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When selecting a particular WAN technology, you should be familiar with the three major categories that represent traditional WANs:

• Circuit-switched— Data connections that can be brought up when needed and terminated when finished. Examples include ordinary PSTN phone service, analog modems, and ISDN. Carriers reserve that call path through the network for the duration of the call.
• Leased lines— A dedicated connection provided by the service provider. These types of connections are point-to-point and generally more expensive. TDM-based leased lines usually use synchronous data transmission.
• Packet- and cell-switched— Connections that use virtual circuits (PVC/SVC) established by the service provider. Packet-switched technologies include Frame Relay and cell-switched technologies such as ATM. The virtual circuits are part of the shared ATM/Frame Relay service provider backbone network. This gives the service provider greater flexibility with its service offerings.

WAN Topologies

When designing a WAN, you should become familiar with the basic design approaches for packet-switched networks. These approaches include hub-and-spoke, partial-mesh, and full-mesh topologies, as shown in Figure 6-1.

Figure 6-1. WAN Topologies

Hub-and-Spoke Topology

A star or hub-and-spoke topology provides a hub router with connections to the spoke routers through the WAN cloud. Network communication between the sites flows through the hub router. Significant WAN cost savings and simplified management are benefits of the hub-and-spoke topology. Hub and spoke topologies also tend to be the most popular WAN topologies.

A major disadvantage of this approach is that the hub router represents a single point of failure. The hub-and-spoke topology limits overall performance when accessing resources at the central hub router from the spoke routers, which affects scalability.

Full-Mesh Topology

With full-mesh topologies, each site has a connection to all other sites in the WAN cloud (any-to-any). As the number of sites grows, so does the number of spoke connections needed. Consequently, the full-mesh topology is not viable in very large networks. However, a key advantage of this topology is that it has plenty of redundancy in the event of network failures. But redundancy implemented with this approach does have a high price associated with it.

Here are some issues inherent with full-mesh topologies:

• Many virtual circuits (VCs) are required to maintain the full mesh
• Issues occur with duplication of packets for each site
• Complex configurations are needed
• High cost

Partial-Mesh Topology

A partial-mesh topology has fewer VC connections than a full-mesh topology. Therefore, not all sites in the cloud are required to be connected to each other. However, some sites on the WAN cloud have full-mesh characteristics. Partial-mesh topologies can give you more options and flexibly as far as where you may want to place the high-redundancy VCs given your specific requirements.