured to request network access from a wireless interface adaptor when a wireless interface adaptor is available, in order to communicate with another personal communications unit and each personal communications unit is configured to commence communications only after access is granted by the wireless interface adaptor (if available). All of the communications between the two personal communications units is routed through the wireless interface adaptor when a wireless interface adaptor is available. 
     According to the preferred embodiment of the present invention, the personal communications units and the wireless interface adaptor are configured to provide automatic switching from distributed control to centralized control of the network when a wireless interface adaptor becomes available and to provide automatic switching from centralized control to distributed control of the network when the wireless interface adaptor becomes unavailable. 
     Optionally, the present invention further comprises a wired intercom system in communication with the wireless interface adaptor. The wired intercom system may either be in wired communication or in wireless communication with the wireless interface adaptor, as desired. The wired intercom system and the wireless interface adaptor are configured to transmit composite voice communications from the wired intercom to the personal communications units. As used herein, the term composite is defined to include voice communications which results from a plurality of different conversations. As discussed above, the use of such composite voice communications results in much more natural communication, wherein each person speaks at any time, without the need to wait for other conversations to end. 
     Preferably, the wireless interface adaptor is configured to transmit a first composite voice communication from the wired intercom to the personal communications units and to form a second composite signal comprised of communications from a plurality of personal communications units and to communicate the second composite signal to the wired intercom system. 
     According to the present invention a method for providing personal communications comprises the steps of defining a network with a plurality of personal communications units, the personal communications units being configured to send and receive digital communications among one another via the network. The network is further defined with a first wireless interface adaptor. The wireless adaptor coordinates access of each personal communications unit to at least one other personal communications unit. The personal communications units cooperate with one another to effect communications therebetween when no wireless interface adaptor is available. 
     Further, according to the methodology of the present invention, the personal communications units use distributed control of the network defined thereby, when a wireless interface is not available. 
     The method further comprises the step of having the personal communications units and the wireless interface adaptor communicating both voice and data. The wireless interface adaptor transmits communications received from a personal communications unit, another wireless interface adaptor, or a wired intercom to another personal communications unit, a third wireless interface adapter or a wired intercom. 
     The number of personal communications units, wireless interface adapters, and wired intercoms defining a particular network may be any desirable number. Thus, for example, a network may comprise several hundred personal communications units, three wireless interface adapters, and five wired intercoms. Thus, those skilled in the art will appreciate that any desired number of personal communications units, wireless interface adapters, and wired intercoms may define a network. 
     According to the preferred methodology of the present invention, the wireless interface adaptor provides centralized control of the network. Each personal communications unit requests network access from a wireless interface adaptor and commences communications only after access is granted by the wireless interface adaptor, when a wireless interface adaptor is available. 
     Each personal communications unit requests network access from a wireless interface adaptor, when a wireless interface adaptor is available, in order to communicate with another personal communications unit and commences communication only after access is granted by the wireless interface adaptor. All of the communications between the two personal communications units is rather through the wireless interface adaptor. 
     The methodology of the present invention further comprises the steps of placing a wired intercom system in communication with the wireless interface adaptor and the wireless interface adaptor transmitting composite voice communications from the wired intercom to the personal communications units. 
     Preferably, the wireless interface adaptor transmits first composite voice communications from the wired intercom to the personal communications units. A second composite signal comprises of communications from a plurality of personal communications units is transmitted therefrom, to the wired intercom system. 
     Thus, according to the present invention, a personal communications system is provided for individual soldiers, military airborne vehicles, factor and warehouse communications and monitoring, construction site communications, mobile customer service, and the like. 
     The personal communication system of the present invention operates in full-duplex mode, such that any person on the network can both listen and talk simultaneously. Each person on the network can hear multiple conversations simultaneously, wherein other people on the network are conversing with each other, as well as the listener. In this manner, natural voice communications is provided and the above described disadvantages associated with half-duplex operation are eliminated. 
     The present invention further comprises a multimode personal communications system comprising a plurality of personal communications units (PCUs), wherein each PCU is configured for: 1) network mode communication; and 2) autonomous (non-networked) mode communication. 
     A universal adaptor interface is in communication with each of the plurality of PCUs during network mode communications. The UAI is configured to receive PCU uplink information from a plurality of PCUs and to transmit UAI downlink information in the form of a composite signal by a plurality of PCUs, the composite signal including information separately addressable to each of a plurality of PCUs. The PCUs are further configured to receive PCU uplink information from non-network PCUs and to forward uplink information from the non-networked PCUs to the UAI in the PCU uplink information from a networked PCU. 
     The networked PCUs and the UAI are syncronized such that the PCU uplink occurs during a first time interval and the UAI downlink occurs during a second time interval. 
     The networked PCUs are further configured such that the autonomous mode transmissions occur during a third time interval. The PCUs are further configured to recognize that a non-networked PCU transmitting in autonomous mode during the third time interval is receiving the UAI downlink information. 
     The PCUs are further configured to recognize that a non-networked PCU transmitting information in autonomous mode during other than the third time interval is not receiving the UAI downlink information. 
     The PCUs are preferrably further configured to receive UAI downlink information intended for a non-networked PCU not receiving the UAI downlink information, and to transmit the received information, and to transmit the received information to the non-networked PCU during the third time interval. 
     These, as well as other advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structures shown and described may be made within the scope of the claims without departing from the spirit of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the personal communications system of the present invention; 
     FIG. 2 is a block diagram of an exemplary network configuration according to the present invention; 
