US 2004161081 A1 abstract an asymmetrical digital subscriber line communication system enhances the signal to noise ratio of the received signals by conducting a pll process with a tone conditioned on the most superior snr instead of a fixed tone an improvement in data rate is thereby achieved application_number US 77032704 A description it should be understood that the description of the preferred embodiment is merely illustrative and that it should not be taken in a limiting sense in the following detailed description several specific details are set forth in order to provide a thorough understanding of the present invention it will be apparent however to one skilled in the art that the present invention may be practiced without these specific details practical embodiments of the invention will be explained in conjunction with the drawings the adsl annex a standard defines the 64 tone as a reference signal to obtain loop timing thus a central station loads a signal with constellation information of 1 i on the 64 tone and then a remote terminal obtains the loop timing between the central station and itself by conducting a pll operation with the 64 tone during such a process of channel analysis it is impossible for the remote terminal to obtain the loop timing from other tones because the central station sends irregular constellation values to other tones that is as shown in fig5 a qam analysis resulting from a pll operation with a tone other than the 64 tone shows that the phases are considerably distance from the 45 degree mark this is because the.