Subject: [IMRG] clock synchronization

From: Augusto Castelan Carlson <[email-masked]>
Date: Sat, 19 Jul 2003 19:28:30 -0300 (BRT)

Hi!

Is there advantages in use GPS clock synchronization for network
measurements?
If so, which GPS product is recommended for it?

Thank you!

--
Augusto Castelan Carlson

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From: "Henk Uijterwaal (RIPE-NCC)" <[email-masked]>
Date: Mon, 21 Jul 2003 10:00:33 +0200 (CEST)

Hi,

> Is there advantages in use GPS clock synchronization for network

Clock synchronization is essential for one way measurements that involve
timing.  The most obvious is one way delay: one box sending packets, the
other one receiving it, if the clocks aren't synchronized then it is
impossible to do these measurements.

There are many ways to synchronize a clock: over the network using NTP or
from an outside source (GPS, CDMA, DCF77 and other long-wave radio clocks,
atomic clocks).

NTP suffers from the problem that it often uses the same network to
synchronize the clocks and do the measurement.  That unfortunately means
that clock accuracy goes down when the network behaves badly (losses,
delay variations) and this is usually the time that you are interested in
accurate measurements.

The advantage of GPS over other solutions is that it works everywhere _if
you can see the sky_.  Units are relatively cheap (500-1000 US$).  CDMA is
a good alternative if you live in an area with CDMA coverage.

> If so, which GPS product is recommended for it?

That depends on several factors.  In theory, any GPS receiver that can
output a pulse-per-second (clock pulse) can be used to sync a clock. In
practice, things like distance between receiver and antenna, availability
of drivers for receiver and O/S, are also important.

Henk

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