Abstract:
A method for installing branches in a protective ducting system in which guide tubes and cables have already been laid, wherein at a desired branch location in the protective duct, a splittable Y-branch connector with an inlet opening, an outlet opening and at least one branch opening is installed by cutting a short section from the duct at the desired branch point and exposing the guide tubes and cables, opening the splittable branch connector and arranging it around the exposed guide tubes and cables, and closing and securing the Y-branch connector with splittable coupling collars in such a manner that the inlet opening and the outlet opening engage in sealing manner over the respective cut ends of the existing duct, and the branch opening of the connector is secured by engagement of a branch stub around a branch duct through which a branch guide tube is laid.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. application Ser. No. 09/750,802 filed Dec. 27, 2000 now U.S. Pat. No. 6,619,697. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   This invention relates generally to optical fiber communication systems, and in particular to a split table branch connector and a method for installing the branch connector in an existing protective ducking system in which channelization guide tubes and/or cables already have been installed. 
   Communication systems employing optical fibers have termination points where optical fiber cross connections, interconnections and terminations are established. The termination points are generally located at a customer&#39;s premises, remote from a central office. To reach each termination point, the optical fiber cables must be separated (to form a branch) from a bundle of cables (a “trunk” or “ring”) and then are routed through a protective branch duct from various junctions or branch locations to remote customer interface access stations. 
   In the access network, the connection from the central exchange office to the customer passes many splices and branches. Splicing and branching in copper twisted-pair has been done for more than a century. However, for optical fiber (where a minimum of splices is preferred) the conventional technology does not suffice. Optical access networks require a high degree of versatility: it is not known when or where a connection will be requested, installation must be fast, new connections should not disturb existing ones, both business and consumer markets may ask for solutions requiring different quality of service, bandwidth provisions and redundant connections. Also excessive digging should be avoided and trench space is limited. 
   Other factors also must be considered when branching is performed in a fiber optic cable network. A major concern is avoidance of damage to the cable during installation. Damage can occur in a variety of ways, namely 1) radial crushing the cable with installation equipment; 2) severe bending, twisting, flexing or stretching damage due to excessive forces applied during installation; 3) damaging the protective cable jacket, such as by abrasion, cracking or cutting the outer protective layer; and 4) long term exposure of the cable to environmental factors which cause thermal cycling. 
   Another concern is ease of installation and reduction in the amount of time needed to install the cable. Also, there is a concern to avoid splices in the cable as much as possible. Splices are time consuming to make and incur considerable installation costs. Generally, it is desirable to install the longest continuous length of cable possible to reduce the number of splices needed for the desired cable run. Moreover, it is not desirable to have a large number of splice joints in view of the relatively substantial signal damping caused by each joint in proportion to the total signal damping of the overall signal path. 
   Protective cable ducts have been channelized in an effort to satisfy these concerns. For this purpose a tube, whose interior may have a lower coefficient of friction than the existing duct, is installed in the existing protective duct, (direct buried is also possible), thereby establishing a separate channel in which cable, optionally at a later time, can be blown or pulled through the protective duct over a greater length. It may also be desirable to install in an existing duct a larger number of tubes with a smaller cross section than that of the existing duct if it is desired to use each of the smaller tubes as a separate channel or sub-duct for single-core or multi-core copper or glass fiber cables. Further, it may be necessary to install in an existing duct a protective tube with a water barrier, so that in the existing duct, whose interior gradually fills up with water through diffusion, a waterproof conduit is created by means of the second tube, this waterproof conduit allowing the routing of cables without a water shield. 
   U.S. Pat. Nos. 4,850,569 and 4,934,662 to Griffioen et al. describe combining high speed air flow with a pushing force applied at the entry end of the conduit to install a traditional (i.e., with non-negligible stiffness) cable. These techniques also have been used advantageously for installing channelizing guide tubes in an existing protective duct. For example, in U.S. Pat. No. 5,884,384 to Griffioen, channelization is achieved by installing a bundle of guide tubes or sub-ducts in an existing protective duct by means a fluid under pressure, for instance compressed air, together with a pushing force exerted on the guide tubes as they enter the protective duct. 
