10 GbE en MASse PMD
Rich,
I don't think anyone is going to be convinced on MAS
either way before I start my Thanksgiving Holiday
Friday evening. So try to keep the nerdherd moving forward
while I am out, and I'll pick this bone with you on 1 Dec.
Meanwhile, some things for all of you to chew on
as a side dish:
1. Historically, Ethernet has utilized the optical
technologies developed by at least several companies
with some key fielded product.
2. Our efforts are directed at producing a standard.
This assumes (1) above. Otherwise, we ought to define
the 10GbE HSSG as a technology development or research body.
3. The primary objective for 10GbE must be the LAN environment.
Solutions already exist for the MAN/WAN. If 10GbE is not
successful in the LAN, it will have no base for penetrating
the private WAN much less the public network. The reason we
are debating how to extend 10GbE into the WAN is because it
has been so succesful as a LAN technology.
4. There are big differences between the GbE LAN and the SONET
public network. Two important ones which come to mind are
latency and scalability. These two network topologies have
been designed with different priorities in mind.
When (1) and (2) are considered together, it becomes clear we
probably ought not standardize a new signalling technology which
has not been fielded in the past.
Number (3) forces me to acknowledge one important plank of Rich
Taborek's platform. Namely, most of the LAN is multimode fiber.
Therefore, careful consideration must be given to any way we can
possibly extract more bandwidth from the installed fiber base.
Otherwise, we limit ourselves to the MAN/WAN where single mode
fiber is predominant, or to those users willing to pull new fiber
to accomodate the new 10GbE LAN technology.
Number (4) tells me we are always going to be a little bit behind
the TDM guys and their 40Gb X 50 channel 600km solutions in terms
of raw bandwidth. We ought not to be embarassed, because TDM
networks are so much easier to scale. All they have to do is figure
out how to make faster and narrower linewidth lasers and better
fibers where cost is no object. No need for bandwidth envy however,
because Ethernet dominates in the raw numbers of connections due
to the accessibility and cost advantages it provides. Besides, we
get to use their components when we upgrade our networks.
So I'll get down to business here with the real meat of my comments
intermingled with the sweet sauce of Rich's original recipe text.
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At 12:24 AM 11/18/99 -0800, you wrote:
>
>Patrick,
>
>The flip side of the argument is that traditional optical communications
links are
>
>among the last holdouts to fall to modulation methods more efficient than
binary
>signaling. For all telecom and datacom applications to date there has been no
>reason to deviate. Fiber (SMF) has unlimited bandwidth for all practical
purposes.
>
>However, 10 Gbps datacom applications require low cost, not just cost
effective
>optical communications links. This is the primary reason for investigating
>alternative modulation methods.
>
>I don't understand your comment about fundamental and practical issues of
laser
>physics. Semiconductor lasers exhibit excellent small-signal modulation
>performance and are generally limited in practice to a bandwidth of 10 GHz
because
>
>of electrical parasitics. However, since semiconductor lasers are almost
>exclusively employed in optical communications links and binary modulated,
>small-signal performance is not applicable and only large signal
performance is
>relevant. Tradition certainly has its place, but...
>
(original text omitted)
Points well taken. Actually, the self resonances of many
semiconductor laser cavities limit their usability as optical
sources well below 3.5GHz. However, it is no stretch to assume
we will have access to lasers with bandwidths in excess of 10GHz
as you have stated.
You have also hit the mark with your statement about packaging
parasitics as the major obstacle to enhancing laser bandwidth.
I also agree with your object of investigating MAS and other
bandwidth enhancing techniques, but with a different motivation.
Many a transmitter is better when properly enhanced, but....
As I stated in my ambling preamble, the reason for bandwidth
enhancement must be the installed base of fiber, not just cheap
lasers. There are no compelling cost advantages to a surface
emitting data communications VCSEL over a edge emitting Fabry Perot,
otherwise a VCSEL wouldn't cost twice as much as a CD laser. The
same goes for a DFB or a FP. The chip costs the same, but the
packaged DFB laser could easily cost 10 times as much. The real
issues are volume, packaging, testing, and certification.
So there is no reason to rule out external modulators for instance.
If you look at the bill of materials for an external modulator, you
might want to wonder, "Is that all there is?" Apologies to all you
Lithium Niobaters out there, but the same material with a different
cut is used as a crystal in that cheap clone you play games with every
night.
