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PPPoA vs. PPPoE

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Swifty

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Dec 2, 2009, 1:28:58 PM12/2/09
to
I'll admit upfront that this is nothing to do with AAISP but the right
sort of people are found here.

A colleague who lives in a very rural part of Wales is getting ADSL
drops. They started happening about a year ago, with nothing else
changing (that we know about) so an external influence.
BT have detected intermittent white noise on the line, but have so far
not traced its origin.
It is also affecting his "neighbours" who live within a few miles. So
the problem may be nearer the exchange than the customers.

He's been fiddling with router settings and switched between PPPoA and
PPPoE (I don't know which way). His ADSL drops ceased.

So my question: Is there a difference in the robustness of the two
encapsulations?

I wouldn't have thought that he could switch like that without the
"other end" getting involved, but I don't know anything about
encapsulation. I'm just passing on the question that I was asked. I'm
protecting my reputation as "the chap who can get the answers" here.

--
Steve Swift
http://www.swiftys.org.uk/swifty.html
http://www.ringers.org.uk

Rodney Pont

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Dec 2, 2009, 2:44:57 PM12/2/09
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On Wed, 02 Dec 2009 18:28:58 +0000, Swifty wrote:

>He's been fiddling with router settings and switched between PPPoA and
>PPPoE (I don't know which way). His ADSL drops ceased.
>
>So my question: Is there a difference in the robustness of the two
>encapsulations?
>
>I wouldn't have thought that he could switch like that without the
>"other end" getting involved, but I don't know anything about
>encapsulation. I'm just passing on the question that I was asked. I'm
>protecting my reputation as "the chap who can get the answers" here.

I hope I've got this right:

BT 20CN exchanges use PPPoA(TM) but the 21CN exchanges use
PPPoE(thernet) but translate to PPPoA for modems running that. I
suspect he's switched from PPPoA to PPPoE and that has dropped the
translation/encapsulation(no idea what it actually does).

Of course he could be on an LLU operator who would run whatever they
liked but you did mention BT involvement :-)

--
Regards - Rodney Pont
The from address exists but is mostly dumped,
please send any emails to the address below
e-mail ngpsm4 (at) infohitsystems (dot) ltd (dot) uk


Dave N

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Dec 2, 2009, 4:16:43 PM12/2/09
to
Rodney Pont wrote, on 02/12/2009 19:44:
> On Wed, 02 Dec 2009 18:28:58 +0000, Swifty wrote:
>
>> He's been fiddling with router settings and switched between PPPoA and
>> PPPoE (I don't know which way). His ADSL drops ceased.
>>
>> So my question: Is there a difference in the robustness of the two
>> encapsulations?
>>
>> I wouldn't have thought that he could switch like that without the
>> "other end" getting involved, but I don't know anything about
>> encapsulation. I'm just passing on the question that I was asked. I'm
>> protecting my reputation as "the chap who can get the answers" here.
>
> I hope I've got this right:
>
> BT 20CN exchanges use PPPoA(TM) but the 21CN exchanges use
> PPPoE(thernet) but translate to PPPoA for modems running that. I
> suspect he's switched from PPPoA to PPPoE and that has dropped the
> translation/encapsulation(no idea what it actually does).

Whilst acknowledging your caveat, that description is very interesting
because it opens up issues about optimising throughput and avoiding
packet fragmentation. Presumably PPPoE, if it is implemented on BT's
21CN, will operate with the usual MTU and MRU of 1492 bytes or is that
an assumption too far?

