Re: Wide Area Networking for the Rest of US - the debate on BER and other issues
- To: "bill.st.arnaud" <bill.st.arnaud@xxxxxxxxxx>, l_d_miller@xxxxxxxxxxxxx, stds-802-3-hssg@xxxxxxxx
- Subject: Re: Wide Area Networking for the Rest of US - the debate on BER and other issues
- From: Paul Gunning <paulg@xxxxxxxxxxxx>
- Date: Sat, 05 Jun 1999 14:39:43 +0200
- Organization: Futures Testbed, BT Laboratories
- References: <NBBBJIMEPHPGCNGAHPMFEEDNENAA.bill.st.arnaud@xxxxxxxxxx>
- Sender: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
Hi Bill,
I think the BER knob is a bad idea. Isn't one of the points
of the OSI "layering" model to "hide" or abstract-away a lot
of details about lower layers? Why not imagine (and aim
for) the physical layer as being an "error-free" medium.
Period!
By that I mean external noise effects that affect the
physical transmission of data are controlled or minimised
appropriately so that a "bit-error" i.e. the physical
tranformation of a data '1'-bit to a data '0'-bit
(and vice versa) just doesn't occur.
It then follows that physical layer-derived frame errors
don't occur simply because all the physical bits within
the frame are uncompromised by external physical effects.
Any "errors" prompting re-transmission can then be assumed
to occur at L2, L3 & L4. These are non-physical errors
because they are due to the effects of statistical multiplexing
like contention, buffer overflow, and so on.
With this in the back of ones' mind it follows that realistic
values for L2, L3 and L4 errors can be used to specify the BER
value required at L1 to render the transmission medium error-free.
Maybe I'm mixed up, but might it be the case that two culture are
meeting
for the first time? On the one hand there are those with a datacoms
background who have a bag of useful techniques to deal with physical
layer errors by using redundancy and appropriate coding techniques
to identify and correct transmission errors. On the other hand there
are those with an optics comms. background who always aim to provide an
error-free transmission medium via the appropriate use of data
regenerators.
To put this into perspective. I was horrified (and amazed) that
mobile telephony worked despite physical layer error rates of 1x10^-4!!!
I was equally amazed when an optics chap from Nortel, during an MSc
lecture
I attended, told the assembled class some anecdotes about how
"error-free" (i.e. 1 x 10^18 or better) optical fibre-based transmission
systems can be---and how the affect of a butterfly flapping its wings in
Tiannamen Square could lead to a power surge in the mains supply in an
optics lab in the south of England...
With the application of Moores law after adoption "lower cost" lasers
etc.
become higher cost in the long run because of the compromises that have
to be adopted.
If you want a BER "knob" then consider what I wrote earlier (which
mysteriously doesn't appear on the Email Archive.) This suggests the
use of optical modules (850nm, 1300nm, & 1550nm) akin to GBICs
appropriate to the transmission distance required for serial 10xGbE.
All can be specified to a certain BER for the transmission media
(i.e. single mode, multimode) and distance:
Date: Fri, 28 May 1999 19:32:01 +0200
From: Paul Gunning <paulg@xxxxxxxxxxxx>
Organization: BT Laboratories
To: Iain Verigin <Iain_Verigin@xxxxxxxxxxxxxx>,
bill.st.arnaud@xxxxxxxxxx
CC: bin.guo@xxxxxxx, "rtaborek@xxxxxxxxxxxxxxxx"
<rtaborek@xxxxxxxxxxxxxxxx>,"dwmartin@xxxxxxxxxxxxxxxxxx"
<dwmartin@xxxxxxxxxxxxxxxxxx>, "stds-802-3-hssg@xxxxxxxx"
<stds-802-3-hssg@xxxxxxxx>,sachs@xxxxxxxxxxxxxx,widmer@xxxxxxxxxx
Subject: Re: Wide Area Networking for the Rest of US - the debate on
BER and other issues
Why not...
Layer 3 IP
Layer 2 Ethernet (10GbE = serial 10Gbit/s -> 12.5GBaud
with 8B/10B)
Layer 1 Optics (N * 12.5 GBaud via N WDM channels)
then have a generic PHY termination onto which one of three possible
optical "modules":
a) 850nm VCSELs for short reach using multimode fibre (<300m)
b) 1300nm Fabry-Perot or VCSELs for short-to-intermediate reach
(<30km)
c) 1550nm temperature controlled DFB lasers for long reach (>30km)
can be plugged? Want to increase capacity? Use WDM! (Just consider
each WDM channel as a "virtual" fibre!)
