the hi - sd cbs includes a termination unit and pluralities of distributed customer premise units connected through the cable tv network . the radio signals in hi - sd cbs are propagating inside the co - axial cable . to insure the linking quality between the termination unit and customer premise devices , preamble procedures are required to adjust the transmitting radio power , the receiver sensitivity , the channel equalization , and to synchronize the transceivers for data transmission between the termination unit and the customer premise devices . to guarantee a secure link between the termination unit and an individual customer premise device or a group of premise devices , the termination unit provides different encryption keys to different customer premise device groups .