BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to a casing/shelter for sheltering electronic components, to a network element and to an air passage. The casing comprises an air inlet at a first position of the casing and an air outlet at a second position of the casing. The second position is usually different from the first position. 
     The air inlet may comprise a first plate with at least one first aperture, typically with a plurality of first apertures. The apertures allow air to flow in the casing for cooling purposes. The cooling may be active using a fan or passive using only convection. 
     The casing may also comprise a filter element that is arranged near the air inlet and prevents for instance dust particles from entering into the casing. The filtering element may comprise synthetic fibers, for instance glass fibers or polypropylene fibers. 
     Especially, casings for the sheltering of network elements and other telecommunication equipment have to fulfill special demands with regard to, for instance:
         fire resistance,   earthquake resistance,   protection against rain water or sprinkling water, and   protection against undesired touching.       

     Furthermore, there may be other demands that result from standards or de facto standards, for instance from:
         ATCA (Advanced Telecommunications Computing Architecture) that was developed by the PCI industrial computers manufacturers group (PICMG), and/or   NEBS (Network Equipment-Building system) that was developed by Regional Bell Operating Companies (RBOC).       

     BRIEF SUMMARY OF THE INVENTION 
     Therefore, there is a need for a casing that fulfills as much as possible of the demands that were mentioned above, especially the demands for touching protection and/or preventing particles from outside coming in and/or against sprinkling water. Furthermore there is a need for a corresponding network element and/or air passage. 
     This need is accomplished by the subject matter of the independent claims. The sub claims refer to specific embodiments of the present invention. 
     The casing may comprise in addition to the parts which were mentioned at the beginning:
         a first plate comprising at least one first aperture therein, and   a second plate arranged between the first plate and an inner part of the casing and the second plate comprising at least one second aperture.       