     FIG. 3 is a block diagram of a personal communications unit according to the present invention; 
     FIG. 4 is a flow chart showing the operational states of a personal communications unit of the present invention; 
     FIG. 5 is a flow chart showing the stand-by mode of a personal communications unit of the present invention; 
     FIG. 6 is a flow chart showing the slave mode of a personal communications unit of the present invention; 
     FIG. 7 is a flow chart showing the autonomous mode (neighboring network) for a personal communications unit of the present invention; 
     FIG. 8 is a flow chart showing the autonomous mode (autonomous network) of a personal communications unit of the present invention; 
     FIG. 9 shows a protocol frame format of the present invention; 
     FIG. 10 shows a protocol slot format of the present invention; 
     FIG. 11 shows the protocol header format of the present invention; 
     FIG. 12 shows a message format for a downlink voice transmission according to the present invention; 
     FIG. 13 shows a message format for an uplink voice transmission according to the present invention; 
     FIG. 14 shows the interpretation of ISMA bits according to the present invention; 
     FIG. 15 shows protocol slot usage for autonomous mode operation with a tapering network, according to the present invention; and 
     FIG. 16 is a graph showing communciations between a UAI and a PCU when the PCU is out of range with respect to the UAI. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiment of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiment. It is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. 
     Referring now to FIG. 1, when at least one universal adapter interface (UAI)  12   a  is available, then all messages transmitted and received by each personal communications unit (PCU)  10   a ,  10   b ,  10   c , are routed through the UAI  12   a  which acts as a wireless network master. If more than one UAI  12   a ,  12   b  is available, then one of the UAI&#39;s  12   a ,  12   b  is designated as the master UAI  12   a  and the other is designated as the group UAI. In this mode, each PCU  10   a ,  10   b ,  10   c  transmits its audio signal to the master UAI  12   a . The master UAI  12   a  then forms a composite audio intercom signal by summing together all of the various uplink PCU  10   a ,  10   b ,  10   c  transmissions received during a given time interval or frame. If any UAI  12   a ,  12   b  is connected to a wired intercom network  13   a ,  13   b , then conversations from the wired intercom  12   a ,  13   b  are included in the overall composite audio signal as well. 
     The master UAI  12   a  transmits this composite audio signal to all of the PCU&#39;s  10   a ,  10   b ,  10   c  within range, either as a general broadcast signal or as a signal dedicated to a specific PCU  10   a ,  10   b ,  10   c , or both. In addition, the master UAI  12   a  may optionally transmit the wireless communications signals over the wired network, thereby providing a bridge between wireless PCU&#39;s  10   a ,  10   b ,  10   c  and wired intercom  13   a ,  13   b.    
     Autonomous mode is a more complex mode of PCU  10   a ,  10   b ,  10   c  operation in which PCU&#39;s  10   a ,  10   b ,  10   c  initiate communications in the absence of a UAI  12   a ,  12   b . A PCU  10   a ,  10   b ,  10   c  that is out-of-range of a UAI  12   a ,  12   b  is referred to herein as an autonomous PCU  10   a ,  10   b ,  10   c . Note that autonomous PCU&#39;s may initiate communications with other PCU&#39;s  10   a ,  10   b ,  10   c  already in a neighboring repeater-based network (one in which at least one UAI  12   a ,  12   b  is available), or with other autonomous PCU&#39;s  10   a ,  10   b ,  10   c.    
     Referring now to FIG. 2 a repeater-based network containing one UAI  12  and four PCU&#39;s  10   d - 10   h  is shown. PCU  10   h  is within range of PCU  10   d  and PCU  10   e , but out-of-range of the UAI  12 . To ensure useful, reliable information exchange: 1) PCU  10   h  preferably does not disrupt the existing repeater-based network, and 2) PCU  10   h  preferably is able to communicate with PCU  10   d  and PCU  10   e.    
     These objectives are achieved through use of control fields contained in the messages transmitted by each PCU  10   d - 10   h . Specifically, an autonomous PCU (such as PCU  10   h ) will search for uplink PCU  10   d - 10   g  transmissions which indicate that a PCU  10   d - 10   g  within listening range (PCU  10   d  or PCU  10   e ) is part of a repeater-based network (a network having at least one available UAI  12 ). Then, the autonomous PCU  10   h  will use the network status fields in these transmissions to determine the network availability and system timing in the neighboring network. 
     The autonomous PCU  10   h  will begin transmitting during time periods in which the neighboring network is inactive, and will include embedded control information that indicates that these messages correspond to an autonomous PCU  10   h . In this way, PCU&#39;s  10   d - 10   g  in the neighboring network and within listening range of the autonomous PCU  10   h  will decode this control information and include the autonomous PCU&#39;s  10   h  messages in their composite audio signal. 
     In the event that an autonomous PCU  10   h  does not detect the presence of other PCU&#39;s  10   d - 10   g , the autonomous PCU  10   h  will begin broadcasting if it has radio data to transmit. Other autonomous PCU&#39;s  10   d - 10   h  in the area will obtain network timing from this signal, and will establish communications using a procedure similar to the one described previously. 
     PCU&#39;s  10   d - 10   h  are also capable of initiating point-to-point voice communications in which two PCU  10   d - 10   h  users converse privately. For point-to-point operation, two PCU&#39;s  10   d - 10   h  use either a user-programmable identification (ID) number, or a read-only manufacturer ID number to provide network addressing. Also, in a repeater-based system (having at least one UAI  12 ), this conversation can use the same wireless medium as the voice intercom without interference. 
     Given that the communications system discussed herein employs a digital wireless medium, applications requiring digital data communications are supported as well. For example, some of the applications facilitated by this system architecture include: remote database access (e.g., for images, maps, etc.); remote report filing; reconnaissance (e.g., transmission of images and sounds to a centralized facility); and user-to-user data transfer. 
     Moreover, in a repeater-based network, digital data communications can occur simultaneously with point-to-point voice intercom communications over the same wireless medium without interference. To achieve this simultaneous operation, control information embedded in the PCU uplink and UAI downlink transmissions are used to route intercom and data messages to the appropriate PCU&#39;s. 
     A communications protocol for the present invention preferably utilizes a Time Division Multiple Access (TDMA) architecture in which each TDMA frame contains five (5) downlink and four (4) uplink slots, as discussed in detail below. Each slot is further divided into a synchronization field, a header field, a data field, and a CRC-16 field. In addition, Inhibit Sense Multiple Access (ISMA) is used as the Medium Access Control (MAC) algorithm for determining slot availability. 
     Referring now to FIG. 3, the device architecture for each PCU  10   a - 10   h  is shown. The major components of a PCU  10   a - 10   h  include: an audio codec  30 , a data port (e.g., an RS-232 interface)  32 , a radio  40 , an FPGA  36 , and a DSP  38 . The audio codec  30  provides an interface to an external headphone  24  and microphone (not shown), and/or an internal speaker (not shown) and microphone (not shown). The radio  40  functions as the interface to other PCUs and/or UAIs. The data port  32  is used to interface the PCU  10   a - 10   h  with an external data terminal, for example, a personal computer  28 . The user interface allows external control of the various PCU  10   a - 10   h  functions, for example, channel selection and volume control. The FPGA  36  acts as a data interface between all of these components and the DSP  38 . Finally, the DSP  38  implements the control and signal processing algorithms used by the PCU  10   a - 10   h.    
