Motorola V186 - Cell Phone - GSM System Planner Manual page 147

Clock Functions and Synchronization
RTU (usually FEP) can act as a server. This enable setting its time via MDLC time sync,
for example, and having other RTUs specify it as an NTP server and obtain their time
from it.
NTP synchronizes clock both in time and frequency. In time means it make its clock
offset as close as possible to the server. In frequency means it learns the server drift (time
between "ticks") in order to avoid polling it every few seconds. An example, not related
to NTP, is ACE3600 send MDLC Sync over radio to another ACE3600. The clock offset
guaranteed to be less than 1 millisecond. However a 30ppm clock drift after 1 minute
offset will be 1.8 milliseconds. NTP prevents that by learning the drift frequency of the
server.
User can set a single NTP server, or several ones. NTP operates under the assumption
that each server's time should be viewed with a certain amount of distrust. NTP really
prefers to have access to several sources of lower stratum time (at least three) since it can
then apply an agreement algorithm to detect insanity on the part of any one of these.
Normally, when all servers are in agreement, NTP will choose the best of these, where
"best" is defined in terms of lowest stratum, closest (in terms of network delay) and
claimed precision, along with several other considerations.
As the below figure shows, at the top of any NTP hierarchy are one or more stratum 0
reference clocks. These are electronic clocks such as GPS signals, radio signals, or
extremely accurate frequency control. Reference clocks are assumed to be accurate. In
ACE3600 a GPS port can be configured, it will serve as a reference clock for that RTU.
In this case RTU will operate on stratum 1 with an accuracy of 200 microseconds.
NTP Architecture
As the above figure shows, time is distributed from an NTP subnet of servers. Each
server comprises a stratum, which designate its location in terms of hops to the UTC
source. The stratum 1 is the most accurate server of which all servers should be
synchronized to. Up to 15 stratum levels may exist. Stratum 16 means server
unreachable.
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