Abstract:
A crosspoint switch is described that has N inputs, M outputs and an array of (N+X)×(M+X), where N M and X are all positive integers. By providing more than the standard N×M switching elements, it becomes possible to utilise the additional switching elements to provide one or more additional protection pathways to compensate for a failure in a switching element.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a crosspoint switch, and in particular to an apparatus and a method suitable for providing alternative pathways through the switch in the event of a failure of a switching element. These alternative pathways have the potential to be engaged automatically and/or remotely.  
         BACKGROUND OF THE INVENTION  
         [0002]    Communications networks are moving towards becoming all optical (photonic) networks, incorporating photonic (optical) switching in which optical signals are switched directly rather than converted to electrical signals, switched electrically, then converted back to optical signals for re-transmission. Photonic switches may be used to switch wavelength division multiplexed (WDM) signals as a group, or the WDM signals may be demultiplexed outside the switch and switched individually as channels, or as groups of channels as desired. Photonic switches are fabricated using a range of technologies and frequently employ a crosspoint (crossbar) architecture. In such architectures light from an input port may traverse a number of switching elements. At each switching element the light may be switched and directed towards an associated output port or alternatively pass though to the next switching element. Once the light has been directed towards an output port it may traverse more switching elements which, in most implementations, must remain inactive so as not to block or disrupt the light path before it reaches its output port  
           [0003]    For example, a recently developed photonic switch using Micro Electro-Mechanical systems (MEMS) technology is described in “Free-Space Micro Machined Optical Switches for Optical Networking” by L Y Lin et al, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5 No.1, January/February 1999; which is incorporated herein by reference. Such MEMS switches typically use moveable mirrors to re-direct optical paths within the switch in order to complete an optical signal or channel connection across the switch.  
           [0004]    [0004]FIG. 1 shows a schematic diagram of a typical MEMS photonic switch  100 . The switch  100  is bidirectional, but for simplicity is assumed to comprise 4 inputs in the form of optical fibres  112 ,  114 ,  116  &amp;  118 , and 4 outputs which are also optical fibres  122 ,  124 ,  126  &amp;  128 . Each input and output has an associated lens  104  which collimates the beams from the inputs and focuses the beam at the outputs. Such a switch is generically referred to as a 4×4 switch (number of inputs×number of outputs).  
           [0005]    The switch  100  is a crosspoint (cross bar) switch, having a switching element (here, a mirror,  106 ) located at each of the points at which optical signals emitted from the input fibres would cross with optical signals emitted from the output fibres. The switch  100  thus has a four by four array of mirrors  106  mounted on a surface  102 .  
           [0006]    In this particular switch, each mirror may be moved between two stable positions. FIGS. 2 a  and  2   b  illustrate these positions. FIG. 2 a  shows the mirror in the inactivated position  106   a , where the mirror is flat i.e. substantially parallel to the surface  102 . FIG. 2 b  shows the mirror having been raised to the activated or upright position  106   b , substantially perpendicular to the surface  102 . This activation may be performed by a variety of means e.g. by micro actuators causing the mirror to be rotated about the hinges  108 . The mirrors are typically formed of materials such as polysilicon, the reflectivity of which is increased by providing a reflective coating  107  such as gold. In the inactivated state, it is typical for the relatively non-reflective surface  109  of the mirror to lie adjacent to the surface  102 , so that the reflective coating  107  does not contact the surface  102 .  
           [0007]    [0007]FIG. 1 shows a typical operation of the switch  100 . By raising the appropriate mirrors, an optical signal from each of the inputs  112 ,  114 , 116  &amp;  118  is directed to a respective output  128 ,  126 ,  122  &amp;  124 . For instance, an optical signal originating from input fibre  112  is formed into a collimated beam  132  by lens  104 . The beam  132  then reflects off the front reflective surface  107  of a raised mirror  106   b  into a further lens  104  which focuses the beam  132  into the output fibre  128 . It will be appreciated that by appropriate control of the array of mirrors  106 , any one of the signals originating from the inputs  112 ,  114 ,  116  &amp;  118  can be switched into any one of the outputs,  122 ,  124 ,  126  &amp;  128 .  
