Paul A Vixie <vixie@wisdom.home.vix.com> wrote:

DC power is what the box wants internally, and there's a lot less heat generated by making 5V and 12V out of 48VDC instead of 110VAC or 220VAC. if you visit a telco switching center and surround yourself with 10 Northern Telecom OC48 racks you should notice how quiet it seems.

Well, it is not exactly true. The way modern power supplies work is to rectify AC to DC, modulate it with high frequency (1KHz or so) with some transistors to feed a small transformer (the higher frequency, the smaller core you need), and then rectify it again. The power loss on transistors working in saturated modes is mostly due the voltage loss (which is constant determined by physics of the PN-junction or field-effect) multiplied by the current. With lower voltage you need higher current to get the same power, so the power dissipation on switching transistors is higher. Therefore, high-voltage power supplies are more efficient than low-voltage. The reason why sync mux racks are quiet is because they don't do any fancy high-speed stuff (after all, those muxes are nothing more than glorified shift registers) and so they don't need any power-hungry circuitry. --vadim /