|  | 3.3 | How to Identify and Select Networking Capabilities
| 3.3.2 | A hierarchical WAN design model
Network designs tend to follow one of two general design strategies: mesh or hierarchical. In a mesh structure, the network topology is flat; all routers perform essentially the same functions, and there is usually no clear definition of where specific functions are performed. Expansion of the network tends to proceed in a haphazard, arbitrary manner. In a hierarchical structure the network is organized in layers, each of which has one or more specific functions. Benefits to using a hierarchical model include the following: Scalability -- Networks that follow the hierarchical model can grow much larger without sacrificing control or manageability because functionality is localized and potential problems can be recognized more easily. An example of a very large-scale hierarchical network design is the Public Switched Telephone Network. Ease of implementation -- A hierarchical design assigns clear functionality to each layer, thereby making network implementation easier. Ease of troubleshooting -- Because the functions of the individual layers are well defined, the isolation of problems in the network is less complicated. Temporarily segmenting the network to reduce the scope of a problem also is easier. Predictability -- The behavior of a network using functional layers is fairly predictable, which makes capacity planning for growth considerably easier; this design approach also facilitates modeling of network performance for analytical purposes. Protocol support -- The mixing of current and future applications and protocols is much easier on networks that follow the principles of hierarchical design because the underlying infrastructure is already logically organized. Manageability -- All the benefits listed here contribute to greater manageability of the network.