Systems and methods are provided for navigating an information hierarchy using a mobile communication device. The light beam is directed along one axis of the fiber. A notch cutter portion has a plurality of rotatable cutters for cutting a plurality of retaining notches into the retaining clip. The safety valve includes a conductive contact portion projecting toward an inside of the sealed cell, a peripheral portion in a periphery of the conductive contact portion, and a pin projection in the peripheral portion. Insulative sleeves are placed between the wall panels. The modulation and coding set determination unit determines a modulation and coding set, as a downlink modulation scheme for packet communications, such that a prescribed communication condition corresponding to the channel quality is satisfied, and that padding added to the data buffered in a transmission buffer for subsequent transmission to the mobile station becomes the minimum.