   In U.S. Pat. No. 5,971,035 to Griffioen a method is provided for installing a ducting system with branches, wherein at the point of a branch in an existing duct of the system a tubular branch element with an inlet opening, an outlet opening and at least one branch opening is arranged by removing a duct portion from the existing duct at the point of the branch, by sliding the branch element on one of the free ends of the existing duct resulting from the interruption, replacing the removed duct portion or a portion identical in shape in the interruption and moving and securing the branch element in such a manner that the inlet opening and the outlet opening engage in sealing manner over the respective ends of the existing duct. In the duct with branches, over the entire length thereof, a bundle of sub-ducts can be arranged, which sub-ducts can then be branched-off in a simple manner at the location of a branch. In a duct thus branched-off, for instance a continuous fiber optic cable can be installed. 
   The method and branch apparatus of Griffioen U.S. Pat. No. 5,971,035 have achieved excellent results for new installations where branch locations are known in advance and the branch connections can be installed at the known locations. A limitation on this method is that such branching is intended for installation before cables are laid in, since it is necessary to cut and completely separate the free ends of the protective duct to allow serial attachment of the branch connector and end couplings. This means that any existing plant components, e.g., guide tubes and/or cables already laid, would also have to be cut and separated to allow installation of the branch connector. It would then be necessary to splice and restore the existing fiber cables (which would require some over-length provision and two splices to restore each fiber path), followed by water-proofing and mechanical restoration of the duct. This naturally would cause an interruption of existing cable services, also causing some signal loss and degradation at each splice point. For these reasons such interference with existing plant equipment is to be avoided as much as possible. 
   It will be appreciated that building optical access networks with conventional methods and equipment is challenged by the uncertainties imposed by growing demand. For example, to splice a branch-cable to a feeder cable it is required to build over-length (window cut) in the feeder cable, in order to allow splicing above the trench. This is done at a predetermined fixed branch position, close to the customer. If all next customers were known in advance, over-length and branches could also be built close to them. But this is hardly ever the case. The location where branches and over-length may be needed is just a guess. In practice new customers are far away from these locations. To avoid digging again along the feeder route extra tubes are laid parallel. A lot of trench space is consumed and much money is invested in outside plant. Also, the number of fibers installed from the beginning means high initial costs. Moreover, to avoid numerous splices for every length extension more length must be installed than needed for the first customer. 
   Consequently, there is a continuing need for improvements in outside plant equipment and installation methods that can provide versatility to meet growing, unpredictable demand, reduce the number of splices required, and provide mid-span branching access at any place, any time, even after cables have been laid in existing protective ducts. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides a solution that makes it possible to install branches in an existing duct in which channelization guide tubes and/or cables have already been laid wherein the existing duct can be cut at any point where a branch is desired, without disturbing existing services. This is accomplished by a “splittable” branch connector that is mountable in an operative branching position on the free end portions of an existing duct that has been cut at any mid-span location where a branch is desired. 
   The branch connector includes a pair of separable housing members that are engagable with each other, each having end couplings for clamping onto the cut ends of the duct. The housing members include duct sidewall portions that can be installed in spaced relation around existing guide tubes and cables, thus forming a serial extension of the existing duct. When clamped together, the sidewall portions form a protective duct restoration passage through which the existing guide tubes and cables extend, undisturbed. The restoration housing members, when clamped together in the protective restoration position, also form an inlet opening and an outlet opening for receiving the cut free end portions of the main duct. Separable fasteners releasably hold the housing members in clamping engagement with each other and with the free end portions of the protective duct. 
   Either one or both of the restoration housing members include sidewall portions forming a tubular branch stub in communication with the restoration duct passage. According to this arrangement, the splittable branch connector restores the existing protective duct and provides a protective branch stub permitting serial connection of an existing guide tube to a branch guide tube through which a continuous drop cable can be installed by pushing and/or blowing. 
   According to an important aspect of the invention, best results are obtained when the guide tubes have been installed in a loose bundle that does not entirely fill up the protective duct to offer sufficient mechanical protection, to make all tubes accessible at any place and for ease of installation of the bundle. By this arrangement, each guide tube is readily available and easily maneuverable at any desired mid-span location. A short segment of the protective duct is cut away and removed, thus exposing the loose bundle of guide tubes. A selected (empty) guide tube is then cut, and the cut-free end is routed through the branch stub for serial attachment to a drop-branch guide tube. The other cut end of the selected guide tube is sealed. After connecting the drop-branch guide tube, a protective branch duct is attached to the branch stub by a screwable connector, and a cable is then installed through the branch by blowing and/or pushing. 
