Patent Publication Number: US-8538369-B2

Title: Multi-wideband communications over power lines

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims benefit of and priority to European Patent Application EP 05 256 179.2, entitled “Power line Communication Device and Method,” filed Oct. 4, 2005 under 35 U.S.C. 119. The disclosure of the above patent application is hereby incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to systems and methods for power line communication and in particular, systems and methods for wideband power line communication. 
     2. Related Art 
     With the growing use of digital content (e.g. MP3 audio, MPEG4 video and digital photographs) there is a widely recognised need to improve digital communication systems. Power line communication (PLC) is a technology that encodes data in a signal and transmits the signal on existing electricity power lines in a band of frequencies that are not used for supplying electricity. Accordingly, PLC leverages the ubiquity of existing electricity networks to provide extensive network coverage. Furthermore, since PLC enables data to be accessed from conventional power-outlets, no new wiring needs to be installed in a building (or different parts of a building). Accordingly, PLC offers the additional advantage of reduced installation costs. 
     Referring to  FIG. 1 , a household  10  typically has a distributed mains wiring system consisting of one or more ring mains, several stubs and some distribution back to a junction box  12 . For the sake of example, let the household  10  comprise four rooms  14 ,  16 ,  18  and  20 . Every room  14 - 20  may have a different number of outlets and other mains connections. For example, room  14  may have only one connection  22 , room  16  may have two connections  24 ,  26 , room  18  may have three connections  28 ,  30 ,  32  and room  20  may have six connections  34 ,  36 ,  38 ,  40 ,  42 ,  44 . 
     Accordingly, there are a variety of distances and paths between different power outlets in the household  10 . In particular, the outlets most closely located to each other are those on multi-plug strips, and the outlets furthest away from each other are those on the ends of stubs of different ring mains (e.g. power outlets in the garden shed and the attic). Communication between these furthest outlets typically pass through junction box  12 . Nonetheless, the majority of outlets associated with a particular application (e.g. Home Cinema) are located relatively close together. 
     Because the channel capacity of a power line and connectors attenuates according to, amongst other features, the frequency of a transmitted signal, current generation PLC systems have been developed to transmit signals at relatively low frequencies (i.e. below 30 MHz) and thereby obtain suitable transmission distances. However, the use of such low transmission frequencies limits the maximum data throughput obtainable by PLC systems. 
     The processes of receiving analog signals and injecting modulated signals are treated differently by different PLC standards. Current approaches perform some analog conditioning to the signal-path (e.g. low-pass filtering for anti-aliasing or smoothing, or AC coupling to remove the low-frequency [&lt;&lt;1 KHz] high voltage content of the electricity mains). However, there are no analog systems available for combining two or more broadband PLC technologies that can work simultaneously. 
     A number of power line communication standards have been defined. These include the Homeplug 1.0/1.1 standards, the Homeplug AV standard, the CEPCA standard and the Digital Home Standard. 
     In common with most communication systems, one of the main problems with prior art PLC systems is obtaining high throughput and wide coverage at reasonable implementation cost, whilst maintaining compatibility with existing technologies. Although a few PLC systems that provide transmission rates of hundreds of megabits per second are currently on the market, these systems have high implementation costs as they employ high bps/Hz modulation schemes (i.e. approximately 10 bps/Hz) which require high accuracy data converters, extremely linear interface electronics and increase the cost of the digital implementation due to the computational complexity of the modulation. 
     There is, therefore, a need for improved PLC systems that overcome the above and other problems. 
     SUMMARY 
     Various embodiments include systems and methods of communicating over a power line by simultaneously sending and/or receiving data over a plurality of wideband frequency ranges. Various embodiments include a power line communication network comprising at least one power line communication device configured to use a plurality of wideband frequency ranges. 
     Some embodiments include a system for communicating over a power line comprising a device for communicating over a power line, systems for connecting to the power line, and systems for transmitting data from the device to appropriate applications or transmitting data from the applications to the device. 
     In some embodiments, the power line communication device is configured to improve the throughput/coverage/cost performance trade-off of a power line network, when compared with current-generation PLC networks, by spreading the transmission of data into a plurality of independent wideband frequency bands that can be operated simultaneously and independently. 
     Furthermore, the power line communication device also optionally facilitates inter-operability by using one or more of the frequency bands to facilitate communication with nodes employing previous power line technologies. In this way, the power line communication device provides a way of creating a scalable implementation of a network where nodes of previous technologies work, without loss of performance, together with new-generation power line technologies. 
     More particularly, the power line communication device can enable the use of frequencies above 30 MHz whilst maintaining compatibility with current worldwide EMC regulations and standards. This is achieved by using a signal of frequency less than 30 MHz (as currently used by the power line standard and/or regulations) and at least one other signal of frequency greater than 30 MHz without compromising the performance of any of the signals due to interference. 
     The result is a new PLC system that facilitates interoperability with a pre-existing power line communication technology within a wideband (currently in the frequency range of 1 MHz to 30 MHz) and provides the ability to extend the system into new significantly higher frequency widebands (e.g., frequencies between 30 MHz and 1 GHz) to improve the overall throughput of the resulting communication system while simplifying the implementation of any single given wideband. 
     The power line communication device comprises a network interface device that employs an analog signal separating device (e.g., analog filter) to separate the paths of different wideband signals received from the power line before converting them to their digital representation. The analog signal separating device also separates the paths of different wideband signals to be transmitted on the power line (after their conversion from their digital representation). The network interface device optionally employs TDMA (time division multiple access) and/or FDMA (frequency division multiple access) as a scheme for enabling co-existence, synchronisation and/or bi-directional transmission. 
     The analog signal separating device is configured from block elements comprising discrete and/or integrated electronic components and the natural characteristics of the wiring and/or printed circuit board traces used to interconnect said components. 
     In some embodiments, the power line communication device is employed in a system that is configured to be expandable to provide greater overall bandwidth, but with widely differing injected power levels in the different frequency ranges, and/or to coexist and be inter-operate with other pre-existing technologies on the same network. 
     In various embodiments, each of the pre-existing and new-generation power line technologies is configured to implement different modulation schemes (e.g. OFDM, CDMA (code division multiple access) and/or OWDM (orthogonal wavelet domain modulation)), either alone or in combination). Depending on its channel state, the power line communication device can send data through any or all of the widebands. Furthermore, depending on its network function, the power line communication device can distribute the data from a single source or mesh it together with data repeated from another node on the network. 
