The cable clock card UBR-CLK-T1 enables the cable router to synchronize to an external T1 timing reference and distribute the clock to its Cisco MC16S and MC16E cable modem cards. The synchronized Data Over Cable Service Interface Specification (DOCSIS) time stamps are then passed downstream to all cable modems. The clock card is designed for cable networks running Voice-over-IP (VoIP) applications where synchronized timing is vital for maintaining voice quality.Because voice streams are real-time and originate at a synchronized fixed rate, it is important to configure synchronous clocking for voice applications; otherwise overruns and underruns could occur at the cable modem coder-decoder (codec), resulting in data corruption and loss. By ensuring synchronized data transmission, the clock card enables high-quality delivery of IP telephony, fax, and analog modem services over DOCSIS-based networks. The clock card locks to an external T1 timing signal supplied by a global positioning satellite (GPS) receiver or a T1 Building Independent Timing Source (BITS) reference from the Public Switched Telephone Network (PSTN). It drives the time stamp clock for its cable modem cards, which ultimately sends the time stamp downstream to the cable modems, providing synchronized timing among the cable modems regardless of location. In addition, the clock card automatically provides the timing reference for the midplane TDM clock.