   Through the method of the present invention, at any desired mid-span point in the main duct, even after as well as before the installation of a bundle of guide tubes and/or cables, a branching point can be realized, whereby, due to the restoration of the continuous inside wall of the existing duct at the interruption, a continuous conduit for protecting the existing guide tubes and cables is maintained, and moreover, a branching stub conduit is provided for routing a drop cable, all without damaging or disturbing the existing guide tubes and/or cables. 
   The invention also provides an improved branch connector for use in practicing the method of the present invention, characterized by separable, complementary housing portions, each in the form of a concave shell, and when assembled together forming an internal duct passage, an inlet opening, an outlet opening and at least one branch opening, with the housing portions forming the inlet opening and the outlet opening also including coupling means for connecting them in sealing engagement onto the spaced-apart cut end portions of the existing duct, and the housing portions forming the branch opening also including coupling means for connecting them in sealing engagement with a protective branch duct. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawing is incorporated into and forms a part of the specification to illustrate the preferred embodiments of the present invention. Various advantages and features of the invention will be understood from the following detailed description taken in connection with the appended claims and with reference to the attached drawing figures in which: 
       FIG. 1  is a simplified routing diagram showing multiple distribution tubes and redundant fiber optic cables laid in drop tubes running to business customers from multiple branching points; 
       FIG. 2  is an underground sectional view showing the splittable branch connector of the present invention installed at a mid-span branching location in a protective distribution duct; 
       FIG. 3  is a side elevation view, partly broken away, of a section of the distribution duct shown in  FIG. 2 , containing a loose bundle of guide tubes and/or cables; 
       FIG. 4  is a view similar to  FIG. 3  with a window section of the distribution duct removed and exposing the loose bundle of guide tubes; 
       FIG. 5  is an exploded perspective view of the splittable branch connector, ready for assembly and clamping onto the cut ends of the distribution duct; 
       FIG. 6  is a perspective view of the branch connector, with coupling collars removed and the distribution duct and branch duct partly cutaway; 
       FIG. 7  is a side elevation view of the branch connector of the present invention; 
       FIG. 8  is a longitudinal sectional view thereof; 
       FIG. 9  is an elevation view of a male coupling component of the end coupling collars shown in FIG.  7  and  FIG. 8 ; 
       FIG. 10  is an elevation view of a female coupling component that is configured for complementary slide-fit interlocking engagement with the male coupling component shown in  FIG. 9 ; 
       FIG. 11  is a front elevation view showing the coupling components in interlocking engagement, forming a threaded coupling collar; 
       FIG. 12  is a rear elevation view of the threaded coupling collar; and 
       FIG. 13  is a side elevation view of the female coupling collar component taken along the line  13 — 13  of FIG.  10 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Preferred embodiments of the invention will now be described with reference to various examples of how the invention can best be made and used. Like reference numerals are used throughout the description and several views of the drawing to indicate like or corresponding parts. 
   The splittable Y-branch connector  10  of the present invention is shown installed in a simple access network in FIG.  1  and FIG.  2 . Here four feeding 12-fiber cables  12  are spliced at multi-joint splice points MJ to six 2-fiber branching drop cables  14  that make the drop to customer interface access stations  16 , everything redundantly connected. The drop cables  14  are guided from the splice points MJ through the customer Y-branch connectors  10  to the customer stations  16 , thus allowing one-shot blowing installation of cable to the customer. The entire feeding bundle and Y-branch drop connector  10  fit into a single 40 mm protective duct  18  as shown in FIG.  2 . With this configuration it is possible to connect thirty customer access stations  16  in a redundant way. It is also possible to reserve some extra capacity at the branch points. 
     FIG. 2  shows the installation of the Y-branch connector  10  in an existing protective duct  18  for routing a drop cable  14  through a branch duct  20  that leads to the customer station  16 . The loose bundle  22  of guide tubes  24  are previously installed and are intended to channelize the protective duct, with each guide tube  24  functioning as a separate cable duct. Feeder cables  12  and other drop cables  14  may already be laid in one or more of the guide tubes  24 . 