           [0008]    In any system switching information, it is desirable to provide alternative pathways for the information in the event that the original pathway “fails” and is unable to transmit the signals as desired. Such alternative pathway provision is commonly referred to as “protection” when these pathways may be engaged remotely and/or automatically.  
           [0009]    It will be appreciated that a failure in any of the internal switching elements (mirrors  106 ) would impair the functionality of the switch. For instance, any of the mirrors could be jammed in either the raised  106   b  or flat  106   a  position, and this would prevent a connection between the input and output corresponding to that mirror. In addition a mirror which is jammed in the raised position has the potential to prevent a connection between the associated input and another output and between another input and the associated output. This is because the raised mirror may act as a block to such light paths.  
           [0010]    The present invention aims to address such problems.  
         SUMMARY OF THE INVENTION  
         [0011]    In a first aspect, the present invention provides a crosspoint switch comprising N primary inputs, M primary outputs and an array of (N+X)×(M+X) switching elements, where M, N and X are all positive integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.  
           [0012]    A typical crosspoint switch having N inputs and M outputs will have an array of N×M switching elements. By providing the additional switching elements in the array, it becomes possible to provide alternative connectivity between the inputs and outputs to compensate for any failures in the part of the array normally utilised for switching.  
           [0013]    Preferably, said switch is a photonic switch. Photonic switches can have switching elements such as reflective surfaces (mirrors), refractive media, or interferometers.  
           [0014]    Preferably, said additional switching elements comprise at least one column at an outermost edge of the array, and at least one row at an outermost edge of the array.  
           [0015]    Preferably, the additional switching element located at the intersection of each of said row and said column is located in a fixed position so as to redirect incident signals in a predetermined manner. Such a switching element can act to redirect an incident signal from said row along said column, or from said column along said row.  
           [0016]    Alternatively the switch can further comprise X additional inputs and X additional outputs, each of said additional outputs being transmissively connected to a respective additional input.  
           [0017]    Preferably, said switch is a photonic switch, and said additional outputs are connected to said additional inputs by an optical fibre or other form of optical waveguide.  
           [0018]    Optionally, at least one of said additional outputs is coupled to a tap for the monitoring of signals passing through said output.  
           [0019]    Preferably, N=M.  
           [0020]    Preferably, X=2. If X=2, or any even number, protection can be provided for one or more switching elements that fail in the active position and act to block signals.  
           [0021]    Preferably, said array is substantially rectilinear.  
           [0022]    In another aspect the present invention provides a node for a telecommunications network comprising a crosspoint switch comprising N inputs, M outputs and an array of (N+X)×(M+X) switching elements, where M, N and X are all positive integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.  
           [0023]    In another aspect the present invention provides a transmission system comprising a transmitter and a receiver, and a transmission line connecting the transmitter to the receiver, the system further comprising a crosspoint switch comprising N inputs, M outputs and an array of (N+X)×(M+X) switching elements, where M, N and X are all positive integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.  
           [0024]    In a further aspect the present invention provides a method of operating a crosspoint switch comprising N inputs, M outputs and an array of (N+X)×(M+X) switching elements, where N, M and X are all positive integers, the method comprising detecting that switching element has seized to function correctly; and providing control signals to the switching elements for configuring the switching elements to provide the same connectivity as the incorrectly functioning switching element.  
           [0025]    Preferably, said switch is a photonic switch arranged to switch optical signals, the method further including the step of providing control signals to the switching elements for configuring the switching elements so as to ensure that no optical signals are blocked by the incorrectly functioning switching element.  
           [0026]    In another aspect, the present invention provides a computer program arranged to perform the method of a method of operating a crosspoint switch comprising N inputs, M outputs and an array of (N+X)×(M+X) switching elements, where N, M and X are all positive integers, the method comprising detecting that switching element has ceased to function correctly; and providing control signals to the switching elements for configuring the switching elements to provide the same connectivity as the incorrectly functioning switching element.  
