Network Time Protocols (NTP) are a hierarchical protocol and is divided into stratum which define the distance from the reference clock.
网络时间协议(Network Time Protocol，NTP)是一种分层协议，它被划分为定义与参考时钟的距离的分层协议。级数越低，时钟越准确，解释如下：
- Stratum 0
A reference clock source that relays UTC (Coordinated Universal Time) and has little or no delay is known as a stratum-0 device. Stratum 0 servers cannot be used on the network, instead, they are directly connected to computers which then operate as primary time servers.
Stratum 0是一种参考时钟源，它中继UTC（协调通用时间），并具有很少或没有延迟被称为零层级时钟。Stratum 0不能在网络上使用，而是直接连接到计算机，然后作为主时间服务器运行。
- Stratum 1
主时间服务器，这些服务器与阶层0设备相连，在几微秒误差内同步系统时钟。阶层1服务器之间可以互相连接，进行完整性检查和备份。
- Stratum 2
这些计算机通过网络和阶层1服务器同步。每个计算机可以查询多个阶层1服务器，阶层2计算机之间也可以互相连接。
- Stratum 3
3级时钟可以驱动3级、4E级或4级时钟。
- Stratum 3E
3E级时钟可以驱动3E级、3级、4E及或4个时钟。
关于参考时钟精度级别Stratum 3与Stratum 3E详解
- Stratum 3
Stratum 3 tracks an input over a wider range than Stratum 2. A Stratum 3 clock requires a minimum tracking range of ±4.6 ppm. The short term drift for Stratum 3 is less than ±0.37ppm in 24 hours. That’s about 255 frame slips in 24 hours during holding. A Stratum 3 clock may drive strata 3, 4E or 4 clocks.
Stratum 3的自由运行稳定性为20年以上的偏差在±4.6 ppm之间，24小时以上的持续偏差要求在±0.37ppm范围内，这两个要求包括在所有条件下的频率误差。如晶诺威产恒温晶振OCXO-6X电气参数如下：
- Stratum 3E
Stratum 3E is an upgraded standard that takes into account SONET equipment requirements that are not available from Stratum 3 itself. 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±0.01 ppm in 24 hours. This is a big difference from Stratum 3. Stratum 3 is about 255 frame slips in 24 hours, while Stratum 3E is only 4 slips. A Stratum 3E clock may drive strata 3E, 3, 4E or 4 clocks.
Stratum3E是Stratum 3更加精确的版本，具有相同的自由运行稳定性，即±4.6 ppm，但24 小时以上的持续偏差仅为±0.01 ppm，是普通Stratum 3 性能的37倍。
