WAN DEVICES

WANs are groups of LANs connected together with communications links from a service provider. Because the communications links cannot plug directly into the LAN, it is necessary to identify the various pieces of interfacing equipment.

LAN-based computers with data to transmit send data to a router that contains both LAN and WAN interfaces. The router will use the Layer 3 address information to deliver the data on the appropriate WAN interface. Routers are active and intelligent network devices and therefore can participate in network management. Routers manage networks by providing dynamic control over resources and supporting the tasks and goals for networks. Some of these goals are connectivity, reliable performance, management control, and flexibility.

The communications link needs signals in an appropriate format. For digital lines, a channel service unit (CSU) and a data service unit (DSU) are required. The two are often combined into a single piece of equipment, called the CSU/DSU. The CSU/DSU may also be built into the interface card in the router.

A modem is needed if the local loop is analog rather than digital. Modems transmit data over voice-grade telephone lines by modulating and demodulating the signal. The digital signals are superimposed on an analog voice signal that is modulated for transmission. The modulated signal can be heard as a series of whistles by turning on the internal modem speaker. At the receiving end the analog signals are returned to their digital form, or demodulated.

When ISDN is used as the communications link, all equipment attached to the ISDN bus must be ISDN-compatible. Compatibility is generally built into the computer interface for direct dial connections, or the router interface for LAN to WAN connections. Older equipment without an ISDN interface requires an ISDN terminal adapter (TA) for ISDN compatibility.

Communication servers concentrate dial-in user communication and remote access to a LAN. They may have a mixture of analog and digital (ISDN) interfaces and support hundreds of simultaneous users.