Building a future-proof fibre optic infrastructure is as much about the business model you will follow, as it is the technical decisions you face

FTTx is vital if we are to fulfil the huge demand for large bandwidths in tomorrow's world. One of the options is to use FTTC (Fibre-to-the-Curb) to bring the DSL port closer to the customer. However, transmission by copper wire with DSL is as far as it goes. On the other hand, fibre optic transmission to the customer using FTTH (Fibre-to-the-Home) will provide sufficient bandwidths for the next 20 years.

The telecommunications industry has had more than ten years' experience with active and passive optical networks. And debates about the advantages and disadvantages of these networks have been running for at least that long. Fibre optic networks can be laid directly to households (Fibre-to-the-Home [FTTH]) by using Passive Optical Networks (PONs) and Active Optical Networks (AONs.)

The key technical difference between active and passive access technology is that a passive splitter for passive optical networks is used, whereas active optical networks function with Ethernet-Point-to-Point architecture. The objective of both passive and optical networks is to bring the fibre optics as close as possible, or ideally right to the subscribers' houses and apartments. This FTTH-solution is technically the best option as regards the transmission quality and the bandwidth.

Business case challenge
Using fibre optic cable promises virtually unlimited bandwidths, however the network operator only ever has just the copper wire line in the last mile, apart from a very few exceptions. So if DSL technology is no longer adequate, new optical cables always have to be laid.

The high investment costs of setting up this infrastructure, combined with telecommunications providers' falling revenue, mean it is often difficult to put a business case to investors and network providers' management boards. Nowadays, the ICT industry is still spoilt with returns on investment of one to three years. But expansion of FTTH and FTTB networks, (regardless of whether PON or Ethernet-Point-to-Point technology is used), sometimes takes more than 10 years before a return on investment is seen. Nevertheless, depending on the application and conditions at the time, business cases vary greatly, depending on whether passive or active access technology is used for FTTH rollouts.

Passive Optical Networks (PONs)
As regards the core network, the first network element of a PON network is the OLT (Optical Line Termination Unit), that provides n x 1 Gbit/s and n x 10 Gbit/s Ethernet interfaces to the core network and PON interfaces to the subscribers. The PON types used here today are usually Ethernet-PON (EPON), Gigabit-PON (GPON) and in future Gigabit-Ethernet-PON (GEPON) or WDM-PON. EPON installations are currently primarily found in the Far East, GPON on the other hand in the US and Europe.

In PON's case, the signal on the fibre optic to the subscribers is partitioned by a passive splitter into optical subscriber connections. The splitter is either located in an outdoor housing or directly in the cable run, for example in a sleeve. In other words, the network structure is a Point-to-Multipoint structure (PMP).

In an FTTH network architecture, subscriber access is implemented via optical network termination (ONT) that terminates the optical signal and feeds it into one or more electrical interfaces, such as for example 100BaseTx, POTS, or ISDN. ONTs with VDSL interfaces are available for FTTB to bridge the existing subscriber access lines in the property. In this case, each subscriber receives a VDSL modem as network termination.

Ethernet-Point-to-Point (PtP)
As regards Ethernet-Point-to-Point network structures, every subscriber gets their own fibre optic that is terminated at an optical concentrator (AN = Access Node.) Metro-Ethernet switches or IP edge routers are normally used here that were not originally conceived for the FTTH/FTTB environment. KEYMILE designed MileGate, the Multi-Service Access Node (MSAN), for this type of application. MileGate can be called an optical DSLAM because the system has a very high density of optical interfaces and at the same time fulfils all the demands of a DSLAM. MileGate uses standard optical Ethernet interfaces based on 100 Mbps (for example 100BaseBX) or Gigabit Ethernet. Because of this transmission interface, mini or micro DSLAMS that ensure distribution of data in individual properties, can be used in FTTB architectures too.

All network topologies can be implemented with PON and Ethernet -PtP. However, a network operator should decide early on which architecture will still be in a position to respond to demands in 15 - 20 years. Because infrastructure investments should have an ROI of about 10 years, so that modifications do not have to be made after just five years.

Initially, network operators save real money with a Point-to-Multipoint structure (of the type required for PON systems,) as they have to lay fewer fibre optics than if they used a Point-to-Point structure from the very beginning. However, the optical splitter is a weak point. This network component might have to be replaced if customers need greater bandwidth, or if the worst comes to the worst even be bypassed with additional fibre optics to upgrade the Point-to-Point structure.

A comparison of passive optical and Point-to-Point structures:
PtP technology is much better in terms of bandwidth per subscriber. The maximum bandwidth per subscriber is a lot higher. The flexibility to allocate different bandwidths to individual subscribers is also higher (e.g. for corporate customers) than when PON systems are used. Depending on the splitting factor, a PON connection via fibre optics supplies less bandwidth than a VDSL2 connection via copper wire. Even if it is a question of increasing the bandwidth, PtP architecture is superior to the PON's PMP architecture. Just by converting boards, subscribers can obtain an upgrade, without the network architecture or the service of other subscribers having to be changed.

Within a PON tree, all the subscribers are on the same optical point. If an ONT causes faulty synchronisation, or produces an optically indefinable signal, a remote localisation of the malfunction in the ONT involved might not be possible. Where PtP is concerned on the other hand, due to the PtP architecture, both the fibre optic path and the end customer's ONT can be clearly assessed. In the worst case scenario, the laser on the AN for each subscriber can be deactivated by the control centre. As regards availability, the PON is at a disadvantage compared with PtP because to date, there are no plans to connect customers redundantly in one PON.

Currently, when the same functions are offered, there are no significant differences in the costs of the subscribers' terminal equipment (CPEs, ONTs.) Because PtP Ethernet installations use standard Ethernet interfaces however, substantial falls in prices are to be expected as more and more flood the market. Despite standardisation, ONTs in today's PON environment are not interchangeable between different manufacturers' systems. Which means the selection of models is restricted and the savings provided because a larger number is produced, are negligible. However, in terms of price per subscriber and because the optical paths can be used in several ways, PON is at an advantage compared with Ethernet-PtP.

This advantage is eaten up by the subsequent costs for upgrades. An entire PON tree is affected by an upgrade. Because of the better granularity of the ANs and the separation of the customers (PtP), customised upgrades can be carried out in the active optical network. The advantages of PtP flexibility really bear fruit where business customers are concerned. Requirements from bulk customers are always highly individual, but PON network concepts tend to be more static. Therefore, in this case the active approach is a lot better.

A generic comparison of technology can only serve to gain an initial overview. While network operators in Asia prefer passive optical networks, a study by the FTTH Council Europe showed that in Europe over 80% of the FTTH/FTTB installations are based on Ethernet-PtP.

About the author: Klaus Pollak is Head of Consulting & Projects, Keymile.

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