ADC KRONE makes Physical Layer Management (PLM)

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Joachim Brunzel Product Manager Fibre Optic / Carrier EMEa Joachim.Brunzel @adckrone.com FTTH – Let there be light F or network operators and service providers Europe represents a market that simultaneously offers growing opportunities along with a crucial requirement for compatibility with existing installed network infrastructure. With established copper-based customer access networks transmission technologies such as ISDN or ADSL already hitting the limits of performance they will be completely swamped by new services. It is beyond argument that the demand for greater bandwidth is rising. In a typical household with two or three television sets, new-generation video services will soon be consuming more bandwidth than most xDSL connections can provide today. Applications such as HDTV, IPTV, Video on Demand (VoD), IP telephony (VoIP), digital radio, e-Learning, e-Medicine and high-end gaming will soon overreach the capacity of even the highest data rates achievable currently with high speed VDSL technology (50 MBit/s). Once the capacity of the copper cabling is fully exhausted the only route is to install enhanced optical transmission systems. This is the challenge facing both established incumbents and the new alternative carriers. The assessment of the Fibre-to-the-Home Council Europe is that by 2010 every household will require symmetrical data access at 100 MBit/s in each direction—in other words twice the data rate that VDSL can deliver now at maximum downstream speed. In the light of these developments network operators are asking the question which of the new transmission technologies are best suited to creating the futureproof, robust and flexible infrastructures essential for delivering new services. Most of the answers can be summed up in the acronym FTTx (fibre to the “x”)—optical fibre cables reaching at minimum into customers’ neighbourhoods and ideally laid all the way to individual end users, underpinned by the latest optical transmission technology. Signal delivery over optical fibres offers the advantage over copper cables of unequalled higher bandwidth: capacities per subscriber of 100 MBit/s to 1 GBit/s or more are easily achievable. In this way carriers can also gain new customers who up to now had to put up with relatively slow data access over copper DSL lines. This is because xDSL technologies have a constant battle with line lengths; data rates fall significantly as delivery distance increases. Serving subscribers at a typical range of three to six kilometres from the central office is certainly feasible using xDSL but achievable data rates will lag in the low MBit/s—a trickle compared with the data torrent desired and far too little for the new-breed services that will bring home the real revenue. To deliver higher data rates to the end user, say 50 MBit/s, VDSL is a solution, but only if the copper cabling measures no more than around 400 metres. The pressure is on to bring optical fibres closer to the end user. At this moment some two million households in the EU plus Switzerland and Norway are already connected by FTTH (Fibre to the Home) via high capacity optical fibres, a trend that’s climbing steeply. Even pessimistic observers concede that the number of connections (homes passed) will climb to around 4.5 million subscribers by the year 2010. More positive forecasts speak of around ten million connections and the most optimistic estimate is even 19 million. Put plainly, the day is dawning for a market with massive potential, both for operators and infrastructure providers. FTTN aNd FTTH/P FTTN (Fibre to the Node) is one of the evolutionary stages along the route to the all-optical connection to end users known as Fibre to the Home or Fibre to the Premises, FTTH/P for short. A key advantage of FTTN technology is that it enables provision of maximum VDSL data rates to a major proportion of subscribers. With FTTN the fibres are initially laid only as far as the multiservice access node (MSAN), where individual subscriber lines are connected into the core network. The final link or ‘drop’ to subscribers is made over existing copper telephone access cables. In this situation the various xDSL technologies are employed almost exclusively, using copper cables into users’ homes. ‘Sweating the assets’ of existing copper in the ground like this has the key advantage that no ground has to be broken to connect the subscriber (no digging up of paths or gardens). With MSAN access points seldom far from subscriber premises, new connections can be made without delay. Making this ‘big picture’ possible are the separate network elements that actually make it work. Just as important as the network components installed within the central office is the active equipment which it becomes necessary to deploy in outside plant or external cabinets. Items such as DSL Access Multiplexers (DSLAMs) make new demands for power supplies and ventilation. To make room for new components such as cable splitters and flexibility points outside cabinets will need a space footprint somewhat larger than the old, purely passive cross-connect cabinets that they replace. Modern high-density cabinets simplify the space reclamation procedure. Should bandwidth demands rise even further, at a later stage of roll-out the copper cables in the access network can make way for optical fibres. In this case the carrier lays the fibres all the way to the subscriber’s premises, typically into the basement of a residence or office building. Creating the FTTH topology in the form of a passive optical network (PON) is particularly beneficial, as this guarantees the operator maximum flexibility and future-proofing in return for a relatively low investment outlay. By definition a PON is built without any active electronic or optical components in the fiels and the signals fed into it at the central office are transmitted New 6 ADC KRONE Connecting With Our Customers – Vol.2 No.2 2007
