Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!sdd.hp.com!hplabs!hpl-opus!hpccc!hpcc01!hpcuhb!hpscdc!rkarlqu
From: rkarlqu@hpscdc.scd.hp.com (Rick Karlquist)
Newsgroups: sci.electronics
Subject: Re: time accuracy in telephone service billing
Message-ID: <5770028@hpscdc.scd.hp.com>
Date: 6 Jun 90 20:16:36 GMT
References: <285@sphere.UUCP>
Organization: Hewlett-Packard, Santa Clara Div.
Lines: 15

All major phone systems have "Stratum 1" clocks employing 
Cesium Atomic clocks, which are typically accurate to a
microsecond a day (for the cheap ones the phone companies
buy).  There are any number of fairly easy methods of
resynching them to NIST time periodically.  They need to
have these clocks to generate accurate sync for the digital
transmission schemes now in nearly universal use.  (The 
signals are *not* self clocking and clock wander causes
chucks of data to be lost).  We happen to know that they
also use these clocks for time-stamping purposes.  Hence
you don't have to worry about accurate time vis a vis billing
when using a major phone company.  Now these mickey mouse
"alternate operator services" that own "private" pay phones
I wouldn't trust at all, because they *want* to make errors
(as long as they are in their favor!).