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RE: [EFM] 10 km objective, Optical PMD perspective


I think you have uncovered a potential "rat hole" that we may me leading ourselves in and that is to overspecify the PMD in EFM.  Why not just specify the wavelength allocation over single mode fiber?  Why does it have to be an FP?  Using an alternative laser type not only extends the reach, but enables other fiber types besides G.652.  So why would I use a dispersion shifted fiber i.e. G.655?  Many reasons....
*	Upgradeability for downstream data rates way above 1.25 we can think about all those HDTV channels in the future or downloading a 7 Gb movie in a matter of seconds.
*	Allows the use of DML without dispersion compensation in the HE for analog video.
*	Reduces customer fiber inventory between metro and access fiber types.
*	DWDM possibilities for FTTB 

My point is simple...the standard should avoid overspecifying the PMD to allow network engineers the flexibility to use the right electronic, opto-electronic, and fiber mix to extend the data rate and/or distance to enable the applications of the future.

Best regards,

Rob Carlisle

> ----------
> From: 	Vipul Bhatt[SMTP:vbhatt@xxxxxxxxxxx]
> Sent: 	Tuesday, September 25, 2001 8:01 PM
> To:
> Subject: 	[EFM] 10 km objective, Optical PMD perspective
> Dear colleagues,
> 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)
> links.
> 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.
> Thanks,
> Vipul
> vbhatt@xxxxxxxxxxx
> 408-542-4113