Option b) can make use of mature 1300nm TWSOA (Travelling Wave
Semiconductor Optical Amplifier) technology to extend the link length.
Option c) can make use of mature EDFA technology to extend the
transmission span. Moreover the use of temperature control "locks"
the position of the DFB wavelength i.e channel, within the EDFA
bandwidth which is 50 THz of bandwidth (anyone for Terabit Ethernet?)
assuming 1535nm <-> 1565nm; 1580nm <->1610nm are used.
Consequently a long-distance, optically-amplified, fibre
transmission path could happily carry a mixture of
SONET/SDH _AND_ 10 GbE traffic simultaneously. For example,
1535nm <-> 1565nm would carry 16 * 10 GbE channels;
1580nm <-> 1610nm would carry 16 * OC-192/STM-48 channels?
Each distinct channel is assigned a distinct wavelength.
Use simple passive "blue/red" WDM couplers to MUX/DEMUX
the SONET wavelength band from the GbE wavelength band
at the termination points at either end of the trabsmission
span.
That's 320 Gbit/s of data!!!
For the 10GbE standard is it _REALLY_ necessary to map 10 GbE
into SONET/SDH frames? Why not aim for simplicity? For long distances
over grey fibre (i.e. partially "lit"= available optical amplifer
bandwidth not fully exploited; dark fibre is "unlit") this is
desirable. Remember SONET and 10 * GbE can co-exist happily. Moreover
1300nm intermediate haul 10 GbE could also be carried along with the
1550nm traffic mix!
Has anyone considered exploiting Air Blown Fibre e.g.
http://www.vector-resources.com/abf.htm
as a simple method for installing and/or upgrading the fibre
plant within a building or campus to single-mode?
Finally, serial 100 GbE will be possible soon. It will use a
pragmatic mix of optics and electronics. Therefore the choices
that are adopted for 10GbE will have a crucial bearing on the
ease with which serial 100GbE will become a reality.
(Note that by using WDM as "virtual" fibre 10 * 100GbE = 1 TbE !)
So to conclude...
Keep it serial, keep it simple. Keep the physical layer "error-free"!
Paul.
Bill St. Arnaud wrote:
>
> Larry:
>
> What you say may be true for other types of networks. But dropped packets
> and re-transmissions are an essential feature of Internet networks. The
> TCP/IP congestion control mechanisms uses dropped packets as a mechanism to
> signal the source to throttle back the data flow.
>
> In fact many ISPs use a utility called RED ( Random Early Discard ) or WRED
> ( Weighted Early Random Discard) to deliberately drop packets as a mechanism
> to throttle traffic on congested links. Yes this does cause a
> re-transmission, but TCP automatically drops down to a lower speed when this
> happens. As a result on most Internet links about 1-3% of the traffic is
> dropped packet and re=transmissions. However, most of these dropped packets
> are not due to RED but to buffer overflow at the destination receiver.
> SIGCOMM'98 has some excellent papers documenting this behaviour on the
> Internet.
>
> If I have to do packet discard in any event I might as well do it a layer 1
> just as well as at layer 3. More importantly if I am already dropping
> packets for other reasons, then as long as the number of dropped packets
> from BER is less than the number of dropped packets from TCP congestion
> control then the actual BER (whether it is 10^-15 or 10^-8) is irrelevant to
> me.
>
> I am assuming that if 10XGbE is used in the long haul the primary
> application will be to carry Internet traffic. That is why it would be nice
> to have an option for those of use who are running Internet networks to have
> a BER Knob. With a BER knob I may be able to extend my repeater distance,
> use lower cost lasers, etc etc. However, as I said before this may still
> may not be practical because of other issues particularly with respect to
> the non-linear factors that affect BER. But it still might be worth a
> cursory investigation.
>
> Bill
>
> -------------------------------------------
> Bill St Arnaud
> Director Network Projects
> CANARIE
> bill.st.arnaud@xxxxxxxxxx
> http://tweetie.canarie.ca/~bstarn
>
>
>
>
>
> > -----Original Message-----
> > From: owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> > [mailto:owner-stds-802-3-hssg@xxxxxxxxxxxxxxxxxx]On Behalf Of Larry
> > Miller
> > Sent: Wednesday, June 02, 1999 11:38 AM
> > To: stds-802-3-hssg@xxxxxxxx
> > Subject: Re: Wide Area Networking for the Rest of US - the debate on BER
> > and other issues
> >
> >
> >
> > I think the bit is that when you report bad frames upward to higher layers
> > they have to do some work to re-request those frames and that takes much
> > longer than the time actually burned by the dropped frames. Hence, if you
> > get too low of a raw BER you spend all (or maybe more than all)
> > of your time
> > with higher layer thrashing and never get through with the (say) file
> > transfer.