   The function of the protective duct  18  is to guide and protect the guide tubes  24  and their optical cables. In order to allow installation of a bundle  22  of loose guide tubes by blowing, the protective duct should resist the required pressure differences. The protective duct may be buried directly into the ground as shown in FIG.  2  and is constructed of commonly used duct material, e.g. high-density polyethylene (HDPE). It should be circular and uniform in cross-section throughout its length. The outer and inner surfaces should be free from irregularities such as cracks, pinholes, joints, water splash marks, die-marks, repairs and all other defects. 
   The function of the guide tubes  24  is to form dedicated channels to guide the drop cables  14  through the access network without splicing the cables. In order to allow installation of both the optical cables and the bundle of guide tubes by blowing, the guide tubes should resist the needed pressure, both internal and external (to avoid implosion the guide tubes  24  are also pressurized during installation of the bundle). The guide tubes preferably are formed of high-density polyethylene (HDPE). Other material, e.g. nylon or ABS, is also possible. The guide tubes are preferably circular and uniform in cross-section throughout their length. Their outer and inner surfaces should be free from irregularities such as cracks, pinholes, joints, water splash marks, die-marks, repairs and all other defects. 
   The outside of the guide tubes  24  should be free from lubricant or other contamination. The diameters of the guide tubes used in the loose bundle installation are 7/5.5 and 10/8 mm. The requirement for the number of guide tubes is determined as follows: the total cross-sectional area of the outside of the guide tubes  24  should be about half of the cross-sectional air flow area inside the protective duct  18 . Preferably, the guide tube bundle fill factor is approximately 50%, that is, the effective cross sectional area occupied by the guide tube bundle  22  should be about one-half of the total cross sectional air flow area of the protective duct  18 . Less than optimum values are obtained but relatively trouble-free installation is nevertheless realized when the guide tube bundle fill factor is less than 50% but not less than about 30%, and more than 50% but not exceeding about 60%. 
   Branches, joints and terminations are needed to make a network of the guide tubes and cables. To accomplish this a number of components are required. A drop cable  14  can branch off from the bundle  22  without making a splice. Branching can be done at any place and any time by simply opening a man-hole or hand-hole  11  at the desired branch location, as shown in  FIG. 2 , to expose the protective duct  18 . The protective duct  18  is then cut along the cut lines L 1  and L 2 , for example as shown in  FIG. 3 , and a short duct section  18 S is removed, exposing the loose bundle  22  of guide tubes  24  between the cut end portions  18 A,  18 B. In this example, cables may already be laid in one or more of the guide tubes and one or more of the guide tubes are empty. 
   It is also possible to just dig the protective duct up at any place, making the Y-branch direct buried, even without the need to use a hand-hole. This is even to be preferred since placing the Y-connector will be easier and less trench space consuming. 
   Before installing the drop cable, the exposed section of a selected one of the empty guide tubes  24  is cut at  24 C and the free end  24 A is then connected to a branch guide tube  24 B as shown in FIG.  4  and  FIG. 5. A  serial guide tube connection is made by a permanent or push/pull guide tube coupling  26 . The remaining cut end portion of the empty guide tube  24  is sealed with a plug or cap  28 . 
   The branch guide tube  24 B is installed in a protective branch duct  20  that leads to a customer interface station  16  (FIG.  1  and FIG.  2 ). The main protective duct  18  at the branch point is restored and coupled to the branch protective duct, by using the clip-on splittable Y-branch connector  10 . The Y-branch connector  10  includes a tubular branch coupling stub  10 S. The protective branch duct  20  is joined in at least a mud-tight union with the coupling stub  10 S so that a continuous, dedicated channel is established. 
   For straight coupling of the protective branch duct  20  a connection can be made by using simple straight “screwable” coupling, for example the threaded coupling collar  32  shown in  FIG. 2 , FIG.  7  and FIG.  8 . The coupling collar is slipped onto the free end of the branch duct prior to insertion of the branch duct into the throat of the branch stub  10 S. The coupling collar unions with the branch stub  10 S and the branch duct  20  are sealed preferably water-tight but at least mud-tight using a compression ring  34 . The function of the compression ring is to firmly grip the duct-end  20 , such that a sufficiently high pull-out strength of this duct-end is obtained. The guide tube serial connections  26  are sealed by gas-tight and water-tight O-ring seals. In this way dedicated channels are formed by the guide tubes  14 ,  24 A,  24 B in which small, but outside-plant resistant, optical cables  14  are pushed or blown in through the branch connection to the customer without splicing. 
