Abstract:
A network system comprises a first logic block configured to provide a link to a first network via an access point of a wireless LAN and a second logic block configured to communicate with a first node of a second network and further configured to provide a link between the first node and the first network via the access point, wherein the network system is configured to maintain continuous connections to both the access point and the first node of the second network while receiving power. The second network can be a wireless personal area network with the second logic block configured to communicate with the first node using a modified communication protocol that is only partially compliant with an 802.11x communications protocol. Also provided is a wireless hub for integrating a wireless Personal Area Network (WPAN) seamlessly into a wireless Local Area Network (WLAN) includes, in part, a wireless circuit compliant with the WLAN standard, a processor coupled to the wireless circuit and a memory module that is coupled to the wireless circuit and the processor. The WLAN standard can be an 802.11x standard. The software platform allows the wireless circuit to connect to the WPAN, without losing connectivity (such as association and synchronization) to the WLAN. The wireless circuit can be configured to connect to the WLAN and WPAN alternately.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/376,729, filed Mar. 14, 2006, which claims the benefit of and is a non-provisional of U.S. Patent Application Ser. No. 60/661,763 filed on Mar. 14, 2005, which is incorporated by reference in its entirety for all purposes, and incorporates by reference herein application Ser. No. 11/376,753, filed Mar. 14, 2006, hereinafter referred to as “Vleugels 1”. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to wireless communications. More particularly, the invention relates to seamlessly integrating short-range wireless Personal Area Networks (PANs) into longer-range Wireless Local Area Networks (WLANs). 
     BACKGROUND OF THE INVENTION 
       FIG. 1  depicts some parameters associated with a few existing and emerging standards for wireless connectivity. Based on targeted range and supported data rates, these standards can be grouped into four categories: Wireless Wide Area Networks (WWAN), Wireless Metropolitan Area Networks (WMAN), Wireless Local Area Networks (WLAN) and Wireless Personal Area Networks (WPAN). 
     An example of a Wireless Local Area Network (WLAN) is an 802.11x (x=a, b, g, n, etc.) network. An 802.11x NIC (network interface card) or 802.11x built-in circuitry might be used for networking an electronic device to the outside world, or at least to devices at other nodes of a WLAN 802.11x network. 
     The 802.11x specifications uses unlicensed, free spectrum in either the 2.4 GHz or 5 GHz frequency bands, supporting data rates of up to 54 Megabits per second (Mbps) and ranges of 300 feet and more. The 802.11x standard, also known as Wi-Fi, was adopted several years ago, and is now being widely deployed for WLAN connectivity in homes, offices and public places like airports, coffee shops and university campuses. 
     The adoption and deployment of 802.11x-compliant equipment has experienced tremendous growth in recent years. The majority of laptops manufactured today include a built-in wireless circuit compliant with some variant of the 802.11x standard. While originally devised for enabling wireless network connectivity (“wireless Ethernet”), WLAN connectivity based on the 802.11x standard is rapidly finding its way in new applications like mobile phones—primarily driven by the adoption of Voice-over-IP (VoIP)—and consumer electronics (home entertainment, video streaming etc.). In addition, with the development of the new 802.11xn specification, and the proliferation of citywide 802.11x deployment initiatives, the 802.11x standard is expanding into longer range applications. 
       FIG. 2  illustrates a typical 802.11x WLAN configuration in infrastructure mode  1 . Although the 802.11x standard supports two modes of operation, namely ad-hoc mode and infrastructure mode, the infrastructure mode is used more often. In the infrastructure mode, a dedicated 802.11x wireless circuit, also called an access point (AP), is necessary for and manages an infrastructure network. AP  2  is configured specifically to coordinate the activities of the infrastructure network and to enable connectivity to, for example, the Internet or other WLANs via an Internet router  3 , which may be disposed in AP  2 . Other 802.11x-compliant wireless circuits, hereafter alternatively referred to as stations (STAs)  4  can become a member of the infrastructure network by going through an authentication and association procedure. Additional security procedures may be required as well. Once associated with the infrastructure network, a STA  4  can communicate with AP  2 . A STA  4  may communicate with other STAs  4  of infrastructure network  1  via AP  2 . Furthermore, a STA  4  may communicate with STAs of other infrastructure networks (not shown) via AP  2 . On a regular basis, the STAs listen to the beacons and pending traffic from the AP  2 . 
