Re: CRC check indication of bad fiber

[Jaime Kardontchik <kardontchik.jaime@xxxxxxxxxxx>]

I do not think so.

Once the descrambler at the receiver has been synchronized, single bit errors
in a frame during transmission will usually generate single bit errors in the
receiver (or a few errors, depending on the additional coding using in the
transmitter). These single-bit errors (or few bit errors) will be detected by
the CRC. And the descrambler will remain usually synchronized even in the
presence of isolated transmission errors. So, no avalance of errors will be
produced as long as the descrambler remains synchronized.

In order for the descrambler to loose synchronization something catastrophic
has to occur, for instance the closening of the eye at the equalizer in the
receiver (if there is an equalizer there) and the failure of the timing
recovery circuit of the receiver.

Jaime E. Kardontchik
Microlinear
San Jose, CA 95131
email: kardontchik.jaime@xxxxxxxxxxx


Mick Seaman wrote:

> 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.