     Typically, there may be a plurality of second apertures. The first position may be for instance in a front door of the casing or in a front side of the casing. The second position may be for instance in an upper part of the casing. Alternatively the first position and the second position may be at other places, for instance at side walls of the casing. 
     By using only two plates it is possible to have a simple construction which is able to fulfill demands with regard to touching protection and/or sprinkling water protection. 
     The first plate may be arranged in parallel to the second plate. If those plates are arranged in parallel only a little space is needed. A distance between the two plates is for instance smaller than 25 mm (Millimeter) but greater than for instance 5 mm. 
     The first plate may comprise at least a first slat which extends to the outside of the casing thereby forming a part of the border of a first aperture. In addition, the second plate may comprise a least one second slat. The second slat extends also towards the outside of the casing. If there are a plurality of first slats and a plurality of second slats each plate resembles a Venetian blind in that it is provided with a plurality of elongated slats arranged in parallel, spaced relation and also in columns. Such an arrangement gives excellent protection against water, dust particles or even intrusion of mechanical parts from the outside of the casing to the inside of the casing. 
     The first slat may extend from the first plate downwardly, especially at least with a portion of the first slat that extends to the outside of the casing. Furthermore, the second slat may extend from the second plate upwardly, especially at least with a portion thereof that extends towards the outside of the casing. By directing the slats to different direction, it is possible to prevent in a simple manner the intrusion of long mechanical devices, for instance of a screwdriver or of a long wire. 
     The terms “downwardly” and “upwardly” refer to the usual operation position of the casing or of the network element. The slats of the first plate may have the same shape as the slats of the second plate. However, the second plate may be arranged top down compared to the first plate. 
     The first slat and the first plate may include a first acute angle pointing in a first direction. The second slat and the second plate may include a second acute angle pointing in a second direction which is opposite to the first direction. The angle may include a range of 15 degrees to 45 degrees. The angle may be well defined if the slats are flat or planar. However, it is also possible to define an angle if the slat are non-planar, for instance if they are bent in the vertical direction. This will be explained in more detail below with reference to  FIG. 4 . 
     The first plate and the second plate may be made of aluminum or consist of aluminum or an aluminum alloy containing at least 90 atom percent aluminum. Aluminum is preferred because it has comparably low specific weight, for instance compared to steel. However, it is also possible to use other materials than aluminum, for instance steel, copper etc. 
     The casing may comprise a filter element which is arranged between the second plate and an inner part of the casing. This effects, that the protection that is created by the arrangement of the two plates extends also to the filter element, i.e. it is not possible to damage the filter element using a thin wire or a screwdriver, for instance. 
     It may be not possible to insert a wire with a diameter of 2.5 mm or below or alternatively of 1 mm or below through the air inlet  110  using an inserting force of 1 Newton or below, for instance. The diameter of the wired is for instance greater than 0.5 mm. By fulfilling these demands it is possible to conform with international protection classes, for instance according to EN (European Norm) 60 529, class IP4. or IP5. or higher. The “.” in the name of the IP class is a space holder for a second digit that relates to a different kind of protection demands. 
     The casing may comprise a mechanical frame having the shape of a parallelepiped, i.e. there are six surfaces, wherein each surface may have a rectangular shape. The square is understood here as a special case of a rectangular shape. Furthermore, the surfaces of all three pairs of opposite surfaces are parallel to each other, respectively. 
     There may also be a network element which comprises a casing according to the invention or its embodiments. Furthermore, the network element comprises electronic components of a switch, a router or of a telecommunication device. The switch performs forwarding of data packets at a level two according to the ISO (International Organization for Standardization) network protocol stack, for instance using MAC (Medium Access Control) addresses. A router performs forwarding of data packets at level three of the stack, for instance according to the internet protocol (IP). The telecommunication device may perform circuit switching or packet switching. The telecommunication device may comprise units for xDSL (x Digital Subscriber Lines), especially for ADSL (Asymmetrical DSL) or other DSL standards. 
     There may also be an air passage, i.e. an air inlet or an air outlet, that comprises the features of the embodiments mentioned above. 
     The foregoing has outlined rather broadly the features and technical advantages of embodiments of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the embodiments of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  illustrates a perspective view of a telecommunication cabinet, 
         FIG. 2  illustrates a first embodiment of an air inlet of the cabinet, 
         FIG. 3  illustrates a second embodiment of an air inlet, 
         FIG. 4  illustrates a third embodiment of an air inlet, 
         FIG. 5  illustrates an exploded view of an air inlet mechanical shelter, and 
         FIG. 6  illustrates a perspective view of the air inlet. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. Moreover, the same reference signs refer to the same technical features if not stated otherwise. As far as “may” is used in this application it means the possibility of doing so as well as the actual technical implementation. 
     The present invention will be described with respect to preferred embodiments in a specific context namely a telecommunication cabinet. The invention may also be applied, however, to other technical devices, for instance transformers for high voltages of greater 200 volts or even greater 1000 volts. 
       FIG. 1  illustrates a perspective view of a telecommunication cabinet  10 . The cabinet  10  includes a frame  12 , two doors (only right hand side door  14  is shown), a top cover  16  and covers for side walls and for the back wall that will be explained in more detail below. 
     The frame  12  comprises:
         a front bottom rail  20 ,   a right bottom rail  22 ,   a back bottom rail  24 ,   a left bottom rail  26 ,   left front rails  30   a ,  30   b ; alternatively only one rail may be used instead of rails  30   a ,  30   b,      right front rails  32   a ,  32   b,      right back rails  34   a ,  34   b,      left back rails  36   a ,  36   b,      front top rail  40 ,   right top rail  42 ,   back top rail  44 , and   left top rail  46 .       

     This means that the frame  12  has the shape of the edges of a parallelepiped. There is a casing  11  that comprises:
         a left side panel  2 ,   a right side panel  4 ,   a back panel or back panels  6   a ,  6   b,      front doors, only the right front door  14  is shown, and   a bottom panel  8 .       