     In various embodiments, each node on the power line communication network is an apparatus that integrates the analog signal separating device and modem converters (e.g. DFE (Digital Front End), MAC (Media Access Control), etc.) as part of the power line network interface device; and an application such as a computer, mass storage device, display device, speaker, DVD (Digital Versatile Disc) player, PVR (Personal Video Recorder), etc.; and/or an interface to connect an application such as a digital audio interface, digital video interface, analog audio interface, analog video interface, Ethernet interface, IEEE1394/Firewire/iLink interface, a USB (Universal Serial Bus) interface, and/or the like. 
     In some embodiments, the power line communication device is configured to use a signal (in line with the current standards and injected power regulations) of frequency less than about 30 MHz and at least one other signal of frequency greater than 30 MHz without compromising the performance of any of the signals due to interference. This feature can enable the power line communication device to increase throughput whilst enabling interoperability with previous PLC technologies. 
     An advantage of using a low frequency band is the possibility for higher coverage (e.g., communication over greater distances) than that achievable with a high frequency band, due to the greater injected power allowed by the regulations and the lower channel attenuation. An advantage of using a high band is the higher throughput achievable due to the greater available bandwidth. 
     In some embodiments, the power line communication device is configured to exploit the natural topology of power line networks in a home, wherein a group of related devices and sockets are typically clustered close to each other (e.g. plasma screen, DVD player and speakers in a living room) and other clusters of devices and sockets are clustered elsewhere (e.g. desktop printer, scanner and ADSL router in a home office). Such household topologies can benefit from the high throughput short-range coverage provided by the high band (which is simultaneously and independently available within each of the clusters) whilst the low band can be used to carry the majority of data communications between the clusters. It will also be appreciated that some communication nodes may benefit from communications on both bands. 
     In some embodiments, the parallel use of multiple widebands enables the use of different injected power levels, receiver sensitivities, transmission times, symbol lengths and modulation techniques to optimise the performance and cost of each wideband, leading to a better cost performance solution even though it is necessary to provide more than one analog and digital front-end. Part of the implementation cost advantage arises from the ability to reduce the bps/Hz in each wideband, but still maintain throughput performance because of the additional bandwidth available. This effect non-linearly compensates for the cost of implementing more than one wideband communication technology. Furthermore, the reduced coverage of the high band(s) is offset by the parallel use of the low band (with its greater allowable injected power). For instance, in various embodiments, the power line communication device may be used to provide Gbit/s performance at a lower cost than current 200 Mbps systems, by using lower bps/Hz modulation schemes (i.e. approximately 5 bps/Hz rather than 10 bps/Hz) over multiple widebands. 
     In various embodiments, a network interface device is configured to employ analog signal separation and include multiple analog front-ends. The use of analog signal separation, based on frequency, enables each wideband technology to optionally operate independently, and may include one or more of the following features: 
     (a) The network interface device is configured for independent performance optimization of the analog to digital converters (ADCs), digital to analog converters (DACs) and PGA or line drivers employed in processing the signals from a given frequency band, wherein the optimisation is performed for the required bandwidth, linearity and dynamic features of the frequency band and optimised for the power levels required to match EMC regulations and/or coverage of the frequency band; 
     (b) The network interface device is configured for the power line communication device to maintain compatibility and inter-operability with existing standards using one of the frequency bands, whilst exploiting independently (and without causing prohibitive interference to) another frequency band for additional communication (for example, the Homeplug AV standard which uses frequencies in the range of 2 MHz-28 MHz could work simultaneously with another standard that uses frequencies greater than 30 MHz); 
     (c) The network interface device is configured to increases the capacity of the power line communication device by allowing the inclusion of additional widebands that do not need to use the same modulation technology, but can use modulation technologies that best match the new frequency bands&#39; power line channel characteristics; and 
     (d) The network interface device is configured to allow different widebands to operate without synchronisation to or dependence on other widebands. 
     In various embodiments, the power line communication device is also configured to allow other network technologies to be layered independently on top of it, for example:
         (a) combinations of data from different bands at various different communication levels whether in the digital front-end, the MAC layer or the application layer;   (b) using notches in the modulation scheme to limit emissions of certain frequencies within one of the analog defined wideband frequencies;   (c) using repeaters in a node to re-transmit on the same or a different frequency band;   (d) using a range of modulation schemes such as OFDM, CDMA and/or ODWM; and/or   (e) forming point-to-point, point-to-multipoint and or multipoint-to-multipoint communication patterns.       

     In some embodiments, the network interface device can combine and partition data communicated across different paths to maximise performance, coexistence and interoperability whilst minimizing system cost. 
     Various embodiments of the invention include a method comprising receiving digital data form one or more applications, encoding a first part of the digital data into a first signal within a first wideband frequency range, at least part of the first wideband frequency range being less than 30 MHz, encoding a second part of the digital data into a second signal within a second wideband frequency range, at least part of the second wideband frequency range being greater than 30 MHz, combining the first signal and the second signal to generate a combined signal, and sending the combined signal over a power line. 
     Various embodiments of the invention include a method comprising receiving a signal over a power line, separating the received signal into a first signal component in a first wideband frequency range and a second signal component in a second wideband frequency range, each of the first wideband frequency range and the second wideband frequency range being at least 10 MHz wide, separately processing the first signal component and the second signal component to extract digital data, and providing the digital data to one or more applications. 
     Various embodiments of the invention include a communication network comprising a first communication node configured to communicate using a first wideband frequency range at least 10 MHz wide and a second wideband frequency range at least 5 MHz wide, a second communication node configured to communicate with the first communication node over a power line by simultaneously using both the first wideband frequency range and the second wideband frequency range. 
     Various embodiments of the invention include a communication device comprising a coupling configured to communicate data over a power line, a first part of the, data being communicated using a first wideband frequency range and a second part of the data being communicated using a second wideband frequency range separate from the first wideband frequency range, the first part of the data being independent from the second part of the data, first logic configured to process the first part of the data, and second logic configured to process the second part of the data. 
     Various embodiments of the invention include a communication network comprising a first communication node configured to communicate using a first wideband frequency range, a second communication node configured to communicate using a second wideband frequency range separate from the first wideband frequency range, and a third communication node configured to simultaneously and independently receive communication from the first communication node over a power line using the first wideband frequency range and from the second communication node over the power line using the second wideband frequency range. 
