?– Communication servers: Communication servers concentrate dial-in and dial-out
user communications.
Presentation
Application
OSI Model
WAN Services
Session
Transport
Network
Data Link
Physical
Frame Relay, ATM, HDLC
Electrical, Mechanical, Operational Connections
Understanding WAN Technologies 351
?– Modems or digital service units (DSU)/channel service units(CSU): In analog
lines, modems convert the digital signal of the sending device into analog format for
transmission over an analog line and then convert this digital signal back to digital form
so that it can be received and processed by the receiving device on the network. For
digital lines, a DSU and a CSU are required. The two are often combined into a single
piece of equipment, called the DSU/CSU. The DSU/CSU can also be built into the
interface card in the router.
?– WAN networking devices: Other devices, such as ATM switches, Frame Relay
switches, public switched telephone network (PSTN) switches, and core routers, are
also used within the cloud to support the access services.
Devices on the subscriber premises are referred to as customer premises equipment (CPE).
The subscriber owns the CPE or leases the CPE from the service provider.
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