Drop Cables For Customer Premises FTTH Installation
A FTTH PON network consists of 3 main segments: OLT (Optical Line Terminal), ODN (Optical Distribution Network), and ONT (Optical Network Terminal). OLT is the main element of the system, which provides triple-play services to all subscribers. All ONTs are connected to the OLT via ODN. Subscribers have ONTs, which enable services.
ODN is a completely passive optical network, which is composed of optical cables, optical distribution boxes, optical closures, optical splitters, etc. Each ODN consists of 3 segments: feeder segment or feeder optical cable, distribution segment or distribution optical cable, and drop segment or drop optical cable. Each of these ODN segments has its own specificities, especially when we talk about the types of optical cables. Today's article describes the drop segment.
Drop Optical Cable
The last optical segment of the ODN is the drop segment. The drop optical cable is located between the optical access point and ONT. The optical access point is usually an optical splitter in the optical distribution box. At this point, the drop cable must be connected to the distribution network with fusion/mechanical splice or optical connectors. On the other side, near customers, there is ONT, which requires only an optical connector.
Drop optical cables have usually 1 or 2 fibers, or sometimes 4 fibers. Cables can be installed aerially, underground (standard ducts or micro tubes), and directly buried. Typically G.657.x (bend insensitive) fibers are used since they may require complex routing inside buildings. Drop optical cables can be without connectors or with optical connectors on one or both ends (pre-terminated or “plug & play” solution). All of these cables are characterized by small dimensions, light weight, high flexibility, simple structure, easy installation, etc.
We can divide them into two large groups: indoor and outdoor drop cables.
Indoor Drop Cables
Indoor drop cables can have 1, 2 or 4 fibers, most often G.657.A2 standard. Usually, this type of cable is fig. 8 ("number 8"), where there are two parallel FRP reinforcements at the ends of the cable and fibers in the middle of the cable.
This cable does not have factory-installed optical connectors and requires splicing on both ends. One end of the cable is spliced in the optical box/cabinet in the corridor of the building, and the other end of the cable is spliced in the ATB (Access Terminal Box), in the subscriber's apartment.
In the optical box in the corridor of the building, most often fiber is spliced to fiber. Sometimes, the fiber of the drop cable is spliced into a pigtail and this way it is terminated in the cabinet/box. On the other hand, in ATB, the fiber is always finished in a pigtail. Most telecommunication operators use SC/APC connectors, very rarely SC/PC or LC connectors. Due to reflection problems, it is especially important to use SC/APC if there is a CATV service in the system. This type of cables are mainly used in new buildings. They are installed in PVC pipes, which are located in the walls.
There are also round indoor drop cables with or without factory connectors on one or both ends. If there are connectors at both ends, then the connection is made in the optical box to the optical splitter and the subscriber is connected directly to the ONT. In case where there is one factory-installed connector, that end of the cable terminates to the ONT and the other end of the cable is spliced into the box. This type of cable is most often used in old buildings. It can be installed through PVC channels, ducts, etc.
Special Type of Indoor Drop Cables – Transparent Drop Cables
In the last few years, transparent or invisible drop optical cable has started to be used. A transparent drop cable has one fiber, and it can be with or without a connector (usually SC/APC). The most common standard is G.657.B3. There are several ways to install it. For this purpose, the Corning company has produced a transparent adhesive channel in which this cable is installed. A special tool - "small car" is used for cable installation. We can see this solution in the next figures.
For the installation of this cable, the Chinese company Huawei uses a special tool that directly sticks the optical cable to the wall. This solution is shown in the next figures.
And many other companies have joined the production of these types of cables. These solutions are especially good for old buildings. People don't like to see new cables, channels, etc.
Outdoor Drop Cables
There are different types of outdoor drop cables. They have significantly bigger protection then indoor cables. They have to withstand large changes in weather conditions and must provide mechanical protection. For air installation, fig. 8 and round drop cables are used.
Different types of round drop cables are used for installation in PE tubes and direct burial. Cables that are buried directly have layers of metal wires or tapes to protect the fibers from mechanical damage - armored duct cable.
Fig. 8 can have 1, 2, or 4 fibers and without factory-installed connectors. The most common standard is G.657.A2. This drop cable requires splicing at both ends. At one end, it is spliced into an optical box, while at the other end, it terminates at ATB (pigtail).
In the last few years, more systems have used pre-terminated round drop cables. These drop cables are G.657.B3 standard and have SC/APC optical connectors on both ends. In this way, a very easy, and fast installation is enabled. At one end, the cable is connected to an optical splitter (optical distribution box), while at the other end, it is connected directly to the ONT.
