Broadband Related > ADSL Issues
ADSL / PPP / ATM / Hawk Tester
kitz:
--- Quote ---I am led to believe that errors on the line can cause the PPP session to drop, while sync still stays up.
--- End quote ---
Highly unlikely if you are talking about a fault related to the physical side.
Any CRC type errors go through an escalation process which will eventually cause loss of sync way before the PPP session drops.
A PPP session can (& will) actually survive a resync. A session is negotiated and only drops when told by either the ISP or modem. It's why at one time stale sessions were problematic for many users because PPP remained up despite the line resyncing.
To avoid this ISPs must now restart any PPP sessions upon a line retrain by ensuring L2TP/PPP timeout is set at a minimum of 20 seconds. PPP will either drop when forced by ISP or modem... or when PPP idle timeout has been detected by either side.
CRCs cause Errored seconds, get enough errored seconds and the modem enters into an alarm state. The modem will go through a series of error alarm states eg An LOS defect is triggered after just 20ms.. and 2 consecutive defects will cause an LOS event. Most alarm state deactivation periods are 10 seconds.. so youre likely going to enter loss of link and a retrain of the line way before the PPP drops. - See the various error states for more info.
If it helps, think of the PPP session as like a cookie which doesn't expire until its told. Most of the recurring PPP session drops are usually down to a router misconfig issue.
The engineer who visited is correct, he can't do anything about PPP unless the line is also experiencing sufficient CRC & Err Sec type errors to cause the line to retrain. :/
What modem/router are you using?
ejs:
CRC errors can cause the PPP session to drop, without dropping the underlying DSL link, if the PPP / LCP messages are lost. This can and does happen. It's even possible to rack up CRC errors at the maximum possible rate without losing the DSL connection. It probably depends on the modem, some even have low-level configurable settings to control when it drops the DSL link.
LOS has nothing to do with CRC errors.
kitz:
>> CRC errors can cause the PPP session to drop,
and the chances of it repeatedly 'just' being the LCP echo messages that drop without causing the modem to go into an alarm state or showing any of the other more serious error types and/or changes in SNRM if thats being recorded?
Yes perhaps it could happen on an odd occasion, but constantly without any of the other symptoms?
I still maintain that its far more common to see constant PPP drops through either the modem or ISP misconfiguration. Some can be resolved by changing settings in the router, but its not unknown to be a problem at the ISP end either for example it was happening a few years ago with one of Zens gateways. I asked for the type of router because it tends to be own bought routers rather than ISP preconfig ones.
The other thing to bear in mind is that from some router logs the PPP down may be more obvious than loss of sync, Ive seen many people say loss of PPP when the line is also losing sync.
I just chose LOS because its one of the more common ones we see recorded in the line stats, indicating there is a problem with the physical link. There are plenty of others which is why I deliberately said eg and linked to the full list :/
As BS said, we need to see more info from line stats really.
burakkucat:
I would definitely like to know the make/model of the CPE in question.
ejs:
I'm not convinced the modem has an "alarm state" merely for lots of CRC errors. Obviously it wouldn't be just the LCP echo messages being lost, packet loss in general would be fairly high for anything else attempting to access the Internet over the connection.
In any case, if the CRC errors can be monitored and plotted on a graph, it should be simple to see if the PPP drops happen when there are large bursts of errors.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version