Italy is equipping itself with a public fiber optic network, with construction sites open in thousands of municipalities in market failure areas, in addition to private investments, with the direct intervention of Open Fiber and the Flash Fiber consortium of TIM and Fastweb.In all these cases, the technology chosen to bring the optical fiber directly into Italian homes is the one called GPON or Gigabit Passive Optical Network.But how is this network made and what are the real performances?Let's see in more detail how it works and why it was chosen by those who decided to invest in FTTH, including the Italian State.The benefits of optical fiber are known to most: with a relatively inexpensive plastic cable and using light instead of electrical signals, very large amounts of information can be transmitted over long distances, with great scalability and low maintenance.What perhaps many do not know is that it is possible to create an optical network without any active component, that is, without electronic devices that need to be powered by electricity.A junction box typically installed in a road pit.It can serve up to 48 homes.This is the basic idea behind GPON technology, in which the distribution network is made up entirely of fiber cables and optical-only equipment, splitters and couplings.The first is a device that copies the incoming light onto many outputs and vice versa;the second allows two sections of optical fiber to be joined together.The real electronics are at the two ends of the network, where the devices that transform light into electrical signals and vice versa are connected, which take the name of Optical Line Termination or OLT (operator side) and Optical Network Termination or ONT (user side) .This architecture has several benefits: the infrastructure is "lighter" because it does not require electrical power and its implementation is easier.Furthermore, the absence of electronic components also simplifies maintenance and reduces the possibility of breakdowns.These are the reasons why in Italy and beyond most operators have chosen this solution for the development of FTTH networks, compared to point-to-point infrastructures that require active equipment.However, there are also some disadvantages.As we have just seen, the network is composed only of optical equipment, with splitters that copy the incoming signal on a fiber to multiple outputs.But the network must reach more users, each with their own traffic.How is it possible to reconcile the two?At the topology level, the GPON network is made up of several point-to-multipoint trees, with a main branch coming from the network entrance "nozzle", typically installed in the Point of Presence or POP, and which gradually opens up with different branches according to a certain multiplication or split factor.CNO of Open Fiber for the white areas, with split on 16 fibers.In white areas, for example, Open Fiber uses a split factor of 16, meaning that each fiber, starting from its OLT installed in the POP, reaches 16 users at the other end.But the tree starts from a single cable, so those 16 termination points share the capacity of that starting point.Each branch of the GPON network has a download capacity of 2.5 Gbit / s.This means that if all 16 users registered to the same tree occupied the resource at the same time, they would have a theoretical maximum bandwidth of about 150 Mbit / s.It is no coincidence that for the Ultra Broadband Plan, whose objectives speak of a network capable of carrying at least 100 Mbit / s to all users, that multiplication factor was chosen.Usually, to give the idea, a factor of 1 to 64 is used instead, ie each tree is shared by a maximum of 64 users.The fact that the network is passive implies that in download all the traffic that starts from the main branch of the tree propagates in cascade towards all the smaller branches.That is, each user sees the same signal arriving on his fiber, including traffic destined for other homes.Each packet transmitted over the network will naturally be addressed to a specific user ID and the ONT installed in the house is programmed to read only the packets addressed to it and discard everything else.To prevent a malicious person from reprogramming his ONT to "listen" to traffic destined for others, clearly any communication sent on the PON branch is encrypted (in AES128).It must be said that the security of the GPON network is one of its most criticized aspects of this type of architecture, especially in light of the vulnerabilities often brought to light by the ONTs provided by the operators.If when the network downloads, in the absence of particular limitations imposed by the operator, it is legitimate to expect to be able to enjoy a 1 Gigabit / s connection, when the load increases, even an FTTH connection can see the maximum download speed drop significantly based on the number of active users registered on the same PON tree and the traffic generated by them, precisely because the resource is shared.In a historical period like the one we are experiencing, in which domestic users are more used than usual, even the performance of the fiber network can suffer.Fortunately, GPON networks are designed to last up to 40 years, with the potential to increase bandwidth as technology advances.In the direction of the upload, ie from the user to the operator's network, the situation is reversed and only the signal generated by the single user passes on each "branch", effectively establishing a point-to-point connection.But when the individual beams are aggregated together it is clear that being a shared resource not everyone can speak at the same time, which is why even in upload traffic is regulated by a multiplexing technique.In any case, downstream and upstream travel simultaneously on the same fiber optic cable, this is because two different wavelengths are used for the two signals, in more intuitive terms two different "colors".This technique, which takes the name of frequency division multiplexing, is the one that will be implemented to evolve the GPON network into the NG-PON2 (Next Generation Passive Optical Network) which promises to bring the bandwidth of the network up to at least 40 Gigabit / s precisely using different wavelengths to multiply the number of users served by the same fiber, with the possibility of offering users symmetrical connectivity services at 10 Gigabit / s.The important thing to underline is that the transition to the NG-PON2 network takes place without modifying the existing GPON network, simply by replacing the terminal devices of the network, ONT and OLT.Open Fiber, for example, has already activated 10 Gbit / s symmetrical services in experimentation in XGS-PON technology, which coexist with normal 1 Gbit / s users using the same PON infrastructure.It is for this reason that these are future-proof networks.The GPON network (and its subsequent evolutions) provides connectivity in the last mile, starting from an entry point where operators install their equipment, the aforementioned PoP, up to the real estate units.The fibers that start from the PoP (called PCN, Neutral Delivery Point in the white areas), travel towards the CNO, an acronym that stands for Optical Nodal Center, where the devices that perform the splitting from one to several fibers reside.Example of topology of a GPON network, in this case the one used by Opne Fiber in the Ultra Broadband project which affects the white areas and which will remain public property.In these areas, an NCP is expected to serve more municipalities.These are the lockers that we can see in our streets and depending on the splitting factor of the network, there may be two in cascade before arriving at the real estate unit.Each cabinet can receive multiple fibers connected to different OLTs and accommodate multiple splitters.From here the fiber travels to the wells where the joints reside, from which the fibers for the Building Optical Splitter or ROE depart, the "boxes" that are installed on the facade or inside a building from which the fibers depart for The flats.In the case of the network that is being built in the white areas by Open Fiber, the ROE is expected to be installed directly in the manhole in the street near the houses or on existing public poles, with the last section being built only upon activation request. of a user.Finally, the fiber from the ROE rises in the homes towards the optical socket, to which the ONT will be connected, where the optical fiber enters on one side and the Ethernet connection to the user's router or provided by the operator on the other.The network remains of the optical type up to the apartment and becomes of the Ethernet type only at the output of the ONT, the small box that the operator installs when the service is activated.When operators are ready to increase the capacity of the network, it will be enough to replace the ONT.Copyright © 2022 DDay.it - ​​Scripta Manent publishing services 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