Abstract:
A hybrid IP/ATM DSLAM and method are provided for hybrid IP/ATM DSL access multiplexing. While traversing a DSLAM, ATM traffic over ATM network infrastructure and GigE/IP traffic over GigE/IP network infrastructure may be recast into crossover GigE/IP traffic and crossover ATM traffic respectively, and routed to the opposite kind of network infrastructure.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to digital subscriber line access multiplexers (DSLAMs), and more particularly to a hybrid IP/ATM DSLAM and the provision of hybrid IP/ATM DSL access multiplexing.  
       BACKGROUND OF THE INVENTION  
       [0002]     In providing services to customers, network service providers are constantly trying to provide faster, more robust services, and to provide more bandwidth to customers of their communications networks. ATM is currently deployed heavily for DSL services. Service providers who possess large ATM based network infrastructures are moving towards GigE (Gigabit Ethernet)/IP based infrastructure because of the benefits provided by a GigE/IP based infrastructure including the delivery of enhanced features, more bandwidth, faster service, and more features to customers.  
         [0003]     Referring to  FIG. 1A , a known ATM DSLAM  22  employed in an ATM based infrastructure is discussed. The ATM DSLAM  22  has ATM based network interfaces  12  such as OC12, OC3, DS3, and DS1, and others. The ATM DSLAM  22  is designed to work with ATM network rules. The ATM DSLAM  22  typically provides limited functionalities and relatively low speed services over its user interfaces  25  such as HSI (High Speed Internet) over ADSL (Asymmetric Digital Subscriber Line), and SHDSL (Symmetric High-bit-rate Digital Subscriber Line) to end users. An ATM DSLAM  22  includes a number of ATM LT (line termination) cards  20  connected by an ATM point to multipoint bus  18  to an ATM switch  14  of an ATM NT (network termination) card  16 . The ATM based network interfaces  12  are connected to the ATM NT  16 .  
         [0004]     Existing ATM DSLAMs  22  utilize existing ATM hardware, shelving and ATM LTs  20 , are designed to work with existing ATM system interfaces, and form part of an existing ATM network infrastructure.  
         [0005]     Referring to  FIG. 1B , a known GigE/IP DSLAM  62  is discussed. In general, a GigE/IP DSLAM provides more bandwidth both on its network interfaces  52  and user interfaces  65  than an ATM DSLAM does on its network interfaces  12  and user interfaces  25  respectively. GigE/IP DSLAMs also provide more enhanced features in comparison to ATM DSLAMs. GigE/IP DSLAM  62  provides multiple GigE/IP or  10 / 100  Ethernet IP based network interfaces  52  towards the network which allows it to provide very high speed DSL services such as SHDSL, Voice, and VDSL over user interfaces  65  which allow for such capabilities as TV broadcasting using phone lines. A GigE/IP DSLAM  62  is designed to work with GigE/IP rules and hence existing network equipment based on ATM technology cannot be used with it. The GigE/IP DSLAM  62  includes a number of GigE/IP LT cards  60  connected by a GigE/IP star bus  58  to a GigE/IP switch  54  of a GigE/IP NT card  56 . The GigE/IP based network interfaces  52  are connected to the GigE/IP NT  56 .  
         [0006]     Service providers have thousands of ATM DSLAMs that they want to migrate to GigE/IP DSLAMs in order to migrate from an ATM network infrastructure to a GigE/IP network infrastructure. Service providers would rather not have to resort to dropping new GigE/IP DSLAM systems into their networks as replacements of the ATM DSLAM systems due to the cost of each new GigE/IP DSLAM itself, the cost of empty slots in those GigE/IP DSLAMs during the transition, and the amount of new space required for the new GigE/IP DSLAMs. The service providers would prefer a solution that could support both ATM and GigE/IP system interfaces and help them continue to utilize the shelves and ATM line cards that they have already paid for to facilitate a gradual migration from an ATM network infrastructure to a GigE/IP network infrastructure.  
       SUMMARY OF THE INVENTION  
       [0007]     According to one aspect, the invention provides for a hybrid IP/ATM DSLAM comprising: a GigE/IP communications portion for traffic flowing between a first user interface and a GigE/IP network interface; an ATM communications portion for traffic flowing between a second user interface and an ATM network interface; and an IP/ATM bridge for passing traffic between the GigE/IP communications portion and the ATM communications portion.  
