ext step is to determine the topology of the backbone net- work. The topology is determined by three basic considerations. 1. Physical considerations. This includes existing or planned fa- cilities and geographical preferences as well as considerations such as access to communications facilities. 2. Cost considerations. This includes the physical facilities, labor and communications costs. 3. Requirements of the network. This would include the pattern and volume of the network traffic as well as considerations of redundancy and network growth. The decision on backbone topology is the most important and often the most difficult part of network design because of physical Fig. 6-16. Logical matrix. Each square is filled in with the number of connections required between intersecting cities. 106 TELECOMMUNICATIONS and cost considerations. If traffic patterns and volume were the only considerations, then network topology could be generated by com- puter software. This would lead to the best network solution. How- ever, physical and cost considerations play an important part in the design, forcing the use of a combination of techniques to arrive at a solution. This means that given all existing considerations, the final network design is only a compromise. Configuring redundancy to back-up lines and equipment failures always involves a tradeoff between availability and cost. Even minor changes to the redundancy philosophy can have a major impact on network cost. Redundancy requirements within the network are usu- ally assessed from three perspectives. 1. Node hardware redundancy. This includes redundancy of the processing hardware and communication port hardware. 2. Trunkline redundancy. This includes alternate routing of trunkline traffic around backbone failure. 3. Netline redundancy. This takes the form of alternative access paths to the backbone nodes. SATELLITE AND CARRIER SERVICES 7.1 SATELLITE OVERVIEW At an altitude of 36,000 kilometers above the equator, telecommun- ications satellites are by no means isolated