Abstract:
An optical data network including an optical communications medium, adapted to convey optical signals of multiple different wavelengths; a plurality of optical data transceivers, which are adapted to transmit and receive the optical signals over the medium and at least one optical coupler. The optical coupler is coupled to the medium between the transceivers, and is adapted to filter the wavelengths conveyed over the medium so as to convey the optical signals in a first subset of the wavelengths only to a first group of the transceivers, and to convey the optical signals in a second subset of the wavelengths only to a second group of the transceivers, which is different from the first group.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/325,248, filed Sep. 26, 2001, which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to data communication, and specifically to data transference by wavelength multiplexing of optical carriers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Passive optical networks (PONs) use fibre optic cables which may be coupled together in a number of different configurations, for example, star, tree, bus, ring or mesh topologies, or combinations of these topologies. Protection against failure of a section of the network may be provided inherently, as in a ring topology, or by adding redundant fibre optic links between elements of the network.  
           [0004]    U.S. Pat. No. 5,003,531, to Farinholt et al., whose disclosure is incorporated herein by reference, describes an optical network having all nodes of the network coupled together by fibre optic cables. The cables are configured so that a failure of a cable or of a node is overcome by protection switching to a standby link formed from the cables.  
           [0005]    U.S. Pat. No. 5,982,517, to Fishman, whose disclosure is incorporated herein by reference, describes an optical fibre optic network comprising a plurality of synchronous optical network (SONET) rings coupled to a wavelength division multiplexing (WDM) link. The WDM link is protected against failure by rerouting WDM link traffic through a dedicated protection ring selected from the SONET rings. The rerouting is performed by “color-blind” optical fibre-to fibre switches.  
           [0006]    U.S. Pat. No. 6,327,400, to Harstead et al., whose disclosure is incorporated herein by reference, describes a method for protecting a point-to-multipoint network against fibre and/or network terminal failure. Terminals are coupled to a ring via a 1:2 optical switch, and protection is provided in the case of a failure by switching the optical switch.  
           [0007]    U.S. Pat. No. 6,327,400, to Dyke et al., whose disclosure is incorporated herein by reference, describes a passive optical network (PON) arrangement having a plurality of optical splitter/combiners. Each splitter/combiner comprises a pair of through ports and one or more “drop” ports which transfer a portion of radiation conveyed between the through ports to downstream optical terminals. The PON arrangement may use WDM to separate transmit and receive signals in the network.  
           [0008]    U.S. Pat. No. 6,414,768, to Sakata et al., whose disclosure is incorporated herein by reference, describes -an optical communication system in the form of a loop network. The system utilizes an active transceiver and a standby transceiver at a head end. The pair of transceivers communicate with terminals coupled via star couplers to the loop. A test signal is transmitted in the loop to detect a fault, and when a fault is detected, a control unit at the head end operates the standby transceiver to recover communications that have been unsuccessfully transmitted because of the fault.  
           [0009]    Bandwidth of an optical network may be increased, without changing the physical configuration of the network, by using wavelength division multiplexing. However, power budget within the network is a factor limiting the size and/or complexity of any network, whether or not WDM is used. For example, in star or tree networks using point-to-multipoint transitions, each transition significantly attenuates power transferred through the transition. In such point-to-multipoint transitions much power may be wasted, since what may actually be needed is a point-to-point transition.  
           [0010]    Thus, a more flexible and efficient method for configuring passive optical networks is desirable.  
         SUMMARY OF THE INVENTION  
         [0011]    Preferred embodiments of the present invention seek to provide a passive optical network (PON) which is able to be configured flexibly and which is also able to significantly reduce power wastage. The network is operated in a wavelength division multiplexing (WDM) mode, and is divided topologically into autonomous regions by wavelength-dependent splitter/combiners. By dividing the network into regions, substantial sections of the network may be protected against failure within the network. Furthermore, dividing the network into regions improves power distribution within the network and increases overall bandwidth of the network.  
