Jan Hoevers schreef:
Jan Hoevers wrote on 18-9-2007 1:22:
Someone answered me off list that the systematic offset of a stratum 2
(or higher) server on an adsl is cancelled out when another host gets
time from that server.
I think that was a smart and interesting remark too.
If this is true, it would mean that a stratum 1 server on an adsl line
shows a systematic offset, and a stratum 2 server does not.
This is probably true, but how much it will actually affect you is a
different matter.
For example, on my 6Mbit/768kbit ADSL line the delay time to an NTP
server located at the ISP is below 6 ms. Traceroute indicates 4 hops to
that server, so there are two more links behind the ADSL network.
Likely they are Gbit ethernet and the router between them is a Juniper.
Let's assume the delay of the ADSL line is about 4 ms. That is
roundtrip, so the asymmetry has to come out of this, e.g. by being 3/1
ms instead of 2/2 ms.
I think it is unlikely that you will have more than a millisecond of
systematic offset out of that. This will be difficult to notice on a
server with only network sync.
On the other hand, there are ADSL connections with much worse latency
than this KPN ADSL line. The KPN ADSL network is very fast. So it may
affect others more than what is described above.
My experience with ADSL over time shows that latency is affected most by
the "line mode", which can be "interleaved" or "fast". It should be
"fast" for best latency, but some providers set it to "interleaved". I
think that means that subsequent packets to be sent over the modem are
interleaved at the ATM level, and so a burst error on the line will not
introduce one long error burst in a single packet, but several short
error bursts in several different packets. The result is that the
forward error correction is more likely to be able to fix the faulty
bits, and so there are fewer packets dropped in the case of interference
bursts.
This of course improves things only for customers that experience errors
on their line. However, it appears that in ADSL most parameters can
only be set at the DSLAM side, and the customer modem cannot be
configured to negotiate parameters at all. So, when your provider has
chosen to use "interleaved" mode, there is usually little you can do as
an individual subscriber as they won't change the profiles in the DSLAM
just for you.
(this is a bit of a nuisance. for example, the competition on the
broadband market has made the ADSL providers here to increase the
bitrate on existing subscriptions several times. those with long lines
sometimes saw their bitrate moved into the region where a reliable
connection is no longer possible and the connection became errattic.
they had to ask their provider to downgrade to a lower class with a
lower speed, instead of being able to set a maximum bitrate in their
local modem. with "fast" vs "interleaved" it seems to be the same
thing. classical telephone modems had no problem with setting and
negotiating training parameters at the local modem)
When your provider does not pay full attention to latency aspects,
and/or their internal network is saturated at some point, of course you
could experience very bad delays that are asymmetrical as well. There
is little you can do about it, except get a time receiver. There are
solutions that need not be very expensive, especially when you can
tinker a bit with it yourself.
For example, in the past I have taken several cheap DCF77 based alarm
clocks (they receive a longwave transmitter located in Germany and can
be used in western Europe) that are available for 10-20 euro, and
connected the receiver module to a serial port. There is support for
this in ntpd, and you have your own stratum 0 clock.
Better is a GPS receiver, that is available well under 100 euro these
days as well. Unfortunately those receivers often do not have a "pulse
per second" output and interface via USB, which makes microsecond
accuracy impossible. But maybe you can find an old model that has RS232
and puts the PPS signal on DCD of the RS232 interface. Those can be
used with ntpd, if wanted via gpsd (so you can still use it as a
positioning device at the same time).
Rob
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