[Cake] pppoe-ptm overhead calculation question

[Sebastian Moeller]

Hi Jonathan,


On Sep 3, 2015, at 20:02 , Jonathan Morton <chromatix99 at gmail.com> wrote:

> I'm getting the distinct impression that VDSL1 & VDSL2 are completely different technologies with the misfortune of sharing an acronym and physical medium.

	That is a decent way of putting it. VDSL2 has the advantage of coming late to the party and avoiding some of the then obvious warts of VDSL1; also VDSL1 ist not compatible with ADSL-lines in the same DSLAM and hence probably on the define world wide. There are some deployments in Asia and parts of the rest of the world (according to wikipedia) VDSL2 seems to have struck mor resonance with telco ISPs and hence seems more prevalent..


>  It's certainly far too easy to confuse data about one for the other.

	It certainly is, it does not help that ITU documents are decidedly not fun to read...

> 
> Ideally, I'd like to see measurements on a real line to corroborate one way or the other.  

	The good thing is that I outlined the approach to measure this: create a traffic shaper close to line-rate, saturate the shaper with to-be-expanded-bytes packets and measure latency under load. For VDSL2 all should be well (or as well as with not-to-be-expanded bytes as payloads), for HDLC latency should rise quickly and badly (my first approximation would be that the on-the-wire-size doubles, so the shaper is toast by then).


> However, the use of a fixed-size word frame does sound much more elegant than HDLC.

	64/65 encoding is not too bad actually 1.5% overhead independent of the number of packets is decent…

Best Regards
	Sebastian

> 
> - Jonathan Morton