G
LOSSARY AND
A
BBREVIATIONS
8029HEPTA-V2/GPS GPS - NTP Time Server with LAN Interface - V08.00
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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]
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.
Algorithms 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
comparison 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
transmission 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.
With reference to the above mentioned error circumstances, the delivered
time offset
of the
NTP should be considered to be at best the most favourable case and in no way to be a value
that takes account of all possible errors.
In order to resolve this problem, NTP delivers the maximum possible error in relation to the
offset. This value is designated as the synchronization distance ("
LAMBDA
") and is the sum
of the
Root Dispersion
and half of the
Root Delay
of all NTP servers used. This value
describes the worst possible case and thus the maximum error that can be expected.
Finally, please note that the user of the Time Server is responsible for the network conditions
between the Time Server and the NTP clients.
As an example, we mention the case where a network has a delay of 500msec and an
accuracy shift (asynchronous) of 50msec occurs. The synchronised clients will therefore
NEVER achieve accuracy values of one millisecond or even microseconds!
The accuracy value in the GENERAL tab of the WebGUI is designed to help the user to
estimate the accuracy.
