The Evolution Of Fibre Optics In Cable Tv Systems

Abstract

The evolution of fibre optics in cable TV systemsJohn R FoxBritish Telecom Research Labs, Martlesham Heath, Ipswich, IP5 7REIntroductionOutline of the paperThis paper examines the application of fibre optics in cable TV networks in 3 timeframes, namely:-- its use to the present time.- its use in the immediate future, in particular its application in the newswitched -star type of network being introduced.- its long -term use in the anticipated all- fibre, all- digital integrated wideband localnetwork.Although cable TV is the prime subject of the paper, often it will be sensible to referto the network , which is simply a more general term referring to the final stagesof communication to the customer. It therefore includes telecommunications services aswell; however, for the time being, cable TV is the only wideband service to the domesticcustomer likely to justify fibre optics in the local network.After reviewing the limited use to date of fibre optics in cable TV networks, the paperconsiders in some depth its basic attributes with reference to this application, and thepossible transmission techniques, since these are the essential basis from which to judgehow use of fibre optics will evolve in this field.The traditional Cable TV networkAs background, it is useful to briefly describe the traditional tree and branch cablenetwork which is shown in Figure 1. The central source of programme material in thenetwork is termed the head -end, though often part of that material is fed in on a numberof links from their actual source (eg a remote aerial site). From the head -end themultiplex of all the channels is fed out on a trunk coaxial cable, continually branchingalong its route until the customer is reached. In the customer's home the network oftenneeds a set -top box to convert the required channel to the TV input frequencies, andperhaps to allow decoding of scrambled Pay TV channels.The newer Switched -Star networkIt is the introduction of this type of cable TV network that has opened the door toexploitation of fibre in the local network. Figure 2 shows the structure of the networkbeing installed in Westminsterl. This scheme, which the author is involved with, is alarge system with potential to serve 100,000 customers. It is due to start service in thesecond half of 1985.The network is characterised by having 3 distinct transmission stages, namely:-- Super -primary links: potentially long (10 km +) trunk routes, using fibre, whichtake channels out to intermediate hub -sites.- Primary links: shorter fibre routes (5 km maximum) fanning out to the switch points,which serve up to 300 customers in Westminster (the switch size varies in other systems).- Secondary links: the star part of the network, presently short coaxial cable links(less than 500 m) in Westminster.Fibre links are additionally employed for the feeds into the head -end from externalsources.Switched -star offers a number of advantages over tree and branch, basically stemmingfrom the individual customer feed. Some of these are:-- easier provision of dedicated services, such as videotex and video library.