Laptop Atomic clock sync via GSM?

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Jeff Spicoli - 29 Aug 2005 18:57 GMT

We have rugged tablets which connect to our network via GSM for field
technicians. We are looking for the most efficient way to keep the tablets
internal clocks in sync.

Does any recommend a way to use the atomic clock servers to keep these
times in sync via GSM?

Any other recommendations?

Thanks in advance.

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DevilsPGD - 29 Aug 2005 20:15 GMT

>We have rugged tablets which connect to our network via GSM for field
>technicians. We are looking for the most efficient way to keep the tablets
[quoted text clipped - 4 lines]
>
>Any other recommendations?

Many/most GSM networks broadcast the time, this would keep you in sync
to within 1-2 seconds (my understanding is that the towers need to be
kept in exact precision with each other, but that the resolution
transmitted to the handset is +/- 1 second)

Alternatively, if you have GRPS available, you can run software to
synchronize against any publicly available timeserver, or run your own.

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John Henderson - 29 Aug 2005 21:40 GMT

> Many/most GSM networks broadcast the time, this would keep you
> in sync to within 1-2 seconds (my understanding is that the
> towers need to be kept in exact precision with each other, but
> that the resolution transmitted to the handset is +/- 1
> second)

For GSM, this is news to me, and I'd like more details.
Certainly, they may broadcast _timezone_ information, including
the date-time of the last timezone update (GSM 04.08, section
9.2.15a).

CDMA cells need accurate timekeeping, and broadcast time
information. Locally, I can tell which sites have CDMA -
they're the ones with the discrete GPS antenna on the roof.

> Alternatively, if you have GRPS available, you can run
> software to synchronize against any publicly available
> timeserver, or run your own.

There will be more latency doing this over-the-air. The
viability of this approach depends on the desired accuracy.

John

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DevilsPGD - 29 Aug 2005 22:38 GMT

>> Many/most GSM networks broadcast the time, this would keep you
>> in sync to within 1-2 seconds (my understanding is that the
[quoted text clipped - 6 lines]
>the date-time of the last timezone update (GSM 04.08, section
>9.2.15a).

I'm not sure of the details, but my V3 is able to set the date/time on
it's own, I just set "AutoUpdate" to "On"

I assumed this was just a basic GSM feature, iDEN phones (closely
related to GSM) don't even let you set the time, it's pulled from the
network. I've never set a clock on a cell phone in the last 6 years,
and I've only ever carried iDEN or GSM phones.

I'm still annoyed that my pager doesn't set the time on it's own too,
especially since it's clock floats +/-1 minute a day.

>> Alternatively, if you have GRPS available, you can run
>> software to synchronize against any publicly available
>> timeserver, or run your own.
>
>There will be more latency doing this over-the-air. The
>viability of this approach depends on the desired accuracy.

True. NTP should be able to handle latency though, it calculates
latency and factors it into the update.

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John Henderson - 29 Aug 2005 22:52 GMT

> I'm not sure of the details, but my V3 is able to set the
> date/time on it's own, I just set "AutoUpdate" to "On"

On GSM phones that I'm familiar with, that function just does
timezone corrections, when travelling or changing to/from
daylight savings.

> I assumed this was just a basic GSM feature, iDEN phones
> (closely related to GSM) don't even let you set the time, it's
> pulled from the network. I've never set a clock on a cell
> phone in the last 6 years, and I've only ever carried iDEN or
> GSM phones.

I'm not familiar with iDEN. And perhaps you've just been lucky
with the accuracy of your GSM phones.

Easy to test though - just set your GSM phone clock to be wrong
by 25 minutes (this could never be a timezone error), and see
if it corrects.

>>There will be more latency doing this over-the-air. The
>>viability of this approach depends on the desired accuracy.
>
> True. NTP should be able to handle latency though, it
> calculates latency and factors it into the update.

I wasn't aware of that. I've read recommendations to carefully
choose a local time server to avoid latency. That's a problem
with the internet - the need to separate the wheat from the
chaff :)

John

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DevilsPGD - 30 Aug 2005 01:35 GMT

>> I assumed this was just a basic GSM feature, iDEN phones
>> (closely related to GSM) don't even let you set the time, it's
[quoted text clipped - 8 lines]
>by 25 minutes (this could never be a timezone error), and see
>if it corrects.

When I first got the phone, the date/time were completely wrong, but as
soon as I set AutoUpdate on, it was correct.

At 5:21PM MST, I set my phone to 01-Jan-2001 12:00am. Then I went back
to the main screen, confirmed it saved. I rebooted the phone. It was
still back in time.

Next I turned on AutoUpdate, went back to the main screen, and it's back
to the correct time/date.

Oh, and the time/date settings disappear completely when AutoUpdate is
enabled.

Maybe I've just been lucky so far though -- But I get very annoyed at
clocks that don't match (different time being displayed on my phone vs
Palm vs PC vs Pager vs Satellite receiver vs Cable receiver vs bedroom
clock vs everything else) -- I have virtually everything in my house
setting their clocks themselves, and virtually everything stays in sync.

