Which outdoor fibre optic cable to use for an underground FTTH/P rollout?

In urban or in low density areas with recent dwellings, the customer connection to the ultrafast broadband network is performed in an underground network configuration. Whatever the architecture of your network, as soon as an optical cable is foreseen for an installation in underground configuration, even when ran on short distances, it is important to make sure that the selected outdoor cable is built with swelling elements. Thus, in case of flooding of the telecommunications manhole, these swelling yarns would prevent the penetration of humidity if the cable sheath were to be damaged.

To select the right fibre optic underground cable, there are two considerations to keep in mind: the network layout and the associated constraints. To help you conducting successful underground rollout projects, we set forth the following analysis focused on 3 case studies corresponding to 3 types of network layouts generally met in the field:

By following the cable route, we will get a better insight on the constraints to which the fibre cable will be subject to, assess the risks and, finally, make an informed choice to guarantee reliable and durable telecommunications networks.

Benefits of an underground installation:

  • • Aesthetic and unobtrusive networks
  • • Secured networks. Unlike the overhead cable installations, underground cables are less submitted to various hazards such as tearing or crushing

1. Underground installation exclusively: which indoor/outdoor drop cable to use?

FTTH/B rollout with a double-sheathed FO underground cable

In urban and suburban areas, for buildings with more than 8 dwellings an underground FTTH deployment is generally carried out. New Single Dwelling Units (SDUs) in residential areas are also subject to this type of rollout. Indeed, for the construction of recent homes, the supply of telecommunications lines has been planned to be provided via underground ducts and tubing. Thus, in urban areas the subscriber connection to very high-speed networks is performed starting from a telecom manhole located on the public space (whether this is placed under a sidewalk, a roadway or a park). The optical cable is ran into a split corrugated tubing up to the building’s entry point or, in some cases, all the way up to the customer’s premises if the cable is a multi-applications optical drop such as Droptic® LM3 or the outdoor double sheathed cable LM4 .

An underground optical cable depending on the distance to be covered

DTIo To enable FTTH connections into recent buildings, the optical signal can be routed all the way to the room where the fibre optic outlet has been installed with the use of an indoor drop cable. This latter is connected upstream to an intermediate Optical Telecommunications Outlet such as the Eline® DTIo (Device for the Termination Indoor of optical fibres). This optical box is usually installed inside a residential communication gateway. Then, outgoing from this DTIo , an optical cable is routed throughout a duct or tubing towards the Demarcation Point (DP or DPO). The DP is generally located in a manhole at the limit of the subscriber’s property.
Depending on its internal structure, the same underground drop cable can be used to link via ducts and tubing the telecom manhole placed on the subscriber’s property to another manhole located on the public space and so on until reaching the telecom operator’s manhole.

Technical advice: it is recommended to check out if the duct or tubing is already encumbered in order to determine the maximum allowable diameter for the cable to be installed. It is important to consider the compatibility between the cable’s construction and the distance to be covered. Fibre optic drop cables such as Droptic® LM4 and LM3 are engineered to be rolled out into ducts over several hundred meters, while optical drops such as LM2 or LM8 are only compatible with deployments over short distances, up to 50 meters.

Installation of a buried underground optical cable.

Among FTTH underground layout configurations, we can also observe fibre deployments carried out by using buried optical cables. This technique is more restrictive, as the cable is subject to the risk of tearing in case construction works were to be conducted on the subscriber’s property or at the subdivision level. Also, when a buried cable is damaged, trenches must be recreated to install a new cable. Thus, FTTH deployments using underground optical cables rolled out into ducts are more convenient. This latter installation technique provides a longer cable lifespan, as it benefits of mechanical protection against the elements, abrasion, etc.

Aerial-underground installation: which optical cable to run the fibre overhead and underground?

The aerial-underground configuration may be implemented when the fibre is available only on one side of the street while the subscriber to be connected is located at the opposite side and has an available underground infrastructure, extended to the property line. In this particular case, while subscribers located on the side of the overhead line will be connected via an aerial-façade configuration, the optical signal will be routed in an aerial-underground configuration for the rest of FTTH subscribers. Thus, from the telecom pole located near the house to be connected, a cable down-lead will be installed to further route the drop cable into ducts up to the subscriber’s premises.
Despite the apparent complexity of the jobsite, this type of FTTH project can be successfully carried out with one single FO cable part number. Thus, for greater distances than 50 meters in underground layout, Droptic® LM4 and LM3 cables are more convenient, while for shorter distances, inferior to 50 meters, both LM2BK and LM8BK drop cables are appropriate.
The choice between LM4 and LM3 depends on the requested cable diameter. The selection between the LM2BK and LM8BK has to be made according to the cable’s application. It is important to note that the LM2BK is engineered with a single module for different capacities (1FO, 2FO and 4FO), while the LM8BK is built with two fiber optic strands in two separate modules, enabling an independent fiber management.


Multi-application cables are the most appropriate solutions for this type of network configuration as they allow to:

  • simplify the management of product references
  • reduce OPEX by cutting off the expenses related to splicing applications performed by long trained technicians
  • minimise the risk of signal loss or overburdening the optical budget due to poorly performed splices

3. Underground-facade installation: which cable for facade and underground rollouts?

In this type of network configuration, a fibre optic drop cable is run in underground layout, from a Fiber Concentration Point (FCP) or a fiber splicing closure up to a telecom manhole located at the building’s entry or at the premises’ line. When the infrastructure is no longer available and the cable cannot continue its way in underground configuration, the FO drop will be fusion spliced with another optical cable compatible with facade layout applications (respect of bending radius, UV resistance, etc.). The underground drop can also be deployed seamlessly on a facade and indoor, if specifically built for multiple applications (in duct, façade, indoor) such as the LM4 outdoor cable or the LM3 fibre drop.

In an underground-facade fibre network layout, the very first meters of optical cable will be protected by a compatible cable cover. Then, inside the Optical Telecommunications Outlet used as an outdoor-indoor transition point, the telecom technician will strip off the LM4 outer sheath in a couple of seconds to access the LSZH-FR LM1L indoor cable. This last one will be run inside the premises, alongside doors, windows or baseboards. When using a LM3 drop cable, installation can also be performed uninterruptedly.

To aspects must be observed in order to distinguish between the LM3 and LM4 drop cabling solutions: the size of their outer diameter and their inner cable. Thus, while the LM4 is developed with an outer diameter of 4mm and includes a LM1L indoor cable (Ø 2.8mm), the alternative LM3 is engineered with an outer diameter of 6mm and a built-in LM1 drop cable (Ø 3.3mm). The choice between these two multi-application drops is to be made based on the the constraints observed in terms of bending radius, available space for an installation in a cable raceway, etc.