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squircle

join:2009-06-23
Oakville, ON
reply to xsbell

Re: [Internet] Per packet overhead on Bell's VDSL? ATM based?

said by xsbell:

said by squircle:

VDSL2 doesn't use ATM.

It most certainly does, with an option to use PTM (HDCL encapsulation)

VDSL2 supports (and Bell uses) PTM (similar to 802.3ah) instead of ATM over VDSL2 links.
said by xsbell:

said by squircle:

There's no ATM/AAL5 to worry about (i.e. your 1500 byte packets aren't being chopped into 31 (and a bit) ATM cells), removing the layer that has historically made ADSL connections "lower than advertised."

said by squircle:

VDSL2 doesn't use ATM. The only overhead is from the PPP headers (which is why your MTU is 1492 instead of 1500), but your sync rate usually compensates for this (e.g. my 25/10 connection syncs at 27/11.3).

Then why are they giving you 27/11.3?

That extra sync speed is for line encoding, just like all other xDSL technologies. I may have misspoke when I said "removing the layer that has historically made ADSL connections "lower than advertised"; it just makes the problem a lot better. Now one doesn't have to worry about adjusting their IP MTU to fit a full packet inside ATM cells without needing extra padded zeros.