9 – Wide area networks

Wide area networks (WANs) occur when two or more local area networks (LANs) are joined over a long-distance link. This link requires a digital access from the local telephone exchange, and this can be provided by a conventional modem, a digital subscriber line (DSL) modem, or integrated services digital networks (ISDN) terminal. High-speed digital subscriber line (HDSL) provides a means of extending 2 Mbit/s E1 or 1.544 Mbit/s T1 data to subscribers when the only access cable is twisted pair. Asymmetric digital subscriber line (ADSL) system allows incumbent telephone operators to use existing twisted pair cables for providing television services. Very high speed digital subscriber line (VDSL) provides a very high data rate into offices and buildings. Major standards organizations have recognized asynchronous transfer mode (ATM) as the preferred standard for broadband integrated services digital networks (BISDN). ATM operates at the data link and physical layers of the ISO model for open systems interconnection (OSI). Some higher level activities (such as routing) are also provided by ATM.