POL Basics - GPON (Part 1)
Photo by Umberto

POL Basics - GPON (Part 1)

Continuing the series on POL Basics, Passive Optical LAN isn't just using optical fiber, it is based on GPON or Gigabit Passive Optical Network. As the article I wrote for my blog, which you can find in full at https://poldigitaltransformation.blogspot.com/2020/04/pol-basics-gpon.html turned out to be rather long, I decided to publish it here in LinkedIn in two parts.

Part 1 described : What is GPON? - the ODN structure - splitters and attenuation - other sources of attenuation

Part 2 will describe: GPON specifics and Data transmission

What is GPON?

Passive Optical LAN doesn't just use optical fiber, it is based on a widely used technology called GPON or Gigabit Passive Optical Network.

The basic standards defining the GPON technology are:

  • ITU-T G.984.1: Gigabit Passive Optical Networks (GPON): general characteristics
  • ITU-T G.984.2: Gigabit Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specifications
  • ITU-U G.984.3: Gigabit Passive Optical Networks (GPON): Transmission convergence layer specification
  • ITU-U G.984.4: Gigabit Passive Optical Networks (GPON): ONT management and control interface specification (OMCI)

GPON was originally designed and deployed in the carrier space: access providers connecting residential (and potentially business) users to the central office. The solution is typically called FTTH (Fiber To The Home), FTTU (Fiber To The User) or FTTP (Fiber To The Premise).

More recent GPON is also used by access or internet providers with solutions like FTTC (Fiber To The Curb), where a very short copper loop is still used (which would allow for VDLS2 or even G.Fast) from a modified ONT towards the end-user's house.

In the enterprise sector, GPON is used more and more as a superior alternative to LAN implementation. With proven use cases in many segments: hospitality, hospitals, corporate offices, banks, airports, age care facilities, ...

Below is the high-level overview of what a GPON system looks like, specifically used for a POL scenario.

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(the above diagram comes from the Association for Passive Optical LAN)

It contains the following elements:

  • OLT: Optical Line Termination: an access multiplexer, aggregating multiple dozens or hundreds of GPON terminations. Vendor specific active equipment.
  • ONT: Optical Network Termination (unit): small end-user side device, which terminates GPON on the network side and offers several (typically copper based) end-user interfaces, like Ethernet (RJ45), POTS (RJ11) or Coax. Vendor specific active equipment.
  • ODN: Optical Distribution Network: the passive part of the solution, a tree based optical structure, based on the GPON technology.
  • The other elements of Core Switch and Workstation will be describe when we come to the POL specific concepts in a future article.

In this article we concentrate on the ODN part of the POL solution.

ODN Structure

The basic structure of GPON is a tree-based (point-to-multipoint) passive topology and consists of the following parts:

  • Feeder cable : Single Mode Fiber (SMF) optical fiber, single core, going from the GPON port on the OLT towards a passive splitter
  • Passive Splitter: splits the single feeder cabling into several outgoing SMF cores going towards the ONTs. The split ratio (number of outgoing SMF cores downstream) is always a power of 2. We talk about 1:2 splitters all the way up to 1:128 splitters.
  • Drop Cables: SMF optical fibers connecting the passive splitter with the ONT
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The structure as it is shown in the diagram above, with one feeder cable, splitter and several drop cables towards ONTs, is called a PON port.

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Splitters and Attenuation

The traditional procedure to create a splitter was to fuse two fibers together and cut one of the four legs away. This way you end up with a 1:2 splitter. See example below, if we cut away 'Arm 2' you have 'Arm 1' coming in as the feeder cable and 'Arm 3' and 'Arm 4' going downstream towards two ONTs.

You then repeat the process of fusing (hence splitting) two fibers at each of the arms ('Arm 3' and 'Arm 4') to end up with a 1:4 splitter, etc... until the desired split ratio is attained.

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This original technique was called Fused Biconic Taper (FBT). These days, splitters are implemented as a Planar Lightwave Circuit (PLC), see below, with the glass waveguides (SMF) attached to a substrate (similar to a circuitboard in modern day computers).

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Within the ODN, the splitter represents the most important element of signal attenuation. With each split from one to two fibers, half of the optical budget follows one downstream SMF, the other half of the budget the other SMF. This amounts to a loss of 3dB per split.

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In other words, is you have a 1:32 split ratio, you have internally in the splitter 5 levels of splits (since 32 == 2^5), and hence the splitter adds 5*3 == 15dB of attenuation.

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Other Source of Attenuation

While the splitter is by far the most important source of attenuation, in order to calculate the loss budget, you will need to be aware of all sources of attenuation:

  • connectors : at the very least at the both extreme ends of the fiber (connecting into the OLT and ONT), potentially also at splitter. The amount used for calculations may vary, in our example below we will be using 0.3dB per connector.
  • splices: 0.1dB per splice
  • ageing of the fiber: 1dB
  • attenuation per km: maximum of 0.42dB/km (for upstream)
  • WDM coupler (if any is present): can be used either when analogue video is being modulated over GPON or when a mix of GPON and XGS-PON is deployed (Both scenarios are beyond the scope of POL Basics)
  • splitters: as mentioned earlier, for each 1:2 split, you have a theoretical loss (attenuation) of 3dB. However, most tools will take a slightly higher number to compensate for any inaccuracies. 3.5dB is a fair number to use.

In order to calculate the loss budget, you need to first know what class of SFP you are using. There are 2 types relevant for POL:

  1. Class B+ : optical budget of 28dB
  2. Class C+: optical budget of 32dB

Let's consider a theoretical example, based on a FTTH deployment with Class B+ optics.

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In the above diagram, we can easily confirm the optical budgets:

  • downstream: 1.5 - (-27) - 0.5 = 28 dB
  • upstream: 0.5 - (-28) – 0.5 = 28 dB

In our example we consider the following:

  • 16 way splitter loss: 14 dB (theoretical: 12dB)
  • Connector + splicing loss: 3 dB (24*0.1 dB + 2*0.3 dB)
  • ageing: 1 dB
  • downstream attenuation: 0.30 dB/km
  • upstream attenuation: 0.42 dB/km

This leads to a maximum distance : (28 – 14 – 1 – 3) / 0.42 = 23.8 km (for a 1:16 splitter)

In POL, the issue is sometimes that there is no enough attenuation. Consider a floor, connected to one PON port, which only requires some 64 Ethernet ports. Assuming we use a 4 port ONT, we can cover the floor with 16 ONTs, hence a 1:16 splitter is sufficient.

Referring back to our optical budget diagram above, you saw that both the OLT and ONT have a range for the Rx and Tx budget. Consider the maximum Tx power at OLT for downstream and the maximum Rx power level for the ONT. Further, the ONT will assume a margin of error on it's own measurement of 1dB.

This gives us the minimum attenuation: 5 - (-8) – 0.5 + 1 = 13.5dB

Taking into account:

  • high quality splitter, hence we 'only' loose 12dB at the splitter
  • short distance (few hundred meter) that will barely give any additional attenuation on the fiber
  • short distance probably means fewer splices and connectors
  • ageing factor is only a theoretical attenuation and might not be a factor in reality

We might barely get to the minimum attenuation that is required so that the ONT can correctly read the incoming signal from the OLT.

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To Be Continued ..... in Part 2

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