[Oops, intended to CC the list.] An extra 15-20ms of latency, which is what you're seeing there, shouldn't be caused by packet overhead in the actual VDSL modem part of the device. Worst case the implementation of the PTM-TC layer (the equivalent of ATM) might round the packet up to a multiple of 64 bytes (although it's not supposed to), but unless your line rate is 25kbps, that should not cause 20ms of latency.
VDSL2 operates at 4Khz or 8Khz symbol rate, and while it contains and interleaving or retransmission layer which can add up to 64ms in its most high-latency configuration, those layers do not operate at the level of packets and I'm pretty sure that that latency cannot be affected by packet size in a conformant implementation. This latency is being caused by something else. The best way to figure out the per-packet overhead in your VDSL2 modem is probably to count the number of packets that get through, not measure latency. Alex ----- Original Message ----- > From: Dan Siemon <[email protected]> > To: bloat <[email protected]>; [email protected]; > [email protected] > Cc: > Sent: Tuesday, December 4, 2012 3:13 AM > Subject: Re: [Bloat] [Codel] [Cerowrt-devel] FQ_Codel lwn draft article review > > On Mon, 2012-12-03 at 15:58 +0000, David Woodhouse wrote: >> ADSL is basically just ATM with a strange PHY. You have a bunch of >> options for how you use this ATM link. Mostly it's RFC2364 PPP-over-ATM >> or it's PPPoE on top of RFC2684 Ethernet-over-ATM. > > Speaking of xDSL, does anyone on the list happen to have a good > understanding of how much per-packet overhead there is on VDSL2? I've > been tweaking the buffering and shaping on my upstream link and noticed > unexpected behavior with small packets. > > The link below (use wayback machine version) has a good description of > per-packet overhead for various forms of ADSL but I haven't found > something similar for more modern DSL variants. > http://www.adsl-optimizer.dk/thesis/ > http://web.archive.org/web/20090422131547/http://www.adsl-optimizer.dk/thesis/ > > I started a discussion on DSLReports > http://www.dslreports.com/forum/r27565251-Internet-Per-packet-overhead-on-Bell-s-VDSL-ATM-based- > but experimentally the overhead discussed there doesn't appear to be > correct > http://www.coverfire.com/archives/2012/11/29/per-packet-overhead-on-vdsl2/ > > > > _______________________________________________ > Bloat mailing list > [email protected] > https://lists.bufferbloat.net/listinfo/bloat > _______________________________________________ Cerowrt-devel mailing list [email protected] https://lists.bufferbloat.net/listinfo/cerowrt-devel
