I guess where I have
difficulty is the following: Even if one packet is resent every 10 sec, on a
100G link, this is a very tiny proportion of all the packets being transmitted.
Also, in the vast majority of cases, the traffic will consist of millions of
flows, so only one flow in several million will see a single packet
retransmission in any ten second period. Is this really detrimental?
From: Marcus Duelk [mailto:duelk@xxxxxxxxxx]
Sent: Tuesday, August 29, 2006
Subject: Re: [HSSG] BER Objective
I guess you have to look at it also from the "packet world". A
means that TCP does not get an acknowledgment which means that the packet
gets resend. If we target 100 Gb/s (1E11 b/s) and you have a BER of 1E-12
then you have an error every 10 seconds. This is a little too high and will
too many packets being resent. I don't know exactly what the right or good
numbers are but I would guess that BER goals in the range of 1E-14 to 1E-15
will be practical. BER 1E-15 will mean an error every 1E4 (10,000) seconds,
that should be good enough for Carrier Ethernet equipment.
The interesting thing about the time it takes to accumulate an error is
that it depends a lot on the PHY. If you use 100G serial PHY then the
numbers I have given above are correct. If you use 10x10G PHY then
the BER accumulate rather at 10G rate, i.e.
BER100G = [BER10G(1)*BER10G(2)*....BER10G(10)]/10
where BER10G(i) is the BER on each of ten 10G lanes. If they
are all the same then BER10G = BER100G, which means that
the error accumulation time is given by the 10G speed, not the 100G
of this raises the following question: If this is so hard to measure, how
much impact can it really have in the real world? Why not back the BER
requirement off to 10e-10?
I agree with Howard. It is impractical and expensive to test for very
low BERs - the specs should be such that the power budget is capable of
achieving BER =1e-15, yet the testing can be some kind of accelerated BER at
lower value that is derived from the curve interpolation.
the as with any extrapolation of testing results one has to be careful, so in
this case it will be manufacturers' responsibility to guarantee the BER=1e-15.
1101 Kitchawan Road,
Rte. 134 (shipping)
Yorktown Heights, NY 10598
For the 100 Mbps EFM fiber optic links
(100BASE-LX10 and 100BASE-BX10)
we specified a BER requirement of 1E-12, consistent with the
for gigabit links. We recognized that this would be
impractical to test in a
production environment, so we defined a means to extrapolate
a BER of 1E-12
by testing to a BER of 1E-10 with an additional 1 dB of
58.3.2 and 58.4.2.
Sent: Monday, August 28, 2006 1:54 PM
Subject: Re: [HSSG] BER Objective
Prior to 10G, the BER standard (for
optical communications) was set at 1E-10 (155M-2.5G). At 10G, the BER
standard was revised to 1E-12. For unamplified links, the difference
between 1E-12 and 1E-15 is only a difference of 1dB in power delivered to the
PD. However, the larger issue is one of margin and testability (as the
duration required to reliably verify 1E-15 for 10G is impractical as a factory
test on every unit) especially since we’d want to spec worst case product
distribution at worst case path loss (cable+connector loss) and at EOL with
margin. Thus in reality, all products ship at BOL from the factory with a
BER of 1E-15 and in fact nearly all will continue to deliver 1E-15 for their
entire life under their actual operating conditions and with their actual cable
Thus, if by “design
target”, you mean a worst case-worst case with margin to be assured at
EOL on every factory unit, then this is overkill. I might be willing to
entertain a 1E-13 BER as this would imply that same number of errors per second
(on an absolute basis; irrespective of the number of bits being passed; this
takes the same time in the factory as verifying 1E-12 at 10G although this is
in fact a real cost burden which adversely product economics); however, this
would not substantially change the reality of the link budget. It would
make for a sensible policy for the continued future of bit error rate specs
(should their be future “Still-Higher-Speed” SG’s).
From: Martin, David
Sent: Friday, August 25, 2006 12:22 PM
Subject: BER Objective
the discussion on Reach Objectives there didn’t appear to be any mention
of corresponding BER.
the comments from the floor during the July meeting CFI, regarding how 10GigE
has been used more as infrastructure rather than as typical end user NICs. And
that the application expectation for 100GigE would be similar.
on that view, I’d suggest a BER design target of (at least) 1E-15. That
has been the defacto expectation from most carriers since the introduction of
need for strong FEC (e.g., G.709 RS), lighter FEC (e.g., BCH-3), or none at all
would then depend on various factors, like the optical technology chosen for
each of the target link lengths.
David W. Martin
+1 613 765 2901 (esn 395)
Bell Labs / Lucent Technologies
Data Optical Networks Research
Crawford Hill HOH R-237
791 Holmdel-Keyport Road
Holmdel, NJ 07733, USA
fon +1 (732) 888-7086
fax +1 (732) 888-7074