III. Aerial versus Underground routes - Issues to consider

ANACOM sought to identify some of the issues to be considered in respect of communications cable routes using the two methods: aerial routing and underground routing.

  • Comparison or Selection Requirements

In order to compare the different methodologies of construction, installation and protection of optical fibre network infrastructure in terms of costs, it is important to consider not only investment costs (CAPEX), but also costs that result from operation, maintenance and functioning (OPEX). As such, account must be taken of the duration of the useful life cycle that is to be guaranteed for the infrastructure concerned.

As far as investment costs are concerned, the least expensive options are necessarily those making use of existing suitable infrastructure, which, at the time of installation, takes advantage of circumstances where another entity intends to carry out a construction project which allows sharing and consequently a reduction in costs, or which imply the construction of above-ground infrastructure, in particular as regards aerial communications cables.

As regards operation, maintenance and functioning, it is important to consider, for the duration of the infrastructure lifecycle, costs related to improvement and protection, which in the case of forest fires include above-ground infrastructure costs related to clearing and removal of vegetation, as well as rents payable for the use of suitable infrastructure or sites belonging to other parties or costs incurred due to the supply of goods and services. Further to these costs are costs which stem from the modification of infrastructure where adaptations are necessary due to changes in infrastructure requirements or objectives not foreseen during planning, and finally recovery and repair costs resulting from damage and other losses caused, in particular, by forest fires and other extreme events.

However, the cost perspective is just one of the requirements to be taken into consideration when it comes to comparing or selecting techniques of construction and installation of outside optical fibre network infrastructure.

According to the ITU1, for the purpose of comparison or selection of technologies to be used in support of sustainable and low-cost telecommunications infrastructure, several requirements are identified, besides cost (CAPEX and OPEX), including (i) reliability; (ii) data rate; (iii) flexibility; (iv) scalability; (v) energy efficiency and (vi) environmental impact.

In the same recommendation, the ITU considers that the reliability of the solution should be enhanced as far as possible to cope with harsh natural environments.

  • Best Practices in Underground Route Construction

According to the ITU, best practices in construction and installation techniques for underground routes of optical fibre network infrastructure requires some alteration and adaptation of procedures and methodologies traditionally used in the construction and installation of cement duct networks.

Following the installation of underground piping infrastructure, optical fibre cabling is blown in2, i.e. fibre cable is no longer pulled and stretched.
In terms of planning, a Geographic Information System (GIS) should be employed, with comprehensive information on subsoil infrastructure existing in the area of work; the SIIA - Sistema de Informação de Infraestruturas Aptas (Suitable Infrastructure Information System)3 is particularly suitable for this purpose. In addition, especially in the case of techniques that do not involve trenching, it preliminary analysis of the subsoil must be performed along the intended path, by means of soil radar penetration 4 5.

Infrastructure construction and installation techniques are divided into two types, depending on whether a trench is necessary or, if not, a subsoil drilling or boring technology is used.

In the case of trenching techniques, the choice of the specific technique depends, among other things, on whether the intervention is made on asphalted ground, i.e. on the roadway or on earth. Intervention on the roadway depends, inter alia, on articulation between the entity seeking to install the fibre with the entity that regulates access to the asset of the public domain - the roadway. The SIIA is especially focused on facilitating this articulation. So-called micro trench techniques6 or mini trench techniques7 are particularly suited for interventions on asphalted terrain, and the ITU-T's work covers intervention in urban areas regarding the use of low-impact trenching techniques8.

Subsoil installation techniques are often used in cases where an obstacle, such as a road or a river, is traversed by crossing underneath, although their use is not limited to these cases. The techniques used employ boring methods, in the case of guided horizontal boring, or drilling, by means of directional drilling methodology9.

1 Recommendation ITU-L L.1700 (06/2016) - Requirements and framework for low-cost sustainable telecommunications infrastructure for rural communications in developing countries.
2 Recommendation ITU-T L.57 (05/2003) / L.156 (02/2016) - Air-assisted installation of optical fibre cables.
3 Sistema de Informação de Infraestruturas Aptas (SIIA) - Suitable Infrastructure Information System (SIIS)https://www.anacom.pt/render.jsp?categoryId=384522.
4 Recommendation ITU-T L.39 (05/2000)/L.257 (02/2016) - Investigation of the soil before using trenchless techniques.
5 Recommendation ITU-T L.84 (07/2010) / L.260 (02/2016) - Fast mapping of underground networks.
6 Recommendation ITU-T L.49 (03/2003) / L.154 (02/2016) - Micro-trench installation technique.
7 Recommendation ITU-T L.48 (03/2003) / L.153 (02/2016) - Mini-trench installation technique.
8 Recommendation ITU-T L.155 (11/2016) - Low impact trenching technique for FTTx networks.
9 Recommendation ITU-L.38 (09/99)/L.152 (02/2016) - Use of trenchless techniques for the construction of underground infrastructures for telecommunication cable installation.