     The data flow and signal processing algorithms are as follows. The PCU  10   a - 10   h  processes two separate, asynchronous data streams: the audio input/output and the radio input/output. The timing for the radio input/output data stream is determined by the TDMA framing. Received radio data is buffered in the FPGA  36  and read by the DSP  38  once a complete TDMA slot is received. The DSP  38  decodes the header field and determines if the data field should be processed or ignored. If processing is required, the DSP  38  stores the data field in a circular buffer corresponding to the current TDMA slot. Conversely, radio data to be transmitted is written by the DSP  38  to a buffer in the FPGA  36 . The DSP then writes control information to the radio  40 , causing the radio  40  to transmit this data during the correct TDMA slot. 
     The sampling rate of the audio codec  30  determines the timing of the audio input/output data stream, and ultimately the timing of the PCU  10   a - 10   h . Specifically, the audio codec  30  writes audio data received by the microphone to the DSP  38 . The DSP  38  processes this data using a Voice Activated Switch (VOX) algorithm, or a push-to-talk (PTT) switch, to determine if speech is present. At the start of an audio frame the PCU  10   a - 10   h  determines which received radio data slot buffers contain valid data. Any valid radio data is summed with the input audio signal, to form a composite audio output. The audio output is written to the codec for transmission to the headphones  24  or speaker. Finally, if the VOX algorithm indicates that speech is present, or the PTT switch is actuated, the DSP  38  will use the audio input signal as the radio data to be transmitted during the next appropriate TDMA slot. 
     Besides data flow and signal processing, the DSP  38  also implements the control algorithms that cause the PCU  10   a - 10   h  to operate in and transition between the various protocol states, as well as the radio control processing (e.g., frequency hopping, transmit and receive timing output power control, etc.). 
     Referring now to FIG. 4, each PCU  10   a - 10   h  can be in one of four possible protocol states or modes: Standby mode  50 , Slave mode  56 , Autonomous mode—Neighboring network  54 , and Autonomous mode—Autonomous network  52 . 
     A PCU  10   a - 10   h  is in Slave mode  56  if it is linked to a master UAI  12   a  (FIG.  1 ). A PCU  10   a - 10   h  is in Autonomous mode—Neighboring network  54  if it is linked to PCU&#39;s  10   a - 10   h  that are members of a repeater-based network. A PCU is in Autonomous mode—Autonomous network if it is linked to PCUs that are not members of a repeater-based network. A PCU  10   a - 10   h  is in Standby mode  50  if it is attempting to determine the current network configuration. Referring now to FIGS. 5-8, the possible state/mode transitions, and the control processing that occurs within each possible state/mode are shown. 
     According to the preferred embodiment of the present invention the length of an audio frame equals the length of a TDMA frame. Since the audio and TDMA frames are typically offset with respect to one another, block data processing is used in which the start of the audio frame indicates the start of the block of data to be processed. 
     An example of a system architecture for versatile, wireless voice and data communications is depicted in FIG.  1 . In general, this system contains two main components: repeaters, or Wireless Interface Adapters (UAIs)  12   a ,  12   b , and Personal Communications Units (PCUs)  10   a ,  10   b ,  10   c . In its standard mode of operation, a UAI  12   a ,  12   b  is a device that provides centralized control of the wireless network by receiving, processing and routing incoming PCU  10   a ,  10   b ,  10   c  transmissions. A PCU  10   a ,  10   b ,  10   c  is a device that provides an individual user with access to the wireless network. In its standard mode of operation, the PCU  10   a ,  10   b ,  10   c  requests network access from the corresponding UAI  12   a ,  12   b , and begins transmitting once access is granted. In this mode, messages transmitted and received by the PCU  10   a ,  10   b ,  10   c  are routed through the UAI  12   a ,  12   b . However, the PCU&#39;s  10   a ,  10   b ,  10   c  are also capable of forming a network in the absence of a UAI  12   a ,  12   b . In this case, the PCUs use distributed control to establish the network and grant network access. Note that the switch between centralized and distributed controls occurs automatically and seamlessly as the network topology changes. 
     UAIs  12   a ,  12   b  in a wireless intercom network may also be connected to wired intercom. As such, the UAI  12   a ,  12   b  acts as a bridge between users of the wireless intercom and users of the wired intercom. The UAI  12   a ,  12   b  sends the composite wired and wireless intercom voice signal to the wireless users during its downlink transmissions. Conversely, the UAI  12   a ,  12   b  combines all uplink wireless messages to form a composite signal that is transmitted to all wired intercom users. A more detailed discussion regarding the intercom communications is provided in the next section. 
     System operation for the case of wireless voice intercom communications is discussed below. In this case, the system components (UAI  12   a ,  12   b  and PCUs  10   a ,  10   b ,  10   c ) may operate in one of three modes: repeater mode, autonomous mode and group mode. 
     Repeater mode is the standard mode of network operation in which the UAI  12   a ,  12   b  is the wireless network master. In this mode the UAI  12   a ,  12   b  receives multiple uplink transmissions from a number of PCUs  10   a ,  10   b ,  10   c . The UAI  12   a ,  12   b  then forms a composite audio intercom signal by summing together all of the uplink PCU transmissions received during a given time interval, or frame. If the UAI  12   a ,  12   b  is also connected to a wire intercom network, it can include those conversations in the overall composite audio signal as well. Finally, the UAI  12   a ,  12   b  transmits this composite audio signal to all PCUs  10   a ,  10   b ,  10   c  in range. In addition, the UAI  12   a ,  12   b  may transmit the wireless intercom signals over the wired network, thereby providing a bridge between wired and wireless intercom users. 
     Autonomous mode is the more complex mode of network operation in which PCUs  10   a ,  10   b ,  10   c  initiate communications in the absence of a UAI  12   a ,  12   b . A PCU  10   a ,  10   b ,  10   c  that is out-of-range of a UAI is called an autonomous PCUs  10   a ,  10   b ,  10   c . Note that autonomous PCUs  10   a ,  10   b ,  10   c  may initiate communications with other PCUs  10   a ,  10   b ,  10   c  already in a neighboring, repeater-based network, or with other autonomous PCUs  10   a ,  10   b ,  10   c.    
     Given that the system architecture discussed herein employs a digital wireless medium, applications requiring digital data communications are supported as well. For example, some of he applications facilitated by this system architecture include: 
     Remote database access (e.g., for images, maps, etc.); 
     Remote report filing; 
     Reconnaissance (e.g., transmission of images and sounds to a centralized facility); and 
     User-to-user data transfer. 
     Moreover, in a repeater-based network, digital data communications can occur simultaneously with point-to-point voice intercom communications over the same wireless medium without interference. To achieve this simultaneous operations, the UAI uses control information embedded in the uplink and downlink transmissions to route intercom and data messages to the appropriate PCUs. 
     The system architecture preferably has the characteristics listed below: 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 System Characteristics 
               
