There are three basic WAN technology categories:
• Circuit switched
• Packet switched
Leased lines, also known as point-to-point links, are permanent connections established between two customer end points through a carrier's WAN. Usually these types of links are for private use by the customer for the duration of their lease or contract.
Circuit switching is a technology used by ISDN. This is where the carrier establishes a physical circuit for the length of the session. This type of technology is similar to establishing a phone call.
Packet switching, used by ATM, Frame Relay, Switched Multimegabit Data Service (SMDS), and X.25, can share a single physical connection through the carrier network. In these types of networks, broadcast traffic can greatly affect the network's performance. However, a technology such as X.25 and SMDS that does not support broadcasting is called a nonbroadcast multi-access (NBMA) network. Networks such as Frame Relay and ATM that do support broadcast are called multi-access networks.
Beyond meshing topologies, there are two other WAN configuration or connection types that need to be discussed. The first is a point-to-point connection. This is a description of a type of logical connection. There is a distinction between the point-to-point WAN technology and the point-to-point connection. A Frame Relay network can have one core router that only allows communication between the core and the remote offices. This means that the remote offices could not talk directly to each other, so logically the Frame Relay network is operating as multiple point-to-point connections. This type of connection is very common in T1 WAN configurations where one end system is connected to another. This connection is also seen in Frame Relay and ATM.
The other type of connection, which is often used in packet-switched networks, is the point-to-multipoint type of connection. This is similar to the star topology configuration, where one core or root device is connected to multiple end devices or leaves. These types of connections, as well as the meshing topologies, pose problems in routing because of loops.
Using the split-horizon update prevents a common problem with routing loops. Split horizon is a method of preventing a router from sending routing information out to the same interface from which it received the update. However, in the case of point-to-multipoint topologies, also known as hub-and-spoke topologies, where one central point handles all the traffic, how is this problem resolved? This problem is addressed in Frame Relay by using subinterfaces. That means that one physical interface has multiple virtual interfaces. Routing information from one virtual interface is sent out another virtual interface, thereby avoiding the split horizon problem.
After reviewing the WAN technology you want to implement, the next step is to select the right hardware.
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