|
|
2.1 | |
WAN Technologies Overview | |
| 2.1.2 | |
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 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.
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.
|
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.
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. 
Web Links