The master clock in Hillsboro also provided nationwide timing for the Bell system up until“Does anybody really know what time it is … does anybody really care?” With apologies to Robert Lamm and '70s rockers Chicago, the answer from successful wireless carriers is an unequivocal yes. They are ardently focused on minimizing complaints about blocks, drops and cut-outs.
How do they do it? They have a few available options. Plesiochronous timing actually refers to tying each cell independently to an external clock source, usually a GPS pulse. Global positioning satellites and receivers require a super-accurate timing mechanism to perform their primary positioning function to expected accuracy. Cells and switches will piggyback onto this to tune themselves, guaranteeing seamless call control.
A hierarchical source-receiver system ties an entire network to a limited number of external sources, whose timing references are passed down sequentially through the systems. It's the timing equivalent of a PTA telephone tree. Fifteen years ago, according to Steve Clark, U.S. Cellular (www.uscc.com) vice president of network operations, atomic clocks served as communal master references for the entire North American network. There were two: the Naval Observatory, and a cesium clock in Hillsboro, MO, controlled and shared by AT&T (www.att.com). Although accurate at the source, they degraded ever so slightly as they were passed down through each network.
Mutual synchronization is a third common timing method. Basically, each cell sends and receives timing references from all other cells in the system, continuously. Each cell determines official network time-stamp by averaging the pulses it constantly receives from other cells.
Loss of timing can be visible to customers in a number of ways. Shutdown of a master timing source could bring down a digital network. Short-lived audio anomalies result from faulty timing contact between handset and base station in areas of fringe or obstructed coverage. Most commonly, timing problems between switches will manifest themselves to users as those brief, clean, silent cut-outs, which Clark called the “Max Headroom” effect.
You can hear a mistimed analog signal, however corrupted with cross-talk or static. A mistimed digital call is completely, if temporarily, cut out. This is jitter. Lengthier or more severe timing differentials can then cause blocks, lost handoffs or failed call setup.
the last 20 years or so. See..Hillsboro has a
claim to fame!