           [0012]    In preferred embodiments of the present invention, an optical communications system comprises optical data transceivers, also herein termed network elements, which communicate with each other over a PON using WDM. The network elements are coupled to fibre optics within the PON, and the fibre optics are in turn coupled to wavelength-dependent splitter/combiners, herein termed couplers. The couplers act as partial separators, so that the PON is effectively split into a number of substantially independent regions, each region comprising a sub-set of the network elements present in the complete network (the PON). Network elements within each region may communicate with each other using wavelengths common to the region, but are shielded from communication with other regions which use different wavelengths. However, any network element may be a member of more than one region, depending on how the PON is configured, and also on the wavelengths that couplers connecting to the network element are configured to transmit. Thus, communication between any elements of the network may be implemented. Furthermore, by dividing the network into multiple regions, multiple paths may be formed between network elements, improving the reliability of network operation.  
           [0013]    There is therefore provided, according to a preferred embodiment of the present invention, an optical data network including:  
           [0014]    an optical communications medium, adapted to convey optical signals of multiple different wavelengths;  
           [0015]    a plurality of optical data transceivers, which are adapted to transmit and receive the optical signals over the medium; and  
           [0016]    at least one optical coupler which is coupled to the medium between the transceivers, and which is adapted to filter the wavelengths conveyed over the medium so as to convey the optical signals in a first subset of the wavelengths only to a first group of the transceivers, and to convey the optical signals in a second subset of the wavelengths only to a second group of the transceivers, which is different from the first group.  
           [0017]    Preferably, the at least one optical coupler includes a passive optical coupler.  
           [0018]    Further preferably, the passive optical coupler includes at least one of an element chosen from a coated beamsplitter, an un-coated beamsplitter, a diffractive optical element, a waveguide, and an optically active material.  
           [0019]    Preferably, the passive optical coupler is operative as at least one type of filter chosen from a narrow band filter, a broad band filter, a long pass filter, and a short pass filter.  
           [0020]    Preferably, the optical communications medium includes one or more fibre optic cables, each fibre optic cable having one or more strands.  
           [0021]    Preferably, the optical data transceivers are configured in at least one configuration chosen from a star, tree, ring, bus, and mesh structure.  
           [0022]    Further preferably, the first group of the transceivers and the second group of the transceivers are configured in substantially the same configuration.  
           [0023]    Alternatively, the first group of the transceivers and the second group of the transceivers are configured in different configurations.  
           [0024]    Preferably, the first group of the transceivers and the second group of the transceivers include at least one common transceiver.  
           [0025]    Further preferably, the at least one common transceiver includes a head end transceiver adapted to operate as a controller of the optical signals in the first group and the second group of the transceivers.  
           [0026]    Preferably, the first group of the transceivers is adapted to operate according to a first communication protocol, and the second group of the transceivers is adapted to operate according to a second communication protocol, different from the first protocol.  
           [0027]    Alternatively, the first group of the transceivers and the second group of transceivers are adapted to operate according to substantially identical communication protocols.  
           [0028]    Preferably, the at least one optical coupler includes a three port coupler having a first port which is adapted to transfer the first subset and the second subset of the wavelengths, a second port which is adapted to transfer only the first subset of the wavelengths, and a third port which is adapted to transfer only the second subset of the wavelengths.  
           [0029]    Alternatively, the at least one optical coupler includes a three port coupler having a first and a second port which are adapted to transfer the first subset and the second subset of the wavelengths, and a third port which is adapted to transfer only the second subset of the wavelengths.  
           [0030]    There is further provided, according to a preferred embodiment of the present invention, a method for configuring an optical data network including:  
           [0031]    coupling a plurality of optical data transceivers to exchange optical signals of multiple different wavelengths over an optical communication medium;  
           [0032]    coupling at least one optical coupler to the medium and the optical data transceivers; and  
           [0033]    filtering wavelengths conveyed between the optical data transceivers using the at least one optical coupler so as to convey the optical signals in a first subset of the wavelengths only to a first group of the transceivers, and to convey the optical signals in a second subset of the wavelengths only to a second group of the transceivers, which is different from the first group.  
           [0034]    Preferably, the at least one optical coupler includes a passive optical coupler.  
           [0035]    Further preferably, the passive optical coupler includes at least one of an element chosen from a coated beamsplitter, an un-coated beamsplitter, a diffractive optical element, a waveguide, and an optically active material.  
