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ANALYSIS & OPINION CABLE TV NETWORKS


In advance


Vanesa Diaz discusses building


advanced cable TV networks for gigabit services


O


ver the past several years, cable operators have seen their networks transform into the premier platform for transmission of data


services, both for residential and business customers. In order to accommodate the growth of services and transmission speeds, the networks have been divided into smaller and smaller clusters of customers forming independent service groups. When fibre came into widespread use in


cable television, it was common for one node to serve a cascade of several trunk amplifiers, with each amplifier feeding dozens of line extenders before the RF signals would finally arrive at a customer. Tat architecture could involve thousands of homes per node. Today, most nodes serve about 500 homes passed and do so via line-extender amplifiers only.


Future-ready In order to be ready for the near future, these nodes are being sub-divided into smaller service groups, typically feeding 125 or fewer homes each. Tese smaller numbers allow for gigabit services to be delivered effectively along with legacy video services. Looking towards the longer-term future,


fibre to the home (FTTH) options will demand even smaller service groups, typically 32 or 64 homes each. With all of these architectures having one


thing in common – the need for fibre transmission paths between the headend and service group – let’s take a look at the access network choices and technologies available to help deliver gigabit services.


Supporting a fibre-deep network Initial system design, and how demands have been addressed to date, are two key areas in assessing how many fibres are available to serve


18 FIBRE SYSTEMS Issue 20 • Summer 2018


a collection of service groups. Take, for example, a typical Node+3 HFC (Hybrid Fibre-Coax) system designed for 500 homes per node in the early 2000s (Figure 1). Since fibre cable at that time was bundled in buffer tubes or binders of six fibres each, many systems designed six fibres per node. Matching the nodes to the buffers made splicing and organising the fibres easier. Each node would require one fibre for downstream, and one for upstream, leaving four fibres for spares. Business services may have used two more fibres for upstream and downstream dedicated fibre service to a business location, or perhaps all four to feed a ring. Preferably, the ring would involve only two fibres from any single node location for route diversity.


When fibre came into widespread use in cable television, it was common for one node to serve a cascade of several trunk amplifiers


If a node split was required, any spare fibres


available could be used, however a 4x4 split requires more than that. We need four service groups where there was just one. To solve this, 1310/1550nm WDM, coarse


wavelength division multiplexing (CWDM) or dense wavelength division multiplexing (DWDM) techniques were, and still continue to be, employed. A single spare fibre could be used to replace the fibre feed to four or more service groups.


Divide further With passive analogue techniques, this method worked well enough to do the job. But, what happens when we want to divide further, as would be required for a Node+0 design, or to consider a PON for FTTH? In this case, there may not be enough fibre from the headend to the aggregation point to do the job. PON requires a single dedicated fibre from the


optical line terminal (OLT) to the splitter for each service group. In a true all-passive-plant PON, the OLT is located at the headend; we would need a fibre from that headend out to each splitter location. Te required fibre count leaving the headend could be in the thousands. In addition, the loss budget required to feed the splitters might also require a maximum 16-way split, rather than the usual 32, further increasing the count. Asking this infrastructure to support a new


fibre-deep architecture may be asking too much, but what about DWDM, both conventional and coherent (recently developed by CableLabs)? As an example, let us take the above Node+3


system and attempt to convert it into a Node+0 system, and also a PON system. Te access network can be broken down into


three distinct sections: Headend to Distribution Point (DP); DP to Node or OLT; and Node or OLT to Premise. When faced with insufficient fibres to feed the


aggregation point, the only option was to overlay more fibre cables from the headend out. Now there are more options, and regardless of the option chosen, we want to make certain that we do not have to face this choice again in the future. Let us examine what happens when we


convert a Node+3 system into a Node+0 system (Figure 2). Each former Line Extender location becomes a new mini-Node location. Tere are other options to reduce the node count, such as


Table 1: The access network can be broken down into three distinct sections Type of Network Coax


Headend to DP/OLT Fibre, DWDM or CWDM


Fibre


Fibre, DWDM or CWDM to OLT


DP to Node/Splitter Fibre – Discrete


Wavelengths to Node


PON Fibre to Splitters


@fibresystemsmag | www.fibre-systems.com


Node/Splitter to Premise


Fibre to Node, Coax to Premise


PON Fibre - one per Premise


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