Abstract:
A network arrangement for aggregation of groups of tunable sources in a photonic network is disclosed. The network arrangement includes transmit edge elements having a plurality of tunable optical transmitters, an optical switch and a cyclic optical multiplexer, and receive edge elements having optical demultiplexers, optical switches and a plurality of optical receivers. A variety of arrangements are disclosed including protected and unprotected network architectures. The network arrangement disclosed is particularly useful for overcoming the problem of scaling a photonic network.

Description:
FIELD OF THE INVENTION 
     The present invention relates to aggregation of groups of tunable sources in a photonic network and is particularly concerned with provision of optical network connections between elements in a photonic network without the use of optical-to-electrical-to-optical conversion. 
     BACKGROUND OF THE INVENTION 
     Presently existing optical networks include optical-to-electrical-to-optical (OEO) conversion at many points. A typical wavelength switch used today converts the input light signal into an electronic signal to detect the routing information, switches the electronic signal, and then eventually reconverts it back into a light signal for further transmission. This device, commonly referred to as an Optical-Electrical-Optical (OEO) switch, not only depends on current semiconductor technologies and processes, but also requires a transmitter and a receiver for each transmission port. These factors cause OEO switches to be large in size, to have high power consumption in the range of kilowatts, to be network protocol and transmission rate dependent, to lack scalability, and to be costly. 
     All-optical networks encounter issues of aggregating and disaggregating communication channels. Power combiners may be used to aggregate channels, but have the inherent problem of introducing high loss as channels are aggregated. Overcoming losses with all-optical amplifiers introduces noise. This issue is exacerbated as networks are scaled to larger sizes and denser frequencies, from systems with 32 optical channels with 100 GHz channel spacing, through 128 channel systems with 25 GHz channel spacing. This trending towards denser and denser frequencies is expected to continue as systems expand beyond the existing use of C-band into L-band. 
     Therefore, what is required is a method or system which would allow the network to establish photonic connections without the necessity of intermediate OEO conversion yet also allow effective scaling of the network to larger and larger networks. 
     SUMMARY OF THE INVENTION 
     The architecture of the invention allows ready optical connectivity between any transmitter and any receiver. The network contains transmit edge elements having tunable optical transmitters, optical switches, and cyclic multiplexers; as well receive edge elements having demultiplexers, optical switches and optical receivers. In general a default wavelength is associated with a particular receiver and the remainder of the network attempts to establish connectivity via that wavelength. Receivers are grouped into customer groups, and a set of wavelengths associated with a customer group forms a wavelength group. Where a particular default wavelength is already in use, connectivity with a receiver may be established using an alternative wavelength in the wavelength group. 
     Therefore, according to an aspect of the invention there is provided an optical network having a transmit edge element and a receive edge element. The transmit edge element has a plurality of tunable optical transmitters, an optical switch having separate inputs each optically connected to an output of one of the plurality of tunable optical transmitters, and a cyclic optical multiplexer having separate inputs each optically connected to a separate output of the optical switch. The output of the cyclic optical multiplexer constitutes the output of said transmit edge element. The receive edge element has an optical demultiplexer wherein the input of the optical demultiplexer constitutes the input of the receive edge element, an optical switch having separate inputs optically connected to the separate outputs of the optical demultiplexer; and a plurality of optical receivers. Each of the optical receivers is optically connected to a separate output of said second optical switch. The input of the receive edge element is optically connected to the output of the transmit edge element. In some applications the multiplexer in the transmit edge element need not be cyclic. 
     In some configurations, the optical network may have multiple transmit edge elements feeding an optical combiner and a band demultiplexer connected to the optical combiner. The outputs of the band demultiplexer are connected to multiple receive edge elements. 
     In an alternate configuration, the optical network may have a central optical switch node having inputs connected to multiple optical combiners, and outputs connected to multiple band demultiplexers. The inputs of the optical combiners are connected to multiple transmit edge elements, and the outputs of the band demultiplexers are connected to multiple receive edge elements. 
     The central optical switch node in certain configurations contains a plurality of optical channel demultiplexers, optical switch and a plurality of optical channel multiplexers wherein the optical switch cross-connects the outputs of the optical channel demultiplexers to the inputs of the optical channel multiplexers. Under certain conditions the central optical switch may have additional ports, and/or a Optical-Electrical-Optical (OEO) switch. 
