Re: 10xGbE on DWDM

[Paul Gunning <paulg@xxxxxxxxxxxx>]

Hi Bill,

Bill St. Arnaud wrote:
> Paul:
> > I fully concur. I would welcome a pointer as to where I could
> > find out some more information about the SDL protocol(s). It(they)
> > sounds very, very interesting.
> 
> Lucent has been doing a lot of work in this area.  I can't point you to a
> specific paper, but I have seen numerous presentations from Lucent on SDL.
> Their web site might be a good place to start.


Cheers. I'll take a look.


> > I realise that. But is there a requirement for additional
> > frame overhead in that you need a contiguous sequence
> > of preamble "training" 'bits' at the front of the frame
> > to phase sync. the Rx PLL?
> 
> If you have loss of sync that may be required, but in general I don't
> believe there is a requirement for "training" bits


OK my misunderstanding. Apologies. I thought I read in one of the GigE
books
that the start of a frame had a contiguous sequence of '1'-bits for 
synchronising the Rx PLL. 


> > Agreed. But there ARE clock management issues between
> > adjacent nodes on a DPT ring in that you have to
> > explicitly nominate a master and slave relationship.
> 
> Hmmm.  I will have to check into this.  DPT and Nortel's IPT which is very
> similar allow for auto-discovery etc which implies that there is no
> master/slave relationship.
> 
> > Unfortunately I can't attend the meeting in Idaho
> > but I would be very interested to learn of any proposals
> > for 100xGBe.
> 
> I remain skeptical that we will see serial speeds beyong 10Gbps or "all"
> optical switches in the near future.  The problem with serial speeds in
> excess of 10 Gbps is the processing required at switching or routing nodes.
> Every router manufacturer I have talked to has found it exceedingly
> difficult to build router interfaces that can work at 10 Gbps line speeds.
> To do this a router must process an incoming packet, perform a 256,000 entry
> address table lookup and the forward the packet across a backplane in a few
> nanoseconds.  At these speeds we are closely the physical limitation of
> light propagation across the phsical dimensions of the box.


Well the ability is there. When you speak of interfaces I assume that
you
mean electronic/optoelectronic. I was referring to all-optical
techniques:

	http://www.labs.bt.com/pressoffice/archive/1995/ns6-95.htm

I believe you'll see all-optical switches sooner rather than later..


> Right now the I believe the propagtion time of an electrical signal for
> "copper on copper" semi-conductors is comparable to the speed of light
> through glass. 


It is.

> But more importantly is the small component size of
> semi-conductors versus existing opto-switching devices.  At these speeds the
> size of components and the length of their interconnection path becomes very
> critical for high speed processing.  As I understand it when you try to
> reduce optical components to the size and density as semi-conductors you are
> approaching the wavelength distance of the actual light path which causes a
> whole set of new refractive and light bending problems.  There was an
> excellent article in the last issue of Scientific American on this topic.


I agree. Electrons with their short wavelengths can (and do) make use of 
the "natural periodicity" of the atomic lattice, making use of
"forbidden"
band gaps etc.. Photons being "bigger" require micron-sized feature
sizes. 
However several research groups are researching this area for the
reasons you state
and are looking at ways of produding photonic bandgap (PBG) structures
that do not
suffer from the size-limitations you describe. I would emphasise that it
is 
VERY early days, but it may become feasible in time to exploit PBGs.
Certainly
the fact that light doesn't interfere with itself, unlike electrons, may
allow
a better exploitation of the 3-rd spatial dimension. Actually this is
drifting
a little away from the topic at hand.


> Finally when you have serial speeds in excess of 10 Gbps you run into a
> whole set of new non-linearities in the fiber itself. Polarization mode
> dispersion, for example becomes very significant and now cannot be ignored.
> Jitter and BER becomes increasingly tougher and tougher as well.


Absolutely agree. PMD is a bag of worms.


> I think someday we will reach these speeds but maybe not serially,  perhaps
> through Silkroad's or Transact's technology solutions.  But I don't see it
> happening for several years.
> 
> >
> > Yes. But deployment economics would tend to prefer DWDM
> > so as to maximise bandwidth per physical fibre. DWDM allows
> > you to have many additional "virtual" fibres per physical fibre.
> 
> The same economics apply to supercomputers.  The cost per memory bit of a
> supercomputer is significantly less than that of a a standard PC.  It would
> be more economical to move all our applications to supercomputers. But you
> don't see many people doing that.


Sure but the cost in terms of your electricity bill would be
astronomical.
Anyway supercomputers will be available from a toy store near you anyday
now:

	http://www.pbs.org/cringely/pulpit/pulpit19990401.html

http://www.consoledomain.com/psx/articles/Playstation_2_Its_Official.html

Chances	are they'll want to communicate with one another!!!!

> I think the same rationale will apply to DWDM.  Yes, the cost per bit is
> signficantly less than CWDM and yes you can pack in many virtual paths.  But
> the upfront capital is very high.  More importantly you are tied to the
> fortunes of your favourite carrier.  Whereas if I can get access to dark
> fiber I can install my own CWDM system at significantly less upfront cost
> and I am in control of my own destiny.
> We have actually been going through a detailed economic exercise on this
> very topic for a 1700 km regional network here in Canada.  Even a very big
> manufacturer of DWDM equipment had to admit that CWDM, particularly with
> 10GbE is "significantly" cheaper than DWDM.  Because the regional network is
> in a small province the local carrier could not justify the upfront capital
> cost of a DWDM system.  But even if the carrier had the business case to
> justify the upfront capital cost,  the prorated DWDM cost was not much
> better than the total CWDM 10GbE cost!!


Very, very interesting...and thought-provoking.

 
> Bill


Paul.
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