WAN Topologies


WAN Topologies.  The phrase WAN Topology refers to the arrangement or relative positioning of links and nodes.  This webpage will present several major types of WAN topologies.  These topologies can end up being analogous, at greater levels of abstraction/hierarchy, to LAN topologies.

The Point To Point WAN.  A Point-To-Point WAN has a remote access link for each major node in the WAN.  The link can be anything from a T-3 line to a 56 Kbps dial up line.  The following diagram represents a intercity WAN.



Its major advantage is that
  • it is inexpensive relative to the other options

Its major disadvantages have to do with

  • vulnerability to failure in key components
  • limited scalability
    • number of hops and alternative routes are made worse

Thus the point-to-point WAN is best when there are only two or three major locations.

The Ring WAN.  The Ring WAN is developed by having point to point connections that connect the major nodes in a ring.  This is illustrated in the next image.



Its major advantages over other topologies are
  • This is an improvement over the point-to-point WAN in that it provides alternative routes
  • It is less expensive than all but the point-to-point WAN

Its biggest disadvantages relative to other topologies are

  • it is slightly more expensive than the point-to-point
  • it has slightly worse scalability problems the point-to-point

The Ring WAN is best when used to connect only a few sites.

The Star WAN.  A WAN star is laid out in a star configuration with one location as the "hub".  In this case this hub will make use of something called a concentrator router.  In the following figure the concentrator router is located in Chicago.



Its major advantages relative to other topologies are
  • it is more scalable
    • relatively easy to add nodes
  • each node is at most two hops away from any other

Its major disadvantage is

  • it has a single point of failure at the concentrator router

To get by some of these problems, network administrators tend to add in some alternative links between sites not at the hub.  This increases costs slightly, but gives alternative routes in case of failure or congestion.  Providing direct links between all possible pairs of nodes, a full mesh topology, requires a huge expense and is not at all scalable.

This leads to the most typical high volume implementation.

The Multitiered WAN.  A Multitiered WAN makes use of

  • concentrator routers like the star WAN,
  • it also links these concentrators together
  • it is also likely to have each major node connected to more than one concentrator router
  • it may also have direct links between other nodes

It is scalable because at worst just one link needs to be added to some concentrator.  Though obviously, performance is improved if there are more links.  In addition these links can be made due to traffic demands.

The following diagram represents a multitiered WAN.