         [0008]     In some embodiments the GigE/IP communications portion comprises a GigE/IP switch for switching GigE/IP traffic, in which the ATM communications portion comprises an ATM switch for switching ATM traffic, and in which the IP/ATM bridge comprises an inter-working function (IWF) element connected to said GigE/IP switch and connected to said ATM switch, said IWF element for passing network traffic between the GigE/IP switch and the ATM switch.  
         [0009]     In some embodiments the IWF element is adapted to: recast GigE/IP traffic received from the GigE/IP switch into crossover ATM traffic; recast ATM traffic received from the ATM switch into crossover GigE/IP traffic; transmit crossover ATM traffic if any to said ATM switch; and transmit crossover GigE/IP traffic if any to said GigE/IP switch.  
         [0010]     In some embodiments the ATM connection portion is adapted to accept standard ATM hardware and provide a standard ATM system interface and in some embodiments the ATM communication portion comprises a standard ATM LT card slot and a standard ATM network interface.  
         [0011]     According to a second aspect the invention provides for a hybrid IP/ATM DSLAM comprising: a GigE/IP switch for switching traffic flowing between a first user interface and a GigE/IP network interface; an inter-working function (IWF) element; a GigE/IP connection coupling said GigE/IP switch to said IWF element; an ATM switch for switching traffic flowing between a second user interface and an ATM network interface; and an ATM connection coupling said IWF element to said ATM switch; wherein the IWF element is adapted to: receive GigE/IP traffic from said GigE/IP switch over said GigE/IP connection; recast said GigE/IP traffic into crossover ATM traffic; pass crossover ATM traffic to said ATM switch over said ATM connection; receive ATM traffic from said ATM switch over said ATM connection; recast said ATM traffic into crossover GigE/IP traffic; and pass said crossover GigE/IP traffic to said GigE/IP switch over said GigE/IP connection.  
         [0012]     According to another aspect the invention provides for a method of hybrid IP/ATM DSL access multiplexing comprising: receiving ATM traffic at an ATM communications portion of a DSLAM; recasting said ATM traffic into crossover GigE/IP traffic at an IWF element of the DSLAM; and transmitting said crossover GigE/IP traffic from a GigE/IP communications portion of the DSLAM.  
         [0013]     In some embodiments of the invention the step of receiving ATM traffic comprises receiving ATM traffic at one of an ATM network interface and an ATM LT card.  
         [0014]     Some embodiments further provide for receiving GigE/IP traffic at the GigE/IP communications portion; recasting said GigE/IP traffic into crossover ATM traffic at the IWF element; and transmitting said crossover ATM traffic from the ATM communications portion over one of an ATM network interface and an ATM LT card.  
         [0015]     According to another aspect, the invention provides for a method of hybrid IP/ATM DSL access multiplexing comprising: receiving ATM traffic at one of an ATM network interface of a DSLAM and an ATM LT card of the DSLAM; recasting said ATM traffic into crossover GigE/IP traffic at an IWF element of the DSLAM; transmitting said crossover GigE/IP traffic from a GigE/IP communications portion of the DSLAM; receiving GigE/IP traffic at the GigE/IP communications portion; recasting said GigE/IP traffic into crossover ATM traffic at the IWF element; and transmitting said crossover ATM traffic from the DSLAM over one of the ATM network interface and the ATM LT card.  
         [0016]     According to another aspect, the invention provides for a method of providing hybrid IP/ATM DSL access multiplexing in an ATM network infrastructure, the method comprising: removing ATM hardware from an ATM DSLAM of the ATM network infrastructure; installing said ATM hardware in a hybrid IP/ATM DSLAM; and replacing said ATM DSLAM with said hybrid IP/ATM DSLAM.  
         [0017]     In some embodiments of the invention the ATM hardware comprises an ATM LT card.  
         [0018]     In some embodiments of the invention the step of replacing said ATM DSLAM comprises disconnecting ATM network interfaces from said ATM DSLAM, connecting said ATM network interfaces to said hybrid IP/ATM DSLAM, and connecting a GigE/IP network interface to said hybrid IP/ATM DSLAM.  
