[ntp:questions] Re: Synchronization with mobile devices and different time zones

Richard B. Gilbert rgilbert88 at comcast.net
Mon Oct 25 11:53:32 UTC 2004

Sergio wrote:

>I see. Thanks a lot for the clarification, I certainly was confused
>about the time zones issue.
>Well, then it's possible. But I assume there is a certain overload
>involved. If a communication takes place once every seventeen minutes
>and that communication is lost (e.g., no network connection
>available), either the clock will not be synchronized until the next
>synchronization point (17 minutes later) or the communication will be
>retried until it is successful, right?
>"Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message news:<uuydnUGqlKT1heTcRVn-vQ at comcast.com>...
>>Sergio wrote:
>>>Hi everybody,
>>>does anyone have any experience at using NTP to synchronize the clock
>>>of computers in different countries (time zones)?
>>>How about when some of the computers to synchronize is a portable
>>>computer with a wireless (and intermittent) connection? Is it possible
>>>to use NTP then or some similar protocol to keep the clocks
>>>As you can see, I am interested in knowing the limitations and
>>>practical applications of the protocol.
>>>Thanks in advance,
>>NTP does not know or care about time zones!!  NTP keeps UTC (formerly 
>>known as GMT).  Local times are a matter of presentation; the O/S adds 
>>or subtracts the appropriate offset to display the time zone of your choice.
>>Wireless network connections should not present a problem.  As long as 
>>you are within range of the access point, a wireless network should work 
>>just as well as a wired network.
>>If your connection is intermittent, whether it's dial-up telephone, or 
>>because you move in and out of range of an access point, NTP will not 
>>work as well.  The quality of the time you can keep will depend more on 
>>the quality of your local clock (usually poor and out of your control 
>>anyway; computers were not designed to serve as clocks).  It will also 
>>depend on how often ntpd is able to get a response from a server.  If 
>>everything is working very well, once every seventeen minutes is enough.
>>If your computer goes into some power saving mode, NTP may not work very 
>>well or may not work at all.
Remember that NTP uses UDP; UDP does not guarantee delivery of packets.  
If ntpd can't get a response from a server, it will try again at the 
next poll interval.

If you get eight consecutive failures, your clock is no longer 
synchronized.  Timekeeping is now entirely dependent on the quality of 
your local clock.  It may be several hours before your clock drifts 
significantly or it may be only a few minutes.  If you need the time 
accurate to within +/- 500 microseconds you are not going to get it this 
way.  If +/- 500 milliseconds is acceptable, it may work.  (Note that 
+/- 500 microseconds is possible, if at all, only with a low latency 
connection to a very good server.)

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