Muni fiber: L1 or L2?

Fri Feb 1 21:59:54 UTC 2013

On Feb 1, 2013, at 1:43 PM, Jason Baugher <jason at 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

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 = ½ 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.

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.

Owen

> CO-based splitter:
> +5dBm - 16dB - (10km x .26dB) = -13.6
> 
> Splitter at 9km:
> +5dBm - (9km x .26dB) - 16dB - (1km x .26dB) = -13.6
> 
> 
> If someone can explain why this math would be wrong, I'd love to hear it and I'd be happy to run it past our vendor to see if they agree.
> 
> 
> On Fri, Feb 1, 2013 at 3:16 PM, Owen DeLong <owen at delong.com> wrote:
> Actually, this is an issue… I should have seen it.
> 
> 
> You have 3 loss components… Power out = (Power in - loss to splitter - splitter loss) / nOut - loss-to-customer
> 
> So, if the loss to the splitter is 3db and you have 20db (effective 320db on a 16x split) loss on each customer link, that's
> a radically worse proposition than 20db loss to the splitter and 3db loss to each customer (which is effectively 48db
> loss on a 16x split).
> 
> It's still do-able, but you either need amplifier(s) or very short distances between the customer and the MMR.
> 
> Given this consideration, I think the situation can still be addressed.
> 
> Put the splitters in the B-Box and allow for the possibility that each subscriber can be XC to either a splitter or
> an upstream dedicated fiber. The provider side of each splitter would be connected to an upstream fiber
> to the MMR.
> 
> So, each B-Box contains however many splitters are required and each splitter is connected upstream to a
> single provider, but you can still have multiple competitive providers in the MMR.
> 
> This setup could support both PON and Ethernet as well as other future technologies.
> 
> Owen
> 
> On Feb 1, 2013, at 1:04 PM, Jason Baugher <jason at thebaughers.com> wrote:
> 
>> I should clarify: Distance x loss/km + splitter loss. = link loss.
>> 
>> 
>> On Fri, Feb 1, 2013 at 3:03 PM, Jason Baugher <jason at thebaughers.com> wrote:
>> I disagree. Loss is loss, regardless of where the splitter is placed in the equation. Distance x loss + splitter insertion loss = total loss for purposes of link budget calculation.
>> 
>> The reason to push splitters towards the customer end is financial, not technical.
>> 
>> 
>> On Fri, Feb 1, 2013 at 2:29 PM, Scott Helms <khelms at zcorum.com> wrote:
>> Owen,
>> 
>> You're basing your math off of some incorrect assumptions about PON.  I'm
>> actually sympathetic to your goal, but it simply can't work the way you're
>> describing it in a PON network.  Also, please don't base logic for open
>> access on meet me rooms, this works in colo spaces and carrier hotels but
>> doesn't in broadband deployments because of economics.  If you want to
>> champion this worthy goal you've got to accept that economics is a huge
>> reason why this hasn't happened in the US and is disappearing where it has
>> happened globally.
>> 
>> 
>> > Bottom line, you've got OLT -> FIBER(of length n) -> splitter ->
>> > fiber-drops to each house -> ONT.
>> >
>> 
>> So far you're correct.
>> 
>> 
>> >
>> > All I'm proposing is making n really short and making "fiber-drops to each
>> > house" really long.
>> > I'm not proposing changing the fundamental architecture. Yes, I recognize
>> > this changes the economics and may well make PON less attractive than other
>> > alternatives. I don't care. That's not a primary concern. The question is
>> > "can PON be made to work in this environment?" It appears to me that it can.
>> >
>> 
>> 
>> Here is where you're problems start.  The issue is that the signal *prior
>> to being split* can go 20km if you're splitting it 32 ways (or less) or
>> 10km if you're doing a 64 way split. AFTER the splitter you have a MAX
>> radius of about 1 mile from the splitter.
>> 
>> Here is a good document that describes the problem in some detail:
>> 
>> http://www.ofsoptics.com/press_room/media-pdfs/FTTH-Prism-0909.pdf
>> 
>> 
>> Also, here is a proposed spec that would allow for longer runs post
>> splitter with some background on why it can't work in today's GPON
>> deployments.
>> 
>> http://www.ericsson.com/il/res/thecompany/docs/publications/ericsson_review/2008/3_PON.pdf
>> 
>> --
>> Scott Helms
>> Vice President of Technology
>> ZCorum
>> (678) 507-5000
>> --------------------------------
>> http://twitter.com/kscotthelms
>> --------------------------------
>> 
>> 
> 
> 