Abstract:
The invention provides a method and system for robust topology wireless communication using access points. A wireless communication system includes a communication cell having a base station controller and at least one set of customer premises equipment. The customer premises equipment is disposed within a sector of the cell, and communication between the base station controller and the customer premises equipment is controlled by the base station controller is so as to prevent interference between multiple sets of customer premises equipment. When customer premises equipment is subject to relatively larger amounts of multipath effect, or interference or noise, one or more access points are disposed within the sector, so as to propagate or route communication between the base station controller and one or more sets of customer premises equipment. The access points may range in complexity from simple reflectors, to repeaters, to routing devices operative within the cell.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   This invention relates to robust topology wireless communication using access points. 
   Wireless communication between a sender and a receiver includes sending information using a wireless communication link, in which the sender modulates information onto a wireless communication channel (such as a frequency band reserved for wireless communication between the sender and the receiver), and the receiver demodulates that information from the wireless communication channel, so as to recover the original information. 
   One problem with known systems is that wireless communication channels are subject to several communication problems that make them relatively unreliable in comparison to wireline communication channels. Among these communication problems are interference, random or pseudo random noise, and multi-path effects. For a first example, a wireless communication channel can be subject to information sent on other wireless communication channels having nearby frequencies or other similar modulation formats. For a second example, a wireless communication channel can be subject to random or pseudo random noise, such as might be generated by ambient noise sources. For a third example, the information sent on a wireless communication channel can be reflected or refracted so as to arrive at its destination at more than one time and with varying degrees of attenuation. Of course, a most serious communication problem involves complete lack of communication between sender and receiver due to an obstruction on the line of sight path between sender and receiver. 
   One known method to account for these communication problems is to attempt to determine the nature of the communication problems likely to plague the wireless communication channel between a selected sender and receiver, and to attempt to reverse the effects of these communication problems. For a first example, the effects of interference and noise can be alleviated in part by error correcting codes and other known techniques of communication using noisy channels. For a second example, multi-path effects can be alleviated in part by attempting to determine an impulse response of the wireless communication link, and to invert that impulse response at either the sender or the receiver, so as to cancel those multi-path effects in the signal eventually presented to the receiver. 
   Although these known methods generally achieve the result of improving communication on wireless communication links, they are subject to several drawbacks. First, these known methods are severely limited in their ability to provide wireless communication to locations not reachable by point to point links, such as locations behind obstructions like buildings or mountains, or locations at nodes in an interference or reflection pattern at the selected communication frequency. Second, these known methods are severely degraded with increases in interference or noise (of any kind), and can at best achieve communication capability associated with a noiseless communication link. Moreover, none of these known methods achieve the capability of solving the problem of complete lack of communication between sender and receiver due to an obstruction on the line of sight path between sender and receiver. 
   Accordingly, it would be advantageous to provide a technique for wireless communication, such as for use in a wireless communication system, that is not subject to drawbacks of the known art. 
   SUMMARY OF THE INVENTION 
   The invention provides a method and system for robust topology wireless communication using access points. In a preferred embodiment, a wireless communication system includes a communication cell having a base station controller and at least one set of customer premises equipment. The customer premises equipment is disposed within a sector of the cell, and communication between the base station controller and the customer premises equipment is controlled by the base station controller is so as to prevent interference between multiple sets of customer premises equipment. When customer premises equipment is subject to relatively larger amounts of interference or noise, one or more access points are disposed within the sector, so as to propagate or route communication between the base station controller and one or more sets of customer premises equipment. In aspects of the invention described herein, the access points may range in complexity from simple reflectors, to repeaters, to routing devices operative within the cell. 
   The invention provides an enabling technology for a wide variety of applications for wireless communication, so as to obtain substantial advantages and capabilities that are novel and non-obvious in view of the known art. Examples described below primarily relate to cellular wireless communication systems, but the invention is broadly applicable to many different types of wireless and other communication systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a first block diagram of a system for wireless communication using robust topology. 
       FIG. 2  shows a second block diagram of a system for wireless communication using robust topology. 
       FIG. 3  shows a third block diagram of a system for wireless communication using robust topology. 
       FIG. 4  shows a fourth block diagram of a system for wireless communication using robust topology. 
