Emerald Time is a very nifty little app, and yes, I've used it a number of
times to check my phone's displayed time. It's often off by more than two
seconds but never more than three, so far.
There are several other apps that can check the time with NTP, and display
it in different formats, some more attractive/useful than others.
However,they will not alter the phone's internal clock. I used to think
this was due to the communication protocol needing accurate time in the
mobile unit, but it's more likely just some kind of legal thing with Apple
that they cannot mess with the clock settings (which _maybe_ could be
construed as tampering with the OS).
Brian
-----Original Message-----
From: Bill Dailey
Sent: Monday, August 04, 2014 6:09 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] How are iPhones' clocks set under LTE?
For informational purposes I will show what I use to compare with my stock
Iphone. It is an app called emerald time. Screenshot at:
https://www.dropbox.com/s/zt6tjrsylrrtrc3/2014-08-04%2008.06.02.png
You can set it up to sync with you own ntp server. I think. You can just
spot check it.
I have never done a rigorous analysis but it appears to be within 1.5s or
better most of the time.
Doc
Sent from mobile
On Aug 4, 2014, at 7:38 AM, BIll Ezell <[email protected]> wrote:
LTE does support the long-standard NITZ (network information and time
zone) service. It's an easy way to find out just where you are without
having to change your TZ settings constantly. In fact, if you go to time
settings on HTC Android phones, the 'automatic time update(NITZ)' setting
turns on NITZ syncing. iPhones also use NITZ, as do most 3G or LTE phones.
But, not necessarily for time.
NITZ implementation is carrier-optional, although almost all do support
it. I know that Vodafone-Austrailia and a handful of other carriers at
least at one point didn't support it. Additionally, the standard doesn't
specify how accurate the time has to be, and it varies widely across
providers. It's usually within a few seconds, but this isn't a
high-precision time reference and can be off by minutes. But, a phone can
use the timezone information to then localize time from some other time
service.
An alternative to determine what your physical location location is uses
lower-level information such as the ECGI (extended cell group identity) or
location information from the MME (Mobile Management Entity). Don't you
just love telecom? Everything's an acronym and frequently an acronym^2 or
^3.
Anyway, the phone then looks up the physical location from whatever id it
uses, then uses a time service to get the actual time, then localizes it
based on the physical location.
Clearly, just using something like NTP directly isn't all that useful
because you have to know your physical location to know what timezone
correction to apply.
I work on cell infrastructure, mostly 3G and LTE (Ericsson), and it just
amazes me that phones work at all. It is incredibly complicated and
convoluted.
Unlike CDMA (where time distribution was an automatic part of the
low-level protocol) I suspect the time displayed on many modern phones is
not set by the telephony synchronous protocol but rather by IP-over-Wifi
packets.
And the packets don't seem to do a very effective job keeping the clock
ont he phone correct. My employer gave me a Nokia Lumia 630 "Windows
Phone"
and its clock has always been off by at least a minute.
There was a few years ago, a very nice article about the effort to repair
the clocks in clock towers in many cities. What rang most true to me was
"if you visit a town they can't even keep the clock correct, who else
knows
what else is wrong there?".
Tim N3QE
--
Bill Ezell
----------
The day Microsoft makes something that doesn't suck
will be the day they make vacuum cleaners.
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