     Various embodiments of the invention include a method comprising communicating first data between a first communication node and a second communication node over a power line, using a first wideband frequency range, and communicating second data between the first communication node and a third communication node over the power line, using a second wideband frequency range separate from the first wideband frequency range, the first data and the second data being communicated simultaneously. 
     Various embodiments of the invention include a method comprising sending a first communication from a first communication node over a power line using a first wideband frequency range, the first communication including data configured to identify a second communication node configured to communicate in the first wideband frequency range, receiving a response to the first communication from the second communication node, sending a second communication from the first communication node over the power line using a second wideband frequency range, the second communication including data configured to identify a third communication node configured to communicate in the second wideband frequency range, receiving a response to the second communication from the third communication node, and determining a communication strategy based on the response to the first communication and the response to the second communication. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Multiple embodiments of the invention will now be described by way of example only with reference to the accompanying Figures in which: 
         FIG. 1  is a block diagram of a prior art residence; 
         FIG. 2A  is a block diagram of an exemplary network comprising a plurality of nodes, some of which have multiple wide-band capabilities, according to various embodiments; 
         FIG. 2B  is a block diagram of the exemplary network of  FIG. 2A , depicting two simultaneous, bi-directional communication links therein, according to various embodiments; 
         FIG. 2C  is a block diagram of the exemplary network of  FIG. 2A , depicting three simultaneous communication links therein, according to various embodiments; 
         FIG. 2D  is a block diagram of a first stage of a one packet data transmission procedure implemented on the network of  FIG. 2A , according to various embodiments; 
         FIG. 2E  is a block diagram of a second stage of the one packet data transmission procedure shown in  FIG. 2D , according to various embodiments; 
         FIG. 3  is a block diagram of the hardware architecture of a modem in a power line communication device, according to various embodiments; 
         FIG. 4A  is a block diagram of signal paths in a single coupling unit prior art power line transmission system, according to various embodiments; 
         FIG. 4B  is a block diagram of signal paths in a dual coupling unit prior art power line transmission system, according to various embodiments; 
         FIG. 4C  is a block diagram of signal paths in a first embodiment of the power line communication device; 
         FIG. 4D  is a block diagram of signal paths in a second embodiment of he power line communication device; 
         FIG. 4E  is a block diagram of signal paths in a third embodiment of the power line communication device; 
         FIG. 4F  is a block diagram of signal paths in a fourth embodiment of the power line communication device; 
         FIG. 5A  is a block diagram of a first integrated circuit embodiment of the power line communication device; 
         FIG. 5B  is a block diagram of an alternatively partitioned second integrated circuit embodiment of the power line communication device; 
         FIG. 5C  is a block diagram of a further alternatively partitioned third integrated circuit embodiment of the power line communication device; 
         FIG. 6A  is a circuit diagram of an exemplary capacitive coupling unit used in the power line communication device, according to various embodiments; 
         FIG. 6B  is a circuit diagram of an exemplary inductive coupling unit used in the power line communication device, according to various embodiments; 
         FIG. 7A  is an exemplary power transmission spectrum of three power line technologies, according to various embodiments; 
         FIG. 7B  depicts the frequency characteristics of an exemplary set of analog filters for use in separating the widebands used by the three power line technologies depicted in  FIG. 7A , according to various embodiments; 
         FIG. 7C  shows the signal isolation provided by a second analog filter Filt B  (shown in  FIG. 7B ) to the Tech B  signal (shown in  FIG. 7A ), according to various embodiments; 
         FIG. 8A  is a block diagram of a household with a simple initial installation of the power line communication network of the third aspect of the invention, according to various embodiments; 
         FIG. 8B  is a block diagram of a household with a more complex installation of the power line communication network, according to various embodiments; 
         FIG. 9  illustrates a method by which one communication node can discover other communication nodes on a network using different wideband frequency ranges, according to various embodiments; 
         FIG. 10  illustrates a method in which a first communication node communicates simultaneously with both a second communication node and a third communication node, according to various embodiments; 
         FIG. 11  illustrates a method in which a communication node receives simultaneous signals using at least two different widebands, according to various embodiments; and 
         FIG. 12  illustrates a method in which a communication node sends simultaneous signals using at least two different widebands, according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     For the sake of clarity, the term “power line” will be used herein to refer to low voltage household mains distribution cabling (typically 100-240 V AC power) or any other distributed electrically conductive cabling (i.e. AC or DC), that is capable of passing power to appliances connected to it. Furthermore, the term “power line technology” will be used herein to refer to a specification that when implemented as a series of network interface devices connected to a power line, enables the devices to bi-directionally communicate with each other using signals superimposed on the power distribution signal already present on the power line. 
     The term “network interface device” will be used herein to describe an apparatus that implements either fully or partially, a communications technology, such as a power line technology, to enable the apparatus to communicate with other devices connected to the same network (such as a power line), regardless of whether or not the apparatus is integrated with other apparatuses or functions within a single enclosure. For the sake of clarity, a device connected to a power line network will be generically known herein as a “node”. 
     The term “coverage” will herein be used to refer to the maximum distance between two nodes at which data transmitted therebetween is still detectable by either node. In addition, the term “throughput” will be understood to represent the rate at which nodes send or receive data on a network. 
     For the sake of clarity, in terms of explanation of operation of the power line communication device around current power line technologies, a wideband frequency band used in the power line communication device whose frequency of less than about 30 MHz, will be known herein as a “low band”. Similarly, a wideband frequency band(s) used in the power line communication device whose frequency is greater than about 30 MHz will be known herein as “high band(s)”. 
     For completeness and since the present invention relates to wideband communication, the term “wideband” will be used herein to refer to a frequency band or range used by a power line technology signal, characterised by having a bandwidth of greater than, or equal to, 5 MHz from the first (lowest) frequency to the last (highest) frequency of the band irrespective of the presence of notches. However, in various embodiments, wideband may have bandwidths of at least 7, 10, 12, 15, 20, 100 or 250 MHz. Similarly, the term “narrowband” will be used to refer to a frequency band used by a power line technology signal, characterised by having a bandwidth of less than 5 MHz. A wideband may include many different carrier channels used to convey data. For example, in various embodiments, widebands include more than 25, 50, 100 or 200 data channels. 