       FIG. 5  shows a fifth block diagram of a system for wireless communication using robust topology. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the following description, a preferred embodiment of the invention is described with regard to preferred process steps and data structures. Embodiments of the invention can be implemented using general-purpose processors or special purpose processors operating under program control, or other circuits, adapted to particular process steps and data structures described herein. Implementation of the process steps and data structures described herein would not require undue experimentation or further invention. 
   RELATED APPLICATIONS 
   Inventions described herein can be used in conjunction with inventions described in the following documents.
         U.S. patent application Ser. No. 09/475,642, Express Mail Mailing No. EL524 780 018 US, filed Dec. 30, 1999, in the name of of Varma, Subir, Ngo, Khuong, Fuentes, Jean, Truong, Paul, and Majidi-Ahy, Reza, titled “Adaptive Link Layer for Point to Multipoint Communication System.”   U.S. patent application Ser. No. 09/475,716, Express Mail Mailing No. EL 524 780 021 US, filed Dec. 30, 1999, in the name of Majidi-Ahy, Reza, Hakim, Joseph, and Varma, Subir, titled “Integrated Self-Optimizing Multi-Parameter and Multi-Variable Point to MultiPoint Communication System.”       

   Each of these documents is hereby incorporated by reference as if fully set forth herein. These documents are sometimes collectively referred to herein as the “Incorporated Disclosures.” 
   Lexicography 
   The following terms refer or relate to aspects of the invention as described below. The descriptions of general meanings of these terms are not intended to be limiting, only illustrative.
         access point in general, a device for performing message forwarding (either passively or actively) within a wireless communication cell. There is no particular requirement that an access point must be a single device; in alternative embodiments, the access points can each include a portion of a single device, a combination of multiple devices, or some hybrid thereof. Access points are sometimes referred to herein as “broadband” access points, to indicate their generic ability to handle broadband communication instead of, or in addition to, narrowband communcation.   base station controller—in general, a device for performing coordination and control for a wireless communication cell. There is no particular requirement that the base station controller must be a single device; in alternative embodiments, the base station controller can include a portion of a single device, a combination of multiple devices, or some hybrid thereof.   cell—in general, a region associated with a base station controller in which customer premises equipment communicate with the base station controller and in which the base station controller exercises control over communication parameters. Although in a preferred embodiment, cells are each contiguous and compact, there is no particular requirement for either property; cells may include non-contiguous sub-regions and are not necessarily compact.   communication link—in general, an element for sending information from a sender to a recipient. Although in a preferred embodiment the communication links referred to are generally wireless line of sight point-to-point communication links, there is no particular requirement that they are so restricted.   customer premises equipment—in general, a device for performing communication processes and tasks at a customer location, and operating in conjunction with the base station controller within a wireless communication cell. There is no particular requirement that the customer premises equipment must be a single device; in alternative embodiments, the customer premises equipment can include a portion of a single device, a combination of multiple devices, or some hybrid thereof.   MAC parameters—in general, with reference to a wireless communication link, a set of characteristics or parameters relating to media access control of a communication link. For example, MAC parameters can include (a) a number of payload data bytes assigned per message, (b) a frequency of acknowledgement messages and a number of message retransmission attempts, (c) a fraction of the communication link allocated to downstream versus upstream communication, and the like.   physical parameters—in general, with reference to a wireless communication link, a set of characteristics or parameters relating to physical transmission of information on a communication link. For example, physical characteristics can include (a) a symbol transmission rate, (b) a number of payload data bits assigned per symbol, (c) a number of error detection or correction bits assigned per symbol, and the like.   propagate or route—in general, to forward a message from a sender toward a destination using an intermediate point, such as an intermediate reflector, repeater, or routing or switching element   sector—in general, a portion of a cell that is not the entire cell. For example, a cell can be divided into multiple sectors in response to an angle of direction or a distance from the base station controller. There is no particular requirement that sectors must be disjoint; they may be overlapping.   wireless communication system—in general, a communication system including at least one communication link that uses wireless communication techniques.   wireless transport layer—in general, a set of protocols and protocol parameters for sending and receiving information using wireless transport. In a preferred embodiment, the wireless transport layer is part of a multilayer systems architecture, in which the wireless transport layer is built using a physical transport layer, and the wireless transport layer is used by a logical transport layer such as IP.       