     According to another aspect of the invention there is provided an optical network having an alternative transmit edge element. The alternative transmit edge element has a plurality of tunable optical transmitters and an optical switch. The optical switch has separate inputs each optically connected to an output of one of the plurality of tunable optical transmitters. Half of the outputs of the optical switch connect to the inputs of a first cyclic optical multiplexer, and the other half of the outputs of the optical switch connect to the inputs of a second cyclic optical multiplexer. The outputs of the first and second cyclic optical multiplexers constitute the first and second outputs of the transmit edge element. These outputs have different paths through the network and are connected to the inputs of a 2:1 optical switch. The output of the 2:1 optical switch connect to the input of a receive edge element. The receive edge element has an optical demultiplexer wherein the input of the optical demultiplexer constitutes the input of the receive edge element. The receive edge element also has an optical switch having separate inputs optically connected to the separate outputs of the optical demultiplexer; and a plurality of optical receivers each optically connected to a separate output of the optical switch. 
     According to yet another aspect of the invention there is provided an optical network having a ring connection joining multiple transmit edge elements and multiple receive edge elements to the central optical switch node. Optical power combiners in a chain configuration are used to join the multiple transmit edge elements. Optical band-droppers, comprising either thin-film band filters or power splitters, are connected in a chain configuration to join the multiple receive edge elements. 
     Conveniently, protection arrangements may be made for the network, either at the fiber level or at the central network switch level by appropriate connection of optical power splitters and n:1 optical switches. 
     The present invention will now be described in more detail with reference to exemplary embodiments thereof as shown in the appended drawings. While the present invention is described below with reference to the preferred embodiments, it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognise additional implementations, modifications, and embodiments which are within the scope of the present invention as disclosed and claimed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further understood from the following detailed description of embodiments of the invention and accompanying drawings, in which: 
         FIG. 1  is a diagram of a power combiner aggregating optical channels as is known in the art. 
         FIG. 2  is a diagram of an example transmit edge element and an example receive edge element according to an embodiment of the invention. 
         FIG. 3  is a diagram of an example cluster of transmit edge elements connected to a cluster of receive edge elements according to an embodiment of the invention. 
         FIG. 4  is a diagram of an example photonic network of multiple clusters of transmit edge elements connected to multiple clusters of receive edge elements according to an embodiment of the invention. 
         FIG. 5  is a diagram of the example photonic network of  FIG. 2  with fiber protection according to an embodiment of the invention. 
         FIG. 6  is a diagram of the example photonic network of  FIG. 3  with fiber protection according to an embodiment of the invention. 
         FIG. 7  is a diagram of an example alternative transmit edge element providing fiber protection connected to a receive edge element according to an alternative embodiment of the invention. 
         FIG. 8  is a diagram of the example photonic network of  FIG. 4  with fiber protection and network switching protection according to an alternative embodiment of the invention. 
         FIG. 9  is a diagram of an example photonic network in a branch configuration according to an alternative embodiment of the invention. 
         FIG. 10  is a diagram of an example photonic network with fiber protection according to an alternative embodiment of the invention. 
     
    
    
     In the figures, like elements are given like reference numbers. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is illustrated a transmit cluster  10  having a means of combining optical signal channels as is known in the art. Transmit cluster  10  has a plurality of tunable wavelength optical transmitters  12 . These typically may be lasers with associated optical modulators. The outputs of the transmitters  12  are optically connected (note: for the purposes of this specification an optical connection means an optical pathway between devices is established according to means well known in the art including such interconnecting devices as necessary to appropriate optical signal transfer, examples of which include optical fiber, connectors, amplifiers, attenuators, dispersion compensators, and the like) to the inputs of optical power combiner  14 . The output  16  of optical power combiner  14  provides an aggregated set of optical channels, however has difficulty scaling to larger numbers of optical transmitters. 