     The panels  2 ,  4 ,  6   a  and  6   b  are fastened by hinges but may be also fastened by other fastening means, for instance by screws. Panel  8  as well as top cover  16  may be fastened by screws or by other fastening means. 
     The cabinet  10  includes for instance a support plate  50  for an electronic device, for instance, for a rectifier. The support plate  50  is fastened to the frame  12  by screws  54  to  68 . 
       FIG. 1  also shows ventilation channels  70 ,  72  in the lower part of cabinet  10 . Electrical fans  76 ,  78  may be arranged in the upper part of the cabinet  10 . An air flow may be directed through slots in the front door  14  and through a channel  84  which contains a filter element to the fans  76 ,  78  and then through air outlets  100  in side walls of top cover  16 . Air outlet  100  may be formed similarly or identically to air inlet  110 . Other arrangements of the air inlets and the air outlets are also possible, for instance in side walls of cabinet  10 . 
     Other electronic devices  82  may be arranged in slots  80  that are situated on the left side of the cabinet  10 . 
     Digital logic units (DLU)  83  may also be inserted in cabinet  10 . These DLU&#39;s are connected or may be connected to subscriber lines that lead to DSL (Digital Subscriber Line) subscribers which use for instance ADSL (Asymmetrical Digital Subscriber Line) or other kinds of xDSL. Therefore, a unit  83  is also named an electronic splitter. 
     Other telecommunication equipment  85  may be arranged left from plate  50 , for instance DSLAMs (DSL Access Multiplexer) HiX 5630 or HiX 5635 DSLAMs manufactured by Nokia Siemens Networks may be used. 
       FIG. 1  also shows a locking mechanism  86  for locking door  14 . Furthermore, a telescope arm  88  is shown which is connected to frame  12  and to door  14 . 
       FIG. 1  also shows mounting rails  90 ,  92  that are arranged above compartments for batteries. Both mounting rails  90 ,  92  are fastened on a horizontal rail  94  by means of holding elements. Vertical rails  96  and  98  are arranged at the left side and at the right side of the support plate  50 . 
       FIG. 2  illustrates a first embodiment of an air inlet  110  of cabinet  10 . Air inlet  110  is part of the right front door  14  of cabinet  10 . Air inlet  110  comprises of the following parts, arranged in parallel with regard to each other:
         an outside panel  112  or plate,   an inside panel  114  or plate, and   a filter element  118  that is held by the channel  84 .       

     Channel  84  forms a frame that circumferences filter element  118  and is fastened to the inside panel  114  by a fastening element, for instance by a screw  116 . 
     Outside panel  112  comprises the following parts:
         slats  120  to  128  or lamellas,   slits  130  to  138 , and   horizontal ribs  140  to  148 .       

       FIG. 2  shows only five slats  120  to  128  although there are typically much more than these five slats. Slats  120  to  128  extend from outside panel  112  to the outside of casing  11 .  FIG. 2  shows a cross section in the vertical direction. In this cross section the slats  120  to  128  have straight section lines with the cross section. The slats  120  to  128  each have an essentially rectangular shape or contour, especially in the main portion thereof. There is a horizontal bending only at end portions of the slats  120  to  128 . It is possible to punch the slats  120  to  128  out from the panel  112  thereby creating the slits  130  to  138  and the ribs  140  to  148 . 
     In the embodiment shown in  FIG. 2  the slats  120  to  128  are wider compared to the height of slits  130  to  138 . This can be reached by an appropriate punching. For instance, slat  120  has an upper edge that corresponds to a bending edge  150  that extends in a horizontal direction. Slat  120  also has a lower edge  152  that extends parallel to bending edge  150 . The lower edge  152  is a free edge of slat  120 . The width of slat  120  equals the distance between edge  150  and edge  152 . 
     According to  FIG. 2  the slats  120  to  128  extend from panel  112  downwardly, see arrow  196 . Therefore, rain water or sprinkling water will drop downwardly to the ground but not inside the casing  11 . Furthermore slats  120  to  128  prevent dust from falling into casing  11 . 
       FIG. 2  shows the following distances:
         a distance D 1  between inside panel  114  and outside panel  112 , for instance in the range of 7 mm to 20 mm or of 10 mm to 13 mm,   a distance D 2  that corresponds to the height of the slits  130  to  138 , for instance in the range of 2 mm to 15 mm or of 3 mm to 10 mm,   a distance D 3  between the outside panel  112  and the lower edge of slats  120  to  128 , for instance in the range of 1 mm to 10 mm or of 2 mm to 6 mm, and   a distance D 4  which corresponds to the height of horizontal ribs  140  to  148  preferably in the range of 2 mm to 20 mm or of 5 mm to 10 mm.       

     The horizontal ribs  140  to  148  are as flat as outside panel  112 . The slits  130  to  138  and the horizontal ribs  140  to  148  have rectangular contours in the present embodiment. 
     The part of inside panel  114  that is part of air inlet  110  has the same shape compared to the part of outside panel  112  that is part of air inlet  110 . However the part of inside panel  114  is turned top down compared with the corresponding part of outside panel  112 . Therefore, there are the following parts of inside panel  114 :
         slats  160 ,  162 ,   slits  170 ,  172 , and   horizontal ribs  180 ,  182 .       