           [0036]    The method preferably also includes operating the passive optical coupler as at least one type of filter chosen from a narrow band filter, a broad band filter, a long pass filter, and a short pass filter.  
           [0037]    Preferably, the optical communications medium includes one or more fibre optic cables, each fibre optic cable having one or more strands.  
           [0038]    The method preferably also includes configuring the optical data transceivers in at least one configuration chosen from a star, tree, ring, bus, and mesh structure.  
           [0039]    Preferably, configuring the optical transceivers includes configuring the first group of the transceivers and the second group of the transceivers in substantially the same configuration.  
           [0040]    Alternatively, the first group of the transceivers and the second group of the transceivers are configured in different configurations.  
           [0041]    Preferably, the first group of the transceivers and the second group of the transceivers include at least one common transceiver.  
           [0042]    Further preferably, the at least one common transceiver includes a head end transceiver, and the method includes controlling the optical signals in the first group and the second group of the transceivers using the head end transceiver.  
           [0043]    Preferably, the method includes operating the first group of the transceivers according to a first communication protocol, and operating the second group of the transceivers according to a second communication protocol, different from the first protocol.  
           [0044]    Alternatively, the method includes operating the first group of the transceivers and the second group of transceivers according to substantially identical communication protocols.  
           [0045]    Preferably, the at least one optical coupler includes a three port coupler having a first port which is adapted to transfer the first subset and the second subset of the wavelengths, a second port which is adapted to transfer only the first subset of the wavelengths, and a third port which is adapted to transfer only the second subset of the wavelengths.  
           [0046]    Alternatively, the at least one optical coupler includes a three port coupler having a first and a second port which are adapted to transfer the first subset and the second subset of the wavelengths, and a third port which is adapted to transfer only the second subset of the wavelengths.  
           [0047]    There is further provided, according to a preferred embodiment of the present invention, an optical data network for coupling a plurality of optical data transceivers to communicate, the network including:  
           [0048]    an optical communications medium, adapted to convey optical signals of multiple different wavelengths between the transceivers; and  
           [0049]    at least one optical coupler which is adapted to be coupled to the medium between the transceivers, and which is adapted to filter the wavelengths conveyed over the medium so as to convey the optical signals in a first subset of the wavelengths only to a first group of the transceivers, and to convey the optical signals in a second subset of the wavelengths only to a second group of the transceivers, which is different from the first group.  
           [0050]    Preferably, the at least one optical coupler includes a passive optical coupler.  
           [0051]    Preferably, the optical data transceivers are configured in at least one configuration chosen from a star, tree, ring, bus, and mesh structure.  
           [0052]    Further preferably, the first group of the transceivers and the second group of the transceivers are configured in substantially the same configuration.  
           [0053]    Alternatively, the first group of the transceivers and the second group of the transceivers are configured in different configurations.  
           [0054]    Preferably, the first group of the transceivers is adapted to operate according to a first communication protocol, and the second group of the transceivers is adapted to operate according to a second communication protocol, different from the first protocol.  
           [0055]    Alternatively, the first group of the transceivers and the second group of transceivers are adapted to operate according to substantially identical communication protocols.  
           [0056]    There is further provided, according to a preferred embodiment of the present invention, a method for re-configuring an optical data network having a plurality of optical data transceivers coupled to exchange optical signals of multiple different wavelengths over an optical communication medium, the method including:  
           [0057]    retro-fitting at least one optical coupler to the medium and the optical data transceivers; and  
           [0058]    filtering wavelengths conveyed between the optical data transceivers using the at least one optical coupler so as to convey the optical signals in a first subset of the wavelengths only to a first group of the transceivers, and to convey the optical signals in a second subset of the wavelengths only to a second group of the transceivers, which is different from the first group.  
           [0059]    Preferably, the at least one optical coupler includes a wavelength dependent passive optical coupler, wherein the optical network initially includes a wavelength independent coupler, and wherein retro-fitting the at least one optical coupler includes replacing the wavelength independent coupler with the wavelength dependent passive optical coupler.  
           [0060]    Preferably, the at least one optical coupler includes a passive wavelength dependent optical coupler, wherein the optical network initially includes an active wavelength dependent coupler, and wherein retro-fitting the at least one optical coupler includes replacing the active wavelength dependent coupler with the passive wavelength dependent optical coupler.  
