Imagine an enterprise that is operational in every country of the European Union and has a branch in every town with a population over 10,000. Each branch has a LAN, and it has been decided to interconnect the branches. A mesh network is clearly not feasible because nearly 500,000 links would be needed for the 900 centers. A simple star will be very difficult to implement because it needs a router with 900 interfaces at the hub or a single interface that carries 900 virtual circuits to a packet-switched network.

Instead, consider a hierarchical design model. A group of LANs in an area are interconnected, several areas are interconnected to form a region, and the various regions are interconnected to form the core of the WAN.

The area could be based on the number of locations to be connected with an upper limit of between 30 and 50. The area would have a star topology, with the hubs of the stars linked to form the region. Regions could be geographic, connecting between three and ten areas, and the hub of each region could be linked point-to-point.

This three-layer model follows the hierarchical design used in telephone systems. The links connecting the various sites in an area that provide access to the enterprise network are called the access links or access layer of the WAN. Traffic between areas is distributed by the distribution links, and is moved onto the core links for transfer to other regions, when necessary.

This hierarchy is often useful when the network traffic mirrors the enterprise branch structure and is divided into regions, areas, and branches. It is also useful when there is a central service to which all branches must have access, but traffic levels are insufficient to justify direct connection of a branch to the service.

The LAN at the center of the area may have servers providing area-based as well as local service. Depending on the traffic volumes and types, the access connections may be dial up, leased, or Frame Relay. Frame Relay facilitates some meshing for redundancy without requiring additional physical connections. Distribution links could be Frame Relay or ATM, and the network core could be ATM or leased line.