GPON networks and optical budget

GPON Networks (Passive Optical Network Gigabit-capable) have the basic structure of a PON network, such as active elements at the ends of the network and passive elements that transmit and spread the signal from the central to each of the subscribers' homes.

GPON networks, as defined in ITU-T G.984 standard, with a range of 20Km and a number of users 64. However, the standard is ready to extend GPON networks up to 60Km and increase the subscribers to up to 128 users. However, the main constraint on the deployment of GPON networks is in its own network architecture point-multipoint.

The division of the signal from the central to the users is done through passive optical splitters that divide the incoming signal into 64 different fibers. Being passive and having no amplification of the signal of any kind, the power of the signal at each output is 1/64 of the received power, which applies a large attenuation to the signal (about -18.5 dB).

Besides the degree of splitting, the distance at which the end users are from the central also applies attenuation to the signal. 20km of distance can cause -6.5dB. This, adding the connections makes the signal to arrive too weak to the final users.

Therefore, usually the GPON networks must find a balance between the distance between the central and the user's home and the degree of splitting. This is what is called "optical budget" and is what will help to ensure that users receive a sufficient level of signal to ensure the quality of service in a GPON network.

Scenarios for the regeneration of the GPON signal 

GPON-Extender is an optoelectronic repeater that allows, through signal sampling, to regenerate the GPON signal and re-broadcast it again with the original power. This is possible because, unlike the optical splitters, this is an active element and can regenerate, amplify and broadcast the attenuated signal that it receives in a GPON network.

This can be used to remove the restrictions that we are limited to by the optical budget in both distance and number of users on a network:

Extending a GPON network from 20Km to 60Km

There are times when it is wanted to reach a residential area or a rural area that is located over 20 kilometers from the nearest central where an OLT can be placed. Through the regeneration of the GPON signal, we can extend the length of a GPON network to reach these points, reaching a distance of up to 60Km eliminating the attenuation due to distance.

These 60km is the maximum logical distance that may be in a GPON network between the OLT and tha furthest ONT, and can be obtained through the concatenation of several GPON-Extender. Each of the cards could extend the network about 20Km, always depending on the particular characteristics of each link.

In this model of deployment, we extend the distance to the homes to up to 60Km from the central, which allows reducing both the CAPEX and the OPEX by eliminating an otherwise needed OLT, closer to the deployment area.

If necessary, several GPON-Extender equipments can be chained to get the desired distance and proper signal strength for the later number of users and level of splitting.

Expanding the number of users of a GPON network from 64 to 128

In the following case study, we will assume a GPON network in which we want to provide service to 128 users. This implies that after passing through the passive splitters, the signal would have power 1/128 times lower than the power emitted by the OLT, not counting the additional attenuation by distance and by connections.

In this scenario, it can be placed a GPON-Extender regenerator after the first level of splitting in one or more branches of the network, regenerating and amplifying the signal completely and allowing this way re-dividing each of these branches into other branches, each in a new optical splitting level and reaching this way the 128 users that the standard is prepared for.

This way, we maximize the investment by using the network to provide maximum coverage and service to the maximum number of users that the ITU-T G.984 protocol can support, as otherwise, the number of users that can be serviced would be limited by the optical budget, having to find other solutions for new potential users in the area of the GPON network.

Extending the branches of a GPON network

There are times when a new residential area is wanted to be covered and it is close to another area where there is already an established GPON network and in which users can be added, but the new area is not large enough to profitably fully deploy a GPON network for itself.

In such cases it would not be needed to install a new OLT close the new community. If the distances and the number of available users are enough, we can use a branch of the already deployed GPON network to, through GPON-Extender 1:4, extend the branch and create a new level of splitting that provide service to the new community that we want to include in the GPON network.

This will significantly lower the cost of deployment and operation in the new area to be covered, while allowing amortization and maximizing the investment previously made in the deployment of the existing GPON network by using its infrastructure to provide service to the new community.

Amplification of the 1550nm signal

GPON carriers that multiplex digital signals (1490nm-downsrteam- and 1310nm  -upstream-) with analog and/or digital carriers in the 1550nm lambda, can use the EDFA gain block (optional) integrated in GPON Extender. This module is added to the basic configuration and optically amplifies the third lambda without changing the values of the GPON signals.

A bank of optical filters in the entrance and and the exit of the EDFA  block guarantee a gain of up to 20dB at 1550nm carrier with a low noise factor (NF<5dB).