   As noted above, these descriptions of general meanings of these terms are not intended to be limiting, only illustrative. Other and further applications of the invention, including extensions of these terms and concepts, would be clear to those of ordinary skill in the art after perusing this application. These other and further applications are part of the scope and spirit of the invention, and would be clear to those of ordinary skill in the art, without further invention or undue experimentation. 
   System Context 
   A system using point to multipoint wireless communication in a wireless communication system operates as part of a system in which devices coupled to a network (such as a computer network) send messages, route and switch messages, and receive messages. In a preferred embodiment, devices coupled to (and integrated with) the network send, route, and receive these messages as sequences of packets, each of which has a header including delivery information and a payload including data. In a preferred embodiment, packet format conforms to the OSI model, in which relatively higher-level protocols such as an application protocol (layer 5, such as FTP), use relatively lower-level protocols such as a transport protocol (layer 4, such as TCP), which uses a network protocol (layer 3, such as IP), which uses a media access control (MAC) protocol (layer 2), which uses a physical transport technique (layer 1). In a preferred embodiment, packet format conforms to the OSI model, in which relatively lower-level protocols use higher-level protocols to recover from communication difficulties. 
   The system for wireless communication using robust topology is described herein with regard to layer 1 and layer 2, particularly as it applies to interactions between layer 1 and layer 2 and between those layers and layer 3. However, concepts and techniques of the invention are also applicable to other layers of the OSI model. Adapting those concepts and techniques to such other layers would not require undue experimentation or further invention, and is within the scope and spirit of the invention. 
   System Elements 
     FIG. 1  shows a first block diagram of a system for wireless communication using robust topology. 
   A system  100  includes a wireless communication cell  110 , a base station controller  120 , and one or more sets of customer premises equipment  130 . The system  100  also includes a set of more than one sector  111  within the cell  110 , a set of zero or more access points  140  within each sector  111 , and a set of packets  150  sent among the elements of the system  100 . 
   The wireless communication cell  110  includes a generally hexagon-shaped region of local surface area, such as might be found in a metropolitan region. Use of generally hexagon-shaped regions is known in the art of wireless communication because they are able to tile a local region with substantially no gaps. However, although in a preferred embodiment the wireless communication cell  110  includes a generally hexagon-shaped region, there is no particular requirement for using that particular shape; in alternative embodiments it may be useful to provide another shape or tiling of the local surface area. 
   The base station controller  120  includes a processor, program and data memory, mass storage, and one or more antennas for sending or receiving information using wireless communication techniques. 
   Similar to the base station controller  120 , each set of customer premises equipment  130  includes a processor, program and data memory, mass storage, and one or more antennas for sending or receiving information using wireless communication techniques. 
   Communication among devices within the wireless communication cell  110  is conducted on one-to-one basis between each set of customer premises equipment  130  and the base station controller  120 . Thus, the base station controller  120  communicates with each set of customer premises equipment  130 , and each set of customer premises equipment  130  communicates with the base station controller  120 . Customer premises equipment  130  do not communicate directly with other customer premises equipment  130 . 
   Communication between the base station controller  120  and each set of customer premises equipment  130  is conducted using a time division duplex technique, in which time duration is are divided into individual frames, each one of which includes a “downstream” portion and an “upstream” portion. Unlike existing protocols in which transmissions are controlled by the transmitting side, the base station controller  120  controls tranmissions for both upstream and downstream directions, without specific requests from the customer premises equipment  130 . 
   During the downstream portion of each frame, the base station controller  120  transmits, thus sending information to one or more sets of customer premises equipment  130 . During the upstream portion of each frame, the base station controller automatically allocates slots for upstream re-transmission. This occurs in the absence of an explicit request for a slot from the customer premises equipment  130 . Particular aspects of each frame are described in the Incorporated Disclosures. Time division duplex techniques are known in the art of wireless communication. 
   Access Points  1   
   In a first block diagram shown in  FIG. 1 , each access point  140  includes a reflector  141 . Reflectors are known in the art of wireless communication. 
   Each packet  150  sent from the base station controller  120  to the customer premises equipment  130  can be sent line-of-sight to an access point  140 , which can include a reflector  141 , rather than line-of-sight from the base station controller  120  to the customer premises equipment  130 . The reflector  141  at the access point  140  causes the electromagnetic signal carrying the packet  150  to be reflected from a first outbound path  151  to a second outbound path  152 , so that the packet  150  reaches the customer premises equipment  130  in better form than using a line-of-sight path between the base station controller  120  to the customer premises equipment  130 . 