> >
> > This, I think, is the fallacy in Mr St. Arnaud's notion.
> >
> > Larry Miller
> > Nortel Networks
> >
> >
> > -----Original Message-----
> > From: Mike Dudek <mdudek@xxxxxxxxxxxx>
> > To: Chang, Edward S <Edward.Chang@xxxxxxxxxx>
> > Cc: bin.guo@xxxxxxx <bin.guo@xxxxxxx>; bill.st.arnaud@xxxxxxxxxx
> > <bill.st.arnaud@xxxxxxxxxx>; rtaborek@xxxxxxxxxxxxxxxx
> > <rtaborek@xxxxxxxxxxxxxxxx>; dwmartin@xxxxxxxxxxxxxxxxxx
> > <dwmartin@xxxxxxxxxxxxxxxxxx>; stds-802-3-hssg@xxxxxxxx
> > <stds-802-3-hssg@xxxxxxxx>; sachs@xxxxxxxxxxxxxx <sachs@xxxxxxxxxxxxxx>
> > Date: Tuesday, June 01, 1999 5:42 PM
> > Subject: Re: Wide Area Networking for the Rest of US - the debate
> > on BER and
> > other issues
> >
> >
> > >
> > >Agreed, but the percentage of good frames stays the same. ie the
> > percentage
> > >bandwidth used for retransmissions is the same.
> > >
> > >"Chang, Edward S" wrote:
> > >
> > >> Mike:
> > >>
> > >> If the BER is maintained the same for both GbE and 10xGbE and assume
> > >> everything is equal, the frequency of getting error from 10GbE is 10
> > times
> > >> than GbE from PHY. Of course, the whole system has other factors to be
> > >> included to find the final throughput. In another word, the occurrence
> > of
> > >> frame error will be much more for 10GbE than GbE.
> > >>
> > >> I may present mathematical analysis in July, if my time is allowed.
> > >>
> > >> Ed Chang
> > >> Unisys Corporation
> > >>
> > >> -----Original Message-----
> > >> From: Mike Dudek [mailto:mdudek@xxxxxxxxxxxx]
> > >> Sent: Tuesday, June 01, 1999 10:07 AM
> > >> To: Chang, Edward S
> > >> Cc: bin.guo@xxxxxxx; bill.st.arnaud@xxxxxxxxxx;
> > >> rtaborek@xxxxxxxxxxxxxxxx; dwmartin@xxxxxxxxxxxxxxxxxx;
> > >> stds-802-3-hssg@xxxxxxxx; sachs@xxxxxxxxxxxxxx
> > >> Subject: Re: Wide Area Networking for the Rest of US - the
> > debate on BER
> > >> and other issues
> > >>
> > >> I do not agree that the BER must be improved with data rate increase in
> > >> order to
> > >> obtain the higher throughput. At least for packet based transmission
> > with
> > >> retransmission of errored packets, the throughput increases in
> > proportion
> > to
> > >> the
> > >> data rate for the same BER, assuming that the packet length (in bytes)
> > >> remains
> > >> fixed. I do not think that anyone has proposed changing the packet
> > length,
> > >> but
> > >> if they did then the BER might have to be improved. The
> > throughput is of
> > >> course
> > >> the number of good packets in any interval of time.
> > >>
> > >> "Chang, Edward S" wrote:
> > >>
> > >> > Bin:
> > >> >
> > >> > Yes, I agree. The BER should be improved with data rate increase, if
> > the
> > >> > through put gained from higher data rate is to be maintained. In
> > addition
> > >> > to the retry times wasted, the external sources of noise remain the
> > same,
> > >> > which further requires the lower BER. These are the correct design
> > goals
> > >> we
> > >> > should work on. Although, we also should keep the cost-effectiveness
> > in
> > >> > mind to maintain optimum balance between performance and cost.
> > >> >
> > >> > Ed Chang
> > >> > Unisys Corporation
> > >> >
> > >> > -----Original Message-----
> > >> > From: bin.guo@xxxxxxx [mailto:bin.guo@xxxxxxx]
> > >> > Sent: Friday, May 28, 1999 4:57 PM
> > >> > To: Edward.Chang@xxxxxxxxxx; bill.st.arnaud@xxxxxxxxxx;
> > >> > rtaborek@xxxxxxxxxxxxxxxx; dwmartin@xxxxxxxxxxxxxxxxxx
> > >> > Cc: stds-802-3-hssg@xxxxxxxx; sachs@xxxxxxxxxxxxxx;
> > "widmer@xxxxxxxxxx
> > >> > widmer@xxxxxxxxxx widmer"@us.ibm.com
> > >> > Subject: RE: Wide Area Networking for the Rest of US - the debate on
> > BER
> > >> > a nd other issues
> > >> >
> > >> > Ed,
> > >> >
> > >> > If the specified BER for 1000BASE-X is 10^ -12, then to have
> > the equal
> > >> > error-free period the specified BER for 10G should be at
> > least 10^ -13.
