FTTx fiber optic network topology

Currently the vast majority of Internet users in Spain connect through fiber optics. In Spain the deployment has been really fast, and it has been greatly noticed in the speed of Internet connection that users enjoy today. The most common is to have speeds of 300Mbps symmetric or higher, the star speed being the popular 600Mbps symmetric. Today in RedesZone we are going to explain in detail the different fiber optic network topologies (FTTx) that exist today.

Multimode fiber optic vs singlemode fiber optic

First of all we are going to explain to you that there are two types of optical fiber, we are talking about the so-called multimode and single- mode fibers .

The multimode fibers are fibers that allow the simultaneous transmission of several light beams (modes), each beam is reflected at a different angle within the core of the cable. Unlike single-mode optical fibers, multimode optical fibers, in order to allow this, need to have a larger core diameter than the former. Within multimode optical fibers there are two other types that we are just going to mention: they are those with a stepped index, and those with a gradual index. The latter achieve a slightly higher range. Multimode fibers are cheaper to manufacture (compared to single-mode fiber optics) and their range is relatively small (about 10 km) compared to single-mode.

In contrast, single-mode fibers only allow one mode of transmission. This is achieved by reducing the diameter of the core. It is reduced so much that there are hardly any bounces, the light beam propagates parallel to the axis. Its distance can be up to 400 km, but the cost of its manufacture is expensive. As it is already known, one of the advantages that optical fiber has with respect to the copper pair, is that it is hardly attenuated. In a copper pair we can speak that in a kilometer distance, 25 dB can be attenuated (it is an approximate average, since each pair is attenuated in a different way), but the attenuation level of the optical fiber is much lower, in one kilometer is attenuated 0.30 dB.

FTTx modalities that exist

After these explanations and clarifications we go with the different FTTx modalities. It is clear that for all of you the best known and most listened to is FTTH (Fiber-to-the-home), and it is the only one that can be sold to the subscriber as optical fiber, since, in the rest of the types of FTTx, fiber optical does not reach the user’s home to the PTRO (optical PTR).


It is also known as Fiber-to-the-home, in this type of optical fiber it is reached directly to the home, where we will have an optical PTR to connect the fiber cable directly to the operator’s ONT or to our router if it includes the ONT in the same device. This fiber optic modality is the only one that reaches the subscriber’s home, and is the most used in Spain due to its performance, both in terms of speed and latency.


Also known as Fiber-to-the-building, in this fiber mode, the connection to the building is reached with fiber optics, the fiber ends directly at the RITI or RITU in a CTO. Once the optical fiber has reached the intermediate distribution point, the end subscribers in the building or in the house are accessed using VDSL2 technology, copper pair or directly over twisted pair (Cat 6 network cable). Thanks to this modality, we will have fiber to the building itself, to later carry the connection using different lower-cost technologies.


It is also known as Fiber-to-the-cabinet, in this fiber mode you reach a telecommunications cabin or closet near your home, normally it should be less than 300 meters away.


It is also known as Fiber-to-the-node, this fiber modality is similar to FTTN, but in this case, the telecommunications cabin or closet is further from the home, more than 300 meters away, therefore, we will have worse performance overall.

Other FTTx modalities

After having seen the main FTTx modalities that currently exist, there are other minority denominations that are not usually used, or at least, are used very little, but it is important to know them to know what we are talking about.


Also known as Fiber-to-the-office, it is exactly the same as FTTH, but instead of being domestic “homes”, they are “offices.” This means that the optical fiber reaches the interior of the subscriber’s office itself. Logically, this FTTO could have specific configurations for companies such as embedded videoconferencing platform, VoIP, without TV and with guaranteed Internet connection speeds, but this will depend on each Internet operator.


It is known as Fiber-to-the-premises, it is usually a general term to designate FTTH and FTTB architectures, it is also designated to fiber optic networks that include both homes and small businesses.

What modality can we find in Spain?

Currently the most widely used modality of FTTx is FTTH, fiber optic to the home. Depending on where your house is located, the FTTH could reach you through a CTO hanging from one of the facades of your house, or directly through a RITU in the community of owners if the urbanization or the apartment block is of recent construction.

If it reaches you directly through a hanging CTO, the technicians will pull the outdoor fiber cable through the facade until it reaches your house, and once inside, they will place the PTRO (optical PRT) where you will connect the yellow fiber cable, and it will go directly to the operator’s ONT.

If the fiber reaches the RITU, there the operator or operators will have their different CTOs, and the technician will simply have to connect the fiber cable that comes from your home to the RITU, to the CTO of your operator. The installation is clearly much simpler in this type of FTTH connections, because all the fiber optic cable is already pulled through the different conduits, it is only necessary to connect the cable that arrives from your home to the CTO, and from the PTRO of your home to the operator’s ONT to have an Internet connection.

We must not forget that FTTH technology and also the rest of FTTx, make use of passive optical networks (PON), generally they make use of GPON technology that is the one used by Internet operators, although they are already modernizing the entire fiber network optics incorporating XG-PON technology to provide us with even more symmetrical speed. The fiber optic cabling does not need to be replaced, but it is necessary to change the OLTs and ONTs to make them compatible with the new standard that is about to arrive to offer speeds higher than 1Gbps symmetrical in our homes.

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