         [0019]     According to a further embodiment the invention provides for a method of providing hybrid IP/ATM DSL access multiplexing in an ATM network infrastructure, the method comprising: removing an ATM LT card from an ATM DSLAM of the ATM network infrastructure; installing said ATM LT card in a hybrid IP/ATM DSLAM; disconnecting any ATM network interfaces from said ATM DSLAM; connecting said any ATM network interfaces to said hybrid IP/ATM DSLAM; and connecting a GigE/IP network interface to said hybrid IP/ATM DSLAM.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The features and advantages of the invention will become more apparent from the following detailed description of the preferred embodiment(s) with reference to the attached figures, wherein:  
         [0021]      FIG. 1A  is a block diagram of a known ATM DSLAM;  
         [0022]      FIG. 1B  is a block diagram of a known GigE/IP DSLAM;  
         [0023]      FIG. 2  is a block diagram of a hybrid IP/ATM DSLAM according to a preferred embodiment of the invention;  
         [0024]      FIG. 3A  is a block flow diagram of hybrid IP/ATM DSL access multiplexing in which ATM traffic is recast into GigE/IP traffic according to another embodiment of the invention;  
         [0025]      FIG. 3B  is a block flow diagram of hybrid IP/ATM DSL access multiplexing in which GigE/IP traffic is recast into ATM traffic according to another embodiment of the invention; and  
         [0026]      FIG. 4  is a block flow diagram of provisioning of hybrid IP/ATM DSL access multiplexing in an ATM network infrastructure according to a further embodiment of the invention. 
     
    
       [0027]     It is noted that in the attached figures, like features bear similar labels.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Referring to  FIG. 2 , a hybrid IP/ATM DSLAM  100  in accordance with the preferred embodiment of the invention will now be discussed in terms of structure.  
         [0029]     The hybrid IP/ATM DSLAM  100  has first user interfaces  201  providing relatively low speed services such as HIS over ADSL and SHDSL to end users. First user interfaces  201  are coupled to a number of ATM LT cards  200  connected by an ATM point to multipoint bus  202  to an ATM switch  112 . The ATM LT cards  200  are standard ATM LT cards that could be used in known ATM DSLAMs. The hybrid IP/ATM DSLAM  100  accepts existing ATM based hardware such as existing ATM shelving and ATM LTs. The hybrid IP/ATM DSLAM  100  has ATM based network interfaces  102  which could be for example OC12, OC3, DS3, DS1, or others. The hybrid IP/ATM DSLAM  100  supports existing ATM system interfaces. The ATM based network interfaces  102  are coupled to the ATM switch  112  in the hybrid IP/ATM DSLAM  100 . The first user interfaces  201 , the ATM LT cards  200 , the ATM bus  202 , the ATM switch  112 , and the ATM based network interfaces  102  are collectively referred to as the ATM communications portion  300  of the hybrid IP/ATM DSLAM  100 .  
         [0030]     The hybrid IP/ATM DSLAM  100  has second user interfaces  221 . Second user interfaces  221  are coupled to a number of GigE/IP LT cards  220  connected by a GigE/IP star bus  222  to a GigE/IP switch  122 . The hybrid IP/ATM DSLAM  100  accepts existing GigE/IP based hardware such as existing GigE/IP shelving and GigE/IP LTs. The hybrid IP/ATM DSLAM  100  has multiple GigE/IP network interfaces  104  towards the network. The hybrid IP/ATM DSLAM  100  supports existing GigE/IP system interfaces. The multiple GigE/IP network interfaces  104  are coupled to the GigE/IP switch  122  in the hybrid IP/ATM DSLAM  100 . The second user interfaces  221 , the GigE/IP LT cards  220 , the GigE/IP star bus  222 , the GigE/IP switch  122 , and the GigE/IP network interfaces  104  are collectively referred to as the GigE/IP communications portion  310  of the hybrid IP/ATM DSLAM  100 .  
         [0031]     The GigE/IP communications portion  310  is coupled to an IWF (inter-working function) element  130  by a GigE/IP connection  126  from the GigE/IP switch  122 . The IWF element  130  could be a network processor element typically used in existing GigE/IP LT cards to recast ATM streams into GigE/IP streams with the added capability of recasting GigE/IP streams into ATM streams. It follows therefore that in some embodiments the IWF element  130  can be made using existing designs for hardware typically used elsewhere. The ATM communications portion  300  is coupled to the IWF element  130  over an ATM connection  116  from the ATM switch  112 .  
