A WDM optical packet switching block (10, 110), comprising: a first multiport-to-multiport WSS module (1, 101) comprising: a plurality of WDM input ports each adapted to receive an input WDM signal comprising optical packets carried on a plurality of wavelength channels, and a plurality of transit-side ports each allocable to a wavelength channel, a plurality of packet transit paths (4, 104), each packet transit path comprising: an input section connected to a respective transit-side port of the first multiport-to-multiport WSS module for receiving optical packets carried on the allocated wavelength channel, a selective element switchable to selectively pass or block an individual optical packet carried on the allocated wavelength channel and received in the input section, and an output section for transmitting optical packets passed by the selective element, wherein the first multiport-to-multiport WSS module is operable to route independently each wavelength channel of each input WDM signal between the corresponding WDM input port and any one of the transit-side ports, wherein the first multiport-to-multiport WSS module is configured to independently allocate a wavelength channel to each of the transit-side ports, so that the wavelength channels allocated to two transit-side ports may be different or identical, and a second multiport-to-multiport WSS module (5, 101) comprising: a plurality of transit-side ports, the output section of each packet transit path being connected to a respective transit-side port of the second multiport-to-multiport WSS for transmitting the optical packets passed by the selective element on the allocated wavelength channel, and a plurality of WDM output ports each adapted to pass an output WDM signal comprising optical packets carried on a plurality of wavelength channels, wherein the second multiport-to-multiport WSS module is operable to route independently the wavelength channel allocated to a transit path between the corresponding transit-side port and any one of the WDM output ports, wherein the second multiport-to-multiport WSS module is configured to route the wavelength channel received each of the transit-side ports to a selected one of the WDM output ports. a first multiport-to-multiport WSS module (1, 101) comprising: a plurality of WDM input ports each adapted to receive an input WDM signal comprising optical packets carried on a plurality of wavelength channels, and a plurality of transit-side ports each allocable to a wavelength channel, a plurality of WDM input ports each adapted to receive an input WDM signal comprising optical packets carried on a plurality of wavelength channels, and a plurality of transit-side ports each allocable to a wavelength channel, a plurality of packet transit paths (4, 104), each packet transit path comprising: an input section connected to a respective transit-side port of the first multiport-to-multiport WSS module for receiving optical packets carried on the allocated wavelength channel, a selective element switchable to selectively pass or block an individual optical packet carried on the allocated wavelength channel and received in the input section, and an output section for transmitting optical packets passed by the selective element, wherein the first multiport-to-multiport WSS module is operable to route independently each wavelength channel of each input WDM signal between the corresponding WDM input port and any one of the transit-side ports, wherein the first multiport-to-multiport WSS module is configured to independently allocate a wavelength channel to each of the transit-side ports, so that the wavelength channels allocated to two transit-side ports may be different or identical, and an input section connected to a respective transit-side port of the first multiport-to-multiport WSS module for receiving optical packets carried on the allocated wavelength channel, a selective element switchable to selectively pass or block an individual optical packet carried on the allocated wavelength channel and received in the input section, and an output section for transmitting optical packets passed by the selective element, a second multiport-to-multiport WSS module (5, 101) comprising: a plurality of transit-side ports, the output section of each packet transit path being connected to a respective transit-side port of the second multiport-to-multiport WSS for transmitting the optical packets passed by the selective element on the allocated wavelength channel, and a plurality of WDM output ports each adapted to pass an output WDM signal comprising optical packets carried on a plurality of wavelength channels, wherein the second multiport-to-multiport WSS module is operable to route independently the wavelength channel allocated to a transit path between the corresponding transit-side port and any one of the WDM output ports, wherein the second multiport-to-multiport WSS module is configured to route the wavelength channel received each of the transit-side ports to a selected one of the WDM output ports. a plurality of transit-side ports, the output section of each packet transit path being connected to a respective transit-side port of the second multiport-to-multiport WSS for transmitting the optical packets passed by the selective element on the allocated wavelength channel, and a plurality of WDM output ports each adapted to pass an output WDM signal comprising optical packets carried on a plurality of wavelength channels,