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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