Line rate and optical span
I've not been following up the reflector for a while, and I don't know
where to jump in, but I'd like to make some comments on the statements
made in earlier mails. (I'm giving these commenst without yet knowing
whether the issue has been properly clarified on behalf of me.)
> You hit on another very good reason for the WAN version to use
> scrambled encoding. Let me rephrase it for emphasis. I believe it is a
> requirement that 10 Gb/s Ethernet be able to ride over existing DWDM
> These spans have already been engineered for 10 Gb/s channels.
> the bit rate would increase the optical bandwidth, and would require
> increasing the optical power as well. Thus an 8B/10B 12.5 Gb/s signal
> not be able to ride on most existing spans, but would instead require
> completely new spans to be engineered. This will not be acceptable to
> carriers. Therefore, using scrambling is clearly a hard requirement
> Gb/s Ethernet over DWDM systems.
The optical span cleary depends on the required optical bandwidth by the
chosen line code. Usually, the logic depends on the two popular notions:
N1) The higher the line rate, the more bandwidth;
N2) The more bandwidth, the shorter the optcial span.
However, N1) will not be true anymore due to the new coding cocept as we
introduced, in the Montreal meeting, with the new code MB810. Since the
code requires only the Nyquist bandwidth, it requires far less bandwidth
than usual non-minimum-bandwidth codes. In high-speed optical
transmission environments wherein fine pulse shaping like usual
Raised-Cosine is hard to implement and thus sqaure-wave pulses are
driven at the transmitter, the bandwidth reduction can be as much as
twice; MB810 uses virtually half the bandwith of usual 8B10B, and what
is more, even 40% less than Scrambled NRZ does.
Thus with this new coding technique, 'the higher the line rate, the more
bandwidth' part is no more true.
Once 'less bandwidth', then we can achieve a longer span, against the
fear of most people. And also, with this new coding technique, the
resultant signal spectrum can well fit into the existing install base
WAN SONE/SDH 10G fiber links.
Therefore, using scrambling (Scrambled NRZ) is NOT a hard requirement
10Gb/s Ethernet over DWDM systems.