Network planning and design is an iterative process, encompassing topological design, network-synthesis, and network-realization, and is aimed at ensuring that a new telecommunications network or service meets the needs of the subscriber and operator. The process can be tailored according to each new network or service. A network planning methodology A traditional network planning methodology in the context of business decisions involves five layers of planning, namely: need assessment and resource assessment short-term network planning IT resource long-term and medium-term network planning operations and maintenance. Each of these layers incorporates plans for different time horizons, i.e. the business planning layer determines the planning that the operator must perform to ensure that the network will perform as required for its intended life-span. The Operations and Maintenance layer, however, examines how the network will run on a day-to-day basis. The network planning process begins with the acquisition of external information. This includes: forecasts of how the new network/service will operate; the economic information concerning costs, and the technical details of the network’s capabilities. Planning a new network/service involves implementing the new system across the first four layers of the OSI Reference Model. Choices must be made for the protocols and transmission technologies. The network planning process involves three main steps: Topological design: This stage involves determining where to place the components and how to connect them. The (topological) optimization methods that can be used in this stage come from an area of mathematics called Graph Theory. These methods involve determining the costs of transmission and the cost of switching, and thereby determining the optimum connection matrix and location of switches and concentrators. Network-synthesis: This stage involves determining the size of the components used, subject to performance criteria such as the Grade of Service (GOS). The method used is known as "Nonlinear Optimisation", and involves determining the topology, required GoS, cost of transmission, etc., and using this information to calculate a routing plan, and the size of the components. Network realization: This stage involves determining how to meet capacity requirements, and ensure reliability within the network. The method used is known as "Multicommodity Flow Optimisation", and involves determining all information relating to demand, costs, and reliability, and then using this information to calculate an actual physical circuit plan. These steps are performed iteratively in parallel with one another. The role of forecasting During the process of Network Planning and Design, estimates are made