--
Dave N

Bob Goddard

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Dec 2, 2009, 4:35:23 PM12/2/09
to
Rodney Pont wrote:

> On Wed, 02 Dec 2009 18:28:58 +0000, Swifty wrote:
>
>>He's been fiddling with router settings and switched between PPPoA and
>>PPPoE (I don't know which way). His ADSL drops ceased.
>>
>>So my question: Is there a difference in the robustness of the two
>>encapsulations?
>>
>>I wouldn't have thought that he could switch like that without the
>>"other end" getting involved, but I don't know anything about
>>encapsulation. I'm just passing on the question that I was asked. I'm
>>protecting my reputation as "the chap who can get the answers" here.
>
> I hope I've got this right:
>
> BT 20CN exchanges use PPPoA(TM) but the 21CN exchanges use
> PPPoE(thernet) but translate to PPPoA for modems running that. I
> suspect he's switched from PPPoA to PPPoE and that has dropped the
> translation/encapsulation(no idea what it actually does).
>
> Of course he could be on an LLU operator who would run whatever they
> liked but you did mention BT involvement :-)

When BT first came out with ADSL, it was only PPPoA. However, a year or
so after during one summer, they enabled all the exchanges to handle
PPPoE.

Both 20CN & 21CN exchanges are quite capable of handling both PPPoA &
PPPoE.


B

--
http://www.mailtrap.org.uk/

Rodney Pont

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Dec 2, 2009, 4:38:23 PM12/2/09
to
On Wed, 02 Dec 2009 21:16:43 +0000, Dave N wrote:

>> I hope I've got this right:
>>
>> BT 20CN exchanges use PPPoA(TM) but the 21CN exchanges use
>> PPPoE(thernet) but translate to PPPoA for modems running that. I
>> suspect he's switched from PPPoA to PPPoE and that has dropped the
>> translation/encapsulation(no idea what it actually does).
>
>Whilst acknowledging your caveat, that description is very interesting
>because it opens up issues about optimising throughput and avoiding
>packet fragmentation. Presumably PPPoE, if it is implemented on BT's
>21CN, will operate with the usual MTU and MRU of 1492 bytes or is that
>an assumption too far?

I'm sure it is implemented and that 1492 is the correct MRU to use.
Wasn't there some problems with MRU on 21CN lines not so long ago and
AAISP implemented a workaround?

When I heard that 21CN was going to use PPPoE I asked if that meant my
modem wouldn't work after the switchover until I changed it to PPPoE
and I think it was Adrian who said I didn't have to since it would
negotiate PPPoA for me still.

My exchange has a target date of Q4 2009 for the switchover but I don't
know if it's happened yet. I might try PPPoE tomorrow and see.

Rodney Pont

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Dec 3, 2009, 3:44:36 AM12/3/09
to

--

Rodney Pont

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Dec 3, 2009, 3:51:12 AM12/3/09
to
On Wed, 02 Dec 2009 21:35:23 +0000, Bob Goddard wrote:

>When BT first came out with ADSL, it was only PPPoA. However, a year or
>so after during one summer, they enabled all the exchanges to handle
>PPPoE.
>
>Both 20CN & 21CN exchanges are quite capable of handling both PPPoA &
>PPPoE.

I didn't realise that and thought I would be able to tell when the
exchange was upgraded to 21CN by being able to connect with PPPoE.

I've tried PPPoE and the speed test always shows 7.0 something but with
PPPoA it's always 7.1 something so maybe I'm still on 20CN and it's a
little quicker because the exchange isn't having to convert PPPoE to
ATM.

It's only a small change but it is consistent with the protocol I'm
using. Perhaps PPPoE will be the quicker one when I'm converted.

I've no date for WBC but I think I'll get ADSL2+ when that does finally
make it's way to me so should notice that :-)

Bob Goddard

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Dec 3, 2009, 7:20:41 AM12/3/09
to
Rodney Pont wrote:

> On Wed, 02 Dec 2009 21:35:23 +0000, Bob Goddard wrote:
>
>>When BT first came out with ADSL, it was only PPPoA. However, a year or
>>so after during one summer, they enabled all the exchanges to handle
>>PPPoE.
>>
>>Both 20CN & 21CN exchanges are quite capable of handling both PPPoA &
>>PPPoE.
>
> I didn't realise that and thought I would be able to tell when the
> exchange was upgraded to 21CN by being able to connect with PPPoE.

After the exchange has been upgraded, it will either require that A&A put an
order in to upgrade your line, or you wait until the bulk upgrade.

