PPPoA vs. PPPoE on plusnet?

[#1]

For ages I used a normal ADSL router and used PPPoA to connect to plusnet.
Recently, I wanted more functionality in a router, so bought a more powerful one that had 2 ethernet WAN ports (Draytek Vigor 2920).  This then connects to a ADSL router that's been dumbed-down (set up in bridge mode) to act just as an ADSL "modem".
The vigor router now authenticates with plusnet using PPPoE.
This all seems to work just fine.
However, I've just been speaking to a plusnet support chap who is booking BT to look at some issues on my line, and he tells me that I shouldn't be using PPPoE at all, and said "it is like putting diesel in a petrol car".
He also notes in the support text: "I highly suggest you configure your router to use PPPoA otherwise we will pass any engineer charges incurred onto yourself if this is sound to be the cause of your connection problems."
and: "BT exchange equipment is not compatible with PPPoE therefore will cause errors"
Am I wrong?  Should I not use PPPoE at all?  Surely if it works fine, there isn't an issue here?
This is on BT's 21CN, not a LLU or whatever.
Thanks!
-- gyre --

[#2]

The CS agent is talking complete rubbish. BT exchanges have supported PPPoA and PPPoE for years. Originally when ADSL was introduced only PPPoA was supported but a few years later they upgraded the majority (if not all) exchanges to support PPPoE as well.
The 21CN network was built from the start to support PPPoE and this is actually the preferred connection method but they also detect and support any PPPoA connections for backwards compatibility.
There is a very small overhead when using PPPoE (we are talking a few bytes per packet) so nothing that anyone will notice.
For FTTC I am using PPPoE because we have to use a DSL router to connect to the VDSL modem.
Finally, BT could not care less if you use PPPoA or PPPoE as both are supported.
However..... Not all routers will work with PPPoE due to either issues with the router or compatibility issues when talking to the exchange hardware. So some people may find if they try it, it will not work. PPPoA is what 99.9% of users will be connecting with so that is the one I would recommend people stick with, but if you do have problems try PPPoE and see if it helps.
For a technical reply on how this all works see http://forum.aaisp.net.uk/index.php?t=msg&th=581&goto=2404&#msg_2404 but put simply, your connection to the exchange is always ATM (asynchronous Transfer Mode), the PPPoE or PPPoA just defines what format your data is in when it leaves the router and is contained within the ATM packets to/from the exchange.

[#3]

Some of us can ONLY establish a PPP session using PPPoE because of incompatibility between some router chipsets and exchange equipment...  see Bob Pullen's comment at the bottom of page 1 here...  http://community.plus.net/forum/index.php/topic,78341.0.html
Since that was written, I've migrated to 21CN and ADSL2+ and it still needs a PPPoE connection to re-establish a PPP session if it drops.

[#4]

Nice explanation.  Thank you Peter.
I know that when the Plusnet folks do different tests on my connection to diagnose problems, they always flag the ppp layer as having failed.  I'm guessing that's because I'm using PPPoE from the Vigor router and not PPPoA from the Netgear or Speedtouch or whatever.
The actual ADSL problem I have is that my connection can be rock solid for days on an end... and then it just falls apart.  In particular, incoming phone calls cause an instant connection drop.  No warning.  The problem is.. it's intermittent, and the last time I had a BT engineer around, I couldn't get it to drop for love nor money.
Yesterday, the connection was all over the place... and today it's rock solid again.  I'm wondering if the deluge of rain we had yesterday might have had something to do with it.
Thanks.
-- gyre --

[#5]

Quote from: gyre

In particular, incoming phone calls cause an instant connection drop.  No warning.

That's usually a sign that you haven't fitted the ADSL filters to all the necessary phone sockets.
More generally, it used to be that all UK ISPs specified PPPoA except AOL who specified PPPoE.  That was a few years ago now (and before 21CN) but I remember failing to establish an AOL connection until setting the router to PPPoE, PPPoA didn't work.     

[#6]

Quote from: ReedRichards

Quote from: gyre In particular, incoming phone calls cause an instant connection drop.  No warning. That's usually a sign that you haven't fitted the ADSL filters to all the necessary phone sockets.

That's what I thought.
However, BT master socket has ADSL Nation XTE-2005 faceplate filter.  Have tried 2.  Have also tried filters on all phone sockets and removing all phones.  Same problem.
Could possibly be line card at exchange.  However, it's intermittent and is rock solid when BT guy is here
-- gyre --

[#7]

I had exactly that problem and after numerous fault reports and several engineer visits I gave up.
I was moved to 21CN (ADSL1) which involved changing from DSLAM to MSAN at the exchange - problem completely solved

[#8]

Quote from: Oldjim

I was moved to 21CN (ADSL1) which involved changing from DSLAM to MSAN at the exchange

Can somebody offer a translation?  I presume Oldjim means that the physical equipment being used at the exchange was changed?  So is the implication that the old DSLAM equipment (line card?) was faulty but never changed in prior investigations, or is it something more profound?

[#9]

My line was moved from the old 20CN equipment to the new 21CN equipment
The line card was supposed to have been checked, reseated and changed at various times
With the benefit of hindsight this was the difference so it may not have been the line card but something wrong in the wiring or interference from some neighbouring kit in the board - but I am not an expert so I could be completely wrong

[#10]

DSLAMs are the bits of equipment where the connection is made in the exchange for 20CN circuits, and MSANs are kit for 21CN connections. If you want to read about it in depth there's a lot on Kitz's site here: http://www.kitz.co.uk/adsl/21cn_network.htm
EDIT: as Jim says above

[#11]

Kitz's diagram is not strictly correct as it miss out the EvoTAM http://www.techbroadband.net/wiki/EvoTAM
With DSLAM's there is a direct route on the MDF to the DSLAM with MSAN there is only an indirect route to MSAN via EvoTAM

[#12]

Quote from: Oldjim

I had exactly that problem and after numerous fault reports and several engineer visits I gave up. I was moved to 21CN (ADSL1) which involved changing from DSLAM to MSAN at the exchange - problem completely solved

My problems started when I was moved to 21CN
Had the BT guy around today... he checked and replaced everything around the master socket and said that problem was somewhere else on the line... d-side or e-side or something?  I didn't quite catch what he was saying... but the end result was that he'd gone as far as he could within the premises and left it to me to contact plusnet to get further action taken.
Joy. 
-- gyre --