     In contrast to WLAN, no such unifying standard exists for WPAN. Instead, a number of proprietary and standardized communication protocols have been and are being developed for establishing short-range WPAN connectivity. Standardized protocols include the Bluetooth specification (based on the IEEE 802.15.1 standard), the recently approved Zigbee specification (based on the IEEE 802.15.4 standard), and the Ultra-Wideband (UWB) specification which is still under development. In addition, there are several proprietary protocols in the unlicensed 27 MHz, 900 MHz, and 2.4 GHz frequency bands developed for the sole purpose of providing short-range wireless connectivity. Examples include Cypress Semiconductor&#39;s proprietary wireless USB solution, or Logitech&#39;s proprietary FastRF solution. The lack of a unified standard is hindering the widespread adoption of WPAN technologies. In addition, several WPAN communication protocols co-exist in the same 2.4-GHz frequency band as a commonly used version of the WLAN protocol. Because they use different methods of accessing the wireless medium, and are not synchronized with one another, severe interference may result when devices conforming to such standards are made to co-exist and are positioned in the same physical vicinity. 
     One alternative for avoiding the above mentioned problems when seeking to establish interoperability between WPAN and WLAN networks, is to use network interface circuitry based on the WLAN protocol in WPAN STAs. However, the power dissipation of the resulting STA would be several orders of magnitude higher than what is acceptable in typical WPAN applications. WPAN technologies are typically used to establish communication with a remote battery-operated device for which it is inconvenient, impractical, or may be impossible to replace batteries. Examples include security sensors in windows, wearable or implanted medical monitoring devices or environmental sensors to monitor temperature, humidity or other environmental parameters. To minimize the frequency at which batteries need replacement, maximizing the battery life is of paramount importance, thus placing stringent requirements on the power that can be dissipated in establishing and maintaining the wireless communication link. 
     The power dissipation of a standard WLAN STA is several orders of magnitude higher than what is acceptable in most battery-operated devices for a number of reasons. First, in order to be able to communicate with the AP, which may be, for example, 300 feet away, a standard WLAN STA transmits at high transmit powers (up to 20 dBm) and is also required to receive relatively weak signals, attenuated heavily by the path loss it encounters in the over-the-air transmission. Second, the WLAN must adhere to stringent receiver sensitivity requirements. Both the transmit and receive requirements result in relatively large power dissipation in the network interface circuits. Furthermore, WLANs typically operate at relatively high data rates (up to 54 Mbps). It is thus undesirable to have a STA that is part of an infrastructure network to communicate at lower data rates, since such a STA will slow down the entire infrastructure network. This is the case because some of the communication between the AP and its associated STAs occurs at the lowest common data rate supported by all STAs. The noise and linearity requirements associated with transmitting at high data rates thus result in large power dissipation of the wireless 802.11x wireless circuit. Furthermore, there is significant protocol overhead associated with the services and procedures required to establish and maintain an association with an infrastructure network. This overhead translates directly in higher power dissipation. As a member of an infrastructure network coordinated by an AP, the STA has, on a regular basis, to listen to the beacons transmitted by the AP. Also, although the 802.11x standard specifies power save modes that allow the STA to skip some of the beacons, the STA is still required to wake up on a regular basis to maintain association and synchronization with the AP. 
     Accordingly, a need continues to exist for a method and apparatus that overcome the above-described problems and enable seamless integration of WPAN into WLAN infrastructure, and at power dissipation levels that meet the stringent requirements of battery-operated devices. 
     BRIEF SUMMARY OF THE INVENTION 
     A wireless hub for integrating a wireless Personal Area Network (WPAN) seamlessly into a wireless Local Area Network (WLAN) includes, in part, a wireless circuit compliant with the WLAN standard, a processor coupled to the wireless circuit and a memory module that is coupled to the wireless circuit and the processor. 