>>>There will be more latency doing this over-the-air. The
>>>viability of this approach depends on the desired accuracy.
[quoted text clipped - 6 lines]
>with the internet - the need to separate the wheat from the
>chaff :)

http://64.233.187.104/search?q=cache:hL3DYrpJiYoJ:www-scf.usc.edu/~asyed/papers/
tshl.pdf+ntp+high+latency
isn't bad, although it's not an easy read.

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John Henderson - 30 Aug 2005 21:19 GMT

> When I first got the phone, the date/time were completely
> wrong, but as soon as I set AutoUpdate on, it was correct.
[quoted text clipped - 5 lines]
> Next I turned on AutoUpdate, went back to the main screen, and
> it's back to the correct time/date.

I've got no idea how they do that. I've found nothing in the
GSM radio layer standards so far, but I'll keep looking.

> Oh, and the time/date settings disappear completely when
> AutoUpdate is enabled.

And that's another puzzling thing. I downloaded a Motorola V3
GSM user manual, but there's no mention of any autoupdate
feature in it. In fact, it suggests that the date-time must be
set manually: "You must set the time and date to use the
datebook." But I know that Motorola is legendary for releasing
different model phones, and having the same model number.

It's conceivable that this phone has been flashed with
"enhanced" firmware by your carrier, to use a proprietary
network feature. What carrier are you using (Rogers, Canada as
per a previous post?), and did you buy the phone through them?

I'd genuinely like find out how they do this.

John

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DevilsPGD - 31 Aug 2005 10:07 GMT

>> When I first got the phone, the date/time were completely
>> wrong, but as soon as I set AutoUpdate on, it was correct.
[quoted text clipped - 8 lines]
>I've got no idea how they do that. I've found nothing in the
>GSM radio layer standards so far, but I'll keep looking.

Interesting. It's possible that the phone pulls the time from another
source, or even NTP, although frankly, it happened WAY too quickly and I
think the phone would ask for authorization before firing up GPRS.

>> Oh, and the time/date settings disappear completely when
>> AutoUpdate is enabled.
[quoted text clipped - 5 lines]
>datebook." But I know that Motorola is legendary for releasing
>different model phones, and having the same model number.

I don't recall seeing it in my manual either, but I'm not big on manuals
-- I tend to use the product first, then read the manual, then go back
to the product.

>It's conceivable that this phone has been flashed with
>"enhanced" firmware by your carrier, to use a proprietary
>network feature. What carrier are you using (Rogers, Canada as
>per a previous post?), and did you buy the phone through them?

Rogers branded phone, purchased from Rogers, on the Rogers network.

I can take some digital pictures if you don't believe me or want to see
for yourself :)

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John Henderson - 01 Sep 2005 02:58 GMT

> I don't recall seeing it in my manual either, but I'm not big
> on manuals -- I tend to use the product first, then read the
> manual, then go back to the product.

...

> Rogers branded phone, purchased from Rogers, on the Rogers
> network.

Thanks. I'll keep looking around for how it might be done.

Is there anyone from Europe reading who gets their clock updated
on a GSM phone? I don't mean automatic timezone adjustments -
they just leave the clock wrong by the same "offset".

Is this absolute date-time correction just a Rogers, or North
American, thing?

> I can take some digital pictures if you don't believe me or
> want to see for yourself :)

No need. But if you find any enlightening documentation, I'd
appreciate hearing about it. I'm more inclined to think they
might have done something "outside the strict confines of the
standards" (there's scope for that). I've since downloaded a
V3 user manual from the Rogers site specifically, and it makes
no mention of any auto-update feature.

John

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DevilsPGD - 01 Sep 2005 22:14 GMT

>> Rogers branded phone, purchased from Rogers, on the Rogers
>> network.
[quoted text clipped - 7 lines]
>Is this absolute date-time correction just a Rogers, or North
>American, thing?

I had the opportunity to play with one of my parents' Nokia phones last
night, it has an autoupdate feature too, although I didn't try it (I
have a strict rule against changing other people's phone options without
their permission), but according to my dad, his phone was losing a few
minutes a month before he turned that on.

The next time I see my mom I'll try it on her phone, I can monkey with
all the settings I want on her phone :)

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matt weber - 30 Aug 2005 01:01 GMT

>We have rugged tablets which connect to our network via GSM for field
>technicians. We are looking for the most efficient way to keep the tablets
>internal clocks in sync.
>
>Does any recommend a way to use the atomic clock servers to keep these
>times in sync via GSM?

There are commercial products available that do just that.
Take a look at http://www.brandywinecomm.com/mod_time.html
that once can be ordered to set from either WWVB or GPS.

Read about these devices at
http://www.brandywinecomm.com/net_timeFAQ.html

They aren't the only ones to make this sort of device however.

>Any other recommendations?
>
>Thanks in advance.

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