Re: [Fwd: 1000BASE-T PCS question], Importance of DC Balance

["Simon Sabato" <simons@level1.com>]

Gents, 

One question that I'd like to ask of the transceiver vendors.  If we do
specify a short-run-length coding, are we going to pay for the more complex
solution anyway?  I heard yesterday a comment to the effect that at least
the PLLs between a particular vendors short-run-length and long-run-length
transceivers was practically identical.  

It would be a shame to specify a short-run-length code on the basis of
saving cost if market considerations (i.e. targetting parts to several
different areas) will mean that we still pay for technology that we don't
need.  Conversely, if we're going to pay for it, we may as well at least
consider using it to reduce the overhead in our coding scheme (whether or
not we go with scrambling or just a coding scheme with longer worst case
run-lengths). 

Finally, I think that any discussions of an "attack" should include the
consideration that there are other ways in which systems are (in the real
world) regularly compromised.  Ensuring a reasonable margin of safety
compared to other methods in which the system could be attacked should be
possible, I'd imagine. 

Just a thought (not even worth 2 cents), 

-Simon

----------
> From: widmer@us.ibm.com
> To: rtaborek@transcendata.com
> Cc: mritter@us.ibm.com; jfewen@us.ibm.com; dlrogers@us.ibm.com;
soyuer@us.ibm.com; meghelli@us.ibm.com; mwsachs@us.ibm.com;
stds-802-3-hssg@ieee.org
> Subject: Re: [Fwd: 1000BASE-T PCS question], Importance of DC Balance
> Date: Thursday, June 03, 1999 5:54 AM
> 
> 
> How important is DC Balance? This question is best answered by the
engineers who
> design the critical three circuits (Laser Driver, Receiver Preamplifier,
Clock
> Recovery), the persons who package the electrical and optical components,
and
> those who design the verification and production tests. Given an option,
they
> generally prefer a code with DC balance and a short run length. After
> consultation with colleagues active in those endeavors, I can offer the
> following list of circuit related advantages of a transmission code such
as the
> Fibre Channel 8B/10B code:
>    Level settings of the laser driver bias point  and the receiver
threshold can
>    be based on the average signal level which is simpler and more precise
than
>    using level restoring circuits. The receiver level restore circuits
usually
>    require some type of peak detection circuits which are difficult to
implement
>    if the electronics is pushed to its limits. Peak detector noise may
cause
>    higher noise levels than otherwise expected because of the peak
detectors
>    tendency to capture occasional large noise excursions. The design of a
peak
>    detector which is both accurate and fast requires difficult inherent
>    compromises.
>    Thermal cycling of lasers or LED's is eliminated.
>    Capacitive coupling  and level shifting is possible without
complications to
>    accommodate various package, grounding, and power supply
configurations at
>    the transmitter or receiver. At frequencies above 5 GHz it is hard to
find
>    capacitors which work well with unbalanced bit patterns for various
reasons.
>    Capacitive differential coupling at the front end of optical receivers
with
>    small integrated capacitors is more easily accomplished and provides
better
>    noise margins. At the lower data rates, designers may still include
offset
>    compensation circuits with a balanced code to reduce the capacitance
values
>    to a range compatible with integration in monolithic circuits. Such
>    compensation circuits require less precision and complexity for a
balanced
>    code.
>    Receivers at the end of computer or LAN links generally require a
large
>    dynamic range which is more readily achieved with a balanced code.
>    Attenuation in electrical package interconnects is on the order of 1
dB/cm at
>    the fundamental frequency of 10 GHz and much higher for the
frequencies
>    needed to transmit fast pulse edges. Any transmission line is easier
to
>    equalize for balanced codes because of the lower ratio of the maximum
to
>    minimum frequency content.
>    It is desirable to set the low frequency cutoff of receivers as high
as
>    possible to remove noise from several   sources, such as: power supply
noise,
>    low frequency modal noise arising from movements of multimode fibers,
or 1/f
>    noise of front end devices, especially GaAs devices not optimized for
low
>    noise analog operation . For low and moderate cost highly integrated
designs
>    it is usually not possible to pick the best devices which otherwise
might be
>    used.
>    The shorter run length of a good code allows much relaxed
specifications for
>    the clock recovery circuit. The lower Q of the PLL enables it to cope
with
>    more external noise interference such as digital noise coupling from
>    neighboring circuits, power supply variations,  or totally external
>    electromagnetic interference. It is less problematic to place a  PLL
with a
>    lower Q on a large digital chip with limited isolation for a fully
integrated
>    solution. The low pass filter of the PLL is more readily implemented
with an
>    on-chip capacitor or a totally digital solution (random walk filter)
and the
>    phase comparator is simpler for the coded version.
>    For links carrying scrambled traffic, the link jitter budget expressed
as a
>    percentages of a baud interval  allows much less jitter for the
transmitter
>    which significantly complicates the design of the frequency
synthesizer, the
>    laser driver, and the connection between the driver and the laser.
>    Simpler circuits consume less power in a critical area.
>    Scrambled data requires nearly ideal circuit implementations in the
areas
>    discussed above. Cost considerations, design time  and skill
limitations make
>    the attainment of near perfection for the 10 Gb Ethernet application
an
>    unrealistic goal. Less than perfect circuits have a greater hidden
cost in
>    terms of signal to noise ratio for scrambled data. For a given baud
and error
>    rate, the coded link can span a longer distance with less
sophisticated
>    circuits.
>    The design, performance simulation, test, and trouble shooting is
simplified
>    for a well constrained code. The robust operation of the coded link
depends
>    on no assumptions about the data pattern of the traffic. The
performance can
>    be proven with a few well defined worst case patterns tailored for
stressing
>    the major performance parameters. There is no exposure to hacking via
the
>    data pattern. This is in contrast with scrambled links for which
performance
>    in practice  can only be verified for a statistical consensus pattern
and
>    where it is always possible to come up with data patterns which cause
the
>    system to fail.
> 
>