     Referring to  FIG. 2 , there is illustrated a transmit edge element  200  for a photonics network having a plurality of tunable wavelength optical transmitters  202 . These typically may be tunable lasers with associated optical modulators. The outputs of the transmitters  202  are optically connected to the inputs of optical switch  204 . The outputs of switch  204  are then optically connected to the inputs of cyclic multiplexer  206 . Cyclic multiplexer  206  may be any cyclic multiplexer such as a dielectric thin film, but would typically be an arrayed waveguide grating (AWG) cyclic multiplexer. The cyclic multiplexer output  208  constitutes the optical output of the transmit edge element  200 . The inherent capability of the transmit edge element as described is for any transmitter among transmitters  202  to provide a wavelength at a desired frequency, say λ 1 , at any of the inputs to optical switch  204 . The optical switch  204  then may switch λ 1  to the appropriate port corresponding to λ1 of the inputs of cyclic multiplexer  206 , where it will be multiplexed onto the output  208 . Output  208  of transmit edge element  200  is connected to output fiber  209  and thence to a network. Due to the cyclic nature of the multiplexer, it handles wavelengths of a cycle higher than λ 1  in the same manner as λ 1 . As an example, a cyclic AWG with a cycle of 8 channels would handle λ 1 , λ 9 , λ 17 , and λ 25  in a like manner upon having any of them connected to the same input port of  206 . Under certain applications, it may be possible to use a non-cyclic multiplexer in place of cyclic multiplexer  206 , for example where a wavelength plan does not require such. 
     Turning now to the other part of  FIG. 2 , there is illustrated a receive edge element  210  for a photonics network having a demultiplexer  216 . The input  218  of demultiplexer  216  constitutes the input of the receive edge element  210  and is optically connected via input fiber  219  to a network. As discussed for the cyclic multiplexer  206 , the channel demultiplexer  216  may be any demultiplexer such as a dielectric thin film, but would typically be an arrayed waveguide grating (AWG) demultiplexer. The outputs of the channel demultiplexer  216  are then optically connected to the inputs of optical switch  214 , and the outputs of optical switch  214  are connected to a plurality of optical receivers  212 . An effect of the aforedescribed arrangement is that any wavelength arriving on receive edge element input  218  may be connected to any of the plurality of optical receivers  212  as the demultiplexed wavelength will appear on one of the outputs of the demultiplexer  216  and may then be switched by the optical switch  214  to the desired optical receiver. In some applications the channel demultiplexer  216  could be cyclic, however this is not required for the present networks. Alternatively, it is contemplated that certain applications could use a non-cyclic multiplexer in the transmit edge element. Coupled with the use of a non-cyclic multiplexer would be a larger optical switch to compensate for the non-cyclic nature. 
     The optical switches  204  and  214  of  FIG. 2  are square in terms of having as many outputs as inputs, thus being 4×4 or 8×8 type optical switches for example. In practice, optical receivers  212  will each be assigned the frequency of the particular channel demultiplexer  216  output port is has connection to when the optical switch  214  is performing a straight-through connection. When it is desired to establish a connection between one of the optical transmitters  202  and a given receiver  212 , the optical transmitter  202  will be tuned to the appropriate frequency, and a connection established through the network as described previously. The simple photonic network of  FIG. 2  suffices for scenarios wherein a limited number of optical channels are required, e.g. 4 or 8 as matched by the optical switches  204  and  214 . 
     Referring to  FIG. 3 , there is illustrated a scaled-up photonic network in comparison to the network of  FIG. 2 , according to an embodiment of the invention. In this network, transmit edge element cluster  340  comprises a set of transmit edge elements. The outputs of these transmit edge elements are combined via power combiner  346  and connected via the output of power combiner  346  to the fiber  347 . Fiber  347  connects to the input of band demultiplexer  348 . The outputs of band demultiplexer  348  are connected to the inputs of the receive edge elements comprising receive edge element cluster  350 . In operation, each receive edge element of receive edge element cluster  350  will be assigned a sub-band of frequencies, and optical receivers within each receive edge element will each be assigned a default frequency within the sub-band of that receive edge element. When it is desired to establish a connection between one of the optical transmitters of the transmit edge element cluster  340  and a given receiver, the optical transmitter will be tuned to the appropriate frequency, and a connection established through the network as previously described. The photonic network given in  FIG. 3  scales upward in terms of optical receivers and transmitters to the point that the number of available frequencies is exhausted. In this example the optical switches in the receive edge elements are typically not required, however in anticipation of further network expansion they are included. 
