G
LOSSARY AND
A
BBREVIATIONS
8030HEPTA/GPS GPS - NTP Time Server with 2x 10/100/1000 MBit LAN Interface - V04.00
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hopf
Elektronik GmbH
Nottebohmstr. 41
• D-58511 Lüdenscheid • Tel.: +49 (0)2351 9386-86 • Fax: +49 (0)2351 9386-93 • Internet: http://www.hopf.com • E-Mail: [email protected]
14.5
Accuracy & NTP Basic Principles
NTP is based on the Internet protocol. Transmission delays and errors as
well as the loss of data packets can lead to unpredictable accuracy data
and time synchronization effects.
NTP protocol neither defines nor guarantees the accuracy or correctness
of the time server.
Thus the QoS (Quality of Service) used for direct synchronization with GPS or serial interface
does not apply to synchronization via NTP.
In simplified terms, accuracies of between 1msec and 1sec can be expected, depending on
the accuracies of the servers used.
The accuracy of IP-based time synchronization is depending on the following criteria:
•
Characteristics and accuracy of the time server / time signal used
•
Characteristics of the sub-network
•
Characteristics and quality of the synchronization client
•
The algorithm used
NTP has a variety of algorithms to equalise the possible characteristics of IP networks. Algo-
rithms also exist to equalise the offset between reference time source and the local clock.
However, under some circumstances it is not possible to provide an algorithmic solution.
For example:
1. Time servers which do not deliver any correct time cannot be detected at all. The only
option available to NTP is to mark these time servers as FALSETICKERS in compar-
ison to other time servers and to disregard them. However, this means that if only 2
time servers are configured, NTP has no way of determining the correctness of the
individual times and clearly identifying which time is incorrect.
2. Asymmetries in the transmission between NTP servers and NTP clients can neither
be measured nor calculated by NTP. NTP works on the assumption that the transmis-
sion path to the NTP server is exactly as long as the return path. The NTP algorithm
can only filter out changes on a statistical basis. The use of several servers makes it
possible for the combining algorithm to pick up and filter out any such errors. However,
there is no possibility of filtering if this asymmetry is present on all or most of the NTP
servers (faulty routing etc).
3. It goes without saying that the accuracy of the synchronised time cannot be better
than the accuracy resolution of the local clock on the NTP server and NTP client.