             
          
           
               
                   
                 Parameter 
                 Value 
                 Comment 
               
               
                   
                   
               
               
                   
                 Range 
                 1500 ft. typical 
                   
               
               
                   
                 Channel type 
                 Time division 
               
               
                   
                   
                 duplex, frequency 
               
               
                   
                   
                 simplex 
               
               
                   
                 Data rate 
                 1 Mbps 
               
               
                   
                 Physical layer 
                 Frequency hopping 
                 FCC complaint in 
               
               
                   
                   
                 spread spectrum 
                 the 2.45 GHz ISM 
               
               
                   
                   
                   
                 band 
               
               
                   
                 No. of hopping 
                 75 
                 Per FCC 15.247 
               
               
                   
                 Operating 
                 2400-2483.5 
                 Per FCC 15.247 
               
               
                   
                 frequency range 
                 MHz 
               
               
                   
                 Number of 
                 64 
                 Each channel is 
               
               
                   
                 distinct 
                   
                 assigned a 
               
               
                   
                 channels 
                   
                 distinct hopping 
               
               
                   
                   
                   
                 pattern 
               
               
                   
                 Physical layer 
                 Time division 
                 4 uplink slots; 1 
               
               
                   
                 frame format 
                 multiple access 
                 repeater downlink 
               
               
                   
                   
                 (TDMA) 
                 slot; 4 dedicated 
               
               
                   
                   
                   
                 downlink slots 
               
               
                   
                 Number of 
                 4 PCUs 
                 unlimited number 
               
               
                   
                 simultaneous 
                 transmitting, and 
                 of listeners 
               
               
                   
                 intercom talkers 
                 a wired intercom 
               
               
                   
                   
                 if connected to 
               
               
                   
                   
                 the UAI; 
               
               
                   
                 Medium access 
                 Inhibit sense 
               
               
                   
                 control (MAC) 
                 multiple access 
               
               
                   
                 layer 
                 (ISMA) 
               
               
                   
                 Networking modes 
                 Repeater-based, 
               
               
                   