     The term “transmission time” is herein used to describe the maximum amount of time it takes to transmit a single co-existent message. The transmission time includes, but is not limited to, a start of transmission marker time (if any), a synchronisation time (if any), a channel access resolution time (if any), a negotiation time (if any), a message transmission time, an acknowledge transmission time (if any) and an end of transmission marker time (if any). 
     The term “notch” will be used herein to refer to a frequency band where the energy level of a power line technology signal has been deliberately reduced to prevent interference with other users of the spectrum (whether on or off the power line). Notches are characterized by having a narrower bandwidth than the power line technology signal itself and are generally implemented by digital or analog signal separating device within a single digital signal processing block or analog front end. 
     For the sake of clarity, the term “sub-band” will be used herein to refer to a frequency band where a power line technology signal characteristic differs from the characteristics of the power line technology signal in the remainder of the signal&#39;s bandwidth. Such differences can include the optional or mandatory presence of the sub-band, the signal power level of the sub-band and the directionality of the sub-band. Sub-bands are characterized by having a narrower bandwidth than the power line technology signal itself. The use of overlapping sub-bands in OFDM enables notches to be created, wherein a sub-band is disabled if the reception of the sub-band is heavily impaired or the sub-band can interfere with another service. Furthermore, OWDM can simplify the notching out of carriers due to its lower side lobes. 
     For the sake of simplicity, the term “transmitter signal path” will be used to refer to the path of a signal transmitted from an apparatus to the power line. Similarly, the term “receiver signal path” will be used to refer to the path of a signal received by an apparatus from a power line. On a related note, it may not be necessary to perform the isolation on both the receiver and transmitter signal paths (depending on the specifications of the analog components and the modulation techniques employed therein). 
     For the sake of clarity, the power line communication device of the present invention will be referred to herein at times as an “improved power line communication device”. Similarly, the network interface device of the improved power line communication device will be referred to herein as an “improved network interface device”. Finally, the power line communication network comprising nodes that are improved communication devices will be known herein at times as an “improved power line communication network”. 
     The term “separate,” as used herein with respect to widebands, is to characterize widebands that do not use, except incidentally, the same frequencies for communication data or commands. Widebands may be separate but interleaved, e.g., overlapping. 
     The term “simultaneously” is used herein with respect to communicating data to indicate that at least part of first data or commands are communicated using a first wideband at the same time as at least part of second data or commands are communicated using a second wideband. Simultaneous transmission is contrasted with systems that alternate or interleave the use of frequencies, one after the other or hopping from one to the other. 
     The term “independent” is used herein with respect to data transmitted to indicate that data transmitted using one wideband does not depend on data simultaneously transmitted using another wide band. Independent data transmission can include, for example, data sent to or received from different locations. Data in which alternative bits are transmitted using different frequencies is not independent because the bits are dependent on each other to form a useful byte. 
     It will be appreciated that the specific network and other examples described in these sections are used for illustrative purposes only. In particular, the examples described in these sections should in no way be construed as limiting the improved power line communication device. 
     Some embodiments of the improved power line communication network comprise a plurality of nodes of which some employ a network interface device that enables simultaneous and independent communication over two or more widebands, to similar multi-wideband nodes or conventional nodes. A first wideband optionally comprises frequencies of less than 30 MHz, in line with the current standards and injected power levels (and will herein be known as a low band) and the other wideband(s) comprise frequencies of greater than 30 MHz (and will herein be known as high band(s)). Alternatively, both a first and second wideband may comprise frequencies greater than 30 MHz. This enables power line technologies to be optimised for each of the widebands, so that the trade-off between cost, coverage and throughput will be superior to that achieved by a purely mono-wideband approach. 
     In particular, the modulation schemes for each technology used within the improved power line communication network can be optimised for cost. For instance it may not be necessary to use a particularly high modulation density (bps/Hz) in the low band to enhance throughput because the low band can work in parallel with the inherently high throughput high band(s). 
     The improved power line communication network provides inter-operability with prior art power line technologies by also supporting communication between multi-wideband nodes and mono-wideband nodes (that use one of the power line technologies supported in the low band or high band(s) and communicate at frequencies in the low or high band(s). 
     The improved network interface device may be part of an external modem apparatus or embedded within another apparatus (e.g. computer, TV etc.). However, regardless of the manner in which an improved network interface device is included within a node, the device remains physically connected to electrically conductive cabling (that passes AC or DC power) and is capable of transmitting digital data across the cabling using either or all of the low and high bands. 
     In accordance with current regulatory standards, low band signals may be transmitted with a power of approximately up to −50 dBm/Hz whereas high band signals may only be transmitted with a power that causes emissions in this frequency band to be lower than −80 dBm/Hz. Accordingly, signals in the low band may be transmitted with a power approximately one thousand times greater than signals in the high band. Consequently, if signals in both of these bands were to be transmitted simultaneously, without using some form of analog frequency isolation, the dynamic range and voltage compliancy requirements of the high band signals would be significantly increased. 
     However, the potential for interference or saturation of a lower power signal may be even more problematic. In particular, f one of the bands is used to receive a line-attenuated signal whose power level is close to the noise on the power line (i.e. −150 dBm), while at the same time, the other band is used to transmit a signal at its maximum allowable transmission power, the isolation required to prevent the signals from the two bands from interfering with each other would be approximately 100 dB. However, this is beyond the current state of the art analog implementations and would have high implementation costs. 
     In summary the isolation required to effectively allow simultaneous and independent communication of high and low bands falls into three main categories: 
     (1) isolation to prevent the strength of a signal received over the network in one wideband from saturating the receiver of the other wideband; 
     (2) isolation to prevent the transmitter of one band from interfering with the reception of another band; and 
     (3) isolation to prevent the degradation of the transmitter for one band when another band is being transmitted. 
     In view of the above, the improved network interface device employs an analog signal separation device to isolate the paths from the power line connection to an apparatus; to data-converters for each wideband. One of the most efficient ways of providing this isolation is by high-pass filtering or band-pass filtering high band signals, whilst minimising out-of band signals in the low band (using high linearity components and possibly analog low-pass smoothing or anti-aliasing). 
     Signals in the high band and the low band can use the same or different modulation techniques (e.g. OFDM, CDMA and/or ODWM) or time division schemes to facilitate co-existence and/or bi-directional communication. In one possible scenario, the low band could employ a modulation scheme that is inter-operable with one of the existing power line modem standards or proposals, whilst the high band is used for performance expansion beyond previous standards. Data and/or control can be passed through one or both of the widebands simultaneously and via a plurality of nodes in the form of a repeater (e.g., relay) network. 
