Re: Getting time from a GPRS modem

In comp.arch.embedded,
Bernhard Deny <a2973445@xxxxxxxxxxx> wrote:
Am 14.03.2011 11:48, schrieb Stef:
In comp.arch.embedded,
Bernhard Deny<a2973445@xxxxxxxxxxx> wrote:
Am 11.03.2011 12:47, schrieb Stef:
Is there a possibility to get current time (UTC preferred) from a
(connected) GPRS modem? I've been searching through the Telit AT
Commands Reference Guide, but only references to "time" are in
context of SMS. Not sure if it depends on the type of modem, but
a possible candidate is the Telit GE863.

In the past we have used NTP over the dataconnection to get time
on our device. But for a new device, I was hoping to skip the added
complexity of using NTP. Mainly to avoid having to set up a dedicated
NTP server or make arrangements with or another provider.

Forget about using NTP over GPRS Networks if you need to rely on
seconds. This won't work as expected because the latency is in the range
of 0.4 to 30 seconds and more, BTDT.
Using ntpd leads also to excessive data traffic, obviously NTP tries to
correct the errors with additional requests.

Being a few seconds off is not a problem. We have done NTP over GPRS
before and did not experience any problems with that. In that case we
had our own NTP server. We also implemented our own NTP client that
only sent out a single request during each communication attempt, so
no excessive data traffic.

Do you know of an alternative to get somewhat accurate time (lets say less
than 1 minute error) on a remote embedded device with no user interface and
only GPRS connectivity?

Ok, i must say that the GPRS signal level on our location was a bit low.
As an alternative, here (Germany, also ok for central Europe) we use a
DCF77 longwave receiver as a timebase for ntpd. Works like a charm, ntpq
says offset < 1ms.
I other areas, consider WWVB, MSF, JJY, TDF...

Yes, considered this as well, but requires additional hardware. I heard
that there must be some single chip DCF77 receiver/decoders that require
little or no antenna. So far I only found receiver chips with no decoder
and requiring a ferrite antenna. Any suggestions?

I don't yet know if the additional cost is acceptable and it introduces
a second source for reception problems.

GPS is even more accurate, but requires an external antenna with an
unobstructed view to the sky, and is overkill for your application.

Yes, GPS would give the time nicely. But as you say, it would not
always work in a building, not even with the so-called "indoor GPSes".
And adding GPS adds some considerable cost.

Stef (remove caps, dashes and .invalid from e-mail address to reply by mail)

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