An apparatus, comprising: a transponder comprising a number of transponder slices, wherein each of the number of transponder slices comprises: an analog front end configured to receive an analog input; analog to digital converters configured to convert the analog input to digital signals; a digital channelizer configured to process the digital signals to generate a plurality of frequency slices; and a digital combiner configured to assemble the plurality of frequency slices to form output sub-bands; characterised in that each of the number of transponder slices further comprises: a digital switch configured to route the plurality of frequency slices from the digital channelizer of a first transponder slice to a digital combiner of a second transponder slice, wherein the digital switch comprises an at least three stage Clos switch network comprising ingress stage switches, middle stage switches, and egress stage switches, and wherein the ingress stage switches are configured to route the plurality of frequency slices to middle stage switches on the transponder slice and to middle stage switches on a number of other transponder slices. a transponder comprising a number of transponder slices, wherein each of the number of transponder slices comprises: an analog front end configured to receive an analog input; analog to digital converters configured to convert the analog input to digital signals; a digital channelizer configured to process the digital signals to generate a plurality of frequency slices; and a digital combiner configured to assemble the plurality of frequency slices to form output sub-bands; an analog front end configured to receive an analog input; analog to digital converters configured to convert the analog input to digital signals; a digital channelizer configured to process the digital signals to generate a plurality of frequency slices; and a digital combiner configured to assemble the plurality of frequency slices to form output sub-bands; characterised in that each of the number of transponder slices further comprises: a digital switch configured to route the plurality of frequency slices from the digital channelizer of a first transponder slice to a digital combiner of a second transponder slice, wherein the digital switch comprises an at least three stage Clos switch network comprising ingress stage switches, middle stage switches, and egress stage switches, and wherein the ingress stage switches are configured to route the plurality of frequency slices to middle stage switches on the transponder slice and to middle stage switches on a number of other transponder slices. a digital switch configured to route the plurality of frequency slices from the digital channelizer of a first transponder slice to a digital combiner of a second transponder slice, wherein the digital switch comprises an at least three stage Clos switch network comprising ingress stage switches, middle stage switches, and egress stage switches, and wherein the ingress stage switches are configured to route the plurality of frequency slices to middle stage switches on the transponder slice and to middle stage switches on a number of other transponder slices.