     There are many different ways of implementing the improved power line communication device. For example, the improved power line communication device may be implemented on one or more integrated circuits (whether dedicated to the modem function or as part of an application system on a chip), and in combination with the characteristics of passive components and interconnects. However, the implementation of the analog signal separating device to separate the low band and high band(s)) employs a combination of the components in the different signal paths, whether passive or active, integrated or discrete. In particular, it is possible for the widebands: 
     (a) to share part of their paths (e.g. through a coupling unit); or 
     (b) to be joined only at the power line; and/or 
     (c) to be at opposite ends of the apparatus. 
     It is also possible to expand the improved power line communication device to communicate on more than two widebands. Similarly, it is also possible for the widebands to overlap slightly if required, and to be different in frequency ranges or bandwidths to those cited in the specific description. 
     Referring to  FIG. 2A , a network  50  that represents the improved power line communication network, comprises a plurality of nodes  54 - 76 . Some of the nodes comprise the improved power line communication device and accordingly implement more than one PLC technology. Nodes that do not include the improved network interface device can only implement one PLC technology. For the sake of simplicity, nodes that can only implement one PLC technology will be known herein as “mono-wideband nodes”. Similarly, nodes that can implement more than one technology will be known herein as “multi-wideband nodes”. 
     Three different PLC technologies are employed for communication on the network, namely Tech A , Tech B  and Tech C . Nodes  54  and  76  comprise the improved network interface device and are capable of implementing PLC technologies Tech A  and Tech B . Nodes  58  and  66  comprise the improved network interface device and are capable of implementing PLC technologies Tech B  and Tech C . Finally, nodes  60  and  68  comprise the improved network interface device and are optionally capable of implementing all three PLC technologies. 
     The remaining nodes (namely nodes  56 ,  62 ,  64 ,  72  and  74 ) do not comprise the improved network interface device and thus can only implement one of the PLC technologies. In particular, nodes  62  and  70  implement PLC technology Tech A  only, nodes  56  and  72  implement PLC technology Tech B  only, and nodes  64  and  74  implement PLC technology Tech C  only. Optionally, all of the communication between the nodes on the network  50  takes place through a common power line  52 . 
     In some embodiments, the improved power line communication network supports communication between nodes that implement different PLC technologies. In contrast, prior art PLC systems can only support communication between nodes that implement identical PLC technologies (e.g. nodes  56  and  72 ), even if the nodes in question co-exist on a network with nodes that implement other PLC technologies (e.g. nodes  64  and  74 ). 
       FIG. 2B  shows two simultaneous, bi-directional and non-interfering communication links the network  50  of  FIG. 2A . A first communication link  80  is a point-to-point communication link between nodes  54  and  68  which simultaneously uses the high band and low band to enable simultaneous communication of both Tech A  and Tech B  messages between the two nodes. 
     It should be noted that whilst node  68  is optionally capable of implementing all three technologies (i.e. Tech A , Tech B  and Tech C ) only the Tech A  and Tech B  capabilities of node  68  are used in the first communication link  80 . Furthermore, it should be noted that the first communication link  80  can re-distribute data from the Tech A  and Tech B  technologies across the high band and the low band in accordance with the current network characteristics (e.g. channel impairments). 
     A second communication link  82  connects nodes  68 ,  58 ,  60 ,  74  and  64 . Since nodes  64  and  74  are only capable of implementing PLC technology Tech C , the second communication link  82  only supports communications of the Tech C  technology. 
     The presence of the two communication links  80  and  82  allows nodes  68 ,  58 ,  60 ,  74  and  64  to establish communication (through the second communication link  82 ) at the same time that node  68  is communicating with  56  (through the first communication link  80 ). In other words, the network arrangement depicted in  FIG. 2B , enables two simultaneous and concurrent communications to be performed, wherein the first communication link  80  enables dynamic data transmission and reception using technologies Tech A  and/or Tech B  and the second communication link  82  enables dynamic data transmission and reception using technology Tech C . 
       FIG. 2C  shows three concurrent and simultaneous communication links  84 ,  86  and  88  in the network  50  of  FIG. 2A . The first communication link  84  provides a bi-directional point-to-multipoint connection between node  54  and nodes  68  and  70 . Since node  70  is only capable of implementing technology Tech A , the first communication link only supports communications of the Tech A  technology. Meanwhile a second communication link  86  enables communication between nodes  56  and  72  of technology Tech B  with technology Tech A , using a co-existence strategy such as Time Division Multiple Access (TDMA) (i.e. a multiple access technique where only one transmitter transmits on a particular channel at any given time). 
     Finally, a third communication link  88  supports communication of Tech C , wherein these communications are conducted in a different wideband that does not interfere with the other communication links. 
     Referring to  FIG. 2D , let there be a first communication link  90  between nodes  68 ,  58  and  60  of the network  50  depicted in  FIG. 2A . The first communication link  50  is bi-directional and supports communication of Tech B  and Tech C . Furthermore, let there be a simultaneous second communication link  92  between nodes  70  and  62  of the same network  50 . The second communication link  92  supports communication of Tech A . 
     For the sake of example, let there be a message originating in node  58  that to be distributed to the nodes of the network  50 . Nodes such as  68 ,  60  and  66  that support Tech B  and Tech C , can demodulate and receive a message from node  58 . However, the remaining nodes on the network  50  cannot receive the message. To solve this problem, a two-stage communication process is implemented in which: 
     (i) in the first stage, the message is transmitted from node  58  to  68 , using PLC technologies Tech B  and Tech C ; and 
     (ii) in the second stage, the message is re-transmitted (e.g., relayed) by node  68 , using its technology capabilities, so that optionally all of the nodes on the network  50  can receive and demodulate the message. 
     Repeaters can also be used to increase the coverage of a given technology (when a node can detect its neighbouring node, but not further nodes thereafter). 
       FIG. 2E  is a block diagram of a second stage of the one packet data transmission procedure shown in  FIG. 2D . This second stage optionally includes a broadcast made simultaneously in more than one wideband. As such, the broadcast may include more that one communication standard. 
     Referring to  FIG. 3 , a modem  80  in an improved network interface device comprises N blocks  82 A- 82 N corresponding to each of the PLC technologies supported by the node. In other words, block  82 A corresponds with the Tech A  technology, block  82 B corresponds with the Tech B  technology, and so on, until block  82 N, which corresponds with the Tech N  technology. 