   Similarly, each packet  150  sent from the customer premises equipment  130  to the base station controller  120  can be sent line-of-sight to an access point  140 , which can include a reflector  141 , rather than line-of-sight from the customer premises equipment  130  to the base station controller  120 . The reflector  141  at the access point  140  causes the electromagnetic signal carrying the packet  150  to be reflected from a first inbound path  153  to a second inbound path  154 , so that the packet  150  reaches the base station controller  120  in better form than using a line-of-sight path between the customer premises equipment  130  to the base station controller  120 . 
   Access Points  2   
     FIG. 2  shows a second block diagram of a system for wireless communication using robust topology. 
   In a second block diagram shown in  FIG. 2 , each access point  140  includes a repeater  142 . Repeaters are known in the art of wireless communication. Each repeater  142  includes a reconditioning element  143 , such as including amplifiers and noise-removal circuits. 
   Similar to access points  140  including reflectors  141 , each packet  150  sent from the base station controller  120  to the customer premises equipment  130  can be sent line-of-sight to an access point  140 , rather than line-of-sight from the base station controller  120  to the customer premises equipment  130 . At the access point  140 , the packet  150  is received, reconditioned (for example, noise is reduced and the signal is re-amplified), and resent from the access point  140  to the destination customer premises equipment  130 , so that the packet  150  reaches the customer premises equipment  130  in better form than using a non-reconditioned path between the base station controller  120  and the customer premises equipment  130 . 
   Similarly, access points  140  using repeaters  142  can also be used for sending packets from the customer premises equipment  130  to the base station controller  120 . 
   Access points  140  can include both reflectors  141  and repeaters  142 , and can include combinations thereof, so that a packet  150  can sent line-of-sight to an access point  140 , reconditioned at that access point  140  using a repeater  142  or variant thereof, and resent line-of-sight to its destination (either the base station controller  120  or the customer premises equipment  130 ). 
   Access Points  3   
     FIG. 3  shows a third block diagram of a system for wireless communication using robust topology. 
   In a third block diagram shown in  FIG. 3 , each access point  140  includes a routing or switching device  143  operative within a sector  111  or within a cell  110 . Routing or switching devices  143  are known in the art of communication networks. 
   Similar to access points  140  including reflectors  141 , each packet  150  sent from the base station controller  120  to the customer premises equipment  130  can be sent line-of-sight to an access point  140 , rather than line-of-sight from the base station controller  120  to the customer premises equipment  130 . At the access point  140 , the packet  150  is received, routed (for example, the destination customer premises equipment  130  is determined and the packet  150  is resent line-of-sight to that destination customer premises equipment  130 ), and resent from the access point  140  to the destination customer premises equipment  130 , so that the packet  150  reaches the customer premises equipment  130  in better form than using a non-routed path between the base station controller  120  and the customer premises equipment  130 . 
   Similarly, access points  140  using routing or switching devices  143  can also be used for sending packets from the customer premises equipment  130  to the base station controller  120 . 
   Access points  140  can include any one of reflectors  141 , repeaters  142 , and routing or switching devices  143 , and can include combinations thereof, so that a packet  150  can sent line-of-sight to an access point  140 , routed or switched at that access point  140  using a routing or switching device  143  or variant thereof, reconditioned at that access point  140  using a repeater  142  or variant thereof, and resent line-of-sight to its destination (either the base station controller  120  or the customer premises equipment  130 ). 
   Moreover, routing or switching devices  143  can be used to enforce restrictions on communication between the base station controller  120  and the customer premises equipment  130 , using either ACL (access control list) restrictions, QoS (quality of service) restrictions, or other restrictions known in the art of communication networks. Those skilled in the art will find, after perusal of this application, that the possibility of enforcing such restrictions is part of the scope and spirit of the invention, and would not require undue experimentation or further invention. 
   Access Points  4   
     FIG. 4  shows a fourth block diagram of a system for wireless communication using robust topology. 
   Similar to the third block diagram shown in  FIG. 3 , in a fourth block diagram shown in  FIG. 4 , each access point  140  includes a routing or switching devices  143  operative within a sector  111  or within a cell  110 . 
