| Title: | FDDI - The Next Generation |
| Moderator: | NETCAD::STEFANI |
| Created: | Thu Apr 27 1989 |
| Last Modified: | Thu Jun 05 1997 |
| Last Successful Update: | Fri Jun 06 1997 |
| Number of topics: | 2259 |
| Total number of notes: | 8590 |
Hi!
A customer is seeing LEM errors (3'000 in 25 days) and LEM Rejects (85 in
the same time) on one port of the first DECconcentrator 900TH. On the other
900TH 80 LEM errors and no LEM Rejects during the same time. They are
on a hub 900. The remaining ports (one B and one A) are connected through
the hub's backplane. The ring is as follows:
900TH-----900TH
�| |
| |
GS/FDDI-----
However on the oppposite side of the fibers, there are no errors on
the GIGAswitch/FDDI's ports.
This is not a firmware issue. All are up to date. The attenuations have
been measured on the fibers and are OK, although we suspect one fiber.
The question are:
What do LEM Rejects represent?
The number of times the link has been rejected thus making the port wrap
from the primary ring to the secondary ? this doen't seem possible if the
GS/FDDI doesn't have LEM Rejects.
Or could it be that it is always the 900TH that DETECTS the link error and
then wraps (before the GS/FDDI)?
this doesn't sound reasonable either.
One step will be to see what happens if we replace the PMD.
Any advice will be appreciated.
Pedro PAIVA
| T.R | Title | User | Personal Name | Date | Lines |
|---|---|---|---|---|---|
| 2019.1 | LEM Reject = Many link errors | NPSS::KIRK | Fri Apr 19 1996 15:50 | 48 | |
The short answer is that an LEM reject represents a bad link -
too many errors. This can occur if the power into the receiver
is too weak, or for a number of other reasons. Suspecting either
the fiber or the PMD are reasonable things to do. Cleaning all
fiber connectors is also a good idea.
You said that all the attenuations are correct, but that one fiber
is suspect. Can you provide the details? I would need fiber
lengths and attenuations
If you can, try reversing the fiber pair at both ends of the link.
If the fiber is really bad, the errors should be noticed at the
Gigaswitch. If the 900TH PMD is bad, then the errors will still be
reported at the 900TH.
If the fiber is the problem, but is only marginally bad, it may
be that the receiver at the GIGAswitch may be better than at the
900TH, in which case the rate of errors may decrease.
In any case, I would advise use of known good fiber links.
Please provide the fiber details, and see if the problem follows
the PMD, or the fiber.
Regards,
Dick Kirk
Network Product Support
Some additional detail on LEM Rejects:
LEM Rejects can occur if the LerEstimate drops to the LerAlarm
level. When a link is established after the FDDI LCT (Link Confidence
Test), the LerEstimate is set to 15. The LerEstimate only changes if
link errors are detected. It is a measure of the bit error ratio of
the link. Our FDDI links have monitors (LEM) with alarms. If the
LerEstimate gets down to 8 (roughly a BER of 1e-08), then the firmware
declares the link to be 'broken' since it does not meet the required
FDDI bit error ratio. So the connection is rejected by the Link Error
Monitor (LEM).
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| 2019.2 | Thanks | 49575::PAIVA | Hawkeye - Network Support @GEO | Tue Apr 30 1996 08:25 | 7 |
Dick,
Thank you very much for your reply. I'm having trouble getting in
touch with the customer, reason why I didn't post the fiber figures (I
don't have them). Will advise when I will have more information.
Pedro
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