A dispersed Hartmann sensor includes a Hartmann lenslet in combination with a dispersive element, whereby a Hartmann spot formed by light passing through the Hartman lenslet is dispersed parallel to the phase step of the light. For example, an HDD 12-1 makes another HDD to perform long distance seek using this communication line and measures the influence of vibration caused at that time. In an embodiment, the prepreg layer has a thickness of at most about 0. In addition, a method for increasing the time span between Delivery Traffic Indication Message beacons by tracking when Wireless Stations are able to receive messages and instantly delivering messages to those Wireless Stations.