     In some embodiments, the WLAN standard is 802.11x standard. In such embodiment, the wireless circuit is an 802.11x-compliant wireless circuit, and the memory module may be integrated with the wireless circuit. The hub further includes software modules forming a software platform that allows the wireless circuit to connect to both the WPAN and WLAN. In accordance with one embodiment, the software platform allows the wireless circuit to connect to the WPAN, without losing connectivity (such as association and synchronization) to the WLAN. In another embodiment, the wireless circuit is configured to connect to the WLAN and WPAN alternately. In some embodiments, an operating system enables the operation of the wireless hub, thereby enabling users to write application-specific application software. The operating system may be Windows XP, Windows CE, Linux, Symbian, or the like, that may be used to develop additional applications. 
     In accordance with one embodiment, the wireless hub is seamlessly integrated into an electrical power outlet. This allows the hub to be unobtrusively and conveniently integrated in a home, business or industrial setting. Such embodiments are hereinafter alternatively referred to as “Wi-Fi-enabled power outlets”. As is known, “Wi-Fi” is often used to refer to “wireless fidelity”, and refers to 802.11x-based radio technologies. 
     Advantageously, the present invention extends the communication range of power-sensitive battery-operated devices and allows power-sensitive battery operated devices to become part of the larger WLAN infrastructure, thus enabling monitoring and control from any location that is within the range covered by the WLAN In addition, since battery-operated devices are IP addressable and since the AP of the WLAN can be connected to the Internet via an Internet router, the battery-operated devices may be monitored and controlled from any location when access to the Internet is available. The longer communication range and seamless integration into the larger WLAN infrastructure is obtained without incurring the power penalty that is typically unavoidable in longer range communication and is inherent to the protocol overhead of typical WLAN networks. 
     Other objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features throughout the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a number of parameters associated with a few existing and emerging standards for wireless connectivity, as known in the prior art. 
         FIG. 2  illustrates some of different components of an 802.11x WLAN in infrastructure mode, as known in the prior art. 
         FIG. 3  illustrates an apparatus configured to integrate a wireless Personal Area Network into a wireless Local Area Network, in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates a number of wireless Personal Area Networks integrated into a wireless Local Area Network, in accordance with one embodiment of the present invention. 
         FIG. 5  is a simplified high-level block diagram of a power-sensitive station (PS-STA), in accordance with an embodiment of the present invention 
         FIG. 6  is a simplified high-level block diagram of a wireless hub configured for use as a bridge between a wireless Personal Area Network and a wireless Local Area Network. 
         FIG. 7  illustrates a wireless personal Area Network used for remote monitoring and controlling, in accordance with one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 3  illustrates a wireless personal are network (WPAN)  10  integrated with wireless Local Area Network (WLAN)  6  to form an integrated network  5 , in accordance with one embodiment of the present invention. In the embodiments described below, WLAN  6  is compliant with the 802.11x specification. It is understood, however, that the WLAN may be compliant with other protocols, such as WiMax. WLAN  6  may operate either in ad-hoc or in infrastructure mode. Moreover, the following description is provided with reference to the infrastructure mode of operation of WLAN  6 . It is understood that the present equally applies to the ad-hoc or any other mode. The infrastructure WLAN  6  is shown as including an AP  7  and one or more STAs  8 . STAs  8  are associated with and synchronized to AP  7  and periodically listen to beacons from AP  7 . 
     Each STA  8  is configured to include an 802.11x-compliant wireless circuit, such as a wireless enabled computer, a wireless Personal Digital Assistant, a Wi-Fi enabled cellular phone, or the like. The AP  2  can be connected to the Internet via an Internet router  9 . Internet connectivity can be established through any number of communication services, including Digital Subscriber Line (DSL), cable, satellite, or the like, as is well known 
     WPAN  10  is shown as including comprises one or more power-sensitive stations  11  (PS-STA). A PS-STA is defined herein as a device that is battery-operated and for which maximizing battery-life is beneficial to the application and/or user. Examples of PS-STAs include peripherals and accessories for personal computers, cellular phones, home entertainment accessories such as remote controls, monitoring devices for security, automation medical applications, or the like. 
     In accordance with one embodiment, a PS-STA is typically in a sleep mode the majority of the time, only waking up occasionally to communicate and exchange information with the outside world. In some systems described herein, each PS-STA  11  is equipped with a wireless circuit that can communicate directly with a standard 802.11x-compliant wireless circuit. PS-STAs  11  however are not required to be fully compliant with the 802.11x specification; some PS-STAs  11  may have reduced power dissipation thereby extending the battery life. 