       FIG. 2  only shows five slats of inside panel  114  from which only two have reference signs  160  and  162 . Usually, there are is a plurality of slats  160 ,  162  in inside panel  114 . The slats  160 ,  162  of inside panel  114  extend from inside panel  114  upwardly towards the outside of casing  11 , i.e. towards outside panel  112 . There is an angle A 2  between slat  160  or  162  and inside panel  140  which is an acute angle, for instance in the range of 15 degrees to 45 degrees. Angle A 2  has preferably the same value as angle A 1  between slats  120  to  128  and outside panel  112 , for instance in the range from 10 degrees to 50 degrees. 
     As the slats  120  to  128  are directed downwardly and as the slats  160 ,  162  are directed upwardly it is almost impossible to insert a wire or a screwdriver in air inlet  110 . 
     In the embodiment shown in  FIG. 2  slits  130  to  138  of outside panel  112  are aligned with corresponding slits  170 ,  172  of inside panel  114  in a direction that is normal to outside panel  112  or to inside panel  114 , see arrow  198 . 
     In a modification of the embodiment shown in  FIG. 2  slats  120  to  128  and slats  160 ,  162  each have two free edges  150 ,  152 . In this case ribs  140  to  146  and  180 ,  182  are optional. 
     Other arrangements and shapes of slats  120  to  128  and  160 ,  162  are possible as well. 
       FIG. 3  illustrates a second embodiment of air inlet  110 . Same parts are referenced by the same reference signs without detailed explanation of these parts to avoid repetition. However the parts of  FIG. 3  have an appended letter “a”. This means that there are for instance the following parts of air inlet  110   a:  
         an outside panel  112   a,      an inside panel  114   a,      slats  120   a  to  128   a  of outside panel  112   a,      slats  160   a ,  162   a  of inside panel  114   a , and   slit  138   b  that corresponds to slit  138 .       

     The difference between  FIG. 3  and  FIG. 2  is, that there is an offset D 5  between the higher edges of slits  130   a  and  170   a , i.e. neighboring slits  120   a  of outside panel  112   a  and  170   a  of inside panel  114   a  are not aligned with each other. The distance D 5  is preferably in the range of 1 mm to 10 mm or of 2 mm to 7 mm. 
       FIG. 4  illustrates a third embodiment, i.e. an air inlet  110   b . Corresponding parts are again referenced by the same reference numbers. However, the letter “b” is appended to the reference signs of  FIG. 4 . Therefore, there are for instance the following parts of air inlet  110   b:  
         an outside panel  112   b  that corresponds to outside panel  112     an inside panel  114   b  that corresponds to inside panel  114 ,   slats  120   b  to  128   b  that are similar to slats  120  to  128  with regard to the direction in which the slats  120   b  to  128   b  extend from outside panel  112 ,   slats  160   b  and  162   b  that correspond to slats  160  and  162  with regard to the direction in which they extend from inside panel  114   b.          

