Some Gigabit Ethernet history -- why are the links so overdesigned?

[gwinn@xxxxxxxxxx (Joe Gwinn)]

At 12:00 PM 99/5/11, Paul Bottorff wrote:
>
>In the LAN code efficiency is less important since cable is free and
>amplifiers are not required. Never the less 1 Gigabit had great trouble
>making the 500 m spec for multimode fiber. Reductions in reach outside the
>standards for building wiring are expensive for LAN applications.

There is a little bit of Gigabit Ethernet history that may be useful here:

If you recall from the GbE Modal Bandwidth Investigation (MBI) effort, the
key issue in setting the guaranteed link lengths was the wide variability
in fiber characteristics (with uncontrolled laser launches), especially for
the dusty legacy fibers of the installed base.  Because the then objective
was to ensure that at least 99% of conforming links would work with
randomly chosen transceivers, the 1% tail of the fiber bandwidth
distribution wagged the dog, resulting in grossly overdesigned (~4:1) links
in the typical case.  So, if we can find a way to reduce the variability,
even if the average bandwidth is unchanged, we may be able to significantly
increase the guaranteed lengths.

I would suggest that before there is much discussion of possible coding
schemes, the committee should first debate and decide if now is the time to
cut the legacy cord, or not.

If the cord is to be cut, it's best done cleanly.


Joe Gwinn