           [0061]    Preferably, the optical data network initially operates according to a first protocol, and retro-fitting the at least one optical coupler includes replacing the first protocol by a second protocol, different from the first protocol.  
           [0062]    The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0063]    [0063]FIG. 1 is a schematic diagram of a passive optical network (PON), according to a preferred embodiment of the present invention; and  
         [0064]    [0064]FIG. 2 is a schematic diagram illustrating properties of wavelength dependent couplers used in the PON of FIG. 1, according to a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0065]    Reference is now made to FIG. 1, which is a schematic diagram of a passive optical network (PON)  10 , according to a preferred embodiment of the present invention. Network  10  comprises generally similar optical data transceivers  24 A,  24 B, . . . ,  24 J, and  24 K, herein also termed network elements, and generically termed network elements (NEs)  24 . Network  10  also comprises a head end element  22 , which is able to transmit data to and receive data from network elements  24 , and which acts as a controller of the network. Network elements  24  are coupled to each other by fibre optic cables  26 , and head end element  22  is also coupled to network elements  24 A and  24 K by cables  26 . Network elements  24 A,  24 I,  24 J,  24 K,  24 B, and  24 H are coupled to their respective fibre optic cables  26  by respective wavelength independent splitters  21 . Network elements  24 C,  24 D,  24 E,  24 F, and  24 G act as terminators of their respective fibre optic cables  26 . Cables  26  comprise one or more fibre optic strands, each fibre optic strand being able to convey optical radiation within the strand. Each cable  26  may comprise two strands which are used to convey optical radiation in opposite directions. Alternatively, a single fibre optic strand may be used to convey bi-directional radiation.  
         [0066]    Elements  24  and head element  22  transfer data between themselves by using optical radiation as a data carrier. The optical radiation is wavelength multiplexed, and by way of example head end element  22  is assumed to operate at seven different wavelengths λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , λ 6 , and λ 7 , herein generically termed wavelengths λ. Each wavelength λ may be substantially a single wavelength, or alternatively may comprise a group of two or more wavelengths. For example, λ 1  may comprise 1510 nm and 1530 nm, which are typically used as separate receive and transmit carriers.  
         [0067]    Network  10  comprises passive wavelength dependent couplers  12 A,  12 B,  12 C, and  12 D, herein also generically termed couplers  12 , which transfer optical radiation between fibre optic cables  26  to which they are connected, and which are wavelength dependent. Couplers  12  comprise three or more ports connecting to the fibre optic cables, and act as wavelength dependent splitter/combiners of optical radiation. Examples of couplers  12  and their operation are described with reference to FIG. 2 below. Network  10  also, by way of example, comprises a wavelength independent one-to-many star splitter  20 .  
         [0068]    [0068]FIG. 2 is a schematic diagram illustrating properties of passive wavelength dependent couplers, according to a preferred embodiment of the present invention. A first coupler  30  comprises three optical ports  32 ,  34 , and  36 , also referred to herein as ports A, B, and C respectively. Each port  32 ,  34 , and  36  may operate as a unidirectional port or as a bi-directional port. Port  32  receives radiation having two wavelengths λ A  and λ B , and coupler  30  divides the radiation so that substantially all energy at wavelength λ A  is radiated from port  34  and substantially all energy at wavelength λ B  is radiated from port  36 . Coupler  30  may also operate as a combiner of wavelengths λ A , and λ B . For example, port  34  receives wavelength λ A  and port  36  receives wavelength λ B , and substantially all energy at wavelengths λ A  and λ B  is radiated from port  32 .  