without amplification, thus purely passively. Whilst the useful life of most electronic transmission equipment lies around seven years, the PON infrastructure and components can be expected to remain in use for around 30 years. OuTsidE PLaNT When designing FTTH infrastructure network planners must devise their topology to maximise flexibility. At the same time capital outlay and long-term operating costs must be kept as low as possible. Decisions made today will have a major bearing on the future, determining how well the FTTH network structure meets the demands of the future. FTTH networks need to be just as reliable and flexible as robust long-haul optical transport networks. This would require the deployment of redundant, in other words duplicated, fibres which of course may be unreasonable or indeed irrational. The copper outside plant (OSP) networks that have served Europe reliably for well over a century employ a tree-and-branch topology or structure, which reaches out from the switching centres towards end users along trunks, branches or twigs of decreasing size, with many branching-off points along the route. This kind of architecture is clearly unsuitable for the construction of redundant optical fibre networks, which need to be built on the pattern of ring networks. Dispensing with redundancy is certainly an option if predominantly private subscribers are to be served. Neighbourhoods composed mainly of business users do nevertheless require solutions with redundant fibres, which will follow as far as possible the existing duct routes of the old tree-and-branch structure and enable subscriber connections to be made as economically as possible. Network planners must also take a view on how many optical lines are needed to serve each MSAN point, also on how future bandwidth demands can be fulfilled with the passive optical network (PON). Thinner optical cables can offer advantages at construction time; they are easier to draw through existing ducts and also simplify the creation of redundant structures with fibres laid in parallel spans. Particular significance should be attributed to the means of jointing the optical fibres; in Europe most carriers perform this ‘splicing’ function in so-called ‘enclosures’—hermetically enclosed housings, that are buried directly in the ground. This technology finds particular merit where optical cables are laid direct from customer premises right into the central office. Contrastingly intermediate fibre patch-panels offer greater flexibility and reduced fault susceptibility —even in situations of high customer connection density. Patching is a system of terminating optical cables on a flexibility module known as a distribution or crossconnect frame or panel. This enables technicians to reconnect cables flexibly simply by plugging them in and out from one location on the panel to another. A Main Fibre Cross Connect (MFCC) provides patching access to the core network cables and enhances the flexibility of the infrastructure. FiBrE disTriBuTiON wiTHiN THE CENTraL OFFiCE FTTH/P also places critical demands on central office facilities. The assessment that operators must make here is how the optical equipment (optical line terminals, OLTs) is to be jointed to the incoming optical cables from the external network. Efficiency is an important consideration; operators will naturally wish to make maximum use of the OLT interfaces. The greatest flexibility is achieved when all line cards and incoming OSP fibre terminations are connected to an optical distribution frame (ODF). The OMX600-ODF from ADC KRONE offers an example of how a particularly high connection density can be achieved in a compact structure, at the same time providing easy access to cross-connect patch cables for essential cable management. A WDM (Wavelength Division Multiplexer) merges data, speech and video signals into an optical fibre and is thus a key component of the FTTH infrastructure. In this context optical splitters are employed to distribute the signals to the end users. The location of these components in the CO or the OSP—‘centralised’ or ‘cascaded’—is of strategic significance and is a determining factor in both the flexibility of the network and the costs. A further requirement is the ability to test the FTTH network without interrupting or disturbing current services if at all possible. This demands the provision of monitoring modules or optical switch solutions that enable interruption-free remote test and monitoring functions. Distribution of signals from the central office to the MSAN access nodes in the access network is handled by passive splitters located at fibre distribution hubs (FDHs). Two configurations dominate in this situation: cascaded splitters on the one hand and centralised splitters on the other. The cascaded implementation typically employs a 1x4 splitter in the FDH, the input side of which is linked direct to the OLT port in the CO via an ODF. The output side goes direct to inputs of 1x4 or 1x8 splitters installed as a secondary distribution hub. Each splitter output leads to the ONTs (optical network terminal) in subscribers’ houses or apartments. In contrast the centralised implementation employs only one splitter (1x16, 1x32 or 1x64 according to the bandwidth required by each subscriber) at a central point (the FDH). This system has proved more flexible, since enables optimal utilisation of the OLT Line Cards at the CO and provides easier access for test apparatus. The cascaded architecture leaves ports in the line cards unused if not all the households served are actually making use of the access. CONCLusiON FTTH is the logical route forward to serve the growing demands for bandwidth and the copper-based transmission technology in the access network that by now has almost reached the limits of its development. Nevertheless the build-out of FTTH infrastructure presents network operators with new challenges, as much in the external network domain as inside the central office. A passive external network with access distributors and central splitters together with central offices equipped with high-density Optical Distribution Frames represents the most flexible solution. Moreover, this combination creates an attractive migration path towards the Gigabit PON technologies of the future. Joachim Brunzel New ADC KRONE Connecting With Our Customers – Vol.2 No.2 2007 7
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ADC KRONE makes Physical Layer Management (PLM)

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