[time-nuts] Time syncing question

[David I. Emery]

On Wed, Aug 30, 2006 at 07:53:19PM -0500, Didier Juges wrote:
> I have observed that some cell phones set their clock when you power 
> them up, and others set it at regular time. Some automatically change 
> time zone as you travel and some don't, maybe due to the same process.
> 
> Didier KO4BB

	I assume most members of this august group know that all CDMA
cellphones MUST know the correct time to within about 1 us or so in
order to correctly spread and despread the forward and reverse channel
signals.

	There are mechanisms (pilot signals) built into the signaling
format that allow a CDMA cellphone with cheap TCXO time base to acquire
the necessary frequency and then time lock when it is first turned on or
first sees a signal.

	But a CDMA cellphone locked up on a base-station (its normal
state) should know the CDMA system time with microsecond accuracy and
also be able to correct its TCXO time base and lock it to the system
reference so it too should be very accurate.

	And essentially ALL CDMA systems lock system time and frequency
to GPSDOs at the cell sites (easiest way of keeping a whole network of
them locked together so handoffs work) and keep system time set to UTC.

	So the only thing preventing the clock on a CDMA cellphone from
being essentially arbitrarily accurate is laziness or sloppiness in the
GUI firmware that handles the visible clock display - in engine room in
the bowels of the phone there is VERY accurate time of day.

> 
> Glenn wrote:
> > _Most_ cell phones set their time to "network time." Usually within  
> > one second. Although I have seen cell phones set themselves and be  
> > off by nearly a minute.
> >
> >   
> 
> 
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-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."