Re: Fwd: [LARTC] HTB Weird Shaping Question(Bug?). Please Help!

[Andy Furniss <andy.furniss@dsl.pipex.com>]

Taylor, Grant wrote:
>> Err, since when is ADSL half duplex ? News to me :)
>>
>> I think you'll find if you read up on (A)DSL that it is in fact full
>> duplex. Going from memory, the frequency range from 25Khz up to 1.1Mhz is
>> broken up into a fairly large number of subcarriers (52 I think ?) and
>> some subcarriers are used for the downstream direction (the majority in
>> the case of ADSL, or half of them for SDSL) while the rest are used for
>> the upstream direction.
> 
> 
> *nod*  This is indeed the case.
> 
>> Because it uses frequency division muliplexing for each carrier to exist
>> on the line simultaneously there is no need to do any time division
>> multiplexing between upstream and downstream.
> 
> 
> I'm not sure why ADSL is simplex / half duplex as it seems that it 
> should be able to be (full) duplex.  I do know that the the types of 
> service that have been seen here in my town are indicative of simplex 
> traffic.  I don't know if this is a limit that the LEC is putting in 
> place or not.  Also many a provider that I've talked to have agreed that 
> ADSL in particular is simplex.  If you are talking SDSL the circuit is 
> indeed duplex.
> 
>> Besides, even a normal modem dialup connection is full duplex, it's
>> unlikely ADSL would take a backwards step from that ;-)
> 
> 
> Keep in mind that 33.6 kbps modems download at 33.6 kbps and upload at 
> 33.6 kbps where as 56 kbps modems download at 56 kbps and upload at 28.8 
> kbps.  One reason that ADSL my be simplex is that simplex technology is 
> easier to implement and thus has more features or speed on a given 
> circuit.  Thus ADSL could be simplex allowing for a wider band for 
> download sacrificing some of the upload bandwidth / capabilities.  In a 
> residential / small office environment this is typically not a problem.
> 
> Again, I have no paperwork in front of me to back this up, just my 
> experience and conversations with half a dozen or more people in the ISP 
> business.

I've never heard of half duplex adsl - your modem should tell you up and 
down sync rates and that's what you've got.

It is very easy to make it look like it though, just start an upload 
while downloading something and watch your down rate fall apart. This is 
TCP and the fact that adsl modems tend to have huge buffers not the 
link. The first thing to do when making a shaping script is priorotise 
empty acks - then uploading doesn't trash your download.

Andy.
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