[Cerowrt-devel] VDSL

[Sebastian Moeller]

Hi Rich,


On Jan 11, 2014, at 20:06 , Rich Brown <richb.hanover at gmail.com> wrote:

> Hi Sebastian,
> 
>>>> 	Well, that looks like a decent recommendation for the wiki. The SQM configuration page still needs to expose all three values, atm, ethernet, and  none so that people can actually change things...
>>> 
>>> So two questions really:
>>> 
>>> 1) (From my previous note) What’s the difference between the current “Ethernet” (for VDSL) and “None” link layer adaptations?
>> 
>> 	Currently, "none" completely disables the link layer adjustments, "ethernet" enables them, but will only use the overhead (unless you specify an tcMPU, but that is truly exotic).
>> 
>>> 
>>> 2) When we distinguish the Ethernet/VDSL case, I would really like to use a different name from “Ethernet” because it seems confusingly similar to  having a real Ethernet path/link (e.g., direct connection to internet backbone without any ADSL, cable modem, etc.) 
>> 
>> 	On the one hand I agree, but the two options are called "ATM" (well for tc "adsl" is a valid alias for ATM) and "ethernet" if you pass them to tc (what we do), and I would really hate it to hide this under fancy names. I see no chance of renaming those options in tc, so we are sort of stuck with them and adding another layer of indirection seems too opaque to me. This is why I put some explanation behind the option names in the list box…
> 
> Now I see how it works. (I didn’t understand that “None” really meant NONE.)

	Ah, I see, the reason most likely is that initially I had it set up differently, and only later I realized that there are only three relevant states...

> The following choices in the Link Layer Adaptation would have eased my confusion:
> 
> - ATM (almost every type of ADSL or DSL)

	Question, for me DSL is short hand for SDSL, ADSL, and VDSL, but i take it too mean whatever the ISP put in the product name so most likely ADSL, is that correct?

> - Ethernet with overhead
> - None (default)
> 
> Then the text can say:
> 
> —
> You must set the Link Layer Adaptation options so that CeroWrt can perform its best with video and audio chat, gaming, and other protocols that rely on short packets. The general rule for selecting the Link Layer Adaption is: 
> 
> * If you use any kind of DSL/ADSL connection to the Internet (that is, if you get your internet service through the telephone line), you should choose the “ATM (almost every type of ADSL or DSL)" item. Set the Per-packet Overhead to 44. 

	Sounds like a reasonable default, we only sacrifice a bit of bandwidth but keep the latency spiffy.

> 
> * If you know you have a VDSL connection, you should choose “Ethernet with overhead" and set the Per-packet Overhead to 8. 

	This depends on whether there is PPPoE in use (then 8 seems correct), I do not know about potential ethernet heads or VLAN headers, so * might be too small. Also I have not seen much data whatsoever from a VDSL link so I really have no good handle on this (except Germany's lardiest ISP uses PPPoE). Note I might switch to VDSL later this year, then I can see (if I manage to get anything useful out of non ATM links, my overhead detector critically relies on ATM cell quantization)

> 
> * If you use Cable modem, Fiber, or another kind of connection to the Internet, you should choose “None (default)”. All other parameters will be ignored.

	I think this is the status quo; I have no data to argue any change here (I have an intuition though that most likely all of these will also suffer from additional per packet overhead by virtue of being shared media).

> 
> If you cannot tell what kind of link you have, first try using "None", then run the [[Quick Test for Bufferbloat]]. If the results are good, you’re done.

	I will ned to test this with link layer disabled, to better describe what to expect. I fear a typical speediest.net test will not show ATMs small packet volatility; so there might be need for a dedicated test...

> Next,  try the ADSL/ATM choice, then the VDSL choice to see which performs best. Read the **Details** (below) to learn more about tuning the parameters for your link. 
> —
> 
> Would that be better? Thanks.

	Oh, I think that is quite reasonable and helpful. Now I need to put more information into the **Details**

Best
	Sebastian

> 
> Rich