UFiber GPON - Designing a GPON Network


Overview


This article will give the necessary information to create a functioning GPON network using UFiber equipment and accessories. It will also give details for planning a high-scale GPON network while considering optical power, distance, attenuation, and bandwidth capacity.

NOTES & REQUIREMENTS:
Applicable to the latest UFiber firmware on the UF-OLT models. Please see the Related Articles below for more information.
 
Devices and accessories used in this article:

Table of Contents


  1. Introduction and Requirements
  2. Network Design and Setting up Basic Connectivity
  3. Attenuation and Power Level Calculations
  4. Upstream and Downstream Calculation Example
  5. Planning for Capacity vs Client Quantity
  6. Planning with Splitters
  7. Related Articles

Introduction and Requirements


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Optical Distribution Network (ODN) planning is critical to a successful GPON implementation. It is essential to have a well-planned network design to ensure CPEs receive a usable signal, allow for bandwidth capacity and client count on each PON port, and save on costs. This is done by balancing optical power, distance, attenuation, and bandwidth capacity. 

For a properly connected GPON network, whether in a lab environment or for actual deployment, some accessories are required to connect the UF-OLT to a UF-Nano or UF-Loco ONU. At a minimum, the following components are required:

  • SC/UPC to SC/APC Single-Mode fiber patch cable
  • SC/APC to SC/APC Single-Mode fiber patch cable
  • PLC splitter
  • Optical attenuator

There is more information about these components in our UFiber accessories article here.


Network Design and Setting up Basic Connectivity


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On the uplink side, we use a high capacity router such as an ER-8-XG, connected to the UF-OLT using 10Gbps SFP+ or DAC cables. See this page for more information on UBNT branded SFP/SFP+ modules and DAC cables.

On the PON side, we insert a GPON OLT SFP module (such as the UF-GP-B+) into the UF-OLT. This then connects to a PLC splitter using a SC/UPC to SC/ACP Single-Mode fiber patch cable. At the other end of the splitter, we connect a UF-Nano ONU using a SC/APC to SC/APC Single-Mode fiber patch cable. Assuming the power level and attenuation is calculated correctly (see the calculation section), the ONU will be connected. The acceptable optical power level range at the ONU is -8 dBm to -28 dBm.

topology1.png

 

Basic GPON topology using an ER-8-XG on the uplink side and a PLC splitter on the PON side.


Attenuation and Power Level Calculations


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This section focuses on attenuation, which is the most important factor in designing a GPON network. First, we need to become familiar with sources of attenuation so that we can use them in designing the network and calculation the optical power level (dBm). For example, the UF-GP-B+ module starts with an output power of +3 dBm, while supporting a Rx minimum loss of -8 dBm and a maximum loss of -28 dBm. All of the signals must be within this range.

NOTE: There will be zero throughput if the Rx signal is outside of the maximum/minimum loss range With GPON. This differs from AirMAX for example, where you might see a decrease in throughput if the signal is weak.

 Common sources of attenuation are:

  • LengthAttenuation occurs over the distance of a fiber run per kilometer (Km) and differs in the downstream (~0.3 dB per Km on 1490 nm) and upstream (~0.5 dB per Km on 1310 nm) frequencies.
  • SplicesEach splice in a fiber optic run accounts for ~0.1 dB. This seems minimal at first, however the count of splices in a single run can add up considerably.
  • ConnectorsEach connector accounts for a ~0.6 dB loss. This starts from the SC connector at the UF-GP-B+ module and ~0.6 dB is added for each other connector.
  • SplitterSplitters are essential in a GPON networks to connect multiple ONUs to a single PON port. See the splitters section below for more information on using splitters.

These are general attenuation values and different vendors may lead to different values depending on the quality of the product. Each source of attenuation will decrease the starting power level to be within the acceptable range for the ONUs. 

ATTENTION: Connecting a UFiber ONU directly to the UF-GP-B+ in the PON port from the OLT will potentially cause damage to the optics in the ONU and/or the OLT because the power is too high.
 
The acceptable optical power for the UF-Nano is -8 dBm to -28 dBm. The output from the UF-GP-B+ module is +3 dBm. Therefore, there must be attenuation to provide an acceptable level at the ONU. This is also a factor in the upstream optics where the output from the ONU at ~3 dBm would be too high and cause damage to the UF-GP-B+ module.

Upstream and Downstream Calculation Example


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This section focuses on how to calculate the optical power the ONU and OLT will receive. When laying out your fiber distribution network, the goal is to calculate all attenuation in each fiber run to ensure that when connecting the ONU on the customer’s premises it will receive a Rx value of -8 dBm to -28 dBm. It is also important that the received signal from the ONU is also within the same range at the OLT.

Like highlighted in the section above, the common sources of attenuation are:

  • Upstream LengthLoss of ~0.5 dB per Km on 1310 nm. 
  • Downstream LengthLoss of ~0.3 dB per Km on 1490 nm.
  • SplicesLoss of ~0.1 dB per splice. 
  • ConnectorsLoss of ~0.6 dB per connector. 
  • SplitterLoss of log10(split:ratio) x 10 = Attenuation in dB for each split.