         [0069]    A second coupler  40  comprises three optical ports  42 ,  44 , and  46 , also referred to herein as ports D, E, and F respectively. Each port  42 ,  44 , and  46  may operate as a unidirectional port or as a bi-directional port. Port  42  receives radiation having two wavelengths λ A , and λ B . Coupler  40  divides the radiation so that substantially all energy at wavelength λ A  is transferred from port  42  to port  46  and is radiated therefrom. Energy at wavelength λ B  is split so that a first portion is conveyed to port  44 , and a second, remaining, portion is conveyed to port  46 . Typically the first portion is a small percentage of the total energy incident at port  42 . A similar process occurs for radiation at wavelengths λ A  and λ B  initially incident on port  46 , substantially all energy at λ A  being conveyed to port  42 , and a portion of the energy at λ B  being diverted to port  44 . Energy incident on port  44 , at wavelength λ B , may be transferred to port  42 , port  46 , or both ports, depending how coupler  40  is configured. (It will be appreciated that coupler  40  may be implemented as a combination of coupler  30  with a splitter.) Herein it is assumed that energy at λ B  incident on port  44  is transferred to both ports  42  and  46 .  
         [0070]    Those skilled in the optical art will be familiar with other types of passive wavelength dependent couplers, similar in operation to that described above for couplers  30  and  40 , and methods for implementing such couplers. The passive couplers may be formed by combining one or more couplers of the form of coupler  30  and/or coupler  40 , and may comprise wavelength-dependent optical elements such as coated or un-coated beamsplitters, diffractive optical elements, waveguide elements and/or an optically active material such as a ferroelectric, or combinations or sub-combinations of such elements. Each such coupler may be formed as a narrow or a broad band filter, and/or as a long or short pass filter, or as a combination of these types of filters, according to wavelength transmission and reflection requirements of the coupler being implemented. All such types of passive couplers are assumed to be within the scope of the present invention.  
         [0071]    Returning to FIG. 1, couplers  12 A,  12 B, and  12 D are implemented to operate as three port wavelength dependent couplers similar to coupler  30 . Table I below shows characteristics of each of these couplers.  
                                                 TABLE I                                           Ports, elements coupled to ports, and               wavelengths transferred via ports.                Coupler   Port A   Port B   Port C                       12A   NE 24A   NE 24B   NE 24I               λ 1 , λ 2 , λ 3 ,   λ 4 , λ 5 , λ 6 ,   λ 1 , λ 2 , and               λ 4 , λ 5 , λ 6 ,   and λ 7     λ 3                 and λ 7             12B   NE 24B   Splitter 20   Coupler 12C               λ 4 , λ 5 , λ 6 ,   λ 5  and λ 6     λ 4  and λ 7                 and λ 7             12D   NE 24K   NE 24J   NE 24H               λ 1 , λ 2 , λ 3 ,   λ 1 , λ 2 , and   λ 4  and λ 7                 λ 4 , and λ 7     λ 3                        
 
         [0072]    Coupler  12 C is implemented to operate as a three port wavelength dependent coupler similar to coupler  40 . Table II below shows characteristics of coupler  12 C.  
                                                 TABLE II                                           Ports, elements coupled to ports, and               wavelengths transferred via ports.                Coupler   Port D   Port E   Port F                       12C   Coupler 12B   NE 24F, NE 24G   NE 24H               λ 4  and λ 7     λ 7     λ 4  and λ 7                        
 
         [0073]    Most preferably, each NE  24  comprises a respective filter which only allows wavelengths transferred to and from the network element to pass. For example, NE  24 H comprises a filter allowing wavelengths λ 4  and λ 7  to be received by and transmitted from the network element.  
         [0074]    It will be appreciated by inspection of FIG. 1 that couplers  12  divide network  10  into topologically distinct regions, the network elements within each region being able to communicate with each other using one or more wavelengths transmitted within each region. Because the regions are topologically distinct, communications within each region may be performed substantially independently of and in parallel with communications within other regions. Table III below shows regions A, B, C, and D of network  10 , network elements within each region, and wavelengths used for transferring data within each region.  
