A network router is tested for operation according to a given network protocol, by coupling the router under test to a test host device and arranging the device to simulate operative test network topologies which include router nodes, network nodes, and edges between the nodes. Upon the application of a shrinkage agent, either uniformly or selectively to the pile surface, the individual component yarns shrink to different degrees. Among the visual effects that may be utilized is a Moir\xe9 effect. At least a part of the suspension is fed from the first module into the second module where it is cooled and simultaneously subjected to ultrasound.