[Cerowrt-devel] Linksys wrt1900acs rrul traces

Richard Smith smithbone at gmail.com
Fri Apr 8 07:51:11 EDT 2016

On 04/07/2016 12:16 PM, moeller0 wrote:
> Hi Richard,
>> For these tests I had the inbound and outbound limits set to 975000
>> kbps.  975000 was somewhat arbitrary.  I wanted it below 1Gbps
>> enough that I could be sure it was the router as the limit but yet
>> fast enough that I would be able to see the peak transfer rates.
> All of the following might be old news to you, but please let me
> elaborate for others on this list (well, most folks here know way
> more about these things than I do). I believe Gbit ethernet is
> trickier than one would guess, the 1 Gbit rate contains some overhead
> that one typically does not account for. Here is the equivalent
> on-the-wire size of a full MTU non-jumbo ethernet frame: Layer “1+":
> 1500 (payload pMTU) + 6 (dest MAC) + 6 (src MAC) + 2 (ethertype) + 4
> (FCS) + 7 (preamble) + 1 (start of frame delimiter) + 12 (interframe
> gap)) = 1500+6+6+2+4+7+1+12 = 1538 ”Equivalent” in that the
> interframe gap is not really used, but filled with silence but it has
> the duration one would need for 12 bytes.

I knew there was a bit of overhead but thanks for laying out the details 
and why it matters.

> 975000 * (1538/1514) = 990455.746367 (which still is below the 1GBit
> Layer1+ ceiling that GbE has).  At 985000 * (1538/1514) =
> 1000614.26684 you would already have slowly caused the NIC’s buffer
> to fill ;)

So by luck I just barely made it. :)

> Luckily sqm-scripts will allow you to specify any
> additional per-packet overhead so just set this to 24 and things
> should just work out I believe.

I knew that there might need to be some overhead accounting and I looked 
at Link Layer Adaption tab in the options when I set up the SQM but the 
info in that menu isn't quite descriptive enough for for my setup.

It has a box for "Ethernet with overhead, select for eg VSDL2".  I'm 
using Ethernet but I don't know about VSDL2.  If I select it then I get 
a 2nd box that asks for the per packet overhead.  Even if I had tried to 
fill that out then I would have gotten it wrong. :)

The other option is "ATM" which I know I'm not using.

Not knowing what I really should select I just left it at None.

Perhaps there are too many combinations to add items for everything but 
a few more options and more descriptions of scenarios might be helpful.

So now that I have that set at 24 is it worth re-running some of the tests?

Richard A. Smith

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