     In embodiments in which PS-STAs  11  are not fully compliant with the 802.11x specification, the drivers or firmware of the 802.11x-compliant wireless circuit at the other end of the communication link (i.e., the device with which the PS-STA is interacting) may require modification. Thus, in some implementations, both the wireless circuit at the other end as well as the PS STA are both 802.11x-compliant, while in others the wireless circuit at the other end is 802.11x-compliant, but the PS-STA is not a fully compliant 802.11x wireless circuit, while in yet other implementations the driver or firmware of the 802.11x-compliant wireless circuit at the other end of the link requires modifications to accommodate the PS-STA. Integrated network  5  is also shown as including a wireless hub  12  adapted to facilitate seamless communication between the WLAN and the WPAN. The wireless hub  12  includes, in part, a wireless 802.11x-compliant wireless circuit that can communicate with the AP  7  disposed in infrastructure WLAN  6  as well as with PS-STAs  11  disposed in WPAN  5 . If more than one PS-STA is present in the WPAN, the wireless hub coordinates the timing and communication with each of the PS-STAs. In some embodiments, it may be desirable to shift as much as possible of the protocol overhead associated with the communication between wireless hub  12  and the PS-STAs  11  such as, for example, access to the medium, reservation of the medium, synchronization, etc., onto the wireless hub  12 , where power consumption is much less of a concern compared to the PS-STA. In such cases, the driver or firmware of the 802.11x-compliant components disposed in wireless hub  12  may require modification 
     To operate, wireless hub  12  is placed within the range of the AP  7  of the infrastructure WLAN  6 ; this range is typically on the order of 300+ feet. The wireless hub  12  is also be placed within the range of each of the PS-STAs  11  in the WPAN  10  The PS-STAs  11  typically have a range of about 30 feet. This range can be longer or shorter depending on the application. 
     In one embodiment, the wireless hub  12  (alternatively referred to herein below as a “hub”) is seamlessly integrated within an electrical power outlet. In a different embodiment, the hub can be a separate device that can be plugged into a power outlet. The wireless hub  12  can also be integrated inside other electronic devices, such as light bulbs, light switches, thermostats, energy meters, personal computers, Personal Digital Assistants (PDAs), cellular phones, home entertainment equipment and the like. 
     In some embodiments, a multitude of WPANs  13  may be so configured so as to be coupled to and in communication with a single WLAN  14 , as shown in  FIG. 4 . Each WPAN  13  is coupled to the WLAN  14  by using a wireless hub  15 , as described above. If WPANs  13  are configured to operate independently, no additional coordination is required and each wireless hub  15  decides autonomously when to communicate with each of its respective PS-STAs under its control. However, in cases where additional coordination between the different WPANs is desirable, the necessary timing and control information can be exchanged between the wireless hubs  15  via the longer-range WLAN  14 . 
       FIG. 5  illustrates some of the components disposed in a PS-STA  11 , in accordance with one embodiment. PS-STA  11  typically includes, in part, a sensor or stimulus unit  17 , a clock or crystal  18 , a wireless circuit  19  and an antenna  20 . Although not shown, other components like capacitors, resistors, inductors, an external power amplifier (PA) and an external low-noise amplifier (LNA) may also be included in PS-STA  11 . Wireless circuit  19  is configured so as to communicate over the physical layer (PHY) of a standard 802.11x-compliant circuit chip disposed in the wireless hub (see  FIGS. 3 and 4 ). Wireless circuit  19  may be an embedded System-on-Chip (SoC), having disposed therein a radio  21  operating, for example, in the unlicensed 2.4-GHz and/or 5-GHz frequency bands, a baseband modem  22 , dedicated control and datapath logic  23 , a central processing unit (CPU)  24 , a memory module  25  and interface circuitry  26 . CPU  24  and memory module  25  are used to implement the portion of the communication protocol that is not implemented in the dedicated control and datapath logic (also referred to as the 802.11x device drivers), together with any application-specific software. Wireless circuits are well known in the art and are not described herein. 
