RE: 10xGbE on DWDM

["Bill St. Arnaud" <bill.st.arnaud@xxxxxxxxxx>]

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.

>
>
> 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


>
> 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.

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. 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.

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.

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.

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!!


Bill



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