   The function of the guide tube coupling  26  is to allow branching installation of a drop cable. The guide tube coupling  26  should have a smooth inside geometry to allow passing of the cable without sticking and should be airtight and pressure resistant to allow installation by blowing. The guide tube coupling  26  can be permanent or a push/pull type. 
   The function of the splittable Y-branch connector is to elongate or restore the protective duct  10  and to allow installation of additional guide tubes and/or cables. The mounted Y-branch connector  10  should have a smooth inside geometry of large bending radius to allow passing of additional guide tubes and/or cables without sticking and should be at least mud-tight. 
   Referring now to  FIG. 5 , FIG.  7  and  FIG. 8 , the splittable Y-branch connector  10  includes a first housing member  40  ( FIG. 7 ) having a first coupling end portion  42 , a second coupling end portion  44  and a duct sidewall restoration portion  46  extending therebetween. A second housing member  48  is constructed in mirror-image complementary relation, also having a first coupling end portion  50 , a second coupling end portion  52  and a duct sidewall restoration portion  54  extending therebetween. The separable, complementary housing portions, each in the form of a concave shell, when assembled together form an internal duct restoration passage  56 , at least one branch opening  58 , an inlet opening  60 , and an outlet opening  62 . 
   The housing members  40 ,  48  are engagable with each other in the branching position (shown in FIG.  5 ), thereby forming a serial duct restoration passage  56  extending between the duct coupling end portions. As shown in  FIG. 5 , FIG.  7  and  FIG. 8 , the housing members are movably coupled together by a hinge assembly  63 , preferably a living hinge, so that the housing members can open and close together in clam-shell movement around the guide tube bundle  22 . 
   The coupling end portions are engagable with each other in the branching position, thereby defining the inlet and outlet openings  60 , 62  for admitting the free end portions  18 A,  18 B of the existing duct into the duct restoration passage  56 . Preferably, the union of the housing members is sealed water-tight or at least mud-tight by compressible insert seal strips confined in longitudinal seal pockets along the interface between the housing members. 
   A pair of splittable coupling collars  64 , 66  are provided for releasably engaging the first and second housing members  40 , 48  and clamping them together in the operative branching position. Referring to  FIGS. 9 ,  10 ,  11 ,  12  and  13 , the preferred construction of the splittable coupling collars is indicated. Each coupling collar is formed in two separable (splittable) male and female sections  64 A,  64 B and  66 A,  66 B respectively. The male collar sections are provided with coupling flanges  68  equipped with T-profile ribs  70  and the female collar sections are provided with coupling flanges  72  intersected by T-profile channels  74  that are dimensioned to allow sliding axial insertion of the T-profile rib into the complementary T-profile channel, thereby producing a releasable interlocking fit between the collar sections. 
   The collar sections  64 A,  64 B and  66 A,  66 B are also provided with internal threads T for making up a tight union with the threaded end portions of the restoration housing members for engaging the coupling collars  64  and  66  sealed preferably in a water-tight but at least mud-tight union using a compression ring  76 . The function of the compression ring is to firmly grip the duct-ends  18 A and  18 B, such that a sufficiently high pull-out strength of these duct ends is obtained. 
   Referring again to  FIG. 5 , the Y-branch connector  10  is assembled in the operative branching position by opening the splittable restoration housing members  40 ,  48  and then inserting the cut duct end portions  18 A,  18 B into the inlet and outlet openings  60 ,  62  respectively, with the duct end portions being placed in alignment with the longitudinal axis A of the restoration passage  56 , as shown in FIG.  8 . The housing members  40 , 48  are then closed together as shown in FIG.  6 . The housing members are clamped tightly together by the coupling collars  64 ,  66  as shown in FIG.  7  and FIG.  8 . 