     Referring now to  FIG. 4 , there is illustrated a photonic network architecture according to an embodiment of the invention. A first cluster  440  of customer groups  401 ,  402 , and  403  and a second cluster  442  of customer groups  404 ,  405 , and  406  of transmit edge elements as described in  FIG. 2  are optically connected to a central optical node  444 . Combining in a single fiber of transmit edge element outputs for a particular cluster occur via optical power combiners  446  and  447  as illustrated. Central optical node  444  contains photonic switch  464  optically coupled to input demultiplexers  461  and  462  and output multiplexers  465  and  466 . Central optical node  444  also optionally contains electronic switch  467  and additional optical input ports  468  and optical output ports  469 . The additional input and output ports may be used in single channel tributary cards pr sent in the node. The outputs of central optical node  444  are optically connected to band demultiplexers  448  and  449 . The outputs of band demultiplexers  448  and  449  are optically coupled to receive edge element inputs. The receive edge elements are grouped into a first cluster  450  of customer groups  411 ,  412 , and  413  and a second cluster  452  of customer groups  414 ,  415 , and  416 . 
     A typical embodiment employing this architecture could have transmit edge elements supporting customers in groups of 8 channels. A cyclic AWG with a 900 GHz free spectral range is used in both transmit and receive edge elements. Customer groups may be located in different locations with the provision of a miniband amplifier as needed in the optical connection between the transmit edge element and the power combiner. The architecture is scalable as a cluster of 32 customers and does not need to be fully populated at installation, but instead groups may be added over time as long as the optical power combiner is initially present. The port size of the central photonic switch will be a function of network size. As an example, if there are ten clusters, each with four groups of eight customer channels, then the size of the central photonic switch should be at least (320+n)×(320+n) where the first term of the expression is the product of the number of clusters, the number of groups, and the number of channels per group; and the second term represents the additional ports required for single channel tributary cards present in the node supporting services such as channels which need to be dropped or added at this location, and for re-grooming and switching of some channels with sub-lambda customers which have alternative destinations. For simplicity of presentation only one central optical node has been illustrated, however the architecture can be extended to pass through multiple central nodes. Additional amplifiers, attenuators, and dispersion compensators may be placed as required to complete the optical connectivity. 
     The band demultiplexers  448  and  449  could be dielectric thin film filters or AWG if performance permits. Their purpose is to split the signal from the output of central node (the egress trunks) into sub-bands appropriate to the group of wavelengths destined for a particular receive edge element. The sub-bands are unique. 
     By way of example, referring to  FIG. 4 , assume that it is desired to establish connectivity between a customer at optical transmitter  441  in customer group  401  (note that in this discussion edge elements correspond to customer groups) and a customer at receiver  451  in customer group  411 . Assume the customer at receiver  451  is considered customer number  2  in the customer group  411  and therefore uses λ 2  as a default. The optical transmitter  441  would be tuned to λ 2 . The optical signals would exit the transmit edge element, proceed through optical power combiner  446 , be switched through the central optical node  444  to band demultiplexer  448  where as a member of the wavelength group associated with receive edge element  411 , would be delivered to optical receiver  451  with the optical switch of the receive edge element set to a “straight-through” switch setting. Should some other optical receiver in the wavelength group associated with receive edge element  411  be currently using λ 2  then optical transmitter  441  could be tuned to an alternate wavelength of the same wavelength group  411 , for example λ 3 . As the signals arrive at receive edge element  411 , the demultiplexed wavelengths would emerge from the demultiplexer at the λ 3  output port and would be switched to the receiver for customer number  2 . As the receivers are wavelength independent, the arrival of the data encoded on λ 3  will not interfere with reception. 
     The decision of wavelength allocation at each node will be accomplished by local and global management. It is anticipated that each node will have a lookup table containing the wavelengths available at each receive edge element, and the wavelength allocation at a given time noted as free or in-use. The look-up table would be updated on an ongoing basis as wavelengths are claimed or released from use. 
     In any particular architectural configuration, wavelength groups will be preferably established taking advantage of particular component availability. For example, a cluster may comprise four customer groups with each customer group having a total of eight wavelengths per customer group for a total of 32 wavelengths in a cluster. Alternatively, smaller edge elements may be deployed with four wavelengths per customer group, with a total of eight customer groups to a cluster; yielding again a total of 32 wavelengths in a cluster. Within a cluster wavelength management is handled by the optical switches within an edge element. To establish connectivity between clusters the central optical switch is employed. 