         [0032]     Collectively the GigE/IP switch  122 , GigE/IP connection  126 , the IWF element  130 , the ATM connection  116 , and the ATM switch  112  are referred to as the IP/ATM bridge  110 .  
         [0033]     The hybrid IP/ATM DSLAM  100  shown in  FIG. 2  will now be discussed in terms of function.  
         [0034]     The ATM communications portion  300  functions as an ATM DSLAM. The ATM communications portion  300  provides services over first user interfaces  201  to end users, is designed to work with ATM network rules, and communicates with the network over its ATM based network interfaces  102  which could be OC12, OC3, DS3, and DS1, and others. The hybrid IP/ATM DSLAM  100  is capable of accepting existing ATM based hardware such as existing ATM shelving and ATM LT line cards, and provides support for existing ATM system interfaces. As such, the ATM communications portion  300  of the hybrid IP/ATM DSLAM  100  provides all of the functions of a known ATM DSLAM so that the hybrid IP/ATM DSLAM  100  is compatible with ATM network infrastructure and even provides for the re-use of existing ATM hardware such as shelving or LT cards from existing ATM DSLAMs.  
         [0035]     The GigE/IP communications portion  310  functions as a GigE/IP DSLAM. The GigE/IP communications portion  310  provides very high speed services such as SHDSL, Voice, and VDSL to end users over second user interfaces  221  providing for services such as TV broadcasting over phone lines. The GigE/IP communications portion  310  is designed to work with GigE/IP network rules, and communicates with the network over its GigE/IP based multiple GigE/IP network interfaces  104 . The hybrid IP/ATM DSLAM  100  is capable of accepting existing GigE/IP based hardware such as existing GigE/IP shelving and GigE/IP LT line cards, and provides support for existing GigE/IP system interfaces. As such, the GigE/IP communications portion  300  of the hybrid IP/ATM DSLAM  100  provides all of the functions of a known GigE/IP DSLAM so that the hybrid IP/ATM DSLAM  100  is compatible with GigE/IP network infrastructure and even provides for the re-use of existing GigE/IP hardware such as shelving or LT cards.  
         [0036]     The IP/ATM bridge  110  serves to pass or crossover network traffic from the GigE/IP communications portion  310  to the ATM communications portion  300  and from the ATM communications portion  300  to the GigE/IP communications portion  310 . Network traffic that has crossed over the IP/ATM bridge  110  is referred to herein as crossover traffic.  
         [0037]     In the upstream direction from the ATM communications portion  300  to the GigE/IP communications portion  310 , traffic from first user interfaces  201  traverse the ATM LT cards  200  and the ATM bus  202  to the ATM switch  112  as ATM traffic. The ATM switch  112  routes the ATM traffic over the ATM connection  116  to the IWF element  130 . The IWF element  130  serves to recast the ATM traffic in the form of an ATM data stream into a GigE/IP data stream generating crossover GigE/IP traffic. The crossover GigE/IP traffic traverses the GigE/IP connection  126  to the GigE/IP switch  122  where it is routed over appropriate upstream GigE/IP network interfaces  104 .  
         [0038]     In the upstream direction from the GigE/IP communications portion  310  to the ATM communications portion  300 , traffic from second user interfaces  221  traverse the GigE/IP LT cards  220  and the GigE/IP star bus  222  to the GigE/IP switch  122  as GigE/IP traffic. The GigE/IP switch  122  routes the GigE/IP traffic over the GigE/IP connection  126  to the IWF element  130 . The IWF element  130  serves to recast the GigE/IP traffic in the form of a GigE/IP data stream into an ATM data stream generating crossover ATM traffic. The crossover ATM traffic traverses the ATM connection  116  to the ATM switch  112  where it is routed over appropriate upstream ATM network interfaces  102 .  
         [0039]     In the downstream direction from the GigE/IP communications portion  310  to the ATM communications portion  300 , GigE/IP traffic from the upstream network traverses GigE/IP network interfaces  104  to the GigE/IP switch  122  where it is routed over the GigE/IP connection  126  to the IWF element  130 . As discussed above, the IWF element  130  serves to recast the GigE/IP traffic in the form of a GigE/IP data stream into an ATM data stream generating crossover ATM traffic. The crossover ATM traffic traverses the ATM connection  116  to the ATM switch  112  which routes the crossover traffic over the ATM bus  202  to the ATM LT cards  200  and over appropriate first user interfaces  201 .  
