Stratum 1 is defined as a completely autonomous source of timing, which has no other input, other than perhaps a yearly calibration. The usual source of Stratum 1 timing is an atomic standard (Cesium Beam or Hydrogen Maser) or reference oscillator (OCXO). The minimum adjustable range and maximum drift is defined as a fractional frequency offset f/f of 1 x 10-11 or less. At this minimum accuracy, a properly calibrated
source will provide bit-stream timing that will not slip relative to an absolute or perfect standard more than once every 4 to 5 months. Atomic standards, such as Cesium clocks, have far better performance.
A Stratum 1 clock is an example of a Primary Reference Source (PRS) as defined in ANSI/T1.101. Alternatively, a PRS source can be a clock system employing direct control from Coordinated Universal Time(UTC) frequency and time services, such as Global Positioning System (GPS) navigational systems. The GPS System may be used to provide high accuracy, low cost timing of Stratum 1 quality.
A Stratum 2 clock system tracks an input under normal operating conditions, and holds to the last best estimate of the input reference frequency during impaired operating conditions. A Stratum 2 clock system requires a minimum adjustment (tracking) range of 1.6 x 10-8. The drift of a Stratum 2 with no input reference is less than 1.6 x 10-8 in one year. The short-term drift of the system is less than 1 x 10-10 in 24 hours. If one interprets this specification as a drift of 1 x 10-10 each 24 hours, this amounts to a frame slip rate of approximately 1 slip in 7 days when the Stratum 2 clock system is in the hold mode. A Stratum 2 clock with a drift of less than 2.5 x 10-11 per day will result in a time to the first frame slip of more than 2 months. Typical examples of Stratum 2 clocks are Rubidium Standards and Double Oven OCXO’s.