   Similar to access points  140  including routing or switching devices  143  that route or switch packets  150  directly to destination customer premises equipment  130 , each packet  150  sent from the base station controller  120  to the customer premises equipment  130  can be sent line-of-sight to an access point  140 , rather than line-of-sight from the base station controller  120  to the customer premises equipment  130 . At the access point  140 , the packet  150  is received, routed (for example, the destination customer premises equipment  130  is determined and the packet  150  is resent line-of-sight to a second access point  140 ), and resent from the access point  140  to that second access point  140 . From the second access point  140 , the packet  150  is again received, routed (for example, the destination customer premises equipment  130  is determined and the packet  150  is resent line-of-sight to the destination customer premises equipment  130 ), so that the packet  150  reaches the customer premises equipment  130  in better form than using a non-routed path between the base station controller  120  and the customer premises equipment  130 . 
   Similarly, access points  140  using routing or switching devices  143  can also be used for sending packets from the customer premises equipment  130  to the base station controller  120 , by way of intermediate access points  140  (whether including routing or switching devices  143  or not). 
   Access Points  5   
     FIG. 5  shows a fifth block diagram of a system for wireless communication using robust topology. 
   Similar to the third block diagram shown in  FIG. 3 , and similar to the fourth block diagram shown in  FIG. 4 , in a fifth block diagram shown in  FIG. 5 , each access point  140  includes a routing or switching devices  143  operative within a sector  111  or within a cell  110 . 
   Similar to access points  140  described above, those access points  140  including routing or switching devices  143  can route or switch packets  150  among themselves, so as to route or switch packets  150  from the base station controller  120  to a set of intermediate access points  140  to the customer premises equipment  130 . Multi-hop routing of packets  150  is known in the art of communication networks. 
   At each access point  140 , the packet  150  is received, routed, and resent from the access point  140  to a next hop. From the next hop, the process is repeated, until the packet  150  is sent line-of-sight to the customer premises equipment  130 , and thus reaches the customer premises equipment  130  in better form than using a non-routed path between the base station controller  120  and the customer premises equipment  130 . 
   In a preferred embodiment, each access point  140  includes a routing or switching device  143  capable of performing at least some of the following functions:
         IP routing—routing or switching devices  143  can route packets  150  using IP layer routing or switching   MAC routing—routing or switching devices  143  can route packets  150  using MAC layer routing or switching   Re-Adaptation—routing or switching devices  143  can re-adapt packets  150 , such as breaking packets  150  into smaller units and collecting smaller units into larger packets  150     Mini-Scheduler—routing or switching devices  143  can include a scheduling function for determining which packets  150  to give priority to in routing, re-adaptation, and resending       

   Routing and switching devices  143  can also be configured so as to route or switch only within a single sector  111  within a cell  110 , or to route or switch among multiple sectors  111  within a cell  110 . 
   Access Points (Further Generality) 
   Those skilled in the art will recognize, after perusal of this application, that the architectures shown in the figures and described herein can be substantially generalized in several ways. 
   First, the addition of access points  140  within a cell  110  allows the base station controller  120  to add additional capacity for sending and receiving messages in conjunction with customer premises equipment  130 . Thus, the set of access points  140  (using reflectors  141 , repeaters  142 , or routing or switching devices  143 ) can form a mesh network in which messages are routed between the base station controller  120  and customer premises equipment  130 , using the set of access points  140  as a communication sub-network. The communication sub-network is thus simultaneously (1) a distributed network of intermediate nodes in a switching network, and (2) a centralized switching system under control of the base station controller  120 . For example, the set of access points  140  each operate independently to perform routing and switching (possibly including other aspects of routing and switching, such as scheduling, quality of service guarantees, and flow management), while the base station controller  120  controls the PHY and MAC parameters (as shown in the Incorporated Disclosures) of each of the access points  140 . The base station controller  120  can control each of the access points  140  separately, or can determine global parameters for use within a cell  110  or sector within a cell  110 . 
   Second, each access point  140  can perform some or all of the functions of the base station controller  120 , only limited to a portion of the cell  110 . Thus, within a single sector within a single cell  110 , the base station controller  120  and the set of access points  140  within that sector can cooperate to control traffic, so as to form a micro-cell within the cell  110 . There is no particular requirement that any one micro-cell must be entirely within a single sector; micro-cells can span portions of multiple sectors or even portions of multiple cells  110 . This also allows the base station controller  120  and the set of access points  140  to cooperatively maintain a multi-tier cellular architecture, with each micro-cell each controlled by an associated access point  140  and cells  110  including multiple micro-cells controlled by the base station controller  120 . 
