VDSL2 uses PTM. In the UK, the regulator has mandated that VDSL2 must be
run over fibre, normally to an MSAN within 200-300 metres of the user.
So what is the fibre in your cable and fibre category?
Is it FTTC/FTTH as I describe, or fibre using some other transmission
protocol?
On 18/12/13 10:33, Sebastian Moeller wrote:
Hi David,
On Dec 18, 2013, at 05:34 , David Lang <da...@lang.hm> wrote:
On Tue, 17 Dec 2013, Rich Brown wrote:
- From what you’ve said, I don’t have much hope for doing it automagically. But
maybe we can provide clues to help the customer do to the right thing. Perhaps
the first dropdown could be “Link Layer Adjustments (used on DSL or ATM)” with
options for “None/ADSL/SDSL/VDSL over PTM/VDSL over ATM/PPPoATM” and maybe
others. CeroWrt could automatically set the proper link layer adaptations for
each. We could also include a link to the wiki for a flow chart for setting
each of these cases, especially the questions they should ask their ISP.
Let's start with the first question, what is the difference between these as
far as what the config should be?
1) ATM based carriers (ADSL1, ADSL2, ADSL2+, potentially VDSL1): link
layer has to be set to ADSL
2) PTM based carriers (VDSL2): link layer has to be set to ethernet
3) cable, GPON (fiber): link layer has to be set to ethernet
1) and 2) typically have additional overhead to account for, 3) may or
may not
Only 3) with no overhead is fine with no link layer adaptation
mechanism.
forget the GUI or automated settings. If I am configuring a Cerowrt box
mmanually, what should I set differently for the different types of configs?
Current GUI settings (might change)
A) ATM based transports:
1) Which link layer adaptation mechanism: tc_stab
2) link layer: ADSL
3) overhead: see http://ace-host.stuart.id.au/russell/files/tc/tc-atm/
(section: Overhead and MTU Calculations)
B) PTM based transports:
1) Which link layer adaptation mechanism: tc_stab
2) link layer: ethernet
3) overhead: unclear but
see:http://www.dslreports.com/forum/r27565251-Internet-Per-packet-overhead-on-Bell-s-VDSL-ATM-based-
C) cable, GPON (fiber):
1) Which link layer adaptation mechanism: none, assuming no per packet
overhead
otherwise
1) Which link layer adaptation mechanism: tc_stab
2) link layer: ethernet
3) overhead: unclear but
see:http://www.dslreports.com/forum/r27565251-Internet-Per-packet-overhead-on-Bell-s-VDSL-ATM-based-
There should be no need to fiddle with the advanced link layer options, unless you link
MTU is >> 1500. Note for link layer ethernet no size table is constructed unless
MPU > 0.
What is the impact of getting it wrong? (if it's like VPN overhead where
setting the rate just slightly too high results is lots of wasted 'airtime' by
setting it too low results is a amall amount of wasted 'airtime' then a low
enough value to be reasonalbe everywere is a good default)
User on an ATM based link without link layer adaptation: The shaper will
underestimate the relevant wire seize of each packet and hence will not shape enough to
avoid filling the potentially bloated buffers of the DSL modem. This effect gets worse
the more the packet length distribution is skewed towards small packet (the estimate can
be off by around 50% worst case, so this is not good.) But note this basically is the
"status quo" for most users (as far as I know no router/modem sets these
options correctly, but I do not claim to know all such systems). ALSO this in theory is
testable, on such a system buffer bloat/latency increase should be more severe if one
tries to fill the nominal transmit rates with small than with large packets. Misjudging
the overhead either wastes bandwidth or also if too large or increases the likelihood to
see buffer bloat by overestimating the effective link capacity.
Users on a PTM based link, when running with link layer ADSL, will
waste 10% of the bandwidth right there (for taking the 48 in 53 encapsulation
into consideration). Plus they will overestimate the effective size of small
packets and will waste up to 50% of the remaining bandwidth there. Overhead
misjudging has the same effect as on ATM (except overheads on PTM typically
should be smaller I guess so this effect might not be too relevant).
To summarize, using the wrong link layer adaptation will hurt the user, ATM users
will suffer buffer bloat, but will use all available bandwidth (well more actually since
that creates the bloat), PTM users will suffer severe packet-size-dependent bandwidth
decreases. The "status quo" is more or less fine for groups 2) and 3), not so
good for 1). I think most people in 1) caring enough reduced the shaped rates by a larger
amount than people in groups 2) and 3) and just accepted that depending on the packet
size mix latencies got more variable.
Getting it wrong is not advisable… Getting it right requires some
non-obvious information from one's ISP. While VDSL will become more prominent
in the future, ADSL variants will not disappear for a long time, as VDSL only
works (well)
on short loops, so people far away from the DSLAM will stay on ATM (or one day
a new ADSL might learn to use PTM, but I will not hold my breath)
Best Regards
Sebastian
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