Outdoor cables are black, and it is a problem for the user's house/apartment. Because of that, the companies started to think about that topic. Subscribers prefer white cables in apartments. In the next figure, we can see a solution for this problem - the black cable is outside and the white cable is inside.
Special Type of Optical Cables – Hybrid Cables
In situations where the remote power supply of certain ONTs is required, special cables are used – hybrid cables. This type of cable has SM fibers and electrical power wires. They can be with different numbers of fibers, different G.657.x standards, and with or without connectors. We use these cables in different enterprise solutions. An example of this cable we can see in the figure below.
Preterminated Cables vs Standard Cables
As we could see, there are pre-terminated cables and cables without connectors that require fusion or mechanical splicing.
cables are cables that have optical connectors on one or both ends, most often
SC/APC. In this way, the implementation of connecting subscribers is
significantly accelerated. The technical team does not need to have special
tools and a fusion splicer. In addition, no trained staff is required.
Otherwise, this type of cable offers significantly higher reliability than
standard cables without connectors.
Visual inspection and constant cleaning of all optical connectors are very important. The largest number of problems on optical networks are the result of dirty connectors. Visual inspection is checking the surface of the connector using optical microscopes. If the connector is damaged, it must be replaced. In case there is dirt, cleaning is mandatory. The best cleaning method is the wet-dry method. Very popular tools - one-click cleaners - are mostly used for connectors.
Standard drop cables require mechanical or fusion splicing. The optical cable can be spliced to the pigtail or a special tool can be used and splicing the optical connector directly – splice-on connectors. This requires trained personnel in the first place. Then, the technical team must have special tools and a splicer. The reliability of splicing by a technician is significantly lower compared to a pre-terminated solution.
Mechanical splicing is the least reliable connection method - it has more splicing and reflection than fusion splicing. In addition, the performance of the index machining gel decreases over time. And in this way, atenuation and reflection are increased. In general, a very unreliable solution.
Mechanical connection in the drop segment is the use of fast connectors, where technicians install fast connectors on the ends of the cable. Fiber preparation is the same as for fusion splicing. There is a special tool for measuring fiber. After preparation (strip/cleave), the fiber is introduced into the optical connector, and positioning and fixing are performed. For a more reliable connection, VFL can be used. In the next figure (left side) we can see a special tool for measuring fiber and fast SC/APC connector.
Custom Premises FTTH Installation
The installation of the drop cable can be aerial, underground in a PE tubes, and directly buried. All installation methods are used. Buildings are most often connected underground using PE tubes. Houses can connect everything using all methods. In rural and sub-rural areas mostly air installation, while in urban areas the remaining two methods are used. Of course, all methods are applicable everywhere. Due to the economy, underdeveloped and poor countries use aerial installation in most areas.
When doing underground installation for houses, standard procedures are used, whereby the drop cable ends in the house through the wall or on the facade of the house. If it ends on the facade of the house, there is a box with ONT. In the apartment, between the ONT and the user’s devices there are already cables. If the cable enters the house, it is led to the intended place for the ONT.
Drop cable for aerial installation most often ends up in the subscriber's house using different methods. In the next figure, we see one of the ways of entering the house.
The drop cable terminates directly at the ONT in the case of pre-connectorized cables or the drop cable terminates at the ATB in the case of standard cables without connectors. From the ONT to the user’s devices (TV, STB, IPTV box, WiFi router, etc.), there are coaxial RG6 cables and UTP cables (usually cat. 5, cat.5e, and cat. 6). All of these cables are fixed in the apartment using glue or special clamps or plastic channels are installed.
In new buildings, investors in cooperation with telecommunication companies immediately install FTTH network, coaxial cables, and UTP cables in corridors and apartments using PVC tubes. In the next figures, we can see a part of the installation in the new buildings.
During installation in apartments and houses, technicians must work carefully with all cables and be meticulous. It is necessary to reduce the creation of new holes, damage to walls and floors, etc. If the existing cables in the apartment/house are good, they should be used to the max.
There are several different types of drop optical cables, which can be divided into two large groups - indoor and outdoor. They can be with or without factory-installed connectors. For the reasons described above, pre-terminated cables are often the best solution. The installation of the drop segment is similar to the installation of other ODN segments - aerial, underground in a tubes, and directly buried. There is a difference is FTTH installation in buildings and apartments.
Vladimir Grozdanovic is a graduate electrical engineer for
telecommunications with more than 10 years of experience in access networks
(HFC and FTTH) in large cable operators in Serbia (SBB and Jotel).
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