The existing paging infrastructure is used to send commands to operate remotely-located electronic or mechanical devices. The top part has a lid integrally moulded with thermoplastic resin. The focus control mechanism causes a light beam having passed through the sensor lens to have astigmatism, thereby being focused linearly in the Y direction on a front focal line located closer to the sensor lens and focused linearly in the X direction at a rear focal line located farther therefrom. In a signal interpolation type timing recovery digital filter, the coefficients of an inverse interpolation filter are determined in such a way as to minimize an error of a signal after interpolation. In another embodiment, flow control frames allow flow control commands to be applied differently to different service classes such that PPP flow can be controlled on a per-class basis.