                               TABLE III                                           Wavelengths used to               Network Elements comprised   transmit data within           Region   within the region   the region                           A   Head end 22, NEs 24A, 24I,   λ 1 , λ 2 , and λ 3                 24J, and 24K           B   Head end 22, NEs 23A, 24B,   λ 7                 24F, 24G, 24H, and 24K           C   Head end 22, NEs 24A, 24B,   λ 5  and λ 6                 24C, 24D, and 24E           D   Head end 22, NEs 24A, 24B,   λ 4                 24H, and 24K                      
 
         [0075]    The regions defined by couplers  12  may comprise star, tree, ring, bus, or mesh structures, or combinations of these or other topological structures. Each region functions according to properties specific to the region, substantially independent of properties of other regions within network  10 . For example, region C described above comprises point-to-multipoint star coupler  20 , so that network elements  24 C,  24 D, and  24 E may act as downstream optical network units which are controlled by head end  22  operating as an upstream unit. Since elements  24 C,  24 D, and  24 E are coupled by coupler  20  and operate at the same wavelengths λ 5  and λ 6 , head end  22  most preferably controls their operation by one of the time division multiplexed (TDM) systems known in the art, in order to prevent data information collisions within the third region.  
         [0076]    Depending on the type of structure defined, a region may include prevention against a failure within the region. For example, region A described above comprises a ring structure which may be implemented to operate as a token ring. A failure within the ring, for example, a break in cable  26  between network elements  24 I and  24 J, at a point  27 , does not destroy the connectivity between head end  22  and network elements  24 A,  24 I  24 J, and  24 K. Similarly, a failure within one specific region of the network may have substantially no effect on connectivity of other regions. For example, the failure at point  27  has substantially no effect on communications within region B described above. Conversely, a failure at a point  29  between couplers  12 B and  12 C in region B has substantially no effect on communications within region A.  
         [0077]    Network  10  uses head end  22  as an overall controller of the network, and head end  22  and network element  24 A are members of all regions of the network. However, it will be appreciated that there is no necessity for there to be one or more network elements which are common to all regions of networks configured within the scope of the present invention. For example, a network  50  may be configured to be substantially the same as network  10 , but without head end  22 , network element  24 A, and their interconnecting cable  26 . Table IV below shows regions E, F, G, and H of network  50 , elements within the regions, and wavelengths used by the regions.  
                               TABLE IV                                           Wavelengths used to               Network Elements comprised   transmit data within           Region   within the region   the region                           E   NEs 24I, 24J, and 24K   λ 1 , λ 2 , and λ 3             F   NEs 24B, 24F, 24G, 24H, and   λ 7                 24K           G   NEs 24B, 24C, 24D, and 24E   λ 5  and λ 6             H   NEs 24B, 24H, and 24K   λ 4                        
 
         [0078]    By inspection of Table IV it will be observed that there is no common network element to regions comprising network  50 . Communications within each region in network  50  may be controlled by a network element in each respective region which is designated to be a region controller.  
         [0079]    Networks such as network  10  and network  50  have significantly better utilization of power compared to networks which are not divided into regions. For example, referring to Table III, power in region A is divided between head end  22  and four network elements  24 A,  24 I,  24 J, and  24 K, and is not radiated to other elements of network  10 , wherein the power would be wasted. Similar power savings occur for other regions of both networks  10  and  50 .  
         [0080]    It will be appreciated that each region that a network such as network  10  is sub-divided into may operate with a different protocol, unrelated to that operated by any other region. For example, region A may operate using an asynchronous time multiplexed PON (APON) protocol, region B may operate using a carrier sense multiple access with collision detection (CSMA/CD) protocol such as an Ethernet PON (EPON) protocol, region C may operate using a time division multiple access (TDMA) protocol such as a gigabit PON (GPON) protocol, and region D may operate according to any other standard or custom protocol implemented by an operator of the network. Alternatively, since the regions are substantially independent, two or more regions may operate using the same protocol.  
         [0081]    It will also be appreciated that networks such as networks  10  or  50  may be implemented by retro-fitting couplers such as couplers  12 to existing networks. The retro-fitting preferably replaces couplers which are operative as substantially wavelength independent couplers with wavelength dependent couplers such as couplers  12 . Alternatively, the retro-fitting may take the form of adding couplers such as couplers  12  to an existing network. Furthermore, couplers such as couplers  12  may be retro-fitted to networks having active couplers, such as optical add/drop multiplexers (OADMs); in this case a protocol initially operating the network may need to be altered and/or replaced to accommodate the change from an active coupler to a passive coupler. Thus, a network which is initially undivided may be re-configured into regions, defined by couplers  12 , which comprise sub-sets of network elements of the original network.  
         [0082]    It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.