Chapter 4. Stratum Levels Defined

An American National Standards Institute (ANSI) standard entitled "Synchronization Interface Standards for Digital Networks" (ANSI/T1.101-1987) was released in 1987. It is being revised in Committee T1X1 for release as ANSI/T1.101-1998 [Reference 4]. This document defines the stratum levels and minimum performance requirements. Further discussion of the hierarchical connectivity of the network is given in Reference 3. The requirements for the stratum levels are shown in Table A, which provides a comparison and summary of the drift and slip rates for the strata clock systems.

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 or reference oscillator. 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 [Reference 4]. 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. GPS receivers are available for this application, as well as integrated timing system products available from Larus.

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. Larus has a Stratum 2 clock with a drift of less than 2.5 x 10^-11 per day, resulting in a time to the first frame slip of more than 2 months.

Stratum 3 is defined as a clock system which tracks an input as in Stratum 2, but over a wider range. A Stratum 3 clock system requires a minimum adjustment (tracking) range of 4.6 x 10^-6. The short term drift of the system is less than 3.7 x 10^-7 in 24 hours. This amounts to approximately 255 frame slips in 24 hours while the system is holding. Some Stratum 3 clock equipment is not adequate to time SONET network elements.

Stratum 3E, that was defined in Bellcore documents [References 3, 7 and 8], is a new standard created as a result of SONET equipment requirements. Stratum 3E tracks input signals within 7.1 Hz of 1.544 MHz from a Stratum 3 or better source. The drift with no input reference is less than 1 x 10^-8 in 24 hours. Larus provides a Stratum 3E Enhanced clock that provides a drift of less than 5 x 10^-9 in 24 hours, which is better than the 3E designation. This is less than four frame slips in 24 hours, compared to 255 slips for Stratum 3. Typical performance of the Larus 3E Enhanced clock is less than one slip in 36 hours or 9 x 10^-10 /day.

Stratum 4 is defined as a clock system which tracks an input as in Stratum 2 or 3, except that the adjustment and drift range is 3.2 x 10^-5. Also, a Stratum 4 clock has no holdover capability and, in the absence of a reference, free runs within the adjustment range limits. The time between frame slips can be as little as 4 seconds.

Stratum 4E is a proposed new customer premises clock standard which allows a holdover characteristic that is not free running. This new level, intended for use by customer provided equipment in extending their networks, is not yet standardized.
 
Table A. Clock Strata Requirements
 

Notes:
1. Larus Stratum 2 is typically less than 2.5 x 10^-11 /day
2. Larus Stratum 2 drift results in more than two months to a frame slip.
3. Larus Stratum 3E Enhanced.
4. Larus Stratum 3E Enhanced typical performance is less than one slip in 36 hours or 9 x 10^-10 /day.
* To calculate slip rate from drift, one assumes a frequency offset equal to the above drift in 24 hours, which accumulates bit slips until 193 bits have been accumulated. Drift rates for various atomic and crystal oscillators are well known, and are not usually linear or not necessarily continually increasing.