ts to the internal clock source. 5 NETWORK ARCHITECTURE, FACILITIES, AND SERVICES Communications network clocking architecture is multi-level or strata. There are four levels: stratum 1, 2, 3, or 4. Stratum 1 is the most accurate and stable. Stratum 1 clocks, by definition, do not rely on and may not have or need external references but simply an ability to be calibrated on some periodic basis-say on 1-year intervals. Stratum 2, 3, and 4 are less accurate and stable than stratum 1, and they typically have provisions for external reference input. From a strict timing perspective, stratum 1 clocks are at the core of the network, while stratum 4 clocks are at the edge of the network such as in a PBX, channel bank, data multiplex equipment or router. Going back through the chain, stratum 4 clocks depend on a stratum 3 clock, which depends on a stratum 2 clock, which depends on a stratum 1 clock, the master clock for the network. It's a given that long-term, undisturbed connections through a network depend on continuing, long-term, undisturbed common clock signals and their distribution (or linking to every single network element), which carry a digitized payload. Numerous strategies are used or chosen from to formulate clocking in a network. Regardless, the net- work operates synchronous or non-synchronous. T-carrier networks are said to run plesiochronous. Taken literally, and in subjective prac- tical terms, it means nearly synchronous. Therefore, it is not synchron- ous, as understood by an experienced radio or television engineer. Such operation is characterized by the fact that it experiences clock slip behavior sooner or later. Clock slips mean that the clock and everything that uses it vary from its reference by an amount sufficient to cause the payload to be out of coincidence with its original clock by one clock cycle. It's not a matter of whether or not clock slips occur, but when. Even when all the network elements run with external reference to a common, higher order accuracy and stability clock, slips ca