About GPON

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A GPON network is capable of transmitting ethernet, TDM (Time Division Multiplexing) as well as ATM traffic. A GPON network consists of OLT (Optical Line Terminals), ONU (Optical Network Unit), and a splitter. The splitter will divide the signal when needed. The OLT takes in all of the optical signals in the form of beams of light from ONUs and will convert it to an electrical signal. OLTs normally support up to 72 ports. An ONU connects to end users and will send their signals back to the OLT. A GPON network can reach up to 20 km and provide service up to 64 end users. GPON utilizes both upstream and downstream data by means of Optical Wavelength Division Multiplexing (WDM).

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Multiplexing consists of combining two or more information channels into one. There are many types of multiplexing but GPON uses WDM (Wavelength Division Multiplexing) wavelength division multiplexing, that is, multiplexing or superimposing several wavelengths on a single fiber using a laser or LED light beam.
• Downstream in GPON
• The packets are transmitted from the OLT in TDM (Time Division Multiplexing) broadcast mode.
• In TDM it is always being broadcast with a fixed time period, regardless of whether there is data available or not.
• All data is transmitted to all ONTs and the splitters are limited to replicating the data.
• Each ONT filters the data and keeps only those that are directed to its user. The rest of the data is discarded because it corresponds to other subscribers.
• The data is encrypted so that an ONT cannot read the data of another user.

Upstream in GPON
• The packets are sent by the ONT in TDMA (Time Division Multiple Access) mode.
• The data goes from the ONT of each subscriber to the OLT of the operator. The data of all users is collected in the splitter.
• In TDMA the ONT is only transmitted when it is necessary so that collisions do not occur when sending the data to the OLT.
• The OLT is the one that tells the ONT when to broadcast so that these collisions do not occur and it must know the distance of all the ONTs to take into account the arrival time of each one.

https://naseros.com/2017/03/13/como-funciona-una-conexion-de-fibra-gpon-y-ftth/