k categories used throughout the frequency distribution network, and numerous standards organiza- tions have classified these categories. One approach, taken by the American National Standard Institute (ANSI), is to classify the per- formance of clocks into four levels, known as stratum levels, with stratum 1 as the highest and stratum 4 the lowest level of performance. Note that this is not the same definition of stratum used by the Network Time Protocol (NTP). Chapter 2 and Chapter 7 provide more information on For the North American market, an ANSI standard, originally called T1.101, "Synchronization In- terface Standards for Digital Networks," was first released in 1987. It defined the accuracy and formance requirements for a clock hierarchy in the synchronization network. The latest version of T1.101 is referred to as ATIS-0900101.2013(R2018). Figure 3-11 illustrates the hierarchy of stratum levels from T1.101. Stratum 1 Highest quality clock Primary reference Frequency accuracy < 10-11 source Defined in ANSI T1.101 Primary reference source High quality clock Often used in BITS Stratum 2 Defined in ANSI T1.101 and Stratum 3E Telcordia GR-1244-CORE Central offices Network element clock Used in routers, switches, ADMs, etc. Defined in ANSI T1.101, Stratum 3 Stratum 3 Telcordia GR-1244-CORE and Telcordia GR-253-CORE (SONET) Local offices Network element clock Used in access routers, PBX, etc. Defined in ANSI T1.101 and Stratum 4 Stratum 4 Customer premises Telcordia GR-1244-CORE Figure 3-11 ANSI Frequency Distribution Hierarchy The top of the timing pyramid is what T1.101 calls the primary reference source (PRS) or what the ITU-T SDH world calls the PRC-more on the ITU-T approach follows. The job of the PRS is to provide the reference frequency signal to other clocks within a network or part of the network. This is equivalent to a stratum 1 clock, and its output controls other clocks further down the hierarchy, namely stratums 2, 3, and 4. A stratum 1 clock and PRS both have the same accuracy, and the terms are