The deployment of a PON architecture requires a number of outside plant components, including the optical distribution frame, splitters, splicing closures, and wall-mounted boxes on the home or the building. It also requires fibre-optic cables with specific performance characteristics, both in the indoor and outdoor environment.
The selection of cables and associated components, as well as how they are implemented in the network – including the splitting ratio – are key to optimizing capex and to reducing opex. The network owner will require comprehensive expertise in these areas if PON investment is to be maximized.
A key component of the PON architecture is the optical distribution frame (ODF), which houses the OLTs and is located in the central office. It has to be capable of a number of functions, including the connection to ports in different sections of the central office. Patching port connections enables operators to use their existing billing and management systems for PON customers, and requires fibre splicing and fibre coiling. The placing of 1xn splitter – where ‘1’ represents one incoming strand of fibre and ‘n’ the number of splits (in PON configurations, ‘n’ can represent 2, 4, 8, 16, 32 and 64) – is not normally located in the central office as this can increase capex on outside fibre plant. However, the operator may consider installing a 2xn splitter at the central office (where two incoming fibres are accommodated). The 2xn splitter can be used as a means by which operators can introduce a test function on the PON, which can be managed from the central office.
Accessible splicing closures, with a mechanical sealing system, is another key part of a flexible and scaleable PON architecture. The primary function of a splicing closure is to interconnect transport cables onto multiple distribution cables and, in turn, to enable multiple drop cables from the distribution cables themselves. To guarantee these functions while minimising capex, the closure has to be re-accessible and resealable for progressive PON deployment. Part of this flexibility requires the splicing closure to have a scalable splicing tray capability. This, in turn, provides scalability on the splitter ratio.
Fibre-optic cable performance
To maximise the component functions as outlined in 2.3.2, the selected cables need to have complementary features, such as small diameters and mid-span access capability. Mid-span access is where the cable can be stripped in the middle of a length in order to get access only to the selected fibres for splicing. For multi-dwelling units where there is high subscriber density, operators have two choices of fibre-optic cable to go through the vertical riser: Low Smoke Zero Halogen (LSZH) cables with ‘standard pull installation’ or micro-blown cable with an associated Halogen Free Flame Retardant micro duct (see section 4.3.1 on ‘blowing’ fibre through micro ducts).
For low-density buildings, a drop cable for a single user can be installed directly from the basement. But in both cases (high-density and low-density subscriber buildings) the drop cables from the basement (or from the floor) to the subscriber flat – as well as the indoor fibre-optic cables that connect onto the ONT – bendinsensitive fibre has to be used. According to Nexans, the use of bend-insensitive fibre introduces an extra initial cost of ‘a few percent’ but reduces opex. For the drop cable connection, an external transition box can be located against the wall of the subscriber. If no external transition box is used, the design of the drop cable must include double sheath to operate the transition between external environment conditions to internal LSZH requirements. Once the fibre-optic drop cable penetrates the home, it is connected onto a wall outlet box fitted with a connector adapter to connect the patch cord onto the ONT.
All the cables, at this level of the network, have to include bend-insensitive fibre to maximize opex savings.