The routing of data streams is discussed, and particularly routing one or more incoming streams to one or more output destination ports. The ability to merge incoming streams is discussed so that several low bit rate input packet streams can be merged into a higher bit rate output stream. An assignment data structure identifies for each input stream the or each destination to which it is to be routed, and a packet allocation data structure holds information about the packets and information about the destination of the packets to allow a memory holding the packets to be controlled accordingly.