           [0027]    Preferably, said computer program is stored on a machine readable medium. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0028]    In order that a greater understanding of the invention can be obtained, embodiments of the invention will now be described with reference of the accompanying drawings, by way of example only and without intending to be limiting, in which:  
         [0029]    [0029]FIG. 1 shows a typical MEMS switch arrangement (PRIOR ART);  
         [0030]    [0030]FIGS. 2 a  and  2   b  show respectively a mirror from the switch of FIG. 1 in the inactivated state and the activated state (PRIOR ART);  
         [0031]    [0031]FIG. 3 shows a photonic switch according to a first embodiment of the present invention;  
         [0032]    [0032]FIG. 4 shows an alternative configuration of the switch of FIG. 3; and  
         [0033]    [0033]FIG. 5 shows a photonic switch according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]    [0034]FIG. 3 shows a photonic switch of similar construction principles to that show in in FIG. 1, although the focusing lenses  104  have been omitted from the diagram for clarity. The switch comprises four optical wave guide inputs  112 ,  114 ,  116  &amp;  118  and four optical wave guide outputs  122 ,  124 ,  126  &amp;  128 . In this instance, the wave-guide is an optical fibre.  
         [0035]    Switching elements in the form of movable mirrors are arranged in a 6×6 array. This array can be viewed as comprising a 4×4 array (as indicated by A) with additional switching element arranged as two additional columns B adjacent to the inputs, and two additional rows C adjacent to the outputs i.e. the illustrated 4×4 crosspoint switch has an array of (4+2)×(4+2), as opposed to a normal crosspoint switch which would have an array of 4×4 switching elements.  
         [0036]    This particular embodiment of the invention further comprises two additional outputs  222 ,  224  respectively coupled by optical wave-guides  232 ,  234 , to inputs  212 ,  214 . An optical signal entering an output  222 ,  224  would thus re-enter the switch via the respective input  212 ,  214  due to transmission along the respective wave-guide  232 ,  234 .  
         [0037]    Additionally, the optical wave-guide  232  has a tap  242  for allowing the monitoring of signals passing along the wave-guide  232 . In this instance, both  232  and  242  consist of optical fibres, the tap comprising a splice such that part of the optical signal transmitted along the fibre  232  will be transmitted along the fibre  242 . The fibre  242  may hence be connected to an optical detector to allow the monitoring of the signals passing along fibre  232 .  
         [0038]    In normal operation, the switch functions in a similar manner to the switch shown in FIG. 1. Optical signals from inputs  112 ,  114 ,  116 ,  118  can be respectively directed to any of the outputs  122 ,  124 ,  126  &amp;  128  by raising the appropriate mirror into the upright position from the 4×4 array of switching elements denoted by the letters AA. For instance, the optical signal  180  from input  114  is switched to output  124  by the mirror  206   b  being in the upright position.  
         [0039]    [0039]FIG. 4 shows the same switch as in FIG. 3, but where the mirror  206   b  has been jammed in the upright position. It is desired to switch the optical signal from input  114  to the output  128 . However, it will be appreciated that the malfunctioning mirror  206   b  would prevent this switch occurring by using the AA switching elements. Consequently, the signal  290  from input  114  is re-directed by additional switching mirror  506   b  to output  224  and hence via fibre  234  to input  214  where the signal emerges and is denoted by  291 . The signal  291  is then re-directed by the additional switching element  806   b  to the desired output  128 . Consequently the input  114  is connected to the output  128  by the additional switching elements provided in columns B and rows C i.e. by using the alternative protection pathways provided by these additional rows and columns.  
         [0040]    In this particular instance, it is also desirable to connect input  112  to output  124 . As mirror  206   b  is jammed in the upright position, the corresponding crosspoint element in the array AA cannot be utilised to switch the optical signal from input  112 . Consequently, mirror  406   b  is actuated to be in the upright position so as to redirect the input signal  280  from input  112  into the additional output  222 . The signal is then transmitted along the wave guide  232  to additional input  212 , where the emerging signal  281  is then switched by raised mirror  706   b  into the desired output  124 . Hence the desire to connectivity between input and output is once again achieved by using the alternative protection rows and columns.  