Cluess will tell you if you are connecting in with 21CN/ADSL2+.

> I've tried PPPoE and the speed test always shows 7.0 something but with
> PPPoA it's always 7.1 something so maybe I'm still on 20CN and it's a
> little quicker because the exchange isn't having to convert PPPoE to
> ATM.

PPPoE has more overhead, therefore it is slower.


B


--
http://www.mailtrap.org.uk/

Rodney Pont

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Dec 5, 2009, 4:26:07 AM12/5/09
to

--

Rodney Pont

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Dec 5, 2009, 4:33:33 AM12/5/09
to
On Thu, 03 Dec 2009 12:20:41 +0000, Bob Goddard wrote:

I think this is the only thread where I've pressed the send button
before typing anything, and I've done it twice now...

>> I didn't realise that and thought I would be able to tell when the
>> exchange was upgraded to 21CN by being able to connect with PPPoE.
>
>After the exchange has been upgraded, it will either require that A&A put an
>order in to upgrade your line, or you wait until the bulk upgrade.

I'll wait for the bulk upgrade this time. Going from 2meg to MAX was
cheaper for me and it was a few months before the bulk upgrade came
along so I did save by paying :-)

With 21CN I'll get a larger allowance but I wouldn't use it unless my
use changes.

>Cluess will tell you if you are connecting in with 21CN/ADSL2+.

So it does.

>> I've tried PPPoE and the speed test always shows 7.0 something but with
>> PPPoA it's always 7.1 something so maybe I'm still on 20CN and it's a
>> little quicker because the exchange isn't having to convert PPPoE to
>> ATM.
>
>PPPoE has more overhead, therefore it is slower.

Will that still be true with 21CN since it will be using ethernet at
the exchange back haul instead of ATM? I assumed in that case I'd have
the extra overhead of converting from ethernet to PPPoA and thought
that PPPoE would be the most efficient then.

Bob Goddard

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Dec 5, 2009, 5:22:51 AM12/5/09
to
Rodney Pont wrote:

> On Thu, 03 Dec 2009 12:20:41 +0000, Bob Goddard wrote:
>
> I think this is the only thread where I've pressed the send button
> before typing anything, and I've done it twice now...
>
>>> I didn't realise that and thought I would be able to tell when the
>>> exchange was upgraded to 21CN by being able to connect with PPPoE.
>>
>>After the exchange has been upgraded, it will either require that A&A put
>>an order in to upgrade your line, or you wait until the bulk upgrade.
>
> I'll wait for the bulk upgrade this time. Going from 2meg to MAX was
> cheaper for me and it was a few months before the bulk upgrade came
> along so I did save by paying :-)
>
> With 21CN I'll get a larger allowance but I wouldn't use it unless my
> use changes.
>
>>Cluess will tell you if you are connecting in with 21CN/ADSL2+.
>
> So it does.
>
>>> I've tried PPPoE and the speed test always shows 7.0 something but with
>>> PPPoA it's always 7.1 something so maybe I'm still on 20CN and it's a
>>> little quicker because the exchange isn't having to convert PPPoE to
>>> ATM.
>>
>>PPPoE has more overhead, therefore it is slower.
>
> Will that still be true with 21CN since it will be using ethernet at
> the exchange back haul instead of ATM? I assumed in that case I'd have
> the extra overhead of converting from ethernet to PPPoA and thought
> that PPPoE would be the most efficient then.

Think about it... The equipment at both ends is ATM based regardless of
whether it is ADSL or ADSL2+.

--
http://www.mailtrap.org.uk/

Rodney Pont

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Dec 5, 2009, 5:38:40 AM12/5/09
to
On Sat, 05 Dec 2009 10:22:51 +0000, Bob Goddard wrote:

>> Will that still be true with 21CN since it will be using ethernet at
>> the exchange back haul instead of ATM? I assumed in that case I'd have
>> the extra overhead of converting from ethernet to PPPoA and thought
>> that PPPoE would be the most efficient then.
>
>Think about it... The equipment at both ends is ATM based regardless of
>whether it is ADSL or ADSL2+.

