The ‘last mile’ in a network is the final length of cabling (or antenna transmission) from the nearest distribution point to a home or workplace. But branching off individual drop lines from the main fiber line is more difficult than it might appear. There are different ways of getting fiber into the premises. How to make the most practical, cost-effective decision without compromising on data speed, latency, and user experience?

Advertising often suggests ‘high speed’ or ‘broadband’ networks are full-fiber. However, the ‘last mile’ often uses no fiber at all. Although a Passive Optical Network (PON) or FTTH (Fiber to the Home) network runs fiber-optic cables all the way into the subscribers’ premises, most fixed high-speed internet services (partially) use existing copper lines – the legacy of 20th-century telephone networks. Optical signals need to be transformed to electrical signals before reaching end users. Sometimes, wireless signals are used to bridge the ‘last mile’ gap. ‘Fiber to the Node’, ‘Curb’, ‘Antenna’, or ‘Building’ networks, aren’t full-fiber, which means performance is affected. So why are copper-based solutions still used? Simple: last-mile access line represents the largest chunk of average investment per network connection, making it cheaper to use existing infrastructure. Copper-based or wireless technology may provide enough bandwidth for many uses – but uptake of technologies such as 5G and WiFi 6 means this might not remain the case for long.

Fiber is the fastest transmission method available, with the lowest latency. Low attenuation of very high frequencies and very low noise allow it to rapidly, reliably, and efficiently transfer the largest amount of data over a given distance. Wherever networks are business-critical or need to support demanding applications or large numbers of users, fiber is the only choice, clearly surpassing copper-based transmission in the last mile. That brings us to the challenge of actually getting the last transmission link into a building. Which options do we have?

Going underground

Last-mile cables generally run through a conduit. Often, cables that have previously been laid to the building to support other services run through existing ducts. Fiber cables can be added to these. FO cables are thin to offer the capability to transport a large amount of data. And because they are so thin, they can be installed in cable ducts which are already in use. Furthermore, the cable should be able to run around bends without performance being affected.

One way of adding fiber cable to a conduit is by pulling it through. The most important mechanical properties for such cables are tensile strength and dead weight. We recommend Polyethylene (PE)-sheathed cables, as PE has excellent surface properties and sliding properties (low frictional resistance) for all outdoor applications. Universal-use cables of type U-DQ(ZN) or U-DQ(BN) are also suitable for pulling into ducts. For shorter outdoor runes we recommend armored cables with glass roving (which provides a certain degree of rodent protection) of the type U-DQ (BN) loose tube cable equipped with a FRLSZH outer sheath.

Blowing cables into pre-laid ducts is the most economical installation method for buried cables. Cables optimized for blowing should be lightweight, preferable slim, have a certain stiffness and the outer sheath should offer excellent sliding properties. As rodents shouldn’t be able to get into ducts, pulled or blown fiber cables don’t need additional rodent protection. Unlike with pulled cables, strain relief and resistance to high tensile forces are not necessary. Micro cables type A – D2Y with a loose tube of 1.2 mm (4 fibers) or 2.0 mm (12 fibers) / 3.0 mm (24 fibers) are optimized for the blowing process thanks to their low weight, optimum stiffness, and minimal outer diameter.

Cables can also be ‘direct buried’. As these are laid directly into the ground, without a duct, these need to offer a high level of crush resistance and be rodent secure. Longitudinal water tightness is a prerequisite. High-Density Polyethylene (HDPE) sheathing is indicated because PE is sufficiently resistant to all chemical influences that direct-buried cables might conceivably be exposed to. Bearing in mind the likelihood of elevated moisture levels, transverse pressure load, and rodent infestation, we recommend corrugated steel tape armored cables of type A – DQ (ZN) (SR) 2Y or the double sheathed version A – DQ (ZN) 2Y (SR) 2Y.

Aerial cable

Aerial cable installation is the most economical installation method for the last mile. One advantage is the fact you can use existing poles.  However, the requirements for aerial cables are significantly higher than for buried cables, as they are continuously exposed to environmental conditions. In addition to the necessary high tensile force, other influences must also be taken into accounts, such as ice load, vibration, UV resistance, temperature cycling behavior, cable weight, and cable diameter and sag. We recommend air cables are encased in either UV-stable HDPE or then FRLSZH outer sheathing and are designed for a temperature range of -25 °C to +70 °C (a black surface under direct sunlight reaches +70 °C relatively easily). Aerial drop applications, being all dielectrically and self-supporting are essential. U – (ZN) H type cabling with its straightforward construction can traverse span lengths of up to 70 m.

In short: the choice of cable installation method depends on a number of factors, including population density in an area, geology, intended use, and more. Of course, if it’s possible to reuse existing infrastructure, that’s always advisable.

Some product recommendations and graphics can be found in the Brochure ‘Last Mile’ under Downloads: Cable knowledge.

If you’d like to discuss your options for buried or aerial fiber deployments, get in touch!

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