                   
                 autonomous, and 
               
               
                   
                   
                 group 
               
               
                   
                 Echo suppression 
                 UAI controlled 
                 Facilitated by 
               
               
                   
                   
                   
                 the dedicated 
               
               
                   
                   
                   
                 downlink slots 
               
               
                   
                   
                   
                 and an echo 
               
               
                   
                   
                   
                 suppression 
               
               
                   
                   
                   
                 algorithm 
               
               
                   
                 Sidetone 
                 Locally generated 
               
               
                   
                 Audio format 
                 8 bit μ-law PCM 
               
               
                   
                   
               
             
          
         
       
     
     The method for implementing a time division multiple access protocol for digital communications according to the present invention is illustrated in FIGS.  2  and  9 - 15 , which depict a presently preferred embodiment thereof. 
     The protocol described herein was developed for use in a wireless communications system containing multiple Personal Communication Units (PCUs) and multiple repeaters. A PCU is a device that provides a user with access to the wireless network. A repeater is a device that controls the wireless network traffic and may be connected to an additional wired network. To achieve the system features discussed previously, the protocol utilizes a Time Division Multiple Access (TDMA) architecture that provides multiple users with simultaneous access to the transmission medium. 
     Referring now to FIG. 9, the protocol is divided into frames that are transmitted sequentially in time. Each frame contains a number of TDMA slots D 0 -U 4 . Slots D 1  through D 4  and U 1  through U 4  are downlink (transmission from repeater to PCU) and uplink (transmission from PCU to repeater) slots, respectively. Usually, downlink and uplink slots having the same index (e.g., D 1  and U 1 ) are linked to form a downlink-uplink slot pair. In a typical system, multiple PCUs will use a Medium Access Control (MAC) algorithm to access the available uplink slots. Once a PCU obtains access to an uplink slot, it receives transmissions from the repeater during the corresponding downlink slot. Those PCUs that do not have access to an uplink slot will receive broadcast repeater transmissions during the downlink slot D 0 . Finally, a frame gap is provided so that frames do not overlap due to clock inaccuracies, and for synthesizer re-programming in a Frequency Hopping (FH) Spread Spectrum (SS) system. 
     Referring now to FIG. 10, each slot in the protocol frame is segmented into a number of fields. Each slot contains a preamble and slot sync field for radio acquisition, a header field for control information, a data field for audio and digital information, a 16-bit CRC field for error detection, and a slot gap to allow for clock inaccuracies. 
     Referring now to FIG. 11 the header format for all slots is depicted. The generic format of the header permits protocol “layering” and the reserved field permits future protocol growth. Also, the user-programmable ID field permits the point-to-point communications previously discussed. 
     The eight (8) bit Message type field permits 256 possible types of messages. Those currently defined are listed in Table 1A and discussed in the following sections. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1A 
               
             
             
               
                   
               
               
                 Defined Message Types 
               
             
          
           
               
                   
                 MESSAGE TYPE 
                 DESCRIPTION 
               
               
                   
                   
               
               
                   
                 80h 
                 Downlink voice message 
               
               
                   
                 01h 
                 Uplink voice message for 
               
               
                   
                   
                 PCUs in a repeater-based 
               
               
                   
                   
                 network, or in range of 
               
               
                   
                   
                 PCUs in a repeater-based 
               
               
                   
                   
                 network 
               
               
                   
                 08h 
                 Uplink voice message for 
               
               
                   
                   
                 autonomous PCUs out of 
               
               
                   
                   
                 range of a repeater 
               
               
                   
                 00h, 02h-07h, 09h-7fh, 
                 To be defined (for point- 
               
               
                   
                 81h-FFh 
                 to-point conversations, 
               
               
                   
                   
                 data communications, 
               
               
                   
                   
                 testing, etc. 
               
               
                   
                   
               
             
          
         
       
     
     This section describes the message types used to provide voice intercom communications. In the standard repeater mode of network operation, the repeater receives multiple uplink transmissions from a number of PCUS. The repeater then forms a composite audio intercom signal by summing together all of the uplink PCU transmissions received during a given protocol frame. If the repeater is also connected to a wired intercom network, it can include those conversations in the overall composite audio signal as well,. Finally, the repeater transmits this composite signal to all PCUs in range using the appropriate downlink slots. In addition, the repeater may transmit the wireless intercom signals over the wired network, thereby providing a bridge between wired and wireless intercom users. 
     Referring now to FIG. 12, voice intercom transmissions sent by a repeater during a downlink slot use Message Type  80   h . For Message Type  80   h , the Message Subtype, ID, and Reserved fields for standard voice intercom communications are unused and are set to 00h. The “Master” and “Autonomous” bits are interpreted according to Table 2. For downlink transmission, the repeater will send (Master, Autonomous)=(1,0). 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Interpretation of Master and 
               
               
                 Autonomous Bits 
               
             
          
           
               
                   
                 Bit 
                 Bit 
                 Interpretation 
               
               
                   
                   
               
               
                   
                 0 
                 0 
                 A PCU in slave 
               
               
                   
                   
                   
                 mode, or a 
               
               
                   
                   
                   
                 repeater in 
               
               
                   
                   
                   
                 Group mode and 
               
               
                   
                   
                   
                 slaved to a 
               
               
                   
                   
                   
                 master repeater. 
               
               
                   
                 0 
                 1 
                 A PCU or 
               
               
                   
                   
                   
                 repeater (Group 
               
               
                   
                   
                   
                 mode) in 
               
               
                   
                   
                   
                 automous-slave 
               
               
                   
                   
                   
                 mode. 
               
