US Patent 8068430 High quality timing distribution over DSL without NTR support

A method and system in which a Digital Subscriber Line Access Multiplexer (DSLAM) that is part of a DSL (Digital Subscriber Line) system is able to provide synchronous services to end user equipment connected to the DSLAM via a DSL link and Customer Premises Equipment (CPE). A sniffer device having direct or indirect access to a Primary Reference Clock (PRC) of the DSL system calculates the phase difference between the PRC and the DSLAM's Local Timing Reference (LTR). The phase difference is sent to the CPEs requiring accurate reference clock frequency. The CPEs are able to derive the PRC from the phase difference information transmitted by the sniffer device thus enabling synchronous end user equipment operation.