UISP Fiber - Designing a GPON Network

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.

Installation Requirements and Network Design

Back to Top

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. In the example setup below, a UFiber OLT (UF-OLT or UF-OLT-4) is used as the core of the GPON network, connecting the PON side to the rest of the routing and switching domain.

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

The SFP+ uplink port(s) are used to connect UFiber GPON network to the rest of the routing and switching domain. On the uplink side, we use a high capacity router such as an ER-8-XG, connected to the OLT using 10Gbps SFP+ fiber modules or DAC cables.

On the PON side, we insert a GPON OLT SFP module (UF-GP-B+ / UF-GP-C+) into the OLT, supporting up to 128 UFiber ONUs per PON port when using PLC splitters. There are various options available to properly connect the OLT to the UFiber ONUs (UF-Nano / UF-LOCO / UF-WIFI). There components that typically make up a GPON network are:

• Feeder cable Fiber cable (SC/UPC to SC/APC) connecting the OLT to a distribution point, typically a PLC splitter.
• PLC Splitter Fiber splitter that distributes the fiber connection using multiple available split ratios.
• Drop cable Fiber cable (SC/APC to SC/APC) providing the final link by connecting the PLC splitter to the ONU.
• Adapters or Splices Used to inter-connect the different fiber cables and splitters.

The simplest method of connecting UFiber equipment is to use pre-terminated fiber cables with connectors which can simply plug into the other accessories. The official UFiber GPON fiber cable and PLC splitter accessories use SC/UPC and SC/APC connectors which can be used to quickly and easily set up GPON networks. See the Accessories article for more information. There are also other options available such as mechanical and fusion splicing. See the Fiber Connectors and Splicing article for more information.

Using either the official accessories or other available fiber equipment, and assuming the power level and attenuation is calculated correctly (see the Calculation section below), the ONU will be connected. The acceptable optical power level range at the ONU is -8 dBm to -28 dBm.

Attenuation and Power Level Calculations

Back to Top

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.

 Common sources of attenuation are:

• Length Attenuation 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.
• Splices Each 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.
• Connectors Each 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.
• Splitter Splitters 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.

Upstream and Downstream Calculation Example

Back to Top

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:

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

Power level calculation example using an UF-OLT, a 1:32 PLC splitter and a UF-Nano.

Downstream Calculation:

Upstream Calculation:

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

Planning for Capacity vs Client Quantity

Back to Top

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.

Planning with Splitters

Back to Top

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.

Example GPON network using multiple PLC splitter ratios.

Downstream Calculation:

Upstream Calculation: