A very important part of the budget required to build new FTTH networks is dedicated to the installation of cables to distribute the optical signal to all users. In areas with low density of users, these costs can represent up to 80% of the total. In addition to the associated cost, also can appear legal or bureaucratic constraints: application for permission to undertake works, traffic cuts and inconvenience to users.

New techniques to facilitate the installation and to reduce costs are needed to boost new networks' lines.

Installation types

1.    Overhead installation:

The distribution and installation is easier and cheaper, especially if there are posts available on which to fix the cables. It is strongly conditioned by the laws of each country and local regulations, as well as aesthetic criteria.

The cable is fully exposed to the weather, so it must be specially reinforced depending on the weather of the place.

2.    Installation in underground trenches:

The cable can be directly buried or placed inside ducts. If it is taken advantage of the renewal of services or the construction of new ones (sewers, pipelines, roads), the cost of the installation of the cable will not significantly influence the total. Else, installation costs are increased, so infrastructure sharing by operators becomes necessary.

It is a good option when the trench is already open for other facilities, and local authorities usually encouraging the installation of cable or pipeline when a street or highway has to be opened.

Besides the economical cost, it is needed to apply for permissions, conducting roadblocks and traffic cuts that affect the users. There is also the possibility of damage operational services (electricity, water, gas).

3.    Underground installation in galleries and sewers.

It is vital the collaboration with operators of local authorities, as they have public domain properties that can be transferred to permit deployment of new infrastructure. This way it can be prevented the implementation of new work, but the installation is very dependent on the state of channels to use. In some cases, access is possible through these services or they do not fit in conditions..

This option, along with the overhead lines, is the most advantageous from the economic point of view.

4.    Micro-trench installation

The micro-trench installation is an alternative that reduces the impact of engineering work in the installation of new cables. The cost may be 1/3 of a normal channeling.

It consists of making a very small ditch which varies between 1 and 10cm wide and 10 to 20cm deep depending on the diameter of the cable to install. Once the cable is placed inside, protective elements or ducts where the cable will be installed later are placed in, and finally it is sealed with bituminous material.

The layout of the installation must be straight so not to induce bends in the cable, the abrupt changes of direction should be avoided, and when unavoidable should be made angle.

As main advantages:

•    Reduced installation costs
•    Less traffic problems and to the users than the traditional civil works
•    Fast deployment
•    Low production of debris
•    Low penetration in the subsoil
•    Easy maintenance

Disadvantages:

•    Exposure of the cable to heavy traffic vibrations
•    Use of special cables or adequate study of isolation to allow proper installation
•    Susceptible to damage in infrastructure maintenance, as the installation is made next to the surface.
•    Extensions require a new network installation

The phases of installation are:

•    Dig the micro-trench
•    Cleaning by using compressed air or pressurized water
•    Cable Placement
•    Placing protective fillers
•    Closed the micro-trench by applying special bituminous material

Fig1: a) Schematic of cable installation in micro-trench b) making a micro-trench c) Final aspect after the filling process

Cables used for this application should have the following characteristics:

•    Reduced diameter
•    High resistance to crushing and vibration
•    Resistance to high temperatures, sealing material can reach temperatures between 100º and 170º C.

These cables may have not enough mechanical strength on busy roads, being more desirable in a residential installation.

Installation can easily be affected by maintenance work and other underground services work and resurfacing, since the cables are located at shallow depth.

Adequate cables for micro-trench installations

With all mentioned above, the cables must have the following features:

•    Reduced diameter
•    High compressive strength
•    Good resistance to high temperatures
•    Contain material to prevent the spread of water inside

TELNET REDES INTELIGENTES offers two alternatives for this type of installation:

•   PDR fiber optic cable with up to 96 f.o.
•   ESP fiber optic cable with up to 96 f.o.

Using armed with steel cables or not depends on the need for increased crush resistance and high temperatures. These conditions are given by the details of installation and also the use or not of protection elements for cable insulation.

Conclusions

The installation technique of micro-trench allows to significantly reduce the cost of fiber deployment. It is one among the techniques described above, for which is needed an analysis of the installation place and the associated costs.

TELNET has two types of cables suitable for a possible installation using previously described micro-trench.

Also depending on the particularities of the facility it could be offered variations on the design of the cover to provide greater resistance to temperature or crushing special requirements.