On Wed, 2012-12-05 at 12:33 -0800, Alex Burr wrote: > > From: Dan Siemon > > > > 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/ > > > > It almost looks like you're being limited to 5k packets/sec. Now, I > know that some devices will only support a certain packet rate, and > it's not as stupid as it sounds because it's fairly rare to want to > send max rate of solely minimum size packets. But I wouldn't expect it > here, because I would expect a VDSL2 device to work with 100Mbps down, > 50Mbps up, even if your contract and line only support much less. And > the device would need to support more than 5k packets/sec to get > 50Mbps, even at the biggest packet size. I guess you might be able to > tell by plotting packet rate against packet size with more values of > packet size: if it's a rate limit, there will be a sharp corner, > whereas if it's overhead, there will should be more of a curve. The > figure for the 75byte payload suggests a curve. I ran the same test with more small packet sizes: http://www.coverfire.com/archives/2012/12/05/per-packet-overhead-on-vdsl-3/ This looks like it supports the PPS limit theory. > PTM-TC has a minimum overhead of 2 bytes/packet, IIRC. (I'm not > counting the 65/64 cell overhead, because that is not (in a good > implementation) aligned to packet boundaries and is therefore better > thought of as a 1/65 tax on your line rate). I'm not optimistic about > tuning shapers based on these details, however, as unlike ATM/AAL5, > implementations are allowed to insert any amount of padding between > packets, so the per packet overhead is not predictable from the > standard. There are also nonstandard framing implementations. That's disappointing news. My VDSL2 modem (Alcatel Cellpipe 7130) shows: Upstream line rate: 7344 kbps Bearer Upstream payload rate: 6560 kbps Can you shed some light on what causes the difference? Is it the 65/64 encoding and error correction overhead? I assume this does not take into account things like the 802.3 header. Thanks