From: Ronald Miller (firstname.lastname@example.org)
Date: Wed Nov 17 1999 - 08:01:32 PST
Pat et al
Back in 76 we tried to develop a bandwidth limiting data
communications scheme called Partial Response. We violated
the Nyquist criterion by setting our bandwidth at half the required
bandwidth for the data rate and ended up with 7 levels. The signal
could not make it from 0 to 7 in one symbol time so the other levels
The end result was that we never finished because of the complexity
of the filtering and also because of the obvious decrease in noise margin
or in signal to noise ratio. Net result - no go.
It looks like this multi-level signaling may be up against the same signal
to noise ratio or noise margin problem. When one considers ripple
in the rails seen in an eye diagram and assumes that these same ripples
will occur at all the levels of a multi-level system the obvious eye-closure
will be a real problem.
However, if the ripple which is caused by reflection from impedance changes
in the transmission lines can be reduced substantially it may work. Can the
connectors, transmission lines and IC pins be made good enough to clean up
this ripple. That is the gating question for this type of system.
Getting back to your origional problem with the laser linearity, I do not believe
that multi-level optical was the intent. I believe that multi-level signaling is
considered for the electrical connection to the optical device which would then
convert it to 10 GB optical.
Patrick Gilliland wrote:
> It is true there are many advantages to the
> use of multi-level signalling in copper based
> Ethernet systems. There are also many other
> examples to choose from to demonstrate the many
> benefits of multi-level data encoding such as the
> PAM5 or QAM4 you have suggested.
> However, these types of communications systems rely
> on linear components at the transmitter and receiver
> ends of the link. I believe you have correctly identified
> this lack of linearity as one of the major problems to
> be dealt with in any multi-level data communications system
> employing a laser as the active element. My experience
> with lasers teaches away from the direction of MAS. While lasers
> can be relied on to some degree to be linear as FM transducers,
> it is difficult enough to get them to work in a bi-level
> AM application such as the 1.25Gbaud Ethernet data link.
> I admit some prejudice in this regard, but I feel the need
> to comment because at some point, if we continue to work
> along this direction, I might be asked to make such a transmitter.
> I also disagree with the assumption behind the premise this
> type of bandwidth reduction is necessary to accomodate the
> optical fibers. The bandwidth of the fibers is clearly sufficient
> to allow for inexpensive 12.5Gbaud transmission. It is no doubt
> already being done by the telecommuncation companies of the world
> such as Lucent, NEC, Alcatel, Nortel, etc. Their methods employ
> single mode fibers and external modulators in many cases.
> The cost of these types of laser transmitters is relatively high
> as you have stated. However, those in the business of producing
> these components realize many of the cost issues are volume and
> certification dependent. Many efforts are presently underway to
> address the cost problem at the present time which may bear fruit
> in a timely manner to the issuance of a 10Gbit Ethernet specification.
> I apologize for my lack of enthusiasm, especially because of the
> tremendous amount of quality work you have obviously put in on this
> proposal. I will also give your proposal some additional thought.
> Maybe there is a way to overcome some of the difficulties which
> are inherent in any multi-amplitude-signalling (MAS) laser based
> optical link. I remain open minded but skeptical on this issue.
> Best Regards,
> Pat Gilliland
> At 03:33 AM 11/16/99 -0800, you wrote:
> >Not at all. I've been pushing multi-level signaling scheme for 10 GbE since
> >March. There are a number of presentation out there. I suggest working
> >starting with the Kauai meeting. Here are the two best links for info:
> >Patrick Gilliland wrote:
> >> Rich,
> >> I have some questions based on my reading
> >> of your latest posting and the presentations
> >> you cited. In one of the studies, a mention
> >> was made of multi-level optical transmission.
> >> Is this a misread on my part?
> >> Pat Gilliland
> >> email@example.com
> >> ------------------------------------------
> >> At 06:54 PM 11/14/99 -0800, you wrote:
> >> >
> >> >The purpose of this note is to clear up confusion regarding Hari, a
> >> >proposed 4-lane serial interface for 10 GbE and train-up sequences.
> >> >
> >> >It should be clear that NO TRAINING SEQUENCES are proposed for Hari.
> >> >Both the "Hari Coding Objectives" presentation
> >> .pdf)
> >> >and "Word Striping on Multiple Serial Lanes"
> >> df)
> >> >make a point of noting that no train-up is required Hari to deskew.
> >> >
> >> >The Hari Coding Objectives proposal uses the standard Idle sequence
> >> >proposed by Howard Frazier of Cisco to deskew multiple parallel lanes
> >> >while simultaneously acquiring code-group synchronization on all lanes.
> >> >
> >> >--
> >> >Best regards,
> >> >Rich
> >> >
> >> > ----------------------------------------------------------
> >> >
> >> >Richard Taborek Sr. 1441 Walnut Dr. Campbell, CA 95008 USA
> >> >Tel: 408-370-9233 Cell: 408-832-3957 Fax: 408-374-3645
> >> >Email: firstname.lastname@example.org
> >> >
> >> >
> >Best regards,
> > ----------------------------------------------------------
> >Richard Taborek Sr. 1441 Walnut Dr. Campbell, CA 95008 USA
> >Tel: 408-370-9233 Cell: 408-832-3957 Fax: 408-374-3645
> >Email: email@example.com
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