[ntp:questions] Help verifying accuracy of NTP server

Jan Ceuleers jan.ceuleers at computer.org
Thu Dec 3 16:46:45 UTC 2015

On 02/12/15 21:00, Joachim Fabini wrote:
> NTP algorithms rely on symmetric connection delay but DSL delay is
> commonly highly asymmetrical. In measurements for my setup (VDSL;
> 8Mbit/s DL, 768kbit/s UL), the VDSL one-way delay at low packet payload
> averages 12ms for DL and 6ms for UL. Very surprising if you consider the
> DL/UL capacity ratio of larger than 10:1.

For what it's worth, this is what my VDSL2 modem is telling me:

Extended Port Status
Bme: 1 Port: 1
Downstream line rate: 37792 kbps
Upstream line rate: 4960 kbps
Bearer0 Downstream payload rate: 0 kbps
Bearer1 Downstream payload rate: 30064 kbps
Bearer0 Upstream payload rate: 0 kbps
Bearer1 Upstream payload rate: 4048 kbps
Downstream attainable payload rate: 34368 kbps
Downstream attainable line rate: 46112 kbps
Downstream Training Margin: 9.5 dB
Downstream Line Protection (Bearer1 Path): 3.0 DMT Symbols
Upstream Line Protection (Bearer1 Path): 1.0 DMT Symbols
Near-end ITU Vendor Id: 0xb500494b4e530200
Far-end ITU Vendor Id: 0xb500494b4e530200
Downstream delay: 9.7 ms
Upstream delay: 5.4 ms
Tx total power -7.9 dbm
FE Tx total power 10.8 dbm
VDSL Estimated Loop Length : 1901 ft
G.Hs Estimated Near End Loop Length : 3187 ft
G.Hs Estimated Far End Loop Length :1881 ft
Current framing mode: 0x10
Bandplan Type...........: 2
No. of Upstream Bands...: 2
No. of Downstream Bands.: 2
Line Type: 0x00200000

So 9.7ms down and 5.4ms up. These account for the coding, interleaving
and transmission delay; they don't yet account for the delay incurred by
the layers above (but this delay should be the same in both directions).

So do these numbers merit being called highly asymmetrical? I suppose
so, but they're also pretty small in comparison with delays deeper in
the network and they don't end up causing appreciable asymmetry in the
end-to-end paths to and from internet-based NTP servers.


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