> > >> > Based on Rich T and Rich S's BER number:
> > >> >
> > >> > A system BER of 10 E - 8 @ 10 Mbps = a bit error every 10 seconds.
> > >> > (10BASE-T)
> > >> > A system BER of 10 E-12 @ 100 Mbps = a bit error every 166
> > minutes, 40
> > >> > seconds. (100BASE-X)
> > >> > A system BER of 10 E-10 @ 1 Gbps = a bit error every 1
> > minutes, 40
> > >> > seconds. (1000BASE-T)
> > >> > A system BER of 10 E-12 @ 1 Gbps = a bit error every 16
> > minutes, 40
> > >> > seconds. (1000BASE-X)
> > >> > A system BER of 10 E-12 @ 10 Gbps = a bit error every 1 minutes, 40
> > >> > seconds.
> > >> > A system BER of 10 E-13 @ 10 Gbps = a bit error every 16
> > minutes, 40
> > >> > seconds.
> > >> >
> > >> > If the TCP/IP is the only protocol 10G PHY needs to support, then the
> > >> above
> > >> > specified BER may be more than enough. Moving from 1G to
> > 10G, the bit
> > >> > period is scaled 10X smaller while jitter and noise from some sources
> > are
> > >> > not scaled the same way -- much tight control should be applied to
> > achieve
> > >> > even the same BER.
> > >> >
> > >> > Bin
> > >> >
> > >> > ADL,AMD
> > >> >
> > >> >
> > >> >
> > >> >
> > >> > > -----Original Message-----
> > >> > > From: Chang, Edward S [SMTP:Edward.Chang@xxxxxxxxxx]
> > >> > > Sent: Friday, May 28, 1999 12:44 PM
> > >> > > To: bill.st.arnaud@xxxxxxxxxx; Guo, Bin;
> > rtaborek@xxxxxxxxxxxxxxxx;
> > >> > > dwmartin@xxxxxxxxxxxxxxxxxx
> > >> > > Cc: stds-802-3-hssg@xxxxxxxx; sachs@xxxxxxxxxxxxxx;
> > "widmer@xxxxxxxxxx
> > >> > > widmer@xxxxxxxxxx widmer"@us.ibm.com
> > >> > > Subject: RE: Wide Area Networking for the Rest of US - the
> > debate
> > >> on
> > >> > > BER a nd other issues
> > >> > >
> > >> > > Bill:
> > >> > >
> > >> > > 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 [mailto:bill.st.arnaud@xxxxxxxxxx]
> > >> > > Sent: Friday, May 28, 1999 8:52 AM
> > >> > > To: bin.guo@xxxxxxx; rtaborek@xxxxxxxxxxxxxxxx;
> > >> > > dwmartin@xxxxxxxxxxxxxxxxxx
> > >> > > Cc: stds-802-3-hssg@xxxxxxxx; sachs@xxxxxxxxxxxxxx;
> > "widmer@xxxxxxxxxx
> > >> > > widmer@xxxxxxxxxx widmer"@us.ibm.com
> > >> > > Subject: Wide Area Networking for the Rest of US - the
> > debate on BER
> > and
> > >> > > other issues
> > >> > >
> > >> > >
> > >> > >
> > >> > > All:
> > >> > > 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
> > >> > >
> > >> > >
> > >> > >
> > >> > >
> > >> > >
> > >> > > -------------------------------------------
> > >> > > Bill St Arnaud
> > >> > > Director Network Projects
> > >> > > CANARIE
> > >> > > bill.st.arnaud@xxxxxxxxxx
> > >> > > http://tweetie.canarie.ca/~bstarn
> > >> > >
> > >> > >
> > >> > >
> > >> > >
> > >> > >
> > >> > >
> > >> > >
> > >> > > > -----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"@us.ibm.com
> > >> > > > 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
> > >> > > >
> > >> > > > ADL, AMD
> > >> > > >
> > >> > > > > -----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
> > >> > > > > >========================
> > >> > > >
> > >
> > >
> >
> >
> >
--
Paul Gunning
Futures Lab
BT Laboratories
Phone: +44 1473 647049
Fax: +44 1473 646885
http://www.labs.bt.com/people/gunninp
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