The table below show the general attenuation loss for common splitter ratios:

 Split  Formula  Loss in dB 
1:2 log10 (2) x 10 -3.01dB
1:4 log10 (4) x 10 -6.02dB 
1:8 log10 (8) x 10 -9.03dB 
1:16 log10 (16) x 10 -12.04dB 
1:32 log10 (32) x 10  -15.05dB 
1:64 log10 (64) x 10 -18.06dB 

Using the below network diagram as a guideline, we can use these attenuation sources to end up within the acceptable optical power levels.

topology2.png

Downstream Calculation:

 Source  Loss of Optical Power Calculation  Loss in dB
Length .3dB x 10Km + .3dB x 6Km -4.8dB
Splices .1dB x 2 splices -.2dB
Connectors .6dB x 4 connectors -2.4dB
Splitter 15.05dB for 1:32 splitter -15.05dB
  +3 dBm starting power minus loss  -19.45dBm

Upstream Calculation:

 Source  Loss of Optical Power Calculation  Loss in dB
Length .5dB x 10Km + .5dB x 6Km -8dB
Splices .1dB x 2 splices -.2dB
Connectors .6dB x 4 connectors -2.4dB
Splitter 15.05dB for 1:32 splitter -15.05dB
  +3 dBm starting power minus loss  -22.65dBm

Both the upstream and the downstream signals are within the acceptable range.

ATTENTION: The bandwidth will not increase with a better optical power value. Think of this as either on when inside the range or off when outside the range. The UF-Nano has a built in display showing the optical power levels of both Rx and Tx to easily identify whether the level is in the acceptable range.

Planning for Capacity vs Client Quantity


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When planning your network it is important to plan for future customers and calculate the available bandwidth on each PON port compared to the client count. There is often a mix of many clients with relatively low bandwidth needs of around 50 to 100 Mbps, and a smaller amount of clients requiring 500 Mbps to 1 Gbps bandwidth.

High bandwidth design example:
For highest capacity of bandwidth, connecting a single ONU to a single PON port could provide a single client 20km away with the full bandwidth of the PON port. Keep in mind that each of the eight PON ports on the UF-OLT can provide 2.488 Gbps downstream and 1.244 Gbps upstream. In the rare case that a single ONU is used on a single PON port keep in mind that the bandwidth will be limited by the 1 Gbps LAN copper port on the ONU.

High capacity design example:
For highest capacity of clients. A PLC splitter with a 1:128 ratio connected to a PON port could provide 128 clients with equal bandwidth of about 19 Mbps download and 9 Mbps upload when the clients are all within a range of ~8Km.

NOTE: Note: the distance here is decreased from the max 20Km to 8Km due to the attenuation loss ratio of the splitter, fiber length, connectors, and splices. The distance could vary when cascading multiple splitters to give the same bandwidth to customers at a further distance. See the splitters section for more details.

Planning with Splitters


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When designing your network it is important to utilize splitters to reduce costs, greater customer reach and for future expansion. The example below shows a good mix of splitters to cover customer locations while still keeping the values in the acceptable range.

In this illustration, we will calculate the power from the OLT to the ONU and the ONU to the OLT to be sure each customer will have a usable signal. Remember to always calculate in both the upstream and downstream directions.

topology3.png

Downstream Calculation:

ONU  Source  Loss of Optical Power Calculation  Loss in dB
ONU1 Length .3dB x 20Km -6dB
Splices .1dB x 4 splices -.2dB
Connectors .6dB x 4 connectors -2.4dB
Splitters 3.01dB for splitter -3.01dB
  +3 dBm starting power minus loss  -8.81dBm
ONU2 Length .3dB x 16Km -4.8dB
Splices .1dB x 8 splices -.8dB
Connectors .6dB x 6 connectors -3.6dB
Splitters 3.01dB + 6.02dB for splitters -9.03dB
  +3 dBm starting power minus loss  -15.23dBm
ONU3 Length .3dB x 13Km -3.9dB
Splices .1dB x 12 splices -1.2dB
Connectors .6dB x 8 connectors -4.8dB
Splitters 3.01dB + 6.02dB + 9.03dB for splitters -18.06dB
  +3 dBm starting power minus loss  -24.96dBm

Upstream Calculation:

ONU  Source  Loss of Optical Power Calculation  Loss in dB
ONU1 Length .5dB x 20Km -10dB
Splices .1dB x 4 splices -.2dB
Connectors .6dB x 4 connectors -2.4dB
Splitters 3.01dB for splitter -3.01dB
  +3 dBm starting power minus loss  -12.81dBm
ONU2 Length .5dB x 16Km -8dB
Splices .1dB x 8 splices -.8dB
Connectors .6dB x 6 connectors -3.6dB
Splitters 3.01dB + 6.02dB for splitters -9.03dB
  +3 dBm starting power minus loss  -18.43dBm
ONU3 Length .5dB x 13Km -6.5dB
Splices .1dB x 12 splices -1.2dB
Connectors .6dB x 8 connectors -4.8dB
Splitters 3.01dB + 6.02dB + 9.03dB for splitters -18.06dB
  +3 dBm starting power minus loss  -27.56dBm

Related Articles


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UFiber GPON - Getting Started

UFiber GPON - Accessories


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