What is PPP and Why does it drop?

[#1]

I have had a long period of relative stability but recently I've returned to losing PPP sessions repeatedly. What is happening - how can I have a connection to the exchange but then lose the session somewhere else?
I have an old connexant modem which shows these messages:
Ethernet connection to ATM - Passed
Simple PPP Session 0 PPP Layer Session - Failed
My connection is Down:  SNR Margin 9.4 db    Line Attenuation  60.2 db
                              Up                        19.0 db                                    31.5 db

[#2]

PPP stands for Point-to-Point Protocol. It is the part of the internet connection that links your exchange to your ISP. It follows quite a complex set of rules to ensure that you stay connected to the ISP, one of which is a "handshake" where the ISP's server constantly asks if your router is still there. If it ain't it will terminate the connection completely. The link between you and the exchange is via a high-frequency tone down the copper phone line. If this drops then you will obviously lose the link between your router and the ISP's server so your connection will drop. However, the high frequency (DSL) tone is separate from the PPP link.
Have a look elsewhere in this forum. You will see that there has been quite a bit of discussion recently concerning dropping the PPP but not losing sync to the exchange. Your router's log apparently shows that you are not losing the DSL link, but you are losing the PPP, which seems relevant to this problem. It seems to be related to something which BT have done recently and certain Broadcom chipsets which are fitted to certain routers. I'm not sure what kind of chipset your router has. Plusnet is currently taking an interest in this (as are also several other parties) and are carrying out tests to try to bottom it.
So I suppose if you think you are suffering from this then raise a ticket, but MAKE SURE THAT YOU STATE THAT YOUR DSL LINK IS NOT DROPPING! Otherwise you might get sucked into the "test your wiring" routine etc.
By the way, your Downstream attenuation seems very high. What speed are you connecting at?
EDIT: quite a few people (including me) have had success with changing the router's connect settings from PPPoA  and VCMux to PPPoE and LLC/SNAP. Try this if you want to see what effect it has. PPPoA is Point-to-Point over ATM, PPPoE is Point-to-Point over Ethernet. The latter is the "wrong" setting but it works!

[#3]

Hiya,
From the information that I have available to me (ie: BT line checks), it's extremely difficult to determine whether this is a synchronisation issue, or whether your PPP sessions are just dropping randomly.  Our RADIUS reporting also does not clearly identify this (your connection drops are showing as user reset which suggests that your router is dropping the connection rather than the connection being dropped elsewhere).
It's not outside the realms of possibility that your router is dropping synchronisation, then PPP and then the PPP is taking a period of time to reconnect.

[#4]

On rare occasions (mainly connected with heavy rain) the ADSL synch fails (that is to say I catch it in a failed state). Normally it's just the PPP which never seems to get re-established without re-booting the router.
You seem to be implying a different router could help. I did by a Buffalo ADSL2+ combined modem+WiFi bridge but it seemed to sync but I couldn't get anything to connect to the WiFi.
What components are involved in establishing a PPP session and why should it fail when ADSL is working?

[#5]

Try wading through this lot!
http://en.wikipedia.org/wiki/Point-to-Point_Protocol
The fact that you're having to do a power-cycle on the router makes me think that you are indeed suffering from the dreaded PPP-drop syndrome; the session has to be re-established completely, including DSL sync for some reason. This problem hasn't been bottomed yet, but I personally think it's something to do with the way the authentication handshake is passed through the BT backhaul system in combination with how certain Broadcom chipsets "see" the handshake. If you buy/try another router try to check what the chipset is first. 
I don't know any more than that, except to say that I can't get my router to establish a PPP session on PPPoA at all now. Up until recently it used to establish ok and frequently drop out. PPPoE is fine; actually the most stable connection I've ever had!!!!
Did you try using PPPoE.

[#6]

I can't find any Plusnet reference to using in PPPoE instead of PPPoA.

[#7]

That's because generally it's not recommended.
It does seem however that for some of those affected by these phamtom PPP drops, that using PPPoE is more effective.

[#8]

Quote from: Innovations

I can't find any Plusnet reference to using in PPPoE instead of PPPoA.

Reply numbers 14 and 15 in this thread. You might want to read through it completely too.
http://community.plus.net/forum/index.php/topic,78341.0.html