PPPoA LLC versus VC

All the documentation I can find suggests that UK default framing is
PPPoa VC but I discovered my router was set to PPPoA LLC and always has
been.

It shows no diffrence which ever way I set it?
ISP is clara, so a generic BT DSLAM is what I am talking to..

The Natural Philosopher wrote:
> All the documentation I can find suggests that UK default framing is
> PPPoa VC but I discovered my router was set to PPPoA LLC and always has
> been.
>
> It shows no diffrence which ever way I set it?
> ISP is clara, so a generic BT DSLAM is what I am talking to..

BT support both - the difference is 8 bytes more overhead IIRC - which
is going to be hidden to some extent by the padding to make up to whole
cells.

On Mon, 14 Jul 2008 20:20:31 +0100, Andy Furniss passed an empty day by
writing:

> The Natural Philosopher wrote:
>> All the documentation I can find suggests that UK default framing is
>> PPPoa VC but I discovered my router was set to PPPoA LLC and always has
>> been.
>>
>> It shows no diffrence which ever way I set it? ISP is clara, so a
>> generic BT DSLAM is what I am talking to..
>
> BT support both - the difference is 8 bytes more overhead IIRC - which
> is going to be hidden to some extent by the padding to make up to whole
> cells.

Can I ask what a 'cell' is Andy? It's not a phrase I'm familiar with -
well not since my drunken fighting nights as a teenager, that is.

--
begin oefixed_in_2005.exe

Klunk wrote:
> On Mon, 14 Jul 2008 20:20:31 +0100, Andy Furniss passed an empty day by
> writing:
>
>> The Natural Philosopher wrote:
>>> All the documentation I can find suggests that UK default framing is
>>> PPPoa VC but I discovered my router was set to PPPoA LLC and always has
>>> been.
>>>
>>> It shows no diffrence which ever way I set it? ISP is clara, so a
>>> generic BT DSLAM is what I am talking to..
>> BT support both - the difference is 8 bytes more overhead IIRC - which
>> is going to be hidden to some extent by the padding to make up to whole
>> cells.
>
> Can I ask what a 'cell' is Andy? It's not a phrase I'm familiar with -
> well not since my drunken fighting nights as a teenager, that is.
>
its another word for a data frame or data packet.

On 2008-07-16, Klunk <(E-Mail Removed)> wrote:
> On Mon, 14 Jul 2008 20:20:31 +0100, Andy Furniss passed an empty day by
> writing:
>
>> The Natural Philosopher wrote:
>>> All the documentation I can find suggests that UK default framing is
>>> PPPoa VC but I discovered my router was set to PPPoA LLC and always has
>>> been.
>>>
>>> It shows no diffrence which ever way I set it? ISP is clara, so a
>>> generic BT DSLAM is what I am talking to..
>>
>> BT support both - the difference is 8 bytes more overhead IIRC - which
>> is going to be hidden to some extent by the padding to make up to whole
>> cells.
>
> Can I ask what a 'cell' is Andy? It's not a phrase I'm familiar with -
> well not since my drunken fighting nights as a teenager, that is.

With ATM framing, which ADSL uses, all data is sent in fixed-size 48 byte
chunks with a 5 byte header; the 53 byte thing is called a "cell".
When a variable-length IP packet is sent it gets carved into 48 byte
segments, with the last segment padded out with a packet length
and frame CRC added. A couple of bits in the cell header are
used to indicate the first, last, and not-first-or-last, cell
in a packet.

The difference between VC and LLC encapsulation is bigger
than the 8 byte difference would suggest, actually. With VC
multiplexing I think a 40 byte packet will fit in a single cell
while with LLC it requires two cells. As about 40% of the packets
will be 40 byte packets, this actually makes a significant (though
still not huge) difference. The thing is, though, what is set at
your end won't change what the other end is sending so changing the
setting at your router won't change anything in the direction you
probably care most about (i.e. download).

Dennis Ferguson

Dennis Ferguson wrote:

> The difference between VC and LLC encapsulation is bigger
> than the 8 byte difference would suggest, actually. With VC
> multiplexing I think a 40 byte packet will fit in a single cell
> while with LLC it requires two cells.

PPPoA/VC still misses by 2 bytes - which means an 8mbit/448kbit line is
even more horribly asymmetrical than the numbers suggest.

