RE: CRC check indication of bad fiber

["Michael M. Salzman" <msalzman@xxxxxxx>]

Drew and Mick,
Your notion is interesting.  Certainly bad packet indications can be useful
measurements of quality, but of what?  As Mick says, the architecture and
the coding scheme convolve these measures such that they become almost
irrelevant.

In todays highly switched, non-shared network links, CRC errors are more
likely to indicate a problem with the source or switching equipment then
with the link.  The coding scheme may or may not yield a run of errors based
on the CRC error, depending on where in the process the error took place
(before or after the coding).  Furthermore code errors are usually detected
up front in the PHY process and may (if so designed) issue signals to the
MAC layer about the channel.  Usually these channel indications are not
gradual and graceful, on the contrary they are usually a foggy statement
that the channel is not useable.  Why foggy?  Because they need to be
resolved against other indications in order to settle the proximate cause of
failure.  For example a sync loss in an 8B10B system can be an indication of
complete and abrupt link failure, or merely a bit error in the 10B symbol.
If the latter, then the system will quickly recover sync at the next comma,
but by that time the Phy state machine will take the link down, and the Mac
will have to restart it.

So, we come back to the issue that to verify the link operation, we have two
options.  If we want to do it dynamically, while the link is running, we
have to inject some kind of secondary channel or test packets and measure
them.  Or we can do it only upon link startup, at the full 10Gig rate for
some few seconds or minutes and assess the quality of the link at that time
using some clever patterns.  Beyond that level of built in testing, we can
employ specific test equipment.


> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx]On Behalf Of Mick
> Seaman
> Sent: Thursday, May 20, 1999 14:11
> To: 'stds-802-3-hssg@xxxxxxxxxxxxxxxxxx'
> Subject: CRC check indication of bad fiber
>
>
>
> My understanding is, that if the data is scrambled, one physical bit error
> can be turned into a large number (order of the scrambling polynomial?)
> data bit errors.
>
> This could much reduce the degree of error checking/protection provided by
> the CRC, presuming that to be calculated over the data bits
> rather than the
> physically transmitted bits and to remain the same 'end to end' where in
> this case I mean that much interpreted term to mean over a number
> of number
> of .1D bridges or a number of repeaters (are full-duplex repeaters capable
> of all potential media type conversions with the different transmission
> arrrangements under discussion?).
>
> So it may be necessary to add extra information to the frame (or
> to a set of
> frames) to guard against bad fibers (?).
>
> Mick
>
> > -----Original Message-----
> > From:	Chang, Edward S [SMTP:Edward.Chang@xxxxxxxxxx]
> > Sent:	Thursday, May 20, 1999 1:58 PM
> > To:	Drew Perkins; 'msalzman@xxxxxxxxxx';
> > 'stds-802-3-hssg@xxxxxxxxxxxxxxxxxx'
> > Subject:	RE: IEEE 802.3 Requirements
> >
> >
> > Drew:
> >
> > Yes, at MAC level, the CRC of the whole packet is checked; therefore, it
> > can
> > be used for indication of bad fibers.  However, we have to differentiate
> > the
> > normal read errors from the persistent errors generated by a bad fibers.
> > Any way, it can be done.
> >
> > Ed Chang
> > Unisys Corporation
> >
> > -----Original Message-----
> > From: Drew Perkins [mailto:drew.perkins@xxxxxxxxxxxx]
> > Sent: Thursday, May 20, 1999 2:32 PM
> > To: 'msalzman@xxxxxxxxxx'; 'stds-802-3-hssg@xxxxxxxxxxxxxxxxxx'
> > Subject: RE: IEEE 802.3 Requirements
> >
> >
> >
> > Is there any reason that the Ethernet CRC wouldn't make a
> pretty darn good
> > error detection mechanism?
> >
> > Drew
> > ---------------------------------------------------------
> > Ciena Corporation                 Email: ddp@xxxxxxxxxxxx
> > Core Switching Division                 Tel: 408-865-6202
> > 10201 Bubb Road                         Fax: 408-865-6291
> > Cupertino, CA 95014              Cell/Pager: 408-829-8298
> >
> >
> > -----Original Message-----
> > From: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> > [mailto:owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx]On Behalf Of Michael M.
> > Salzman
> > Sent: Wednesday, May 19, 1999 9:28 PM
> > To: stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> > Subject: RE: IEEE 802.3 Requirements
> >
> >
> >
> > Hi Ed, comments offered below on your ideas.
> >
> > > -----Original Message-----
> > > From: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> > > [mailto:owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx]On Behalf Of Chang,
> > > Edward S
> > > Sent: Wednesday, May 19, 1999 10:44
> > > To: Bruce_Tolley@xxxxxxxx; msalzman@xxxxxxxxxx
> > > Cc: stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> > > Subject: RE: IEEE 802.3 Requirements
> > >
> > > First of all, all datacom equipment have built-in error-check routines
> > to
> > > count the number of retries with a given client.  When that number
> > reaches
> > > the preset "water-level", it will give up retry and report
> the problem.
> > > These mechanisms are already in place, and we do not need to
> reinvent or
> > > re-invest.  We may modify the error check routines to fit our purpose.
> >
> > Ed,  802.3 does not have access to any retry counters of any
> higher level
> > protocols.  Furthermore, not all protocols rely on retries.
> The MAC layer
> > has access only to its own activities, which include send
> packet attempts.
> > In a full duplex configuration the send attempts are always successful,
> > unless the entire layer fails.  The only way to measure a live
> error count
> > is to run some kind of OAM channel and to pass test frames over it.
> > Furthermore, some coding schemes, can give abrupt indication of
> sync loss.
> > In summary, at the MAC layer, it is difficult to assess channel
> > deterioration.
> >
> > A practical approach is to detect link failure, shut it down,
> and then do
> > link acquisition which includes link quality testing at full
> rate, and to
> > then either declare the link dead or alive.  That's roughly what is done
> > in
> > 1GE and we can improve upon it for 10GE, or we can add optional
> > improvements
> > for, say, MAN applications.
> >
> > >
> > > Second, we, TIA FO2.2, have studied many of the MM fibers in industry
> > with
> > > varieties of launch conditions; therefore, we should be able to
> > > come up with
> > > a realistic, optimized cable plant design to drastically improve the
> > > performance, and at the same time, reject those DMD, or bad fibers.
> > > Remember, those are defected fibers, which is wrong to be in
> the market
> > in
> > > the first place.  We have pretty good idea how it may shape up.
> > > We are not
> > > talking taking-chance with ignorance, or try-and-error.  We all
> > > have product
> > > responsibility; as a result, reliability and customer satisfaction are
> > > always the first priority.  It implies that the rejection ratio is in
> > the
> > > minimum, limited to those DMD and bad fibers only.
> >
> > Ed, I am not sure what you are suggesting.  Perhaps you can offer a
> > presentation on this idea in the meeting.
> >
> > Mike.
>