One more observation: the reported delay figures suggest that your DL operates interleaved while the UL is non-interleaved - same scenario like shown in the diagrams but with distinct capacities. Interesting is the fact that (in theory) the payload/delay lines for UL and DL will come close but not intersect for your setup. For hypothetical 1500 byte frames the payload-caused uplink penalty will be about 3ms at 4Mbit/s. The initial offset UL/DL (at 0 bytes) being 4.3ms, the UL delay for all Ethernet frames (below MTU) will be lower than the DL delay of 0-size payload frames. Joachim
> Am 03.12.2015 um 22:10 schrieb Joachim Fabini <joachim.fab...@tuwien.ac.at>: > > Comments inserted inline below. > >>>> Am 03.12.2015 um 17:46 schrieb Jan Ceuleers <jan.ceule...@computer.org>: >>> >>> 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). > > The delay figures need some more detail. I'd guess that what is reported here > is the initial delay (equivalent to transfer of a packet with payload size 0 > at link layer). As visible in the diagrams referenced in my previous message, > the 1:10 ratio of UL/DL capacity results in an increase of the gap, i.e., > more pronounced asymmetry for larger payloads. > >> 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. > > The difference to queuing delays in symmetric core network links is the > consistent asymmetry. Whereas NTP algorithms can detect and handle delay > variations (that in symmetrical links might level out), they are not capable > to detect consistent delay asymmetry like for VDSL UL/DL. > > The initial request asked about possible reasons for a consistent 4 ms offset > of network-acquired NTP time vs. GPS/PPS sync. Wrt to this order of > magnitude, I consider the asymmetry of DSL to be substantial and a likely > reason for the offset. Moreover because both NTP servers show the same offset > despite distinct delays. > Joachim _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