     By way of another example referring to  FIG. 4 , assume that it is desired to establish connectivity between a customer at optical transmitter  441  in customer group  401  and a customer at receiver  457  in customer group  415  in cluster  452 . Assume the customer at receiver  457  is considered customer number  8  in cluster  452  and therefore uses λ 8  as a default, the optical transmitter  441  would be tuned to λ 8 . The optical signals would exit the transmit edge element, proceed through optical power combiner  446 , be switched through the central optical switch  444  to, demultiplexer  449  i.e. across to cluster  452  where as a member of the wavelength group associated with receive edge element  415 , would be delivered to optical receiver  457  with the optical switch of the receive edge element set to a “straight-through” switch setting. 
     Should some other optical receiver associated with receive edge element  415  be currently using λ 8  or any other optical transceivers from the cluster  440  be using λ 8  or cyclic variation of λ 8 , then optical transmitter  441  could be tuned to an alternate wavelength of the wavelength group, for example λ 5 . As the signals arrive at receive edge element  415 , the demultiplexed wavelengths would emerge from the demultiplexer at the λ 5  output port and would be switched via the internal optical switch of receive edge element  415  to the optical receiver  457 . As can be seen, the presence of the optical switch within the receive edge element resolves blocking problems at the receive edge element level. 
     Referring to  FIG. 5 , there is illustrated a protected form of the network shown in  FIG. 2 . Transmit edge element  500  is connected to receive edge element  510 . In this particular embodiment, the output of transmit edge element  500  is first connected to optical power splitter  528 . The outputs  524  and  526  of optical power splitter  528  travel different paths through the network and are connected to the 2:1 optical switch  522  which has its output connected to the input of receive edge element  510 . Should there be a network failure on either path  524  or  526 , signalling may be transferred via optical switch  522  to the alternate output maintaining the connection between transmit edge element  500  and receive edge element  510 , thereby providing path protection. 
     Referring to  FIG. 6 , there is illustrated a protected form of the network shown in  FIG. 3 . Transmit edge element cluster  640  comprises a set of transmit edge elements the outputs of which are combined via power combiner  646 . The output of power combiner  646  is connected to optical power splitter  628 . The outputs  624  and  626  of optical power splitter  628  are connected to the 2:1 optical switch  622  which has its output connected to the input of band demultiplexer  648 . The outputs of band demultiplexer  648  are connected to the inputs of the receive edge elements comprising receive edge element cluster  650 . In operation, should there be a network failure on either path  624  or  626 , signalling may be transferred via optical switch  622  to the alternate output maintaining the connection between transmit edge element cluster  640  and receive edge element cluster  650 , thereby providing path protection. 
     Referring to  FIG. 7 , there is illustrated an alternative construction for a transmit edge element and for a receive edge element photonic network. In this embodiment, transmit edge element  770  has a plurality of tunable wavelength optical transmitters  772  optically connected to the inputs of optical switch  774  as in the previous embodiment. However, optical switch  774  is not square as in the previous embodiment, but instead of a form factor n×(2n) where n is the number of inputs. Optically connected to the doubled outputs are cyclic multiplexers  775  and  776 . The advantage of this configuration lies in the redundancy provided by the dual outputs  778  and  779 . As two optical outputs emerge from this variation of transmit edge element, provision for a protected network architecture can be had as any of the transmitters  722  may be connected to either output  778  or  779 . Optical path  724  connected to output  778  and optical path  726  connected to output  779  are combined by the optical combiner  722 . The output of optical combiner  722  is connected to the input of receive edge element  710 . Should there be a network failure on either path connected to either output, signalling will be transferred via optical combiner  722  to the alternate output. 
