Fiber-optic networks are coming to replace traditional coaxial and copper networks. Spain is, in fact, the leading country within the European Union in the deployment of this kind of fiber-optic networks and intends to expand to rural areas. Considering how it is moving forward and their advantages over other models – such as high-speed connection – at Televes Corporation we would like to describe the key components of last-mile fiber networks – commonly known as GPON (Gigabit Passive Optical Network).
Before we take a look at the components making up fiber-optic networks, we should understand what these networks consist of. It should be noted first that this technology has been developed over a long period of time – roughly 60 years. To become today’s technology, it has been necessary to improve the production of the glass used in fiber optics. Nowadays, not only is it used in the telecommunications sector, but also in other fields such as biomedicine, the car industry, aeronautics, among others.
Last-mile fiber-optic networks are made of active and passive elements.
Among passive elements we can find fiber optics, optical attenuators, optical splitters and the optical taps that can be used in the roll-out.
The most common active elements are OLTs – which are installed in the head-end –, optical amplifiers and ONTs – user reception terminals. This kind of network can be described as shown below:
At a conceptual level, this fiber-optic architecture is quite similar to coaxial-cable and twisted pair.
Fiber optics is based on the transmission of data as photoelectric pulses along a waveguide – fibers made of glass or plastic – able to transmit light. Unlike copper, glass fibers can be very thin (125 µm in diameter), which is very advantageous if we take into account that a great amount of fibers could fit in a regular copper cable.
Data is transmitted by means of a light signal – which is invisible in the network – and travels from one end of the fiber to the other. These features allow the signal to be faster than metallic cables while experiencing less losses. Then, information can be transmitted directly within a distance of, for example, 80 km, which would be impossible to do with copper. Because they do not conduct electricity, this technology is not subject to electromagnetic interference, unlike twisted pairs.
Fiber is the main component in a fiber-optic passive network, as it is the core of the wire. Depending on the cable, it can be made up of hundreds of filaments. There are different types of fiber: single mode and multi mode.
Single-mode fiber (core diameter of 9um) is normally used for long distances because of their lower attenuation and their ability to transport extraordinary amounts of information depending on the used light source.
As for multi-mode fiber (core diameter of 50 or 62.5um), it is normally used for shorter distances because of its greater attenuation and inferior ability to transport information compared with single-mode fiber. When transmitted through single-mode fibers, light beams bounce off the core walls inside the fiber, leading to a multipath transmission.
It is single-mode fiber what is used when last-mile networks are being deployed.
These are the passive elements allowing optical signals to be divided in different paths depending on the number of users connected to the network.
When we are talking about this kind of network, the number of users that are included is normally indicated (32, 64…). This makes it necessary to use optical splitters to split the signal so it can reach them all. Distributing TV in a building is quite similar to this, because either splitters or taps are used so the signal can reach every neighbor.
Along with the OLT and the optical amplifier, they are reception elements that will be used if the network provides traditional TV or SAT services.
The OLT joins together every digital service (IP) the subscriber will have access to through their ONT.
In this case, we are referring to the ONTs installed in the final user demarcation point. If the network is providing traditional TV and/or SAT services, it will integrate a specific receiver for such application. If this is not the case, the ONT is a router with a fiber-optic input featuring several Ethernet outputs – where our computers can be connected –, Wi-Fi signal for mobility in the building and twisted-pair outputs, allowing us to place our traditional telephone.