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Chapter 2Overview of current CATV network pThe downstream channels support the legacy analogue broadcast television (80-450MHz, the frequency depends on the country), and multiples of 1-6 MHz or 1-8 MHzchannels in the 450-900 MHz region for the transmission of digital data. The upstreamchannels are also divided into 1-6 MHz channels, but the high level of noise means thatdata capacity is only 1-6 Mbps per channel in EU and 1-10 Mbps in US and Japan asopposed to the 28-40 Mbps available in each downstream channel in US and Japan. Adata rate of up to 50 Mbps can also be supported in EU.In terms of modulation schemes, most manufacturers have implemented 64 and 256Quadrature Amplitude Modulation (QAM) for the downstream and Quaternary PhaseShift Keying (QPSK) for the upstream channels. Japan and US have also implemented a16-QAM modulation. These modulation techniques are described in [6].2.3.2 Upgrade of CATV networks to bi-directional HFC architecturesIn the last few years, CATV networks have been upgraded to improve the quality of thesignal transmission and to increase available capacity. Modern CATV networks areinitially built with Hybrid Fibre Coax technology [64]. The gradual evolution has shownthe technical and economical viability of two-way communication in a CATV network.A combined fibre coaxial CATV network is referred to as an HFC System. Fibre linksare used to transport subcarrier multiplexed signals typical to a group of between 500and 2000 subscribers. Such networks are now standard and provide a bandwidth up to750 MHz (in most systems) in the downstream direction for digital and analoguetransmissions [27]. This bandwidth will be up to 1 GHz in the future [12].In order to deliver data over HFC networks, laser transmitters convert a fixed frequency(6 MHz wide in the US and 8 MHz in Europe) sent from the headend into opticalsignals. At the outskirts of a community, a laser receiver, named as Optical NetworkUnit (ONU), reconverts the signals so that they can again be transmitted over thecoaxial cable, which goes into each individual house. At the Customer PremiseEquipment (CPE) a receiver (set-top box or cable modem) tunes to the appropriate fixedfrequency in order to receive downstream signals.2-5