[EFM] 10 km objective, Optical PMD perspective
Our distance objective for optical links is ">= 10 km". We need to
now examine that in greater detail, and make a difficult decision.
When converting this requirement into Optical PMD specifications, it
is becoming clear that Fabry Perot (FP) lasers will dispersion-limit
the distance to somewhere between 10 and 13 kilometers, depending on
how aggressive we get with the specs. FP laser is the workhorse of
nearly all 1000BASE-LX transceivers. It is cost-effective and field
proven. Unfortunately, it won't support 20 kilometers. Throwing a
bigger power budget at the problem won't help. (Details are being
discussed on the P2P reflector.)
To support longer distances, we have to resort to either temperature
controlling the FP laser, or specifying an additional PMD type with
more expensive or less proven lasers.
Temperature control is worth investigating, but it has its own set
of issues, and we end up cost-penalizing the majority of (short)
Adding a PMD type sounds easy, but we should think again. We already
have 5 PMD types on the table - at least one copper, one temperature
extended LX, and three single-fiber. Adding one PMD type effectively
adds two PMD types, because head-end and subscriber-end need
opposite wavelength plans.
We can make all Optical PMDs capable of supporting 20 km by
specifying expensive or less proven lasers, but that would deny the
majority of links the benefit of using FP lasers.
This begs two questions: Will a number between 10 km and 13 km be an
acceptable limit? What percentage of total deployments are likely to
exceed this limit?
Using regenerators for longer distance links is expensive. But so is
the inclusion of 7 PMD types in a standard.