   The complementary male and female components of each coupling collar are initially pulled apart and separated (( FIG. 9 ,  FIG. 10 ) so that they can be placed around the assembled duct end portions  18 A,  18 B that previously were inserted into the duct restoration passage  56  (FIG.  5 ). (the collars can also be placed around the duct end portions before the Y-connector is placed, and even before the protective duct is opened; the need for the collars to be splittable remains because the guide-tubes that are not branched will not be cut, especially not when a through going cable is already present.). The complementary male and female components  64 A,  64 B are then slipped together and interlocked, forming a complete coupling collar ( FIG. 13 ) that is then ready to be assembled onto the threaded end portions of the clamped-together housing shells  40 ,  48 . 
   The male and female coupling components of each coupling collar  64 ,  66  are mutually engagable with each other in an interlocking union thereby forming a unitary threaded fastener. They also can be pulled apart and separated for individual placement and reassembly about the free end portions of the existing duct after the cut end portions have been inserted into the duct restoration passage and the housing shells have been closed around them in the restoration position. By this separable fastener arrangement, the male and female coupling components can be placed into interlocking union with each other while encircling the clamped end portion of the existing duct, thereby forming a unitary coupling collar for engaging the threaded end portions of the closed-together housing shells. 
   The housing members include concave neck portions  78 , 80  forming the tubular branch stub  10 S in communication with the restoration duct passage  56  when the housing members are assembled together in the operative branching position. By this arrangement the branch stub is also splittable, to facilitate routing of the branch guide tube. However, the branch stub can also be integrally formed with only one of the housing members, and can provide comparable branch service, since the cut end of the guide tube may be easily inserted into the branch passage. 
   The branching (coupled) guide tube  24 B follows a smooth path along the axis B of the branch stub  10 S with sufficiently large bending radius inside the throat passage  58  in order to allow passing of the cable without problems. The unused part of the cut guide tube is closed with a plug. No further installation of guide tubes is needed at the branch connectors. Therefore branch connectors do not need to be smooth-walled and pressure resistant. They should, however, be at least mud-tight. 
   The branch connector  10  has a Y-shape according to  FIG. 2  but may also have, for instance, a T-shape or even several branch openings and/or branch stubs. More than one guide tube can be inserted into the branch duct and guided through the Y- or T-connector and connected to the guide tubes from bundle  22 . It is also possible to connect two guide tubes in the bundle  22  in opposite directions (in T-connector) resulting in a redundant connection from there. 
   Because the Y-branch connector  10  and the coupling collars  64 ,  66  are each separable, the branching connection can be completed without cutting guide tubes or cables that are already laid in the protective duct  18 . This means that new branching points can be established at any mid-span location at any time to provide new cable service with no interruption of existing cable services. 
   It will be appreciated that the guide-tube branching system of the present invention is a new concept for optical access networks that overcomes the limitations of conventional technology. It consists of loose bundles of individual guide tubes running from a protective trunk duct and smaller guide branch ducts branch to the subscribers. Low-cost splittable “clip-on” Y-branch connectors are used which can be placed at any place and any time, without disturbing existing cabling. After connection of the chosen guide tubes to each other, dedicated cable paths are created. Here miniaturized cables can be installed without splicing, e.g. by simply pushing or using compressed air to blow the cables through the branching guide tubes. These cables may be copper twisted-pair (1 quad) or optical (2-48 fibers) cables. Also coaxial cables can be blown in the branched guide tube systems. All cable constructions offer excellent resistance to outside plant conditions and handling. Preferably, the guide tubes are installed in a loose bundle that does not entirely fill up the protective duct to offer sufficient mechanical protection, to make all tubes accessible at any mid-span location and for ease of installation of the bundle. 
   The concept can be completed with optional joint boxes to connect cables from the feeder-, distribution- and drop-portions of the access network. 
   The above-described branching installation is achieved using only one protective duct with 10 multiple guide tubes. Trench space is saved. Customers can be connected any place and any time. No window-cuts are needed. Also one layer of splice-points has been eliminated from the network. Furthermore only those fibers are installed which have been paid for (parallel upgrading). And when new customers appear beyond the installed section a new branching section is simply clicked on, allowing passage of a new cable without making a splice (serial upgrading). 
   Although the invention has been described with reference to certain exemplary arrangements, it is to be understood that the forms of the invention shown and described are to be treated as preferred embodiments. Various changes, substitutions and modifications can be realized without departing form the spirit and scope of the invention as defined by the appended claims.