     Each block  82 A- 82 N comprises the first (PHY) and second (MAC) layers of the OSI (Open Systems Interconnection) stack for each technology. For example, the block  82 A comprises the blocks PHY A  and MAC A . Similarly, the block  82 B comprises the blocks PHY B  and MAC B . The modem  80  further comprises a data distribution block  84 , which distributes data amongst the blocks  80 A and  80 N in accordance with the technology of the signal and current network traffic characteristics. 
     When used for transmission, signals from each technology supported by a node are processed by an analog filter bank (not shown). The operation of the analog filter bank will be described in more detail later. The processed signals  86  are forwarded to the data distribution block  84  for distribution amongst the blocks  82 A- 82 N. The outputs from the blocks  82 A- 82 N are combined in a coupling/decoupling stage  88  from which they are injected into a power line  90 . 
     When used for receiving a signal from the power line  90 , the coupling/decoupling stage  88  decouples the component signals for each supported PLC technology. The decoupled signals arc processed through blocks  82 A- 82 N and forwarded through the data distribution block  84  to the appropriate applications running on the node. 
     Each of the PHY blocks (PHY A -PHY N ) may have a feedback signal  92 A- 92 N which provides information regarding the usage of each of the technology signal paths. This information is used by the data distribution block  84  to redistribute the data flow amongst the N available blocks  82 A- 82 N. It should also be noted that parts of the MAC and PHY blocks (PHY A -PHY N ) and (MAC A -MAC N ) may be capable of sharing resources. 
     Referring to  FIG. 4A , in a first form of a prior art power line transmission system, a power line  100  is connected to a single coupling unit  102 , which has high-pass transmission characteristics to enable the rejection of the AC line frequency of the power line  100 . The coupling unit  102  is, in turn, connected to receiver and transmitter paths  104 ,  106 , which are isolated during half duplex phases using an RX/TX switch  108 . 
     The receiver path  104  typically comprises a band-limiting anti-aliasing filter  110 , a programmable gain amplifier (PGA)  112 , and an ADC  114 . The resulting digital signal  116  is then demodulated  118 . The anti-aliasing filter  110  may be in a different order and may be partially or completely provided by the bandwidth of the PGA. 
     The transmitter path  106  typically comprises a line driver  120  (which may or may not be capable of operating in high impedance mode) and a band-limiting smoothing filter  122 . The band-limiting smoothing filter  122  limits the power of harmonics (in the out-of-band range) in the analog signal (the harmonics being produced by the operation of a DAC  124  on a received digital signal  126  that had previously been modulated  128 ). It will be realised that part of the modulation and demodulation schemes  118 ,  128  could also be performed in the analog domain. 
     Referring to  FIG. 4B  a slightly different form of a prior art single wideband system employs separate transmitter and receiver coupling units  130 ,  132 . A TX/RX switch is not required in this form of the prior art power line transmission system, as either the impedance of the line driver  120  does not significantly represent an extra impedance load to the power line  100  or the line driver  120  itself is capable of going into a high impedance mode. 
     Referring to  FIG. 4C , the improved network interface device comprises two or more analog front-ends separated into two low band analog paths LB 1  and LB 2  and two high band paths HB 1  and HB 2 , by coupling units  142 ,  148 ,  154  and  160  respectively. 
     The analog filtering characteristics of the different paths (including the coupling units  140 - 146  and the active components) are designed to pass the signal of a given band whilst rejecting the signals of the other bands. The modulation schemes of each technology  152 ,  154  may be the same or different, as may be the demodulation schemes  148 ,  150 . 
     Referring to  FIG. 4D  in a second embodiment of the improved network interface device there are two coupling units  166 ,  176 , wherein coupling unit  166  is used for low band communication and coupling unit  176  is used for high band communication. In addition to the optimisation of the low band paths ( 167 - 171  and  172 - 175 ) and the high band paths ( 177 - 181  and  182 - 185 ) for the power, frequency and modulation schemes of different PLC technologies, the coupling units  166 ,  176  can be optimised to have different pass-frequency characteristics. 
     Referring to  FIG. 4E , in a third embodiment of the improved network interface device, there are two coupling units  186  and  190 , wherein coupling unit  186  is used for reception and coupling unit  190  is used for transmission. However, each high band path ( 188 - 191  and  203 - 206 ) is isolated from the low band paths ( 192 - 195  and  197 - 201 ) by deliberately inserted filters  187 ,  202  with high pass or band pass characteristics. 
       FIG. 4F  shows a fourth embodiment of the improved network interface device, applied to two different wideband technologies as in  FIG. 4E . Whilst there are many possible other combinations, it is not necessary for there to be separate paths and converters for the high band paths and low band paths in either the receiver or transmitter, as communications in one direction may benefit more from the improved network interface device than communications in the other direction. 
     The improved network interface device comprises one coupling unit for transmission  218  and one for reception  208 . The high band is isolated from the low-band on the receiver path, by deliberately inserted filters  209  with high-pass or band-pass characteristics. However, the transmitter modulation schemes are combined in the digital domain  222 A,  222 B and then passed through a very high performance DAC (digital to analog converter)  221 , smoothing filter  220  and line driver  219 . 
     Referring to  FIG. 5A , an exemplary integrated circuit  250  implementation of the improved network interface device comprises two analog front ends AFE A , AFE B  for the two widebands of the improved power line communication device. The exemplary integrated circuit  250  also comprises a logic element  226  configured to implement the different power line modem technologies (including DFE and MAC) and provide a digital interface to the next stage application  228  in the device. 
     The high band analog front end (AFE B ) contains high band converters  230 ,  232  and active interface electronics (i.e. a PGA  234  and line driver  236 ) and connects to the power line via a coupling unit along path  238 . The low band analog front end (AFE A ) comprises low band converters  240 ,  242  and active interface electronics (i.e. a PGA  244  and line driver  246 ) and connects to the power line via a coupling unit along path  248 . 
     A digital representation of the signal to be sent on the low band and high band is produced in the logic element  226  and is present at interfaces  250 ,  252  to the analog front ends AFE A , AFE B . 
