PONderings of a Network Engineer
Motivation
Traditional access network media/protocols like DOCSIS over Coax or any DSL variant over 2-wire copper are the most prominent methods how residential subscribers access the internet, but more future-proof fiber based technologies are on the rise, and even in Germany slowly becoming more popular.
I did not know much about the world of access networks up until December 2020, when I started to work in that area. My background was mostly in service provider networks, so the World of Access[tm] was new to me. Since then I was following conversations in the *NOG community where people are talking about access networks, discussing and assess the different protocols and media available. What I noticed while learning about fiber-based access networks is that my service provider mindset was too narrow to fully understand restrictions and challenges of fiber-based access network designs. Proven solutions for networks in data centers or for larger backbones are not necessarily the best choice for access networks.
Going through that learning process myself gave me good insight into those challenges and I wanted to make use of that knowledge and give an introduction into Fiber-based networks in general, and PON networks specifically, for network engineers from other fields.
Having said that, I will make some assumptions on the knowledge of the readers, and won’t explain every bit in great detail. I will for example assume you know how fiber-based networks work in general, that there are different wavelengths for transceivers, what splitters/MUXes are or what attenuation in a fiber network is.
What I’m hoping the readers will gain from this article is a basic understanding of fiber-based access networks, how they differ from other networks and with this knowledge can better understand why some networks might choose to use PON instead of going for a more versatile AON topology.
PON? AON? P2P?
You will see those acronyms all over the article, so I want to define them in the very beginning.
PON stands for Passive Optical Network, and by that we mean a passive distribution layer where subscribers are aggregated with the use of passive splitters, to share one fiber between multiple subscribers and therefore also share one port on an active network element.
A contrast to PON is AON (Active Optical Network), sometimes also calles P2P network (Point-to-Point). In an AON topology each subscriber has their own fiber going to the active network element, and thus their own port on that network element as well.
Contents
When I was jotting down some notes for this topic, I immediately noticed that there is a lot of ground to cover. To make this text more accessible, I will break it down into multiple articles, so if you are interested look out for the whole series.
Sections in this article:
- Introduction
- Network Design AON/P2P
- PON Passive Network Design Principles
- PON Active Network Basics
-
Excursion to G.fast - coming soon.
- Bitstream Access
- Discussion & Summary - coming soon.
I will describe fiber networks from the viewpoint of a fiber carrier. When I say fiber carrier, I mean a company with the specific purpose of building fiber networks and selling access to subscribers on that network to wholesale carriers and service providers. Wholesale carriers and service providers are then providing the actual IP service to the subscribers. The fiber carrier might have their own IP product as well, but I will not talk about that in the course of this article.
Also, if you’re looking for deep technical background I’m afraid this is not the place you will find it. I’m keeping fairly high-level and won’t dive too deep into protocol-specific details. My goal is to provide a big picture overview over fiber networks.
I will describe the basic design of a fiber-based network, then go into some details what PON brings to the table and which active network elements exist in the PON world. With all the theory explained, I will summarise the advantages and disadvantages of AON and PON technologies, and then we are already done.
Spoiler alert: As always in the real world there is no “one size fits all” solution, so there won’t be a clear winner at the end of this text. But hopefully you will have enough background information to make that judgment yourself from now on.