Returning to aerial lines

Outside cities – in what is referred to as remote locations – traditional underground cabling for FTTH connections is often too complex and expensive. It is often impossible to use in rocky regions. Nevertheless, nationwide broadband supply should not fail. This is why municipalities and network operators are returning to the installation of aerial lines.

There is no doubt that aerial cabling is the most cost-effective method of laying for FTTB/FTTH connection. It is quick and relatively easy to execute, especially on the «last mile» to broadband customers. In previous decades, it has proved itself in the construction of telephone and power supply systems. The planning, however, is demanding.

Pre-terminating saves time

Time is money, even when constructing aerial lines. That’s why project managers think about how they can save time. The answer is pre-termination. The trend is moving away from splicing loose tube cables in the field towards using pre-terminated drop cables.

This trend is reflected in the sale of directly terminated cable types. Demand has been increasing for years. These cable types are particularly suitable for connections from the last distribution points to the connection boxes on the buildings. The R&M portfolio for FTTH connectivity offers the ZOONA splice closure, the family of Polaris boxes and cable products manufactured in-house in Europe.

 

 

Criteria for planning

When planning aerial deployment, particular attention must be paid to the installation environment and the climatic conditions. The climate over the entire year has to be taken into consideration.

The environment can be private or public. In private environments, telecom operators have greater control over their infrastructures. This applies, for example, to industrial networks or campus networks.

However, if cabling is in public areas, existing masts are normally used. Several providers may share the existing infrastructure. Experience has shown that this modality is more prone to operational accidents, resulting in downtime.

But there are other challenges to overcome. For example, the question is:

  • What about the capacity on site? How much space is available on the masts and can they withstand the strain of additional cables and connection boxes?
  • What spans and mounting heights are required? To what extent are the cables allowed to sag without being in danger of coming into contact with vehicles?
  • What demands are there from neighbors and local regulations? To what extent must the appearance of the environment be taken into account?

All of this can be taken into account at an early stage in the planning phase. For example, more robust fastening, sufficiently dimensioned splice closures, and suitable cable constructions can be chosen.

Criteria for drop cables

Some criteria for selecting drop cables can be derived from R&M’s international experience.

Drop cables must be small in diameter and weight. Ideally, they are round to ensure a more reliable seal on the connector or box.

When designing cables, care must be taken to ensure that there is sufficient anchoring distance, taking the climatic conditions into account. The coat should be easy to strip and still be weather-resistant. After all, the cable has to withstand changing tensile loads within a large temperature range for years.

The selection criteria can be concentrated on five simple rules:

  • the lighter the better
  • the thinner the better
  • the higher the tensile strength the better
  • the simpler the design the better
  • the darker the better

 

 

The meaning of aramid

Aerial cables should always be equipped with tension relief made of aramid. Glass yarns or glass rovings are not suitable.

The reason is the vibration. Aerial cables are constantly subject to vibrations and movements caused by wind. Glass yarns in the cable jacket would cut through each other and the cable would lose traction. In addition, aerial cables do not require rodent protection, which is ensured with glass rovings for underground cables.

The market offers miscellaneous cable tensioning devices. They are designed specifically for the respective cable construction. R&M recommends using tried-and-tested combinations of cable construction and cable tensioning devices.

Parameters for cable selection

A number of parameters should be used to select the optimum cables. All premises and individual application usually require their own specifications.

Cable and wire manufacturers usually specify the maximum tensile force (MAT) a cable has. They determine the value at room temperature (± 20°C). This value alone does not, however, indicate whether the cable can be used for a specific application.

To select the optimum product, the cable supplier needs details on the following parameters:

  • Maximum anchoring distance the smaller, the easier the solution
  • Minimum sag (% of span length) the bigger, the easier
  • Maximum wind load the lower, the easier
  • Maximum additional load (e.g. birds, ice, snow) the lower, the easier
  • Minimum, maximum operating temperature (D T) the lower, the easier

If the cable supplier knows these values, they can calculate the most cost-effective variant and successfully implement the requirements of the individual application.

Attributes and specifications should be listed in a technical document (e.g. data sheet). Usually, a span-sag table is used, which gives all values taking into account certain climatic conditions.

Basically, there are three standardized ambient conditions for the load capacity of the cables, known as NESC Light, NESC Medium, and NESC Heavy. Here is the table:

Load level Wind Ice Temperature Added Load
Light 95 km/h (430 Pa) 0 mm -1°C 0.7 N/m
Medium 63 km/h (190 Pa) 6.5 mm -10°C 2.5 N/m
Heavy 63 km/h (190 Pa) 12.5 mm -20°C 4.4 N/m

 

Conclusion

Aerial cables for FTTH connectivity in rural areas should not be over-dimensioned or overdesigned. And «one fits all» solutions are not recommended either. In commercial terms, a cable that fully meets all possible parameters would be unbeatable. In other words, too expensive.

It is also not advisable to simply define individually required aerial drop cables from a catalog or online shop. Relevant influencing parameters such as the load levels could be overlooked.

That’s why contacting the cable supplier you trust should always be your first choice.