Even with 21CN? I thought it would all be ethernet based then if I
switch my modem to PPPoE. Sorry I'm a bit slow on the uptake here Bob.

Simon Farnsworth

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Dec 5, 2009, 6:28:44 AM12/5/09
to
On Wednesday 02 December 2009 18:28, Swifty wrote:
>
> He's been fiddling with router settings and switched between PPPoA and
> PPPoE (I don't know which way). His ADSL drops ceased.
>
> So my question: Is there a difference in the robustness of the two
> encapsulations?
>
There shouldn't be. Both turn into ATM cells on the wire - although PPPoE
may create an extra cell where PPPoA doesn't.

> I wouldn't have thought that he could switch like that without the
> "other end" getting involved, but I don't know anything about
> encapsulation. I'm just passing on the question that I was asked. I'm
> protecting my reputation as "the chap who can get the answers" here.
>

The other end can autodetect what encapsulation you're using, by looking at
the incoming ATM cells.

For PPPoA, the incoming ATM cells contain PPP directly. When the PPP link is
not yet up, these will always begin C0-21.

For PPPoE, the incoming ATM cells contain a 6 byte Ethernet over ATM header,
a 14 byte Ethernet header, then the PPP. The Ethernet over ATM header always
begins AA-AA-03.

Incidentally, this means that PPPoE is always going to be slightly slower
than PPPoA - when you're unlucky, it will use one ATM cell more than PPPoA
for a packet, costing you some speed.

There are some differences in how this all works between 20CN and 21CN:

In 20CN, your DSLAM port is an ATM device - it knows nothing about PPP, IP,
Ethernet etc, and just transfers ATM cells to the BRAS, using BT's ATM
cloud. The BRAS does the encapsulation autodetection, handles the PPP to
L2TP translation, and passes L2TP to AAISP.

In 21CN, the ATM cloud is replaced with an Ethernet cloud, running with
jumbo frames. Your MSAN port does the autodetection, removes the ATM cell
framing, adds Ethernet framing if you're doing PPPoA, and sends the
resulting PPPoE frame (which can be oversized - allowing a full 1500 byte
MTU) to the BRAS, which then does PPP to L2TP and passing data to AAISP.

There is never a performance gain on BT ADSL from using PPPoE; in 20CN, it's
just one more block of bytes for the BRAS to strip. In 21CN, you save the
MSAN adding a block of bytes to each packet, but it can do that without
measurable impact at incredible speeds (easily into the gigabits per second
per ADSL port).

If you're really interested in what happens, read on.

ADSL uses ATM cells on the wire to transmit data. An ATM cell is 53 bytes
long, 5 bytes header, 48 bytes payload. A 48 byte MTU is too small, so we
use an encapsulation called AAL5, which takes 8 bytes from the last cell in
a set, and lets you have up to 65535 bytes of payload. So, 100 bytes of
payload in AAL5 is carried in 3 cells; the first two cells have 48 bytes
each of the payload, and the last cell has 4 bytes of payload, 36 bytes of
padding, and 8 bytes AAL5 trailer. For a second example, if you had 96 bytes
of payload (a disaster case), you get 3 cells. Two cells contain 48 bytes
each of payload, and the last cell contains 40 bytes of padding, and 8 bytes
of AAL5 trailer.

PPP adds a 2 byte header to each IP frame - so, to send 1500 bytes of IP
data, you send 1502 bytes of PPP. If you're doing PPPoA, this 1502 bytes
becomes 31 cells containing 48 bytes of payload, and a cell containing 14
bytes payload, 26 bytes padding, and the 8 byte AAL5 trailer. Your sync
speed is measured in kilobits of cells per second, so a 832kbit/s sync speed
is around 1,900 cells per second (832kbit/s divided by (53 bytes per cell))

If you add PPPoE into the mix, the PPPoE header adds another 6 bytes. The
Ethernet header adds a further 14 bytes, for 20 bytes overhead. When doing
Ethernet over ATM, you normally drop the Ethernet checksum, but add 10 bytes
of headers, for a further 30 bytes of overhead. To send 1500 bytes IP in
this setup, you need to send 1532 bytes of payload. This becomes 31 cells
carrying 48 bytes of payload each, a cell carrying 44 bytes of payload and 4
bytes padding, and a cell carrying 40 bytes padding and 8 bytes of AAL5
overhead.