       FIG. 6  shows various blocks of a wireless hub, such as wireless hubs  12  and  15  shown respectively in  FIGS. 3 and 4 , in accordance with one embodiment. The wireless hub acts as a pivot and provides communication between the corresponding WPAN and WLAN. The wireless hub includes an 802.11x-compliant wireless circuit  27 , a processing unit  28  coupled to or integrated with the 802.11x-compliant circuit, a memory module  29  that is coupled to or integrated with the 802.11x-compliant circuit, a crystal or clock  30 , and an antenna  38 . The 802.11x-compliant circuit  27  is shown as including a radio  31  operating, for example, in the unlicensed 2.4-GHz and/or 5-GHz frequency bands, a baseband modem  32 , and dedicated control and datapath logic  33 . Interface circuitry  34  provides an interface to the processing unit  28  and memory module  29 . Wireless hub may be connected to the power grid, in which case no batteries are needed to operate the device. Regulator  35  is adapted to regulate the supply. The wireless hub may further include various passive components like capacitors, resistors and/or inductors and an external power amplifier (PA) and/or external low-noise amplifier (LNA) (not shown). 
     The wireless hub further includes a number of software modules forming a software platform  36  that enable circuit  29  to communicate with both the WPAN and WLAN. In one embodiment, the software platform  36  enables circuit  27  to connect to the WPAN, without losing connectivity (such as association and synchronization) to the WLAN, as described in Vleugels I. Circuit  27  can be connected to the WLAN and WPAN in alternating cycles, however added latency would be incurred. 
     In some embodiments, the wireless hub may further include an operating system  37  that may be used to write application-specific software. The operating system may be, for example, Windows XP, Windows CE, Linux, Symbian, or any operating system that may enable writing of applications. 
     The processing unit  28  and memory module  29  are used to implement that portion of the communication protocol that is not implemented in dedicated control and datapath logic; this portion of the communications protocol is referred to as the 802.11x device driver. If the communication protocol between the wireless hub and a PS-STA is modified to reduce power consumption of the PS-STA, the 802.11x device driver may also require slight modification to accommodate such changes. The CPU and memory module are also used for the implementation of the software platform that enables concurrent or alternating WLAN/WPAN connectivity, and can furthermore be used to run application-specific software. 
     The following example is provided to further aid in understanding the invention.  FIG. 7  illustrates a wireless personal Area Network used for remote monitoring and controlling, in accordance with one embodiment of the present invention. A user desires to check one or more security monitoring devices  39  inside or around his house  40  while at work  41 . Each security monitoring device is a PS-STA and is wirelessly connected to a Wi-Fi-enabled power outlet  42 . The Wi-Fi-enabled power outlet is furthermore within the range of a WLAN infrastructure network  43  which the user is assumed to have set up at his home. 
     The WLAN infrastructure network  43  is adapted to establish communication with the Internet via an Internet router  44  that is coupled to the AP  45 . At the office, the user has access to a laptop  46  that is equipped with an 802.11x-compliant wireless circuit. This circuit is associated with a WLAN infrastructure network  47  that has been set up in the user&#39;s office  41 . The WLAN network  47  is adapted to establish communication with the Internet via an Internet router  48  that is coupled to the infrastructure&#39;s network AP  49 . The connection at the office may be wireless or wired. In a wired office environment, the user&#39;s laptop is hooked up directly through the Internet router  48  with a cable, without making use of the WLAN  47 . 
     Application software on the user&#39;s laptop  46  allows the user to poll information from a specific PS-STA at home. To do so, the user sends a poll request, which contains the information required to unambiguously identify the PS-STA of interest, and possibly additionally information about the data to be retrieved. Destination address information includes the address of the router  44 , the address of the Wi-Fi-enabled power outlet  42  that controls the PS-STA of interest and the address of the PS-STA  39  itself. PS-STA address is typically required where multiple PS-STAs are connected to, for example, a single Wi-Fi-enabled power outlet. The poll request is transmitted over the WLAN  47  in the office, and via Internet router  48  transported over the Internet to the Internet router  44  at the home. At the user&#39;s home, the poll request is directed to the Wi-Fi-enabled power outlet that coordinates the PS-STA of interest. The Wi-Fi-enabled power outlet receives this request over the home&#39;s infrastructure WLAN. If the requested information has already been retrieved from the PS-STA during a previous data transfer event, the Wi-Fi-enabled power outlet responds to the poll request by sending the requested information over the home&#39;s infrastructure WLAN  43  to the Internet router that is connected to the home&#39;s WLAN AP. The requested information is transported over the Internet to the Internet router at the office, and from there directed to the user&#39;s laptop over the offices WLAN infrastructure network. Application software on the user&#39;s laptop receives the information and presents it to the user. In case the requested information has not yet been previously retrieved from the PS-STA, the Wi-Fi-enabled outlet does so during the next scheduled WPAN communication event. The timing of the occurrence of this event, depends, in part, on the power management techniques used for the WPAN communication. 