     Referring to  FIG. 8 , there is provided an alternative embodiment of a photonic network similar to that of  FIG. 4 , with added fiber and network switch protection. In this embodiment, there is a first cluster  840  of transmit edge elements and a second cluster  850  of receive edge elements. Optical splitters  880 ,  881 , and  882  act to split the optical outputs of the respective transmit edge elements. Power combiner  846  combines the first outputs of the optical splitters  880 ,  881 , and  882  for central optical node  844 . Likewise, power combiner  886  combines the second outputs of the optical splitters  880 ,  881 , and  882  for central optical node  884 . The output of central optical node  844  is taken to band demultiplexer  848  and thence to the respective receive edge elements via 2:1 optical switches  890 ,  891 , and  892 . Likewise, the output of central optical node  884  is taken to band demultiplexer  888  and thence to the respective receive edge elements via 2:1 optical switches  890 ,  891 , and  892 . In operation, should there be a network failure on either the paths or network elements connected to one of the central optical nodes  844  or  884 , signalling may be transferred via optical switches  890 ,  891 , and  892  to the alternate input maintaining the connection between transmit edge element clusters and receive edge element cluster, thereby providing both path and network protection. 
     Referring to  FIG. 9 , there may be seen a diagram of a photonic network in a branch configuration according to an alternative embodiment of the invention. Transmit edge elements  900 ,  940 , and  960  are connected to power combiners  995  and  996  in the configuration shown so that the combined outputs are connected to the input of central optical node  944 . The ports of power combiners  995  and  996  may contain taps, variable attenuators, and amplifiers which, operating in conjunction with an optical spectrum analyzer, could ensure that the optical channel power of the added traffic will be equalized to the optical channel power of the incoming traffic. The ratio of the power combiners would be network design dependent, however the amplifiers and attenuators would operate so as to adjust the power of each port such that the power per channel will be matched for all channels at the output port. The output of central optical node  944  is taken to band-drop elements  997  and  998 . These band-drop elements would typically be wide-band thin-film drop filters which drop a specific sub-band corresponding to the frequency allocations of each receive edge element  910 ,  950 , and  970 . However, in an alternative contemplated configuration, band-drop elements  997  and  998  could comprise optical power splitters for the case where there is no defined band structure or alternatively, where the number of receive edge elements exceeds the number of sub-bands available. The branch configuration offers advantages of reduced fiber due to the channel accumulation. 
     Referring to  FIG. 10 , there is illustrated an alternative construction for a photonic network providing fiber and network switch protection. In this embodiment, the transmit edge elements have non-square optical switches and double output multiplexers as described for the embodiment depicted in  FIG. 7 . As well, the receive edge elements are configured in a similar manner with non-square optical switches and double input demultiplexers. In this embodiment, there is a first cluster  1040  of transmit edge elements and a second cluster  1050  of receive edge elements. Power combiner  1046  combines the first outputs of the transmit edge elements for central optical node  1044 . Likewise, power combiner  1086  combines the second outputs the transmit edge elements for central optical node  1084 . The output of central optical node  1044  is taken to band demultiplexer  1048  and thence to the respective first inputs of the receive edge elements. Likewise, the output of central optical node  1084  is taken to band demultiplexer  1088  and thence to the respective second inputs of the receive edge elements. In operation, should there be a network failure on either the paths or network elements connected to one of the central optical nodes  1044  or  1084 , signalling may be transferred via the optical switches within the transmit and receive edge elements to the alternate multiplexers and demultiplexers respectively, maintaining the connection between transmit edge element clusters and receive edge element cluster, thereby providing both path and network protection. 
     In general, for a photonic network having:
         a channel spacing of X GHz;   channel groups of M channels;   channel spacing between channel groups of S skipped channels; and   a total of P channel groups available in an available optical band, the total number of channels available with separate wavelengths may be calculated as the product of M and P, i.e. (M×P). Further, this total number of channels occupies a spectrum of bandwidth which may be calculated as:
 
[((M+S)×P)−S]×X
       

     For specific applications, S may be set to {0, 1, 2, . . . } as per a wavelength plan. Also X may be 100 GHz, 50 GHz, or 25 GHz. The total number of channel groups available, P, may be 4, 5, . . . , 9 or higher. In typical applications M may be 4 or 8, but M may assume other values as needed in specific applications. 
     For the cyclic AWG used for multiplexing in the transmit edge element, the Free Spectral Range (FSR) required may be calculated according to the formula:
 
(M+S)×X.
 
     For the non-redundant edge element the switch size is:
         M by M.       

     The group multiplexer which directs the channels to the receive edge elements has a required bandwidth for each output of:
 
(M−1)×X.
 
     While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.