         [0040]     In the downstream direction from the ATM communications portion  300  to the GigE/IP communications portion  310 , ATM traffic from the upstream network traverses ATM network interfaces  102  to the ATM switch  112  where it is routed over the ATM connection  116  to the IWF element  130 . As discussed above, the IWF element  130  serves to recast the ATM traffic in the form of an ATM data stream into a GigE/IP data stream generating GigE/IP crossover traffic. The crossover GigE/IP traffic traverses the GigE/IP connection  126  to the GigE/IP switch  122  which routes the crossover GigE/IP traffic over the GigE/IP bus  222  to the GigE/IP LT cards  220  and over appropriate second user interfaces  221 .  
         [0041]     The hybrid IP/ATM DSLAM  100  has the capability of forwarding messages from the new faster uplinks towards the ATM devices and vice versa because of the IP/ATM bridge  110  between the ATM and GigE/IP communications portions  300 ,  310  and hence between communications platforms attached to the hybrid IP/ATM DSLAM  100 .  
         [0042]     Referring also to  FIGS. 3A and 3B , hybrid IP/ATM DSL access multiplexing in a DSLAM according to an embodiment of the invention will now be discussed.  
         [0043]     With respect to  FIG. 3A  in which ATM traffic is recast into GigE/IP traffic, ATM traffic is received at an ATM network interface of the DSLAM or at an ATM LT card of the DSLAM at step  400 . As discussed above, receiving traffic at the ATM network interface corresponds to downstream traffic whereas receiving traffic at the ATM LT card corresponds to upstream traffic. This ATM traffic is recast into crossover GigE/IP traffic at an IWF element of the DSLAM in step  410 . In step  420  the crossover GigE/IP traffic is transmitted from a GigE/IP communications portion of a DSLAM. As discussed above, transmission of traffic from a GigE/IP communications portion may be in an upstream (IP network connection) direction or downstream (user interface) direction.  
         [0044]     With respect to  FIG. 3B  in which GigE/IP traffic is recast into ATM traffic, GigE/IP traffic is received at a GigE/IP communications portion of a DSLAM in step  450 . As discussed above, receipt of traffic at a GigE/IP communications portion may be from an upstream direction (IP network connection) or from a downstream direction (user interface). This GigE/IP traffic is recast into crossover ATM traffic at an IWF element of the DSLAM in step  460 . In step  470  the crossover ATM traffic is transmitted over an ATM network interface of the DSLAM or over an ATM LT card of the DSLAM. As discussed above transmission of traffic over the ATM network interface corresponds to upstream traffic and transmission of traffic over the ATM LT card corresponds to downstream traffic.  
         [0045]     Service providers looking to replace their large investment in existing ATM DSLAMs and ATM network infrastructure with GigE/IP DSLAMs and GigE/IP infrastructure are finding it difficult to switch due to the amount of the existing equipment and the cost such a migration from ATM to GigE/IP entails.  
         [0046]     A migration path from ATM DSLAMs to GigE/IP DSLAMs which allows for gradual introduction and switching from the old ATM equipment to new GigE/IP based equipment is preferred as part of a DSL services migration from an ATM network infrastructure to a GigE/IP network infrastructure.  
         [0047]     Service providers can use the hybrid IP/ATM DSLAM  100  as part of such a migration path from ATM DSLAMs to GigE/IP DSLAMs. The hybrid IP/ATM DSLAM  100  immediately brings GigE/IP DSLAM capabilities and support for GigE/IP LT line cards to an ATM infrastructure, while providing for gradual migration to a GigE/IP network infrastructure by allowing customers to re-use their existing ATM DSLAM infrastructure while replacing existing ATM DSLAMs. As hybrid IP/ATM DSLAMs  100  are introduced into the ATM network infrastructure, existing ATM hardware, services, and system interfaces are re-used with the hybrid IP/ATM DSLAMs  100  while new GigE/IP DSLAM capabilities are introduced into the network infrastructure. In this manner service providers can plan to integrate the GigE/IP based network topology into their networks without throwing away existing ATM based network topology. Most of the heavily deployed ATM based DSLAM equipment is re-used during the introduction of next generation GigE/IP DSLAM capabilities avoiding the massive economical burden that would be associated with a massive decommissioning of ATM network infrastructure resources and the simultaneous deployment of all new GigE/IP based network infrastructure.  