IPoATM I think is the the only one with a small enough overhead to fit a
(timestamps off) ack into a cell and no provider in the UK uses it AFAIK.

It means you have to patch/tweak your QOS to work in cells or overly
back off and waste bandwidth.People have been submitting patches to
Linux for this for years - they are just about to get in. Some routers
with Qos may be OK as they work in a different way.

Andy.

Andy Furniss wrote:
> Dennis Ferguson wrote:
>
>> The difference between VC and LLC encapsulation is bigger
>> than the 8 byte difference would suggest, actually. With VC
>> multiplexing I think a 40 byte packet will fit in a single cell
>> while with LLC it requires two cells.
>
> PPPoA/VC still misses by 2 bytes - which means an 8mbit/448kbit line is
> even more horribly asymmetrical than the numbers suggest.
>

Care to exapnd on that. I cannot get anywhere NEAR 448 kbs upload speeds
on speeed tests..about 360 is what I get.


> IPoATM I think is the the only one with a small enough overhead to fit a
> (timestamps off) ack into a cell and no provider in the UK uses it AFAIK.
>
> It means you have to patch/tweak your QOS to work in cells or overly
> back off and waste bandwidth.People have been submitting patches to
> Linux for this for years - they are just about to get in. Some routers
> with Qos may be OK as they work in a different way.
>

Happy to tweak anything and everything to improve it.

> Andy.

On 2008-07-16, Andy Furniss <(E-Mail Removed)> wrote:
> Dennis Ferguson wrote:
>
>> The difference between VC and LLC encapsulation is bigger
>> than the 8 byte difference would suggest, actually. With VC
>> multiplexing I think a 40 byte packet will fit in a single cell
>> while with LLC it requires two cells.
>
> PPPoA/VC still misses by 2 bytes - which means an 8mbit/448kbit line is
> even more horribly asymmetrical than the numbers suggest.

You're right; I forgot about the PPP part. With no PPP and no
TCP options a 40 byte packet will fit with a couple of bytes to
spare, anything else won't.

Of course the real bug is the use of ATM framing by DSL. 12 or 15
years ago, when this stuff was designed, everyone thought ATM
would be the best thing since sliced bread and would be used
for lots of stuff all by itself. In the end, however,
the only thing anyone ever does with ATM is to carry variable
length packets, something ATM is really inefficient for.

Dennis Ferguson

The Natural Philosopher wrote:
> Andy Furniss wrote:

>> PPPoA/VC still misses by 2 bytes - which means an 8mbit/448kbit line
>> is even more horribly asymmetrical than the numbers suggest.
>>
>
> Care to exapnd on that. I cannot get anywhere NEAR 448 kbs upload speeds
> on speeed tests..about 360 is what I get.

The asymmetrical bit is nothing to do with upload speed tests it's just
when downloading and uploading at the same time the acks from the
download eat a fair chunk of your upstream bandwidth.

448kbit is at atm level and your modem will round it down to whole
cells/sec = 1056. A 1500 byte packet will use 32 cells whether you use
VC or LLC = 33/sec.

Speed tests usually measure tcp data which is 1460/packet*33*8 = 385440
bits/sec max. Speed tests probably start timing before the flow gets
going and there may be some loss and retransmission depending on your
modem buffer size and tcp settings for you and the speed test site so
360 is not too bad.

Adslguide/thinkbroadband speed test also uses big K and divides by 1024,
I don't know what others do.



>
>
>> IPoATM I think is the the only one with a small enough overhead to fit a
>> (timestamps off) ack into a cell and no provider in the UK uses it AFAIK.
>>
>> It means you have to patch/tweak your QOS to work in cells or overly
>> back off and waste bandwidth.People have been submitting patches to
>> Linux for this for years - they are just about to get in. Some routers
>> with Qos may be OK as they work in a different way.
>>
>
> Happy to tweak anything and everything to improve it.

You can tweak your MTU to get slightly better use of your line - 1478
for PPPoA/VC fits 31 cells exactly so no padding. Changing it on routers
can be troublesome, so doing it on PCs is probably safer.

If you use a linux box as a gateway to do QOS I can tell you how - it
will be different depending on your kernel/iproute2 versions as they
keep changing things. In the future it should just be another option.

Andy.