[ntp:questions] CDMA and Leap Seconds

Bruce Penrod bmpenrod at comcast.net
Sun Jan 1 07:31:25 UTC 2006


Apparently a little explanation of how the CDMA mobile phone system 
keeps time would be welcome.  As most of you are aware, each basestation 
in the system must maintain absolute synchronization to GPS time to the 
10 us level in order to keep from interfering with the other 
basestations.  To do this, each has a GPS timing receiver with high 
stability oscillator, either quartz or rubidium.  This so as to be able 
to maintain that level of sync even during hours of holdover after the 
antenna has been shot off the tower.

UTC time is not used by the CDMA system for obvious reasons, e.g. 
leapseconds.  The GPS timescale is strictly monotonic.  However, in 
order to make the phones show the correct local time, embedded in the 
sync channel message which all basestations broadcast continuously, is 
the value of the UTC offset to GPS time and the local offset to UTC, 
i.e. timezone.  However, since these offset values do not affect the 
operation of the mobile phone system, great care was not taken to 
implement the changeovers rigorously.

Though until now there have been no leap second insertions since we 
began shipping our CDMA based products, we have had over the years a 
handful of customers report basestations transmitting leap second values 
that were off by one second.  In response, three years ago we created a 
user enterable leap second mode for our CDMA products to allow 
overriding the system transmitted value if necessary.  This mode also 
included the ability to set up the future leap second value prior to the 
next insertion so as to be independent of the behavior of the 
basestation.  This has all been well documented in the product manuals 
and a webpage dedicated to leap seconds has been maintained continuously 
on our site to show the current and future values of the leap second so 
that customers may easily determine how to configure their boxes.

When the IERS Bulletin C arrived in July, we updated our website and 
notified via e-mail all CDMA product customers of the impending leap 
second, and recommended that all users operate their NTP servers in the 
user leap second mode.  All NTP servers we shipped after July 1 were 
preconfigured to user leap second mode with the current and future leap 
seconds set appropriately.

We are pleased to announce that our GPS NTP servers and our CDMA NTP 
servers configured in user leap mode performed the leap properly this 
afternoon.  We are aware that some of the cellular basestations set the 
leap second a day early, and some PCS basestations have still not set 
it.  We regret that some customers apparently had not configured their 
NTP servers to operate in user leap second mode, and apologize for any 
problems this might have caused.

Unfortunately, judging from the other recent posts, it appears to me 
that the long dearth of leap seconds may have resulted in a step 
backwards in the proper handling of leap seconds.  Many products that 
had been debugged back when we were having regular leap second events 
have been replaced by new ones that have no experience with them.  I'm 
not optimistic that we'll ever have a ho-hum leap second event with all 
posting to say how smoothly everything went.

If we're lucky, we'll get rid of the pesky things before there's another 
one...

Bruce Penrod
EndRun Technologies




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