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importance of DC balance

subject: Importance of DC Balance

On June 4, Albert X. Widmer, from IBM, wrote:

> 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 Fiber Channel
> 8B/10B code:

> ... (a list of important technical arguments)


I think that the engineers were not given
the right options nor were asked the right
questions. Restricting the issue to "what coding
would you prefer from the point of view of
DC balance ?" will give you the wrong answer
to the main issues. (Personally, my answer
would be: Manchester encoding. It has one
transition per baud and the clock is already
embedded in the data...).

The answers could be very different if the
same engineers are asked questions of the
following type:

"Give me system architecture alternatives that
can deliver 10 Gbps with a BER of 10^(-12) in a
LAN environment with a cost of about 3 times
the cost of Gigabit Ethernet over fiber and
using the installed fiber for Gigabit Ethernet".

(system objectives ...)

I think that we have to recognize that

 "Bandwidth is not free anymore in optical
  fiber when we go to 10 Gbps"

Installed optical fiber for Gigabit Ethernet
begins to look in some respects as the
telephone wire when we want to increase the
data rate by a factor of 10. Dispersion in
the fiber becomes important. See, for example,
Paul Kolesar's presentation in Idaho, showing
in slide # 15 the catastrophic effects of modal
dispersion in a traditional MMF fiber if used
in a system at 12.5 Gbaud/sec (8b/10b + serial)
or even in a system at 3.125 Gbaud/sec
(8b/10b + 4-WDM, 0.32 nsec baud period): the
modal dispersion in the fiber eats all the
timing budget.

Looking at the system from another point of
view: static SNR. Even neglecting any high
frequency effects on the electronics and optics,
8b/10b+4-WDM incurres a penalty of 8 dB in
SNR compared to 1000BASE-X just because the
bit energy is smaller (smaller baud period,
0.32 nsec vs 0.8 nsec) and the thermal noise
is larger (3.125 GHz bandwidth vs 1.25 GHz
bandwidth of the receiver analog front end).
This increases either the BER and/or reduces
the achievable link length of the 8b/10 + 4-WDM

May be that at 10 Gbps we might begin looking
at the type of techniques used to increase
the data rate over telephone wires: multi-level
signalling and coding gain in order to
restrict the transmitted signal to smaller

We have to judge proposals by defining first
the overall objectives and then looking at the
complete architecture of every proposal and
how it balances the requirements on the
electronics, optics and transmission medium.

I do not say that 8b/10b encoding does not
deserve a place in 10 Gbps Ethernet. We might
define different objectives/options within the
10 Gbps standard (as we already have options
within the present 1000-BASE-X Ethernet standard),
and for some of these options 8b/10b might be
the right choice.


Jaime E. Kardontchik
Micro Linear
San Jose, CA 95131
email: kardontchik.jaime@xxxxxxxxxxx