|  | 2.1 | WANs
| 2.1.1 | WANs and devices

A WAN (wide area network) operates at the physical layer and the data link layer of the OSI reference model. It interconnects LANs (local area networks) that are usually separated by large geographic areas. WANs provide for the exchange of data packets/frames between routers/bridges and the LANs they support. The major characteristics of WANs are: They operate beyond the local LANs geographic scope. They use the services of carriers such as the Regional Bell Operating Companies (RBOCs) and Sprint and MCI. They use serial connections of various types to access bandwidth over wide-area geographies. By definition, WANs connect devices that are separated by wide geographical areas. Such devices include: routers -- offer many services, including internetworking and WAN interface ports switches -- connect to WAN bandwidth for voice, data, and video communication modems -- interface voice-grade services; channel service units/digital service units (CSU/DSUs) that interface T1/E1 services; and Terminal Adapters/Network Termination 1 (TA/NT1s) that interface Integrated Services Digital Network (ISDN) services communication servers -- concentrate dial-in and dial-out user communication