     Referring to  FIG. 5B , in an alternative integration partition  300  of the integrated circuit implementation of the improved network interface device, there are two integrated circuits, namely a digital modem integrated circuit  302  and an analog modem integrated circuit  304  containing the two analog front ends AFE A , AFE B . The analog modem integrated circuit  304  may be configured in several parts, as illustrated in  FIG. 5B , or alternatively as a single component. 
     Referring to  FIG. 5C , in another integration partition  400  of the integrated circuit implementation of the improved network interface device, the analog front end of each wideband is split into data converters Conv A , Conv B  and interface circuits I/Face A , I/Face B . In this case, the converters Conv A , Conv B  are integrated with the digital logic  401  of the power line modem in one integrated circuit  402  while the higher frequency current/voltage interface circuitry is provided in another integrated circuit  404 . 
     It will be appreciated that there are numerous other possibilities including the embedding of all or part of the active electronics within other devices in the system, or the use of discrete blocks for various blocks. 
     As discussed elsewhere herein, a coupling unit can have frequency characteristics. Referring to  FIG. 6A , a capacitive coupling unit  500  comprises X 1  type capacitors  502  that are used to couple a signal source  504  (via an isolating transformer  506 ) onto a power line  508 . In this case, the impedances of the transformer  506 , capacitors  502 , signal source  504  and the power line  508  determine the frequency response of the capacitive coupling unit  500 . 
     Referring to  FIG. 6B , an inductive coupling unit  520  comprises a signal transformer  522  that inductively couples a signal  524  in tandem with a Y 1  type capacitor  526 . The inductive coupling unit  520  is roughly equivalent to the capacitive coupling unit  500  (depicted in  FIG. 6A ), with the respective impedances of the transformer  522 , capacitor  526 , signal source  524  and power line  528  determining the frequency response of the inductive coupling unit  520 . 
     Furthermore, a low-pass filtered version of the power line  508 ,  528  can be used within the improved network interface device to provide a power supply. It is also possible to implement higher order filters in the coupling units with more passive components. Nonetheless, it will be appreciated that it is possible to employ many other types of coupling unit (e.g. optical coupling units) in the power line network interface device. 
       FIGS. 7A to 7C  illustrates the exemplary frequency spectra of a number of different power line technologies, and demonstrates how an analog signal separating device could be used to separate the signals of a given technology (from the signals of the other technologies) into a particular signal path. 
     Referring to  FIG. 7A , a first technology Tech A  has a transmission power P A  and a wide-band  540  delimited by frequencies f A1  and f A2 . The wide-band  540  has internal notches  542  to comply with EMC regulations. A second technology Tech B  has a transmission power of P B  and a wide-band  544  delimited by frequencies f B1  and f B2 . The wide-band  544  also has a notch  546  to comply with EMC (Electromagnetic Compatibility) regulations. It should be noted that f B1  may be greater than f A2  to avoid band-overlapping. Finally, a third technology Tech N , has transmission power P N  and a wide-band  548  delimited by frequencies f N1  and f N2 . The wide-band  548  also has a notch  550  to comply with regulations. 
     Referring to  FIG. 7B  a first, second and third analog filter (namely Filt A , Filt B  and Filt N  respectively) respectively isolate the signals from each of Tech A , Tech B  and Tech N , from the signals from the other technologies. The analog filter characteristics are applicable to the transmitter and/or the receiver of each technology within a node. 
     The first analog filter Filt A  is defined by passband start and end frequencies f A3 , f A4 . Similarly the second analog filter Filt B  is defined by passband start and end frequencies f B3  and f B4 . Finally, the third analog filter Filt N  is defined by passband start and end frequencies f N3  and f N4 . 
     In one embodiment, the start of at least one of the passbands of the analog signal separating device of the power line communication device is between 1 MHz and 30 MHz, and is at least 10 MHz in width. Optionally, at least one of the other widebands includes signals at a frequency greater than 30, 40, 50, 75, 100, 200 or 500 MHz, and optionally less than 1 GHz. The difference between the passband and stopband for any one of the elements of the analog signal separating device may be more than 6 dB. 
     It should be noted that it is possible for different analog filters to overlap (e.g. f A4 &gt;f B3 ), or not to overlap (e.g. f B4 &lt;f N3 ). It should also be noted that it is not necessary for the passbands of all of the analog filters to have the same transmission power, attenuation functions, or other characteristics. The product of the analog filter characteristic and the modulation scheme of a given PLC technology determines the effectiveness of the isolation by each analog filter. The absolute transmission of the filters in their respective passbands is less important than the ratio of passband to stopband, as any attenuation differences in these filters can often be compensated for with more injected power at the pre-filter stage and/or increased receiver sensitivity. 
       FIG. 7C  shows an example of the isolation provided by the second analog filter Filt B  to the Tech B  signal. In use, the second analog filter passes a signal Path B  that is the product of the transmission power (PF B ) of then passband of the second analog filter and the transmission power (P B ) of the Tech B  signal. The signals of the other technologies (Tech A  and Tech N ) are attenuated by the stopband of Filt B  to a power level PF N  that is sufficiently less than PF B  to ensure that they do not significantly interfere with the Path B  signal. 
     In some embodiments, the improved power line communication network includes the ability to provide increased throughput as the number of nodes increase in the network. In particular, such increased throughput demand typically coincides with an increased number of nodes, since more data needs to be transmitted when multiple devices share the network. 
       FIG. 8A  shows a simple initial installation of a single service, in this case IPTV (internet protocol television) delivered to the home  600  via a DSL connection  602 , which is distributed (using the improved power line communication network) from the DSL modem  602  in the office to a TV set  604  in the living room. Since the distance between the DSL modem  602  and the TV set  604  is comparatively long, the connection C 1  therebetween predominantly uses the low band due to its inherently greater coverage. The bandwidth provided by the low band is sufficient for the TV set because only a single TV channel is transmitted. 
     As an improved power line communication network grows (i.e. more nodes are added to it) the average distances between nodes tends to decrease. When a very complex situation is installed like that depicted in  FIG. 8B  (showing a complex in-home multimedia network with multiple simultaneous video and audio streams), connections C 2 -C 7  are predominantly implemented using the high band (due to its greater throughput) over the relatively short distances present. Low band connections C 8 , C 9 , will still be in use when their efficiency is higher than using multiple hops of high band links. In addition, many connections (e.g. C 10 ) will be served by communication using both bands. 
