NTP stratum

What is Stratum 1?

In the world of NTP, stratum levels define the distance from the reference clock. A reference clock is a stratum-0 device that is assumed to be accurate and has lttle or no delay associated with it. The reference clock synchronizes to the correct time (UTC) using long wave radio signals, GPS transmissions, CDMA technology or other time signals such as WWV, DCF77, etc. Stratum-0 servers cannot be used on the network, instead, they are directly connected to computers which then operate as stratum-1 servers.

A server that is directly linked to a stratum-0 device is called a stratum-1 server. This includes all time servers with built-in stratum-0 devices and those with direct links to stratum-0 devices such as over an RS-232 connection or via an IRIG-B time code. The basic definition of a stratum-1 time server is that it be directly linked (not over a network path) to a reliable source of UTC time such as GPS, WWV, or CDMA transmissions. A stratum-1 time server acts as a primary network time standard.

Higher stratum levels are distanced from the stratum-1 server over a network path. Thus, a stratum-2 server gets its time over a network link, via NTP, from a stratum-1 server. A stratum-3 server gets its time over a network link, via NTP, from a stratum-2 server, and so on.

As you progress through different strata there are network costs involved that reduce the accuracy of the NTP server in relation to UTC. A stratum-1 time server will typically have less than 1 millisecond (ms) accuracy to UTC, depending on its reference clock. On the internet, because of network delays, a stratum-2 time server will have anywhere from 10-100 ms accuracy to UTC and each subsequent time server will add an additional 10-100 ms of inaccuracy.

quoted from : http://www.endruntechnologies.com/stratum1.htm