A SpaceWire router connects together many nodes using SpaceWire links, providing a means of routing packets from one node to any of the other nodes or routers attached to the router. A node is simply the source or destination of a SpaceWire packet. A SpaceWire router comprises a number of SpaceWire link interfaces and a switch matrix. The switch matrix enables packets arriving at one link interface to be transferred to and sent out of another link interface on the router.
Each link interface may be considered as comprising an input port (the link interface receiver) and an output port (the link interface transmitter). A SpaceWire router transfers packets from the input port of the switch where the packet arrives, to a particular output port determined by the packet destination address. A router uses the leading data character of a packet (one of the destination address characters) to determine the output port of the router to which the packet is to be routed. If there are two input ports waiting to use a particular output port when the previous packet has finished being sent, an arbitration mechanism decides which input port is to be served.