Physical x Logical Reach in GPON networks

By Tatiane Figueiredo – Training Instructor

When talking about power budgeting for GPON networks, either during the technical training we offer or even during a support session, the most frequently asked questions are: Which GPON module to choose: B +, C + or C ++? The C + module has a longer range (in mileage), correct? Can I use a C + module with a ONU B +?

There are countless doubts about this topic in our day-to-day lives, but before answering them, let's look at some details.

The SFP (Small Form Factor Pluggable) GPON module is a compact device that has a laser internally, which is associated with a certain transmission power (light intensity), defined by ITU-T in recommendation G.984.2, which specifies the physical layer requirements issues. Where it is observed:

What about the famous C ++ laser? This has no recommendation from the ITU-T, that is, there is no standardization, however in August 2019 the recommendation G.984.2 was republished adding the new standard: class D (known in the market as C ++).

The table above, in addition to the transmission power, brings two more information:

  1. Sensitivity: It is the lowest level of optical power that the device can receive or detect;
  2. Overload: It is the highest level of optical power that the device can receive or detect.

Look at the chart below to see the highlighted information better.

The values ​​are applied to the calculations of the power budget, which we also covered in our blog (Power budget), it is worth checking.

The control of the power emitted by the SFP GPON module is performed by the OLT itself and does not allow manual adjustments. This characteristic is also defined in the ITU-T standardization. However, if I cannot perform the power adjustment, what is the degradation of the signal / laser?

Datacom uses feedback technology (closed loop), both in the OLT GPON module and at the ONU. Basically there is a sensor tuned to the same frequency as the transmission and that measures everything that is being transmitted. If the reading is above or below the predicted, the current of the transmitter is altered, to be within acceptable parameters. This compensates for the degradation that occurs over time and also for changes in operating temperature.

The installation of GPON B +, C + or D modules does not impact the operation of the OLT, as they are characteristics of the module itself, that is, all configurations performed (or to be performed) are not impacted by this SFP characteristic. There are also no restrictions on the combined installation of different modules on the different OLT ports, such as: type B + on the GPON port 1, C + on the GPON port 2, D on the GPON port 3, and so on.

The same occurs with ONUs with B + or C + laser, note that the power of the two types of laser is the same. The change occurs in the sensitivity parameter, that is, the more sensitive (the lower the signal level), the lower the power that the device can work with or the greater the amount of losses that can be inserted.

There are nine possible combinations that will impact power budgets:

Having more power or a higher power budget does not mean that it will be possible to exceed the 20 km distance defined by the ITU-T recommendation. It also will not always allow you to use more splitters or open more clients on a PON port. The type of laser will directly impact sensitivity.

So, a C + or D laser will not bring a longer range (or a longer mileage), but will allow you to work on the physical range issue, according to the following definitions:

Physical Range: It is related to the optical characteristics of the network and depends on a series of factors, such as fiber attenuation, splitter, splices and connectors, output power of the transmitter and sensitivity of the receivers.

Logical Scope: It is associated with the communication protocols between the OLT and the ONU, which has as a requirement the maximum time for receiving messages. The difference between the maximum and the minimum distance must not exceed 20 km, so that the ranging protocol works properly and does not impact the size of the transmission windows.

The time required for transmission of optical signals and the time the ONUs receive these signals are different. The mechanism that performs the distance measurement is the RTD – Round Trip Delay, which considers the round trip time of the signal, the speed of light propagation in the fiber, the delay of the optical-electrical components and the processing time of the ONU. This result is the time difference between the transmission of the 1st bit of the ranging message and the reception of its last bit.

The OLT assigns a burst time to each ONU, used for the ONU to send its data. Between the burst time allocated to each ONU, there is a guard interval in order to ensure that the information sent by two consecutive ONUs does not collide, this interval varies according to the difference between the OLT distances for each ONU.

To discover the new ONUs, OLT periodically opens measurement windows, allowing new bursts to be sent upstream (ONU to OLT) to determine the distance and allocation of transmission time.

For this equalization to occur, the ITU-T in recommendation G.984.2 standardized the issue of distance, called the maximum logical differential.

In 2010, ITU-T launched recommendation G.984.7, which extends the differential range from 20 to 40 km, but does not depend on optical power, but rather on enough delay to allow delay in the propagation of the signal received by the ONUs OLT. For differential distances between 0 and 40 km, the propagation delay of 402 µs was defined.

Currently the maximum logical range for GPON technology is set at 60 km and the maximum differential between the first and the last ONU is 20 km.

So, it does not depend on the GPON module, nor on its power to reach a greater range (greater than 20 km), but on software implementations at OLT, so that it can handle this time, described by the recommendation.

DATACOM's OLTs (DM4610 and DM4615) are able and in accordance with recommendations G.984.2 and G.984.7, allowing the provisioning of ONUs of the 40Km differential range, also allowing the manipulation of this value, independently by PON port. Next, we highlight the output of the command:

Setup is simple and easy:

Today, DATACOM has two types of GPON modules available for sale: B + and C +. The D laser is in development by our engineering team. Our OLTs do not have locks for GPON modules from other manufacturers. We believe in the concept of free choice and that the brand decision is made by the Internet Service Provider / ISP itself.

Regarding the ONUs we have 5 models:

  1. DM984 family
    1. The. DM984-100B: C + laser
    2. DM984-420 and DM984-422: B + laser
  2. DM985 family
    1. DM985-100 and DM985-424: B + laser

Just like the concept of free choice for SFP modules, our OLT also does not block ONUs from other manufacturers. Our support and engineering team frequently performs interoperability tests with several ONUs in the market.

Follow our News and the information present in the release notes for more details on a regular basis.

It is worth remembering that Datacom has a complete structure in its headquarters where on-site training is offered. In the training it will be possible to manipulate the equipment, make configurations of different topologies and application scenarios in a complete laboratory environment, in addition to being able to count on the help of our professionals in a series of good practices that will help a lot in the operation of your network.

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If you have questions about these applications, do not hesitate to contact our suporte.prevendas@datacom.com.br team. We are available to assist you in choosing the product that best suits your needs.