               
                   
                 1 
                 0 
                 A repeater in 
               
               
                   
                   
                   
                 master mode. 
               
               
                   
                 1 
                 1 
                 A PCU or a 
               
               
                   
                   
                   
                 repeater (Group 
               
               
                   
                   
                   
                 mode) in 
               
               
                   
                   
                   
                 autonomous- 
               
               
                   
                   
                   
                 master mode. 
               
               
                   
                   
               
             
          
         
       
     
     The six (6) channel bits are used to represent up to 64 distinct wireless channels. The Inhibit Sense Multiple Access (ISMA) bits are interpreted according to FIG. 14, and are used to facilitate MAC processing. The “reserved” bit is currently unused and set to 0. Finally, the “Active Slot” bit field is used to represent the current active slot, and is interpreted according to Table 3. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Interpretation of Active Slot Bit Field 
               
             
          
           
               
                 Active Slot Field 
                 Interpretation 
               
               
                   
               
               
                 100 
                 Repeater transmitting in 
               
               
                   
                 Slot D0 
               
               
                 100 
                 Repeater transmitting in 
               
               
                   
                 Slot D1 
               
               
                 001 
                 Repeater transmitting in 
               
               
                   
                 Slot D2 
               
               
                 010 
                 Repeater transmitting in 
               
               
                   
                 Slot D3 
               
               
                 011 
                 Repeater transmitting in 
               
               
                   
                 Slot D4 
               
               
                   
               
             
          
         
       
     
     Referring now to FIG. 13, voice intercom transmissions sent by PCU (or repeater in Group mode) during an uplink slot use Message Type  01   h , as shown in FIG.  13 . As for Message Type  80   h , the Message Subtype, ID, and Reserved fields for standard voice intercom communications are currently unused in Message Type  01   h , the “Master” and “Autonomous” bits are interpreted according to Table 2, and the six (6) channel bits are used to represent up to 64 distinct wireless channels. 
     Referring now to FIG. 14, the ISMA bit field is interpreted as shown, if (Master bit, Autonomous bit)=(0,0), otherwise the ISMA bits are ignored. The “PTT” equals “1” if the push-to-talk switch on the PCU is depressed, otherwise it is set to “0”. The “Reserved” bit is currently unused and set to “0”. Finally, the “Active Slot” bit field is used to represent the current slot, and is interpreted according to Table 4. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Interpretation of Active Slot Bit Field 
               
             
          
           
               
                 Active Slot Field 
                 Interpretation 
               
               
                   
               
               
                 00 
                 PCU transmitting in Slot 
               
               
                   
                 U1 
               
               
                 01 
                 PCU transmitting in Slot 
               
               
                   
                 U2 
               
               
                 10 
                 PCU transmitting in Slot 
               
               
                   
                 U3 
               
               
                 11 
                 PCU transmitting in Slot 
               
               
                   
                 U4 
               
               
                   
               
             
          
         
       
     