     To conserve power, the PS-STAs are typically mostly in sleep mode and only occasionally wake up as needed to transmit or receive data and/or control signals. When connected to the WPAN coordinated by the Wi-Fi-enabled power outlet  42 , a PS-STA  39  is synchronized to the Wi-Fi-enabled power outlet  42 , which as part of the infrastructure network, is in turn synchronized to the AP  45 . The synchronization between the PS-STAB and the Wi-Fi-enabled power outlet ensures that the Wi-Fi-enabled power outlet is in WPAN mode at the same time that a PS-STA wakes up to transmit or receive. The above example describes an instance where the information from a single PS-STA is remotely accessed, using a Wi-Fi-enabled power outlet. It is understood that the wireless hub does not have to be a Wi-Fi-enabled power outlet, and may be any wireless hub, as described above. Furthermore, it is understood that multiple PS-STAs may be connected to a single as well as to multiple wireless hubs. The present invention may also be used to activate or steer PS-STAs, in addition to monitoring or retrieving information. 
     In some embodiments, rather than having data transfer be triggered by a poll request, the PS-STAs may also transmit data to the wireless hub periodically. In such embodiments, the retrieved data can be stored and/or processed locally on the wireless hub, or, alternatively, be transferred to a different location. 
     The association of a PS-STA with a wireless hub may or may not be static. In some embodiments, the PS-STA may be attached to a moving object, in which case the nearest wireless hub is dynamic and may change over time. This scenario is common in the context of medical monitoring/treatment. In such embodiments, medical sensors and stimulus devices in, on and around a person&#39;s body communicate to a nearby wireless hub that acts as a seamless bridge between the low-power WPAN and the longer-range WLAN. As the person/patient moves around the house, the nearest wireless hub may change over time. In such applications, seamless transitioning between wireless hubs is carried out and includes dynamic association capabilities inside the PS-STA, as well as software on the wireless hub side to seamlessly handle the required hand-offs among wireless hubs. The present invention is also applicable, for example, to the following situations:
         Remote medical monitoring   Medical monitoring/treatment in hospitals   In-house monitoring and control from any location to any location   Industrial monitoring/warehouse monitoring   Home automation   Energy metering   PC, cell phone and home entertainment peripherals and accessories       

     The following are among the advantages of embodiments of the present invention:
         Cost savings associated with infrastructure/hardware re-use   Integration of low-power short-range networks in the ubiquitous WLAN infrastructure results in cost savings since already-present hardware can be re-used. Little or no dedicated set up is required to enable the short-range connectivity   IP-addressable PS-STAs, enabling remote monitoring   Low-power short-range networks typically act as isolated networks. As a consequence, such networks can only be accessed when in close vicinity. This enables access to the WPAN from any location that is within the coverage area of the WLAN, or even from a remote location. Unlike other low-power wireless technologies, the power-sensitive nodes described herein are IP-addressable and, consequently, can be monitored and/or controlled from any location that has Internet access.   Long-range connectivity is achieved, without putting the associated burden on the power-sensitive device   The burden of achieving long-range connectivity is shifted away from the power-sensitive device onto the wireless hub. Since typically, the wireless hub is a powered device, power dissipation is not much of an issue.   As a result, a power-sensitive battery-operated device can be connected to the ubiquitous WLAN infrastructure without having to bear the consequences in terms of power dissipation and protocol overhead that are typically associated with this.
 
Variations
       

     Other variations should be apparent upon review of this disclosure. 
     While the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes, and substitutions are intended in the present invention. In some instances, features of the invention can be employed without a corresponding use of other features, without departing from the scope of the invention as set forth. Therefore, many modifications may be made to adapt a particular configuration or method disclosed, without departing from the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments and equivalents falling within the scope of the claims.