         [0048]     Referring also to  FIG. 4 , provision of hybrid IP/ATM DSL access multiplexing in an ATM network infrastructure will now be discussed. At step  500  ATM hardware such as an ATM LT card is removed from an ATM DSLAM of an ATM network infrastructure. The ATM hardware (ATM LT card) is installed in a hybrid IP/ATM DSLAM at step  510 . Such a step re-uses the existing ATM hardware to save costs while introducing the GigE/IP DSL access multiplexing of the hybrid IP/ATM DSLAM. At step  520 , any ATM network interfaces are disconnected from the ATM DSLAM. The ATM network interfaces are then at step  530 , connected to the hybrid IP/ATM DSLAM. At step  540  any new GigE/IP network interfaces are connected to the hybrid IP/ATM DSLAM.  
         [0049]     The gradual migration path described above may also be facilitated by the hybrid IP/ATM DSLAM at a point in time if and when the ATM network infrastructure is to be replaced with solely GigE/IP based infrastructure. The hybrid IP/ATM DSLAM  100  could be phased out by removing the standard GigE/IP LTs  220  and other GigE/IP hardware from the hybrid IP/ATM DSLAM and installing them in new GigE/IP DSLAMs.  
         [0050]     A hybrid IP/ATM DSLAM  100  incorporated into an ATM network infrastructure accommodates more traffic in its backplane to service the GigE/IP line cards but even the existing ATM LT cards benefit from the increased bandwidth of the backplane. The hybrid IP/ATM DSLAM  100  allows a customer to gain the benefits of more bandwidth, enhanced and extra features of a new GigE/IP infrastructure while allowing them to utilize their existing ATM DSLAM infrastructure, and ATM hardware.  
         [0051]     During a transition from ATM to GigE/IP both ATM and GigE/IP network infrastructures will be present and will require support in the communications network. The hybrid IP/ATM DSLAM  100  accommodates both ATM and GigE/IP based LTs and both ATM and GigE/IP based network connections without changing existing deployment of either architecture.  
         [0052]     The hybrid IP/ATM DSLAM  100  allows for more network interfaces than a single ATM DSLAM or GigE/IP DSLAM because it houses both an ATM communications portion and a GigE/IP communications portion, and hence houses network connections that would be found on both an ATM DSLAM and a GigE/IP DSLAM. The additional higher speed GigE/IP interfaces allow the equipment to have more bandwidth.  
         [0053]     The hybrid IP/ATM DSLAM  100  offers flexibility to service providers as they can tune their equipment depending on the end user traffic demands. The same management of the both ATM and GigE/IP equipment within the same shelf allows the customers to migrate quicker as the learning curve is shorter.  
         [0054]     It should be noted that the IP/ATM bridge  110  may be implemented in many different ways. According to a preferred embodiment, the GigE/IP switch  122 , the GigE/IP connection  126 , the IWF element  130 , the ATM connection  116  and the ATM switch  112  are all part of a hybrid IP/ATM NT (network termination) card. Such a card could have onboard controllers for both the ATM and GigE/IP portions of the card and would be connected to both GigE/IP and ATM network interfaces. In other embodiments the IWF element could be implemented between a GigE/IP NT card and an ATM NT card in which either the GigE/IP NT card or the ATM NT card is a daughter card of the other. Many other implementation possibilities exist for the IP/ATM bridge  110  in accordance with the invention.  
         [0055]     Although a GigE/IP switch, GigE/IP connection and GigE/IP network interfaces have been described in association with the preferred embodiments of the invention it is to be understood that the GigE/IP communications portion may be comprised in general of Ethernet/IP based components having an Ethernet/IP switch, an Ethernet/IP connection, and Ethernet/IP network interfaces.  
         [0056]     The embodiments presented are exemplary only and persons skilled in the art would appreciate that variations to the embodiments described above may be made without departing from the spirit of the invention. The scope of the invention is solely defined by the appended claims.