     In Inhibit Sense Multiple Access (ISMA), the network master (repeater) transmits information regarding slot availability. PCUs use this information to randomly access one of the available uplink TDMA slots. Specifically, the ISMA technique used in this protocol is as follows: 
     1. The repeater transmits a busy/free flag for each TDMA slot in a frame; 
     2. PCUs in the network echo the ISMA information in their uplink header field. 
     3. PCUs do not attempt to access a slot that is busy; 
     4. PCUs will randomly attempt to access slots that are free; 
     5. After accessing a slot, a PCU monitors the busy/free flag for that slot during the next few frames. If the flag is not set to busy after a predetermined number of frames, the PCU stops transmitting since its transmission was not received by the repeater, most likely due to a collision with another PCU transmission. 
     Repeater mode is the standard mode of network operation in which the repeater is the wireless network master. In this mode, the repeater determines the network timing (e.g., frame start and end) and facilitates downlink-uplink slot pairing. 
     Autonomous mode is the more complex mode of network operation in which PCUs initiate communications in the absence of a repeater. A PCU that is out-of-range of a repeater is called an “autonomous PCU”. Note that autonomous PCUs may initiate communications with other PCUs already in a neighboring, repeater-based network, or with other autonomous PCUs. 
     Referring again to FIG. 2, a repeater-based network containing a repeater  12  and four (4) PCUs  10   d - 10   h  is shown. PCU  10   h  is within range of PCU  10   d  and PCU  10   e , but out-of-range of the repeater  12 . As discussed above, to ensure useful, reliable information exchange: 1) PCU  10   h  should not disrupt the existing repeater-based network, and 2) PCU  10   h  should be able to communicate with PCU  10   d  and PCU  10   e . These goals are achieved through use of the ISMA, Master, and Autonomous bits, and the Active Slot field in message type  01   h . Specifically, an autonomous PCU (PCU  10   h ) will look for uplink slot transmissions with (Master, Autonomous)=(0, 0), which indicates that the PCU (PCU  10   d  or PCU  10   e ) within listening range is part of a repeater-based network. Then, the autonomous PCU  10   h  will decode the ISMA bits echoed in the received message header to determine the slot availability in the neighboring network. Finally, the autonomous PCU  10   h  will use the Active Slot field to determine the system timing of the neighboring network, and will begin transmitting during one of the available slots, using Message Type  01   h  with (Master, Autonomous)=(0, 1). PCU  10   d - 10   g , in the neighboring network and within listening range of the autonomous PCU  10   h  will decode this Master-Autonomous PCU&#39;s message in their composite intercom audio signal. 
     Referring now to FIG. 15, the protocol slot contents and resulting intercom audio signals for this example are depicted for the repeater-based network shown in FIG. 2 with PCUs  10   d - 10   g  in range of the repeater and autonomous PCU  10   h  in range of PCUs  10   d  and  10   e.    
     In the event that an autonomous PCU does not detect the presence of other PCUs, the autonomous PCU will start broadcasting Message Type  08   h  with (Master, Autonomous)=(1, 1), and with the ISMA bits and Active Slot field set to indicate that slot U 1  is active. Other autonomous PCUs in the area will obtain network timing from this signal, and will establish communications using Message Type  08   h  with (Master, Autonomous)=(0,1) and a procedure similar to the one described previously. 
     Repeaters in the wireless intercom network may also be connected to a wired intercom. As such, the repeater acts as a bridge between users of the wireless intercom and users of the wired intercom. The repeater sends the composite wired intercom voice signal to the wireless users during its downlink slot transmission. Conversely, the repeater combines all wireless transmission received in the uplink slots to form a composite signal that is transmitted to all wired intercom users. 
     The Group mode of operation can be used to connect two or more wired intercom networks together, along with other wireless intercom users. Specifically, a repeater in Group mode is configured to function as a PCU having the wired intercom as its audio input signal. Since the repeater acts as a PCU, it can communicate with other repeaters and PCUs using the repeater and autonomous modes discussed previously. 
     Point-to-point voice communications in which two users converse privately are also possible within the framework of this protocol. For this mode of operation, two PCUs would use either the user-programmable ID header field, or a read-only manufacturer ID number to direct messages between each other and not over the intercom network. Also, in a repeater-based system, this conversation can use the same wireless medium as the voice intercom without interference. Note that additional message types (i.e., a subset of the types  00 h,  02 h- 07 h,  09 h- 7 Fh,  81 h-FFh in Table 1) will be defined to support point-to-point communications. 
     In the repeater-based network of the present invention, digital data communications can occur simultaneously with point-to-point voice intercom communications over the same wireless medium without interference. Note that additional message types (i.e., a subset of the types  00 h,  02 h- 07 h,  09 h- 7 Fh,  81 h-FFh in Table 1) will be defined to support data communications. 
     Flow charts showing the operation of the present invention are provided in FIGS. 5-8, which are discussed in detail below. 
     With particular reference to FIG. 5, a flow chart of the PCU standby mode is provided. The PCU standby mode  60  may be entered from PCU power up, new channel selection, PCU autonomous mode-neighboring network, or PCU autonomous mode-autonomous network. 
     The PCU receiver monitors  62  the default channel frequency for up to 80 frame intervals or frequency hops. If no preamble is detected  64 , then a check is made to see if more than 80 frame intervals have passed. If not, then the PCU continues to monitor  62  the default channel. If greater than 80 frame intervals have passed, the PCU commences autonomous mode—autonomous network operation  68 . 
     When a preamble is detected  64 , then the channel number and slot number  70  are determined. If the channel is not correct  72 , then the PCU monitors  62  the default channel again. If the channel is correct  72 , then the message type is decoded. If a downlink slot message type  80   h  is received, then the PCU enters  80  to slave mode. If an uplink slot message type  01   h  is received, the PCU enters  82  the autonomous mode-neighboring network. Otherwise, if an uplink slot message type  08   h  is received, then the PCU enters the autonomous mode-autonomous network  84 . 
     With particular reference to FIG. 6, a flow chart of the PCU slave mode is provided. The PCU slave mode  100  is entered from the PCU standby mode, the PCU autonomous mode-neighboring network or the PCU autonomous network. After entering the PCU slave mode  100 , the system decodes the header for slot D 0 , if present, and stores UAI data  102 . If a UAI is not detected  104 , then PCU autonomous mode-neighboring network  106  is entered. If a UAI is detected  104 , then check is made to see if there is radio data to transmit  108 . If not, then the header is decoded for all uplink slots and data stored for autonomous PCU&#39;s  114 . Also, audio data in the buffers for slot D 0  and uplink slots containing autonomous PCU data are summed  120  and audio data is sent  126  to the headset. The system then waits for the end of frame and hops  132  to the next frequency, then resumes decoding headers for slot, D 0  if resent, and storing UAI data  102 . When radio data to transmit is present  108 , or if a beacon interval for autonomous PCU timing has been exceeded, then a check is made to see if the system is currently transmitting  110 . If the system is not currently transmitting  110 , then the ISMA flags received in the UAI transmissions are used to determine uplink slot status  116  and a check is made to see if a slot is available  122 . If no slot is available  122 , then the system decodes the header for all uplink slots and stores data for autonomous PCU&#39;s  114 . If a slot is available  122 , then a transmission is made on a randomly selected available uplink slot  128 . 
