Re: Muni fiber: L1 or L2?

[Robert Bonomi <bonomi () mail r-bonomi com>]

> From nanog-bounces+bonomi=mail.r-bonomi.com () nanog org  Fri Feb  1 16:11:17 2013
> Subject: Re: Muni fiber: L1 or L2?
> From: Owen DeLong <owen () delong com>
> Date: Fri, 1 Feb 2013 13:59:54 -0800
> To: Jason Baugher <jason () thebaughers com>
> Cc: NANOG <nanog () nanog org>
>
>
> On Feb 1, 2013, at 1:43 PM, Jason Baugher <jason () thebaughers com> wrote:
>
> > It's still a 23dB loss for each customer from the CO to the ONT.
> >
> > I have an OLT that launches at +5dBm. At 1490nm, I should see about a 
> > .26dB loss per km. My 1x32 splitter is going to add about 16dB more 
> > loss. Assuming we ignore connector losses, and also assume that the 
> > customer is 10km away:
>
> Nope  The power going into each fiber out of the splitter is 1/16th that 
> of what went into the splitter.
>
> Yes, your total in-line loss is still 10km, but you are forgetting about 
> the fact that you lost 15/16th of the power effectively going to the 
> fiber when you went through the splitter (in addition to the splitter 
> loss itself).
>
> So: CO Based splitter:
>
> Each customer gets (IN - 16dB - (10km x .26db))/32

Wrong.  The 'loss' of the splitter  _includes_ the division into
multiple outputs.  16dB insertion loss for a 16-way splitter is
12dB for the division and 4dB of 'internal' (less than theoretically
perfect) losses.

> Splitter at 9km:
>
> Each customer gets (IN - (9km x .26dB) -16db)/32-(1km x .26db)
>
> If we use 5dBm as our input, this works out:
>
> CO: (5db - 16db - (10km x .26db) / 32
> /32 is effectively -15 db (-3db =   power, 32 = 2^5)
> Substituting: (5db - 16db - 2.6db) -15db = -28.6db to each customer.
>
> Spitter at 9km: (5db - (9km x .26db) -16db)/32-(1km x .26db) 
> Substituting: (5db - 2.34db -16db)-15db-.26db = -28.08db to each customer
>
> So there is a difference, but it seems rather negligible now that I've 
> run the numbers.

Strange, I get -28.6 for both your calculations -- 'bc' output:
    
    (5-16-2.6)-15
    -28.6
    (5-2.34-16)-15-.26
    -28.60

> However, it's entirely possible that I got this wrong somewhere, so I 
> invite those more expert than I to review the calculations and tell me 
> what I got wrong.

a) arithmetic error.   <grin>
b) 'double-counting' splitter losses.  Real-world for a 32-way splitter 
   is around 20db -- 15db for the division, and 5db for the 'internal'
   losses.  giving:
   C.O.:  5-20-2.6      = -17.4 db
   @9km:  5-2.34-20-.26 = -17.4 db