     In use, a node on the network will typically discover the other nodes on the power line through some form of synchronization that is usually defined within the power line technology used in one of the bands. The node will also identify the technology capabilities and virtual network membership of the detected nodes, to determine what communication will be possible and/or allowable (for instance, whilst a detected node may physically have certain technology capabilities, these may be impaired by interference or restricted in use). Having identified the physically possible and allowable communications, the sending node will decide the best path for sending/receiving data, based on factors such as the type of data to be communicated, how it is ranked in the QoS (quality of service) and the available channel capacity. 
       FIG. 9  illustrates a method by which one communication node can discover other communication nodes on a network using different wideband frequency ranges. In a send step  910 , a first communication is sent from a first communication node over a power line using a first wideband frequency range, the first communication including data configured to identify a second communication node configured to communicate in the first wideband frequency range. This first communication is configured to solicit a response from other communication nodes, such as the second communication node, that are configured to communicate over the first wideband frequency. 
     In a receive step  920 , a response to the first communication is received from the second communication node. 
     In a send step  930 , a second communication is sent from the first communication node over the power line using a second wideband frequency range, the second communication including data configured to identify a third communication node configured to communicate in the second wideband frequency range. This second communication is configured to solicit a response from other communication nodes, such as the third communication node, that are configured to communicate over the second wideband frequency. 
     In a receive step  940 , a response to the second communication is received from the third communication node. If the second communication node is further configured to communicate using the second wideband, then receive step  940  may include receiving communications from both the second and third communication nodes. 
     In a determine step  950 , a communication strategy based on the response to the first communication and the response to the second communication. This determination may include selections of widebands or standards to use. This determination may include a strategy for communication between the third and second communication nodes, e.g., should the communication be direct or should the first communication node function as a relay. In some embodiments, Send Steps  910  and  930  include using different standards in addition to or as an alternative to using different widebands. 
     In an optional communicate further step communication is executed according to the determined communication strategy. 
       FIG. 10  illustrates a method in which a first communication node communicates simultaneously with both a second communication node and a third communication node. The communication is optionally independent. The first communication node optionally operates as a relay between the second communication node and the third communication node. 
     In a first communication step  1010 , first data is communicated between a first communication node and a second communication node over a power line, using a first wideband frequency range. In a second communication step  1020 , second data is communicated between the first communication node and a third communication node over the power line, using a second wideband frequency range separate from the first wideband frequency range, the first data and the second data being communicated simultaneously. 
     The method illustrated in  FIG. 10  is optionally employed where the second communication node functions as both the second and the third communication node. In these embodiments a first part of data can be sent using the first wideband frequency and a second part of data can be sent using the second wideband frequency. In these embodiments, the use of the two different wideband frequencies is optionally configured to maximize total data bandwidth. 
       FIG. 11  illustrates a method in which a communication node receives simultaneous signals using at least two different widebands. Data encoded in these signals is optionally independent and may be received from different communication nodes on a network. The data may also be transmitted and/or decoded using different communication standards. 
     In a receive first signal step  1110 , a signal is received over a power line. This signal includes encoded data. The encoding may include any of the various methods known for encoding data on a time dependent signal. 
     In a separate component step  1120 , a first component of the signal, in a first wideband frequency range, is separated from a second component of the signal, in a second wideband frequency range. In various embodiments, each of the first wideband frequency range and the second wideband frequency range are at least 5, 7, 10, 12, 15, 20, 100, and/or 200 MHz wide. For example in one embodiment, the first wideband frequency range is at least 10 MHz wide and the second wideband frequency range is at least 5 MHz wide. In one embodiment, the first wideband frequency range is at least 10 MHz wide and the second wideband frequency range is at least 200 MHz wide. In some embodiments, the separation of signal components is performed using analog bandpass filters. For example, one bandpass filter may be configured to isolate the first wideband frequency range and another bandpass filter may be configured to isolate the second wideband frequency range. 
     In a process step  1130 , the first signal component and the second signal component, separated in separate component step  1120 , are each separately processed. This processing is typically preformed in parallel. For example, one signal component may be processed using low band analog path LB 1  and the other signal component may be processed using high hand path HB 1 , Low band analog path LB 1  and high band path HB 1  optionally share one or more component. Process step  1130  results in two sets of digital data. 
     In a provide step  1140 , the two sets of digital data are provided to one or more applications. The two sets of digital data may be used independently. 
       FIG. 12  illustrates a method in which a communication node sends simultaneous signals using at least two different widebands, Data encoded in these signals in optionally independent and may be intended for different communication nodes on a network. The data may be transmitted and/or encoded using different communication standards. 
     In a receive data step  1210 , digital data is received from one or more applications. This data is optionally independent. In an encode step  1220 , a first part of the digital data is encoded into a first signal within a first wideband frequency range, at least part of the first wideband frequency range is optionally less than 30 MHz. This encoding may be performed using, for example, low band analog path LB 2 . 
     In an encode step  1230 , a second part of the digital data is encoded into a second signal within a second wideband frequency range, at least part of the second wideband frequency range is optionally greater than 30 MHz. This encoding may be performed using, for example, high band path and HB 2 . Encode Step  1230  and Encode Step  1220  may be performed in parallel. 
     In a combine step  1240 , the first signal and the second signals are combined to generate a combined signal. This step is optionally performed using Tx Coupling  196 , or. HB Tx Coupling  154  and LB Tx Coupling  160 . For example, using HB Tx Coupling  154  and LB Tx Coupling  160  the combination may occur as both signals are coupled to a power line. 
     In a send step  1250 , the combined signal is sent over a power line. Different parts of the combined signal may be sent simultaneously and may be intended for different destinations. Several embodiments are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations are covered by the above teachings and within the scope of the appended claims without departing from the spirit and intended scope thereof. For example, the techniques described herein may be in used in household, industrial and/or vehicle power systems. Further various elements illustrated and discussed herein may be embodied in software (stored on computer readable media), firmware, and/or hardware, These element forms are generally referred to herein as “logic.” 
     The embodiments discussed herein are illustrative of the present invention. As these embodiments of the present invention are described with reference to illustrations, various modifications or adaptations of the methods and or specific structures described may become apparent to those skilled in the art. All such modifications, adaptations, or variations that rely upon the teachings of the present invention, and through which these teachings have advanced the art, are considered to be within the spirit and scope of the present invention. Hence, these descriptions and drawings should not be considered in a limiting sense, as it is understood that the present invention is in no way limited to only the embodiments illustrated.