     If the system is currently transmitting  110 , then slot access is verified  112 . If valid slot access for the current slot is yet to be verified, then the setting of the ISMA flag for the current slot is checked  124 . If the flag is not set  124 , then a check is made for another available slot  122 . 
     If the flag for the selected slot busy is set  124  or if the slot access has already been verified  112 , then the system continues to transmit on the selected uplink slot  130 . The system decodes the header for the dedicated downlink and all uplink slots and stores available audio data  134 . Audio data in the buffers for the selected downlink slot and all uplink slots containing autonomous PCU data are summed  136  and the audio data is sent to the headset  126 . 
     With particular reference to FIG. 7, a flow chart for the PCU autonomous mode-neighboring network is provided. The PCU autonomous mode-neighboring network is entered from the PCU standby mode, the PCU slave mode, or the PCU autonomous mode-autonomous network. If present, the header for slot D 0  is decoded and UAI data is stored  152 . If a UAI is detected, then the PCU slave mode is entered  156 . If a UAI is not detected, then the headers for all uplink slots are decoded and data for autonomous PCUs is stored  158 . If no uplink slot is detected  160 , then PCU standby mode is entered  162 . If an uplink slot is detected  160 , then a check is made to see of there is data to transmit  164 . If there is no data to transmit  164 , then audio data in the buffers for uplink slots containing PCU data is summed  170  and the resulting audio data is sent  176  to the headset. Then, the system waits for the end of the current frame and hops  182  to a new frequency. 
     When there is data to transmit  164 , or if a beacon interval for autonomous PCU timing has been exceeded, then a check is made to see if the system is currently transmitting  166 . If the system is not current transmitting  166 , then a determination is made to see if a slot is available via the ISMA control fields received in the uplink slots  172 . If no slot is available  178 , then the system sums audio data in buffers for uplink slots containing PCU data  170 . If a slot is available  178 , then the system transmits  184  on a randomly selected available uplink slot. 
     When a check that the system is currently transmitting is positive  166 , then a determination is made to see if a slot collision has occurred  168 . This utilizes information in the headers of uplink PCU transmissions to signal if autonomous PCU data is received. If slot access is not verified, then a check is made for another available slot  178 . If slot access is verified, then the PCU continues to transmit  180  in the selected uplink slot. 
     With particular reference to FIG. 8, a flow chart for the PCU autonomous mode-autonomous network is provided. The PCU autonomous mode-autonomous network  200  is entered only from the PCU standby mode. After entering the PCU autonomous mode-autonomous network  200 , the system decodes the header for slot D 0 , if present, and stores UAI data  202 . If a UAI is detected  204 , then the PCU slave mode  206  is entered. If a UAI is not detected, then the system decodes the header for all uplink slots and stores data for autonomous PCUs  214 . If an uplink slot is not detected  222 , and no PCUs have been detected for a predetermined period of time, then the system enters the PCU standby mode  240 . 
     If an uplink slot was detected  222 , then a check is made to see if there is a UAI in the area  216 . If there is a UAI in the area  216 , then the PCU enters the PCU autonomous mode-neighboring network  208 . If no UAI is in the area, then the autonomous network timing is determined  224 . If there is no data to transmit, then audio data in the buffers for uplink slots containing PCU data is summed  238  and audio data is sent to the headset  242 . Then the system waits for the end of the current frame and hops  244  to a new frequency. 
     If there is data to transmit  230 ,  232 , then a check is made to see if the system is currently transmitting data  210 . If not, then a determination of slot availability is performed using the control fields received in the transmission of other autonomous PCUs  218 ,  226 . If no slot is available, then the PCU sums audio data in the buffers for uplink slots containing PCU data  238 . If a slot is available, then the system transmits on a randomly selected available uplink slot  234 . 
     If the check to see if the system is currently transmitting  210  results in a positive indication, then a determination is made as to whether or not there is a slot collision  212 . This uses information in the header of the received uplink PCU transmissions to signal if the previously autonomous PCU data was received. 
     A check is made to see if access for the selected slot has been verified  220 . If slot access is not verified  220 , then a check is made for additional available slots  226 . If slot access is verified, then the PCU transmits in the selected uplink slot  228 . 
     Referring now to FIG. 16, the present invention preferably comprises a method for relaying communications from a PCU which is out of transmit range with respect to the UAI, but which is within receive range thereof. That is, the PCU is so far away from the UAI that it cannot transmit messages thereto, but the PCU is close enough to the UAI that it can receive messages therefrom. Those skilled in the art will appreciate that this is a common occurrence, since the UAI will typically have a greater power output than the PCUs. 
     When a first PCU is incapable of transmitting to the UAI, a second PCU which is capable of receiving communications from the first PCU and which is also capable of transmitting to the UAI, relays communications from the first PCU to the UAI. 
     The second PCU may sense the need to perform such relaying of communications from the first PCU to the UAI by monitoring communications between the first PCU and the UAI. During such monitoring of the communications between the first PCU and the UAI, the second PCU may notice that the first PCU is not transmitting to the UAI during its allocated time slot. Alternatively, the second PCU may notice the lack of a communication received acknowledgment in the UAIs transmission to the first PCU. As a further alternative, the first PCU may add a flag or other message to it own header indicating its inability to communicate with the UAI and thereby requesting that another PCU relay communications for it. 
     For example, at time T 1  the first PCU transmits to the UAI. Then, at time T 2  the UAI transmits back to the first PCU. However, if the UAI does not acknowledge receipt of the transmission from the PCU at time T 1 , then the first PCU may not transmit during its next allocated time, i.e., at time T 3 . At time T 4  the UAI attempts to again establish communications with the first PCU. 
     When such inability of the first PCU to transmit successfully to the UAI is noticed by a second PCU, which is capable of communicating with the UAI, then the second PCU relays communications between the first PCU and the UAI. 
     It is understood that the exemplary personal communication system described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. Further, various modifications and additions may be obvious to those skilled the art and may be implemented to adapt the present invention for use in a variety of different applications.