t splitting as well as cable loss. For the trunk portion larger cross promises very high quality over both coaxial cable and fibre within section cable and more sophisticated amplifiers can be afforded reasonable bandwidths. compared to the distribution area. 57.3.4.2 Coaxial 57.3.3.3 Switched star The essence of designing a coaxial tree and branch system is the From the late 1970s an alternative structure was being considered balancing of the cable and splitter loss incurred in delivery to the with a dedicated cable feed to each customer (Figure 57.20). This customers by the use of appropriate amplifiers, placed before signal offered a number of potential advantages, most obviously easier degradation has gone too far. Because street sited amplifiers must interactivity. Control of the service selection via a video switch gave be placed regularly at quite short distances (every few hundred secure control (no need for scrambling to achieve tiering) and metres), and with relatively little tolerance on the distance, design allowed a tailored service package to be delivered to the customer can only proceed with a detailed knowledge of the area to be cabled. not constrained by the capacity of the final coaxial link (since only Cable loss varies according to the square root of frequency, and the wanted channels have to be transmitted along it). Technology hence equalisation must occur in the amplifiers to boost the higher was also a driver, since this network type was seen as an application frequencies compared to the lower. This can be done by both for fibre. compensation to the received signal, and deliberate tilt to the laun- In Europe a limited number of these systems have been installed, ched signal. Given that this is done, the basic loss balancing exercise some all coxial, some part fibre, and some all fibre. Matching the can be done by considering one frequency (usually the highest). low cost of the tree and branch networks for basic TV delivery has Thus a trunk cable might have a loss of 50dB per km at 450MHz. 