What you are terming large signal performance is my small signal
performance. It is all small, and noise is getting to be a problem
ever since we decided to abide by those weak-eyed Europeans' laser
safety rules. Couldn't we just buy them some sunglasses?
-----------------------------------------------------------------
>A simple offset correction to set bias above threshold is generally chosen
by a
>designer of an optical transmitter employing binary signaling. Conversely, a
>designer of an optical transmitter employing multilevel signaling may choose
>instead to set laser bias around average optical power and then employ simple
>optical intensity modulation via a modulation current to set n levels
above and
>below the bias point. For example, for PAM5, one could set 2 levels above and
>below the bias point. The lowest level could correspond to a "0" light
level for a
>
>binary signaled system and the highest level could correspond to a "1".
>
(original text omitted)
Sounds like we need a very fast D/A with a lookup table to linearize
the non linearities of our nasty little lasers
--------------------------------------------------------------------
>
>I certainly don't want to give anyone the impression that I'm an optical
expert,
>and I fully understand that there is a significant amount of research to
be done
>in the area of multi-level optics. However, my gut tells me that
multilevel optics
>
>is an engineering exercise with no real "walls" or "cliffs" in sight. In
addition,
>
>tools such as closed-loop full-duplex feedback, virtually unlimited
functionality
>afforded by CMOS usage within a transceiver, modulation methods which reduce
>direct modulation rates, etc. greatly simplify the engineering exercise. I
warmly
>welcome optical experts to join in with protocol and systems experts to
arrive at
>well engineered multilevel optical communications link.
>
(original text omitted)
I'm was just blowing smoke about the optics thing myself....
We are all students of this thing. I'm hoping to eke out a "B"
in Lab. I just don't want to follow the rest of you lemmings over
the cliffs into a MASsive sea of troubles full of sea-moss.
----------------------------------------------------------------
>You're over my head with the above explanation. If you can, please
translate it
>into an effect which affects the multi-level eye.
>
(original text omitted)
The modal effect results in a jitter penalty primarily. However,
this might be mitigated by the newest class of VCSELs which are
very narrow linewidth, low threshold, and quasi-single mode. Even
so, multimode fiber makes fools of us all.
-----------------------------------------------------------------
>
>My calculations for SNR reduction due to PAM5 are 6 dB electrical and 4.1 dB
>optical. Please explain the difference.
>
>PAM5 is a modulation method which is independent of laser type. It's clear
that a
>"clean" multilevel system can be constructed with DFB/DBR lasers and a
matching
>single mode fiber. However, I see no reason whatsoever to exclude any other
>reasonable combination of laser and fiber. Reasonable means good small-signal
>performance, reasonably low noise, etc.
>
(original text omitted)
I have searched your presentation material and have not found
your mathematical derivation so I am in no position to judge.
You will note my figure of 7dB was poised as an estimate and
indeed it was because I just realized I was analyzing a system
with five eyes, which would indicate six levels. I should have
realized you only have four eyes. Therefore, I must retract 1dB
from my previous offer, which leaves you with only six. This is
my last, best, and final offer.
>Anyone with bare bones lab equipment and a high-speed Arbitrary Waveform
Generator
>
>can put together a working MAS prototype system and experiment with
various lasers
>
>and fibers. I don't know of an AWG which can operate at 5 Gbaud, but you
can get a
>
Sorry, my American Wire Gauge tables only go up to 40. And bare
or not, the bones of my lab equipment are looking quite good to me
right now. Maybe we should ask Howie for a PAR for a High Speed Arbitrary
Waveform Generator Study Group (HSAWGSG).
------------------------------------------------------
>pretty good idea of Gbaud range multilevel operation with this setup. I
don't want
>
>to plug a specific vendors product over this reflector so please do your own
>search for "Arbitrary Waveform Generator" on the web.
>
>Best regards,
>Rich
>
>---dashedlinedashedlinedashedlinedashedlinedashedlinedashedline-----
Actually, I would be quite interested to take a further look.
Possibly you could send me a private e-mail with the name and
phone number for this equipment. My boss is starting to wonder
why I haven't submitted any CARs this week. If anyone has a
good recipe for microwaving a small turkey please post it ASAP.
HANDYGs,
Pat Gilliland
patgil@xxxxxxxxxxx
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