On Tue, 2012-12-04 at 01:23 -0800, Alex Burr wrote: > [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.
Thanks for the info. I guess I'll have to keep digging to figure out where the latency comes from. I did a couple more experiments which appear to confirm the large amount of per-packet overhead: http://www.coverfire.com/archives/2012/12/04/per-packet-overhead-on-vdsl2-2/ _______________________________________________ Cerowrt-devel mailing list [email protected] https://lists.bufferbloat.net/listinfo/cerowrt-devel