As you can see, there are whole ranges of packet sizes where PPPoA takes up
one fewer frame on the wire than PPPoE. 8 to 37 bytes of IP (never normally
seen, as IPv4 is 20 bytes overhead, and TCP adds another 20) are 1 cell in
PPPoA, 2 cells in PPPoE. You then get 18 bytes (38 to 55) where the
overheads are the same. 56 bytes to 85 bytes of IP take up 2 cells in PPPoA,
3 cells in PPPoE, and the cycle continues. Thus, depending on your packet
size distribution, you may see the same speeds with PPPoA and PPPoE, or
slightly more with PPPoA.

This is also where the faulty advice to use a 1478 byte MTU with PPPoA comes
from - when doing bulk transfers, most of your packets are MTU-sized. A 1478
byte MTU, with the 10 bytes of PPP and AAL5 overhead works out to precisely
31 cells, so you don't "waste" 26 bytes in every 1536 bytes of payload on
padding; the downside is a slight increase in number of packets sent, so
more lost to TCP/IP headers. Assuming IPv4, a minimum size IP and TCP header
is 40 bytes, so this ends up costing you more in extra IP headers than it
saves in ATM padding.

Finally, if you're wondering why 48 bytes - there are good technical reasons
to make the payload size a power of 2. The Americans wanted 64 bytes,
because that would be more efficient for data, and still good enough for
voice with echo cancellers in a US-wide network. The French wanted 32 bytes,
because that would allow them to have a France-wide network without echo
cancellers for voice. They refused to agree, so management types compromised
on 48 bytes, a pain for *everyone*.

--
Hope some of this interested people,

Simon

Marco

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Dec 5, 2009, 8:29:15 AM12/5/09
to
Simon Farnsworth wrote:

> The other end can autodetect what encapsulation you're using, by looking
> at the incoming ATM cells.
>
> For PPPoA, the incoming ATM cells contain PPP directly. When the PPP link
> is not yet up, these will always begin C0-21.

>[EXCELLENT technical explanation snipped]

Sorry to jump in here, but I must really THANK YOU for the amazing level of
detail you provided in your explanation. That kind of information is a bit
sparse on the Internet, and it's difficult to find all together and
explained so well as you did.

Thanks again.

Tony Firshman

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Dec 5, 2009, 8:42:33 AM12/5/09
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Marco wrote, On 5/12/09 13:29:
... and here too. It was so good I have printed it out to try and
digest it!


--
Tony Firshman
<firstname>@<surname>.co.uk

Bob Goddard

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Dec 5, 2009, 4:57:36 PM12/5/09
to
Rodney Pont wrote:

> On Sat, 05 Dec 2009 10:22:51 +0000, Bob Goddard wrote:
>
>>> Will that still be true with 21CN since it will be using ethernet at
>>> the exchange back haul instead of ATM? I assumed in that case I'd have
>>> the extra overhead of converting from ethernet to PPPoA and thought
>>> that PPPoE would be the most efficient then.
>>
>>Think about it... The equipment at both ends is ATM based regardless of
>>whether it is ADSL or ADSL2+.
>
> Even with 21CN? I thought it would all be ethernet based then if I
> switch my modem to PPPoE. Sorry I'm a bit slow on the uptake here Bob.

Simon gave a good description on it, but to boil it down...

20CN and 21CN are just marketing names by BT. They swapped ADSL1 with an ATM
backbone on 20CN with ADSL2+ with an ethernet backbone on 21CN. Regardless
though, the packets which fly over your telephone wire are still ATM cells.


B

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http://www.mailtrap.org.uk/

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