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RE: Wide Area Networking for the Rest of US - the debate on BER and other issues


I like your idea of implementing native 10xGBE for intermediate long haul
and WAN, which is a good move.  The advantage you are mentioning will
greatly reduce the cost to users. 

It is true, in a TCP/IP links, the TCP flow control causes more
retransmission than BER. Therefore, the extremely low BER, 10^-15, does not
necessarily gain any more advantage than the specified BER of 10^-12. 

Ed Chang  

-----Original Message-----
From: Bill St. Arnaud []
Sent: Friday, May 28, 1999 8:52 AM
To: bin.guo@xxxxxxx; rtaborek@xxxxxxxxxxxxxxxx;
Cc: stds-802-3-hssg@xxxxxxxx; sachs@xxxxxxxxxxxxxx; "widmer@xxxxxxxxxx
widmer@xxxxxxxxxx widmer"
Subject: Wide Area Networking for the Rest of US - the debate on BER and
other issues

I have been following the interesting debate about BER. Let me bring some
further issues into the debate.

I am assuming that on WAN and long haul GbE the upper layer protocol will
only be IP.

On most IP links, even ones with BERs of 10^-15 there is about 1-3% packet
loss and retransmission.  This is due to a number of factors but most
typically it relates to TCP flow control mechanism from server bound
congestion (not network congestion) and the use of WRED in routers.

So, on most IP links the packet loss due to BER is significantly less than
that due to normal TCP congestion.  As long as that ratio is maintained it
is largely irrelevant what the absolute BER value is.  There will be many
more retransmissions from the IP layer than there will be at the physical
layer due to BER.

Other protocols like Frame Relay and SNA are a lot more sensitive to high
BERs.  IP ( in particular TCP/IP) is significantly more robust and can work
quite effectively in high BER environments e.g. TCP/IP over barbed wire.

I would like to suggest that the 802.3 HSSG group consider an 2 solutions
for 10xGbE WAN:
(1) native 10xGbE using 8b/10b; and
(2)10xGbE mapped to a SONET STS OC-192 frame

For extreme long haul solutions SONET makes a lot of sense as a transport
technology.  However for intermediate long haul (up to 1000 km) and WAN
native 10xGbE is more attractive. Native GbE can be either transported on a
transparent optical network or carried directly on a CWDM system with
transceivers. In medium range networks coding efficiency is not as important
as it is in long haul networks. If coding efficiency is important then in my
opinion, it does not make sense to invent a new coding scheme for 10xGbE
when it would be just as easy to map it to a SONET frame.

The attraction of native 10xGbE for the WAN is that it is a "wide area
networking solution for the rest of us".  You don't need to hire specialized
SONET engineers to run and manage your networks.  The 18 year old kid who is
running your LAN can now easily learn to operate and manage a WAN.

In Canada and the US, there are several vendors who are willing to sell dark
fiber at a very reasonable cost.  Right now the cost of building a WAN with
10xGbE and CWDM is substantially less (for comparable data rates) than using
SONET equipment.


Bill St Arnaud
Director Network Projects



> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx]On Behalf Of
> bin.guo@xxxxxxx
> Sent: Thursday, May 27, 1999 7:28 PM
> To: rtaborek@xxxxxxxxxxxxxxxx; dwmartin@xxxxxxxxxxxxxxxxxx
> Cc: stds-802-3-hssg@xxxxxxxx; sachs@xxxxxxxxxxxxxx; "widmer@xxxxxxxxxx
> widmer@xxxxxxxxxx widmer"
> Subject: RE: 1000BASE-T PCS question
> Rich,
> The DC balance can be directly translated into jitter (when timing is
> concerned) and offset (when threshold slicing is concerned).  You
> only need
> to deal with the former if the signal is 2-level NRZI, while you need to
> deal with both if multi-level signal modulation is used.
> For long term DC imbalance, it translates into low frequency jitter and if
> it's low enough(<1 KHz ?), it's called baseline wonder.  For
> short term, it
> relates to Data Dependent Jitter, which is more difficult for timing
> recovery to handle since it's not from system or channel imparity, and
> therefore it's harder to compensate.
> When you have a lot of jitter margin, for example in lower speed clocking,
> the amount of jitter, translated from DC drift resulted from data
> imbalance
> coupled by AC circuit, percentage wise is a small portion of the clock
> period and therefore does not contribute to much of the eye
> closing.  On the
> other hand, for high speed clocking at 10G (100 ps?), the jitter
> translated
> from the same amount of DC drift can be a significant portion of the clock
> period, so contributes to much large percentage wise jitter which
> results in
> reduced eye opening -- higher BER.
> Dave said in his mail that "The limiting factor is enough RX optical power
> to provide a sufficiently open eye." but you still have to deal with the
> data dependent jitter due to DC imbalance generated after O/E, that can
> close the eye further again.
> Bin
> > -----Original Message-----
> > From:	Rich Taborek [SMTP:rtaborek@xxxxxxxxxxxxxxxx]
> > Sent:	Thursday, May 27, 1999 3:23 PM
> > To:	David Martin
> > Cc:	HSSG_reflector; Sachs,Marty; Widmer,Albert_X
> > Subject:	Re: 1000BASE-T PCS question
> >
> >
> > Dave,
> >
> > Do you know of any research or other proofs in this area? You say that
> > lower speed SONET links regularly achieves BERs of < 10 E-15. I have
> > substantial experience with mainframe serial links such as ESCON(tm)
> > where the effective system BERs are in the same ballpark. SONET uses
> > scrambling with long term DC balance and ESCON uses 8B/10B with short
> > term DC balance. The following questions come to mind:
> >
> > - How important is DC balance?
> > - How does this importance scale in going to 10 Gbps?
> >
> > I'll see if I can get some 8B/10B experts to chime in here if you can
> > get scrambling experts to bear down on the same problem.
> >
> > --
> >
> > >(text deleted)
> > >
> > >The point here is that the SONET scrambler is not the limiting issue in
> > >achieving low error rates. The issue is having enough photons/bit, or
> > >optical SNR (eye-Q) to accurately recover the data.
> > >
> > >...Dave
> > >
> > >David W. Martin
> > >Nortel Networks
> > >+1 613 765-2901
> > >+1 613 763-2388 (fax)
> > >dwmartin@xxxxxxxxxxxxxxxxxx
> > >========================