Telecommunications switch

A telecommunications switch comprising:

• (a) a plurality of buffers each with an input for receiving packets to be transmitted and an output line providing a source of packets to be switched;
• (b) a switching device having a plurality of stages each with a plurality of switching elements, wherein each switching element has two inputs and two outputs and is operable in response to a destination address contained in a packet received by the element to route the packet to one or other output, wherein the first stage has its inputs connected to the source lines, subsequent stages have their inputs connected to outputs of the elements of the preceding stage and the last stage has outputs connected to destination lines of the switch, the links being such that each element of the first stage can serve any destination lines served by the stage which precedes it, each element in the last stage can serve any source line and each element of any preceding stage can serve a subset of the source lines served by the stage which follows it;
• (c) control means connected to receive destination addressed from packets store in the buffer, to select packets which can be transmitted through the switching device without mutual blocking, and to release the selected packets from the buffers, the control means including an optical neural network having a two-dimensional array of light sources disposed to illuminate via a mask a two-dimensional array of photodetectors, and feedback means from the detectors to the sources, wherein each light source corresponds to a respective one of possible paths through the switching device and each photodetector corresponds to a respective one of said possible paths;
• wherein:
• (i) the output of each element of the first stage has a destination index having a value of 0 or 1 according to whether it serves a first or a second partition of the destination lines;
• (ii) the output of each element of each stage after the first has a destination index having a value of 0 or 1 according to whether it serves a first or a second partition of the destination lines served by those elements of the preceding stage to which the element in question is connected;
• (iii) the input of each element of the last stage has a source index having a value of 0 or 1 according to whether it serves a first or a second partition of the source lines;
• (iv) the input of each element of each preceding stage has a source index having a value of 0 or 1 according to whether it serves a first or a second partition of the source lines served by those elements of the following stage to which the element in question is connected;
• (v) for each of said possible paths, the position in the respective array of the light source and of the photodetector which corresponds to that path is determined:
• in a first co-ordinate direction, by a first co-ordinate having a magnitude equal to that represented by a binary number consisting of the destination indices of the element outputs traversed by that path, taken in the same order as the stages, with the index corresponding to an output from the first bank being the least significant bit and the index corresponding to an output from the last bank being the most significant bit;
• and
• in a second co-ordinate direction, by a second co-ordinate having a magnitude equal to that represented by a binary number consisting of the source indices of the element inputs traversed by that path, taken in the same order as the stages with the index corresponding to an input to the first bank being the most significant bit and the index corresponding to an input to the last bank being the least significant bit.