asures are an additional expense. PRE-PLANNING IN TRANSMISSION NETWORK Point-to-Point topology Star topology Chain topology Loop topology Figure 3.6 Transmission network topologies Star topology: This topology resembles a star - hence the name. With this arrangement the capacity can be utilised in a better way by using cross-connecting equipment. This topology is also simple to implement, but if a node fails for any reason then the traffic coming through it towards the BSC is lost. Again, protection is expensive in this kind of topology. Chain topology: This is generally used to connect the sites that extend from one region/city to another region/city. A typical example is connection of sites on a highway. The capacity of the links is usually fully utilised. However, the last link of the chain can be vulnerable. Loop topology: This is an extension of the chain topology, with the last link of the chain being connected back to the BSC. Although this requires an additional link in the chain, it provides protection without additional hardware redundancy. Also, there is a requirement for an additional node, which is able to control loop protection. In practice, a transmission network will employ all of the above technologies. Transmis- sion planning engineers have to balance the technological and cost aspects, and the strategic importance of the site before a decision is made. 3.3.5 Site Selection and Line-of-Sight Survey Site Selection The site selection process is described in Chapter 2 because it is predominantly a ra- dio network planning engineer's domain, but with the active involvement of transmis- sion planning engineers. The types of site that are preferred by radio and by transmis- sion planning engineers have opposite characteristics. Radio planners prefer sites that are not too high, SO as to prevent interference problems from other sites, while transmission planning engineers prefer sites that are very high, SO that connectivity to a large num- ber of other sites is possible. However, both agree that a si