     However, slats  120   b  to  128   b  and  160   b  and  162   b  are non-planar in their main lateral portions. As shown in  FIG. 4  slats  120   b  to  128   b  and  160   b  and  162   b  are bent in the plane in which the cross section in  FIG. 5  is arranged. There may be a circular bending, a non-circular bending or an aspheric bending. Other kinds of bending are possible as well. 
       FIG. 4  also shows a wire  204  that has a diameter of 1 mm or smaller. It is almost impossible to insert the wire  206  into inlet  110   b.    
     An angle A 1   b  is defined between outside panel  112  and slat  120   b . The right side of the angle A 1   b  is adjacent to a bending edge  150   b  of slat  120   b  and to the free edge  152   b  of slat  120   b . Angle A 1   b  may have the same range as angle A 1 . 
       FIG. 5  shows an exploded view of air inlet  110  of the mechanical shelter. In the embodiment shown in  FIGS. 5 and 6  there are five columns of apertures  190  or louvers in each panel  112  and  114 . As shown in more detail in  FIG. 6  there are four columns C 1  to C 4  at the outside panel  112 . Rows R 1  to R 5  and further rows  210  extend in the horizontal direction, for instance 53 rows. 
     As mentioned above, inside panel  114  has the same shape as outside panel  112  in the region that forms air inlet  110 . 
     However the region of inside panel  114  is turned top down compared to the corresponding region of outside panel  112 . A frame  206  defines the distance D 1  between outside panel  112  and inside panel  114 . 
     In other embodiments, other shapes of apertures are used, for instance more circular shapes or more elliptical shapes. It is possible to use a matrix like arrangement of the apertures  190  or some other kinds of arrangement. 
     In other embodiments air outlet  100  is formed in the same way as air inlet  110 ,  110   a  or  110   b . A filter element is optional for air outlet  100 . 
     In other words, an air inlet mechanical structure of a telecommunication outdoor shelter is disclosed, especially an air inlet that fulfills IP55. Therefore the shelter forms an outdoor telecommunication shelter. The IP55 mechanical structure may include one double layer front door in which the air inlets are arranged, one membrane filter may be arranged in the door to prevent dust coming into the shelter. 
     To fulfill the cooling needs of the telecommunication shelter the air inlet forms an air channel for air flow. Sometimes this mechanical structure may be designed in a complex way. 
     There may be a lot of additional supplementary parts. Contrary, the invention simply uses the opposite holes in different sides of the double layer door plus the inside membrane filter to solve as much as possible problems. 
     The basic characterization for air inlets is that there are lots of appropriate hole-openings in the door panels. For IP55 needs, the holes in the outside of the door panels are always opened towards the bottom, to avoid water dropping in. These holes may have a height of about 5 mm (Millimeter), for instance. This height will not pass the test for the prevention of a 1 mm diameter wire from penetrating into the casing. However, to fulfill this test an additional part is included, namely inside panel  114 . By arranging the opposite opening holes towards the top it will prevent the wire from entering. Combined with one membrane filter inside the door it is possible to pass the IP55 test, for instance. 
     Therefore there is a protection mechanical structure in a double layer door with cooling inlet openings in different sides of the door with different opening directions. 
     Although embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claim. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes and methods described herein may be varied while remaining within the scope of the present invention. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the system, process, manufacture, method or steps described in the present invention. As one of ordinary skill in the art will readily appreciate from the disclosure of the invention systems, processes, manufacture, methods or steps presently existing or to be developed later that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within there scope such systems, processes, methods or steps. 
     LIST OF REFERENCE SIGNS 
     
         
           2 , 4  side panels 
           6   a ,  6   b  back panel or panels 
           8  bottom panel 
           10  cabinet 
           11  casing 
           12  frame 
           14  door 
           16  top cover 
           20  front bottom rail 
           22  right bottom rail 
           24  back bottom rail 
           26  left bottom rail 
           30   a ,  30   b  left front rail 
           32   a ,  32   b  right front rail 
           34   a ,  34   b  right back rail 
           36   a ,  36   b  left back rail 
           40  front top rail 
           42  right top rail 
           44  back top rail 
           46  left top rail 
           50  support plate 
           52  rectifier 
           54  to  68  screw 
           70 ,  72  ventilation channel 
           76 ,  78  fan 
           80  slots for printed circuit boards 
           82  electronic devices 
           83  printed circuit board 
           84  channel 
           85  equipment 
           86  locking mechanism 
           88  telescope arm 
           90 ,  92  mounting rail 
           94  horizontal rail 
           96 ,  98  vertical rail 
           100  air outlet 
           110  air inlet 
           112  outside panel 
           114  inside panel 
           116  screw 
           118  filter 
           120  to  128  slat 
           130  to  138  slit 
           140  to  148  horizontal rib 
           150  bending edge 
           152  lower edge 
           160 ,  162  slat 
           170 ,  172  slit 
         A 1 , A 2  angle 
         D 1  to D 5  distance 
           180 ,  182  horizontal rib 
           190 ,  192  aperture 
           194 ,  196 ,  198  arrow 
         C 1  to C 4  column 
         R 1  to R 5  row 
           204  wire 
           206  frame 
           210  further rows
A casing for sheltering electronic components ( 52, 83) is disclosed, comprising:--an air inlet ( 110) at a first position of the casing ( 11) and,--an air outlet at a second position of the casing ( 11). The air inlet ( 110) comprises a first plate ( 112) with at least one first aperture ( 190) therein. The inlet ( 110) also comprises a second plate ( 114) that is arranged between the first plate ( 112) and an inner part of the casing ( 11). The second plate comprises at least one second aperture ( 192).