   Third, the addition of access points  140  within a cell  110  allows the base station controller  120  to add incremental additional capacity for sending and receiving messages and controlling the flow of messages within the cell  110 . This enables the wireless system to grow in incremental steps, rather than large jumps in capacity, so as to create a “pay as you grow” deployable system. There is no particular requirement that the addition of access points  140  within a cell  110  requires the base station controller  120  itself to have additional capacity or functionality. 
   Fourth, there is no particular requirement that communication paths must be line of sight. Although in a preferred embodiment, most electromagnetic communication is line of sight, it is also possible for electromagnetic communication to include non line of sight components. A first example is differential penetration of barriers at different frequencies, such as buildings which pose a substantial reflection problem at a first frequency f 1  but do not pose a substantial reflection problem at a second frequency f 2 . A second example is refraction of electromagnetic communication at known frequencies so as to obtain a non line of sight path. A third example is multipath addition resulting in communication between a first peak and a second peak in a communication standing wave. Those skilled in the art will recognize, after perusal of this application and the Incorporated Disclosures, that the base station controller  120  can manipulate parameters for the PHY, MAC and other layers to improve communication with access points  140  even under non line of sight conditions. 
   Fifth, the addition of access points  140  within a cell  110  presents a new opportunity for the base station controller  120  to control quality of service (QOS) and flow management within the cell  110 . The base station controller  120  can thus control QOS between the base station controller  120  and customer premises equipment  130 , either by (1) controlling specific PHY and MAC parameters for direct communication, (2) controlling specific PHY and MAC parameters for indirect communication using access points  140 , or (3) selection of the number and location of access points  140  for indirect communication with customer premises equipment  130 . Thus, for example, the base station controller  120  can choose, instead of a shortest-path strategy, a best-QOS strategy for routing messages using access points  140  to customer premises equipment  130 . 
   Sixth, those skilled in the art will recognize, after perusal of this application and the Incorporated Disclosures, that the addition of access points  140  within a cell  110  makes it easier to deploy the wireless communication system. The base station controller  120  can be located in many additional locations, as can customer premises equipment  130 . Moreover, since the base station controller  120  controls the PHY, MAC and other parameters, it is easier to deploy the wireless communication system and obtain acceptable communication between the base station controller  120  and customer premises equipment  130  using access points  140 . 
   Sixth, those skilled in the art will recognize, after perusal of this application and the Incorporated Disclosures, that the addition of access points  140  within a cell  110  makes it easier to use the wireless communication system in conjunction with requirements for QOS and flow management. Since the base station controller  120  controls the PHY, MAC and other parameters, it is easier to manage the wireless communication system for QOS and flow management and obtain acceptable communication between the base station controller  120  and customer premises equipment  130  using access points  140 . 
   Generality of the Invention 
   The invention has general applicability to various fields of use, not necessarily related to the services described above. For example, these fields of use can include one or more of, or some combination of, the following:
         The invention is applicable to other forms of wireless communication, such as code division multiple access (CDMA, also known as spread spectrum communication), frequency division multiple access (FDMA), space division multiple access (SDMA), time division multiple access (TDMA), and combinations thereof;   The invention is applicable to wireline (that is, non-wireless) communication, in which now can be achieved from dynamically adjusting communication parameters, such as physical parameters or MAC parameters. For example, the invention can be generalized to wireline communication using modems in which equalization parameters are to be dynamically adjusted.   The invention is applicable to other wireless communication systems, such as satellite communication systems and (microwave tower or other) point to point transmission systems.   The invention is applicable to both fixed wireless communication systems, in which customer premises equipment do not move relative to the base station controller  120 , and to mobile wireless communication systems, and which customer premises equipment move substantially relative to the base station controller  120 .       

   Other and further applications of the invention in its most general form, will be clear to those skilled in the art after perusal of this application, and are within the scope and spirit of the invention. 
   ALTERNATIVE EMBODIMENT 
   Although preferred embodiments are disclosed herein, many variations are possible which remain within the concept, scope, and spirit of the invention, and these variations would become clear to those skilled in the art after perusal of this application.