A wide area network (WAN) is a network connecting geographicallydistinct locations, which may or may not belong to the same organization. WAN topologies use both LAN add enterprise-wide topologies as building blocks, but add more complexity because of the distance they must cover, the larger number of users they serve, and the heavy traffic they often handle. For example, although a simple ring topology may suffice for a small office with 10 users, it does not scale well and therefore cannot serve 1000 users. The particular WAN topology you choose will depend on the number of sites you must connect, the distance between the sites, and any existing infrastructure.
A WAN with single interconnection points for each location is arranged in a peer-to-peer topology. A WAN peer-to-peer topology is similar to peer-to-peer communications on a LAN in that each site depends on every other site in the network to transmit and receive its traffic. However, the peer-to-peer LANs use computers with shared access to one cable, whereas the WAN peer-to-peer topology uses different locations, each one connected to another one through dedicated circuits.
The WAN peer-to-peer topology is often the best option for organizations with only a few sites and the capability to use dedicated circuits--that is, continuously available communications channels between two access points that are leased from a telecommunications provider, such as an ISP.
In a ring WAN topology, each site is connected to two other sites so that the entire WAN forms a ring pattern. This architecture is similar to the ring LAN topology, except that a ring WAN topology connects locations rather than local nodes. The advantages of a ring WAN over a peer-to-peer WAN are twofold: a single cable problem will not affect the entire network, and routers at any site can redirect data to another route if one route becomes too busy. On the other hand, expanding a peer-to-peer WAN because it requires at least one additional link. For those reasons, WANs that use the ring topology are only practical for connecting fewer than four or five locations.
The star WAN topology mimics the arrangement of a star LAN. A single site acts as the central connection point for several other points. This arrangement provides separate routes for data between any two sites. As a result, star WANs are more reliable than the peer-to-peer or ring WANs. As a general rule, reliability increases with the number of potential routes data can follow. Another advantage of a star WAN is that when all of its dedicated circuits are functioning, a star WAN provides shorter data paths between any two sites.
Like an enterprise-wide mesh, a mesh WAN topology incorporates many directly interconnected nodes--in this case, geographical locatinos. Because every site is interconnected, data can travel directly from its orgin to its destination. If one connection suffers a problem, routers can redirect data easily and quickly. Mesh WANs are the most fault-tolerant type of WAN configuration because they provide multiple routes for data to follow between any two points.
One drawback to a mesh WAN is the cost; connecting every node on a network to every other entails leasing a large number of dedicated circuits. With larger WANs, the expense can become enormous. To reduce costs, you might choose to implement a partial mesh, in which critical WAN nodes are directly interconnected and secondary nodes are connected through star or ring topologies. Partial-mesh WANs are more practical, and therefore more common in today's business world, than full-mesh WANs.
Tiered WAN topologies are similar to the hierarchical hybrid topologies used with LANs. In a tiered WAN topology, WAN sites connected in a star or ring formations are interconnected at different levels, with the interconnection points being organized into layers.
Variations on this topology abound. Indeed, flexibility makes the tiered approach quite practical. A network architect can determine the best placement of top-level routers based on traffic patterns or critical data paths. In addition, tiered systems allow for easy expansion and inclusion of redundant links to support growth. On the other hand, their enormous flexibility means that creation of tiered WANs requires careful consideration of geography, usage patterns, and growth potential.