The differences between drop cables, Riser cables and plenum cables

For the connection to very fast speed networks of subscribers in MDUs, several types of indoor optical cables can be used. Depending on the place where these cabling systems are to be installed, we can distinguish between two main categories of indoor cables: those to be run along the walls, with the use of gluing, stapling or pulling techniques and those to be installed inside walls. In the first category are included indoor drop cables with small diameters, fire performance and reduced bend radii enabling simple and reliable connections such as Droptic® LM1 or LM1L.

As for optical cables designed for an installation inside walls, there are three different cabling solutions : riser cables, plenum cables and also drop cables. Riser cables, as their name indicates, are fibre optic vertical cables developed for installation inside technical ducts or conduits passing from one floor to another and serving the entire building. Riser cables are also known by the acronym CMR which comes from Communications Multipurpose Cable.

fiber optic cables for indoor applications in MDUs

Plenum cables on the other hand, are installed in spaces intended for the air circulation such as heating, ventilation or air conditioning (HVAC). These cabling solutions are to be run between a false and a structural ceiling or, in some cases, between a structural floor and a raised floor. Because of their exposition to air circulation and to prevent the risk of fire propagation in case of a fire starting, plenum cables have a higher fire rating than riser cables.

Finally, certain drop cables can also be used for hauling the optical signal inside commercial or residential use buildings up to the subscriber’s optical wall socket.

Indoor optical distribution cables : Riser cable vs drop cable

Indoor optical distribution cables enable the FTTH roll-out in MDUs. They run from the BEP (Building Entry Point) to the customers’ optical outlet. Generally, these cables are installed in riser ducts, originally intended for the run of energy cables, for example. Sometimes, indoor optical distribution cables are also installed in already occupied flexible split corrugated tubing.

Riser cables may present different constructions: single fibre cables, single module cables and micromodule-based cables. The choice between an indoor optical distribution cable and another depends on the FTTH architecture, the number of fibers required to connect each subscriber, as well as the number of flats and ISPs available in the building. According to all these criteria, several types of FTTH deployments are possible:

  • A cable for each subscriber, from the BEP to the optical outlet. Sometimes, for connecting susbscribers to the very high speed networks telecom technicians run a cable from the BEP to the optical FTTH outlet of each end-user. The Droptic® LM1 drop cable is a perfect choice for this type of installation as it has been developed with a diameter of only 3.3mm for a bend radius of 15mm ( for information, generally for a cable installation at the BEP level, the required bending radius is of 30mm and above while for a cable installation at the optical outlet level, the bending radius is of 15mm and above). Thanks to the two FRP included in its construction, LM1 drops present a certain stiffness for fast roll-outs in riser ducts or occupied conduits.
    This type of FTTH architecture is only recommended for fibre optic deployments in buildings with a couple of floors, few flats and when a single ISP is present in this MDU. For fiber optic rollouts in high-density areas and for highly dimensioned MDUs, this choice presents multiple disadvantages. Indeed, a high number of cables can rapidly obstruct the passage inside riser ducts. Also, the slightest change of ISP can lead to important costs and intervention times.
  • A single module-based pushable drop cable from the BEP to the OTO. To bring fiber optic to the subscriber’s premises, telecom technicians can also run pushable drop cables into occupied split corrugated tubing. These micro-cables with reduced diameters are thus pushed into micro-conduits, manually or with the use of a blowing machine. Droptic® LM7 and LM7N drop cables, for instance, are designed for this type of installation. Thanks to their flexible yet rigid construction, Droptic® LM7 drops can be both pushed by a single technician inside flexible tubing and pulled for installation alongside walls. Moreover, Droptic® LM7 and LM7N optical cables are compatible with Field Mountable Connectors. These dielectric cables are CPR (Construction Products Regulation) compliant and enable a fast deployment into pre-installed ducts and conduits.
  • A riser cable connecting the BEP to the Floor Distribution Box (FDB). This type of installation presents numerous benefits: with a single cable up to 144 subscribers can be connected to the ultrafast broadband networks. Basically, a riser cable such as the Eline® optical riser is connected at the ground floor or in the basement inside the BEP (Building Entry Point) to the outdoor optical distribution cable. Then, it is run via a riser duct to the top floor. At each floor level, by splitting the outer sheath of the CMR cable, fibre is collected and then spliced to the drop cable inside the Floor Distribution Box. The drop cables will then further route the optical signal to the subscriber’s optical outlet.
    This FTTH architecture is the most commonly met as it allows considerable time saving while also reducing from one hand, the amount of cable introduced into riser ducts and on the other hand, the management of the cable slack at the FDB level.

What is FTTH Riser cable and how to use it ?

There are two types of FTTH Riser cables : those with a long extractable length and those with a medium extractable length. In the first case, the extracted module is routed inside a tube to the subscriber’s optical outlet. The intervention on Riser cables with a medium extractable length consists in extracting the fiber and connecting it with a drop cable inside a floor optical distribution box, by fusion splicing. For both types of FTTH riser cables, the fibre extraction is performed in three steps:

Riser cable for enabling customer connections to the ultrafast broadband networks

  1. An opening (cable slit) is made in the outer sheath of the Riser cable on the floor level where the subscriber is located.
  2. On the upper floor, another opening is performed in the vertical optical cable so to identify and cut the module to be used for the subscriber’s connection.
  3. The previous operation will enable to rapidly pull out the needed fiber module so to connect a fiber from it with the subscriber’s drop cable.

This fiber extraction technique from the Riser cable enable to minimize the cable slack management inside optical junction boxes. For simple and fast FTTH roll-outs inside MDUs, Riser cables with wider inside diameters and non-stranded constructions are more suitable solutions.