         [0041]    This invention thus utilises two additional rows and two additional columns in order to provide alternative protection pathways in the switch for a single switching element failure. Two such rows and columns are necessary as the switching element had failed in the on (upright) position, and so incident signals on the switching element would be spuriously redirected. However, if the switching element was to fail in the off position (with the mirror flat) then the switching element would not spuriously re-direct signals, and hence one additional row and one additional column would be required to provide protection. Thus, in systems where a switching element failure would not block signals, only one additional row and column would be required to provide protection.  
         [0042]    For instance, if mirror  206   b  had failed in the flat position then any of the inputs  112 ,  114 ,  116  &amp;  118  could be connected to any of the outputs  122 ,  124 ,  126 ,  128  by the 4×4 array of mirrors AA. The only exception to this would be input  114  could not be connected to output  124  due to the failure of switching element  206   b . However, only a single additional row and additional column would be required to re-route this signal if such connectivity was desired.  
         [0043]    [0043]FIG. 5 shows an alternative embodiment of the present invention. This embodiment corresponds generally to the embodiment shown in FIGS. 3 and 4, with the amendments that no additional inputs  212 ,  214 , outputs  222 ,  224  or connecting means  232 ,  234  are present, and instead this functionality has been replaced by mirrors  906   b . The mirrors  906   b  are located along the diagonal of the 6×6 array of switching elements where the additional columns B and additional rows C intersect. Such switching elements  906   b  are arranged to direct signals from a respective one of the columns B along a respective one of the columns C. In the figures as illustrated, a normal mirror (e.g.  106   b ) produces a 90° clockwise rotation of the optical signal in respect of the direction of beam propagation. Switching elements  906   b  have rear reflective surfaces, and hence produce a 90° anticlockwise rotation of the optical signal. In this instance, the switching elements are fixed in the upright position in order to achieve greater reliability (by ensuring that they do not become jammed in the flat position).  
         [0044]    In FIG. 5, it is once again assumed that mirror  206   b  has become jammed in an upright position, and that connectivity is desired between input  112  and output  124 , and input  114  and output  128 .  
         [0045]    This connectivity is achieved by the signal from input  122  being reflected off mirror  406   b , then subsequently reflected off the static mirror  906   b  to mirror  706   b  and hence into output  124 . Equally, a signal from mirror  114  is reflected off mirror  506   b  to the corresponding mirror  906   b  and hence to mirror  806   b  and to input  128 . It will thus be appreciated that the switching elements  906   b  achieve the same functionality as the additional inputs and outputs  222 ,  224 ,  214 ,  212 . Equally, whilst both the switching elements  906   b  have been indicated as lying along the diagonal of the 6×6 array of switching elements, it will be appreciated that such switching elements  906   b  could equally be located at any appropriate intersection of an additional column B and an additional row C.  
         [0046]    Whilst both embodiments of the present invention have indicated that two additional rows and columns of additional switching elements can be utilised, it will be appreciated that any number of additional rows and columns of additional switching elements could be used to provide alternative protection pathways in a crosspoint switch.  
         [0047]    By providing such additional switching elements, the failure of a single one of the normal switching elements of a crosspoint switch can be routed around. This would maintain full switch functionality in the event of a failure of a single (or, if sufficient protection pathways are provided, a plurality) of switching elements within the switch.  
         [0048]    Whilst the present invention has been described in conjunction with a photonic switch, it will be appreciated that equally the invention could be applied to any switch utilising a crosspoint architecture e.g. an electrical switch.  
         [0049]    For the purpose of this specification, the terms “optical” and “light” should be understood as pertaining not only to the visible part of the electro magnetic spectrum, but also to the infra-red and ultra-violet parts that bound the visible part.  
         [0050]    Any range or device given herein maybe extended or altered without losing the effect sort, as will be apparent to a skilled person from an understanding of the teaching herein.