Broadband Rollouts: Cabling for FTTP and Hybrid Fibre Builds

Full fibre rollouts are moving quickly across the UK, with national operators and a growing number of regional networks all building at the same time. The cabling in these networks is simpler than in a data centre, but there are far more joints, more outdoor work and more people installing it. Most faults come from basic things such as bend radius, dirty connectors and poor splices. This post walks through how a typical build is structured, where each component fits and where installations tend to go wrong.

How an FTTP network is structured

Fibre to the premises (FTTP) runs optical fibre all the way to the home or business. Most builds use a passive optical network (PON), where one fibre from the exchange or point of presence is split to serve many premises.

A typical route looks like this:

Exchange or point of presence. The optical line terminal (OLT) sits here, and the feeder cables leave from it.
Feeder cable. High-count singlemode cable runs from the exchange to a distribution point, usually in ducts, with joints along the route.
Splitter or aggregation point. Passive optical splitters divide one fibre between many customers. Common split ratios are 1:8, 1:16, 1:32 and 1:64, depending on the network design.
Distribution cable and joint closures. Smaller cables branch off towards streets and clusters of properties, with closures protecting the splices.
Drop cable. The final link from a pole, chamber or underground box into the premises, either underground or aerial.
Optical network terminal (ONT). The customer-end equipment, connected to the fibre inside the property.

Each stage has its own installation methods and its own ways of going wrong, so it helps to treat them separately.

Fibre and connector choices

FTTP networks use singlemode fibre (OS2), almost always with bend-insensitive grades such as G.657.A1 or A2. The tighter bend tolerance matters in drop cables and in enclosures, where space is limited and fibre is routed around tight corners.

Connector choice matters more than it appears. PON networks normally use angled physical contact (APC) connectors, identified by the green housing, because their lower back-reflection suits the equipment. Mixing APC and UPC (blue) connectors in the same link causes high loss and damaged end faces, as the two do not mate properly. Label and segregate them clearly, both in stores and in the enclosure.

Joint closures and splice quality

The splice enclosure is where a large share of rollout faults start. Common problems include:

Poor sealing. Water entering a closure, whether through a damaged gasket, a badly seated cable gland or a closure that has been opened and closed too many times.
Splice tray discipline. Fibres crossed, over-tight in the tray or left without enough slack for a re-splice. Leave enough fibre length to remake a splice at least once.
Bend radius. Fibre routed around too tight a radius inside the tray shows up as extra loss, sometimes only at 1550 nm, which can pass unnoticed unless tested at that wavelength.
Fusion splice results. Dirty cleavers, worn electrodes and unclean fibre before cleaving all increase loss. Splicer calibration and electrode condition are worth checking regularly, not only when results go wrong.
Poor records. Without a record of which fibre goes where, a later engineer has to trace fibres by hand.

Our post on avoiding failures in external network links covers the weather and sealing side in more detail.

Underground routes, ducts and blown fibre

Much of the UK rollout depends on existing duct, and on microducts installed inside it or directly buried. Microduct is cheaper to lay, and fibre can be blown into it afterwards, which allows cable to be installed in long lengths with fewer joints. If you are new to the method, our post on what fibre blowing is explains it.

Problems usually trace back to the duct rather than the cable:

Damaged or crushed duct from earlier work, which stops the blown fibre part way along
Water in the duct, which affects blowing distances and should be cleared first
Poor joints between duct sections, which leave steps or gaps
Unrecorded duct routes, where the chamber or joint is not where the plan says it is

Prove the duct before installing cable. A pressure test or a sponge or mandrel pass costs far less than a stuck cable. Where duct space is limited, subducting existing routes can add capacity without new digging.

Drop cables and the last few metres

The drop is the most exposed part of the network and is handled by the most people. Points to look at:

Aerial drops. Use cable designed for the span and the tension, fitted with the correct tensioning hardware. Check sag, clearance and attachment points on poles or walls.
Underground drops. Protect the cable at entry points and where the route crosses driveways or boundaries. Leave a service loop at the chamber.
Building entry. Seal the penetration to keep out water, insects and draughts. Respect the minimum bend radius where the cable turns through the wall.
Fibre inside the premises. Keep the route short and tidy, away from sharp edges, and clipped without crushing.

A drop that passes on the day of installation but is later pinched by a door frame or a garden fence will come back as a fault report. Time spent on routing and fixings avoids most of these.

Hybrid fibre builds

Not every property gets fibre to a socket inside the living space. Some builds combine fibre with copper part of the route. Examples include fibre to the cabinet with a copper final section, or fibre to a building with Cat 6a distribution inside, as in flats, offices and shared sites.

Where fibre meets copper, the points of interest are:

The handover point. Position the fibre termination and the copper distribution so that both are accessible and ventilated, and so each can be tested separately.
Copper limits. A permanent link on Cat 6a is limited to 90m, and the internal layout should be planned around that.
Power and earthing. Powered media converters and active equipment need a power supply, and the earthing and bonding should follow the building design.
Future upgrades. If a building is fed with fibre to a central point, leave spare fibre and space so later upgrades can bring fibre to individual units without reworking the riser.

Testing a PON network

Testing a PON network differs from testing a point-to-point link, because of the splitters in the path.

Before the splitter, test the feeder and distribution fibre with an optical loss test set and an OTDR, as you would a standard singlemode link.
After the splitter, the splitter loss is large, around 10 to 11 dB for a 1:8 and around 17 dB for a 1:32, and it has to be accounted for in the loss budget. Use a test method suited to PON, such as a PON power meter, which can handle the live wavelengths.
Test wavelengths. Singlemode testing at 1310 nm and 1550 nm shows up bends and sealing problems that one wavelength can miss. Some specifications may request other wavelengths such as 1490nm and 1625nm.
Records. Store OTDR traces in their native format, tagged with the fibre and cable identifier, so a later engineer can compare against the original result.

Our guidance on getting testing right first time applies to every stage of the build.

Where installations run into trouble

Dirty or damaged connector end faces, and APC and UPC mixed up
Bend radius ignored in closures, chambers and at building entry
Ducts blocked, wet or not where the records say
Splice trays overfilled, leaving no slack for repairs
Records that do not match what is in the ground
Testing at a single wavelength, which hides bend loss

Most of these are quick to prevent. Cleaning and inspecting every connector, checking the duct before the cable goes in, leaving slack in every closure and testing at both wavelengths take little time on the day, and they cut the number of return visits.

A short checklist for installers and ISPs

Is every connector type labelled, with APC and UPC kept separate?
Has the duct been proved before installing or blowing cable?
Is there enough fibre slack in each closure for at least one re-splice?
Are closures sealed and the gland fitting checked?
Are bend-insensitive fibre and drop cable specified for tight routes?
Are results taken at 1310 nm and 1550 nm and stored in native format?
Do the records match what is installed?

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