A synchronous packet switch (10, 40, 60, 70) comprising: a first plurality of output modules (12, 42, 62, 72) each comprising a second plurality of optical receivers each configured to receive optical signals at a different one of a said second plurality of wavelengths; and a said first plurality of input modules (16, 44) configured to receive electrical signals carrying data cells to be routed, each said input module comprising: a said second plurality of optical transmitters each configured to generate an optical signal carrying one or more of said data cells, said optical transmitters being configurable to generate a said optical signal at a different one of the said second plurality of wavelengths; and routing apparatus (20, 46, 32) comprising a plurality of output ports, each said output module having at least one said output port allocated to it, said routing apparatus being arranged to receive said optical signals at said different wavelengths and configurable to route each said received data cell to a selected output port; and the said switch further comprising: a third plurality of optical connections arranged to couple the said output ports to the respective said output modules (12, 42); and a switch control unit arranged to control routing of said data cells from said transmitters to said output modules and to generate a routing control signal (36, 48) for configuring the said routing apparatus to route a said data cell from a said transmitter to a selected one of said output ports, wherein said transmitters are arranged to receive said routing control signal and to provide a destination label according to said routing control signal to each said received data cell to be transmitted, and each said input module further comprises a label extractor arranged to extract said destination labels from transmitted ones of said data cells carried by said optical signals and to generate a further routing control signal for configuring said routing apparatus. a first plurality of output modules (12, 42, 62, 72) each comprising a second plurality of optical receivers each configured to receive optical signals at a different one of a said second plurality of wavelengths; and a said first plurality of input modules (16, 44) configured to receive electrical signals carrying data cells to be routed, each said input module comprising: a said second plurality of optical transmitters each configured to generate an optical signal carrying one or more of said data cells, said optical transmitters being configurable to generate a said optical signal at a different one of the said second plurality of wavelengths; and routing apparatus (20, 46, 32) comprising a plurality of output ports, each said output module having at least one said output port allocated to it, said routing apparatus being arranged to receive said optical signals at said different wavelengths and configurable to route each said received data cell to a selected output port; a said second plurality of optical transmitters each configured to generate an optical signal carrying one or more of said data cells, said optical transmitters being configurable to generate a said optical signal at a different one of the said second plurality of wavelengths; and routing apparatus (20, 46, 32) comprising a plurality of output ports, each said output module having at least one said output port allocated to it, said routing apparatus being arranged to receive said optical signals at said different wavelengths and configurable to route each said received data cell to a selected output port; and the said switch further comprising: a third plurality of optical connections arranged to couple the said output ports to the respective said output modules (12, 42); and a switch control unit arranged to control routing of said data cells from said transmitters to said output modules and to generate a routing control signal (36, 48) for configuring the said routing apparatus to route a said data cell from a said transmitter to a selected one of said output ports,