What makes PON technology so great is that you don't have any active equipment out in the field. That means no power hookups, no UPS units to install and maintain, no swapping out failed gear, no keeping spare units on hand, and no extra staff to send out for node maintenance. All those headaches just disappear.
Of course, PON has its drawbacks too — we'll cover those in a later post. And I'm sure we'll run into some unexpected issues during construction and day-to-day operation.
When we started planning the distribution network, two main questions came up: how to split the signal along the line, and how to make branches from the main trunk. These sound similar, but they're actually completely different problems.
There are several common approaches to signal splitting.
First, the centralized approach. You run a dedicated fiber from the central node all the way to each subscriber. The downside is pretty obvious — it's expensive. Multi-fiber cables cost a lot, and you have to terminate all those fibers at the central office. Sure, there are high-density modular splice trays and frames these days, but they take up a lot of space and aren't easy to fit into existing facilities. In a PON setup, you put the optical splitters right in that same cabinet and feed each subscriber individually. The big advantage is that you can add OLT ports gradually as subscribers sign up. This works especially well in apartment buildings, where cable runs are short and maintenance is straightforward. Plus, signal levels at every subscriber drop end up nearly identical — the only variations come from cable length differences and minor splitter tap loss variations.
Second, the bus configuration. Here you use splitters with uneven tap ratios. For example, if you're serving 64 ONUs from one OLT port and you've got 8 subscribers on the first tap, you'd use a splitter with a 12:88 ratio — 12% goes to those 8 subscribers, and 88% continues down the line. Why 12%? Because each subscriber needs roughly 1/64 of the OLT's signal power, or about 1.5%. For 8 subscribers, that adds up to 12%. Further down the line, you'd use splitters with ratios like 14:86, 17:83, 20:80, 25:75, 33:67, and finally 50:50.
The advantage here is that you only need one trunk fiber for the entire run, which saves a lot on cable costs. You can also select splitters to balance signal levels across all subscribers. The downside is that you end up with many different splitter types — most of which you'd have to order as custom builds. In practice, splitters are pretty robust. In all my years, I've only seen two fail: one was dead out of the box, and another lost a tap after some time in service. I couldn't tell you what caused either one. You'll probably want to keep spare splitters of each type in stock. Also, uneven splitters are still made by fusing fibers, and it's hard to get precise isolation between taps. Manufacturers give themselves a margin of error, and when you cascade several of these, those errors stack up — so what you get at the far end isn't always what you calculated.
Third, the star topology. Here you split the signal into multiple directions at certain points. Ideally, you place these split points in the center of your distribution area, and sometimes fibers from that point go back the way they came. Often those fibers are actually in the same cable. Then each arm of the star can be split further in the same way. It's not really a star at that point — more like a snowflake. But again, all those "arms" might still be bundled in one cable. So why bother? Because this makes it much easier to distribute optical power using just a handful of splitter types. For instance, in our network we plan to use 2-way, 4-way, and 8-way splitters with even splits. Say you've got an 8-way splitter roughly in the middle of a cable run. Half the taps go forward, the other half loop back. On each side, those taps can split into 8, or into 2 then 4, depending on how many buildings are there and where they're located. In total, you get 64 subscribers per OLT port (8×8 or 8×2×4).
This article is a translation of the original Russian-language post.My journey of learning GPON
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